GeraldRosebery

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I have designed a 90" span (50-60 cc size) aerobat, an extension of previous successful smaller designs. I have been advised that thickening the airfoil at the tip, relative to the root will reduce the tendency of the wing to tip stall. Right now the tip is just a shrunken root such that the top of the wing is flat and dihedral is incorporated into the wing because the tip is that much thinner than the root. I was thinking of thickening the tip by about 10-15%. Can anyone comment on this design modification and its likely effects.

Thanks!

destructiveTester

Ah Good Question!

Opinion is a little divided on this subject to my knowledge.

For example In Model Aircraft Aerodynamics, Martin Simons argues a case for thin tips and thick roots - and there are plenty of full-size planes which also have this configuration. I think perhaps for structural rather than aerodynamic reasons - though it could be both.

But my experience is as follows.

My father and I have designed in the region of 15 aircraft over the years and one of our main aims has always to design aerodynamically "vice-less" aircraft.

I chanced upon the Lift/alpha polars for NACA 0012 and NACA 0015 (12percent and 15 percent respectively) symmetrical sections, and noted that NACA 0015 stalls at a higher angle of attack than NACA 0012.

Now as you probably know, tapered wings can have tip stall problems. This occurs because the tip section is operating at alower Reynolds number from the root, and this can result in an earlier stall - it seems as sections get shorter (for a given air speed) their aerodynamic efficiency reduces.

So If you make your tip section NACA 0015 and therefore 15% thick and you keep the root section as NACA 0012 (12% think) you get a very good wing indeed.

We built a 60 inch span low wing aerobatic sports plane with full span flaps and upwards only spoilers for roll control, which flies incredibly well.

No tip stall, no vices.

And the inverted stall is just as good.

Now there are planes out there with no tip stall, but this is a nice way of getting round the problem as you don't have to build any wash out into your tapered wings to avoid tip stall.

the only problem is that when you skin the wing you have to accommodate the twist of the skin between different thickness aerofoils.

The reason why I mention the actual sections here, is because I cannot guarantee you will get the same results on another section family. For example 15% at the root and 18% thick sections which have just been "made up" or drawn round your favourite boot, may not work that well.

I can say for certain that NACA 0012 works great as a wing section - even though it was used in the wing of the B17 Flying Fortress apparently


hope this helps

p.s. you could check this out too if you want another opinion:-

http://www.djaerotech.com/dj_askjd/d...taperribs.html

ww2birds

My Feedback: (14)

Interesting conversation! At the WRAM show last year, Dave Patrick held a session on trimming an aerobatic airplane, and stated that when he designs a tapered wing, he does not vary the entire airfoil, but only the sharpness of the LE .. making the LE blunter at the tip than the root. I suspect that this is another way to achieve the same net effect as outlined more elegantly in the previous post, but is achievable with a sanding block only on most wings even if you don't have control of the airfoil (e.g. pre cut ribs).

Dave stated, as above, that the motivation for this was to make it stall similarly upright and inverted (e.g. for spin entries), and he preferred this to the traditional washout, which is aimed at avoiding tip stall on landing, which is (I hope) usually not done inverted...

Dave

GeraldRosebery

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It would seem that the advice I was given is generally being followed as I have seen some other input in other threads that gives the same advice. Whether one just sands the leading edge rounder or makes the airfoil thicker at the tip the same end is really achieved. I have flown severals ARF's with thin, sharp LE wings - of note is the small Kyosho .40 size CAP 232. It has truly nasty snap tendencies, even when conservatively balanced. I attribute it to that wing.

10-300

Most IMAC planes that i know of and the extra is one of them don't have the problem of tip stalling, heck they hover. if you scaled up a proven design and had no tip stall tendencies then leave it alone. You can't change just thickness in the tip by 10 to 15% it will be noticably bigger then the previous rib, sure to cause some sort of bad habit. The ratio between each rib from root to tip in length an chord needs to stay the same for that airfoil section. remember you have a proven design your only scaling it up. The only thing i would do if you really think this is a problem is change the airfoil.

William Robison

My Feedback: (3)

Gentlemen:

This has been discussed. To read it go here:

http://www.rcuniverse.com/forum/m_11...tm.htm#1116044

Hope this helps, didn't want to retype it.

Bill.

GeraldRosebery

My Feedback: (14)

What's a wing rib anyway? Just kidding! However, I have to admit I have not cut out ribs for almost 20 years. That's what styrofoam was developed for, isn't it? If you use a foam wing and a not too absurd thickness diffrence it's easy to just cut it in when you cut out the airfoil. It seems that the consensus here and in some other threads is that it is a desirable idea in IMAC designs.