Have a set of plans which looked to have a familiar airfoil drawn to the ribs. After subtracting the 3/32 skin, the ribs look to be an N.A.C.A. 2319. Most places the lines on the plan match within less than 0.010", which I figure to be good enough, EXCEPT for one area.

Using some of the 8" to 15" long ribs for example, everything forward of about the 60% lines matches real close, top and bottom. In between the 60% and the 80% lines it gets more rounded and thicker and varies on the upper surface to about 0.030" maximum. Then from 80% to 100% it goes back about to normal again.

Considering this to be a model, anyone think this variance will affect the flying qualities of a scale model?


Wm.

I really don't know how that shape difference will affect you but I do know that 19% is terribly thick for a model. If you can spare a couple of scale fidelity points how about reducing it to 15%? That's still pretty thick but at least it's down to model size.

Bruce, 19% would be "terribly thick" for a model of a P-51, but not for a model of Burnelli's X-plane from the 1930s.

The Ford trimotor had a rather thick section, as did the inspiration for it, made by Fokker.

Have you noticed the section on some of the new 3-D ARFs? Very thick, symmetrical and teardrop shaped. I was taught that reflex should be up only, otherwise you get more drag without the benefit of increased stability.

Ah, airfoil theory, the sweet mystery of flight.
:spinnyeye

Oh, I know there's many full sized aircraft that use very thick airfoils but it's when they are reduced to model sizes that I start to wonder.

I think the 3D stuff gets away with these super thick airfoils because they weight about 4/5's of nothing but if Coos is building a scale model then it's behaviour at "real" wingloadings is going to be more of an issue I suspect.

But then I'm just guessing here...... It would be nice to run some Xfoil plots on the 2319 at some scale model Reynolds numbers.

Coos, can you offer up some wing sizes and expected weights?

Well, the project is for a 1/5 scale Italian designed set of plans for a Macchi 202. The plans look to be drawn by a fellow with only one ink pen and imitating Brian Taylor. I am having numerous problems with it, the most notable at present is that he tends to like HUGE sheets. They don't fit my digitizer tablet. Other is that he assumes we all know the thickness of the wood to be used, as there is nothing stated as to the ribs. Some look to be 1/8" whereas others measure a good 3/32" thick.

Moreover, I probably should have stated the question a bit differently. I realize the forces are small in comparison to a real A/C, but what would the change be? Additional lift, or poorer penetration, or new stresses? Would like to find out ala a real A/C what this change would be like.


Wm.

I suspect that at present the best option would be to digitize the coordinates of the mystery 'foil and run it through Xfoil. I haven't had much chance to use it myself but all the reports seem to point to Xfoil being one of the better low speed analysis tools.

It's just not enough to say it's "sort of like" since it's all about pressure distributions and local velocities and ..... and ..... you know, stuff.....

Ah, X-Foil. The object of many of my curses, pleadings, and affection. A program whose convergence depends on the tides and the phase of the moon. Actually, it's really not that bad, and pretty powerful once you figure it out. You can get it here:

http://raphael.mit.edu/xfoil/

Anyways, it does let you pick points using the mouse and forms an airfoil to those points, so if you want just load the 2319 (XFoil has all the NACA 4 and 5 digit series programmed in, so just say NACA XXXX at the command line), and modify away till you get close to the mystery foil. This will be a lot easier then trying to get the mystery foil into Xfoil coordinates.

Dan Nelson