Compressibility in the works?

[Citabria]

is it planned to be added one day?

[danish]

Just take a 109 son ;=)

danish

[funked]

That's not compressibility Danish, just good old dynamic pressure causing increased control forces.  You 109 drivers are gonna LOVE compressibility.    

[This message has been edited by funked (edited 04-14-2000).]

[-lynx-]

Funked - unless I'm very much confused that's what the darn thing is, is it not?

Good ol' dynamic forces of air flowing over control surfaces with such a speed that pilot is unable to overcome the pressure and move them? Less so for planes with hydraulic assistance to controls or in-flight adjustable trim of all control surfaces?

Hmmm...

------------------
-lynx-
13 Sqn RAF

[funked]

Lynx, compressibility is something that all gases possess, but becomes a factor in air only when the airflow speed approaches the speed of sound (transonic flow).  Part of the airflow over the plane starts moving close to the speed of sound, shock waves form, and strange things happen.  Controls can become harder to move, or easier to move.  But they may have little effect on the plane.  In the case of the P-38, airflow over the horizontal stabilizer was interrupted, and the plane suffered an irrecoverable downward pitching moment.

There is also, of course, a rapid increase in the amount of drag on the aircraft, hence the "sound barrier".  The fact that our planes don't encounter such a barrier is the most telling clue that compressibility effects are not modeled yet.

The heavying up of the controls we feel in this game is a phenomenon that does not require transonic flow to occur.  Remember that in this game, a given stick deflection does not represent a given deflection of a control surface.  Instead a given stick deflection represents a given amount of force on the "virtual control stick" in the simulation.  If the force on the virtual stick is enough to overcome the control forces in the simulation, you get full deflection.  Otherwise you get the amount of control surface deflection allowed by the available force.

As you fly faster, the aerodynamic forces on the control surfaces will increase with the square of airspeed, by a law of incompressibile air flow - you don't need compressibility to get this effect.

Once you reach an airspeed where the control forces exceed the available force, the control surface can no longer be deflected to its limit of travel, and the response of the aircraft is diminished.

In our game this means you get a smaller response from the aircraft because even at full stick travel you may be applying a force on the "virtual stick" that is less than the force required to create full control surface deflection.

Some aircraft, due to design features like balance areas (P-51), balance tabs (F4U), hydraulic servomotors (P-38), can overcome the increased forces.  And some aircraft have small enough control surfaces (e.g. Fw 190 ailerons) that the control forces are not very large at any speed.


[This message has been edited by funked (edited 04-15-2000).]

[Citabria]

what funked said  

------------------
"There are no born fighter pilots. Some are a little better than others, thats about it. But I would say time, training, training, training and more training are the key... to any success."  -Francis Gabreski

Citabria
=357th Pony Express=

[danish]

Thx funked - a relief to know that its not compressibility when the crate lockes up and mother earth embrases you ,=)

Seriously though (cause even though Im certain you have a point, Im too dumb to see the difference) what, in your opinion, will be the practical difference between the current "good old dynamic pressure causing increased control forces" and "compressibility is something that all gases possess, but becomes a factor in air only when the airflow speed approaches the speed of sound".
As an example lets just take the 109 ;=)

Gad i hate physics..

danish

[Minotaur]

For the P-38 you should see a pronounced nose tuck or dive during compression with the inability to pull the nose up.  

The "Aerodymanic Center of a Wing (AC)" and the "Center of Gravity for the Aircraft (CG)" usually coincide in vertical alignment.  Basically the AC is the center of the lift for the wing and CG is a function of fore to aft weighting of the aircraft.

When compression occurs for the P-38 wing, the AC moves aft.  Because the CG remains at the same point, the AC and the CG are not aligned.  This mis-alignment causes a weight imbalance. IE; causing the plane to have more weight forward of the AC than it should.  This makes the nose heavy.

In the case of the P-38, it causes the plane to so nose heavy that maximum counter elevator controls can not counter this additional nose weight.  Consequently the plane dives, until a lower altitude is reached so that the elevator down thrust  can counter balance the heaviness of the nose.

Keep in mind that the maximum deflection of the elevator is effected by Funk's explanation above.  In high speed dives the P-38 had alot going against it.

For the P-38 a dive flap was added.  The purpose of this flap was two fold.  First off, it functioned as a air brake by slowing the plane in a dive.  Second and most important, it served to move the AC of the wing forward.  By moving the AC forward this re-aligned the AC and the CG.  This solved the "Mach Tuck" problem cause by a  overly heavy nose.

You got to love the P-38L.  

------------------
Mino
The Wrecking Crew

[Minotaur]

BTW, you will notice that most modern high speed aircraft and the Me-262 use a swept back wing design.  I believe this is also a compensating measure for compression.  

Perhaps Funked, may describe that as well.

Thanks!  

------------------
Mino
The Wrecking Crew

[danish]

Minotaur:
Thx for your input!
But....*wispers*...I still dont get the difference...

For all practically reasons: what is the difference between "good old dynamic pressure" and compressibility?
When you say :

"When compression occurs for the P-38 wing, the AC moves aft. Because the CG remains at the same point, the AC and the CG are not aligned. This mis-alignment causes a weight imbalance. IE; causing the plane to have more weight forward of the AC than it should. This makes the nose heavy.

In the case of the P-38, it causes the plane to so nose heavy that maximum counter elevator controls can not counter this additional nose weight. Consequently the plane dives, until a lower altitude is reached so that the elevator down thrust can counter balance the heaviness of the nose."

- that is the thing I start feeling in the 109 at well below  the speed of sound ~ 400 m\h.
So is compressibility just a theoretical entity with no practically relevans? - because what it does to the flight caracteristics allready have emerged at much slower speed under some other name ("dynamic pressure")?
And\or is this so for only a few of the planes (in AH setup)?

Sorry if I dont grasp it, and I really dont want to be a royal paine , but...?

danish

[funked]

nm

[This message has been edited by funked (edited 04-15-2000).]

[wells]

Compression = drag

You should see a reduction in terminal dive speed limits, each plane being slightly different!

[Minotaur]

Danish;

No problem.  

Go back and read Funk's description on dynamic forces.   This basically means that the aircraft is flying so fast that the man inside does not have the physical strengh for the control services to reach full deflection.  

This means to the pilot that the faster a plane goes, the heavier that it gets and much more force is needed to manuver it.  This is combined with then greater amount of force needed to move the controls.  Two opposing agendas.  The game simulates this by reducing control effectivesness.

Compression is a different animal.  It has to do with supersonic air passing over a particular part of the airplane.  This does not always have to be a control surface or a wing, but often is.  

The P-38 is a well known case and a good example.  Compression in the P-38 really kept it from being a better figher up until the problem was solved.  Who wants to lawn dart in a power dive?

Keep in mind that each plane will suffer compressability in a different fashion.  Where dynamic forces effect each plane basically the same way, but perhaps at diffent speeds.  EX: slow roll rate, hard to manuver etc...

Also keep in mind that to the pilot, it just means that the airplane flies this way at this speed etc.  You just learn how take care of this problem or prevent it from happening in the first place.

Good Luck!  

------------------
Mino
The Wrecking Crew

[danish]

Thx ;=)

I know that there are many on these boards that juggle with physics\math every day.For us mortal creatures its good to get things simplified.

I read funked's description again: indeed I think I understand it! hehe

danish

[StSanta]

Just a quickie question; since it is very hard to get the 109 up to transonic speeds, why should this aircraft be more affected by it than other aircraft? The controls become so heavy far below that that no one in their right minds attack at them.

When  I have too much of an alt adv, I don't head straight down. I spiral dive and keep the speed down.

At normal atmospheric pressure, the speed of sound is
     v = 331 m/s + (0.6 m/s/C)*T

where T is temperature of the air. At sea level it should be around 343m/s, which is around 750 miles/hour.

The 109G10 becomes hard to control at 500 knots or so (really should check before I wander into a big pilo of poo poo). Let's say 550 to be somewhat safe. Now I hate all these ways of measuring speed J. I am an European using the metric system  ). One knot is around .51 m/s, which would mean that 550 knots = 280.5m/s.

From this I can conclude that compressibility must be a factor below the speed of sound. What I do not know is when and how much it affects an aircraft,. In the case of the 109 (sorry to go on about this aircraft, but it is a sweeeeet one J), should it be affected at 550kts? (assuming altitude is say 14k).

Any answers would be appreciated

level it should be around 343m/s.

------------------
StSanta
II/JG2