Konrad posted 05-13-99 10:45 PM            

What cause compressability exactly? And how do you get around it? I look at a 109 and Ki61 wing and they look rather similar, yet in WBs the ki61 has much better high speed handling. I read the Typhoon's thick wing made it compress. The F4u seems to have a pretty fat wing itself and it had good high speed handling. The 109 has thin wing yet it compresses intolerably. Anything to do with airframe strengh?

Konrad

Jg77

Wells posted 05-13-99 11:10 PM            

Thickness and camber (curvature) are probably the main factors. The velocity of the airflow close to the surface is travelling much faster than the mainstream flow, due to pressure differences (how lift is created). Around the leading edges of flying surfaces and fuselages, the airflow can be at the speed of sound, where shock waves are starting to form, whereas the mainstream velocity is say Mach 0.7. The shock waves tend to disrupt the smooth flow of air, causing vibrations and loss of control, where the plane wants to become a lawn dart! In additon, there is a huge drag increase, control forces are very high and propellers quickly lose their ability to produce thrust above their design point. They may even start 'braking' or 'run away', going beyond the governers ability to govern the rpm!

In the case of the 109, it uses a fairly high lifting airfoil that is commonly used on trainer type aircraft. This was so that Willy could keep wing area to a minimum and minimize high speed drag. Unfortunately, not much was known about compression back then. That brings up the Spitfire again...hehe

It used the opposite approach of having large wing area combined with a thin airfoil. Lifting capacity wouldn't change, but the critical Mach number was higher...

funked posted 05-13-99 11:47 PM            

Konrad - it's the air compressing, not the wing. The density of airflow around an aircraft does not change much until you get near the speed of sound. Near that point weird things happen, including density changes.

Muddy posted 05-14-99 01:32 AM            

Actually I thought it was a term to describe loss of control input due to disruption of normal airflow over control surfaces during high speed flight. Particularly in a high speed dive where loss of elevator effect prevented a pull out. In effect, at very high speed the elevator was literally operating in a partial vacume. The effect was agravated by thick wings.

I believe it was discovered an easy way to get over this was by moving the horizontal stabilizer high up out of the slipstream of the wings and fuselage. The early P38 was especially prone due to the location of its horizontal stabilizer nearly on the same plane as the wings. The Mig 15 had it's HSTAB high for this reason. In fact the feature was necessary on the X-1 so control could be maintained through mach one. Up high on the vert stabilizer kept it in "clean" air where it could do it's job at speed and high angles of attack. Also having the entire HSTAB move instead of just a trailing elevator helped the situation.

This is my understanding mainly gleaned from Yeager's autobiography so all you slide rule types go easy on me. :-)

Muddy

305th BG (H)

dbng posted 05-14-99 04:32 AM            

The p38 compressed as a result of the wing cord/thickness ratio. Due to the large internal fuel requirements of the Air Corps specifications the 38 wing was pretty thick. In a high speed dive near-trans sonic shock waves moved back across the wing impairing the control surfaces, I don't think the elv being on the same plane affected the condition, they tried higher tail designs. The "dive recovery" flaps ont he 38 were on the underside of the winf outboard of the engine and basically just disrupted the high speed flow on under the wing.

paarma posted 05-14-99 04:48 AM            

WB does not model compression, but stick forces, if I have understood that right (note that if:-)

While compression occurs only when airflow speed on airfoil (not necessarily the IAS for the plane) comes near 1 mach, the stick forces are getting constantly heavier as the speed increases; pilot just can't move the controls.

Boosted controls (like in P38L, vs older models) make it lighter to the pilot, but nothing do they help to the compressibility problem.

WB's hi stick forces can always be beated by using trim. But, for example, EAW models something that might be more like compressibility: plane starts to fall down like a brick, no controls, no lift, nothing.. until airspeed can be lowered somehow. (But EAW does not model trim, and trim can't be used to recover high speed dives like in WB.)

//paarma

buile posted 05-15-99 09:55 AM            

Hello,

I seem to remember something about the shock wave moving the center of lift on the wing back further (behind the center of gravity). This causes the plane to nose down which increases speed, changes the position of the shock wave, pushing the center of lift back, causing the plane to nose down even further... &%#(!!! HELLLPP!!!

buile-

scrmbl posted 05-15-99 03:17 PM            

Isn't compression anything to do with the high speed of air passing over the aerofoil that made it difficult/impossible to deflect the controls?

Wells, you're an expert... can you explain why big aircraft like airliners have those vortex generator thingies on the wings near the leading edge?

llbm_MOL posted 05-15-99 03:39 PM            

Scramble- To keep the air from slipping off the wingtips of course. Less wingtip voteces. The air tends to move out to the wingtips and you lose some lift due to this. They put those on em to make them a little more efficient.

LLBM OUT!!!

jedi posted 05-15-99 06:00 PM            

Probably all these things combine together under the heading of "compressability."

As the sonic shockwave moves back over the wing, the area behind the wave and the area in front of the wave are creating different amounts of lift. This moves the center of lift away from the center of gravity, creating a nose-down moment. Eventually, the nose-up authority of the elevator is exceeded, and there is no way to pitch the nose back up to slow down. This is even worse if the elevator itself is being "washed out" by turbulent airflow off of the wings.

Meanwhile, if the shock wave moves far enough back on the wing, the ailerons become affected. At a high enough speed, deflecting the aileron "down" will actually cause the wing itself to twist, resulting in a decreased angle of attack for the "aileron-down" wing, which then drops instead of rising, and you get "aileron reversal."

I think the general "feature" of compressibility is probably a nose-down pitching tendency accompanied by loss of nose-up elevator authority, which makes it hard to slow down, particularly in fast, low-drag designs like the P-38.

Now, as to why the 38 was more "compressible" than the 51 or 190, I'd have to ask Wells that one Wing design delays formation of the shock wave perhaps? Tailplane mounted higher than wing?

-jedi- VF-17

Wells posted 05-15-99 06:14 PM            

LLBM,

I think you are talking about a wing fence, which is placed about 2/3 of the way out to the tip. The airflow only moves out to the tip on the bottom of the wing and then curles up over the wingtip, creating the vortex. The spanwise flow on top of the wing is towards the root. You still get mini-vortices at the trailing edge. The vortex generators re-energize the boundary layer and delay it's breakaway from the surface. Some planes do it a different way, by having small 'holes' in the wing that provide a suction to keep the airflow laminar .

scrmbl posted 05-15-99 06:46 PM            

LLBM, I think you thought i meant winglets> I know those are there to reduce wing "washout". The vortex generators, I believe, are those little tabs no more than half an inch high and about 2 inches long - about 6 of them - along the length of the wing.

JEDI - the scenario you described reminded me of what some airline pilots call "coffin corner"! A combination of going too fast and too slow at the same time. Please do elaborate BTW the upper winglet on the MD11 is as big as the tailfin on my TB10!

Wells posted 05-15-99 07:07 PM            

CC Jedi, airfoil selection. The P-38's wing is 14% average thickness and 3% average camber. Most of the good diving planes had < 12% thickness and < 2% camber. Both thickness and camber will increase the 'curvature' of the surface and increase the velocity of the airflow, causing Mach effects to happen sooner.

jedi posted 05-15-99 08:46 PM            

Scrmbl--

"Coffin corner," as I understand it, is the corner of the flight envelope that you reach at max altitude and max speed. At max, altitude, you begin to get close to the aircraft's stall speed, because the air density is so low that the wing's lift is just about equal to it's weight. At the airplane's "limiting Mach," it's just about to start running into the problems associated with supersonic speed, whether that's Mach tuck, aileron buzz, flutter, whatever.

The U-2 is notoriously difficult at the edge of it's envelope. There's something like 10-15 KIAS between its stall speed and its critical Mach number at its operating altitudes. If you were to stall, and drop the nose to recover, you might quickly exceed the limiting Mach and start losing big pieces The earlier models don't even have an autopilot!

Hehe you'd think they would want only steely-eyed heroic fighter-pilot types to fly such a demanding plane, but actually, they kinda like C-130 pilots Obviously a perceptive and discerning bunch.

Or maybe it's just that fighter pilots are too smart to fly the U-2.

Naaaah!

-jedi- VF-17