Airbrakes should't affect drag or lift when not deployed.

[Galane]

I put four airbrakes on a lander on top of a rocket to test to see if when deployed they'd increase drag for landing on Kerbin. Whyfor? To see if that would reduce the fuel required to land.

The problem is that the airbrakes cause a huge shift in center of lift, requiring enough fins on the bottom end to outfit a fleet of 1959 Cadillacs. Consider airbrakes on real aircraft. They're made to be flush with the surrounding surface so they don't cause drag when retracted.

Can the CFG for that part be edited to make them produce zero lift all the time and zero drag unless deployed? It would be nice to see this fixed as a hotfix not requiring a full re-download.

[Markarian421]

I want to second this, I ran into the same problem, both trying to use them to slow down a lander and just trying to use them normally on aircraft. They have unexpected effects even when control functions are shut off and they aren't deployed.

I was tweaking the config files last night to see what I could figure out, and it appears the problem is that airbrakes are never completely off, they are always at least slightly deployed -- the curve must not go to zero. (The curve unfortunately isn't in the config file.) You can see this by adjusting the parameter:

deflectionLiftCoeff = 0.38

From the variable name one would guess this is only applied when the airbrake is deployed, but it significantly changes the drag when it's not deployed as well.

[sal_vager]

Every part has drag and will generate lift if at an angle to the airflow, wings generate lift even when at a zero angle to airflow, you can edit them if you like but this is by design

[Markarian421]

My point is that the variables that determine drag for a standard part aren't the apparent problem, it's the variable that controls what happens when the airbrake is deployed that's the problem when it's not deployed -- that seems unlikely to be by design. The effect on a craft is far out of proportion from what you'd expect.

- - - Updated - - -

Ah, I see what you mean though -- if the implementation was done as if this was a wing, it might not go to zero at zero deflection. I think though it's not working as intended for this part. (If it was, in the real world, no aircraft would ever have airbrakes.)

Edited May 13, 2015 by Markarian421
clarification

[sal_vager]

There is a drag coefficient value for airbrakes but I think setting it to zero removes all their drag, and there's no per part drag curve, you can see the drag curve in the Alt+F12 debug screen under the physics/dragprofile tab.

Drag for the airbrake should be as low as possible when closed, but as KSP doesn't actually simulate air itself any airbrakes that are at an angle to the velocity vector will have drag, same as any other part.

[Brotoro]

When airbrakes are folded against the ship, they should only have minimal drag of the type body parts have. Squad should fix that.

Note that it is important to right-click the airbrakes in the VAB to disable the pitch and yaw control functions of the airbrakes (leaving only the braking function active), otherwise they WILL pop out a little during flight in an attempt to control pitch and yaw, but just create too much drag.

Edited May 13, 2015 by Brotoro

[Markarian421]

I'll have to take another look . . . probably not tonight. But following what you're saying, I think I did notice the problem is even worse if you mount an airbrake on a surface that's at an angle to the direction of flight. If an airbrake is flush with the surface it's mounted on, no matter what the angle of that surface, it should not add any significant drag. But it may be that the game is only looking at angle relative to flight as opposed to angle of deployment. (If so, that's a bug.) Even if that is the case, I was seeing problems with airbrakes mounted at no angle to airflow, they still act partially deployed. Maybe the angle isn't being calculated correctly? That or it doesn't go to zero drag at zero angle.

The reason I mentioned the deflectionLiftCoeff is that in playing around with pretty much every parameter in the config file, that appeared to be the one related to the problem. Even zeroing out other parameters did not fix it, but increasing this parameter makes it much worse.

[sal_vager]

If airbrakes are made to provide no drag when closed, you would have a situation where they provide no drag even when visually perpendicular to the airflow, just because they are closed.

As you can see they are not perfectly flush with a surface when closed, so some drag can be expected, that more drag occurs when they are at an angle to the airflow is a product of the aero engine, there is no occlusion of radial parts from drag, only heat.

Just how significant is this drag you are experiencing? As long as it's low it shouldn't matter that it is non-zero.

They are working correctly in the manner they have been coded to function, so I don't see how this can be a bug, feedback maybe.

[Markarian421]

If airbrakes are made to provide no drag when closed, you would have a situation where they provide no drag even when visually perpendicular to the airflow, just because they are closed.

Yes, that is how they should work as long as they are flush with the surface. If you fly a brick wall, and put air brakes on the face of the brick wall, they should not add significant additional drag.

As you can see they are not perfectly flush with a surface when closed, so some drag can be expected, that more drag occurs when they are at an angle to the airflow is a product of the aero engine, there is no occlusion of radial parts from drag, only heat.

I don't think it would be possible to make them perfectly flush in the game as they are in the real world. But that doesn't mean they shouldn't try to behave like their real world counterpart.

Just how significant is this drag you are experiencing? As long as it's low it shouldn't matter that it is non-zero.

Enough that I don't consider air brakes as implemented usable. But I think you're missing the point that it appears the problem isn't tied to normal stationary object drag, it's related to the variable drag air brakes are supposed to produce. Those appear to be different parameters.

They are working correctly in the manner they have been coded to function, so I don't see how this can be a bug, feedback maybe.

IRL I'm an R&D physicist working on scientific instrumentation, and I can't tell you how many times I've had a discussion like that with someone in the software group. "Well yes when it comes to using the instrument this code breaks everything, but it works the way we coded it so it's not a bug." So you just made my day! (Luckily their manager is also a physicist who also considers "doesn't do what it should" a bug.)

[Galane]

The way the airbrakes work, they're unusable as drag adders for a lander because huge fins have to be added to the tail of a rocket to shift the center of lift back. When closed and direction control disabled, the airbrakes are being treated as if they are fixed fins instead of having no lift or drag effect.

How do airbrakes work in parts mods that have them?

I was looking forward to having these as an alternative to parachutes. Dissapointing.

[Keith_Bones]

Having done some very basic testing, I am guessing that something non-standard is going on with air brakes, and the drag they generate. They seem to create an otherwise unreasonably high amount of drag when deployed, more so than a part of similar proportion in a similar position.

But that being said, I'm quite fond of an "air brake umbrella" atmo reentry design I've played with a number of times, with closed airbrakes forming the leading nosecone on takeoff, and have never experienced these issues.

Maybe some screenshots, or craft files so we can see the issue you guys are having?

[Claw]

So, I think there's a fundamental misunderstanding about how the aero surface works.

1) It does not produce lift. Period.

2) It produces drag based on the local AoA of the surface relative to the oncoming wind, regardless of deployed state or what part it's attached to. It's drag production is similar to a lifting surface, vice a drag cube. So yes, it will have higher drag than an equivalently sized flat plate, because it's using drag curves similar to an aero surface.

For the aero surfaces, the curves that govern lift and drag can be found in the physics.cfg file, which I've posted below:

	LIFTING_SURFACE
	{
		name = SpeedBrake
		lift
		{
			key = 0 0 0 0
		}
		liftMach
		{
			key = 0 0 0 0
		}
		drag
		{
			key = 0 0.01 0 0
			key = 0.3420201 0.1 0.1750731 0.1750731
			key = 0.5 0.4 4.557837 4.557837
			key = 0.7071068 2.828427 4 4
			key = 1 4 4 0
		}
		dragMach
		{
			key = 0 0.25 0 -0.8463008
			key = 0.15 0.125 0 0
			key = 0.9 0.375 0.7227947 0.7227947
			key = 1.1 1 0 0
			key = 1.4 0.65 -1.29191 -1.29191
			key = 1.6 0.5 -0.4376471 -0.4376471
			key = 2 0.42 -0.1475873 -0.1475873
			key = 5 0.275 0 0
			key = 25 0.4 0.0006807274 0
		}
	}

So if it's not producing lift, why does it cause stability problems? Because it produces drag like a wing. Which means it changes considerably based on the AoA which outstrips similarly sized parts.

I could build a fix for this in my Stock Bug Fixes, if there's enough interest.

Cheers,

~Claw

Edited May 14, 2015 by Claw

[Galane]

I could build a fix for this in my Stock Bug Fixes, if there's enough interest.

Cheers,

~Claw

YES! I want to be able to put a bunch of airbrakes up on the nose of a rocket and have them *not* drag the nose around to the rear when they're not deployed. In other words, airbrakes that work like airbrakes, not combination fin/spoileron/airbrake thinguses. So long as when deployed they will make large amounts of drag, like a similarly sized wing panel sticking out at the same angle.

Alan Aerospace Recycling and Packaging only very reluctantly uses parachutes. Properly functioning airbrakes should help the company continue not relying on flimsy cloth.

Another drag issue some have noted is that the "lego" wing pieces don't drag shield each other edge to edge so building a large wing from them produces far more drag than a single part wing of the same size.

[Claw]

YES!

...

Another drag issue some have noted is that the "lego" wing pieces don't drag shield each other edge to edge so building a large wing from them produces far more drag than a single part wing of the same size.

This request is now included in the StockPlus portion of my Stock Bug Fixes (link in my sig). When stowed (or stowing), they generate zero drag. It's in the ModuleAeroSurface. Although I've received a bug report for ModuleControlSurface, so expect another update to come down later today.

Also, the drag for equivalent lift rating should be the same regardless of how many wing parts it is made out of. Wing drag is generated based on lift rating and not part shape, so there is no need to occlude edges.

Cheers,

-Claw

[Markarian421]

Thanks!