Big-S strake has more drag than Type A Wing?

[zolotiyeruki]

My understanding of KSP stock aero is that wings don't have parasitic drag, only induced drag. But a while back, I was doing some tests for the Circumnavigation Challenge, and found that a craft of the same mass was less efficient/slower using a pair of Big-S strakes vs the Wing Type A. This makes me think that maybe the strakes have some sort of parasitic drag values.

Does anyone know if this is the case?

Edited October 30, 2015 by zolotiyeruki

[Alshain]

Who told you it didn't have parasitic drag? That is ridiculous, it most certainly does. It doesn't have all types of parasitic drag but it most certainly has form drag. I don't think it has Interference and I doubt it has skin friction.

[NathanKell]

It does have skin friction drag, by dint of drag being nonzero and proportional to wing area at 0 AoA.

[Alshain]

It does have skin friction drag, by dint of drag being nonzero and proportional to wing area at 0 AoA.

Really? That is surprising it goes that far, very cool. So does it have interference drag (i.e. places where air collides causing drag)?

[DancesWithSquirrels]

I don't believe interference drag is part of the aero model - nor are deflection effects or blanking (losing control authority because the wing downdraft blocks the elevator at high AoA - very very unpleasant, I will have to say from personal experience). Air movement and its effects are not dealt with.

[zolotiyeruki]

Who told you it didn't have parasitic drag? That is ridiculous, it most certainly does. It doesn't have all types of parasitic drag but it most certainly has form drag. I don't think it has Interference and I doubt it has skin friction.

From Claw's post on the subject, I was led to believe that wings don't have form drag or skin drag.

I guess my question boils down to "does the BigS strake have drag that the Wing Type A doesn't?"

[Alshain]

From Claw's post on the subject, I was led to believe that wings don't have form drag or skin drag.

I'm not an aeronautic engineer and that is a lot of math far above my head but I didn't see that same implication. That discussion is only about induced drag, but that is just the scope of the discussion, it doesn't mean form drag doesn't exist. Form drag is created from drag cubes that are calculated based on the shape of the part and in fact whole craft. This is separate from the lift drag factors defined in the wing configuration and applies to all parts, not just wings.

I guess my question boils down to "does the BigS strake have drag that the Wing Type A doesn't?"

You are talking about two wings pieces that are different in size, shape and lift rating, so of course their drag would be different, even if they are not on the leading edge of the wing, as Nathan said there is skin drag, the bigger piece is going to have more skin drag. For that reason I think the BigS Strake would have drag the Wing Type A does not and also the Wing Type A would have drag the BigS Strake does not.

Edited October 29, 2015 by Alshain

[zolotiyeruki]

I'm not an aeronautic engineer and that is a lot of math far above my head but I didn't see that same implication. That discussion is only about induced drag, but that is just the scope of the discussion, it doesn't mean form drag doesn't exist. Form drag is created from drag cubes that are calculated based on the shape of the part and in fact whole craft. This is separate from the lift drag factors defined in the wing configuration and applies to all parts, not just wings.

You are talking about two wings pieces that are different in size, shape and lift rating, so of course their drag would be different, even if they are not on the leading edge of the wing, as Nathan said there is skin drag, the bigger piece is going to have more skin drag.

If that is so, then this test should have had much different outcomes for the two craft.

Actually, the BigS Strake and the Wing Type A have the same lift rating, and I think they have the same shape. The BigS strake *is* thicker, though. The fact that I saw a difference in performance between the two leads me to believe that the BigS Strake has some aero forces being applied to it that aren't being applied to the wing.

[DancesWithSquirrels]

They are definitely different. From their .cfg files the strake has twice the induced drag:

Structural Wing A:

{
        name = ModuleLiftingSurface
        useInternalDragModel = True
        deflectionLiftCoeff = 1
        [COLOR=#ff0000][B]dragAtMaxAoA = 0.3[/B][/COLOR]
        dragAtMinAoA = 0.0
    }

Big S Strake:

{
        name = ModuleLiftingSurface
        useInternalDragModel = True
        deflectionLiftCoeff = 1.0        // 3.57m^2
        [COLOR=#ff0000][B]dragAtMaxAoA = 0.6[/B][/COLOR]
        dragAtMinAoA = 0.0
    }

Given that the strake is (presumably) thicker to accomodate the fuel tank, this is probably reasonable.

Edited October 29, 2015 by DancesWithSquirrels

[zolotiyeruki]

I'm actually aware of that, but I seem to recall reading somewhere that dragAtMaxAoA was only used in the old aero model. I'll see if I can find the documentation for that.

EDIT: Found it--the answer is actually part of that same Claw post I linked earlier:

The dragAtMaxAOA and dragAtMinAoA is unused in 1.0.X KSP aero.

EDIT 2: Both parts have "dragModelType = none" in their cfg files, which tells me the whole drag cube model isn't applied to them, if Claw is to be believed:

"dragModelType = none" turns off the drag cubes for wings, whereas other parts specify "dragModelType = cube"

Edited October 29, 2015 by zolotiyeruki

[NathanKell]

Correct. If wing pieces are in the same exact orientation (i.e. both are 'flat' to each other) and have the same lift rating, they will behave identically in terms of lift and drag, no matter their visual geometry or location on the craft.

[zolotiyeruki]

I may have to run the vertical climb test tonight to see if I can figure out why (or even if--I might have done something wrong in my earlier test) they're different. If they're the same, then I might fill those strakes with fuel and try to make it 10x around Kerbin...

[GoSlash27]

zolotiyeruki,

I'm not sure which "wing type A" you're referring to; structural, swept, or connector. 2 wings with equal lift ratings will always exhibit the same drag at any given AoA, altitude, and airspeed in KSP.

Trust NathanKell on this; he literally "wrote the book".

Best,

-Slashy

[zolotiyeruki]

I just tested it out with a test rocket (capsule, SRB, 8 wings in radial symmetry), and can confirm that my previous tests were incorrect. The BigS Strake and the Structural Wing Type A have the exact same drag.

...which also appears to be non-zero. I removed the wings and added a fuel tank, and only partially filled the tank in order to get the same mass at launch. Where the winged rockets got to 22.36km, the no-wing rocket of the same mass got up to 28.38 km.

So that tests the 0 degree angle of attack. But what about drag at a non-zero AoA? I took a Mk1 pod w/ parachute, a flea with the thrust limited to 1.6TWR, and two sets of 4 wings, one set twisted 5 degrees one direction, the other 5 degrees in the other. And they perform the same as each other there, too.

As far as I can tell, the two wings are, as far as the game physics are concerned, identical. With the small exception that the strake's moment arm will be a lot smaller than the arm of the Structural Wing Type A.

[GoSlash27]

zolotiyeruki,

The easiest way to quantify wing drag is to take a peek at the physics.cfg file.

Drag as a function of the sine of the AoA:

AoA: Drag

0 (0°): 0.01

0.3420201 (20°): 0.06

0.5 (30°): 0.24

0.7071068 (45°): 1.7

1 (90°): 2.4

drag by Mach

Mach: Drag

0: 0.35

0.15: 0.125

0.9: 0.275

1.1: 0.75

1.4: 0.4

1.6: 0.35

2: 0.3

5: 0.22

25: 0.3

Best,

-Slashy

[cantab]

Regarding the original issue, the overall mass may have been the same, but were the Centre of Mass and Centre of Lift in the same place? If they weren't then the two aircraft would require different amounts of elevator deflection to maintain level flight at a given speed and altitude and that may account for the different performance.

[zolotiyeruki]

Regarding the original issue, the overall mass may have been the same, but were the Centre of Mass and Centre of Lift in the same place? If they weren't then the two aircraft would require different amounts of elevator deflection to maintain level flight at a given speed and altitude and that may account for the different performance.

Yeah, I'm really wondering what I did different between the two craft now. I now have a new version of my 8x circumnavigator that will go around 9x, and maybe 10x.