low reynolds number aerobatics

[paulriseborough]

Something I've noticed with a few of my slow-fly/indoor aerobatic models is the significant reduction in aileron effectiveness at low speeds. Are any of you aware of any reasearch/tests that have been done to determine effective means of re-energising the boundary layer of symmetrical wing sections operating at low <60000 Reynolds numbers?

I have experimented with thinning the wing section, but the stall was abrupt and Cl max reduced. Larger chord ailerons also seem to help, but the addition of turbulator strips seemed to have unpredictable effects (good on some days and bad on others)

My next wing will have the leading edge shape held by stringers rather than 1/32 sheet.

I suspect that the optimum position and size of boundary layer energisers is different with an aileron wing because of the abrupt change in camber at the aileron hinge location with a deflected aileron.

Has anyone done any work looking at the different types of symmetrical wing sections, ie where is the best chordwise location for maximum thickness for best lift and is this in conflict with the best location for effective aileron operation?

Lots of questions ........ oh for some flow visualisation.

Regards,

Paul

[Dave Robelen]

Hi Paul,
This is not a direct answer to your question, but another solution that I have found very effective. A relatively high taper ratio on a moderately low aspect ratio wing reduces the roll damping and the force necessary to generate an acceptable and reasonably linear roll rate. Take a look at my Flap Jack under the photo section. The airfoil is a 15% semisymetrical with the 3/32" sheet ailerons hinged on the back edge. This of course reduces the effective thickness in percentage of the total chord. It is my percepton that using a low aspect ratio constant chord wing is the source of most of the difficulty. Those wide tips and the associated vortices take a lot of energy to roll, and the ailerons are very eager to stall locally when large deflections are used.
The Flap Jack has a maximum roll rate exceeding 720 deg/sec. Another helpful device is the winglet which aids in trapping the flow over the end of the aileron.
Regards, Dave Robelen

[paulriseborough]

Dave,

What aspect ratio and taper ratio does Flapjack use?

Thanks,

Paul

[Dave Robelen]

Hi,
Iam too lazy to crank the numbers. The span is 25", root chord 8", and tip chord 5.5". Those chord numbers include the ailerons.
Regards, Dave Robelen