Lift model

[Tejing]

I've looked and cannot seem to find a precise mathematical model for how lifting surfaces currently work in KSP. There're a lot of qualitative descriptions regarding what's wrong with the model, but what IS it exactly? I'm trying to work back from specific feasibilty goals to the actual attributes of modded parts I'm making, and this gap is becoming problematic. It would also be nice to be able to figure out the expected glide behavior of my spaceplanes when out of fuel without actually flying them. Can anyone enlighten me on this? (Preferably including the specific meaning of the numbers in the part cfg files)

Edited March 16, 2014 by Tejing
Answered

[Hodo]

Thats because there really isn't one.

Unless you are running FAR the aerodynamic model in KSP is pretty much based on the mass of the craft.

[Claw]

I've looked and cannot seem to find a precise mathematical model for how lifting surfaces currently work in KSP. There're a lot of qualitative descriptions regarding what's wrong with the model, but what IS it exactly? I'm trying to work back from specific feasibilty goals to the actual attributes of modded parts I'm making, and this gap is becoming problematic. It would also be nice to be able to figure out the expected glide behavior of my spaceplanes when out of fuel without actually flying them. Can anyone enlighten me on this? (Preferably including the specific meaning of the numbers in the part cfg files)

Well sure. KSP stock aero has some problems, but the lift equations aren't based on craft mass.

To answer your question:

Lift equation for Control Surfaces: sin(AoA)*pressure*speed*lift rating

Lift equation for Wings: sin(AoA)*(1-|sin(AoA)|)*cos(AoA)*pressure*speed*lift rating

As you can see, the lift equations for wings and control surfaces is different. If you look at the math, there's a bug with the control surface equation in that the lift continues to increase all the way to 90 degrees.

The stock drag equation is a weighted sum of the drag coefficients. I don't have that equation off hand. However, because it's based on mass and drag coefficients, you can get away with creating craft that would be extremely draggy in the "real world."

As I understand it, FAR does some modification to Kerbin's atmosphere and fixes the drag model which helps if you want more "realism". Although nothing stops you from making "realistically" smooth aircraft in stock. I don't know how FAR effects the lift equations, but I'm sure it modifies them.

Use FAR if you'd like, use stock if you'd like. Stock has some bugs, but it's still based on a set of rules.

Edited March 15, 2014 by Claw
Clarification

[Hodo]

FAR isn't really a randomly generated model either. But it is far more accurate than the stock model. (No pun intended)

[Claw]

FAR isn't really a randomly generated model either. But it is far more accurate than the stock model. (No pun intended)

Oh sorry. I see how my comment could be read that way. I wasn't trying to imply that FAR is randomly generated. Just that stock, while buggy, still has rules.

[Starwaster]

As I understand it, FAR does some modification to Kerbin's atmosphere and fixes the drag model which helps if you want more "realism". Although nothing stops you from making "realistically" smooth aircraft in stock. I don't know how FAR effects the lift equations, but I'm sure it modifies them.

Use FAR if you'd like, use stock if you'd like. Stock has some bugs, but it's still based on a set of rules.

Far does not do anything to the atmosphere itself. It sets all part drag to zero and calculates and implements drag itself.

[KerikBalm]

So the mass of a part does not affect the lift generated?

I thought Lift/frag would be something worth considering... but thats not true?

The drag coeffcient is multiplied by mass, but the lift rating is not?

[Claw]

So the mass of a part does not affect the lift generated?

I thought Lift/frag would be something worth considering... but thats not true?

The drag coeffcient is multiplied by mass, but the lift rating is not?

Lift is not multiplied by mass if that's what you are asking. However, lift and drag are significant considerations when designing a craft.

In real life, a wing's ability to generate lift is not a function of mass. But mass will affect how much lift you need to generate and, henece, the amount of induced drag created.

Currently drag is a factor affected by mass. My presumption is that it was an easy way to assume a crossectional area early in development that hasn't been updated yet. But that is pure speculation.

Edited March 15, 2014 by Claw

[Red Iron Crown]

I think the drag model was a placeholder from when all we had were rocket parts. I don't think it was ever intended for aircraft or spaceplanes. It really does need updating now that aircraft/spaceplane parts are in the core game.

[KerikBalm]

I think the drag model was a placeholder from when all we had were rocket parts. I don't think it was ever intended for aircraft or spaceplanes. It really does need updating now that aircraft/spaceplane parts are in the core game.

Yes, it does need to be updated, as to the earlier comment, yes I know IRL lift is not a function of the mass of a wing. But then again... neither is the drag.

Also IRL, any given wing has a fixed Lift/drag ratio (for a given angle of attack).

I was operating under the assumption that the wings and control surfaces with the best lift rating/ drag coefficient were going to be the most efficient within atmospheres. I guess I was wrong, and I should be looking for the best lift rating/mass :/

I think that lift does not scale with the 2nd power of velocity, but drag does, can also lead to some stability issues with control surfaces that acheive active stability through varying their "lift"

What may be sufficient at low speeds won't work at high speeds (now this may sound familiar to some IRL cases, but thats only in a very vague sense, when you get down to the details, its very dissimilar)

[Hodo]

Someone should page Ferram4 for this answer, he would be able to explain it in the finest of details. He is probably the one person on the forums not an employee of SQUAD who knows the flight model better than anyone else.

[Tejing]

Thank you, Claw. This is exactly what I was looking for.