Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Widewing on January 21, 2007, 07:24:56 PM
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I was doing some offline testing of the F6F-5 and F4U-1A and stumbled upon a behavior that I can only conclude is a bug.
It seems both of the aircraft stall at higher speeds power-on, than they do power-off. This is unusual for a prop driven aircraft. Especially in light of the fact that actual stall data on both types is readily available. NACA Report No. 829 documents the effect of prop slipstream raising the lift coefficient on both the F6F and F4U. So, power-on stalls should occur at lower speeds than power-off stalls. In the game, the exact opposite occurs.
Immediately below is stall data for the F6F-5 obtained from a Navy flight test document. Below that is data taken in-game and recorded on film.
F6F-5 stall speeds, per the US Navy:
Clean, power on: 61 knots IAS (70 mph IAS)
Clean, power off: 67 knots IAS (77 mph IAS)
Landing configuration, power on: 55 knots IAS (63 mph IAS)
Landing configuration, power off: 60 knots IAS (69 mph IAS)
(Note: At full overload weight, these numbers increase by about 25 to 30 mph across the board)
Here’s the stall speed data for each type. Power-on setting was MIL power. Speeds recorded are where the aircraft fell off on one wing.
F6F-5
Clean configuration, power-on: 91 mph
Clean configuration, power-off: 62 mph
Landing configuration, power-on: 57 mph
Landing configuration, power-off: 51 mph
F4U-1A
Clean configuration, power-on: 95 mph
Clean configuration, power-off: 69 mph
Landing configuration, power-on: 73 mph
Landing configuration, power-off: 57 mph
I have forwarded this data, along with the films, to HTC for evaluation.
Has anyone else noticed this behavior? Perhaps someone would like to do their own testing as well, although I have great faith in my methodology.
My regards,
Widewing
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well u can "force" the plane to stay stable by stopping the wing dippin. with control imput B5N can "fly" at around 45ish mph.... but if i let go of stick one wing drops at around 60-70
the reason its stalling later with no hands on stick with no power is there is very little prop wash pushing the left (or is it right? lol) wing down. so it doesnt dip the wing as early.
with power on max its gona push one wing down (ie it stalls first).
also i dont think full mil power really makes sense? i landed a F4u1d at around 57 mph with a touch of power..... full power would have probably flipped it over.
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just tested with f4u1d.... 50% full ammo load.
67-71mph with around 15-20mp.... if i take throttle out, stall limitor stops buzzing but the plane drops out of the sky in a relatively stable/level stall. hitting full power to recover from this causes the plane to flip.
how are you testing? (sorry if i missed)
level at 500ft? or trying max angle of attack type climb? cos for me, mil power causes the plane to overspeed the flaps and retracts them.
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Originally posted by 0verlag
just tested with f4u1d.... 50% full ammo load.
67-71mph with around 15-20mp.... if i take throttle out, stall limitor stops buzzing but the plane drops out of the sky in a relatively stable/level stall. hitting full power to recover from this causes the plane to flip.
how are you testing? (sorry if i missed)
level at 500ft? or trying max angle of attack type climb? cos for me, mil power causes the plane to overspeed the flaps and retracts them.
I perform the standard stall test defined by Grumman on their test cards, but I add more power, up to MIL power. Standard test is at Normal power. This varies from plane to plane, but is very similar between the F6F and F4U.
Beginning at between 150 and 200 mph IAS, I ease up the nose until air speed begins bleeding (usually at a vertical rate greater than that sustainable at MIL power). I maintain such backpressure as needed to maintain my climb angle. I do not correct or stablize with rudder. When the aircraft falls off on one wing, I record the speed via E6B.
For power-off I perform the same test, but reduce power to idle once a climb angle is established.
Several of us tested various fighters in the TA tonight. These included the P-51D, P-38G, Fw 190A-5, Bf 109F-4, FM-2, N1K2-J, Spitfire Mk.I and Ki-61. All displayed the same tendency to stall at higher speeds power-on than with power-off. All were tested in clean configuration.
To see an actual demonstration of this type of test, use the link below to view the F6F-3 video on Zeno's Drive-in.
F6F-3 video (http://www.zenoswarbirdvideos.com/realg2/HCb.ram)
My regards,
Widewing
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What is the definition of "stall" in this case?
I recall Hitech saying that his definition of a "stall" is the point where a plane cannot maintain level flight at a given altitude and will inevitably start losing altitude. Perhaps there may be discrepancies here?
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Widewing, I'm not sure how much "test technique" is included in the Grumman test cards but believe the test should probably be done without using ailerons but with rudder. With power on, the stall is significantly affected by slipstream (which usually decreases stall speed because of the additional energized airflow) but this is eventually overcome by torque. If you're not countering the torque with rudder to stay wings level you can't be sure if the aircraft is rolling off due to stall or torque, especially with the power these two planes have. (I'll take a wild guess and say both are rolling off on the left wing) Even with rudder you can't be positive which is the predominant cause (without additional data) but the value you're recording takes into account the control power of the rudder and how the airplane would be flown. Aileron is not used to counter the rolloff because they will increase your stall speed due to induced tip stall.
For a stall test I'd be looking for both the stall speed and the mode of the stall (i.e., the nature of the stall...wing drop, nose drop, high descent) and the test should be conducted at a fixed altitude vice climbing. Level off at your test altitude, set your test configuration (gear, flaps as required) and pull power. As you decelerate add aft stick to maintain level flight and as you approach your anticipated stall speed start feeding in power and rudder so that you're established wings level, nose-high, on altitude at full power before the anticipated speed is reached. The stall occurs when you can no longer maintain altitude (or reach some standardized rate of descent), a wing drops off or the nose breaks. Some airplanes will just "mush" and descend wings level but I've never seen this is an AH aircraft and it's unlikely to occur in a single engine prop.
While I'm sure torque is contibuting to the poor results of the power-on tests you also show significant differential in the power-off test but the AH numbers are significantly better than Grumman's. I don't understand these wide discrepancies, particularly in the clean configuration as this should be a pretty straight forward test. The Grumman cards should include aircraft weight and test altitude (actually they should have complete aircraft and test condition data), do they? Differences in either of these (particularly weight) would significantly affect the stall IAS and I suspect prop settings would also have an effect. Also, since stall occurs at a specific alpha regardless of the weight or altitude it would be great if we could correlate the Grumman numbers with HT aircraft but I don't know if the actual test aircraft were instrumented for alpha (they should have been) or if there is some way for HTC to look at film data and determine the alpha for the AH aircraft for comparison.
If you could send me the Grumman cards I'd love to take a look at them. When I get home today I'll go out to my garage and get some of my old manuals, I'm sure there's info specific to techniques for stall testing in prop planes.
Mace
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Originally posted by Widewing
Has anyone else noticed this behavior? Perhaps someone would like to do their own testing as well, although I have great faith in my methodology.
Hi Widewing
Although I never test the aircraft power off, I have produced EM diagrams for those two aircraft recently, I'm mainly interested in how they perform against other aircraft, but here I've overlaid the EM diagrams for each aircraft at MIL power and at WEP.
(http://www.badz.pwp.blueyonder.co.uk/Files/Images/F6F5MILvWEP.jpg)
In this diagram you can see that with WEP, the F6F is faster, and the Ps=0 curve is higher. That means that with WEP, the F6F has a better sustained turn rate, but the stall speeds are almost identical. It doesn't show on that diagram very clearly, but there is a very small difference. That seems to concur with your results.
Here is the diagram for the F4U1 with and without WEP:
(http://www.badz.pwp.blueyonder.co.uk/Files/Images/F4U1MILvWEP.jpg)
In this diagram, the WEP also results in a higher top speed, and a higher sustained turn rate, as indicated by the slightly higher Ps = 0 curve. But in these tests the F4U1 with WEP has a slightly lower stall speed. Of course, I wouldn't expect to see very much difference in the accelerated stall speeds with and without wep, so these results aren't very helpful.
One question about your tests, that might be a potential issue, when you say power off, did turn off the engine, or just throttle back? I think turning off the engine would yield invalid results, accept perhaps for the P-38.
Hope that helps...
Badboy
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Widewing, I pulled out my junk and then took a quick test flight and here's what I think. The constant-alt technique isn't going to work at mil power(but then you knew that already didn't you!). Trying to bring on power and rudder simultaneously is too eratic and you rapidly go into wing rock so the climb method is better for power-on. Level works well for power-off. Assuming the aerodynamic stall should be lower with power-on due to prop wash what I think is happening is that it reaches minimum usable flying speed due to torque before the aerodynamic stall. This is reflected by the slightly lower stall speeds I got with 47MP. Perhaps torque is overmodeled or rudder power is undermodeled.
I don't know what you used but I went with 50% fuel and level tests were flown at 5k and climb tests went as high as 8k. All of these are in cruise configuration with stall limiter off and auto trim off. I did these quick and dirty but will try to do a complete set and adjust my climb start for the power on so stall is closer to 5k ft. I'll also need to know what altitudes and weights you used to more accurately compare numbers.
Clean Power-off: 90MPH
Clean Power-off: 90MPH
Clean Power-off: 90MPH
Clean Mil power: 90mph
Clean Mil Power: 80mph
Clean Mil Power: 90mph
Clean 47mp: 85mph
Clean 47mp: 85mph
Unfortunantly, I forgot the clipboard doesn't show in film so these are best estimates based on reading the steam gauge. BTW, are the film viewer speeds shown on the right in true? I'm seeing considerably higher speeds than Indicated but they show the same relative performance differences. I suppose I'll have to get something like Fraps so I can capture the clipboard during tests.
I was wondering something else however. Doesn't the F6F have a known pitot/static system descrepancy? This would be even worse at high AOA. You'd assume though that Grumman used an instrumented plane or a calibrated chase plane to determine the speeds in addition to applying position error corrections but then you never know.
Mace
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When stall testing, I do not use aileron or rudder to offset wing drop. I allow the aircraft to fall-off, usually with the left wing dropping when under power, but can go either way when the engine is idling.
I perform the power-on stall test at Normal power (check E6B for MAP and RPM settings).
For both the F6F-5 and F4U-1A, Normal power is 44" of manifold pressure @ 2550 rpm.
I start out level. I then set MAP and rpm. I begin easing up the nose until speed begins to bleed off. I hold the nose high until the aircraft stalls and drops a wing.
For power-off stall tests, I fly exactly as before, but once I have established a nose-high attitude, I reduce power to idle (engine is running). I hold the nose high until the plane stalls and drops a wing.
Even at Normal power, both the F4U-1A and F6F-5 both stall at markedly higher speeds than they do with power reduced to idle. This is exactly backwards to test data and contrary to NACA Report No. 829, which tested both the F4U and the F6F in a wind tunnel.
To my knowledge, all propeller driven aircraft (tractor type, not pusher types) stall at a lower air speed when under power, than they do when engine rpm is reduced to idle (minimal thrust). This is due to the slipstream effect.
In the game, all prop driven aircraft stall at higher air speed when under power than they do when engine rpm is reduced to idle. There is no slipstream effect. In fact, there is apparently a lift penalty when under power.
Let's look at a new pilot reporting to a squadron flying Hellcats in 1944. Among the first things he will do on his first flight (or soon thereafter) is to do power-on and power-off stalls. He does this to learn the feel and warning signs of stalls under those conditions. Every new civil pilot is required to do the same thing as part of the training curriculum. It is essential to have stall experience in the aircraft, both power-on (stalling while climbing) and power-off (stalling with a dead engine). The procedure defined above is virtually identical to that which will be used by the new Hellcat pilot.
Here are two very short films showing the procedure:
F6F-5 Clean Stalls (http://home.att.net/~historyzone/F6F-Clean-Stalls.ahf)
F4U-1A Clean Stalls (http://home.att.net/~historyzone/F4U-1A-Clean-Stalls.ahf)
Mace, I'll have to scan in those cards. I have photocopies.
My regards,
Widewing
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Originally posted by Mace2004
Widewing, I pulled out my junk and then took a quick test flight and here's what I think. The constant-alt technique isn't going to work at mil power(but then you knew that already didn't you!). Trying to bring on power and rudder simultaneously is too eratic and you rapidly go into wing rock so the climb method is better for power-on. Level works well for power-off. Assuming the aerodynamic stall should be lower with power-on due to prop wash what I think is happening is that it reaches minimum usable flying speed due to torque before the aerodynamic stall. This is reflected by the slightly lower stall speeds I got with 47MP. Perhaps torque is overmodeled or rudder power is undermodeled.
I don't know what you used but I went with 50% fuel and level tests were flown at 5k and climb tests went as high as 8k. All of these are in cruise configuration with stall limiter off and auto trim off. I did these quick and dirty but will try to do a complete set and adjust my climb start for the power on so stall is closer to 5k ft. I'll also need to know what altitudes and weights you used to more accurately compare numbers.
Clean Power-off: 90MPH
Clean Power-off: 90MPH
Clean Power-off: 90MPH
Clean Mil power: 90mph
Clean Mil Power: 80mph
Clean Mil Power: 90mph
Clean 47mp: 85mph
Clean 47mp: 85mph
Unfortunantly, I forgot the clipboard doesn't show in film so these are best estimates based on reading the steam gauge. BTW, are the film viewer speeds shown on the right in true? I'm seeing considerably higher speeds than Indicated but they show the same relative performance differences. I suppose I'll have to get something like Fraps so I can capture the clipboard during tests.
I was wondering something else however. Doesn't the F6F have a known pitot/static system descrepancy? This would be even worse at high AOA. You'd assume though that Grumman used an instrumented plane or a calibrated chase plane to determine the speeds in addition to applying position error corrections but then you never know.
Mace
Mace, several things... Watch the two films that I posted. Films show True Air Speed. It shows exactly how I test and power settings used. It will also show that I flew the same profile and it shows this too:
F4U-1A
Power-on stall (44" MAP @ 2550 rpm) 101 mph
Power-off stall (0" MAP @ 2550 rpm) 86 mph
F6F-5
Power-on stall (44" MAP @ 2550 rpm) 91 mph
Power-off stall (0" MAP @ 2550 rpm) 73 mph
I used Normal power as this is the setting used for normal climb out. At 5,500 feet, this represents 1,675 hp in low blower (for either airplane)
As to adding more fuel; this will raise the stall threshold, but will have no significant effect on the aerodynamics.
The reason I use a nose-up attitude for both power-on and power-off is two-fold.
1) Angle of attack is similar, thus the breakdown of lift will be similar. Best to compare apples to apples and not introduce a second variable.
2) Level testing results in mushing. It is extremely difficult to pin-point when mushing ends and a true stall begins. A higher angle of attack results in a pronounced wing drop, which is much easier to pin-point.
Also, I do not believe that the stall break is a result of torque. It's too abrupt. If torque was the culprit, you would find it necessary to constantly dial in rudder trim or apply rudder pedal to counter torque as the aircraft slowed. However, I find that I don't need to do that. No rudder input is required and almost no aileron is input either.
I've done testing with combat trim on and off, but the result is the same. I was hoping that this behavior might be isolated to the use of combat trim. It isn't.
One thing I did notice was that the F6F-5 suffers from a dynamic instability along its roll axis, making it extremely twitchy and sensitive to aileron input when near stall speed. On the other hand, the F4U is extremely stable along its roll axis, with nary a twitch. Inasmuch as the F6F was considered to have better ailerons at low speeds (around 100 mph) than the F4U, this is rather unusual.
My regards,
Widewing
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Maybe try using the auto-speed control and see what that does? I don't know if there would be enough trim authority to counteract the torque or not, but who knows?
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Widewing:
Using your stall testing technique here's what I get for the P-51D 50% fuel.
power-on clean: 97 mph IAS (106 mph TAS, 4.5k)
power-off clean: 88 mph IAS (95 mph AS, 3.5k)
No auto-trim on.
Very curious indeed. Something in the testing technique? I haven't tried it recently but using a power-off level flight approach to testing since I was never comfortable with my results. Curious as to what Pyro or HT think.
Tango, XO
412th FS Braunco Mustangs
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Thanks for the details Widewing. I can see some of the discrepancy between our numbers is related to alt and weight. I was 5k to 8k and 50% fuel. Looks like you're taking the film speeds since True and Indicated (assuming no instrument errors) are essentially equal near sea level. I did my testing higher so had to rely on the AS indicator vice the film so didn't have a precise source of speed. When I repeated at the same weight and alt as you did I'm seeing similar numbers to yours, at least for the F6F which is all I have time for tonight.
I do wish we had an accurate measure of alpha as that's the most precise measure. Regarding AOA and mushing, the plane's going to stall at the same AOA regardless of it's nose position (for the same aircraft configuration). The difference in mushing you're seeing is probably due to more rapid decay of airspeed when nose high which results in the more pronounced stall as opposed to the level flight technique which only requires a slight lowering of the nose to recover.
Agree that the fleet would have used the nose-high technique to demo the stall as it's a bit easier to setup and results in the pronounced departure and a more positive recovery technique but when it comes to test data it also adds a greater component of lift due to thrust.
This actually brings up another point. I agree that the prop wash should generate more lift but you also have the thrust component itself. Jet engine thrust provides no wash over the wings yet still contributes to a lower nose-high stall speed (no stall if you have a greater than 1/1 thrust/weight ratio). Combine the prop wash and thrust effects in a very nose high-attitude and you should see an even greater contribution to lift but it doesn't appear to be reflected in the AH models.
As far as torque is concerned, using your technique I had to use considerable right stick (about 1/4 to 1/2 throw) to counteract torque in the F6F and you're right about the roll stability, it's very touchy. This is one of the reasons I was unable to do the level-flight technique by adding power, it was just too hard to coordinate adding power and rudder smoothly enough to get reasonable data given the wing rock. The difference in lateral stick input may just be due to our different control settings of course but the torque effect is there.
Regarding the NACA test report, I assume they were testing full-scale airframes complete with engine. Did they measure or comment on torque effects in the tunnel? Torque (and p factor) are always going to present themselves at the slowest speed, I wouldn't believe they'd discount these effects. Regarding the prop-wash itself, I've always thought that we didn't get the effect we should when doing rudder reversals. In most planes gunning the engine at the top of a climb provides lots of nrg to the rudder and really assists in yawing the nose around but I just don't feel this effect in AH. I think you're right that they've left prop-wash completely out of the model. Just a side note related to your comments in the TA regarding momentum but has anyone been able to do a lomcevak in an AH plane? I haven't been able to get any of the fighters to do this but I'd think they should be capable of it. This would require moments in all three axis, is this left out? Also, did WWII era planes ever experience coupled departures or were the rates too low?
Whatever the individual contributions of prop-wash and torque actually are I agree with you that AH doesn't seem to have it right. By the way, I also did a quick check in the Hurri which certainly isn't a torque monster. It also stalled at higher speed under power but the airspeed delta wasn't as large (about 10mph in my quick look).
Mace
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Whilst I realise your initially testing clean to compare with RL figures......could you not test under AH auto speed setting for power on and power off
You should be able to obtain very accurate comparison between AH's power on and power off stall points then.................
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Originally posted by Mace2004
I do wish we had an accurate measure of alpha as that's the most precise measure. Regarding AOA and mushing, the plane's going to stall at the same AOA regardless of it's nose position (for the same aircraft configuration). The difference in mushing you're seeing is probably due to more rapid decay of airspeed when nose high which results in the more pronounced stall as opposed to the level flight technique which only requires a slight lowering of the nose to recover.
There should be no large difference wether the stall is generated during level flight or in glide, the later is just easier to reach and measure.
AFAIK the speed is directly CAS in the AH.
gripen
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Originally posted by Tilt
Whilst I realise your initially testing clean to compare with RL figures......could you not test under AH auto speed setting for power on and power off
You should be able to obtain very accurate comparison between AH's power on and power off stall points then.................
Tilt - the above tests were done by manually flying, no auto-speed involved.
Tango, XO
412th FS Braunco Mustangs
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Widewing: Your test is not really measuring power on stall. The stall occurs as soon as you see the cockpit buffet. Also you will need to provide control input both roll and yaw to maintain level flight. Other wise you are just measuring our trim settings. Power on stall is very hard to fly and test do to the fact you have to get way behind the power curve before you start adding throttle.
HiTech
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Originally posted by hitech
Widewing: Your test is not really measuring power on stall. The stall occurs as soon as you see the cockpit buffet. Also you will need to provide control input both roll and yaw to maintain level flight. Other wise you are just measuring our trim settings. Power on stall is very hard to fly and test do to the fact you have to get way behind the power curve before you start adding throttle.
HiTech
Thanks for replying HiTech. I appreciate your interest.
If you are talking about stall buffet, then this is an impending stall, or wing root stall, not yet fully stalled. This assumes that the cockpit shake represents stall buffet. This also assumes that your code stalls the wing progressively from root to tip.
I've flown several power-on stalls in the Grumman C-1A. Not especially difficult to do. The S-2 and C-1 gave almost no warning of a stall, thus a stall-shaker was installed on the yoke. This worked via an airflow sensor installed on the upper surface of the outboard port wing. If the shaker began to vibrate it told you that that you were approximately 5 knots above stall.
I've flown the the identical profiles described previously with combat trim off and saw no appreciable difference in stall speed. My results were no different; the planes stall at higher speeds power-on than with power-off.
Note that flying with a nose-high profile reduces shake duration because speed is bleeding somewhat faster than when nearly level.
Even using cockpit shake as the reference and trimming manually, the discrepancy is still there. Power-on stall occurs about 12 mph higher than power-off stall. In the real F4U and F6F, the opposite occurs.
F4U-1A, clean.
Power-on stall to shake (44" MAP @ 2550 rpm) 107 mph
Power-off stall to shake (0" MAP @ 2550 rpm) 95 mph
F6F-5, clean.
Power-on stall to shake (44" MAP @ 2550 rpm) 98 mph
Power-off stall to shake (0" MAP @ 2550 rpm) 86 mph
Here's the films. By the way, if the aircraft seems to bank to one side while climbing, this was due to not having a visual reference of the horizon, not to excessive torque.
F6F-5 stalls, manual trim (http://home.att.net/~historyzone/F6F-Clean-Stall2.ahf)
F4U-1A stalls, manual trim (http://home.att.net/~historyzone/F4U-Clean-Stall2.ahf)
Perhaps Mace can offer his opinion. Mace is a graduate of the Naval Test Pilot School at Patuxent River.
Most folks probably haven't seen NACA Technical Report No. 829. Use this link (http://hdl.handle.net/2060/19930091906) to download a pdf copy from NASA's technical report server. The report is titled: Summary of measurements in Langley full-scale tunnel of maximum lift coefficients and stalling characteristics of airplanes. Planes evaluated were the F6F-3, F4U-1, P-51B and P-63A, among several others.
My regards,
Widewing
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Originally posted by hitech
Widewing: Your test is not really measuring power on stall. The stall occurs as soon as you see the cockpit buffet. Also you will need to provide control input both roll and yaw to maintain level flight. Other wise you are just measuring our trim settings. Power on stall is very hard to fly and test do to the fact you have to get way behind the power curve before you start adding throttle.
HiTech
HiTech,
Who defines the initial onset of buffet as the stall point? Initial buffet is usually caused just by localized flow separation often due to a non-uniformity in the wing such as a stall strip, dog tooth, gun ports, etc. It can also be caused by changing flow patterns over the wing. Some wings will go deep into buffet before losing significant lift but of course some wings will lose lift immediately after or coincidentally with onset of buffet; however this would not be a suitable fighter aircraft wing. I suppose that the test conditions could define stall as the initial onset of buffet if such is the case (or an ultraconservative number is desired such as GA or transport aircraft) but I've never seen stall defined this way. I also don't think I'd want to fight an airplane with such a characteristic.
Also, if you define power-on stall as initial onset of buffet Widewing's numbers would be even worse (higher) and nowhere near Grumman's.
In every case I've ever seen stall is either the point that the nose or wing drops or the minimum usable flying speed. Minimum usable flying speed is identified as a result of some characteristic such as loss of control on one or more axis or high rate of descent (mushing). A "mushing" stall is very subjective, susceptible to pilot perception, and difficult to repeat since there is no definitive stall point, that's why a uniform, objective value of ROD or specific AOA can be defined. If the limit is minimum usable flying speed that becomes the defined stall.
I agree that control input should be made; however, the preferred control would be rudder followed by the minimum amount of aileron (because aileron inputs can induce tip stall).
In any case and regardless of the test procedure I see no way that the power-on and power-off stall speeds could possibly end up reversed per the actual flight-test data which supports Widewing's arguement that something is amiss.
Edit: I just finished going over the NACA report and it's very interesting stuff. They never address the effect of torque but it does look like the left roll-off for the F4U is significantly driven by stall induced by increased local AOA resulting from the prop. They added a stall strip to the right leading edge but it's not clear if this is addition to the stall strip installed in later production models. From what I can see it appears significantly larger than what I recall on the production planes.
Mace
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Originally posted by gripen
There should be no large difference wether the stall is generated during level flight or in glide, the later is just easier to reach and measure.
AFAIK the speed is directly CAS in the AH.
gripen
I must not have been clear. I was comparing the difference in nose-high (climbing) stalls and level flightstalls. Many (probably most) aircraft will either climb or descend a bit just prior to stall when doing the level test technique but the differences in recorded speed is very minimum. I agree that a level test compared to a descending test would give essentially the same results.
That said, a climbing stall will result in a more rapid reduction of airspeed which will skew the numbers downward. This, coupled with the lift due to thrust (the helicopter effect) will result in artificially low stall speeds. Great if you're a contractor trying to meet the stall requirements of a government contract but sucks when you're the pilot and find you stall five knots above the posted stall speed on approach.
As far as the speed in film viewer it appears to clearly be true airspeed. The E6B correctly shows identical true and indicated speeds on the deck but much higher true at altitude which also agrees with the film viewer speeds.
Mace
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Mace:
well since HT wrote the code he get's to define what exceeding aircraft clmax looks like so I take it from his comment that in AH physics cockpit buffet indicates that a stall has occured :).
Also, I'm certain Pyro and HT are aware of the NACA report 829 that Widewing is referring to. We've used it in the past to discuss other clmax data of particular aircraft. If you look at that doc it discusses a variety of things including the concept of initial stall vs. the progression of the stall over the wing. I know back when AH2 was coming out I had some conversations with Pyro that they were going to include more force vectors along the wings in AH2 which would enable finer modeling of lift across the wings. Back then I inferred from Pyro's comments that one of the things this would allow them to do was to model stall progression across the wing among other things (e.g. increased lift due to propwash etc. etc.) since Cl doesn't just drop to 0 when Clmax is exceeded, and also that Clmax may be exceeded on certain parts of the wing prior to others.
******
HT/Pyro:
I'm curious as to what you meant by "getting behind the power curve" in relation to performing a power-on stall in AH. I'm guessing you mean the power-required curve (drag) but not sure exactly what you were getting at in your statement. Is there a way that we should be conducting power-on stall tests for AH planes? I'm with Widewing that I would think power-on stalls should occur at lower airspeeds vs. power-off stalls.
Tango, XO
412th FS Braunco Mustangs
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Originally posted by Mace2004
As far as the speed in film viewer it appears to clearly be true airspeed. The E6B correctly shows identical true and indicated speeds on the deck but much higher true at altitude which also agrees with the film viewer speeds.
I mean that the AH speeds (from cockpit or film viewer) should be without any error caused by AoA. This means that the IAS from cockpit is directly CAS and the TAS from film viewer is TAS regardless the AoA. The both being same at sealevel as you noted.
gripen
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Rgr that. I agree that we should not have to worry about position/instrument error and the numbers, as presented within AH, are accurate.
Mace
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Originally posted by dtango
Mace:
well since HT wrote the code he get's to define what exceeding aircraft clmax looks like so I take it from his comment that in AH physics cockpit buffet indicates that a stall has occured :).
Also, I'm certain Pyro and HT are aware of the NACA report 829 that Widewing is referring to. We've used it in the past to discuss other clmax data of particular aircraft. If you look at that doc it discusses a variety of things including the concept of initial stall vs. the progression of the stall over the wing. I know back when AH2 was coming out I had some conversations with Pyro that they were going to include more force vectors along the wings in AH2 which would enable finer modeling of lift across the wings. Back then I inferred from Pyro's comments that one of the things this would allow them to do was to model stall progression across the wing among other things (e.g. increased lift due to propwash etc. etc.) since Cl doesn't just drop to 0 when Clmax is exceeded, and also that Clmax may be exceeded on certain parts of the wing prior to others.
******
HT/Pyro:
I'm curious as to what you meant by "getting behind the power curve" in relation to performing a power-on stall in AH. I'm guessing you mean the power-required curve (drag) but not sure exactly what you were getting at in your statement. Is there a way that we should be conducting power-on stall tests for AH planes? I'm with Widewing that I would think power-on stalls should occur at lower airspeeds vs. power-off stalls.
Tango, XO
412th FS Braunco Mustangs
dtango,
Yeah, he's talking about being below L/Dmax. To do level testing you need to be somewhere below L/Dmax so you can add power without accelerating, how far below is based on your specific excess power/ desired test power. You fly level with reduced power settings and decelerate till reaching the back side of the curve. At that point you need more power to fly slower and still stay level so you can start feeding in power until you reach test power. You need to be stabilized at full power before the stall point is reached.
The problem I'm seeing is in coordinating the various control inputs to counteract the increase in power during a relatively short period of time. This is going to be highly dependant upon your specific control set up (response curves, deadbands, damping, etc.) so some may be able to do this easier than others.
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Mace2004: Don't confuse aces high buffet with the real thing. I assume we can agree to define stall to be the point at which more AOA produces less total lift?
In AH the buffet starts at that AOA .
And on behind the power curve , maces description is what I was referring to.
Widewing: The problem is in our testing (which pyro has been doing) we get the opposite results.
HiTech
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It's very useful to know that about the buffet in AH. It also makes it a lot easier to do testing to have a nice clean indication of when stall is happening.
I've done some testing for various planes with power off and landing configuration. I'll do more in clean configurations and post what I find here. I'll also note vertical climb or descent at stall. If it's less than about 1000 fpm, the stall speed won't be different by more than about 2% from what it would be at 0 fpm climb or descent.
I'll also post the data from the pilots' manuals (taking into account corrected indicated airspeed and correct weight of the aircraft to match the tested configuration with AH).
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Originally posted by Mace2004
You fly level with reduced power settings and decelerate till reaching the back side of the curve. At that point you need more power to fly slower and still stay level so you can start feeding in power until you reach test power. You need to be stabilized at full power before the stall point is reached.
The problem I'm seeing is . . .
Yep, I have difficulty, too, getting into slow-flight regime. I can manage it with some power, but I'm having trouble getting up to full power on the back side of the power curve. It's much harder than doing it in a Cessna 152 for me. :)
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Originally posted by hitech
Mace2004: Don't confuse aces high buffet with the real thing. I assume we can agree to define stall to be the point at which more AOA produces less total lift?
In AH the buffet starts at that AOA .
And on behind the power curve , maces description is what I was referring to.
Widewing: The problem is in our testing (which pyro has been doing) we get the opposite results.
HiTech
HiTech, could Pyro e-mail a film of his procedure so that I can try to duplicate it?
My regards,
Widewing
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If the buffet is the sign of the beginning of less lift with more AoA, would it be safe to assume that turn fighting into the buffet is counterproductive? I would assume that this would mean pulling up to, but not including the buffet, would be the max AoA you should pull in a tight turn or the vertical?
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To me it feels like. Easing up the stick with the onset of buffet, the turn tightens.
I might be wrong though.
Widewing would know better than I.
Bronk
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Originally posted by hitech
Mace2004: Don't confuse aces high buffet with the real thing. I assume we can agree to define stall to be the point at which more AOA produces less total lift? In AH the buffet starts at that AOA .
Sure, that's certainly a valid approach but it sacrifices a bit of realism. Max performing an airplane means to get the absolute most of the turn performance and it's not unusual to ride in mild to moderate buffet (depending on the plane) to get best turn performance while using the seat of the pants feel for how close you are to stall. This is especially important when you're twisting all around the cockpit keeping track of your opponent and you're not looking at your airspeed or AOA indicator. If it's as you're describing then I'd think turn performance would suffer anytime there was any buffet at all. The difference would probably be noticable to some of the pilots that fly with the stall limiter off but maybe not to the average Joe. Of course you have a stall buzzer that gets louder the closer you get to stall, maybe this is enough warning. Another thought though, since there are some airplanes known for a nasty stall with little/no warning wouldn't tailoring the onset of buffet to each plane be more representative?
This is what I got today in the F6F after a lot more work, using buffet onset as stall and tweaking my technique. The numbers I present are the avg of 10 stalls in each configuration. One of the biggest "tweaks" was for the power-on stalls where I went to idle to get behind the curve and put just enough rudder to roll the airplane a bit to the right. Adding power rolled the airplane back to wings level flight. This let me get ahead of the roll. Just for comparison I put in Grumman's numbers (in parenthasis) and the delta. Power-on was 44inchesMP:
Power-off, landing: 71.9 (69) Delta: +2.9
Power-on, landing: 75.0 (63) Delta: +12
Power-off, clean: 89.5 (77) Delta: +12.5
Power-on, clean: 91.6 (70) Delta: +20.6
Again, while all stall speeds are above Grummans' the power-on stalls are much higher as Widewing contends.
I'll mention that power-on, clean is a very tough stall to perform, the airplane likes to just slowly nose over with full aft stick and the plane picks up speed. You have to be around 95-97mph just to get behind the curve and the plane will many times power out of it. It also has the broadest spread of stall points, from a low of 85 to a high of 97. The difficulty in actually getting the plane to stall reliably in this configuration also resulted in the greatest difference in pitch angle at the stall which accounts for the wide variation in results. This makes the results of the power-on, clean tests less reliable because of the inconsistency and lack of solid repeatability. Also, just a side note but I noticed that on numerous occasions I got the stall buzzer and buffet but it disappeared below 70mph.
Mace
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The buffet actually kicks in when any portion of the wing begins to stall- not necessarily at peak lift. However, it's a razor thin edge to maximize turning while in the buffet. If you go deep into the buffet you will degrade your turn.
I gave the autopilot full control authority and did the following power-on and power-off stall tests at 1/5 and 1/10 speed to take screenshots. Test subject was an F4U-1A at full internal load- 12,904 lbs. Atmospheric conditions were hard-coded to SL which is why TAS and IAS always match in the screenshots.
Power on test. Speed auto pilot was used with the speed setting constantly reduced until stall horn was blaring. Speed was then reduced in 1 mph increments and allowed to settle before the next reduction.
Lift vectors will turn white when max AOA is exceeded. Plane is in steady flight at 102 mph. As soon as I input 101 as the target speed, the stall began. Plane snaps pretty cleanly in this condition.
(http://hitechcreations.com/pyro/poweron01.jpg)
(http://hitechcreations.com/pyro/poweron02.jpg)
(http://hitechcreations.com/pyro/poweron03.jpg)
(http://hitechcreations.com/pyro/poweron04.jpg)
(http://hitechcreations.com/pyro/poweron05.jpg)
(http://hitechcreations.com/pyro/poweron06.jpg)
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The following series is a power off stall. Autopilot set to level, throttle brought back to maintain flight at about 115 mph before being fully closed. Notice how the plane wallows around in the stall unlike the clean break in the power on stall. Also notice that the stall speed is higher than the power on stall.
(http://hitechcreations.com/pyro/poweroff01.jpg)
(http://hitechcreations.com/pyro/poweroff02.jpg)
(http://hitechcreations.com/pyro/poweroff03.jpg)
(http://hitechcreations.com/pyro/poweroff04.jpg)
(http://hitechcreations.com/pyro/poweroff05.jpg)
(http://hitechcreations.com/pyro/poweroff06.jpg)
(http://hitechcreations.com/pyro/poweroff07.jpg)
(http://hitechcreations.com/pyro/poweroff08.jpg)
(http://hitechcreations.com/pyro/poweroff09.jpg)
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Hi Widewing
I just watched your films, and I think I can see one possible explanation for the figures you are quoting. However, I can't be certain, because the film doesn't show the buffet or stall horn, so it isn't easy for me to pick out the precise point when you are recording the stall speed.
However, I've been paying attention to the load factor, and it looks to me as though you might be measuring accelerated stalls, because the needle doesn't appear to stabilize at 1g, it fluctuates a little above and below.
Even if it did stabilize at 1g, there are two other points that have a bearing on this. Firstly, the definition on the scale isn't fine enough to be accurate, and secondly, even if it were, we know that the cockpit graphics aren't always drawn accurately enough for precise readings.
Does that even matter? Unfortunately it does, and it is important, because even tiny differences in the load factor have a significant effect on the stall speed. For example:
Suppose you were measuring the stall speed on an aircraft that had a stall speed of 90 mph. If you stalled at 1.2g the stall speed would read 99 mph. If you stalled at 1.1g the stall speed would read at 94mph. If you stalled at 0.9g the stall speed would read at 85mph.
So if even if you could guarantee holding the load factor within 0.2g you are probably only going to get a stall speed within 10mph of the true value, and even getting within 0.2g is probably optimistic using the reading in the cockpit.
There is another way to measure stall speeds, that I believe yields far more accurate results (doesn't depend on reading the gauges) that I use when testing.
Hope that helps...
Badboy
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Originally posted by hitech
In AH the buffet starts at that AOA .
HiTech
geez have I had it wrong or what............... the amount of time I spend in buffet when in a turn fight.......to now realise that that is not optimum turn rate or minumum turn radius...............
Great Graphics Pyro............ thanks for sharing HTC
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Tilt: I miss spoke, Buffet starts when any 1 wing section is stalled.
HiTech
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Holy cow Pyro, those are like test pilot porn. Ahhhummm....I need to light some candles, put Barry White on the stereo, open a nice bottle of Merlot and spend a little time alone to study them.
Seriously though that's alot of data. Good news that the buffet is correlated to a local stall vice the entire wing. Just out of curiosity can you tell me what the data are on the wing cut lines? Looks like local values for lift and alpha but don't know what the last is. Some measurement of drag?
Badboy, I don't think Widewing is taking the speeds from the aircraft instruments, he's probably doing like I am and using the clipboard values. Regarding the g's I averaged my numbers over 10 stalls and this would pretty much eliminate any variation of such small g deltas. As far as the specific accuracy is concerned, I think you're assuming a much larger error (10mph) than should be realistic. Pilot technique and perception (i.e., what constitutes a stall?), slight manufacturing variations, calibration of instruments, even the quality of the paint job could all affect the numbers but the only variation in AH would really be pilot technique and perception. Also, don't forget that the stall speeds are for an average pilot flying an average plane on an average day. It wouldn't be very useful to publish stall speeds that can only be achieved with a highly instrumented aircraft flown on a specific and precise profile. If plus/minus .2g is how the plane flies then the average is what's going to be published.
Mace
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Originally posted by hitech
Tilt: I miss spoke, Buffet starts when any 1 wing section is stalled.
HiTech
Some (slender) hope for me yet then............ :)
By section do you mean any of the grouped coloured lines on Pyros graphics or do you mean one wing in "net" stall
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Pyro - thanks for posting the results. That is indeed just a few more lift vectors more than AH1 :) (at least what I remember seeing in the past in Grapevine)!
Badboy - would you mind sharing your stall testing techniques? I had always assumed in the past you were doing level power-off stall flight tests but that's obviously not how you are doing it. Might be fun to try to figure out the physics you might be applying but alas my brain cells are pretty much spent at work and then my family consumes whatever ones I have left when I get home! I've actually tried using the "wind" feature offline as a way of doing it but only tinkered with that approach a few times but never spent the time to try and refine that process!
Mace- the difference is pretty much in the technique. With the auto-pilot with full control authority it must be maintaining much finer controls than we are able to do by hand at normal time.
Tango, XO
412th FS Braunco Mustangs
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Originally posted by Mace2004
Badboy, I don't think Widewing is taking the speeds from the aircraft instruments, he's probably doing like I am and using the clipboard values. Regarding the g's I averaged my numbers over 10 stalls and this would pretty much eliminate any variation of such small g deltas. As far as the specific accuracy is concerned, I think you're assuming a much larger error (10mph) than should be realistic.
Agreed, it is always better to use the clipboard values, I assumed that was what Widewing was doing. And even though small differences in the load factor does make a significant difference to individual stall speeds, you are of course, absolutely correct that averaging several tests should get you much closer to the true value.
Interestingly, the results Pyro posted are in good agreement with the EM diagram posted earlier, which shows a slight reduction in stall speed with wep for the F4U1. But I've known that the stall in Aces High began at the onset of buffet and that flying deep in buffet degraded turn performance, I published as much in a .pdf file some time ago and I've been conducting my own tests accordingly, which may be why I get similar results to Pyro.
So, if it isn't the variation in load factor, do you think the reason your tests and Widewing's show the opposite result to Pryo's and mine is just that you have been using the wrong visual cue for the stall? It will be interesting to to see what results you get from your next set of flight tests.
Badboy
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What's the difference in Pyro's speeds and the historical POH speeds for the F4U-1A?
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Originally posted by Badboy
So, if it isn't the variation in load factor, do you think the reason your tests and Widewing's show the opposite result to Pryo's and mine is just that you have been using the wrong visual cue for the stall? It will be interesting to to see what results you get from your next set of flight tests.
No Badboy I don't think that's it. My last set of tests where I averaged 10 runs in each configuration and using initial buffet as the stall indication I still have reversed numbers. The only differences between what Pyro is doing and what I've done is that obviously he's using the F4U and I did the F6F, he's heavier (100% fuel vice 25%), and he's letting the AP fly the plane which will react quicker than we can. If you've tried to duplicate this and have gotten similar results to Pyro's I can only assume there is some difference in technique or more likely a difference in the way our flight controls react.
Here are my current theories:
AP vs pilot/control system reaction times. Think of it this way, in RL, if a wing begins to drop off a bit the AP reacts very quickly with a very small adjustment. I don't know how AH does it, it could be that AP works simply by saying the wing is level so it's level, there may be no AP action actually taking place. When we're doing the flying we have a delayed reaction time and the wing will drop further so our response is going to be late and therefore, by necessity, relatively large. Whether we're using rudder or aileron each work by increasing the local AOA. Since the AOA is affected most at the wingtip we may actually be initiating the stall followed by rapid stall progression. Of course the same can be said about pitch, particularly in light of your observation of the delta G. If stalls are consistently started by control inputs that's what we'd report out to the fleet and the speed that occurs would become the effective stall speed published even if it would stall slower with absolutely perfect control. Who knows what a real airplane would do with the precise AP control that AH has but would it really matter to the pilot? No, it wouldn't. He'd just know that he can't fly the plane as slow as the computer can and the computer controlled stall would be irrelevant to him.
Weight/CG. Since Pyro did his test at 100% fuel, not only is he heavier, his CG is probably different. Changes in weight without changes in CG will just raise/lower both stall speeds in proportion but changes in CG effects the Cl at which stall occurs but I just don't think this change could be significant enough to reverse the stall speed relationship.
Delta V/Delta T. I considered the affect of rate at which stall is approached but discounted it. Basically the more rapid your deceleration to stall the lower the stall speed. If this were a problem in these tests we'd actually be artificially lowering the power-on stall speeds and making them closer to what Pyro got but then he's approaching stall very slowly so it doesn't make sense that this could be the culprit.
Engine thrust. I considered this and discounted it also. Basically, at the nose-high attitude required for the power-on stall you have a thrust component contributing to lift. This would also artificially lower the stall speed which isn't the problem so this isn't it either.
These are only my current assessment. If I had to pick one, I'd go with AP vs pilot but I'll look at it again, maybe I can find something I'm doing or haven't considered. BTW, love your charts.
Mace
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Pyro, after looking at your data, I find that I can duplicate your clean power-on stall speed both in auto-angle and under manual control. If I steepen my climb angle a few degrees, I can get a slight lower speed of 98 mph.
Where I find a discrepancy is in clean power-off stalls.
Pyro's images show a big rate of descent. It appears that the aircraft was in level flight at the beginning of the test.
If I do the same test, but set an angle (about 8-10 degrees nose high) that produces a stall as the rate of climb reaches approximately zero, the air speed is 9 mph slower than what you show. Why does this occur?
I watch the films at very slow speeds and wait until I see a marked increase in airspeed to record the lowest speed immediately before that happens.
I also tested the F6F-5. Using auto-angle, power-on, I was able to get 92 mph at stall. This is consistent with every test I've done, in manual, with and without combat trim. I obviously don't the ability to decrease speed in 1 mph increments.
Power-off, in auto-angle, using a slight nose up attitude that results in a stall at a rate of descent of 800 fpm, I recorded 87 mph. In manual flight, I recorded a speed of 85 mph. If I start out level (in auto) and pull off power, I see a stall at about 90 mph, but the plane is dropping like a brick.
Pyro, set the nose about 5 degrees high (I use auto angle) and see what you get. Stall speed should be slower than when starting level. Then repeat the test with the nose about 10 degrees high. This should produce an even lower speed.
So, my tests tonight demonstrates a split of 2 mph best case, 7 mph worst case.
How about the difference between Navy stall data for the F6F-5 and that in the game?
Power-on stall speeds for F6F-5, clean configuration.
AH2: 92 mph (25% fuel, approximately 11,300 lbs)
Power-off stall speed for F6F-5, clean configuration.
AH2: 85-87 (or even 90) mph (25% fuel)
Navy Stall data from NAVAIR 1335D, Standard Characteristics of the F6F-5:
Power-on clean: 72.2 knots (83 mph)
Power-off clean: 79.2 knots (91 mph)
Navy stall data for full load, 12,420 lb from Navy test report NA-83/44177:
Power-on clean: 96 mph
Power-off clean: 98 mph
Power-on Landing: 79 mph
Power-off Landing: 84.5 mph
Grumman stall data "normal takeoff weight", actual weight not specifically defined:
Power-on clean: 71 knots (81.7 mph)
Power-off clean: 77 knots (86.6 mph)
There is apparently no general consensus of agreeing data for stall speeds of the F6F-5. The differences are substantial. Training films for the F6F-3 and F6F-5 state stall speeds well below that of the manufacturer. One can see why it's hard for you to select performance criteria....
My regards,
Widewing
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Originally posted by Mace2004
Delta V/Delta T. I considered the affect of rate at which stall is approached but discounted it. Basically the more rapid your deceleration to stall the lower the stall speed. If this were a problem in these tests we'd actually be artificially lowering the power-on stall speeds and making them closer to what Pyro got but then he's approaching stall very slowly so it doesn't make sense that this could be the culprit.
Don't discount it yet... I think you hit on it here. I can lower the stall speed significantly by increasing the rate of deceleration and do it consistently.
If I raise the nose 5 degrees above level, stall speed drops by 3 to 4 mph. I described this in my post above, and asked why this is the case. Has Pyro modeled stall characteristics so accurately that Delta V / Delta T behavior is modeled too? It seems so.
My regards,
Widewing
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Ok, here's some power-off clean tests using auto-angle instead of level.
This series is at 7.3 degrees.
(http://hitechcreations.com/pyro/7stall01.jpg)
(http://hitechcreations.com/pyro/7stall02.jpg)
(http://hitechcreations.com/pyro/7stall03.jpg)
(http://hitechcreations.com/pyro/7stall04.jpg)
This series is at 20 degrees.
(http://hitechcreations.com/pyro/20stall01.jpg)
(http://hitechcreations.com/pyro/20stall02.jpg)
(http://hitechcreations.com/pyro/20stall03.jpg)
(http://hitechcreations.com/pyro/20stall04.jpg)
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love reading this thread, but for the less brilliant minds of Aces High, I would think that there are quite a few who are wondering in laymens terms, how this data and knowing/understanding this data will help them "In - Game"
if it helps any at all.............
back to the office ........
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TequilaChaser: It is not about helping people in game, rather it is us trying to figure out if a problem in modeling exists.
HiTech
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Those pictures are really something, and they explain why Aces High II has the best stall modelling of any simulator. I've always said as much. But I didn't know that airflow was modelled so thoroughly.
My one problem is, isn't the definition of stall that the angle of attack is too high for the wing to produce enough lift to maintain level flight?
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Widewing:
The reason stall speed is dropping a few mph when you go with higher nose attitude is because the weight of the aircraft that the wings need to counteract is lower because of the vector relationships where L and W aren't 180 degrees to each other (where L=W), rather it's now defined by L=cos(y)W.
(http://brauncomustangs.org/images/fig119a.jpg)
The greater the angle (y) in a climb, the less weight the wings have to bear which = lower lift needed.
You can see this in Pyro's screenshots where at 7 degrees AOA the weight of the aircraft is 12904 lbs where the lift is 12063 lbs just before stall - then at 20 deg AOA weight is still 12904 but lift is 10465 lbs before stall.
Tango, XO
412th FS Braunco Mustangs
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My one problem is, isn't the definition of stall that the angle of attack is too high for the wing to produce enough lift to maintain level flight?
That definition only works for 1 speed. And is equivaltent to mine for 1g stalls.
Stalls can happen at any speed and my definition works for any speed.
HiTech
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Originally posted by dtango
You can see this in Pyro's screenshots where at 7 degrees AOA the weight of the aircraft is 12904 lbs where the lift is 12063 lbs just before stall - then at 20 deg AOA weight is still 12904 but lift is 10465 lbs before stall.
Not to nitpick but that's a potentially confusing statement. You mean 7 and 20 degrees nose up attitude, not AOA. They both are at the same AOA at the onset of the stall.
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Regarding the reason the power-off tests flown by Widewing, Mace, and myself appear to have lower stall speeds than power-on conditions, analyzing it a bit more I believe that Widwing and Mace are close to the mark regarding the whole dv/dt affect and the relationship to the stall.
(http://brauncomustangs.org/films/FARStall2.jpg)
The image above is a typical record of a dynamic stall maneuver, power-off. Notice the 1-g stall speed (Vs1g) being higher than the FAA stall (Vs). The reason this is occurs is because the actual normal force acting on the plane is less than 1-g. Essentially the plane is dropping some which creates this condition. One thing to clarify - it's not actually the rate of deceleration that results in this but because the aircraft is experiencing less than 1-g load due to aircraft dropping.
Reviewing Widewing's films and reviewing my own films I've noticed that the accelerometer is actually below 1-g in the power-off cases when the departure of controlled flight occurs which tells me that we are in this dynamic situation.
Tango, XO
412th FS Braunco Mustangs
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Originally posted by Pyro
Not to nitpick but that's a potentially confusing statement. You mean 7 and 20 degrees nose up attitude, not AOA. They both are at the same AOA at the onset of the stall.
Ah yes, my bad. Tried correcting it but I've gone beyond the 120 minutes edit limit on the bbs :).
Tango, XO
412th FS Braunco Mustangs
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I've completed my study of F4U-1 stall speed in Aces High vs. the stall speeds listed in the F4U-1 pilot's manual for clean, power off; clean, power on; dirty, power off; and dirty, power on. I find excellent agreement between the manual and Aces High.
I did some math to figure out a technique that would be repeatable, that would allow steady-state measurements (which are much easier to take), and that would allow reasonably easy avoidance of changing g loads on the aircraft.
My results and analysis are posted here:
http://www.electraforge.com/brooke/flightsims/aces_high/stallSpeedMath/stallSpeedMath.html
If anyone finds errors with my analysis or data, please let me know. I've checked it a couple of times, but that doesn't mean there aren't errors, and it would be better if the analysis and data stands the test of being looked at by others.
I'll be interested to see if anyone repeats my tests or applies it to aircraft other than the F4U-1.
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Brooke:
Outstanding write-up :). Thanks for taking the time to write it up. Great idea by using the rate of climb/descent to figure out what stall speed is!
Just to make sure I remember this myself, so the key equations are (in conditions for steady state velocities):
(1) cos(theta) = sqrt [ (1-Rate_of_Climb) / Vairspeed]
(2) Vs1g = sqrt(1/cos(theta)) * Vairspeed
Tango, XO
412th FS Braunco Mustangs
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Originally posted by Brooke
I've completed my study of F4U-1 stall speed in Aces High vs. the stall speeds listed in the F4U-1 pilot's manual for clean, power off; clean, power on; dirty, power off; and dirty, power on. I find excellent agreement between the manual and Aces High.
I did some math to figure out a technique that would be repeatable, that would allow steady-state measurements (which are much easier to take), and that would allow reasonably easy avoidance of changing g loads on the aircraft.
My results and analysis are posted here:
http://www.electraforge.com/brooke/flightsims/aces_high/stallSpeedMath/stallSpeedMath.html
If anyone finds errors with my analysis or data, please let me know. I've checked it a couple of times, but that doesn't mean there aren't errors, and it would be better if the analysis and data stands the test of being looked at by others.
I'll be interested to see if anyone repeats my tests or applies it to aircraft other than the F4U-1.
A rather complete analysis. Thanks for taking the time.
A couple of points, if I may...
I noticed that you were using 18" @ 2,400 rpm for power-on stalls. I had a conversation with an older friend who did a test pilot stint an Langley in the early '40s. He stated that power-on stalls were generally conducted at Normal Climb power. The purpose being to familiarize pilots with stall behavior at a power setting and configuration that they would commonly experience.
The down side to this is that to induce a stall at the Normal climb power setting (44" @ 2,550 rpm for the F4U), you must assume a nose-up angle of approximately 20 degrees. I used auto-angle. With 25% fuel, zero burn rate, this consistently results in a stall break at 94 mph TAS.
For power-off stall, I set the nose-up angle at about 6 degrees. This resulted in consistent stall breaks at 94 mph TAS. Increasing the angle to 15 degrees resulted in a stall break at 93 mpg TAS, but is a bit tricky to catch unless viewing film at very low frame rate.
As for the F6F-5; the Pilot's Manual provides stall speeds far lower than actual test data recorded in Navy test report NA-83/44177, for a weight of 12,420 lb.
Power-on clean: 96 mph
Power-off clean: 98 mph
Compare this to the Manual (converted to mph from knots, rounded). These speeds are IAS, and they probably reflect stalls performed above 10,000 feet.
Power-on clean: 71 mph
Power-off clean: 74 mph
These numbers very likely represent a different test method and weight than that used in test NA83/44177. To see how this test is done (you'll need Real Player), watch this F6F training film. (http://www.zenoswarbirdvideos.com/realg2/HCb.ram) You will notice that the aircraft was held in a nose-high attitude during all stall demonstrations and that minimum beginning altitude was 10,000 feet.
Beginning at 10,000 feet, my power-on and power-off stalls were 88 mph IAS and 88 mph IAS. Hmm.... That's still way above the Manual's numbers and those presented in the training film.
My in-game data, using 25% fuel and the 20 and 6 degree nose-up attitudes beginning at 2,000 feet (taken from film) results in 88 mph TAS power-on and 88 mph TAS power off.
As I stated previously, there are significant differences between the manual, training films, Navy test data and Grumman test data. Thus, establishing exact stall figures for the flight model requires selecting data deemed most reliable.
Other planes tested, power-on and power-off, clean, 25% fuel:
P-38L:
98 mph
98 mph
The P-38 Manual gives the following power-off, clean figures
15,000 lb: 94 mph IAS
17,000 lb: 100 mph IAS
19,000 lb: 105 mph IAS
P-51D:
99 mph
100 mph
Fw 190A-5:
102 mph
102 mph
Spitfire Mk.VIII
87 mph
89 mph
I'm satisfied that the most significant contributor to generating variances in stall speeds is the result of increasing or varying climb angle.
My regards,
Widewing
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Widewing, are you converting the manual figures to CAS?
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Originally posted by dtango
Just to make sure I remember this myself, so the key equations are (in conditions for steady state velocities):
(1) cos(theta) = sqrt [ (1-Rate_of_Climb) / Vairspeed]
(2) Vs1g = sqrt(1/cos(theta)) * Vairspeed
Tango, XO
412th FS Braunco Mustangs
Yes, but in your first equation you forgot to square the two speed values, and the inner brackets are in the wrong place, it should look like this:
(1) cos(theta) = sqrt [ 1- (Rate_of_Climb^2 / Vairspeed^2)]
But you can actually do it with only one equation and all you need are two items of data, the climb rate, and the stall speed in the climb. If you use the same notation as Brooke that would be:
Climb rate = v_climb
Stall speed in climb = v_SC
Stall speed in level flight = v_stall
From that you can get the stall speed in level flight in one step from this equation:
v_stall = v_SC^1.5 / (v_SC^2 - v_climb^2)^0.25
The results come out exactly the same as before, but it just means you can do it in one step in a spread sheet without using trig.
Badboy
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Hi Brooke
Originally posted by Brooke
If anyone finds errors with my analysis or data, please let me know. I've checked it a couple of times, but that doesn't mean there aren't errors, and it would be better if the analysis and data stands the test of being looked at by others.
Just found a couple of notational errors:
In this coordinate system, the magnitude of the weight vector opposite the lift vector (its projection onto the aircraft's coordinate system) is W * cos(theta). So, at stall speed in a steady-state climb we have:
W * cos(theta) = 0.5 * rho * v_stall^2 * S * C_L_max
Here you are using v_stall for the stall speed in a steady state climb, but later you use that for the stall speed in level flight. It is clear from later equations you should have typed v_SC instead.
Also:
cos(theta) = sqrt(1 - v_climb^2 / v_sc^2).
Everywhere else you capitalized v_SC.
Just nit picky stuff, but that does make it difficult to read for anyone not fluent in our favorite brand of gobbledygook :)
I only looked at it quickly and it looks otherwise ok. I'll take a longer look later.
Badboy
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Originally posted by hitech
Widewing, are you converting the manual figures to CAS?
For which numbers?
My regards,
Widewing
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Thanks for the corrections and the combined equation Badboy.
Tango, XO
412th FS Braunco Mustangs
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Originally posted by Brooke
I've completed my study of F4U-1 stall speed in Aces High vs. the stall speeds listed in the F4U-1 pilot's manual for clean, power off; clean, power on; dirty, power off; and dirty, power on. I find excellent agreement between the manual and Aces High.
I did some math to figure out a technique that would be repeatable, that would allow steady-state measurements (which are much easier to take), and that would allow reasonably easy avoidance of changing g loads on the aircraft.
My results and analysis are posted here:
http://www.electraforge.com/brooke/flightsims/aces_high/stallSpeedMath/stallSpeedMath.html
If anyone finds errors with my analysis or data, please let me know. I've checked it a couple of times, but that doesn't mean there aren't errors, and it would be better if the analysis and data stands the test of being looked at by others.
I'll be interested to see if anyone repeats my tests or applies it to aircraft other than the F4U-1.
Brook, I agree with dtango, excellent write-up. In particular, your description of the differences in the literature is on the nose. I have also been working on refining the test technique and am now able to come within a few mph of Pyro's numbers. Changes I've needed to make to do this includes using 100% fuel and tailoring my stick response curves to allow for very precise pitch adjustments at slow speed; however, this has a detremental affect on normal flying.
Overall, I think what we're butting heads up against the "engineering" answer and the "practical" answer. An "engineering" answer derived from precise instrumentation and very specific test conditions is required for an understanding of aircraft design and may be the basis for fulfiling the contract but, from a practical standpoint, it is often of little use to the pilot in the cockpit. I'll talk about what that means at the end.
Many times you'll find "engineering" data in flight handbooks that can never be met by the average pilot flying an average airplane on an average day. I think it's probably a mix of engineering vs practical information (and occasionally some factually incorrect data) in literature that results in at least some of the confusion. The differences between calibrated vs indicated lose their subtleties when the pilot can only see indicated and is in no position to apply position error corrections. Also, errors sometimes make it into publications and are never corrected. For instance, NATOPS for 40 years has listed the roll-rate of the A-4 Skyhawk as being 720 degrees per second. That has been taught in flight training the entire time and is even the roll-rate the Blue Angles announcer used to describe the A-4 to audiences during airshows. The problem is it's wrong and not just a little wrong. The correct number is 270 degrees per second which is easily demonstrated with a stopwatch. A simple typo turned 270 into 720 and has remained there to this day.
Based on the numbers provided by Pyro and yourself I believe you've demonstrated that the modeling of the actual stall point is probably pretty accurate. I think the discussion and data provided by everyone contributing to this thread also demonstrates the difficulties and significant variations resulting from different techniques and, in particular, the use of airspeed as the principal measurement of stall when in fact AOA is the culprit, airspeed is only a secondary indicator. Variations in loading, variations in environmental conditions and instrument errors shows why the military has switched to AOA and as the principal indicator of stall. The nice thing about AOA is that it's a constant and, for the same configuration, the wing always stalls at the same AOA regardless of these other factors. For instance, in the F-14, 18 units AOA, not airspeed, always indicates the point at which rudder becomes your primary roll control rather than lateral stick. Doesn't matter what your weight, speed or altitude is, it'll always be 18 units.
Regarding your write-up, there are a few things you should look at. In your description of "Usual Method" you say that accelerating or decelerating along the plane's direction of travel doesn't affect the stall speed as long as you remain level. Actually, it does and it can be quite significant but not because of instrument lag, it's because of nonsteady state flow effects. Yes, there are delays in instrument response but if you're deccelerating at a rate that causes significant lag then you aren't going to get good data anyway. Changes in flow over the wing take some finite time and a rapid deceleration, even if remaining level, affects the stall point because the flow has not stabilized. Depending on the wing, the difference between approaching stall at -5mph/sec vs -.5mph/sec can be 5 to 10mph in observed stall speed with the faster deceleration equaling a lower stall speed. That said, Pyro's technique is probably the technique with the absolute smallest deceleration and therefore (at least from an engineering standpoint) the most accurate measurement.
Another area you should take a look at is your discussion of stalls in climbs and "Figure Climb". It does not include the vertical (i.e., lift) component of thrust. This can have a very significant affect on the result just as does G and, the greater the thrust the higher the climb angle and the more stall speed will be thrown off. To take an extreme example, assume the aircraft has a 1 to 1 thrust to weight ratio. The airplane would be able to park 90 degrees nose-high at O airspeed and never stall. In another, less extreme example, make a constant speed climb with a 45 degree pitch angle (assuming thrust is coincident to the aircraft centerline) and 1/2 your lift is coming from your engine and only 1/2 from the wing. The greater the thrust the greater the climb angle and the less useful the results are.
These are the two reasons why I've duplicated Pyro's test condition of 100% fuel. The greater weight decreases the thrust to weight ratio and makes it easier to conduct the power-on test with minimum vertical thrust component and it makes it easier to control the rate at which you approach stall. Assuming no (or small) changes in CG the results at higher weights can be extrapolated to the lower weights. Also, slight excursions in G can be negated by taking the average of multiple runs while the same cannot be said of either pitch angle or high deceleration rates to stall.
All that aside, where does that lead us regarding the AH stall speeds? We are pretty sure the specific stall speeds are accurate, at least from an engineering perspective, and they can be replicated under very specific, controlled circumstances. So here's the rub...why can't we duplicate them or even come close in the case of the power-on numbers without this engineering approach? Remember, when it comes to flight manuals (versus engineering studies) they are attempting to provide numbers which the average pilot in an average plane on an average day can use using the instruments he has in the cockpit (i.e., indicated AS). If, as we're seeing here the stall speeds are accurate but not achievable except under very exacting conditions the flight manuals would (or should) say so. One way to take these practical issues into account would be in the determination of recommended approach speeds and acceleration/climb schedules for launch but I have no information on these so don't know if they take into account a higher minimum usable flying speed; however, since they don't mention difficulty in reaching these numbers (or at least nobody has pointed out where they do this) maybe we're asking the wrong question. Is there something about the way that approach to stall is modeled that is incorrect? Is this sensitive to specific flight controllers and their setup (I suspect it is to a degree)? I don't really know the answer at this point but I do know the "effective stall speed" is much higher than the actual speed.
Mace
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Originally posted by dtango
Just to make sure I remember this myself, so the key equations are (in conditions for steady state velocities):
(1) cos(theta) = sqrt [ (1-Rate_of_Climb) / Vairspeed]
(2) Vs1g = sqrt(1/cos(theta)) * Vairspeed
Tango, XO
412th FS Braunco Mustangs
Thanks, dtango.
Yep, those are the key ones, although as noted by Badboy, the first one should be:
cos(theta) = sqrt[1 - (Rate_of_Climb^2 / Vairspeed^2)]
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Originally posted by Widewing
A couple of points, if I may...
I noticed that you were using 18" @ 2,400 rpm for power-on stalls.
Thanks, Widewing.
I used 18" and 2400 for the clean, power-on and 23" and 2400 for the dirty, power-on only as that's what the pilot's manual lists as the power settings for its data.
The technique for riding the stall during a steady-state climb works, too, so one could apply it with climb settings of the power to check against sources that list power-on stall speeds at different power settings.
As for the F6F-5; the Pilot's Manual provides stall speeds far lower than actual test data recorded in Navy test report NA-83/44177, for a weight of 12,420 lb.
Yep -- the F4F pilot's manual is pretty crappy in its quality of data, I think. Some of the stall-speed data it lists seems like it they might be way off (like the 50-53 knot stall speeds in the text).
As I stated previously, there are significant differences between the manual, training films, Navy test data and Grumman test data. Thus, establishing exact stall figures for the flight model requires selecting data deemed most reliable.
Indeed.
I'm satisfied that the most significant contributor to generating variances in stall speeds is the result of increasing or varying climb angle.
I agree, too. That's why I like the method of a steady-state climb or descent, riding the stall. In that situation, nothing is varying other than a gradual change (very gradual) in altitude -- climb rate is steady, g's are steady at 1 g, airspeed is steady, angle of attack is steady, etc.
All that remains then is to correct the stall speed one is riding to level-flight conditions (and to correct for calibrated airspeed of the literature and make the aircraft weights match).
Thanks, Widewing, for the comments and info.
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Originally posted by Badboy
Hi Brooke
Just found a couple of notational errors:
Badboy
Thanks, Badboy. I'll get on them and post an update today.
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Originally posted by Widewing
For which numbers?
I've been fighting the flu all weekend and my concentration hasn't been up to par.
To answer Hitech's question; stall speeds defined in the manuals are almost certainly not corrected for positional error. I did not apply any correction, but simply converted knots to mph (F6F) or verbatim from the manual (P-38).
My regards,
Widewing
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Originally posted by Mace2004
Regarding your write-up, there are a few things you should look at.
Thanks, Mace. Good points and comments. I'll correct the description of significance of horizontal acceleration.
Another area you should take a look at is your discussion of stalls in climbs and "Figure Climb". It does not include the vertical (i.e., lift) component of thrust.
Figure Climb is mapping everything to the airplane's coordinate system, where lift is perpendicular to velocity vector. In that coordinate system, thrust is along the velocity vector (well, there is sin(alpha) of it in the lift direction, but that is negligible for WWII aircraft), and drag in that coordinate system is opposite the velocity vector.
So, thrust does not matter in the calculation. Angle of climb does matter, but that is taken into account. All that matters is that everything is steady state, and then the equations correct everything back to what the level-flight stall speed is.
In your example of thrust-to-weight of 1:1, for full power on, v_stall is 0 mph (i.e., there is no stall) -- the aircraft isn't going to stall -- so that is correct. For T:W such that climb angle is 45 degrees, in the coordinate system of the plane, there is no lift component that is from thrust. The weight, though, in that coordinate system is not W, it is W * cos(45 deg), and the equations correct for all of that.
So here's the rub...why can't we duplicate them or even come close in the case of the power-on numbers without this engineering approach?
I think some of it might be due to not feeling the g's, so in level flight, we aren't aware when we are pulling 1.2 g's vs. 1 g, etc., and it is because the literature varies and, in some cases, has inconsistencies or lack of data (like what weight the stall is at, what power setting the engine is at, and especially is it indicated or calibrated airspeed).
The AH F4U-1 seems to agree very well with the pilot's manual, and the F4U pilot's manual seems pretty good (unlike, say, the F6F pilot's manual, which seems pretty crappy in data quality). Over time, I'll do some more testing and see what I get for other aircraft.
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Summary: For P-38J, clean, power off has excellent correspondence; dirty, power off is different by 9 mph.
From PILOT'S FLIGHT OPERATING INSTRUCTIONS FOR ARMY MODELS P-38H SERIES, P-38J SERIES, P-38L-1 L-5 AND F-5B AIRPLANES, on page 28, we get that the stall speed in clean condition, power off is 100 mph indicated, and for flaps and gear down, power off, 74 mph. From page 33, we get a table for calibrated airspeed. From the lowest two speeds in the chart, for clean, we get CAS = 0.870 * IAS + 27.8; for dirty, we get CAS = 0.741 * IAS + 34.1. Thus, clean, power off is 115 mph calibrated, and dirty, power off is 89 mph calibrated. This is at 17,000 lbs gross weight.
In Aces High, at 16,360 lbs gross, we ride the stall at 109 mph CAS (113 mph true) and 2700 fpm = 30.7 mph descent. cos(theta) = sqrt(1 - 30.7^2 / 113^2) = 0.962. v_stall = sqrt(1 / 0.962) * 109 = 111 mph calibrated. At a weight of 17,000 lbs, v_stall = sqrt(17000 / 16360) * 111 = 113 mph calibrated. So, Aces High and the Pilot's manual are only 2 mph different for clean, power off.
In Aces High, at 15,724 lbs gross with gear and flaps down, we ride the stall at 91 mph calibrated (93 mph true) and 2700 fpm = 30.7 mph descent. cos(theta) = sqrt(1 - 30.7^2 / 93^2) = 0.944. v_stall = sqrt(1 / 0.944) * 91 = 94 mph calibrated. At a weight of 17,000 lbs, v_stall = sqrt(17000 / 15724) * 94 = 98 mph calibrated. So, Aces High and the Pilot's manual are 9 mph different -- not as close.
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Originally posted by Brooke
(well, there is sin(alpha) of it in the lift direction, but that is negligible for WWII aircraft)
Hi Brooke,
Mace2004 is correct, there is always a thrust contribution to lift, and for modern fighters it can't be ignored. I agree that for WWII fighters with very low T/W ratios its omission represents a small error of less than 1% which is negligible, but may account for part of the difference you are seeing in your results.
Badboy
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Originally posted by Badboy
Hi Brooke,
Mace2004 is correct, there is always a thrust contribution to lift, and for modern fighters it can't be ignored. I agree that for WWII fighters with very low T/W ratios its omission represents a small error of less than 1% which is negligible, but may account for part of the difference you are seeing in your results.
Badboy
Yep, there is a thrust component to lift in the coordinate system I picked of T * sin(phi) where phi is angle between thrust centerline and direction of travel of the aircraft. I ignored that in my calculations, but you guys are right, it might not be negligible.
For example, angle of incidence of wings in WWII aircraft is about 1-2 degrees, and max angle of attack (at stall) is about 12 degrees clean and about 8 degrees with full flaps, so phi_max is about 11 degrees, say.
From propeller theory, T = 550 * HP * eta / v, where T is thrust in lbs, HP is the engine HP, eta is the propeller efficiency, and v is the velocity of the aircraft (in ft/sec). Typical total propeller efficiency is about 0.85. For the F4U-1, HP is about 2000 HP and speed at stall for a 11,500 lbs F4U-1 is about 100 mph true = 147 fps, and we get thrust to be 6360 lbs. Then sin(11) * T/W = 0.106, which could be significant.
I'll take a look at how it all rolls out in the equations.
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Originally posted by Brooke
From propeller theory, T = 550 * HP * eta / v, where T is thrust in lbs, HP is the engine HP, eta is the propeller efficiency, and v is the velocity of the aircraft (in ft/sec). Typical total propeller efficiency is about 0.85. For the F4U-1, HP is about 2000 HP and speed at stall for a 11,500 lbs F4U-1 is about 100 mph true = 147 fps, and we get thrust to be 6360 lbs. Then sin(11) * T/W = 0.106, which could be significant.
Hi Brooke,
Don't forget that 85% is only going to be true at a point near to the top end speed. Near the stall the prop efficiency for the F4U is probably much closer to 60% and may be lower. I'm about to leave for work, I'll check my prop charts when I get back. But I'd say at the stall you are probably looking at a T/W closer to 0.4 or below and that reduces your figure from 0.106 to 0.07.
Badboy
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Good point, Badboy. I'll work on taking that into account, too.
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Originally posted by Brooke
Good point, Badboy. I'll work on taking that into account, too.
Here is a prop chart for the F4U-1 that you might find helpful...
(http://www.badz.pwp.blueyonder.co.uk/Files/Images/F4U1PROP.jpg)
At 100mph (close to the stall speed) the advance ratio is 0.55, so the efficiency can be read from the chart at 59% just a little less than I thought. But I think the AH flight model gets it a tad closer to 60% at that speed. You will notice that the maximum efficiency in this configuration is just over 83% at an advance ratio equivalent to about 300mph.
The Aces High flight model just gets better and better, as we get better and better at measuring it :)
Badboy
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Originally posted by Brooke
Yep, there is a thrust component to lift in the coordinate system I picked of T * sin(phi) where phi is angle between thrust centerline and direction of travel of the aircraft. I ignored that in my calculations, but you guys are right, it might not be negligible.
For example, angle of incidence of wings in WWII aircraft is about 1-2 degrees, and max angle of attack (at stall) is about 12 degrees clean and about 8 degrees with full flaps, so phi_max is about 11 degrees, say.
From propeller theory, T = 550 * HP * eta / v, where T is thrust in lbs, HP is the engine HP, eta is the propeller efficiency, and v is the velocity of the aircraft (in ft/sec). Typical total propeller efficiency is about 0.85. For the F4U-1, HP is about 2000 HP and speed at stall for a 11,500 lbs F4U-1 is about 100 mph true = 147 fps, and we get thrust to be 6360 lbs. Then sin(11) * T/W = 0.106, which could be significant.
I'll take a look at how it all rolls out in the equations.
I like Diz Dean's simplified equation as it is more easily applied and understood by non-engineers by using mph instead of ft/sec.
Thrust = 375 x prop efficiency x horsepower / speed.
Naturally, you need to know the available hp and the efficiency of the propeller. Horsepower can be found in commonly available tables. Prop efficiency is tougher to determine. For max thrust at max level speed, you can fudge it some and simply use 80%. This will produce a reasonable "ball park" figure that is useful when comparing various aircraft.
For the F6F-5, flying at Normal power (44 in/hg @ 2550 rpm), the engine is making 1,675 hp at 5,500 feet (very near to the altitude I was testing at).
Thus, 375 x .6 (60%) x 1,675 (hp) / 88 (speed in mph) = 4282.67 lb
Using the normal equation:
550 x 1,675 x .6 / 129.0666 = 4282.67 lb
So, when flying 20 degrees nose up, less 3 degrees (F6F-5 incidence) we can calculate: Where T = 4282.67, W = 10,980
sin(17) x 4282.67 / 10,980 = 0.114 !!
This, I think, tends to account for the lower stall speed when flying into a stall at higher AoA.
My regards,
Widewing
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Good points, Widewing.
The formulas I'm using, though, are all in the coordinate system of the airplane. Thus, in T * sin(phi), phi isn't the angle between line of thrust and the horizon, it's the angle between line of thrust and direction of travel (it is basically angle of attack minus angle of incidence of the wings, so alpha - 2 degrees or so).
All of this will not affect the power-off numbers where T is taken to be approximately zero. (More accurately, in AH, it is probably a small negative number, but I suspect it is still negligible in magnitude.)
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I have updated all the calculations to include the effect of the lift component of thrust. It didn't change the values I calculated for the F4U-1, which still then has excellent correspondence with AH.
For the P-38, I only have power-off data, so it's calculation is not affected, and it remains not as close in AH compared to pilot's manual for power-off, dirty.
Here's the new writeup with the new contributions from thrust.
http://www.electraforge.com/brooke/flightsims/aces_high/stallSpeedMath/stallSpeedMath.html
If anyone spots an error, please let me know.