What design to prevent tip stalls?

[hul]

I'm building an aileron trainer for the kids. It'll be based on the Al Eastmans's Ampmaster, scaled down to about 39" span, AUW approx. 600-650grams (21-23oz), wing area 20dm2 (310sqin). Root chord will be 240mm, tip chord 160mm (up from 132mm on the original). I'd like to use an airfoil that gives reasonable inverted (i.e. not the original flat bottom).

What would be the best way to avoid tip stalls keeping in mind the requirement to fly inverted?
- washout (worried this will affect inverted)
- thicker airfoil at the tip (worried this won't work with the lower Re number at the tip)
- different airfoil at the tip

On my bigger warbirds I typically use thinner tips and washout, but they don't have to fly inverted.

Thanks, Hans

[Ollie]

With a wing platform, the tip of the wings are over loaded because the tips are too narrow or too tapered. With a straight wing platform and no mean camber airfoil and no wing twist there is no tip stall problem but, such wing will have a slower rolling snap.

Pick the airfoil for tip which has higher maximum lift coefficient than root airfoil which has less maximum lift coefficient. The maximum angle of attack at the tip is higher than the maximum angle of attack at the root . That is usually with more thickness at the tip than the root airfoil.

[hul]

thanks, Ollie. I was thinking the same but wasn't sure.

http://www.aerodesign.de/profile/profildicke_aero.htm (in German, sorry) recommends to use thinner airfoils at the tip, because they have more usable lift at lower Reynolds numbers than thicker foils.

Hans

[dottney]

Choose carefully Hans. Ollie's explanation about too narrow or too swept may indicate a problem with the original design. I built the plane to the plans using a wing jig to insure a straight wing. It tip stalled like mad and I banged the plane a couple of times. I eventually made a constant chord wing. While its not as aerobatic as the original wing it doesn't tip stall any more.
Good luck,
Dave

[BMatthews]

Hans, all else being equal and provided you do not take it to extremes a thicker airfoil will stall at a higher angle of attack than a thin one and the stall will be softer. Also choosing a tip airfoil with the high point slightly further ahead will also delay the stall.

Granted the thicker option may not make as much lift and produce more drag but the stall is delayed and that seems to be what is important for you at this point.

[hul]

I decided to go with a thicker 14% foil at the tip, 12% at the root. Profili reckons they should stall at about the same AOA. I'll try without washout first and add a bit if it tip stalls.
Perhaps aerdesign.de's recommendation was based on very small chords with Re numbers well below 100000. Thin foils seem to work better in that case.

Thanks, Hans

[Ollie]

Why guess. Find the polars for the two airfoils at the airspeed near stall for the Re's for tip and root chords. See:
http://www.profili2.com/eng/default.htm
I would not recommend washout because it becomes washin for inverted flight.

[hul]

that's what I did. The root stalls at about 12 degrees AOA, the tip at 13.

Thanks, Hans

[BMatthews]

Actually that sounds about perfect. You want to avoid any nasty surprises but you also want the wing to stall on demand for snaps of various sorts. Having a wing that stalls pretty much all at once sounds like a great compromise.

[mwraight]

You could just make those special things that attach to the leading edge of the wing...and are removable. They keep you from tip stalling and make the aircraft pretty stable. When you want better performance you just remove them and away you go. I don't remember what they are called, but I recall seeing a recent trainer plane with them installed. The Hobbico NexSTAR -- http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXEXK2**&P=3 will give you an idea.

[mharms]

Looks like the "NASA droop"

[Tom Harper]

Hans,

The scheme only works if you align the zero lift angle of the root and tip sections - not the LE and TE points.

[Pinecone]

Actually that sounds about perfect. You want to avoid any nasty surprises but you also want the wing to stall on demand for snaps of various sorts. Having a wing that stalls pretty much all at once sounds like a great compromise.

Actually for controlability you want a progressive stall from root to tip.

Even full sized aerobatic airplanes use this to make the snap entry and exit easier.

In many full sized aerobatic airplanes what they do is taper the wing in planform, but not in thickness (or not as much). This gives you a rounder leading edge nearer the tips and a thciker (percentage) airfoil, both of which delay the stall.

[hul]

finished the plane with this wing:
root: 240mm chord, Ritz 1-30-12 (12% thick)
tip: 160mm chord, Ritz 1-30-14 (14% thick)
980mm span, no washout.

It flew fine, no tip stalls and good inverted. Unfortunately ended up underpowered (about 110W, Axi 2212/26 on 3s, 9x5 folder) for the weight (700grams, 24.5oz) which made it hard to fly for my inexperienced son. This resulted in him crashing it on the first day out.
Not sure what to do with it now, the damage is easily repaired, but I don't have a motor and packs that fit.

There is nothing wrong with the design, just needs more power, 200W should do. And a longer nose, the original was designed for a heavy brushed motor and NiCds. Only just got the CG right with the pack in the frontmost position.

I have CAD plans if you're interested.

Hans