Wing spar bushings

[Larry Lyons]

I have just acquired newly built spars. They are beautiful, a work of art! However one spar (and one spar only) has been drilled and fitted with bushings. I had already decided some time ago to do this mod. as I thought I would have build to my own spars from scratch. Now I have this set that the previous owner has started with the bushing mod. and I want to continue. My question is what are the specs. of this modification, what are the bushings made of, etc? I know it is to increase the gross load, however I do not plan on applying for that. I just want to do it, as right now, it is much easier than it will be later after the wings are rebuilt and recovered. I believe the paper work includes welding a small plate on the wing attach fittings too. I really want to do all of these things just so if someone in the future wanted to upgrade the bird, all of the internal work is done. It will also give me a bit comfort when I am inverted pulling out of that botched barrel roll. Another words is this an STC or something else, and can I just get a copy of the paper work, do the mod. and note it in the log book and not change the gross loading, or do I have to buy the whole kit an caboodle, (certificate, whatever)? I have no plans on changing the Cont. 65 to anything else.
Thanks, Larry

[T.Taylor]

Re: Wing spar bushings

Hi Larry, I see the factory lists a couple of spar bushings #2570-4 and 2570-7 I don't know if those are the larger ones or not. You really shouldn't be doing any welding without some kind of approval, like the STC. the FAA has tried to stop people from "getting copies" of STC's and making modifications, that is why when you purchase the STC it comes with a letter showing N number and serial number of the aircraft it is to be used on. Best to you, T.T.

[VictorBravo]

Re: Wing spar bushings

The bushings are standard "Micarta" or phenolic composite bushing material. ACS sells it and so does McMaster-Carr and many others.

It would be kinda weird to have one wing using the spar root bushings and the other one not using them.

While you have it all apart, I strongly suggest you do the other bushing and the modifications, even if you never plan on going to 85HP. It will be very easy to do when you can just put the spar in a drill press and bore out the hole. Use a Forstner Bit because it supposedly makes the cleanest possible hole. (an 11/16" Forstner Bit is not a common thing to find BTW)

If I had the luxury of the spars unassembled into a wing, I would drill the hole halfway through from one side and turn the spar over... then drill the other half of the hole. This means you would never get an "exit wound" where the drill pokes out the other side.

The STC tells you to weld the little plates onto the four upper strut fittings. Again, this will be easier to install and deal with while you have the wings apart in kit form. I had to cut up a rib to get the newly welded fittings onto the spars (working on an assembled and covered wing)

Forrest and perhaps others disagree with my preference for gluing the bushings into the spars with structural epoxy. I say it makes the spar stronger, the bushing cannot act like a splitting wedge under bolt loads. They say don't glue in the bushings. You choose what you think is best... either what seems safest or what seems most obedient to the STC paperwork.

In strict legal terms you must purchase the STC, which comes with your N number and serial number, in order to install this modification. In mechanical terms (which the aircraft itself is more interested in), the bushings and welded plates can be added now and if you ever want the STC you can buy it and the difficult parts have already been done. Realisticly, nobody should bother you if you put spar bushings in and welded the fittings but still kept your 65HP engine and your 1200 pounds.

If you truly want to keep the options open for a future buyer to upgrade to the full STC, then while you have the airplane apart you will also want to solder in a bunch of additional tube nipples in the fuel system, you have to add two wing tanks and associated plumbing, run some more fuel vent lines, and other stuff. If you think that a future buyer will want to have the 1500 pound gross weight, then you would want to weld in a couple of tubes here and there while it's uncovered.

With a fair amount of certainty, I can say that the reason yo u don't want to go to 85HP is very likely that you haven't flown one with 85HP yet

[flyguy]

Re: Wing spar bushings

Larry,
There are people who have the STC, but have never used it, and of course they have all the drawings. I would suppose if the information was going to just be used to do the mods without leagalizing them it might be ok to share such information. Why do I feel there is something wrong with that statement? Oh well, someone will let me know. Chuckle.
My 85 is only a 1200 lb version, if you get my drift.
Darryl

[Ray36048]

Re: Wing spar bushings

Grizzly Tool has the 11/16 bits.

http://www.grizzlyimports.com/

[Ragwing nut]

Re: Wing spar bushings

If the hole is already drilled, then a forstner bit is not the correct tool as you will not be able to center it. You will need to get a piloted end mill to make sure your holes are dead on. The bushings were installed at the factory on later s/n's so no paperwork should be required to install those. The wing attach fittings should already have the strap if you have a 46 or later. The strut attach fittings is not to difficult to change later if you provide inspection plates.

[Larry Lyons]

Re: Wing spar bushings

As I have a "41" I do not have the bushings, I did not have the bushings in my old spars. I just wanted to have that upgrade in my rebuilt bird. If I could document it enough for someone to use later I would. That was the only reason for looking for the STC, just so I could make sure I did everything I needed to do. My problem is I do not want the increased gross nor do I want to add a left wing tank and/or a bunch of fittings. My goal is to keep her simple (light) but do some of the things that would help the over all flight characteristics, i.e. increased G loading. I plan on doing simple aerobatics with her and just wanted to do the bushings and attach fittings because I knew they were a part of this. That is why I was looking for the STC to see what else I might be missing and what I could/should add. However, I do not want to add any weight. My goal is to be less then 700 lbs empty when I am through with the rebuild. Yes Bill, you are right I have not flown in a 85hp T-Craft. Maybe I should be at least looking at that option, but I am kind of old school here and would probably opt for a new/different bird rather then modify this one. This one, was and is a complete plane, no mods., just way out of flying condition. If I did want to make that change where would I start to acquire the proper paper work to do so, even if it is to be much later? Another words I have no problem buying the STC, if it is necessary, required, or helpful towards my goal of having a desirable bird when all is said and done. Another T-Craft I am helping on with a rebuild right now and will be flying long before mine is, is getting an increase in HP as we speak, so the flight characteristics are still unknown to me. Larry

[VictorBravo]

Re: Wing spar bushings

My goal is to be less then 700 lbs empty when I am through with the rebuild.

Yes Bill, you are right I have not flown in a 85hp T-Craft. Maybe I should be at least looking at that option, but I am kind of old school here and would probably opt for a new/different bird rather then modify this one.

Larry

Larry, consider that the T-craft would not need any significant external changes for 85HP at all. I believe there are quite a few field approvals and 337's for the 85 engine with only the bare minimum modifications (the strut fitting plates and the spar bushings and the -6 fuel line). The full STC requires a lot more work and is not in keeping with what you want to accomplish, so I believe the field approval route is the easiest/cheapest/quickest move for you.

There is no reason you could not be under 700 pounds with an 85 by the way, but getting any T-craft under 700 is a significant effort with two doors and two yokes. The performance with 85HP makes the 700 pound goal less important. For what it's worth, opting for a new bird as opposed to the light to moderate effort it would take for the 85 does not make sense IMHO. Upgrading to 85 with the airplane apart is a piece of cake compared to doing it on a flying airplane (trust me).

Based on personal experience, the easiest way for 85HP is to

1) Get a copy of someone's field approval for 85/short mount/minimal mods, go to your FSDO and negotiate with them on approving it

2) If that does not work, get a DER to sign off on it (for 1280 pounds gross weight by the way... matching option 1 of the STC), based on the known fact that the strut fittings and the spar bushings are already FAA approved data to bring the structure up to 1500 pounds. This should be an inexpensive and hassle-free thing to do because it is already approved data and no additional risk for anyone.

Do the strut fittings and the spar bushings, put in a -6 fuel line from the gascolator to the carburetor, have at least one wing tank, and you should be good to go.

You will not have to make any changes to the baggage compartment, if you have one wing tank you're fine (no more fittings), you don't have to screw with the cowlings, etc. etc.

If you happen to have Eisemann magnetos on the 85, hammer in a small depression on the firewall where the mags are so they fit easier (don't ask me how I know this!!!).

You can also split the difference and upgrade your 65HP engine to 75HP, once the right paperwork is found per another thread on this forum. That would require zero airframe modifications, but since you want to do the bushings and the fittings anyway it's a moot point.

Bill

[astjp2]

Re: Wing spar bushings

They cannot do a field approval anymore from 65 to 85, power changes are limited to less than 10%, that was a change from a few years ago that now requires some form of an STC. Tim

[Larry Lyons]

Re: Wing spar bushings

I have found the phenolic tube at McMaster-Carr, however there are several choices and costs! Any leads on what would be appropriate would be appreciated. Larry