How to read Junkers airfoils

[Skyraider3D]

Does anyone know how to read Junkers airfoil designations.

For example the Ju 287 has profile "Ju 1 23 12,5-0,825 40/1,425"

I am sure there is a lot of info in that designation, if only I knew how to read it. I'm gonna venture a guess that 12,5 is the thickness (12.5%) and possibly 40 is the thickest part at 40% of the chord, but other than that I am unsure.

 

[Skyraider3D]

Hmmm that tricky huh?

 

[gwiz]

If you want my guess, max thickness 12.5% at 23% chord, max camber 1.425% at 40% chord would give a more typical shape for the period. Not sure about the 0.825, might be trailing edge thickness %, though seems rather high for that, or else leading edge radius %.

 

[Skyraider3D]

Thanks! Some interesting suggestions!

Plotting these against a period drawing of the airfoil, I get an almost perfect match for the lower half, but the upper half should be decidedly fatter.
* red lines are 0 reference lines;
* blue lines are max camber and max thickness;
* purple line at 23% chord, which seems to be unrelated to the max thickness, which seems to be at around 1/3rd of the chord.

This airfoil and the designation come from a 1944 windtunnel model drawing.

The Incomplete Guide to Airfoil Usage gives different figures:
root: Ju 0.9 23 10.7-0.825-40
tip: Ju 0.9 23 12.7-0.825-40

It would seem likely the 10.7 and 12.7 (and thus the 12.5 in my original post) are indeed the airfoil thickness figures in % of chord, as these are the only figures that change (although I'd have expected them to be reversed).

NB. The 12.5% airfoil from my original post is measured at the fuselage sides. Since the fuselage is 2m wide, it's therefore 1m out of the centreline.
I already suspected the root and tip foils to be swapped and this seems to confirm it, because (using the half-span without dihedral of 9m85) the deviation in thickness over 1m is 1/9.85 x (12.7-10.7) = 0.2; or 12.7-0.2 = 12.5% at the fuselage side. Makes sense hopefully?

 

[Skyraider3D]

Ah, at last, the answer from Horst Lommel's Ju 287 book (attached).

So for who doesn't speak German, using the fuselage side wall airfoil as quoted in this book, it says:
* camber of 1% at 23.3% chord
* max thickness of 12,5% at 40% chord
The final figure is indeed related to the trailing edge angle (but I am not sure how to read it?).

I am unsure but I am guessing the 0.825 figure gives the camber at max. thickness.

 

[sienar]

This system Junkers is following is essentially NACA 4 series with a different numbering system, something many other German companies at the time were following as was the DVL.

The number will be something like 1.5 30 16-0.915-35.6

1.5 = % max camber
30 = location of max camber
16 = max thickness
0.915= nose
35.6 = max thickness location


As far as that airfoil with 0.150, I'm not sure what it could be. I'd guess its either a reflexed trailing edge of trailing edge size.

 

[Skyraider3D]

Thanks very much, that makes sense!

 

[Aeroweanie]

These airfoils are uniquely described by the six numbers of their designations:

• The first number indicates the maximum value of the mean-line ordinate (also called the "camber"), in percent of the chord.
• The second number indicates the distance from the airfoil leading edge to the location of the maximum camber in tenths of the chord.
• The third number indicates the section thickness in percent of the chord.
• The fourth number indicates the leading edge radius parameter. The normal NACA 4-digit leading edge radius is designated by a value of 1.1.
• The fifth number indicates the position of maximum thickness in tenths of the airfoil chord.
• The sixth number indicates the trailing edge angle parameter.

Sometimes, these airfoils are given without the last two numbers. In this case, the checkbox can be clicked on and the display will change to this format. The fifth number will be assumed to be 30 and the NACA 4-digit modified expression will be used for the sixth number.

Thus, an airfoil designated "NACA 1.8 25 14-1.1 30/.50" has 1.8% camber, at 25% of the chord from the leading edge and is 14% thick. The leading edge radius is 1.1 times the thickness to chord ratio squared, the position of maximum thickness is now 30% of the airfoil's chord and the trailing edge angle is 2 * arctangent(.5 * t/c). A designation of "0012-.55" indicates that the camber is zero, so the remaining terms are not listed.