FSXA Finding Aircraft Stall Speed

#1
Hello,

I have a quick question... I'm building a custom catapult, to launch aircraft at more realistic speeds than the default cat. To accomplish that goal while keeping it simple, I'd like to launch each aircraft at its stall speed plus a margin of, say, 20kts.

So the question is: how can I find the stall speed (say, at empty weight) for the user aircraft, without resorting to a lookup table by aircraft type? Checking the A:var reference shows "Stall Alpha", but no "stall speed clean" - or better yet "stall speed dirty" (which is of course what I'm actually after)... Can you guys think of any way to calculate the landing configuration stall speed at some weight (I can scale speed for loading change using a simple formula), using either information from A:vars/simconnect, or by reading from aircraft.cfg?

Thanks as always,
Farley
 

Roy Holmes

Resource contributor
#2
Farley,
This post is more appropriate in the flight dynamics forum, however here is how to do what you want.
There are several A Vars that help: Note the SDK gives them as per radian etc but you can use per degree instead which I find easier to grasp.
A:LINEAR CL ALPHA,per degree. This is the slope of the air file Record 404/1545 lift coefficient in the region of 0 to 10 degrees. A typical value is 0.1 per degree.
A:STALL ALPHA,degrees. This is the angle that corresponds to the highest value of CL in the air file Record 404/1545 table, starting from the -180 end. If you had edited an existing 404 and it had a higher CL in the minus angle range than your peak around 15°, you will get a permanent stall warning.
A:ZERO LIFT ALPHA,degrees. This tells you where CL is zero in the Record and completes the picture.

Now, to work out stall speed you also need the wing area and air density and weight.
A:WING AREA,Square feet
A:AMBIENT DENSITY,Slugs per cubic feet
A
:TOTAL WEIGHT,Pounds

The lift equation is L=CL*0.5*RHO*V^2*S
Where L= Lift, =TOTAL WEIGHT,Pounds in level flight
CL= STALL ALPHA*LINEAR CL ALPHA, for example 15*0.1 = 1.5
RHO= AMBIENT DENSITY,Slugs per cubic feet this is 0.002377 at sea level
S=WING AREA, Square feet
V=True Air Speed, feet per second.
This will give you the stall speed clean.
To get the stall speed with flaps you need the flap lift per radian coefficient from the air file CLdf. A typical value is 0.4. This means that you add 0.4 to the clean wing CL if you have 1.0 radians (57.3 degrees) of flap deflection. 30° flap would then give about 0.2 additional CL. That coefficient can be found in Record 1101 or 1539

I'll let you do the math now you have the data
Roy
 
#4
Checking the A:var reference shows "Stall Alpha", but no "stall speed clean" - or better yet "stall speed dirty" (which is of course what I'm actually after)...
Farley,

Did you take a look at these?
Code:
(A:DESIGN SPEED VS0, knots)
and
(A:DESIGN SPEED VS1, knots)
The variable name doesn't include the word STALL so it's easy to miss.

These apparently come from the [Reference Speeds] in aircraft.cfg and should be max weight speeds, but if you have arbitrary and variable loads on the aircraft, maybe Roy's formula is a better way to go.

Bob
 

Roy Holmes

Resource contributor
#5
The reference speeds in the aircraft.cfg have no bearing on how the aircraft stalls, they are numbers that anyone can change at will. I believe the cruise speeds are used by AI aircraft, but stall speeds of AI are derived through the air file as I described. The basic aerodynamic equations have been around for a lot more years than I have.
Roy
 

tgibson

Resource contributor
#7
The clean stall speed is used by AI aircraft for the speed of rotation at takeoff, etc.

The cruise speed values are also used by the Flight Planner to get its time info, AFAIK.
 
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#8
Thanks so much, folks!

For what it's worth, I'm with Roy on this one... It would seem to make more sense, if possible, to use numbers which may or may not be in the Aircraft.cfg... I haven't actually written the code yet. But the launch speeds will be overrideable in the carrier aircraft.INI file that I'm already using for the custom arresting gear anyway. So It makes sense to me to use the deepest-level source for the default values, to ensure everybody gets a reasonable launch speed out of the box.
 
#9
The lift equation is L=CL*0.5*RHO*V^2*S
Where L= Lift, =TOTAL WEIGHT,Pounds in level flight
CL= STALL ALPHA*LINEAR CL ALPHA, for example 15*0.1 = 1.5
RHO= AMBIENT DENSITY,Slugs per cubic feet this is 0.002377 at sea level
S=WING AREA, Square feet
V=True Air Speed, feet per second.
Hi Roy,

This may not be the right place to ask the question, but as it relates to this subject....

How to multiply "degrees" by "per degree" in a variable L having an "enum" or "number" unit?
Example : (A:LINEAR CL ALPHA, per degree) (A:STALL ALPHA, degree) * (>L:CoeffLift, number).
I always get 0, while the values are:
(A:STALL ALPHA, degree) = 16.04
(A:LINEAR CL ALPHA, per degree) = 0.1...

There must be a simple way to do this...! But how do I get 1.604 in my variable (L:CoeffLift, number)? Any help is welcome....
 
#11
Hi Bob...

I'm a donkey...! Indeed, my problem came from an unfulfilled condition...... a little higher in the code...!

A thousand apologies,... and have a good flight!
 
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