Winglet lift/drag coefficients, and how wings work

[etmoonshade]

I'm quite fond of spaceplanes, and I'd love to make one - problem is, the SDK isn't of too much help when it comes to wings.

I've got a few questions for anyone in general, but probably one of the coders:

A: Where is the lifting force applied on a wing as far as KSP's code goes?

B: Does the 3D model affect the wing's performance at all, or is it calculated based on a static length? If the latter, is there a way to change the length?

I figure with these two answers, I could make a far better attempt at making or modifiying a wing.

[etmoonshade]

Reading over this, it's obvious I wasn't thinking well when writing this. Two clarifications:

For A:

Is the lifting force applied at the center of mass of the 3D model as calculated by the program, or is it applied at the origin of the model (i.e. 0, 0, 0?)

For B:

Area. Not length, area. Does the 3D model affect performance, or is the area static?

[Capt'n Skunky]

The aerodynamic physics in the game are very rudimentary currently. This will be improved in future builds. For now we just have to make do.

Cheers!

Skunky

[etmoonshade]

The aerodynamic physics in the game are very rudimentary currently. This will be improved in future builds. For now we just have to make do.

Cheers!

Skunky

Indeed, and I don't expect things to be terribly complex. Winglets -do- seem to generate lift of some description, though - all I'm wondering is how and where those forces are applied currently, since it could help in designing an effective wing.

[Jack]

Wings only add weight and drag at the moment.

[HarvesteR]

Winglets do add deflection lift force. It's a force perpendicular to the velocity direction, that is affected by the angle of attack of the winglet.

This lift force is applied to the center of mass of the winglet. The liftCoeff parameter determines how much force will actually be applied. The model itself has almost no influence over the winglet dynamics, except for the position of the center of mass.

The winglet's angle of attack will also influence how much drag is produced. The lifting force will also start to decline as the winglet enters a stall AoA.

Now, what winglets don't do yet is simulate airfoil lift. They are about as effective as a 2x4 as a source of lift. They are meant mostly for stabilization. Later the idea is to add a Wing module, that will actually simulate airfoil lift, and will have control surfaces as well.

Cheers

[etmoonshade]

Winglets do add deflection lift force. It's a force perpendicular to the velocity direction, that is affected by the angle of attack of the winglet.

This lift force is applied to the center of mass of the winglet. The liftCoeff parameter determines how much force will actually be applied. The model itself has almost no influence over the winglet dynamics, except for the position of the center of mass.

The winglet's angle of attack will also influence how much drag is produced. The lifting force will also start to decline as the winglet enters a stall AoA.

Now, what winglets don't do yet is simulate airfoil lift. They are about as effective as a 2x4 as a source of lift. They are meant mostly for stabilization. Later the idea is to add a Wing module, that will actually simulate airfoil lift, and will have control surfaces as well.

Cheers

Thank you for this - much appreciated. I can work with this.

[rdfox]

Now, what winglets don't do yet is simulate airfoil lift. They are about as effective as a 2x4 as a source of lift. They are meant mostly for stabilization. Later the idea is to add a Wing module, that will actually simulate airfoil lift, and will have control surfaces as well.

So for now, a winged spacecraft will glide about as well as the Space Shuttle? ;D

(Seriously, I remember an engineer stating, when Columbia was first delivered to the Cape in 1980, that the Shuttle had the glide ratio of 'a pair of pliers.')

[Capt'n Skunky]

So for now, a winged spacecraft will glide about as well as the Space Shuttle? ;D

(Seriously, I remember an engineer stating, when Columbia was first delivered to the Cape in 1980, that the Shuttle had the glide ratio of 'a pair of pliers.')

I believe they referred to it as a 'flying brick' as well.

Arrr!

Capt'n Skunky

[rdfox]

The experimental lifting bodies of the 60s and 70s were even worse, of course.

The pilots of THOSE claimed that if you dropped a cinderblock out of the bomb bay of the NB-52 mothership at the same time as you released the lifting body, the lifting body would reach the ground first...

[Capt'n Skunky]

The experimental lifting bodies of the 60s and 70s were even worse, of course.

The pilots of THOSE claimed that if you dropped a cinderblock out of the bomb bay of the NB-52 mothership at the same time as you released the lifting body, the lifting body would reach the ground first...

Well, they did give us one good thing... the Six-Million Dollar Man!

Arrr!

Capt'n Skunky

[DoctorEvo]

Winglets do add deflection lift force. It's a force perpendicular to the velocity direction, that is affected by the angle of attack of the winglet.

This lift force is applied to the center of mass of the winglet. The liftCoeff parameter determines how much force will actually be applied. The model itself has almost no influence over the winglet dynamics, except for the position of the center of mass.

The winglet's angle of attack will also influence how much drag is produced. The lifting force will also start to decline as the winglet enters a stall AoA.

Now, what winglets don't do yet is simulate airfoil lift. They are about as effective as a 2x4 as a source of lift. They are meant mostly for stabilization. Later the idea is to add a Wing module, that will actually simulate airfoil lift, and will have control surfaces as well.

Cheers

Besides an absence of pitching moment modeling, I fail to see what is missing from your description here that a true 'airfoil-lift' model would have. Airfoil lift coefficients vary with AoA (which you seem to have already done)... and they drop off suddenly at a critical AoA (which, according to your description, you have already done)... and drag also varies with AoA/lift (which you ALSO claim to have done)... Seems to me that - as far as your description goes - you've got your bases covered.

Now, what I've found on my own is that there are several (unmentioned) issues with the winglet's behavior:

-Pitching moments are not modeled properly, often doing the precise opposite of what they are supposed to

-Stall (or post-stall) model seems flawed; maximum lift can be achieved at 90 degrees AoA (where lift should be falling back to zero in a sinusodial fashion)

-Induced drag seems off. Sometimes it seems nonexistent, other times it seems to be much too severe. I suspect the configurable 'dragCoef' parameter has influence over this.

In any case, I understand that this is still in development and am very much looking forward to seeing the 'wing module' when it comes out. Hopefully the behavior will be properly nailed-down by then.

[Dragon029]

What they're saying is that if the wing is horizontal, it will produce very little lift, as it will only be in relation to the falling of the rocket.

In other words - if you want to fly horizontally, you need to have the rocket up at an angle right now, whereas with improved physics you would be able to have lift with only pure, horizontal thrust pushing the craft forward.

Also, wings (if they use the generic airfoil shape and not a symmetrical one) should only generate lift in one direction unless at negative AoA's exceeding like 5-10 degrees.

[rdfox]

Actually, as a note, you'll find that very few airplanes actually fly at a zero alpha, at least in their cruise configurations.

Jetliners, as a rule, are designed to fly about 3-5 degrees nose up in cruise, because it's a more fuel-efficient configuration (closer to maximum lift:drag ratio) than flying nose-level. Smaller aircraft, too, tend to have at least a slight nose-high attitude in cruise, because deflection lift is, honestly, the most important contribution to an airplane's lift; airfoil lift is not really enough to get off the ground by itself, no matter how much you've heard people throw Bernoulli's Principle around.

'Then why,' I hear you cry, 'don't I find myself climbing a hill to go to the lavatory on an airliner?' That's because the designers, knowing that passengers would find a cabin that's got a significant pitch attitude in cruise rather uncomfortable, will mount the wings so they're not actually parallel with the cabin floor. If an airplane is designed for a three degree nose-up attitude in cruise, they may design it with the cabin floor being pitched *down* 1.5 degrees in a wings-level configuration, both making it more comfortable for passengers and giving the pilot better visibility over the nose on approach. It's the same trick as some early rocket designers used to spin-stabilize their rockets by having the fins slightly canted off the vertical, essentially.

In short, deflection lift (the stuff you get when you stick your hand out a car window and 'fly' it, because the hand is a HORRIBLE airfoil) makes flight possible; airfoil lift makes it more efficient. Thus, don't expect to *ever* be able to fly a spaceplane at a 0-degree pitch attitude to maintain altitude. (Granted, it would be nice to be able to do so with less than a 30-degree pitch attitude, but hey.)

(I wonder if Peter Garrison ever did sketch out that light airplane with 2x12s as wings, like he always joked one could do with the amount of engine power now available...)

[DoctorEvo]

Actually, as a note, you'll find that very few airplanes actually fly at a zero alpha, at least in their cruise configurations.

'Zero' alpha is merely an arbitrarily-picked reference angle. If the engineers want it to be at the zero-lift axis, they'll put it there. But in the real world, you'll find this is quite often not the case, and they often WILL place 'zero' along the cruising AOA (for instance, this is the way I set up my Dynon primary flight display in my RV-6).

Jetliners, as a rule, are designed to fly about 3-5 degrees nose up in cruise, because it's a more fuel-efficient configuration (closer to maximum lift:drag ratio) than flying nose-level. Smaller aircraft, too, tend to have at least a slight nose-high attitude in cruise, because deflection lift is, honestly, the most important contribution to an airplane's lift; airfoil lift is not really enough to get off the ground by itself, no matter how much you've heard people throw Bernoulli's Principle around.

Well put.

But it's worth noting that the angle you're talking about NOW is another arbitrary reference-angle picked this time by the airfoil designer. It is most often the same as the chord line by convention, rather than the cruise attitude (which the airfoil designer couldn't possibly know unless it was given to him) or the zero-lift axis (which he WOULD know). In the case of symmetrical airfoils (like the ones most likely to be modeled in KSP), however, the zero-lift axis IS in fact the same as the chord line, which permits a number of simplifications.

'Then why,' I hear you cry, 'don't I find myself climbing a hill to go to the lavatory on an airliner?' That's because the designers, knowing that passengers would find a cabin that's got a significant pitch attitude in cruise rather uncomfortable, will mount the wings so they're not actually parallel with the cabin floor. If an airplane is designed for a three degree nose-up attitude in cruise, they may design it with the cabin floor being pitched *down* 1.5 degrees in a wings-level configuration, both making it more comfortable for passengers and giving the pilot better visibility over the nose on approach. It's the same trick as some early rocket designers used to spin-stabilize their rockets by having the fins slightly canted off the vertical, essentially.

Well, if you're going to go THAT in-depth, then you might as well tell them what this angle is called...

http://en.wikipedia.org/wiki/Angle_of_incidence

(I wonder if Peter Garrison ever did sketch out that light airplane with 2x12s as wings, like he always joked one could do with the amount of engine power now available...)

BRB, looking up wind-tunnel data on a 2x12 rectangular section @ Re = 1000000...

[molo]

You've got an RV? Nice. I'm half-owner of a two seat Grumman with a big motor.

[rdfox]

Pah! I used to be 1/470,000th owner of a big two-seat Grumman with TWO big engines!

And here's a photo of it!

Not that I ever got to use it personally, mind... ;D

[DoctorEvo]

You've got an RV? Nice. I'm half-owner of a two seat Grumman with a big motor.

Don't tell me... it's some kind of cat.

Not that I ever got to use it personally, mind... ;D

That happens all-too frequently with these fractional ownerships...

[molo]

Technically it's not a cat... the AA-1A was the 'Trainer', which is pretty lame compared to the rest of them! The 1C was the 'Lynx', though.

[DoctorEvo]

Technically it's not a cat... the AA-1A was the 'Trainer', which is pretty lame compared to the rest of them! The 1C was the 'Lynx', though.

Darn. I was hoping it was a Lynx or a T-cat.

Interesting airplane though... bonded metal wing... and who'da thunk to put fuel INSIDE the spar?

[molo]

The T-Cat was a posh version of the -1A, so we can go with that. It's basically a mini RV now with the modifications. Can't complain about 1500fpm climb fully loaded!

[DoctorEvo]

The T-Cat was a posh version of the -1A, so we can go with that. It's basically a mini RV now with the modifications. Can't complain about 1500fpm climb fully loaded!

Mini RV? Just how big do you think RVs are?

[molo]

Haha, my max takeoff is 1,579 lbs, and stock fuel capacity is 22 gallons... but yeah, they're essentially identical in all other respects.

[DoctorEvo]

Haha, my max takeoff is 1,579 lbs, and stock fuel capacity is 22 gallons... but yeah, they're essentially identical in all other respects.

Technically an RV-6 is only supposed to gross at 1,600 lbs... but it's experimental, so whatever you write down is legal.

But yeah, I've got more fuel capacity.