Graupner Cirrus X 3

[ingrahal]

I'm starting my summer build project. I hole up in the summer the way most people do in the winter. This summer I'm building not one but three Graupner Cirrus. Don't ask. The kits are very rare now but demand is still high. Luckily there are individuals making parts and pieces that allowed us to put together enough for three planes. By us, I mean, my friend Larry, his friend Terry and myself.

Terry found rib kits in the UK and the original Graupner elevator bellcranks somewhere.




Larry found fiberglass fuselages in Florida.



Terry scored a complete kit that is in excellent condition and copied the plans for me. He is keeping the kit for his collection though. I scored a truckload of balsa at an auction last year so the wood is mostly covered.
The plans I'm working from are for a 1/6th scale version of the Open Class Cirrus which make it a 3 meter wingspan. We also have the building instructions.



The first challenge is to translate all the building notes on the plans. Luckily there are numbers on each note and translation to English in the book.



Next I had to figure out the dimensions of all the various pieces of wood. There is a list of all the parts by number in the building instructions with approximate dimensions but you still have to make some judgments. For instance converting 13/64" to a common size. All the dimensions are oddball sizes.



Once I got that done I realized that the trailing edge stock was not a standard trailing edge that anyone makes. So I scrounged around and found some that could be cut and then thickness sanded to work.
My plane is going to have ailerons and a tow release so it's going to be more complicated and is the one I'll document here. Besides adding ailerons I will decrease the dihedral from 6 deg. to 3 deg. Which is what the full scale plane has I believe. I will also modify the ribs and wing tubes to accept a straight wing rod instead of the bent ones in the original kit.



That's it for now gotta go get some flying in. The first wing panel is well underway and I'll be back with an update shortly.

[Steve P]

Nice! Thanks for sharing!

[ingrahal]

Update time.

The wing is your typical D tube construction with a cap and web main spar and a diagonal false spar.


Top and bottom sheeting forms the D tube and 3/16" of washout is incorporated into the trailing edge at the tip. The planform has a symmetrical taper of both leading and trailing edges from about 2/3 of the length out to the tip. Actually the entire wing tapers but the taper increases at about 2/3.


One interesting thing, at least to me, is the angle of the aileron spar. They angled it forward toward the main spar so that it intersects each rib at a constant height. So even though the wing tapers toward the tip the spar is not tapered. This allows you to glue the ribs in flush with the top of the spar and get the built-in washout in the wing. Some pretty thoughtful engineering.


The root rib is set to a -3 deg. because I'm taking out 3 deg. of dihedral.


I stacked and drilled the ribs for the servo wires.


I followed the plan for the aileron option which was on a separate sheet that had no text translations. I had to use a online translator to figure out what was going on. Once I got my head around it, it was pretty straight forward.



Next I'll pull the wing panel off the board to trim the false leading edge and to set the wing tube sleeves. This will involve joining the wing to the fuselage to insure a good fit and proper dihedral.

[Slappy]

Very cool. I'll be following with interest.

[ingrahal]

Hey Steve, hope all is well with you. Thanks for joining along.

Thanks to you too Slappy. hope you enjoy the build.

[ingrahal]

Update.

Here I have drilled the fuselage and inserted the wing rod tubes. Then I mounted the wing and supported it for a nice flush fit to the fuselage and CA glued the tubes in the wing temporarily.



Now with the tubes in place I epoxied them in after filling all voids with scrap wood. You have to get a little creative on the diagonal false spar.


All of the shear webs have been glued in and it's ready for the top sheeting.


Top sheeting glued in place with Titebond II.


That's it for now. Next will be the rest of the top sheeting and the leading edge. Then I'll tackle the aileron and spoiler.

[ARUP]

Nice! That Cirrus is a good looking sailplane!

[lenb]

Art,
Tell me when you're done and I'll ship my next project to you to build. Beautiful work, very clean and thought out.

[ingrahal]

Hey Michael, I agree. Guess I should put a photo up so folks will know what I'm building.

[ingrahal]

Thanks Len!!! Well, Larry ropes me into these deals but I make sure I get an airplane out of the deal.

[ingrahal]

I got a little bit done today. The sheeting, leading edge and wing tip are all done.



Next are the spoiler and aileron.

[ingrahal]

Ok guys I need some advice. I haven't worked with the mechanical airbrake type spoilers before. What size would be appropriate for this sailplane (3 meter w.s.)? I have a set of Multiplex 350 mm spoilers and can use them if they will work. Also when installing how do I tell which is right and which is left? I don't see any indication on these.

[lenb]

I can't help with sizing, but the flat blade side is the front, the black arms are the rear. The mechanism typically points to the fuse.
In this picture the bottom spoiler is for the left side.

[ingrahal]

Thanks Len!!!

[ingrahal]

Got the aileron made. This is a very custom made piece. I used my homemade thickness sander and some tapered stock along with a little creativity to mill the aileron. This minimized the hand sanding.


I'll be taking a break from this project to work on the DG 808S club plane. That shouldn't take too long though.

[ingrahal]

Ok I'm back on this project. I've been working on the right wing panel.

Both wing panels are done except for the spoilers.



I stuck the wings on the fuselage to see what the 3 deg. dihedral looks like. Looks good to me, just like the full scale. The original models had 6 deg. and looked a little odd to me.



Next will be the spoilers.