Re-covering a Citabria

J

John Franklin

New member
Greetings,

I'm a new member and also new to working on full-size airplanes. I'm going to embark on re-covering a 1966 Citabria with its owner and need to get some publications or website links that have information on covering aircraft. This Citabria has wooden spars and we may convert to aluminum ones.

Also, I live in the Houston, TX area if anyone knows of someone who is doing aircraft fabric work, please let me know. I realize that hands-on experience is the best, but I would like to read up on it first. Any help would be appreciated! FWIW, I have built and fabric-covered dozens of R/C model aircraft.
 
Hi John,

Each system,Ceconite,ect.,have there own procedures that must be followed. Once you decide which system your going to use,you can purchase their manual or you can by one for each. I think you can get them from Aircraft Spruce or the system maker your going to use. Another good reference is the FAA's 43.13. Hope this helps.

Woodie
N29763
 
John

I'm curious as to why you are thinking of changing out the wood spars, are they damaged? If not, don't buy into the metal spar hype being fostered by ACA. The wood spars are just as good, maybe superior to the metal.

Ron86654
 
Woodie / Ron,

Many thanks for responding...first, we have decided to recover with Polyfiber and I have found a wealth of info on that system, plus they make a kit that you can cover two ribs to learn on.

Ron, since my first post I've done some research but don't have all the answers. The guys that sell the Al spars tend to scare you about the cracks, but I tend to agree with you that wood spars are OK if they haven't been damaged. One thing I've noticed that has been ignored in the argument is the fact that the load on the spars decreases the closer you get to the wingtip. Model airplanes have taken advantage of that fact for a long time, but it appears the AD doesn't cut you any slack on inspections on any part of the spars. (FWIW, I'm not an aeronautical engineer!) However, the AD is the exact reason we would consider the Al spars, in fact if the owner is forced to put inspection covers on the top of the wing, he may decide to use the Al spars just for that reason. Once we get the old fabric removed we will have a good look at the spars and this plane did suffer a bent strut long ago from a low-speed bump into a fence. The plane has done ZERO aerobatics since 1974 or so.

So, I guess when we get the wing stripped we'll be able to make a much more informed decision, but I do appreciate and seek others'
opinions. Thanks again!
 
Another option is the Milman spar. Aluminum, so you don't have to worry about the extra inspections holes. Downside...it doesn't give you the 100 lb gross weight increase. And apparently, they fit right in without any modifications. I still have my wood spars, but that's what I've heard about Milman.

Erik
 
John,

Check all the great information on the wood spars and inspections methods that are on this site. One of the most informative and helpful sites for information on the AD.

Ron
 
You don't need the top inspection holes. I don't have them. Get advice on that from a mechanic with a lot of citabria experience.

Also, the millman spars are about 1/3 to 1/2 the price of the new ACA ones. One more item to consider, there have been many complaints about the paint on the ACA aircraft. Be careful if you buy the wings from them already painted.

Dave
68 7ECA
 
You may also want to consider simply rebuilding with wood.

I did, the airplane is much lighter than it would be with aluminum, and it is much nicer to fly when the weather is a little bumby, the bumps are not so hard edged.

Alot of folks I know consider the weight savings of spruce to be a good feature from a performance perspective.

My 7KCAB with wood has a useful load of 520 lbs. The new GCAA, which should be a little more load because it does not carry the extra weight for inverted fuel etc...only has a useful load of 550 lbs.

So, with the aluminum wings, a 100lb increase in gross weight only gets you 30lbs more in load. I know people claim better climb, but fully loaded I have never seen less than 1200 FPM....

Also keep in mind that the load increase does not use a different landing gear, or fuselage, so you have an extra margin on those parts if you keep the old weight at 1650.

Ahhhh the wisdom of the original engineers is beautiful...maybe spruce is an ideal material, combining light weight and strength!
 
We recently rebuilt our 7KCAB wings with the Milman spar kit and all I can say is we feel it was money well spent. The aluminum spars are 75% stiffer and several times stronger than the wood spars and you don't ever need to do the compression crack inspection again. There are no rib nails to pull out and the spars slide in without completely disassembling the wing structure.

The plane for some reason also seems to fly about 3-5 knots faster...go figure. Maybe it was the aerothane that we could afford because we did it locally.
 
75% Stiffer? Bravo Sierra!

I know that alot of people think that aluminum is stiffer dimensionally, but it is not true, notice that none of the spars give you an increased acro load ability.

Some of the strongest wings today are still built on sitka spruce spars..Pitts, One design, Eagle, Viking, etc...
When you speculate and circulate information which is wrong you scare others about planes that are perfectly airworthy.

How about the wooden spacer blocks between your spar and ribs? Will that hold moisture against the 2024 AL? If it does will it start a pit or corrosion?...how will you inspect that?

If you recently recovered the plane you could have installed inspection covers that would make inspection simple and easy...when you do the AD adds no expense to the annual.
 
I agree that the wood spar inspection can be done without adding significant cost to the annual. However, the easier you make the inspection, the more likely you are to find cracks. Any crack fails the spar. Believe it. I spent five hot months rebuilding with Milman spars after having my wings condemned for what was probably an insignificant blemish. In fact, it was not even visible during the wetter months. (Wood spar owners take note: If you don't want to see cracks, don't schedule a summer annual.)

Bravo Sierra? That's a service bulletin rubber stamped into an airworthiness directive as a form of corporate welfare. Ok, ok. ACA probably didn't twist any congressmen's arms, and I'm sure they don't even show up on the pork barrel radar. But come on... How many wood spar failures are we preventing here?

Wood is an incredible, natural composite material, and obviously it makes great spars. But if you're going to inspect per the AD, it may cost a lot more than aluminum.

Anyway, I'm quite happy with my Milman sparred wings and the wrap-around leading edge. How much stiffer are they? A lot. Am I worried about corrosion and pitting? No, there's enough aluminum there to last until the next recover. Do I miss the wood? 1) No, I'm going to make it into a coffee table. 2) No, I miss the summer I could have spent flying. Does the aluminum fly better? After five months grounded, I don't even remember what the wood felt like.
 
Some people my think that wood can keep with aluminum. I am a ME and ran the numbers on the Milman spars. They are, per the numbers, 75% "stiffer" which agrees with what Milman told me before buying the kit (meaning less deflection). If you run the strength numbers you will find that it takes an 18-inch deep wood spar to be as "strong". Which means they are several times stronger than wood. Milman does not even mention the strength increase since the FAA apparently rates the increase capability in terms of deflection under a test load. Some my think this is "Bravo-Sierra" but I feel a lot better with my aluminum spars.
 
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