I'm going to throw out a bunch of generalities regarding my limited understanding of aerodynamics.

1. Tapered wings tend to tip stall more easily than constant chord.

2. Washout is a physical twist in the wing to minimize unwanted snaps. Less incidence at the tip.

3. Flat bottom airfoils stall sooner than symmetrical, all things being equal.

4. Full size aircraft with tapered wings use washout as well as progressive airfoils. The airfoil is flat bottomed at the tips and more symmetrical at the root.

IF I'm correct in assuming that flat bottom airfoils stall sooner than symmetrical, why would the tips be flat bottom? Would this not be counter productive and cause the tips to stall sooner and cause a snap? That, of course assumes that one tip will stall before the other.

Curious about this, when I built my .75 powered Chipmunk, I made the root semi-symmetrical and this progressed to a fully symmetrical airfoil at the tips. No washout was used.

I've only flown this plane three or four times due to a lack of a proper flying field. but the flights I put in showed a very solid airplane. Loops, rolls, stall turns, snaps (deliberate) spins, knife edge all normal and I'm very happy with the plane. I did some stall tests at low power (at altitude) and no unwanted snapping tendencies. Stall was straight ahead and recovery is normal.

Any comments?

Andy,

1. Nearly but not quite. It has as much to do with localised Reynolds Number as anything else.

General form Rn = k*velocity*chord

If a wing has a 50% taper root to tip then the local Rn at the tip will be half that of the Rn at the root. That fact alone directly affects the performance of that part of the wing.

<e>The general rule as I understand it for changing sections between root and tip is to match local Rn with the performance charateristics of the airfoil at that point.

So, as an example, on my (old fashioned) F1A gliders I use a foil at the root which has a camber of 6.5% at 45%C. Over the tapered outer panel, that changes to 3.5% camber at 35%C. The effect is what I understand as "aerodynamic washout". The tip foil will stall "later" than the root and this has everything to do with the combination of alpha, Rn, camber, thickness, and everything else<e>


2. Washout does reduce the tendancy to tip stall - no question. It makes the tip operate at a lower Cl than the more inner portion of the wing.

Think about a Cl/alpha polar for a given Rn and you will see why. As alpha reduces, so does Cl, and so also does the tendancy to stall.

3. Not necessarily. It depends far more on all the other factors. For instance, if you drive a Go495 (which is about 14% thick) at the optimum speed for a Naca symmetrical 6% section what will happen? I certainly dont know for sure but I don't think it would be pretty.

This is one of the difficulties of aerodynamics - the clue is in the "dynamics" portion. If you take one factor in isolation it is easy to say "This will happen because..." It is in the nature of the beast that you can not take any one factor in isolation. You gotta have all of them there for it to work proper.

4. Yes, but again not all. The wing design is specific to the performance requirements of each specific aircraft. That more than anything else determines the way in which the wing works. A microlight would not work at all with the wing properties designed into a 747 wing, nor vice versa. Obvious but no less true.

All are good questions, each one deserves a much longer answer. I have tried to be as brief as possible.

I'm gonna try my own hand at this too...

1) Yes. If the section is the same and there is no washout then it will. This doesn't apply as closeley to full sized aircraft though as they don't suffer from as large a "scaling" factor. (the Reynolds number stuff that probligo explained). So for our models it's a good idea to keep the taper ratio low. Tips should be no smaller than .7 to .8 times the root chord unless you use large(r) amounts of washout to compensate and avoid tips stalls.

2) Yes again. Less incidence means lower angle of attack at the tip which means it's not working as hard as the center areas (lower lift coefficient or Cl) so it won't stall as soon.

3) Nope, totally wrong here. Take a NACA 0009 (9% symetrical) and a NACA 6409 (that same shape but with a 6% camber line at the 40% point). The cambered airfoil will stall at a much higher angle of attack and achieve much higher lift coefficients before it stalls than the symetrical one. BUT.... thickness also comes into play. Up to a point thicker airfoils will show a later and softer stall in many cases. This may have been what led you astray. You can't compare thin cambered airfoils to a thick aerobatic airfoil. Apples and oranges I'm afraid. This is perhaps why the tip airfoils on many commercial aircraft show signs of more camber as per your flat bottomed observation. The higher camber delays the tip stalling. Especially in concert with lots of washout.

4) Often they follow this pattern in full sized aircraft. It's much easier to study lower speed aircraft for this rather than the super critical airfoils of many high speed jet airliners. Some of the super critical stuff looks like our model types but upside down. Different purposes...... But something like an Aero Commander certainly follows your pattern.

You need to remember that if one of our models tip stalls and crashes it's not a big deal. But if a full sized aircraft does this head will roll. Full sized stuff has anti stall design elements that we would never dream about modelling but they need to save people from themselves. To do that they will happilly give up some flying efficiency.

Thanks guys for the replies.

I built the Chipmunk wing based on my assumptions. Afterwards I had the opportunity to inspect a full size Harvard and I noticed that the wing was semi-symmetrical at the root, progressed to flat bottom at the tips and had washout. I do understand that the Harvard was designed as a trainer and not an aerobat but based on my understanding, it looked like they went the wrong way. Now I know better.

But, my Chipmunk flys just great. I guess it could be that the mysterious Reynolds numbers are in play mitigating what might be a bad situation if it were applied to a full size aircraft. To test this, I suppose I could just build two more wings. One with the progressive going the other way and one with a fully symmetrical foil non progressive. But, maybe I'll try that on a smaller, purpose built aircraft. Something in a 1/2A size.

OK, here's another question. I've recently received two kits. One is actually an ARF from Wattage, the Extra 330L. Extremely well made, it looks very close to scale including a nice semi-symmetrical foil that looks exact scale as far as thickness. Should I twist in some washout?

The other kit is a Cap 232 from StevensAero. This is a gorgeous laser cut kit intended for electric power. The airfoil is much thicker in comparison to the Extra. Looks much thicker than scale. I've seen the videos, performance is spectacular, even as an electric. A very low wing loading seems to be the key.

It would seem to me that model aircraft could benefit from much thicker airfoils than we normally see today. I'm talking about aerobats. With our high revving motors that bog on the up lines and unload on the down lines, wouldn't a really thick airfoil mediate this effect?

Maybe I'll build an extra uh, Extra wing with a thicker, fully symmetrical foil and see what's what.

FWIW, the Harvard (T-6) wing went thru 3 evolutions... to eliminate the "vicious stall characteristics"... Made them merely unpleasant.
It began life as the BC-1 with NACA 2215 root, 2209 tip. No washout.
Then a leading edge slat was addded, BT-9, and then the tip washed out 2 degrees SNJ-1.
The final wing is 2215 root, 4412 tip, 2 degrees of washout.
Note the B-25 apparently has this same geometry.

Keep in mind that if you twist in washout that it becomes washin when inverted. A aerobat machine is best if it feels the same upright and inverted. The twist would not allow this.

On airfoil thickness, many years ago there was a design trend toward thick airfoils. There were no snap maneuvers in the patterns at that time. Some airplanes went to 18-20% thick with a big leading edge radius. The Tarus is an example of this. Wether or not the goal of constant speed flights were achieved I am not too sure.

With the present day emphasis on snap maneuvers (neat stuff) the thicker airfoils are not going to deliver any performance improvements.