Aces High Bulletin Board
Help and Support Forums => Help and Training => Topic started by: TweetyBird on July 01, 2004, 10:03:41 PM
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Someone needs to explain the law a physics that allows a plane with no thrust to continue to compress. Ever hear of buoyancy?
Pressures will do everything in their power to equalize. With the lack of thrust, compressing planes should slow down almost immediately with no thrust. What is up with this missile to the earth crap and 20 seconds of compression with no thrust?
PLEASE someone post that equation. There is only so much speed gravity will give per pound/mass, and then air displacement and drag comes into the picture.
Either the gravity is WAY too strong or the compression algorithm is - well - gamey.
I gota run more test, but I find it ridiculous that a plane with no thrust at 20k will slow down faster than a plane with no thrust in dense 3k air.
Edit - hope this wasn't shrill :D
but some things in the FM seem a little off
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https://ewhdbks.mugu.navy.mil/freefall.htm
if the terminal velocity of the aircraft is faster than the compression speed, and you had the alt to fall that long, then youre going to compress no matter what throttle settings you have.
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Thanks for the link. I have to go over it more closely, but I think it even states it ignores drag in its calculations. With bombs, that might no be a problem, but an aircraft is going to have a lot more drag.
I have in mind a simple test. Take a plane up to 30k, kill engine, point nose straight down and record the speed at diff alts. Then do the same with different flap settings. That should show the influence of drag.
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Unless you start with full flaps and stay well away from the vertical. You will build up speed, the flaps will autoretract & You'll end up back in compression anyway.
Exception of course those planes (Not the p38) that had true dive flaps. ie SBD, ju87 etc.
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>>Unless you start with full flaps and stay well away from the vertical. You will build up speed, the flaps will autoretract & You'll end up back in compression anyway.
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Oops - didn't think about that - ty. Maybe drop tank and no drop tank? That big keg handing below should cause drag. And rocket rails...
Have to burn out the dt and shoot the rockets to make sure I'm getting mostly drag and little weight.
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Originally posted by TweetyBird
>>Unless you start with full flaps and stay well away from the vertical. You will build up speed, the flaps will autoretract & You'll end up back in compression anyway.
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Oops - didn't think about that - ty. Maybe drop tank and no drop tank? That big keg handing below should cause drag. And rocket rails...
Have to burn out the dt and shoot the rockets to make sure I'm getting mostly drag and little weight.
Weight doesnt affect the ~9ft/s/s accelration that is 1 G. I believe Newton proved this some ungodly number of years ago. (I dont mean this with any kind of attitude...just trying to save you some headache)
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Originally posted by Tails
Weight doesnt affect the ~9ft/s/s accelration that is 1 G. I believe Newton proved this some ungodly number of years ago. (I dont mean this with any kind of attitude...just trying to save you some headache)
ya mean 32 ft/s/s ?
ah you put the number for meters instead of feet
it,s more like 9.803 m/s squared
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Sorry. I was low on caffine when I wrote that, but yeah. Weight doesnt affect acceleration by gravity.
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>>Weight doesnt affect the ~9ft/s/s accelration that is 1 G. I believe Newton proved this some ungodly number of years ago. (I dont mean this with any kind of attitude...just trying to save you some headache)
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oops again. Good thing - fat skydivers would always land first :D
Old science fair project - 2 identical containers - fill one with sand - drop both at the same time - which hits ground first..
Thanks for the correction
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An Apollo video also comes to mind, where they dropped a hammer and an eagle's feather from the same height on the moon. Pretty neat the theories you can prove without a pesky atmosphere to mess things up.
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Tweety, ya need to change ~9ft /s/s to~9m/s/s
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Acceleration on earth is essentially a constant.. (It does vary by altitude, but hardly worth worrying over)
The values are:
32 ft/sec/sec Or stated 32 ft/second squared
This is the same value as the metric equivalent:
9.8 meters/sec/sec
All this means is that a falling body (ABSENT DRAG) will increase speed at a rate of 32 feet per second for every second it falls....so after two seconds, it will be moving 64 feet per second. After 3 seconds, it will be moving 96 feet per second, and so on.... Clear?
Now, drag reduces this value..... and terminal velocity is when the drag exactly matches the forces of gravity....hence no more acceleration. A falling body has increasing drag until drag = gravity...........
Hard to imagine a terminal velocity greater than the compression speed of many airplanes from WW2.......................
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I am not an aero engineer but this is how I see it ...
Once a plane enters compression, drag, I believe is not in the equation anymore cause you have already accelerated to the compression point. Its not how fast you are going, but at what point do you lose control to change your angle to stop the compressed state.
Once into compression, there is nothing that will slow the airplane down or change its angle outside of the normal control surfaces.
Some planes, as mentioned, had special control surfaces (outside of ailerons, flaps, and elevators) that helped break the flow of air over the wing and slow the plane down or kept the plane slow to the point where the normal control surfaces worked.
So if the speed at which compression starts is greater than the speed at which the control surfaces no longer work, your plane is now trimmed for that angle and you will continue your downward flight path until terra firms interrupts it.
I would assume that the steeper the angle at which compression begins, the harder it would be to maybe naturally pull out of a compressed situation.
If your control surfaces are only good up till 100 mph and you enter a dive that brings you to 150 mph, your screwed. If you cannot somehow slow the plane back down to 100 mph or below, at the angle that you are currently diving, you will lawn dart.
That is how I view it ... I could be all wet.
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I was addressing entering compression through "falling" straight down...I doubted you could easily get going fast enough to enter compression simply by pointing the nose down.
Once you are in compression, the forces are many and varied....I am far less expert on that topic other than to say it used to cause a sensation of locked controls, etc........so rigidly did these forces hold the airplane.
I read of pilots standing on the stick to force a negative G pull out from compression.
In any case, once in, getting out is hard.
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>>Hard to imagine a terminal velocity greater than the compression speed of many airplanes from WW2.......................
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Surely there must be a list somewhere of the terminal velocities of WW2 planes - gonna search google.
Also, I wanted to test the air in AH2 as it seems its the same density at 30k as 4k (at least in some respects). Where you do have to stay faster to perform certain maneuvers, I notice if you have a plane going 360 mph true air speed at 30k and kill the engine, it takes the exact same time to slow down to 150 as it does at 4k. Now that doesn't make sense does it?
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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.
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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! :)
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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
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>>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.
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Doubtful it's modelled for free falling bodies.... What's modelled is the well understood speed vs. altitude relationship.
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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
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This is a potentially ugly subject. :D
If you want some fairly straightforward explanations, without too much math, this site (http://adg.stanford.edu/aa241/drag/cdcintro.html) , and this site (http://naca.larc.nasa.gov/reports/1939/naca-report-646/) 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... :D
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.
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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
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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
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that pesky gravity is over rated, can it please be turned off?