Author Topic: High speed vertical attack causes control surface lock up. (Read 2371 times)

Krusty · « **Reply #15 on:** April 20, 2007, 03:43:35 PM »

I'm pretty sure it's been proven that no WW2 airplane could near the speed of sound. Not even the fastest jets. Especially not props (not with the huge air brake of a prop on the nose). They would shear the prop/engine/wings off and fall apart well before nearing mach1.

EDIT: I've not read anything about wing shearing, but some hurricanes came pretty damn near to it, had to be written off. Props would rip off at the gearbox, damaging the engine in the process, possibly taking it with.

EDIT2: Sorry, don't mean to hijack a problem-solving thread, I'm just goingto let it drop.

Murdr · « **Reply #16 on:** April 20, 2007, 04:17:12 PM »

I know what he was getting at, but he was correcting something that was not said. Just pointing out that NHawk said "airflow over the wings of the aircraft can actually exceed the speed of sound" which is exactly what happens when an airfoils critical mach is reached. That is correctly stated what compression is. Speed of the airflow, and speed of the aircraft are two different things.

Hazard69 · « **Reply #17 on:** April 21, 2007, 12:11:45 AM »

You dont need to go beyond mach1 to have supersonic airflow over an airfoil.
The air is accelerated over the cambered upper surface and is much faster than the aircraft. Most jet airliner's today are limited to around Mach 0.8-0.9 cause at this speed the wing is already approaching mach1. Strap jets on a Cessna 152 and it will probably enter supersonic speed airflows at around Mach 0.4

NHawk · « **Reply #18 on:** April 21, 2007, 07:01:23 AM »

Thank you Murdr and Hazard. I didn't want to start a battle of knowledge.

The airflow over the wings begins to reach supersonic speeds at around mach 0.7. Which is around 532mph at sea level. That speed decreases with altitude. That's why you're usually already in compression long before you feel it's full effect. You're diving from high altitude, where the compression speed is much lower thinking all is good until you try to pull up.

Many WWII aircraft actually reached compression at much slower speeds than others. The 109 and Zero come to mind right away. The basic rule of thumb is the lighter the aircraft the sooner it compresses. However wing and control design is probably the main factor. Hence why a Cessna would compress earlier.

Oleg · « **Reply #19 on:** April 21, 2007, 07:57:12 AM »

Even if supersonic airflow is possible for ww2 planes, its definitely not a factor what causes "regular" compression. Airflow pressure on controls and human's strenght is a factor.

Take a thought, why trims ever help against compression? Or why P-38L dont suffer from compression so strong as earlier versions?

NHawk · « **Reply #20 on:** April 21, 2007, 01:03:38 PM »

Quote

Originally posted by Oleg
Even if supersonic airflow is possible for ww2 planes, its definitely not a factor what causes "regular" compression. Airflow pressure on controls and human's strenght is a factor.

Take a thought, why trims ever help against compression? Or why P-38L dont suffer from compression so strong as earlier versions?

When an aircraft is in a high speed dive, the airflow over the wings of the aircraft can actually exceed the speed of sound. This transonic airflow creates a shockwave and a barrier that disrupts the flow of air over the control surfaces. This causes a dramatic loss in control efficiency and is known as compression.

That is all I'll say.

Oleg · « **Reply #21 on:** April 21, 2007, 01:50:45 PM »

Probably we interpret words "control efficiency" in different ways

My interpretation is "can move but dont works", yours is "cannt move" if i understood you correctly.

Anyway, supersonic airflow absolutelly unnecessary for compression. Early war planes was compressed at ridiculous speed, like ~450-500 kmh, where is no way airflow can exceed speed of sound i believe.

Murdr · « **Reply #22 on:** April 21, 2007, 06:02:58 PM »

Quote

Originally posted by Oleg
Even if supersonic airflow is possible for ww2 planes, its definitely not a factor what causes "regular" compression. Airflow pressure on controls and human's strenght is a factor.

Take a thought, why trims ever help against compression? Or why P-38L dont suffer from compression so strong as earlier versions?

See "critical mach"

If you look at what happens with the airflow in the figure, you'll might be able to picture that there is significant air pressure on top of the fore of the wing. On the aft of the wing, there is a separation of airflow which allows increased positive pressure on the bottom aft of the wing.
So you have a net twisting force on the wing which causes the plane to nose down. This is the source of high speed control problems for P-38s and P-47s. To deal with that problem, dive flaps were added under the wings. The dive flaps change the shape of the wing, and therefore the airflow around it. While the transsonic shockwave still forms with the dive flaps deployed, the change in airflow moves its formation further aft on the wing. So the net pressure forces on the wing are not pushing the aft of the wing up.

The shape of the foil and air density are what determines at what speed the shockwaves will start to form. The P-38 and P-47 have thick cords, resulting in a low critical mach. Air is being displaced by a solid object, and the shape of the object is going to determine how fast the air is going to accelerate while being displaced. But it can be accelerated to a speed faster than what the object itself is traveling when the shape is forcing it "out of the way" faster than it can efficently flow.

You can have transonic shock waves form around stabs and control surfaces too which can cause them to be "heavy". This I believe is what happens in the case of a 109 for example.

Compression is often used genericly for any high speed control problem, but from a technical stand point it's often misused.

I hate to hijack threads, but I also hate to see good information being cast into doubt, when I know the info is correct. And its not a matter of a knowledge battle either. It's a matter of the passer-by not being left with bad information.

NHawk · « **Reply #23 on:** April 22, 2007, 06:52:00 AM »

Murdr, I'll just add one more thing that probably should have started your last post.

Air is always flowing over the top of the wing faster than it is flowing below the wing. This is what causes lift. Now,.... If you look at what happens with the airflow in the figure, you'll might be able to picture that there is significant air pressure on top of the fore of the wing......

Thanks again Murdr.

swareiam · « **Reply #24 on:** April 24, 2007, 08:37:35 AM »

Thanks for the aerodynamics lesson guys. But, has any of you experienced the initial problem of control surface lock up? I have also experienced it in some cases recently in straight and level flight during LOW speed dog fight. The control always seem to come back after approximately 10 sec, which in most cases is too late.

Cheers:aok

NHawk · « **Reply #25 on:** April 24, 2007, 08:45:54 AM »

Quote

Originally posted by swareiam
Thanks for the aerodynamic lesson guys. But, have any of you experienced the initial problem of control surface lock up? I have also experienced it in some cases recently in straight and level flight during LOW speed dog fight. The control always seem to come back after approximately 10 sec, which in most cases is too late.

Cheers:aok

Sorry we got so far off track.

There's only one thing I can think of and that's if your connection is through a USB port and your stick is on a USB port. The two may be conflicting or overloading the USB part of your system.

One other thought, do you get the dreaded "Don't move your controls so quickly" (or something like that) message in the text buffer? The 10 second freeze is about right for that.

Just a couple of thoughts. Probably not right, but it's worth a shot.

swareiam · « **Reply #26 on:** April 24, 2007, 09:24:49 AM »

Quote

Originally posted by NHawk

One other thought, do you get the dreaded "Don't move your controls so quickly" (or something like that) message in the text buffer? The 10 second freeze is about right for that.

I know the message that your speaking of, but not in this case. It usually happens as you're zeroing in and shooting off your first volley. A wide turn of sorts manuvering to lead your shot onto your opponents nose.

Anyway, however infrequently it happens it has definetly cost me a few kills at this point. Otherwise, I really do enjoy this game and the comradeship that you gentlemen provide. See you in the arenas and thanks for your input.

Cheers:aok