Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Urchin on December 30, 2002, 02:41:05 PM
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All I have to say is WOW.
It is possible to pull out of a 500 mph (True) dive in less than 1,000 feet. That is some impressive elevator authority.
Now, the bigger question is: Is this modelled correctly? Mandoble doesn't seem to think so, but I haven't seen any arguments either way.
How fast did the Typhoon "compress"? Furthermore, what exactly is 'compression"?
I understand that the controls 'lock up' when the plane is compressed, but what exactly happens? And why? And how does it effect different planes differently? I guess a clearer way to ask that is what plane attributes (wing span, etc.) have an effect in delaying the onset of 'compression'?
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Hi Urchin,
>It is possible to pull out of a 500 mph (True) dive in less than 1,000 feet. That is some impressive elevator authority.
What was the dive angle? :-) For 90°, this would require 16 G, but for 45° it would only require 5 G.
>How fast did the Typhoon "compress"?
Gripen provided the Typhoon's critical Mach number as 0.64 (see below). That works out to 487 mph TAS at sea level, or 422 mph TAS at 20000 ft. Eric Brown's "Testing for Combat" on the other hand gives the critical Mach number as 0.81 and the limiting Mach number as 0.79, which is suprisingly high.
>Furthermore, what exactly is 'compression"?
"Compressiblity" is a term for the difference between the behaviour of a gas and a fluid. You might have seen "water tunnels" that can be used for aerodynamic experiments just like wind tunnels because air behaves very similar to water.
As airspeeds get higher, this similarity gets lost because the forces become high enough to compress the air. This leads to a sharp drag rise, which is a criterium for determining the critical Mach number, and changes the air flow around the aircraft which - depending on the design - can screw up the control.
The P-38 and the P-47 had some problems with this phenomenon as both aircraft became extremely nose-heavy and broke up due to excessive Gs (P-38) or dived into the ground out of control (P-47).
The critical Mach number of "trouble-free" fighters like the Spitfire, Mustang, Messerschmitt or Focke-Wulf was around 0.75, while it was around 0.70 for the P-47D and around 0.65 for the P-38.
Mach 0.64 would mean the Typhoon was quite dangerous to dive, while Mach 0.79 would put it into the group of top performers. I'm somewhat confused by the discripancy between these values.
Regards,
Henning (HoHun)
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HoHun,
Mach number 0,64 for the Typhoon is from "RAF fighters part2" by Green and Swanborough (same number is also mentioned in couple articles from the 40s like Aeroplane, March 1945). It is not real tactical limit but just a speed where the wing entered the compressebility region, tactical limit should be somewhat higher. In the case of the Tempest similar number was mach 0,73 but the tactical limit was mach 0,8.
gripen
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Originally designed to defeat the FW-190 series fighters, the XP-47J certainly would have exceeded this requirement. In point of fact, with its critical Mach of .83:cool:, it had the potential to chase down Me-262's by utilizing a shallow dive, taking advantage of its superior service ceiling.
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To add to what HoHun has stated -
The closer airflow approaches the speed of sound the more it becomes compressed. Compressibility problems arise for aircraft when airflow in a local region of the aircraft gets very close to or hits the speed of sound. This tends to occur where sudden contoured changes occur for an aircraft as explained by bernoulli's law. For instance recall on a wing lift is created by the pressure differential caused by the higher velocity airflow across the top of the wing due to the change in contour to create more surface area vs. the bottom of the wing. Because of this airflow at a region on the top surface of the wing can actually hit the speed of sound before the freestream air velocity does. The velocity at which this occurs is called the critical mach number. When critical mach is hit the formation of shockwaves occur resulting in extremely turbulent airflow behind the shockwave resulting in loss of lift, severe trim problems, and violent vibrations.
For WW2 a/c at critical mach all sorts of nasty things (control column flailing about, controls frozen, a/c nose "tucks under" even more etc. etc.) could occur and generally meant the loss of control.
Critical mach number for a wing is mainly determined by it's thickness ratio. The lower the thickness ratio, the higher the critical mach number is (higher the velocity before the a/c experienced "compression").
Keep in mind that critical mach can occur on other parts of the a/c besides the wing.
I'm with HoHun on the descriprancies of critical mach numbers for WW2 a/c. E.g - for the P-51D instead of .75 I have .81 and even up to .83 for critical mach. For the P-38 I have anywhere from .65 to .675. I don't have any data on the P-47, the Typhoon or any of the LW birds.
Perhaps one of the explanations is that these results were usually done by flight testing and quoted critical mach numbers for a/c were more conservative? For instance in the case of the P-51D according to my source "Initial dives showed the onset of the problems to occur at just under Mach .75. Additional dives were made, using three test pilots, which carried the aircraft to successively .77, then .79, and on up to Mach .81, and finally to Mach .83. As the dive Mach number was increased the compressibility effects became more violent, but the aircraft was still controllable and it was possible to fly it out of the problem when desired. At Mach .83 the shaking and buffeting of the aircraft was so strong that it was decided to explore no further."
Tango, XO
412th FS Braunco Mustangs
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Well, I did film it ( a couple times). It 'felt' like 90 degrees, looking at the film I'd guess it was closer to 70 or so. But it was definately closer to 90 than 45, and I didn't even start pulling out until the alitimeter swung past 1,500 feet (at 500 mph thats tough :)).
I think something may be wrong with the Typhoon. If someone has webspace, I'll mail the film to them to post. It is about a meg (little bit more). I did the dive numerous times, until I finally tried to pull out at about 750 feet and crashed.
I pulled well over 9G's I would think in pulling out of that dive, my pilot was so blacked out he didn't recover until the nose was pointed straight up again. Looking at the G meter it was pegged at 9, but it had to have been more than that.
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Well it had to be less than 12 or you would have died :)
J_A_B
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Hi Dtango,
>The closer airflow approaches the speed of sound the more it becomes compressed.
Compressibility is only one side of the medal, the other could be called expandability :-) The air density around the aircraft just doesn't stay constant any longer at high speed.
>For WW2 a/c at critical mach all sorts of nasty things (control column flailing about, controls frozen, a/c nose "tucks under" even more etc. etc.) could occur and generally meant the loss of control.
Here's a selection of symptoms:
P-38: Aircraft pushes negative Gs ("tuck under")
P-47: Aircraft freezes in dive
P-51: Heavy vibrations, stick thrashes around
Me 109: Vibrations, aileron overbalancing
Me 163: Tuck under
>I'm with HoHun on the descriprancies of critical mach numbers for WW2 a/c. E.g - for the P-51D instead of .75 I have .81 and even up to .83 for critical mach. For the P-38 I have anywhere from .65 to .675.
Probably it's because so many different values are quoted - critical Mach number is not the same as maxium achievable Mach number or as maximum tactical Mach number.
Critical Mach number has an aerodynamic definition like "speed of sharp drag rise", but there are different definitions so that Mach numbers can't be compared precisely.
Maximum achievable Mach number can include an aircraft diving out of control, like Mach 0.73 for the P-47D (without dive flaps) and Mach 0.79 for the Me 109. Recovery is possible only at lower Mach numbers, but fortunately, as you dive into lower, denser air, the Mach number drops a bit allowing pull-out if you're not too fast :-)
The P-38 Mach numbers you provide are the critical Mach numbers for two different configurations, without dive flaps (Mach 0.65) and with dive flaps (Mach 0.68). Dive flaps counteracted the tuck under, and I've heard maximum achievable Mach was about 0.72 as a result.
Maximum tactical Mach number requires the ability to do something useful with the aircraft, like pointing its guns, which requires a certain amount of control. This could vary much according to the control characteristics - for example, most early jets were subject to yaw oscillations at high mach numbers, making them useless as gun platforms. The best of them was the He 162 with Mach 0.84, but the Me 262 wasn't bad either with Mach 0.82. The Gloster Meteor would lose aiming capability to this effect at just Mach 0.7 - if you ever wondered why it wasn't used in the fighter role in WW2, I'd say this might be the answer. (These problems were only solved after the war.)
Regards,
Henning (HoHun)
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Hm... Typhoons Mach 0.64 number means the speed where the compressebility wave starts to develop ie the plane enters to the compressebility region. It's not a similar critical mach number as given in the manuals etc. Similar number for the Tempest is mach 0.73 while it's critical mach number is around mach 0.8.
gripen
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Dunno about all the mathematics about it, but RAF pilots found the Typhoon to be an extremely stable aircraft and able to pull out of very steep dives easier than many other aircraft. Guess it was at its most effective doing that sort of role.
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To expand on HoHun's note, excerpt from 'Testing For Combat'. Not sure how well this relates to the AH flight model, just providing it as information. (Oh man. I only fly them, I don't know what makes em work)
'The speeds given in the Typhoon's table (of 'limiting indicated airspeeds against height bands') were, after allowing for position error, equivalent to a Mach number of 0.79, which was higher than for any contemporary piston-engined fighter except the Spitfire IX. Our job at RAE Farnborough was to determine how critical this limiting Mach number was if taken to the ultimate loss of control. These tests were normally started at the highest possible altitude, so that if loss of control did occur in the dive the Mach number would automatically reduce as height was lost, provided the dive angle was kept constant, and thus allow control to be regained.
The aircraft to be used for the compressibility dive tests was Typhoon IB EK154, fitted with a Machmeter and powered by a 2,200hp Sabre IIA. THe aircraft was climbed to 32,000 ft and after a 3 min level run at full throttle at that height was half rolled and the nose allowed to drop 30 degrees before half rolling again to maintain that dive angle. The indicated Mach number (IMN) had built up to 0.82 by 27,000ft, with moderate buffeting, then at 0.83 a noticeable nose-down change of trim occurred and at the same time the buffeting inceased. Finally at IMN=0.84, the nose-down trim change increased dramatically and even a two-handed pull on the stick could not effect recovery. I could just manage to keep the dive from steepening, and held on with considerable effort until, at 20,000ft, the nose began slowly to rise; by 18,000ft recovery was complete. From these tests it was clear that the true limiting Mach number of the Typhoon was 0.79 and the true critical Mach number was 0.81.'
The rest of the passage relates to buffeting when carrying incendiary 1000lb bombs.
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I too have "duels in the sky" and on page 201 it lists the maximum safe mach number as .80 for the tempest mk V.
from americas hundred thousand it lists the maximum safe mach limit of the 51 as .75 this is also in all the pilots manuals. this part of the book has been used above, but the paragraph wasnt finished. it left out the part where the mustang had to be scrapped due to high speed diving damage.
"In July 1944 Wright Field test pilots explored the high speed dive characteristics of a Merlin powered Mustang. A series of dive tests were made starting from about 35,000 ft. in a test airplane equipped with a mach meter. The idea was to explore the effects of compressibility such as buffeting, vibration, control force changes, and so on. Initial dives showed the onset of the problem to occur at just under mach .75. Additional dives were made, usiung three test pilots, which carried the aircraft sucessively to mach .77, then .79, and up to mach .81, and finally to mach .83 (605 mph) As the dive mach number was increased the compressibility effects became more violent, but the aircraft wsa still controllable, and it was possible to fly it out of the problem when desired, at mach .83 the shaking and buffeting of the aircraft was so strong that it was decided to explore no further. The airplane had suffered considerable structural damage and was written off."
to refresh what the dive limit placard on the P-38 says about mach, .0675 with out dive flaps is listed as safe and you may safely exceed this number by 20 mph IAS. from the book titled "Pilot" by Tony LeVier with John Guenther, the states that with the dive flaps in place he could dive at 60 degrees and achieve mach .72 safely with the dive recovery flaps. martin caiden's book "The forktailed devil" also lists the same data for the 38 as well as bodie's book and quite a few others list this same data.
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Could someone post a chart of 'mach numbers' and speed? Like I have no idea how fast Mach 1 is at 5k, 10k, 15k, 20k.
Although to be quite honest, I don't think the Typhoon is modelled incorrectly now. Just would like a chart showing how fast Mach 1 is at different altitudes so I can compare em to my film showing how fast the Typhoon was going and at what Alt. I can do the division myself :).
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bolillo_loco:
Yeah, I left that last sentence off because the Mustang that hit .83 had structural damage but was still recoverable while experiencing compressibility effects. I didn't think it was germane to the discussion since AH doesn't model things such as scrapping an engine after using WEP.
America's Hundred 1000 lists just above the quoted paragraph the following on the P-51D - "Maximum permissible dive speed for a P-51D was 505 mph IAS below 9000 ft and 300 mph IAS (539 mph TAS and Mach .81) at 35000 ft."
As HoHun, Gripen, and Duma have referred to I think it's clear that the effects of compressibility start at some point (limiting mach #?) and then gets worse as airspeed increases. For the P-51D it appears effects start to be felt near mach .75 (limit mach) and then get worse (crit mach .81-.83?).
I think it's also clear that problems due to compressibility can be pretty complex since we're talking about very turbulent airflow with the formation of shockwaves which makes it hard to predict the precise impact to a/c flight therefore hard to model as well.
The question is how does AH model the progression of compressibility for aircraft and the impact on flight control as well as structural damage. As to comparative onset of compresssion I think a reasonable place to begin would be thickness ratio of wings with thicker wings experiencing compression earlier than thinner wings. So if I were following this logic I would expect the Typhoon (thickness ratio ~19%?) for instance to experience effects of compression before say a P-51D (thickness ratio ~14%).
As to what the effects of progressing into "deeper" compression are on flight controls and structural damage I don't know how one would go about coming up with the "rules" as to how this would happen in a sim for different a/c besides referring to anectodal reports from pilots or flight tests (as the above post by Duma on the Typhoon).
Tango, XO
412th FS Braunco Mustangs
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static float _SpeedOfSoundFPS[_MAX_ALT_ENTRY] =
{
1116.89f, /* ALT 0.000000*/
1114.97f, /* ALT 500.000000*/
1113.05f, /* ALT 1000.000000*/
1111.12f, /* ALT 1500.000000*/
1109.19f, /* ALT 2000.000000*/
1107.25f, /* ALT 2500.000000*/
1105.31f, /* ALT 3000.000000*/
1103.37f, /* ALT 3500.000000*/
1101.43f, /* ALT 4000.000000*/
1099.48f, /* ALT 4500.000000*/
1097.53f, /* ALT 5000.000000*/
1095.57f, /* ALT 5500.000000*/
1093.61f, /* ALT 6000.000000*/
1091.65f, /* ALT 6500.000000*/
1089.68f, /* ALT 7000.000000*/
1087.71f, /* ALT 7500.000000*/
1085.74f, /* ALT 8000.000000*/
1083.76f, /* ALT 8500.000000*/
1081.78f, /* ALT 9000.000000*/
1079.80f, /* ALT 9500.000000*/
1077.81f, /* ALT 10000.000000*/
1075.82f, /* ALT 10500.000000*/
1073.83f, /* ALT 11000.000000*/
1071.83f, /* ALT 11500.000000*/
1069.83f, /* ALT 12000.000000*/
1067.82f, /* ALT 12500.000000*/
1065.81f, /* ALT 13000.000000*/
1063.80f, /* ALT 13500.000000*/
1061.78f, /* ALT 14000.000000*/
1059.76f, /* ALT 14500.000000*/
1057.73f, /* ALT 15000.000000*/
1055.70f, /* ALT 15500.000000*/
1053.67f, /* ALT 16000.000000*/
1051.63f, /* ALT 16500.000000*/
1049.59f, /* ALT 17000.000000*/
1047.55f, /* ALT 17500.000000*/
1045.50f, /* ALT 18000.000000*/
1043.45f, /* ALT 18500.000000*/
1041.39f, /* ALT 19000.000000*/
1039.33f, /* ALT 19500.000000*/
1037.26f, /* ALT 20000.000000*/
1035.19f, /* ALT 20500.000000*/
1033.12f, /* ALT 21000.000000*/
1031.04f, /* ALT 21500.000000*/
1028.96f, /* ALT 22000.000000*/
1026.88f, /* ALT 22500.000000*/
1024.79f, /* ALT 23000.000000*/
1022.69f, /* ALT 23500.000000*/
1020.59f, /* ALT 24000.000000*/
1018.49f, /* ALT 24500.000000*/
1016.38f, /* ALT 25000.000000*/
1014.27f, /* ALT 25500.000000*/
1012.15f, /* ALT 26000.000000*/
1010.03f, /* ALT 26500.000000*/
1007.91f, /* ALT 27000.000000*/
1005.78f, /* ALT 27500.000000*/
1003.64f, /* ALT 28000.000000*/
1001.51f, /* ALT 28500.000000*/
999.362f, /* ALT 29000.000000*/
997.251f, /* ALT 29500.000000*/
995.062f, /* ALT 30000.000000*/
992.905f, /* ALT 30500.000000*/
990.743f, /* ALT 31000.000000*/
988.577f, /* ALT 31500.000000*/
986.405f, /* ALT 32000.000000*/
984.229f, /* ALT 32500.000000*/
982.048f, /* ALT 33000.000000*/
979.862f, /* ALT 33500.000000*/
977.672f, /* ALT 34000.000000*/
975.476f, /* ALT 34500.000000*/
973.276f, /* ALT 35000.000000*/
971.070f, /* ALT 35500.000000*/
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};
in FPS. note speed of sound varies with temp, not pressure.
HiTech
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Just dragged up some tests done by Bullethead
Anyway, I took off from this NW field in the DA. Headed N to get out over the water 11k below. All planes at 25% fuel, went on autolevel as soon as the wheels left the ground, got to level max w/out WEP, then dove vertically with throttle full open still. Maintained vert dive until impact w/out trying to pull out, because compression is what I'm interested in. Results:
Plane Max Level (IAS/TAS) Buffet Onset (IAS) Parts Break (IAS)
51D 315/385 495 510
Typh 290/385 470 480
Dora 315/395 490 N/A
Yak9U 320/410 500 N/A
Jug30 300/375 475 N/A
The buffet onset has a margin of error of +/- 5 knots or so due to the difficulty reading the gauge while buffeting. Those planes where parts actually broke off before impact maxed out at about the indicated speed where the parts started coming off and retained that speed into the water. Those planes that didn't break continued to accelerate to about 600 IAS before impact.
At least between the Typhoon, Yak9U and the P-51D onset of buffet from compression would be in the realm of comparative difference I would expect. Wing thickness ratios from highest to lowest - Typhoon, P-51D, Yak9U.
Tango, XO
412th FS Braunco Mustangs
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Here's a conversion to MPH from HT's chart:
Alt, MPH
0 761.52
500 760.21
1000 758.90
1500 757.58
2000 756.27
2500 754.94
3000 753.62
3500 752.30
4000 750.98
4500 749.65
5000 748.32
5500 746.98
6000 745.64
6500 744.31
7000 742.96
7500 741.62
8000 740.28
8500 738.93
9000 737.58
9500 736.23
10000 734.87
10500 733.51
11000 732.16
11500 730.79
12000 729.43
12500 728.06
13000 726.69
13500 725.32
14000 723.94
14500 722.56
15000 721.18
15500 719.80
16000 718.41
16500 717.02
17000 715.63
17500 714.24
18000 712.84
18500 711.44
19000 710.04
19500 708.63
20000 707.22
20500 705.81
21000 704.40
21500 702.98
22000 701.56
22500 700.15
23000 698.72
23500 697.29
24000 695.86
24500 694.43
25000 692.99
25500 691.55
26000 690.10
26500 688.66
27000 687.21
27500 685.76
28000 684.30
28500 682.85
29000 681.38
29500 679.94
30000 678.45
30500 676.98
31000 675.51
31500 674.03
32000 672.55
32500 671.07
33000 669.58
33500 668.09
34000 666.59
34500 665.10
35000 663.60
35500 662.09
36000 660.59
36500 660.59
37000 660.59
37500 660.59
38000 660.59
38500 660.59
39000 660.59
39500 660.59
40000 660.59
40500 660.59
41000 660.59
41500 660.59
42000 660.59
42500 660.59
43000 660.59
43500 660.59
44000 660.59
44500 660.59
45000 660.59
45500 660.59
46000 660.59
46500 660.59
47000 660.59
47500 660.59
48000 660.59
48500 660.59
49000 660.59
49500 660.59
Tango, XO
412th FS Braunco Mustangs
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Why does the speed of sound not change any more above 36000 feet? Temperature doesn't change any more above that altitude?
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The first layer of atmosphere is the Troposphere from 0 to ~36,000 feet assuming you're mid latitude on earth. Temperature decreases with altitude in the Troposphere.
The 2nd layer of atmosphere is the Stratosphere from 36,000 feet to around 30 miles up. Temperature in the stratosphere remains constant from 36,000 ft to 82,000 ft and then begins to increase with altitude.
Here's a nice little URL with a mapping of atmospheric layers and temperature variation with altitude:
Temperature Variation with Atmospheric Layer (http://www.meteorology.gov.ws/Education/Q&A/Q1.htm)
Tango, XO
412th FS Braunco Mustangs
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Originally posted by J_A_B
Well it had to be less than 12 or you would have died :)
J_A_B
does the game actually model pilot death at 12g's or more?
how about Kurt Tank, how many g's does it take to kill him. And how many perks to draw Kurt Tank as your pilot? :)
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come to think of it, is there any model aircraft in the game that can actually *take* 12 g's without breaking up? I dont think even the heavy American iron could explore that part of the flight envelope and survive
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Originally posted by -duma-
'The speeds given in the Typhoon's table (of 'limiting indicated airspeeds against height bands') were, after allowing for position error, equivalent to a Mach number of 0.79, which was higher than for any contemporary piston-engined fighter.
The aircraft to be used for the compressibility dive tests was.
The indicated Mach number (IMN) ...
The problem whether the instruments showed true mach number will allways remain. I seriously doubt that the instruments showed correct mach numbers in ww2. It doesnīt surprise me anymore that according to instruments(!) most RAF fighter seem to be superior at high mach numbers, though they were definitly not the best aerodynamic designs. Itīs just a question of instrument accuracy.
But for flight safety accuracy wasnīt necessary, when the pilot reads an instrument the true airspeed isnīt of interest, only what the needle shows and what test pilots found out as a limit with the same instruments.
It would be really interesting to take those old instrument out of a museum and run tests in modern wind tunnels to check their correctness.
niklas