Author Topic: Compressibility studies on NASA's website, Good Information? (Read 387 times)

Walker42 · « **on:** February 15, 2003, 12:01:10 PM »

I've been doing alot research on Compressiblity the last few weeks. Mainly because I fly the P-38 in game, and alot of different sites qoute totally different Information.

I stumbled upon NASA's website, and they go into great detail about compressiblity, and wind tunnel tests. Check it out.

Research in Supersonic Flight and
the Breaking of the Sound Barrier

WIND TUNNELS OF NASA :Chapter 3 - Through the Barnstorming Days to World War II, The First Big High Speed Tunnel

Both Go into detail of the P-38, and compressiblity. In my opinion NASA would probably have the most acurate data. They don't say anything on Assumptions.

I took the P-38 up in game offline (no external Ordinance/Full fuel load), and ran a couple dives from 20,000 feet to test how it held up in game. I'll try it online later tonight.

In the offline tests I experienced buffeting, and Compressiblity problems earlier then expected, and the Dive flap slowed me down more then corrected Airflow.

Is this new Reliable data that might not have been taken into consideration?

akak · « **Reply #1 on:** February 15, 2003, 08:37:30 PM »

Don't make the mistake of confusing compressability and high speed buffets. A lot of people do and they are two different things.

I don't know about other planes but compressability in the P-38 only happens in dives started above 20,000ft.

Compressability in the P-38 is caused when the air flow over the leading edge of the wing would reach the sound barrier (not the aircraft itself but the speed of the airflow over the wing) which would then cause a shockwave over the trailing edge of the wing. Only at altitudes above 20,000ft where the threshold for the sound barrier is less than at sea level does the airflow over the leading edge of the plane exceed the sound barrier when in a high speed dive.

You might want to do a search of the AH message boards for P-38 and Compressability threads. One of the threads explains the compressability issue in a little more detail in regards to the P-38. Another thread has a little formula on how to figure out the approximate compressability speeds for the P-38 at high altitudes.

The dive flaps will not slow you down, they are not dive brakes but instead are designed to give the P-38 a 3-4 degree nose up attitude to help pull out of high speed dives. It's how they worked in real life and how they work in AH. Using the dive flaps to 'slow' your P-38 down just won't work because they just don't do that. Also, combat trim and/or improperly manually trimmed plane will effect the dive flaps when they are lowered, which cause some pilots to think it's slowing them down (the nose up effect of dive flaps are fighting against the constant adjustments of combat trim or the effects of an improperly trimmed aircraft) or not providing them enough nose up attitude to pull out or regain control of their plane in a dive.

Ack-Ack

Walker42 · « **Reply #2 on:** February 16, 2003, 09:16:29 AM »

Ack-Ack,

Thanks for the response! I'm new around here, and just assumed the buffeting was compressibility starting.

I have a question about the buffeting then. Why does th P-38 start buffeting so early in a full power dive from 20k feet? In the dives I did it started at 375mph. I took up a 109-E4, and the P-47D-30. These didn't experience the buffeting until 500mph, and 475mph respectively.

Thanks!

ccvi · « **Reply #3 on:** February 16, 2003, 10:19:19 AM »

IHMO the buffeting isn't part of the flight model. It's just some cockpit shaking and sound that is added when a certain TAS (why not IAS?) is reached.

akak · « **Reply #4 on:** February 16, 2003, 11:51:08 PM »

Quote

Originally posted by Walker42
Ack-Ack,

I have a question about the buffeting then. Why does th P-38 start buffeting so early in a full power dive from 20k feet? In the dives I did it started at 375mph. I took up a 109-E4, and the P-47D-30. These didn't experience the buffeting until 500mph, and 475mph respectively.

Thanks!

I'm no aerodynamic engineer but I would think that was do to air density, the air being less dense at higher altitudes. Start your full power dive at 10,000ft and you'll start to buffet past 400mph. I've been able to reach 500mph at lower altitudes and am barely starting to buffet. If you do find yourself in a high speed buffet, just decreasing throttle and maybe some rudder movement to slip the plane will decrease your speed enough to get out of a buffet quickly.

ack-ack

maxtor · « **Reply #5 on:** February 17, 2003, 08:50:45 AM »

Quote

Originally posted by ccvi
IHMO the buffeting isn't part of the flight model. It's just some cockpit shaking and sound that is added when a certain TAS (why not IAS?) is reached.

I have often wondered about same thing

Walker42 · « **Reply #6 on:** February 17, 2003, 09:22:06 AM »

The buffeting makes sense about not being apart of the flight model.

I was recording my tests with the flight recorder, and none of the buffeting was in the recording. It is almost like your point of view in the cockpit just freaks out while in game.

Ack-Ack,

Shouldn't the Higher Air density at low altitude increase buffeting? If the air flow is causing buffeting I'd figure with lower altitude the buffeting would occur sooner. I'm no engineer though either.

I've read a few instances where the 38 had buffeting problems. One was before the fillets was added near the wing, and gondola section. The Second is of course when compressiblity starts to occur, and the third mentioned buffeting would occur if the window was open.

Any one know why it's caused early in the 38 in AH? Is this simulating the pilots fear of dying because of the P38's reputation, or is there something physical that's happening to the Airframe, and if so what causes it?

N8DOG · « **Reply #7 on:** February 17, 2003, 09:23:19 AM »

Your report was pretty good but a few things were a little misleading in it. First off, just as TAS and IAS change with altitude increase , so does the speed at which n aircraft is supersonic. You mentioned that it was at Flt lvl 200. Well at 10 ft the speed of sound will be slightly different than at 11ft, not much but it does change. And on the reason it starts to compress based on TAS is beacause your aircraft TAS starts to differ than the IAS as you gain altitude. Your TAS is higher at higher altitudes due to air density and temp. Also on a hot day the sound barrier is different than a cold day. The sound barrier isnt a "golden airspeed" that you read off your instrument but more of a TAS that you would have to figure out with a flight computer. If you had a E6B or equiv. you could figure out your TAS, GS and other cool facts but this is a game and who cares hehehe, sorry for rambling on.
NICE ARTICLE THOUGH

ccvi · « **Reply #8 on:** February 17, 2003, 01:05:22 PM »

Quote

Originally posted by maxtor
I have often wondered about same thing

I've never thought about it before flying a Komet at 35k / cute 250 IAS and shaking like mad...