I asked about airfoil design about a month ago, and the responses seemed to boil down to a few sensible ones: make it so thick, use so much % flap, etc.

But the "make it so thick" part was confusing me -- do I want an airfoil that is 18-20% of the thickness from the leading edge to the trailing edge of the flap, or that thick from the leading edge to the hingeline?

I would assume that I want the thicker shape, but read them as I might I couldn't get that straight from the responses I got.

So: how thick relative to what chord?

Thanks, and sorry for being so thick :-).

   The chord of the wing (including the flap, if any). Including the flap is the standard method. Al Rabe uses the chord of the fixed portion when specifying the thickness, but he is almost unique in that. It led to some interesting misunderstandings but as long as everybody knows what each other is doing you can easily convert from one to another.

    Brett

Rather than the drafting board, you might consider http://www.profili2.com/eng/default.htm .

Rather than the drafting board, you might consider http://www.profili2.com/eng/default.htm .

Well, the virtual drafting board -- I'm using QCAD.

I'm not CAD drafter, that's certain, but I've been using QCad to doodle. It works pretty well.

But in the end, I'm a paper and pencil guy.

I'm rather puzzled by the comment that airfoil percentage thickness "includes" the flap chord.
I always thought (and I do it myself) that the percentage thickness was only to the flap hinge line?

Who else thinks the way I do?

Floyd

That's why I suggested Profili.  It will tell you what the air thinks.

Al Rabe, apparently.

Speaking as an engineer, I would take the chord length to include the flap.  Any analysis (purely mathematical or using a computer) would take the flap into account.   Certainly if you were looking at an airplane with nicely integrated ailerons you'd calculate the chord as going out to the end of the aileron, and an aileron is nothing but a flap.  Ditto if you're calculating the chord of a tail surface -- you'd take it to the trailing edge of the movable surface, not to the hinge line.

So I think the normal way to do it -- at least if you take all of aviation into account, and not just control line stunt -- would be to include the flap in the overall "wing".

   I agree, there's no reason to invent a new standard just because it's toy airplanes, or someone in the past was unaware of it and used a different definition.

     But I hasten to point out (before Al rises from his hospital bed to smite us all for heresy) - it's really not too important which way you do it as long as everybody understands the method you are using.

    Brett

But I hasten to point out (before Al rises from his hospital bed to smite us all for heresy) - it's really not too important which way you do it as long as everybody understands the method you are using.

I can't agree more -- and I'm not even worried about Al finding out where I live.

While our "flaps" should be included in the wing's chord measurement, I ask how many full sized aircraft have a similar wing airfoil contour to our stunt ships? Can you show me a picture of just one?

I use the measurement to the flap leading edge, for the simple reason that I create the thicker front portion of the wing using a "standard" (?) simple airfoil shape (e.g. NACA00xx) which I chop off as needed ahead of its zero height trailing edge, and rescale in chord, to get the non-zero height trailing edge, a place for our "flap" control surface to connect..

Since that is the portion of the wing where I'm setting the maximum thickness of the wing, it doesn't seem unreasonable not to include the flaps.

I don't think it makes much difference, does it? As long as we are clear on what we are talking about? In conjunction, I haven't seen much problems due to confusions about the definition of chord. Have there been? Just curious as to WHO was confused and subsequently designed a stunt airfoil and built a wing of the wrong thickness? ;->

There are a few "standard" measurements we use in CL Stunt that don't make much sense in the grown up airplane design world. Like back of spinner to wing leading edge ("nose length" or "nose moment" <??>) and hingeline to hingeline distance ("tail moment" <??>). These are not really aerodynamic parameters, rather what has evolved for the convenience of modelers.

I'm aware of efforts of several technically oriented folks here who have tried to help by describing such things as mean aerodynamic chord, and I respect their efforts, but it has very little impact.

Consider this - quite a few of us have quoted wing area values, which may or may not include the portion of the wing hidden inside the fuselage, are frequently coarse guesses, and at best are perimeter outline area measurements. Considering the exaggerated thickness of our modern stunter wings, wing VOLUME might be more meaningful. What's the value anyway, beyond some relative comparative wing size?

I mention it because when I first used AutoCad to measure the actual perimeter area of a Roadrunner wing and arrived at ~593 square inches (which included flaps and the portion inside the fuselage), I was pounced upon by a couple of good friends who just were completely sure it was a lot larger.. This measurements seems somewhat subjective to me.

While perhaps we *ought* to be settling on more meaningful measurements, only guys like our recently deceased Wild Bill (one of my heroes) would find any real application with them. They're simply not terribly useful to those of us lacking aeronautical design credentials. Remember what Bill said about Stunt News readers "not wanting the math".. (I submit that we "design" are model airplanes largely by cut and try, and/or tweaks to well established "numbers". Those "numbers" are what we created for comparative measurements.)

Sorry, I'm with Jimmy Stewart's character Frank Towns in  "Flight of the Phoenix" on this one, who finally concluded "He's crazy Lou, he builds toy airplanes." But a "toy airplane" is pulled on the floor by a string and moves along the ground! And I do remember what Heinrich Dorfmann said about model airplanes needed to be better designed than the Big Jobs..

I think it is time to chop very accurate models of our stunt models into thousands of tiny triangles and get with some serious finite element analysis. Either that, or take the actual model out and fly a pattern or two with it.. ;->

L.

"Mr. Towns, you behave as if stupidity were a virtue. Why is that?" -Heinrich Dorfmann

While perhaps we *ought* to be settling on more meaningful measurements, only guys like our recently deceased Wild Bill (one of my heroes) would find any real application with them. They're simply not terribly useful to those of us lacking aeronautical design credentials.

But you are advocating using parameters that are not useful to anybody and are named with technical terms that mean something else. 

Has anyone taken a look at Keith Trostle's Fock Wolfe that he won a NATS with.  The flap is part of the airfoil until deflected.  Started one and was up to the hingng point when I gave up on it.   

No, they are not useful to anybody.  You can measure the distance between flap and elevator hinge lines, but it has no aerodynamic significance for comparing one design with another.  Something like "tail volume", simplistic as it is, does have some meaning.   

Just out of curiosity, how would you define and/or measure the chord of a Chief with the poliwog airfoil??
 
Troublemaker

Augh!  My thread, it has been overrun by nit-pickers!  Nothing sensible remains!

Good thing I already have my airfoil design, even if it's wrong.

Just do what Jack Sheeks suggested! Draw around the side of his size 9 shoe!

But I don't have any of Jack Sheeks's shoes.  Am I doomed?