Paragon Chordwise Spoiler Location

[SoarNeck]

Hi all,

Just finishing up my new Paragon, and I'd like to add spoilers to the model. Tradition says to add the spoiler just behind the wing spar, but I noticed that models like the AL and Bubble Dancer have pushed the chordwise location of the spoiler rearward. Any advice as to what to do? I plan to seal the bay whereever it goes.

I built this model with a Bubble Dancer-type spar modified for the dimensions of the Paragon, so it should be a hoot. I even tried laminating layers of uni-c/f instead of using pre-cured strips, and I'm happy with the turnout so far. I just hope they hold up on the winch!

[Ollie]

The reason Dr. Drela puts the spoiler so far aft on his designs is that the boundary layer is thicker back there and the flow can better tolerate the little bumps associated with spoilers without the flow seperating.

Watch out for torsional flutter on vigorous winch launches. After strengthening the spar to bending loads, the next weakness to reveal itself will likely be lack of torsional stiffness in the open bay wing structure.

[the bone]

The reason Dr. Drela puts the spoiler so far aft on his designs is that the boundary layer is thicker back there and the flow can better tolerate the little bumps associated with spoilers without the flow seperating.

Watch out for torsional flutter on vigorous winch launches. After strengthening the spar to bending loads, the next weakness to reveal itself will likely be lack of torsional stiffness in the open bay wing structure.
Ollie:

Just read your post on spoilers on the Paragon. I'm getting ready to build one. Would you put the spoilers back towards the trailing edge or go with the plans? Also do you know of any good articles on spoilers useing one servo with strings?

Thank You The Bone

[Ollie]

From Dr. Drela to me:
> 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.

"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 added spoilers to a modified Spectra ( mainly a v tail ) with disappointing results. I started with the Spirit instl and moved it inboard one rib ( felt I could with v tail ). At the same time I moved the hinge point back from 36% chord ( spar ) to 58%. The spoiler is 14% chord and extends up about 60 degrees. I don't notice much difference with it extended. I flew most of a flight with them deployed and never noticed the difference. It climbed just fine.

[Sparky Paul]

I have spoilers on my Spirit wing at the kit location.
With 100% up, the plane wants to go vertical -down-...
It's quite happy at 50%.
Adding similar sized spoilers to other wings have shown the same response.
House of Balsa 2Tee with a vee-tail
The location just behind the spar is perfectly OK.

[Al M]

The spoilers on my 2M Kestrel are hinged on the spar and are very effective. At full opening the nose drops more than the elevator can handle. I guess if you move them back you should make them longer (spanwise) or wider or both. I am not sure which approach to take.

[SoCalGliderFlyr]

Pulling up with opening spoilers is not their correct use. In full size sailplanes spoilers are used during the landing glide to increase sink rate. Landing speed is faster than thermalling speed by ten to twenty percent. You don't use full spoilers during the flair out and wheel touch. You will use full spoilers to stop your roll out as the wheel brake is ususally on the full open side of the spoilers.

If you want slow landing speeds you need to have flaps.

[Al M]

They don't noticably kill lift. When opened the rate of decent doesn't seem to change. It thermals just fine with them deployed. It seems like the chordwise width should be increased when they are moved aft. You are correct about flaps but it is more of a mod than I wanted to do and the radio that I had at the time was limited in its capabilities. I am looking for a steep decent so I can land in a hole.

[Ollie]

You don't want to kill lift unless you want to dive. You want to increase the drag (force) a lot to steepen the glide. The lift (force) does not change but by air speed (unless you dive).

[SoCalGliderFlyr]

They don't noticably kill lift. When opened the rate of decent doesn't seem to change. It thermals just fine with them deployed. It seems like the chordwise width should be increased when they are moved aft. You are correct about flaps but it is more of a mod than I wanted to do and the radio that I had at the time was limited in its capabilities. I am looking for a steep decent so I can land in a hole.


If you want to land in a hole you need 20% to 25% of cord half span flaps at 90 degrees. Not spoilers.

[Al M]

What should I do to an existing plane with spoilers? The spoilers on my 2M Kestrel do get it whowed down and on the ground in a hurry. I find that full deployment is not advised as I don't have enough elevator to keep the nose up. Fortunately our ground is wet and soft enuf so the nose sticks in with no damage. I really do not want to attack anything but the spoilers on the Spectra.

[SoCalGliderFlyr]

Don't flair out for landing with the spoilers open.