How many questions will I be allowed to ask?

The purpose of blending airfoils is to get a wing with the characteristics that you want. Right? Soooo... What would blending a Clark YH to a blunt symmetrical airfoil do for things like stall. What other things should I be looking for? (root clark yh to tip symmetrical)

I have recently done this on a delta and ended up with a perfect airplane. It had no bad characteristics at all. Is that rare or is it difficult to make a bad flying model airplane?
Any examples of commercial models with blended airfoil?

Many models intended for high level competitions are designed with blended airfoils... depending on the purpose the result can be easily seen or more subtly done, with camber changes instead of gross profile changes.
Deltas use both reflexed or symmetrical, or a combination of both as you've found.
I expect that in refining the combination you have you might finalize on a nearly symmetrical but slightly reflexed airfoil.
Depends on what -you- want.

It is SO simple to a make a bad flying airplane!
Ignore the c.g. is the easiest and most common method.
Just build it, stuff it full of stuff, and then stuff all the stuff into the ground on the first (and only) flight attempt.
I bet that happens to many ARFs...

If you are going to blend a CLARK Y into a symmetrical airfoil I suggest you put the more cambered airfoil (CLARK-Y) at the tip and give the wing a couple of degrees of wash-out. In this way you will get a wing with very nice stalling qualities. The washout is needed because the cambered airfoil has a zero-lift angle of attack that is below zero whereas the symmetrical airfoil has zero lift at 0 degrees.

/Red B.

hehe, I meant aerodynamically speaking...

This is probably not a nice thing to say, but... I hate ARF's. They take almost as long as a kit to build and I am never satisfied with the finished product.
I know how to CAD and I found a guy willing to laser cut my parts. Airfoil choice is kinda difficult. I have a friend that has a Masters in Aero that helps me some, but I don't like to bother him too much. (not that he'd mind, I just dont't want to wear out my welcome.) And I like to be able to talk intelligently with him. (not look like a dumb***) I was wondering what kind of experiences people have had with blending airfoils and how necessary it is in the model world.

Red B.
So I went about it in reverse? [X(] That's typical for me.

I am designing an ME 329. It's a flying wing with two pushers that was in development by the germans at the of WW2. Is there a benefit to a blended airfoil in a case like this? Or should I just go with a fully symmetrical airfoil. (it will be powered by .30 to .50 glow four-strokes) pics at http://www.luft46.com/mess/me329.html

The -329 resembles a plank more than a delta. A moderately reflexed airfoil would probably be a better choice than pure symmetrical.

Another weird one is on the Avro Arrow. Looks like the wing is on upside down. That's them Canucks for you. Top of the wing is flat. All of the curve on the bottom. When you stop and think of it, makes sense. Have an EDF in the works to try it out.

Or if he leaves it the way it is, he doesn't need to add washout because the wing already has aerodynamic washout due to the change in zero lift angle between the cambered root and symmetrical tip.

-David

You can get away with a lot when it comes to delta wings. The are not very sensitive to stalling problems and can operate at very high AOA.
The wing planform you have choosen compensated for the negative effects of using the blended airfoils in the way you described. If on the other hand you had chosen a high aspect ratio planform (e.g. glider wing) you might have had some problems.


David, IMHO the result will not be the desired one.

Assuming a Clark-Y profile with a zero lift AOA of -2 deg. and an uncambered symmetrical airfoil (NACA009) with zero lift at 0 deg AOA:
If the cambered airfoil is placed at the tip and the symmetrical airfoil at the root, the wing need to be twisted geometrically by 2 deg (washout) so that there is no aerodynamic twist, i.e. maintaining a constant lift coefficient along the wing. The more cambered airfoil will stall at a higher lift coefficient than the uncambered airfoil and the result will be a safe stalling behaviour where the wing root will tend to stall before the wing tip.

If, as you suggest David, the cambered airfoil is placed at the root, the uncambered section at the tip and the wing is built without twist a problem occurs. If one looks at the polar curves (lift coefficient versus AOA) it becomes apparent that both airfoils will stall at roughly the same AOA (approx. 10 deg at Re=100000). This will not give the wing a safe stall behavior as it will be hard to tell which part of the wing that will stall first.

Furthermore, because the wing is built without twist, at high speed (at low angle of attack) different portions of the wing will operate at different lifting coefficients which is bad from an efficiency point-of-view, especially at high speeds where the lift coefficient of the wing as a whole is small (let's assume 0.1 as a typical example).
With an untwisted wing at 0 deg AOA, the lift coefficient is approx 0.4 for the Clark-Y and 0 for the NACA 009. To achieve an average of 0.1 (for the high speed flight example) the wing will need to operate at a slightly negative AOA. The Clark-Y will be producing an upward lifting force at a lift coefficient that is positive but less than 0.4. The NACA 009 however will produce a downward force with a NEGATIVE lift coefficient. This adds to the induced drag of the wing and is highly inefficient.

Summary: In general, a wing with a cambered root and an uncamberd (symmetrical) tip airfoil will not produce a safe stalling behaviour and is inefficent at high speed.

/Red B.

Red B.,
You're right, of course... but a delta is not the first thing to come into my mind when I think of an efficient airplane.

-David

It's not just our Arrow. The Delta Dagger and Dart used the same airfoil.... or at least ONE of them did. I think you'll find the same airfoil in a few of the delta or other extreme chord full sized airplanes as well.

Even airliners use these sorts. Looking through Profili shows you a few ailiner root airfoils that look like they are mostly upside down. Sort of spits in the eye of the Bernulli principle.