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View Full Version : Why the P38 'Compesses'



03-06-2004, 08:48 PM
Its a simple question really. In AEP the controls will freeze up over 400mph, probably like it did in real life But other planes dont suffer from this as bad, Why?

Sure if she wasn't a nazi and if she was still alive - I'd Hump Her.
http://hazardous.50free.net/Hannah.JPG

03-06-2004, 08:48 PM
Its a simple question really. In AEP the controls will freeze up over 400mph, probably like it did in real life But other planes dont suffer from this as bad, Why?

Sure if she wasn't a nazi and if she was still alive - I'd Hump Her.
http://hazardous.50free.net/Hannah.JPG

Gibbage1
03-06-2004, 09:45 PM
From what I remember, A shock wave formed over the elivator. It happenes on all aircraft, just more-so on the P-38 because of the twin boom design. They had a "swordfish" prototype of the P-38 with an extended center gondola and a proposed bubble canopy that flew extensive test's and extendng the center fixed the compresability.

Gib

03-06-2004, 10:07 PM
Lemme guess, that aircraft never saw combat or was never completed? I mean I love the P38 but diving over 400mph = death without that airbrake. Sounds like a crappy way to die and Im sure many died from it... "OK you dive I dive. HAhahah youre mine now.. Uh-oh! wtf cant pullup and ground looks alot closer. damn japanese must have airbrake on a slid"- Boom.
- BTW thanks for the information and if youre still around, wouldnt the P38 fare better with 4 bladed props for higher rpm/accel? its not an important question... dont even know why i asked.. The p38 is just an awkward looking plane to me that, in AEP, has high durability and speed AND power. (Not enough .50 cal ammo for me though http://ubbxforums.ubi.com/images/smiley/16x16_smiley-very-happy.gif ) Well if youre still reading this, GJ on the planes man, never cease to impress this gamer Or this gaming community for that matter.

Sure if she wasn't a nazi and if she was still alive - I'd Hump Her.
http://hazardous.50free.net/Hannah.JPG

horseback
03-06-2004, 11:19 PM
As I understand it, compressibility refers to a condition that occurs due to the fact that at high altitudes, where the speed of sound is lower, airflow over some parts of the aircraft is going to be higher than over others, due to their shapes. As you may have noticed, the P-38 is a 'bumpy' airplane, with large gaps in the airframe.

So a P-38 entering a dive from high altitude would build up speed quite quickly, due to its intitial speed and weight, and the airflow over portions of the airframe goes supersonic, and the shockwave from that supersonic airflow 'compresses' the control surfaces behind the supersonic shockwave front, rendering them extremely stiff, if not immovable. In the case of the P-38, the elevator is locked up, and the nose starts to tuck under, as in an outside loop.

At an unchecked diving speed, the results were generally catastrophic. The plane would just tear itself apart. Some pilots found their way around this by trimming nose up before the tuck under, but this was not a sure fire escape by any means. Part of the problem was that there was a lot of dispute about what the problem really was-was it buffeting, or a harmonic vibration (the engines vibrating at the harmonic frequency of a part of the airframe, which would cause that part of the structure to shake itself apart-like the tails of the early 109F and the Typhoon), or compressability? Since compressability was largely only a theoretical possibility until 400mph a/c were developed, they ended up addressing that problem last.

There was some problem with buffeting, but that was solved with the fillets at the wing root to gondola junction with the first YP-38s. When the problem persisted, Army Air Force engineers and scientists (yeah, the same guys who pulled the turbosupercharger out of the P-39) insisted in injecting themselves into the problem. The result was the usual thing that comes out of commitees: delayed solutions. Kelly Johnson claimed to his dying day that he wanted to try the dive brake solution by the time the P-38G was going into production, but the bureaucrats had him tied up in red tape for another six months to a year.

A means of limiting dive speed to where no part of the airflow would go supersonic at high altitudes was agreed to pretty quickly once the problem had been identified. That was the airbrakes installed just outboard the engine nacelles under the wings.

I haven't tried this yet (I just got AEP this morning, and I have a son who had his first day of Little League today, which limited my time to experiment), but I suspect that if you try selecting dive brakes as you roll into a dive on the 38L, your diving speed will stay manageable, and your controls won't freeze up.

cheers

horseback

"Here's your new Mustangs, boys. You can learn to fly'em on the way to the target. Cheers!" -LTCOL Don Blakeslee, 4th FG CO, February 27th, 1944

Gibbage1
03-06-2004, 11:32 PM
The Swordfish did exist. But it was only a test-bed. Lockheed would not stop production long enough to implament the design change no matter how much better it made the aircraft.

http://www.wpafb.af.mil/museum/research/p38-7.jpg

zomby_woof
03-06-2004, 11:41 PM
horseback 07-03-04 01:19 wrote:
:
:As I understand it, compressibility refers to a condition that occurs due to the fact that ...
:
---cut---

Excellent, very informative easy to understand reply. Now I know exactly what is compressibility. Thank you sir!

zw [lifetime P-38 fan]

http://web.newsguy.com/zomby_woof/images/IL2AirRacing.gif
Flying on HL as Sky^King

___________

PS: to Gibbage1

Outstanding work on your P-38J & L. http://ubbxforums.ubi.com/infopop/emoticons/icon_smile.gif

[This message was edited by zomby_woof on Sat March 06 2004 at 10:52 PM.]

Blottogg
03-07-2004, 07:35 AM
Horseback, nice explanation. I'll elaborate a bit on compressibility though. Like you said, because the air is displaced as it flows around "bumps" and other airframe bits, it is accelerated, and can go supersonic before the aircraft as a whole does. This is a big problem with the wings in particular. As the air accelerates over the top and bottom of the wing, it will at some speed (critical Mach number, below Mach = 1) form a local shock wave. This shock wave will move aft along the wing as speed increases and a second shock will form further forward (there's something called an expansion fan in between the two, which is the opposite of a shock wave.) Shock waves are nothing but pressure waves really, with lower velocity and higher pressure behind the shock (expansion fans have higher velocity and lower pressure behind them.)

All these pressure changes wreak havoc with the wing's lift distribution. The net effect is that subsonically lift is centered about 1/4 of the way back from the wing's leading edge (a point known as the quarter chord.) Supersonically, the lift is centered further back, at about the half chord. Transonically, where the flow is partially supersonic and part subsonic, lift is somewhere in between the two chord references.

The problem with the lift center moving aft is that it requires more elevator downforce to trim out. This may be more force than the elevator can generate, especially if the elevator isn't hydraulically boosted, since the high speeds will tend to make the controls more difficult to move. There may also be problems with shock waves forming on the horizontal tail, but that usually manifests itself as "Mach buzz", when the shock waves form at different chord positions on the top and bottom of the horizontal stab. Here the pressure differences behind the top and bottom shocks start messing with the elevator, pushing it up or down. As it moves, it changes the airflow, moving the shock waves, changing the pressure distribution, moving the elevator and starting the cycle all over again. This all happens very quickly, manifesting itself as a "buzz", which can quickly become destructive.

With the P-38, I don't know what the specific problem was, but looking at the Swordfish picture Gibb posted (thanks BTW, I'd never seen that) I'd guess it was the wing, not the elevator that was causing the problems. A lot of what determines critical Mach is in the airfoil choice, and that's hard to determine just looking at pictures. The P-38 was also one of the first aircraft to get far enough into the upper left hand corner of the performance envelope to encounter compressibility, regardless of airfoil choice. I've got to go through Warren M. Bodie's "The Lockheed P-38 Lightning" when I get a chance and maybe find out more.

Blotto

"Speed is life." - Anon
"Sight is life. Speed is merely groovy." - "Junior"

SkyChimp
03-07-2004, 07:49 AM
The dive speed limits on the P-38 were as follows:

30,000 feet - 290mph IAS or 440 mph TAS
20,000 feet - 360 mph IAS or 460 mph TAS
10,000 feet - 420 mph IAS or 460 mph TAS

The manual says "Outside the above limits buffeting and dive tendancy my be experienced."

Regards,
SkyChimp
http://members.cox.net/rowlandparks/skychimp.jpg

PBNA-Boosher
03-07-2004, 07:55 AM
John Lowell, the greatest P-38 ace in the european theater, who Adolf Galland said equalled and even surpassed himself in combat, reports that when someone dived down against him in a dogfight, he'd follow in a diving barrel roll with rudder to keep his speed down, when he got the new 38's, he used the dive flaps, but it's good advice:

barrel roll down with rudder.

Magister__Ludi
03-07-2004, 08:20 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by SkyChimp:
The dive speed limits on the P-38 were as follows:

30,000 feet - 290mph IAS or 440 mph TAS
20,000 feet - 360 mph IAS or 460 mph TAS
10,000 feet - 420 mph IAS or 460 mph TAS

The manual says "Outside the above limits buffeting and dive tendancy my be experienced."

<HR></BLOCKQUOTE>


You forgot the dive angles:

max 15 degree dive without using dive brakes - is this a dive? http://ubbxforums.ubi.com/infopop/emoticons/icon_biggrin.gifhttp://ubbxforums.ubi.com/infopop/emoticons/icon_biggrin.gif

max 45 degree dive using dive brakes - outstanding!! http://ubbxforums.ubi.com/images/smiley/35.gif

p1ngu666
03-07-2004, 08:20 AM
it was partly the wing
a p38 with merlins and that wing, how sweet it would have been http://ubbxforums.ubi.com/images/smiley/16x16_smiley-sad.gif

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