Stall Speed Bug?

[Widewing]

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

[0verlag]

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.

[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.

[Widewing]

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

My regards,

Widewing

[Kweassa]

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?

[Mace2004]

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

[Badboy]

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.



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:



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

[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

[Widewing]

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
F4U-1A Clean Stalls

Mace, I'll have to scan in those cards. I have photocopies.

My regards,

Widewing

[Widewing]

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

[Stoney74]

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?

[dtango]

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

[Mace2004]

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

[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.................

[gripen]

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