Spars - Wood or Metal

Everything about your Aeronca, not Chief or Champ or Sedan specific.
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hangerash
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Spars - Wood or Metal

Post by hangerash » Tue Nov 25, 2014 21:22

Faced with the need to rebuild a wing (or two), I am challenged with the decision to use wood or metal spars. There are several sources for wood available and their prices are very reasonable. As I understand the AD, even with new spars the repetitive inspection of the spar is required. Metal is available from two sources and the cost is three to four times that of wood. Resale value is appreciably higher with the metal spar wings and validates the initial investment. One metal spar vendor claims higher airspeed reported when using wings converted with their spar. I would appreciate hearing opinions for both wood or metal expressed.

Paul Agaliotis
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Re: Spars - Wood or Metal

Post by Paul Agaliotis » Tue Nov 25, 2014 21:57

I like the wood spars. Much nicer ride and last much better than the metal spars.
I don't see much resale difference when compared equally. A 1967 Citabria with metal or wood spars commands a similar value. If you try to compare it to the ACA aircraft (they never made a wood wing) that isn't a fair example.
The metal wing also has a front spar fitting that requires a reoccuring condition inspection when using ACA wings along with new style struts. I don't know if the aftermarket suppliers have an inspection requirement for the fittings or just ignore it.
Do what satisfies you.
I would fly a million miles on wood it actually gets stronger with age, not so for the metal.
Paul
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MikeB
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Re: Spars - Wood or Metal

Post by MikeB » Wed Nov 26, 2014 09:10

I think the track record for wood spar wings (especially for the Aeronca) are pretty well proven. I've looked at several with 60 plus years on them and still serviceable. Ground loops and wing dragging the exception but the main inspection points are pretty well known, too. My understanding is the laminated (factory) spars are even more durable than the 'one piece' although anyone who makes their own will no doubt 'whittle' them out of a one piece. They're also able to be spliced and repaired within reason.
MikeB

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hangerash
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Re: Spars - Wood or Metal

Post by hangerash » Fri Nov 28, 2014 08:44

Thanks Mike and Paul I greatly value your input.

Paul would you elaborate on your comment "Much nicer ride and last much better than the metal spars"?

I just sold my 1966 7ECA that had metal wings from ACAC and it flew great.. Docile and positive handling. A broker that looked at it remarked he was only interested because it had metal wings.

My question now is are we too comfortable with working with wood so as not to investigate metal?

Is this like always working with one paint system because that is what we know instead of learning another that might be more ecologically friendly?

I had to replace the rear metal spar on my 7CCM Champ and it required drilling out a lot of rivets and the spar attach fitting. I must admit re installing the fitting had me worried because of the size of the rivets, but they can be easily removed it you don't buck them correctly.

I now am faced with a Chief wing that has the Milman spars and the front is severely damaged. I haven't taken it fully apart, but I suspect the rear spar also sustained some damage.

I respect your individual opinions so much I must reconsider my options at this point.
Richard

Carl Prather
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Re: Spars - Wood or Metal

Post by Carl Prather » Sat Nov 29, 2014 00:30

Hangrash,

I can attest to the ride difference in a Super Cub metal spar and a KCAB with a wood spar. I used to tow gliders and switch among four, sometimes five planes a day. While the Super Cub and the Citabria aren't a fully direct comparison. They are both semi-cantilevered tube-and-fabric airplanes with similar gross weights, horsepower wing loadings and wing spans and I was flying both types in a specific situation. The Super Cubs clearly had the harsher ride, something like a pothole effect. The Citabrias tended to "stretch" its response to turbulence giving a smoother ride, more a "wavy-ride" effect if that makes sense. All things being equal, Wood flexes more easily than metal (the material property is called modulus of elasticity) and it's pretty dang hard to get wood to fatigue. I'm going drive Super Cub guys crazy but apart from about 5 - 7 mph extra in final approach and departure speeds and 100 − 150 feet in runway length, the Citabria was just more civilized. The Citabria is bigger inside, has better visibility, easier to re-trim and less tiring to fly and ride than the Super Cub. In fairness to the Cub, Aeronca had the advantage of using the Cub as a benchmark for improvement.

After paying through the nose for an annual that included that spar AD before I could honestly ask Paul Agaliotis to ferry my Champ home, I found out later that the wood spar concerns were demonstrably overblown. Since then, I believe there are cases where AD and STC requirements represent more of a marketing opportunity than serving the interest of life safety.

Carl
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bob turner
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Re: Spars - Wood or Metal

Post by bob turner » Mon Dec 08, 2014 14:17

I own a Super Decathlon wood spar. One of my students owns a 150CS Decathlon of the same vintage with metal spar wings. I suspect that it would sell for ten grand more than mine.

My impression is that the ride is the same so long as we are talking about straight and level. The metal spar climbs almost as good, is faster in cruise, and much more sensitive to control inputs.

If it were economically feasible, I would go metal spar in a heartbeat. The yearly inspection takes a half a day, and is simply painful. I know I am not going to find defects, but still I feel obligated to follow the instructions in the AD as closely as possible. For those not having a mechanic's license, a proper inspection should cost upwards of a half grand.

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Re: Spars - Wood or Metal

Post by Aerco » Wed Dec 24, 2014 12:21

I find it difficult to believe that a metal spar would have the slightest effect on speed, given that all else is equal. If there is a slight weight difference, it would be almost impossible to detect this by the effect on speed. Even if you can make direct comparisons, there are enough other variables that you cannot possibly attribute a faster speed to a metal spar.

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Re: Spars - Wood or Metal

Post by bob turner » Fri Dec 26, 2014 12:11

I agree with you - just having a metal spar should not make any performance difference. What I meant was, this particular airplane, with a smaller engine, is faster than mine, is much more responsive in roll, and climbs almost as good. Which reminds me - I ought to fly it today - it sits way too much.

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