how to estimate lift-to-drag ratio?

[JtPB]

the title simply says it...

[GoSlash27]

Are you talking just wing surfaces, or the entire craft?

-Slashy

[numerobis]

Lift is linear with speed. Drag is quadratic with speed.

Both lift and drag for aerodynamic surfaces depend on the angle of attack. Higher angle of attack => more drag.

For control surfaces, higher AoA => more lift.

For winglets, AoA increases lift until about 26 degrees, then lift falls.

For lifting surfaces, not sure, haven't worked those out yet (they're new).

[RexKramer]

Lift to drag ratio is dependent on airspeed.

A simple way to estimate this ratio is go flying. Reduce thrust to zero, and maintain a constant speed.

Horizontal distance traveled divided by vertical altitude lost is your lift to drag ratio for the airspeed you maintained.

A complication to this method of finding L/D is the fact that in KSP infiniglide is going to skew your results.

[JtPB]

so is there an add-on to calculate this?

what about the velocity that the plane could take off of the runway at?

[KerikBalm]

Unless using NEAR/FAR, for a given AoA L/D varies with speed (and with FAR, it varies with speed as well when you get into higher mach numbers)

What you can do is simply find a glide slope where you maintain your speed (although, given how fast the atmosphere gets thicker, this can also be hard for steeper glideslopes), and then look at the angle that your flight vector is at on the navball (and take the tangent of that angle)

[JtPB]

Unless using NEAR/FAR, for a given AoA L/D varies with speed (and with FAR, it varies with speed as well when you get into higher mach numbers)

What you can do is simply find a glide slope where you maintain your speed (although, given how fast the atmosphere gets thicker, this can also be hard for steeper glideslopes), and then look at the angle that your flight vector is at on the navball (and take the tangent of that angle)

at the end you want the angle of the flight anyway, so i dont have to do the tangent, do i? or is it useful for any more things?

so what you all saying is that i can check that only through experiment and trial-and-error procedure? isnt there any mod to simulate that(except FAR)?

[mhoram]

Have a look at this Post of the "How to calculate lift?"-Thread where L/D ratios are given:

http://forum.kerbalspaceprogram.com/threads/29788-How-to-calculate-lift?p=606231&viewfull=1#post606231

It might be however that the numbers are no longer valid since the time of writing.

[numerobis]

so is there an add-on to calculate this?

what about the velocity that the plane could take off of the runway at?

This python module can help you calculate the lift and drag of a given airfoil.

This python script shows how to calculate building a jet that can get to Laythe and back. I don't actually really remember what's going on in there exactly, but it includes reportLiftoffSpeed which is probably related to what you need.

There's also the plane designer module, but I forgot to give any examples of calling it.