Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Simaril on August 09, 2007, 09:24:11 AM
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Question for those with aviation or engineering background --
Does RELATIVE turn data change with altittude? In other words, would the ratio between two planes' minimum circle diameter, or degrees per second, stay fairly stable as altitude changes?
ANd in a related question, does the optimum turning INDICATED speed stay stable as altitude changes?
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Originally posted by Simaril
Question for those with aviation or engineering background --
Does RELATIVE turn data change with altittude? In other words, would the ratio between two planes' minimum circle diameter, or degrees per second, stay fairly stable as altitude changes?
ANd in a related question, does the optimum turning INDICATED speed stay stable as altitude changes?
Complicated issues...
Everything depends upon indicated air speed. True air speed is almost meaningless when it comes to high altitude handling.
Suppose you are flying an Fw 190A-8, and you climb up above 30,000 feet to attack bomber formations flying around 27k. Aside from the fact that the 190s can barely climb at all at 30k, they have other issues that will dictate how they fight.
If you have never flown a dogfight at 30,000 feet, you are in for a rude shock. At 30k, a turn radius is not measured in feet, but in yards. This is due to the fact that indicated air speed is the governing air speed. For example, a 190A-8 flying at 32,000 feet requires 4 minutes to accelerate to 200 mph IAS (and stalls at 170 mph IAS in a 2 g turn). The difference between stall speed and max speed is only about 40 mph. Should you bleed off speed maneuvering, you will very quickly find yourself flying on the edge of a stall. Bad news if there's enemy fighters near by (P-47, Spit IX, P-38 or P-51) as they have a large advantage in power loading at 30k. They will quite literally fly circles around the 190s at 30k. Think about this... The P-47s are making 2,000 hp at 30k, while the 190s have less than half that.
I've done some turn radius and turn rate testing at 25,000 feet. Fuel load 25%, zero burn. No flaps and stall limiter turned on to remove pilot skill from the data and create a sense of relative performance. At this altitude, fighters turn huge circles, 4 to 5 times larger than at sea level.
Turn radius / speed in mph TAS / Average circle in time / Turn rate in degrees per second
190D-9: 2,563.3 / 237 mph / 46.31 sec / 7.77
109G-14: 2,109.6 / 200 mph / 43.15 sec / 8.34
P-51D: 2,184.9 / 219 mph / 42.72 sec / 8.43
P-47D-40: 2,119.2 / 226 mph / 40.15 sec / 8.97
P-38J: 2,151.2 / 241 mph / 38.22 sec / 9.42
As you can see, the 190D-9 is outclassed. The 109G-14 actually turns with the American iron, but cannot sustain this without dropping the nose below the horizon after little more than one complete turn. Of the planes listed, the P-38J is the best in terms of combined turn radius and rate.
If you log into the Training Arena, switch to Bishops and go to field A3. This is a 30k field. You can take off or you can use the South East runway to spawn at 30k over A1. Be advised that you spawn with the gear down and the engine off. Get the gear up and nose down. Start the engine after getting the nose down. Level off at 25k and try doing some tight turns. You will quickly see how important it is to concentrate on indicated air speed.
My regards,
Widewing
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Widewing,
I had posted earlier but deleted it, where Istarted off with
the higher you are, the wider you turn or something like that and mentioned the relation of IAS vs TAS and how I rely on IAS for Maneuvering, and how far apart the 2 speed measurements grow as you gain alt and vice versa
the speeds you have listed:
I've done some turn radius and turn rate testing at 25,000 feet. Fuel load 25%, zero burn. No flaps and stall limiter turned on to remove pilot skill from the data and create a sense of relative performance. At this altitude, fighters turn huge circles, 4 to 5 times larger than at sea level.
Turn radius / speed in mph TAS / Average circle in time / Turn rate in degrees per second
190D-9: 2,563.3 / 237 mph / 46.31 sec / 7.77
109G-14: 2,109.6 / 200 mph / 43.15 sec / 8.34
P-51D: 2,184.9 / 219 mph / 42.72 sec / 8.43
P-47D-40: 2,119.2 / 226 mph / 40.15 sec / 8.97
P-38J: 2,151.2 / 241 mph / 38.22 sec / 9.42
you tested for best sustained turnrate/speed for 25,000 ft
can you give the comparison of these same planes and the same speed but for a lower alt like 5,000 &/or SL? so it shows in what relation the difference is as far as Air Density, quicker turnrate, smaller or wider turn radius?
I simply asking to show the difference so its much easier for Simaril & others to understand...if they had a side by side comparison of the differing alts but same speeds ( and would these speeds also be TAS? )
I know just about everyone always test and gather data using TAS, but when some of us fly we use IAS , as you mentioned for maneuvvering
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Regarding the relative turn performance, there is two main factors:
1. Output at given altitude.
2. Reachable max lift coefficient.
Generally the planes with advanced supercahargers did relatively better at altitude because the available power limits the sustained maneuverability.
There was also some variation how the mach number affected to the Clmax; as an example the P-51 maintained ability maneuver relatively well when the mach number increased while in the case of the P-38 compressibility limited maneuverability considerably.
Notable thing is that relatively high Clmax does not help much if there is not enough power available; in practice max sustained turn performance at high altitude tend to happen at Cl values well below the Clmax due to limited power.
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Widewing.. did you do that G14 testing with 30mm or 20mm?
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Hi,
two factors influence the turn with increasing altitude.
1. Thrust
2. Air density
The thrust different is related to the power and air density and although the power of an engine often increase to a certain altitude, the thrust rarely follow this, cause the thinner air make the propeller less effective.
The air density also reduce the lift and thats also the reason why the slow speed stall, while a horizontal flight, remain almost at the same indicated airspeed.
NOTE: This is different with a accelerated stall.
The slow speed stall while a horizontal flight is mainly related to the gravity, and this is almost the same in all altitudes, while the accelerated stall get influenced by the inertia of the plane masses and the inertia is related to the true airspeed. As higher you fly as higher the different between IAS and TAS will be.
As result the turn increase more than someone would expect and the stall behaviour with 250mph IAS isnt the same in low and high alt.
Greetings,
Knegel
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Questions. Questions that need answering.
----Gandalf the Grey
Simaril:
As you can see from the response of others the answers are a little more complicated.
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Relative turn performance does change with altitude between aircraft because engine power output, lift, and drag all change with altitude. Therefore we can't extrapolate that relative turn performance differences at sea level will be the same at 25k.
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Does indicated airspeed for optimum turning stay stable? For sustained turns, no. For corner velocity if we don't factor in compressibility impact on lift then yes they would be the same.
However as gripen has pointed out compressibility reduces the maximum lift obtainable which means IAS for corner velocity also changes.
Tango, XO
412th FS Braunco Mustangs
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Originally posted by Simaril
Does RELATIVE turn data change with altittude? In other words, would the ratio between two planes' minimum circle diameter, or degrees per second, stay fairly stable as altitude changes?
Generally speaking no, the relative performance between two aircraft won't stay the same with altitude, sometimes the difference will increase, and sometimes it will decrease and even reverse. It all depends on the particular aircraft involved. I can illustrate that better with an example. Some time ago I was looking for an aircraft that could be used to fight the P51B, I wanted something that the P-51B couldn't run away from, and couldn't out turn and so I made some likely comparisons. First up, was the F4U1. Here is an EM diagram for both aircraft at Sea Level, at full War Emergency Power (WEP). The diagram also shows the situation with full flaps.
(http://www.badz.pwp.blueyonder.co.uk/images/F4U1vP-51BSL.jpg)
The key features were that both aircraft had a similar top speed, and although the F4U1 has slightly lower Ps performance across the range, which means that it would bleed speed more quickly in the turns, it had a similar sustained turn rate and a slightly smaller turn radius. However, you can also see that with full flaps the F4U1 is dominant with more than 3dps turn rate advantage and almost 200ft smaller turn radius. So this looked like a good candidate P51B killer.
However, the that situation doesn't stay constant as the altitude increases. Take a look at the EM diagram for the other altitude extreme, 30,000ft, shown below.
(http://www.badz.pwp.blueyonder.co.uk/images/F4U1vP-51B30k.jpg)
Notice now that the situation has reversed, at this altitude the P51B is not only faster, it has almost a 2dps better sustained turn rate and almost a 400ft better turn radius.
In that example, the advantage reverses, and so there will be an altitude above which the P-51B has the advantage and below which the F4U1 has the advantage.
However, that isn't always the case for every pair of aircraft. For example, I did a comparison for the last Battle of Britain Scenario and provided a comparison for all the fighters from Sea Level up to 30,000ft. The EM diagrams are in Annex A at the back of this file: (right click and save target as)
http://www.badz.pwp.blueyonder.co.uk/Files/pdf/AH-BOB.pdf
If you look at the EM overlays you will notice for example that the Spitfire Mk1 is superior to the Bf109E-4 at sea level, and all the way up to 30,000ft where the difference between the two decreases as the altitude increases, but remains advantageous to the Spitfire.
One other interesting point you may notice from those diagrams is that while the Bf109E4 is the superior German fighter at lower altitudes, at 30,000ft, the Bf110C matches it in the turn, and is faster.
Anyway, that should give you an idea of how things can vary with altitude.
ANd in a related question, does the optimum turning INDICATED speed stay stable as altitude changes?
Yes, but with one caveat. Things like the stall speed and corner velocity should occur at the same indicated airspeed, however, at higher altitudes it may not be possible for the aircraft to reach those speed. For example, if you check the corner velocity of the Battle of Britain aircraft again, you will notice that while all of the aircraft can generate 6g at 20,000ft, none of them can generate enough lift to reach 6g at 30,000ft and the amount they can reach is limited by their top speed and possibly Mach effects if they can go fast enough, so at that altitude their corner velocities will vary.
Hope that helps.
Badboy
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Originally posted by Widewing
Complicated issues...
Everything depends upon indicated air speed. True air speed is almost meaningless when it comes to high altitude handling.
Suppose you are flying an Fw 190A-8, and you climb up above 30,000 feet to attack bomber formations flying around 27k. Aside from the fact that the 190s can barely climb at all at 30k, they have other issues that will dictate how they fight.
If you have never flown a dogfight at 30,000 feet, you are in for a rude shock. At 30k, a turn radius is not measured in feet, but in yards. This is due to the fact that indicated air speed is the governing air speed. For example, a 190A-8 flying at 32,000 feet requires 4 minutes to accelerate to 200 mph IAS (and stalls at 170 mph IAS in a 2 g turn). The difference between stall speed and max speed is only about 40 mph. Should you bleed off speed maneuvering, you will very quickly find yourself flying on the edge of a stall. Bad news if there's enemy fighters near by (P-47, Spit IX, P-38 or P-51) as they have a large advantage in power loading at 30k. They will quite literally fly circles around the 190s at 30k. Think about this... The P-47s are making 2,000 hp at 30k, while the 190s have less than half that.
I've done some turn radius and turn rate testing at 25,000 feet. Fuel load 25%, zero burn. No flaps and stall limiter turned on to remove pilot skill from the data and create a sense of relative performance. At this altitude, fighters turn huge circles, 4 to 5 times larger than at sea level.
Turn radius / speed in mph TAS / Average circle in time / Turn rate in degrees per second
190D-9: 2,563.3 / 237 mph / 46.31 sec / 7.77
109G-14: 2,109.6 / 200 mph / 43.15 sec / 8.34
P-51D: 2,184.9 / 219 mph / 42.72 sec / 8.43
P-47D-40: 2,119.2 / 226 mph / 40.15 sec / 8.97
P-38J: 2,151.2 / 241 mph / 38.22 sec / 9.42
As you can see, the 190D-9 is outclassed. The 109G-14 actually turns with the American iron, but cannot sustain this without dropping the nose below the horizon after little more than one complete turn. Of the planes listed, the P-38J is the best in terms of combined turn radius and rate.
If you log into the Training Arena, switch to Bishops and go to field A3. This is a 30k field. You can take off or you can use the South East runway to spawn at 30k over A1. Be advised that you spawn with the gear down and the engine off. Get the gear up and nose down. Start the engine after getting the nose down. Level off at 25k and try doing some tight turns. You will quickly see how important it is to concentrate on indicated air speed.
My regards,
Widewing
Now Widewing, what happens when we throw in a TA-152? Just curious, It seems to be a High-alt bird.
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Great info! Thanks, guys!
Always appreciate the E/M diagrams, though I am still learning to read them proficiently. I knew the community would come through!
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Originally posted by FrodeMk3
Now Widewing, what happens when we throw in a TA-152? Just curious, It seems to be a High-alt bird.
Well, the Ta 152 doesn't show a performance advantage until around 40k (in relation to the D models). If we add the P-47N, the Ta 152 doesn't have an edge until well above 40k. At 40k, the P-47N still does 465 mph.
Frankly, the AH2 Ta 152 is not especially impressive, even up high.
My regards,
Widewing
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Well, the Ta 152 doesn't show a performance advantage until around 40k (in relation to the D models). If we add the P-47N, the Ta 152 doesn't have an edge until well above 40k. At 40k, the P-47N still does 465 mph.
Yeah, I hear ya there WW. However, as we've seen with the other info provided, the TA is the best Hi-alt german ride, would'nt you agree?
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Originally posted by FrodeMk3
Yeah, I hear ya there WW. However, as we've seen with the other info provided, the TA is the best Hi-alt german ride, would'nt you agree?
I agree, with the 109K-4 being second best.
My regards,
Widewing
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Well, maybe except the Me-262, and the Me-162. :rofl
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Originally posted by FrodeMk3
Well, maybe except the Me-262, and the Me-162. :rofl
If you fly the Me 262 up to 30k, you'll find it's not happy up that high. Acceleration is horrible and turn radius is huge. P-47s would be all over a 262 up at 30k, he would have to dive to get away.
My regards,
Widewing
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Me 262 is still a lot faster than the best P-47 at 30k. And a 262 would not be interested in fighting a P-47, only evading it to get to the bombers.
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Originally posted by Viking
Me 262 is still a lot faster than the best P-47 at 30k. And a 262 would not be interested in fighting a P-47, only evading it to get to the bombers.
Well, the problem for the Me 262 is that it takes a very long time to build that speed.
If you attempt to take a 262 off from a 30k field in the TA, you run off the end of the runway, with only about 70 mph IAS showing. That's all the speed it can build on the takeoff run.
On the other hand, the P-47 of airborne by mid field and climbing out at a pretty good rate.
If the 262 should burn some E off maneuvering at 30k and find itself at 350 mph TAS, it would be chased down very quickly by any of the P-47s, with the 476 mph N model being a major threat. The 262 driver would have no other option but to dive. This would effectively take the 262 out of the fight as it would take nearly forever to nurse it back up to 30k.
Frankly, 30k is simply too high for the 262 to be fun as it takes a very long time to get there (and uses much of its fuel to do so) and replacing speed requires diving. It just cannot fight at 30k, and is reduced to hit and run.
I'd rather fly the Ta 152H.
My regards,
Widewing
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Hi,
the 262 got high with speed. The climb speed of the 262 was close to or above Vmax of the jug. Climb speed was around 540km/h ias with the jumo engine, it had to be reduced for compressibility problems in higher altitudes and this compessibility problems was the main enemy of a 262 (once fast).
Furher more, the jet engine dont lose as much thrust with altitude like the propeller engine.
The Vmax of the 262 only is in rather low(between 5 and 6 km) altitude, cause the mach related problems, which increase with altitude, hinder the plane to fly faster up there.
In game all this might be different. :)
Greetings,
Knegel
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Widewing, - the 262 will on full power easily outdive any prop plane, - shallowly I mean. ?
Now I shall mention an engagement between pilots of 65 sqn RAF equipped with P51C's. One fine day, a 262 tried to fight them. (They were flying either sweep or escort). It didn't take long, - it was very high up, and the 262 made the mistake of trying to stay in a turn. P51 easily saddled behind and made a kill.
This is from an anecdote though and I would have to get some special "key's" to get to the source. But the incident, I belive, is true, and should be correct anyway, - power vs lift, wingloading, acceleration, and just a pair of seconds to think.
In short, the key for high altitude performance is "widewing", - wingloading and aerfoil, as well as power-to-weight.
POWER,WINGLOADING-SPANLOADING VS WEIGHT.