Increasing camber over a tappered span? what is the implication?

[rcjetpilot]

(Edited, see next post which is more clear as to the question)

Given a standard NACA 0010 airfoil for a tapered wing, what is the implication of increasing the camber to 15% at the tip?

It is my understanding that strongly tapered wings require some washout to counter the increased AOA at the tip. Would increasing the camber achieve the same results?

Bob

[Ollie]

Symmetrical airfoils have no mean line camber. I think you may mean increasing the thickness of the symmetrical airfoil from 10% to 15%.

Referring to Theory of Wing Sections by Abbott and Von Doenhoff, The NACA 0009 at reynolds numbers above 3 million, stalls at a coefficient of lift of about 1.2. The NACA 0012 at the same reynolds numbers stalls at a coefficient of lift of about 1.6. the conclusion is that thickening a symmetrical section toward the tip, in this case, will considerably improve the tip stall margin of a tapered wing without resorting to washout. Similar effects probably apply to models at much smaller reynolds numbers.

It is a good idea to use taper ratios to above 0.4 or 0.5 unless you have quick snap rolls as a higher priority than tip stall margin.

High taper ratios load the tip beyond its ability to carry the load. Increasing the maximum lift coefficient at the tip is a better tactic than washout if tip stalling when inverted is to be avoided. Washout upright becomes washin when inverted.

[rcjetpilot]

Ollie,
Thanks for the response. Sorry for the confusion, I was thinking of two related issues at the same time...as you quickly pickup on, but I typed in one mixed thought. If I could, let me restate my question:

Your information on symmetrical airfoils is correct so here's what I'm wonder, if increasing the thickness over a span to the tip, and given a cambered airfoil, would it also be true that decreasing the camber at the tip equally helps?

Seems to me that reducing the camber at the tip would work equally well on wing forms, especially high speed planes. Generally, would this be correct?

Thanks, Bob.

[FlightofSong]

Reducing camber at the tip is referred to as "aerodynamic twist" as it changes the absolute angle of attack (which is referenced to the zero-lift angle) to a lower value at the tip (assuming no geometric twist). This is a technique that's in common use in many airplanes, and works well for improving upright characteristics. Also, it doesn't do as much damage to negative-lift handling (inverted flight) when compared to geometric washout, as a less-cambered airfoil can often acheive a "more negative" Cl. Compare the S1223 (really, really cambered airfoil), which can't even make negative lift, to the NACA 0012, which can make jsut as much negative lift as positive lift...

blah, blah, blah - tell me to shut up if necessary.

[Sparky Paul]

Originally posted by FlightofSong
... Compare the S1223 (really, really cambered airfoil), which can't even make negative lift, to the NACA 0012, which can make just as much negative lift as positive lift...

blah, blah, blah - tell me to shut up if necessary.
.
I have flown an empty SAE lifter with an S1223 inverted... It wouldn't fly level, but descended, but not rapidly.

[FlightofSong]

Interesting... I thought the S1223 wouldn't go negative at all, but if the plane didn't fall out of the sky, it was obviously making some negative lift...

[rcjetpilot]

Thanks folks for the input, here's what generated my inquiry...

First let me say that I really enjoy this group, and as a long time rc guy, this new area for exchanging information is priceless.

I've designed several scratch-built planes over the years but never delved into the "higher end" performance arena. What I'm looking at in CAD these days is a low drag, swept wing conard powered by a midi fan. I had been looking at some eppler airfoils, 197 aft, 214 fore plane but recently decided that the NACA 6 digit specs seem better. Currently under consideration is:

Aft wing - (root)63Ax210 (tip)63Ax012

Conard - (root)63Ax410 (tip)63Ax212

Thanks again for the help.

Bob

[Ollie]

Actually airfoils make lift for almost the full 360 degrees of the possible range of angle of attack. Even wings that are mounted trailing edge first and even wing that are in a deep stall. The lift may be a lot less than normal and the drag may be vastly greater than normal but lift is almost always not equal to zero except for 4 particular angles of attack in the 360 degree range.

Bob,

High speed planes have drag reduction as a high priority. Pylon racers or other speedsters that fly laps pull high angles of attack in the turns and induced drag becomes a consideration. The decambering of airfoils not only results in aerodynamic twist as FlightofSong explained but also shapes the lift distribution which affects the induced drag. For a speedster that needs an efficient wing at a variety of angles of attack, the planform, twist, sweep back and taper all have to be harmonized for the best results. If you just consider one aspect of the wing design in isolation the design will not converge on a very good result.

See:
http://aero.stanford.edu/WingCalc.html
Use this on line applet to test your wing configuration candidates for the desired properties. You will notice that the analysis is independant of airfoil differences except for zero lift angle of attack. The range of angles of attack should be limited to that which applies to the specific airfoil and aspect ratio up to stall. Beyond the onset of stall the analysis isn't valid. A wing experiences an induced angle of attack in addition to the angle of attack associated with the airfoil's wind tunnel measurements. The induced angle of attack is inversely proportional to the aspect ratio and proportional to the square of the coefficient of lift.

I mention all these complexities so that you will realize that pat answers about one wing design consideration don't apply without side effects. In other words, everything affects everything else.

[FlightofSong]

Originally posted by Ollie
Actually airfoils make lift for almost the full 360 degrees of the possible range of angle of attack. Even wings that are mounted trailing edge first and even wing that are in a deep stall. The lift may be a lot less than normal and the drag may be vastly greater than normal but lift is almost always not equal to zero except for 4 particular angles of attack in the 360 degree range.


VERY good point, Ollie. Upon cogitating, I realize that what was probably going on is that the S1223 doesn't make any negative lift before stalling - the point when most investigations into lift curves terminate, and the point when xfoil throws up its hands and walks away...

However, what I forgot is that the curve always turns around again and heads off into lifting territory if you keep going - the drag is just a huge penalty.

Thanks for the mental adjustment - just goes too show that too much education can be a dangerous thing - time for me to drop out of grad school! (just kidding)