A possible method of reducing drag

[peter anson]

Some time ago I thought of a possible way of cutting down on drag by filling in the control surface gap opposite the hinge - see the diagram below.

The idea was to use a strip of plastic which would cover the gap, but being flexible, would bend to allow control surface movement. Having thought about it, I had no idea how to incorporate the plastic strip. I imagined fitting it between the skin and the rear spar, but that would reduce the strength of the riveted joint. Recently I have seen the same idea used on a Bristell. The Bristell is a pretty sleek all aluminium 2-seater low wing aircraft built in the Czech Republic. Like the Sonex, its control surfaces are hinged at the top skin. The plastic covering strip appears to be simply glued to the painted lower skin.

So here's my question. Can anyone see any drawbacks to incorporating a cover strip?
I can think of 2 problems:
1. There is obviously a danger of the strip becoming disbonded.
2. If used on the ailerons it might create greater adverse yaw in a turn. It might be that with the standard set-up, the widening lower gap on the UP aileron side creates a bit more drag, thus matching the increased drag of the DOWN aileron. The standard Sonex doesn't exhibit much adverse yaw in a turn - one of its nice handling characteristics.
Peter
Sonex 894

[Bryan Cotton]

Peter,
My Hummelbird plans call for similar gap seals but made of thin aluminum. When I got to that point I was going to execute in plastic. Many gliders I've flown have them. The issue I can see when doing this on a Sonex is the rivets to the aft spar. It would be better if you had solid flush rivets back there. Regarding disbonding, yes it can be bad and I've seen it happen. Some cheater rivets might help.

Should you try it I know you will get good before and after data. It would be interesting to know if there was a quantifiable benefit.

I would not rivet the plastic between the skin and spar. Every plastic gap seal will need to be replaced someday. They crack when old.

[Bryan Cotton]

Here is the stuff:
http://wingsandwheels.com/tapes-seals/m ... -seal.html

[NWade]

Here is the stuff:
http://wingsandwheels.com/tapes-seals/m ... -seal.html

Yep, that's the stuff we use on gliders! BTW, acquaintances of mine run Wings & Wheels - good people. I myself have thought about this for my Sonex, but I have a concern (which I'll detail at the end).

Mylar will last a really really long time before any kind of cracking occurs. The more-likely item to wear out is the tape you use to attach it - once the top tape starts peeling up you'll need to strip it all off and replace it.

The best way to attach Mylar is to use a 3-layer method: "Transfer Tape" (special double-sided tape), the Mylar itself, and then PVC anti-peel tape over the top. Then on the control surface where the end of the mylar will rub as you move the controls, you want to put down a layer of Teflon tape - that'll keep the rubbing from eating into your control surface over time with dust & grit; and it'll make sure the wiping action of the mylar doesn't add to the control circuit friction.

There are some diagrams and articles on this page at W&W: http://wingsandwheels.com/tapes-seals.html
And Craggy Aero (who sells Avionics as well as glider supplies) also has an article on mylar application, detailing the steps: http://www.craggyaero.com/User%20Guides ... 0Seals.pdf

The biggest issue is the width of the gap - the Sonex control surfaces are fairly thick compared to gliders, so the distance that the "unhinged" side moves is quite large, when moving the controls through their entire range. I don't know if there is curved mylar out there that's wide enough to span the entire gap (and also rigid enough to hold shape against the air pressure).

Take care,

--Noel
Sonex #1339

[SonexN76ET]

I had considered gap seals when I was building. If others have good luck with them I would be tempted to try them as well. The rudder could use them as well. One guy posted on here a couple or few years ago about trying gap seals and he said he got some vibrations and worried about flutter so he removed them. It could have been his implementation of the gap seals that caused the trouble for him. I will look forward to hearing how others fare with the gap seals. I understand race planes use them as well.

Thanks,

Jake

[peter anson]

Thanks to all for the information. There has obviously been some research in this area, some of it available by wading through many pages of research reports and some available by spending lots of money to buy copies of research reports and then wading through many pages. I was skimming (and not spending money), but if my understanding was correct there should be a couple of percent drop in parasitic drag but surprisingly accompanied by a small reduction in lift coefficient and consequently a slight increase in stall speed. It's all a bit mind boggling.

Because of the wide gap with the Sonex, the Hummelbird idea of using thin Al for the gap seal might be the best option. 2024-T3 is available down to 0.016" which should flex pretty nicely. Not something I will be rushing into though.

Peter

[Bryan Cotton]

Peter,
The Hummelbird plans suggest going to a print shop as their process uses thinner flexible aluminum, I believe less than .010. I wonder if they have those or not anymore.

[gammaxy]

My understanding is that the main benefit of control surface gap seals is to prevent air from leaking from the high pressure side to the low pressure side, reducing the effectiveness of the surface.

The Sonex control surfaces have full-length piano hinges that already block most of this flow.

The marketing anecdotes I've read mention 2-3 mph speed increases and improved roll rates, but all seem to be on aircraft with different hinge designs that don't already block this flow. If this is true, I'd expect any improvement on the Sonex to then be much smaller.

I wonder if applying it to one aileron and paying attention to any change in yaw trim would be a convincing way of measuring the effect.

[sonexsteve]

At the risk of being the 'bummer in the kitchen at the party' on this topic I urge considerable caution trying this, as far as I know there have been no flight tests carried out of any similar configuration, and what is actually a fairly major aerodynamic modification could result in some very different flight characteristics.

I would suggest starting with just the ailerons, try hard to ensure both sides are identical, then fly on a very benign day, preferably with someone at the controls trained and experienced in test flying the untried. If flight characteristics in normal flight feel ok, feel out the low airspeed range in increments of say 5 knots lower each time, assessing lateral control and stability at each test point.

Throughout the exercise mentally prepare for spin recovery, especially the potential difficulty determining which way the aircraft is spinning once it all suddenly starts gyrating unexpectedly. Have in the front of your brain 2 different spin recovery actions, say 'normal - release controls, close throttle, look for spin direction, full opposite rudder, pause, stick forward, recover' and 'release controls, pause for a count of 5, pump throttle if no recovery'.

If that all goes well, and only in SMOOTH, non turbulent air, have a careful and incrimental look at high speed, being extremely aware of any buzzing or unusual vibration through the airframe, your butt, or the controls. If you get any vibration, gently close the throttle whilst slowly and gently reducing speed by pulling back on the stick. Get the aircraft back under smooth controlled flight, land, go and have a scotch/beer/coke to celebrate surviving flutter. I doubt you'll get any, in view of a demonstrated Vd of 230mph, but you're test flying a different beast to a Sonex as designed and test flown. If you get to Vne with everything remaining smooth gently assess stability and control, primarily in roll, but have a brief look at yaw and pitch, being careful not to exceed Vne.

Flutter is very nasty, it can literally shred a control surface, or if it sets off a sympathetic airframe vibration, a whole aircraft, in very few seconds, so smooth but quickly initiated recovery necessary. Smooth air is required as a gust (aka turbulence) can set off flutter.

If the shrouds are tacked on temporarily, one may detach leaving the other effective - this could result in very different flight characteristics than usual at any or all points of the envelope.

All good so far? Best way to progress would be to remove the aileron shrouds, apply similar to the rudder (alone), and repeat the flight test series for the rudder.

Then remove the rudder shroud, install elevator shrouds. This will be the acid test, with the highest likelihood of flutter on the elevators - they have large area, and no mass balance unlike the ailerons. Stall characteristics may be very different due to the changed airflow over your primary control, so both high and low speed areas should be approached very carefully.

All good? Great, now reinstall the aileron and rudder shrouds, test fly them together. If ok, install the elevator shrouds and finally prove it all works well together with no untoward stall or high speed characteristics.

This missive, and the described flight test program, sound, look, and are lengthy - but that is your insurance policy, mitigating risk by incremental steps, leaving you with an 'out' if things go pear shaped.

Here in the UK doing such a modification would require applying for a mod authorisation, and subsequent professional flight test if the request were approved. That may well be over the top like so much else here, but the seriousness with which the LAA would approach the issue illuminates the potential risks.

As always, just my 1 pence worth, your mileage/kilometerage may, and probably will, differ.

Happy to expand further (!) if you wish, PM me if you want to do so.

Steve Moody, Sonex 1383, Leamington Spa, England.

[airscribe]

Gap seals work....and they work on all sorts of aircraft...Gap seals are on a number of LSA aircraft with hinges like those on the Sonex/Waiex flaps and ailerons...and remember, the hinges don't actually "seal" the problem area -- that gap between the control surface and the wing.

That gap creates an eddy that contributes to drag....sealing it can help -- but as pointed out here, the help will be small. Installed flap and aileron gap seals on a Comanche I used to own; totally different hinging, but they helped -- particularly in slow-speed roll response.

The Sonex/Waiex flaps aren't really problematic, since the hing is on the bottom. The gap seal over that hinge will do most of what you want when flying with flaps stowed. But the ailerons, elevators/ruddervators could all benefit.

Plan to fabricate aluminum strips for my Waiex -- ailerons only, at first, with input from an aircraft engineer on how to mount them. If we can make that work as expected, we'll tackle the ruddervators...IF that works, too, may try to fabricate wing-root fairings, which can also reduce drag...

One step at a time...

Dave