Compression with no thrust?

[SlapShot]

I doubted you could easily get going fast enough to enter compression simply by pointing the nose down.

Jump in a P-38 or Me-262 and climb to 25K. Put the nose down, not at a very steep angle.

On the P-38, as you approach 500 mph you will enter compression, and you will be far from nose pointing straight down.

Can't remember the speed for the 262, but it too will enter compression without nose pointing straight down.

In either one, if you don't get the compression under control, early, you will continue, and become a jart.

[TalonX]

There is a huge difference in density of the air at 4K versus 30K.   In level flight, the drag at 4K would be substantially higher......on the order of 2.5 times as high........

They can't model everything!  

[dtango]

In an unpowered dive, Force = Weight (sin (dive angle - y)) - Drag.

With the above relationship:

W*sin(y) < D means the aircraft would decelerate
W*sin(y) = D means the aircarft would be at terminal velocity
W*sin(y) > D means the aircraft would still accelerate

For a quick approximate study of this by putting some realistic values to it all, lest's assume a P-51D (parasite drag coef. ~.018, wing area 233 sq-ft) weighing 7500 lbs, flying at 550mph (in compressibility range) at 5000 ft (air dens. 0.0021), in 45 degree dive.  Using the figures in calculations we get the following.

W*sin(y) = 5300 lbs
D = 2850 lbs

W*sin(y) > D

The P-51 would continue to accelerate downward if the nose stayed pointed at 45 degrees.  This is a rough study and neglects other variables like prop drag etc. but this gives you an idea of the physics involved and the result.

Air density is modelled in AH.  Take for example the differences in TAS for max level speeds of the P-51D on WEP: At 0' ft ~ 365 mph, at 24k ~ 435mph.

Tango, XO
412th FS Braunco Mustangs

[TweetyBird]

>>Air density is modelled in AH. Take for example the differences in TAS for max level speeds of the P-51D on WEP: At 0' ft ~ 365 mph, at 24k ~ 435mph. <<

I think its modeled some but not all. I have to get a stop watch to prove it, as the time on the film is not accurate. But there should be less drag at 30k than 4k in level flight. Unless auto pilot is introducing side effects, I don't know why the planes should slow at the same rate at these two alts upon killing the engine.

[TalonX]

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Doubtful it's modelled for free falling bodies.... What's modelled is the well understood speed vs. altitude relationship.

[dtango]

Tweety:

But there should be less drag at 30k than 4k in level flight.

This is only true when comparing the same TAS at different alts.  This is not true if the aircraft is flying at the same lift and drag coefficients which results in the same amount of drag but higher velocities with increasing alt.  E.g. the same plane flying at max level speed at 4k compared to one flying at 20k has the same drag at both alts but at 20k would be at a higher velocity.

Unless auto pilot is introducing side effects, I don't know why the planes should slow at the same rate at these two alts upon killing the engine.

Hard to say about this since you're comparing apples and oranges if you don't know what the rate of change in drag should be for a given set of parameters.  It's conceivable that if you did a test by flying at max level speeds for different alts and then cut the engine that the time to decelerate from max level speeds to a certain airspeed would be the same since the drag coefficient for the plane doesn't change while the decrease in air-density is offset by the wider band of velocity to decelerate over at higher alts.

TalonX:

Doubtful it's modelled for free falling bodies.... What's modelled is the well understood speed vs. altitude relationship.

Not sure what free falling bodies has to do with modelling air density since air density is the same for a given altitude regardless if an object is free falling or not.  

In discussing with Pyro in the past, what isn't modelled though is local geographic changes in air density due to things like climatic differences etc.

Tango, XO
412th FS Braunco Mustangs

[Spitter]

This is a potentially ugly subject.  

If you want some fairly straightforward explanations, without too much math, this site , and this site have some good info.  

The second one is actually a PDF file of a NACA report, a bit on the technical side.  

With aircraft like the P38, one main reason it had the compressibility issues it did was because it had a relatively thick airfoil, so you are looking at blunt body compressibility.  The shock waves as the Mach number locally approaches M=1.  It is quite possible that unpowered you could put an aircraft into a dive, and easily reach compressibility, especially at altitude. Remember the higher altitude, the lower true velocity is for the same Mach number.

There is no way AH could model all of the affects of changing air density and atmospheric characteristics in a game like this.  I done a few CFD (computational fluid dynamics) problems, on large mainframe workstations, you are looking at 8-40 hours of solid computation time to solve a steady state, compressible fluid flow around a relatively simple body.  Talk about low frame rate...

Everything in the game is an approximation, it has to be.  

Having said that, take a look at those web sites, and google compressibility airfoil for some good pages.  Some of them are very technical, and brought back a lot of bad memories for me.

Basically, though, it boils down to comparing a simulation (and more importantly, GAME) to the real world, and yes, at altitude, it would not be at all difficult to get a relatively streamlined shape, like an aircraft to reach compressibility in a dive.  

The higher you go, the easier it gets.  At extreme altitudes (80k feet) where the U2 flies in the subsonic regime, the range between stall speed and Mach was less than 40 knots, IIRC.  

Cheers,
Spitter, the rocket scientist.

[DamnedRen]

Actually our resident rocket scientist justy about has it!

Spitter, you left out weight.

Weight limits your alt. It also can bring your stall buffet margin down to unacceptable limits. Like having your airspeed within a few (under 5) knots of stall buffet.

I wonder that the discussion really asks for a fix for induced  compression in a dive. It might be interesting to note that in a GAME all things may be possible. Including your ability to recognize the onset of compression and pulling out/away from it.
If you notice the more experience gained in this game the less people have a tendency to go into compression. Its the same as being able to ride the ragged edge of a stall or black out. The more you do it the closer you can come to the absolute edge.

Just a thought.

Ren

[hitech]

TweetyBird: At speed Thrust from the engine in a dive is a relativly small force.

A P51D is aprox 9000 lb, At max speed it produces around 1500lb thrust.

So if it is going straigth down it is produce 11500lb force with full throttle. Or at no throttle it would be producing 9000 lb of force.

Now as an aproximation at max flat level speed the p51 would have 1500lbs of drag (drag = thrust when at max speed).

Since drag vary's aproximatly with the square of the speed, you would have 4 times as much drag at twice speed. So even at sea level where drag is the greatest and max speed is around 360,gravity alone would be enough force to take you over 720 , i.e.1500x4 = 6000 drag and 9000lb force.

And we do completly model atmospher. It just is always the same atmospher.

DammedRen: We already do what your asking, the cockpit begins to shake BEFORE reaching compression.


HiTech

[AmRaaM]

that pesky gravity is over rated, can it please be turned off?