Graupner Cirrus _ How would you build it today? - RC Groups
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May 15, 2008, 04:29 PM
OpenTX University Staff
maguro's Avatar
Discussion

Graupner Cirrus _ How would you build it today?


I built a Cirrus in the late 70s and loved it. The plane was built absolutely stock. I didn't even add ailerons. She did well in local competitions too. Sometime in the 80s a friend gave me a Cirrus kit in return for my building a different plane for him. The kit has been sitting in in the shop waiting for me to do something with it. It has been waiting nearly 30 years, and I think I am ready to build her.

I know of a lot of Cirrus mods people have done over the years, such as adding ribs to extend the wings, increasing the dihedral or building polyhedral into the wings, adding ailerons and flaps, etc. These mods were all done before anyone ever heard of carbon fiber and Kevlar. So here is my question to all of you. Given the materials we have available today, the light powerful servos, etc., how would you build the Cirrus? What would you do to improve it's performance, or correct it's weaknesses, without destroying the beauty of a real classic?

I'm hoping this thread will generate some interesting discussion. Please contribute your ideas and opinions.
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May 15, 2008, 04:51 PM
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Ib Jensen's Avatar
Interesting thought. I built a Cirrus in the early 70s. It was my first radio control model. I was told I should have built a polyhedral ship for my first, but I loved the looks of the Cirrus.
I found the structure fairly robust, but the fuselage in my kit was made from ABS and it was very brittle when the temperature got below 40 deg. All of our launching was with a hi-start so there was not a lot of stress. The wing rod deflected a lot on takeoff so carbon might be a good substitute. I was always putting my finger through the balsa sheeting at the root while assembling or disassembling. I fill the first rib bay with foam and that fixed it.

Ib Jensen
May 15, 2008, 05:02 PM
daveosoar
I still have mine - now much modified! Surely the fuz is PLURA not ABS, and very resilient.
Ihave incorporated ailerons with an Hitec81 in each wing, stiffened the wing joining system, lessened the dihedral to give only 5degrees at rest (requiring a little attention to tip stall tendency) and it still flies as well as many 'modern' mouldies. A real classic!!
Daveosoar.
May 15, 2008, 05:10 PM
yyz
yyz
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Maguro,

I have Cirrus on the board right now. Like most Cirrus owners, I have very fond memories of the Cirrus I had as a kid (paid $70 of hard-earned lawn mowing money for the kit!) so I decided to build mine pretty close to stock. I compromised a bit by cutting foam wings and stabilizers but am sheeting them with balsa and keeping the original airfoil. It will definitely have ailerons but I'm undecided on whether to add flaps or just go with spoilers.

The NEXXXXXXXT one, however is going to have a more modern airfoil (MH32, HQ, etc) and have flaps and ailerons. The stabilizer is pretty oversized on the glider so you could probably trim that as well.

With the already-slender (low cross-section) fuselage and a modern wing, I bet the "old girl" would really be impressive. The epoxy glass Cirrus fuselages I've made (molded from the original) are much lighter, and I would imagine stronger, than the original plastic ones so there's definitely some areas where you could get some weight savings from the original design.

That fuselage seems pretty well sized to the 3 meter span so I'm not sure how much more span you could add without giving up on the classic handling characteristics of the glider.

I have an original set of plans and it's hilarious look at how they had to wedge those old radios in. The modern flight packs could swim in that fuselage.

Mike
May 15, 2008, 05:24 PM
OpenTX University Staff
maguro's Avatar
YYZ, Post some pics of your Cirrus. Heck. Everyone who has potos of their Cirrus should post them. What is a discussion of a rare old bird without photos?

I would love to see one of your glass fuselages. I'l bet they look nice.

I still have some of those old radius. Do you remember the Blue Max? I built my first 6 channel radio from a Blue Max kit. Still have it.
May 15, 2008, 06:12 PM
yyz
yyz
Registered User
yyz's Avatar

Couple of photos


Attached are a couple of photos of the Cirrus fuselage. If someone is interested in building one with the original airfoil, the SPICA is about as close as I could find. Needs to be thickened to about 12-13%:

http://www.ae.uiuc.edu/m-selig/ads/coord/spicasm.dat

Mike
May 15, 2008, 09:19 PM
OpenTX University Staff
maguro's Avatar
That Fuselage is beautiful. You do very nice work.
May 15, 2008, 09:50 PM
yyz
yyz
Registered User
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Quote:
Originally Posted by maguro
That Fuselage is beautiful. You do very nice work.
Thanks for the compliment. That particular fuselage and his brother had tragic UPS "incidents" that resulted in insurance claims. Never mark a long, thin package "Fragile". I think both were used for batting practice at the airport.

Mike
May 16, 2008, 02:36 PM
Registered User
adodad's Avatar

Cirrus today


Maguro:
I'll give you my thoughts for what they are worth...I'm beginning a build of an old Graupner Cirrus kit I recently acquired. I built two of these as a teen in the early 70's. I'll build mine with ailerons with hs 125 servos in each wing driving the ailerons with a RDS (internal, no control horn, Rotary Drive System) system and gapless hinges. Dihedral will be 4 degrees. I fly from a hi-start so won't add any carbon but will increase the spar to 1/4"x1/8" and change the wing rods to 3/16" (I think the kit supplied rods are 4.4mm). Would like to increase spar to 3/8"x1/8" and use a hardened 5/16" main joiner (straight rod with dihedral built into the wing tubes) but this would require a major rework of the bulkhead in the plastic fuselage so is not worth the effort (If I had one of those cool glass Cirrus fuselages this would be done in a heartbeat!). I considered starting with the stock Cirrus airfoil at the root and blending to a S3021 at about 9" out from the root but decided this was too radical (too thin to give necessary strength with this type structure and too much change in too little span). As a compromise I will blend from the stock airfoil to a 12.96% thick Clark-Y (true Clark Y, not "flat bottomed"). This should improve the speed range without changing other flying characteristics too much. The resulting thickness is similar to the stock airfoil to provide necessary strength to this high aspect ratio wing. I think it best to use the stock airfoil at the root to match the profile built into the plastic fuselage to keep things "clean" at this critical area. I'll also incorporate a pair of Graupner "blade" style spoilers driven by a HS51 in each wing. That's about it. Just enough change to have my fingerprint on it but not so much that it won't look every bit like a stock Cirrus. It has such beautiful lines and is a thing of beauty in the air. It will never perform like a modern moldie but it is what it is.
May 16, 2008, 03:04 PM
OpenTX University Staff
maguro's Avatar
Adodad, you have some very nice ideas indeed. I would love to see a drawing or some photos of that aileron setup.

I know exactly what you mean about the fiberglass fuselages that yyz made. As soon as I saw the photos, it was lust at first sight. What a heart breaker to hear they were destroyed.

Hey yyz, do you have any plans to make more? I'm sure you could be kept busy making them. I would like to be first on the list.
May 18, 2008, 12:01 AM
Registered User
I recently aquired a Graupner Cirrus, built complete but with minor damage,
The fuse is no problem some sanding and a fresh coat of paint and its good to go. But, The canopy has some irrepairable damage, The bule tinted plastic ripped and the ABS frame is warped and broken. I can build a new frame from 3/32 or 1/16th plywood and I can (I think) build a plug to vacuum form the new canopy, but would rather not. Has anyone out there pulled a few canopies and be willing to sell one or two. Or have an idea of an easier way to do it?
Last edited by Geyserflyer; May 18, 2008 at 12:03 AM. Reason: spelling
May 18, 2008, 12:04 PM
Kurt Zimmerman ≡LSF 4461≡
kzimmerm's Avatar
Looking at that fuse I'd swear that it looks like an Aquila fuse.... just my 2cents.

Kurt
May 18, 2008, 12:09 PM
Registered User
I Identified it by the Graupner on the fuse just forward of the wing and the Cirrus decal on the vert fin
May 18, 2008, 01:44 PM
OpenTX University Staff
maguro's Avatar
It is most definitely a Graupner Cirrus. The Aquila has a much thicker and more square fuselage.
May 18, 2008, 11:26 PM
yyz
yyz
Registered User
yyz's Avatar
Quote:
Originally Posted by maguro

I know exactly what you mean about the fiberglass fuselages that yyz made. As soon as I saw the photos, it was lust at first sight. What a heart breaker to hear they were destroyed.

Hey yyz, do you have any plans to make more? I'm sure you could be kept busy making them. I would like to be first on the list.
Because it's the Cirrus, I could probably be convinced to make a couple of fuselages if folks are interested. My schedule is full through June so it probably wouldn't be until then.

To the poster looking for a Cirrus canopy, the Astro Jeff canopy is a good match. Ray Hayes used to sell them but I think another fellow has taking over kitting the Astro Jeff. Contact Ray and he could probably point you in the right direction.



Mike


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