Aeroynamic effect of having spoilers that "don't quite" close

[Andy W]

Assembling an ARF sailplane for review, the design of the spoiler causes it's edges to stick up above the surface of the wing - even though the servo is capable of pulling it close in the middle, the extreme edges stick up about 1/16th inch.

From previous experience, I've found this has little effect (no noticible increase in sink rate) on my sailplanes. Could it be that there is no critical airflow that close to the upper surface of the wing to cause any noticible amount of drag or disturbance of lift?

Expert opinions solicited.. :)
..a

[Andy W]

Pic of the "offending" area.

Although you can push them down a little more by hand, the servo - pulling in the middle of the spoiler - only pulls them closed as shown.
..a

[Ollie]

Ollie said,
> I've wondered for a long time about appropriate tolerances for
> the construction and maintenance of spoilers. Is there any
> information available that relates the size of hinge bumps, edge
> gaps and closed position errors to performance degradation or
> changes in stall characteristics? I would like to be able to form
> some judgements about how much effort to put into spoiler
> construction and maintenance.

Dr. Drela said:

"A hinge bump or edge gap will cause additional drag in several ways:

1) It might trip the flow early. For this not to happen, the glitch must be considerably smaller than the local boundary layer thickness. This is not much of an issue on the AL spoiler, where the flow is either turbulent, or about to undergo transition anyway.

2) The step will cause a thickening of an already-turbulent boundary layer. But if the height is less than 10-50% of the BL thickness, then you will see little effect. Where in the 10-50% range you can be depends on...

3) The local velocity just above the step relative to the freestream. The lower this velocity, the larger the step can be without causing significant losses. The absolute worst place for a step, or any proturberance, is on the front 1/3 of the airfoil top surface, while a step near the TE is relatively benign. This is why my AL's front wing bolt head is buried, and the rear one isn't.


The AL's spoiler TE is at 65% chord, where the flow is turbulent (at high CL) or about to undergo transition (at low CL), so tripping is not much of an issue. At a mid-range CL, the local boundary layer at 65% chord is about 0.4" thick. If the spoiler TE is lifted off the airfoil less than about 0.06", I won't worry about it. If the spoiler was mounted off the spar, I would worry about a 0.01" glitch.

- Mark"

[Al M]

I have a somewhat modified Spectra to which I added spoilers but aft of the location of the Spirit unfortunately. The hinge is at 59% and the te is at 73%. they open about 60 degrees and I couldn't notice any effect. I took off once and when I tried to deploy them on landing I was surprised to find them open. That flight lasted about 45 minutes. A piece of 1/32 ply screwed to the top of the spoilers extending to 90% does the job. It does not fit perfectly but I can't tell the difference. They do work now. Don't worry about them especially if they are hinged any distance behind the airfoil high point.

[Al M]

I would hate the looks though.