Feb 112020

That’s the way we used to (and still mostly do) design our stuff. If it looks right, it will fly right. Works since the days of the Wright brothers (and all those that went before and came after them.)

However, fun has to be had! I need to sort of start thinking about the wings in order to build the center section so that I can finish the pcovering of the front section. Given that the plans show a nice 1:1 rib, and given it is often stated to be a ‘Modified M6 profile’, I want to find out what Lou Stolp was thinking.

The short story: Take a picture, turn it into a black and white image, remove all noise, fiddle some more and import the result in Profili. Then say you want to find similar airfoils, job done.

It does not take long to come up with a list. We’ve got a NACA0013, a RAF30, a Goetingen 459 and an Ultimate profile, that all are very similar. Mix NACA0013 with RAF30 and you are 99% close to what Lou made. Running the results through XFoil shows characteristics that are somewhere between the 2.

note: on sheet 15 of the plans there is mention of a ‘osborne a-2’ airfoil. Can’t locate anything about it at the moment. (http://aerofiles.net/airfoils.html shows it, but no data. Jim Osborne was of course the guy who drew the plans 😉

full size profile drawing.

Before anybody asks: I haven’t got a clue what I am doing, but the curves look pretty! But I did have to go and find mr. Reynolds to figure out what I was looking at. (He says his number is somewhere between 300 and 500k for the speeds I guess this will fly.)

Given that the Ultimate is a known well flying Biplane, and the RAF30 profile is:

The basic symmetrical section ( R.A.F. 30 ) was calculated by the method described in R. & M. 911, using the constants k = 1.08, n = 1.95, B = 0. The aerofoil shape so obtained ends in a sharp angle, and so the last 1 percent of the chord was cut off in order to avoid a thin trailing edge. The form of the aerofoil was also adjusted slightly towards the trailing edge in order to remove a slight reflex curvature. The aerofoil has a maximum thickness of 0.13 of the chord at a distance of one third of the chord from the leading edge, and its shape approximates closely to the symmetrical Gottingen section 459 which was known to possess good aerodynamic characteristics.

Yup, that’s good enough for me..

In my younger days I did not have all this fancy stuff, I drew profiles that looked nice, and all those contraptions flew, so actually, this is just a fun exercise, that probably does not add anything to the overall results. But it looks good when you say you figured out what Lou Stolp did!

However, how we go from a M6 profile to a symmetric one remains a mystery for the moment. Searching some more I find this remark on kitplanes.com: The new airfoil is the RAF 34, which is similar to the M6 airfoil used on the Pitts. Searching for RAF34 on Airfoiltools.com indeed shows a profile similar to an M-6. The original Pitts did not have a symmetrical airfoil! Ok, happy with that. I’ll stick to what I have 😉

I could figure this out if I want to, but at the moment I’m not interested. I want a symmetrical profile, that is what I have, and how they got there is not really important.

 Posted by at 6:39 pm

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