1:4 1928 RRG Professor

[ARUP]

It's time to start another build! I'm a sucker for antique aircraft and especially appreciate those with an historical significance. Alexander Lippisch designed the Professor which first flew on 06 August 1928. It was piloted by Robert Kronfeld, a brilliant pilot. (Kronfeld had to escape when Nazis came to power.) The Professor was to be a step up in performance from the primaries but was still simple in design and manufacture. Most importantly, it was the first sailplane to employ the use of a variometer! True soaring was born on this day!

I am using three views from Martin Simons book: Sailplanes 1920-1945. I am going to build the prototype version named Rhongeist which was a nickname given to Lippisch. Lippisch spent a lot of time in the Rhon Mountains. It has a plywood skinned open cockpit fuselage, fabric covered flying surfaces, 'pendulum' elevator, struts and skid. It will span 4.025m (~159") with a chord of 0.375m (over 14") and the length will be 1.75m (~69"). The rudder and elevators will be removable to facilitate storage, along with the three piece wing, in a nice and cozy custom made box.The wing will be in three parts. I am waiting for the delivery of wing tube and socket joiners so the rib patterns can be finalized. I am still drawing the plans.



This picture is an internet grab of a Professor. Stay tuned!

[Al Clark]

Cool! Ought to look lovely in the air.

[ARUP]

Al, thanks for looking!

[Swiss1]

Hey Jelly, " Rhongeist" would mean Rhon the area and Geist is ghost, maybe because of it's see through wing structure which would make it look ghost like in flight, just my thought!

[ARUP]

Thanks, Jeremy! Yes... Lippisch was nick named Rhongeist because he trekked the Rhon mountains so much by himself. I guess he looked 'ghost;y' from afar.

[Tango Juliet]

Exciting! Is this your largest glider now?

[ARUP]

Hey TJ, other than Charlotte this will be the largest I've built! I've been dreaming of this build for a couple of years, now! Thanks for looking!

[French1]

4m should be a sweet scale mike, will be following in the background!

Stéphane

[ARUP]

Stephane, welcome aboard! What's nice is I can put a vario in this one and it will be 'scale'!

[Xroadie]

Foam core wings, I'm sure

[lenb]

I understood they were Composite?

[ARUP]

I'm gonna use that 'stealth' foam. It's there but you can't see it!

[ARUP]

How do you make tapered ribs if: 1) your threaded rods for doing the 'carve and sand' method aren't long enough, 2) the taper is too great to really sand accurately and you don't wanna waste wood or foam as spacers, 3) your dog ate the computer, 4) your Foil Plotting software was accidentally deleted and you don't want to buy it again, 5) you have Foil Plotting software but your cat coughed up a fur ball into the printer, 6) the printer is fine but you used all the paper for the bird cage?

Tah-dahhhhhh! Draw an egg crate! It's basically that simple.



This is a straight taper wing. The root rib is known. I'm using the scale Gottingen 549 foil but the tip foil will be symmetrical. If I build the tip at the proper incidence it will help with tip stalling. The thicknesses of the tip and root were obtained from the 3-view. If you erect the tip on one end of the board and erect the root on the other it is a simple matter to measure the height and length for a spar to fit between them, right? Now, you don't want the spar to go from the TE of one rib to the LE of the other 'cause it would have a weird curve develop on its edges. So that this doesn't happen the tip and root ribs need to be divided into equal 'segments'. But... you say... the tip is shorter than the root rib so you can't divide them equally. That's correct! Linearly they can't be divided equally but they can be divided into an equal number of parts! The tip length was divided into the root length to get a unit-less factor of 3.08 sump'n or other. So, for example, 1mm length on the tip rib is ~3.08 sump'n or other on the root rib and ad infinitum! (that's for JimD) Plotting a bird's eye view of the tip rib and root rib onto a datum line, in this case, the rear edge of the 'D' tube portion of the wing we get a nice 'fan looking' effect. Now, count up the needed ribs and space them equally if the wing had them spaced equally and draw them between the tip and root at the proper spacing. For the professor this was 79.3 sump'n or other mm. I just stored that in the calculator.



More to come!

[Al Clark]

That brings back memories from my days with pencils and T squares (I still have my old drawing board and T square). Thank God I have Compufoil and CAD now to save me!

[Swiss1]

I'll take the Drawing Board, T Square, Set Square and French Curves any day over a computer!

[ARUP]

The rib lengths are all known, now. Also, even though we don't yet know the shape or thicknesses of the intermediary ribs just yet the 'thickness stations' are now known because of all of the 'spars' we drew in 'bird's eye view'! Isn't that awesome? We know the symmetrical foil has a chord length line that bisects it into two equal 'top' and 'bottom' parts. It's symmetrical! The root rib has a chord length line from a midway point on its nose to the TE and it is NOT symmetrical! Trace the shape of your tip and root ribs somewhere onto the plan. I drew the root rib on the right, next to the root end of the 'fan'. Draw perpendiculars across the tip and root rib chord length lines at the just found 'thickness stations', i.e., where the 'spars' intersect tip and root ribs. Also, remember when we drew the line across the 'fan' that represented the rear edge of the 'D' tube? Use this station to align tip and root ribs relative to each other. Draw a convenient line with your straight edge. Mine was above the 'fan' design (the right angle ruler in pic above was covering some of the drawing) and the same width. It was easy to just extend the verticals upward. Measure each relative station on the tip and root for thickness and put the tip thickness on one end and the root thickness on the other. 'Connect the dots' and it will look like a tapered spar going from the tip to the root ribs!

One thing will be absolutely essential in order to plot the ribs and that is the location of the chord length line. At the tip of the 'spar' it will be a midpoint location because the tip rib is symmetrical. Measure it on the root rib and transfer this to the 'spar' at the root end then connect those two dots with a straight line from the tip to the root.



I numbered my 'spars' with Roman Numerals and made 14 of them. You can make more (more foil accuracy) or less... it's up to you! The left pic above shows the complete root 'spar' ends of 'X' and 'XI' and tip 'spar' ends of XII, XIII and XIV. Not so easy to see are how I numbered the ribs onto the 'spars'. Remember, these ribs were just the lines I extended up from my 'fan'. You could draw them elsewhere. I had to draw some elsewhere because my paper wasn't big enough. In left pic above for 'spar' X rib 1 (circled at top) is the known Go 549 root rib. The second rib station has numbers 34 and 40.5. These are the thicknesses I measured with a mm ruler. 34 is from the top to the chord length line and 40 is the total thickness. I put these thicknesses on a datum line. After plotting them all I used a French curve and English curse words to draw the foils!





See how the drawing develops? Four done so thirteen more to go!!! When all of these are done I'll trace them so that the tracings can be put onto wood for cutting. The originals can be referenced for placement of the 'D' tube rear edge, etc. Drawing ribs isn't difficult... just onerous!

[patte de loup]

Please find below a building link of this beauty:

http://www.retroplane.net/forum/viewtopic.php?t=2030&postdays=0&postorder=asc&highlight=professor&start=0

Pat'

[ARUP]

Pat, thanks for the link! Sure is a beauty!

[stihac]

Love it Mike!
Old school bird build old school way. I'll be following the build for sure.

Petr

[ARUP]

Hello Petr, where've ya been? Glad to see you're still around! Old school... that's me. I don't 'cotton' to computers very much. I enjoy figuring things out and learning how to do things without them.

[stihac]

Hi there. I'm still around. Just have not much time to build or fly. I always enjoy your builds. keep it up!

[ChrisL]

Awesome stuff Mike. Everyone of your builds is a work of ART!

Chris

[ARUP]

ChrisL, thanks for the very kind words! I really appreciate it! Did I see a vid of that Parabola Flying wing you are holding once upon a time? THAT'S cool!

[ChrisL]

I was thinking of doing another (better) one for next years Monticello tow. Uploaded a short video to Youtube of the first one.

https://www.youtube.com/watch?v=-giZmOz_N0w

Keep working Mike, you have a lot of parts to draw!

[Tango Juliet]

Parabola 2.0 sounds great! As a former B-2 Crew Chief, flying wings really get my attention.

[Al_M]

Mike, you should have been an engineer! Excellent work. Eagerly lurking.

[ARUP]

Al, how are ya, buddy? I started off in engineering but absolutely could not stand the computer programming stuff... punch cards, FORTRAN, MSDOS, PASCAL, you name it... I hated it! I learned 'em all. Looking back, maybe, architecture would have been more suitable??? Design??? Anything that didn't require computer stuff if that's possible. I always liked 'art stuff' but my color vision is off. Oh well... I sure like building these model airplanes!

[JimD]

After plotting them all I used a French curve and English curse words to draw the foils!

A Kentucky CAD technique..."Cursed At Drawlings"

[ARUP]

Good one, Jim!

[Dion Dunn]

Really like the lines of this one Michael. Less quirky than your previous classics, but just as unique!

[ARUP]

Thanks for looking, Dion! If they end up being 'quirky' well... blame the original designer!

[ARUP]

All drawing is done! I have wing tubes and sockets in hand. The stab pass through spar of CF tube with attendant bushings and elevator control horn is at hand. Patterns are cut. I have ribs for the center section cut. More wood chopping and some pictures to come tomorrow!