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Aircraft Builder's Frequently Asked Questions
(Builder FAQ and How-To Section)

The FAQ section is intended to answer commonly-asked questions and provide a forum to share helpful techniques. While we obviously can't cover every topic here, we would like to hear from you on anything you wish to know. If it's of interest to others, the question and answer will likely be added here. Remember, there is no such thing as a dumb question, except the one that doesn't get asked... we're here to help. Send us your questions!

Currently showing the General Info / Wings category (last updated 11/24/03)
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  · Hale used to offer routed spars. Why are they no longer available?
  · Why are laminated spars stronger than solid wood?
  · What are the tradeoffs between built-up and routed-ply wing ribs?
  · What are the tradeoffs between aluminum and ply leading edges?
  · I bought a project and the fuselage is an inch shorter than the plans. Is this a problem?
  · Who is Steen Aero Lab?
  · When will parts kits for the Pitts Model 14 be available?
  · When will plans for the Pitts Model 14 be available?
  · I understand the Skybolt Radial drawings have been done for some time. It seems you guys are holding back on them. What's up?
  · What are the historical engineering changes for the original Skybolt?
  · Can the new three-piece wing be used with an existing Skybolt fuselage project or rebuild?
  · Can an existing upper wing be adapted for use in the three-section wing?
  · Can an existing wing set be used to construct a three-section wing for the Skybolt Radial having a 2250 gross weight?
  · What are the advantages of the new three-section wing for the Skybolt?
  · Can standard Skybolt wing wire sets be used with the Skybolt Delta or Radial.

Hale used to offer routed spars. Why are they no longer available?

We don't believe they are safe. The structural analysis Hale had done on the routed spar only addressed the major axis of the spar. Indeed, the analysis shows that the spar is not significantly altered in strength along the major axis. Routing will substantially reduce the stiffness of the spar in the minor axis however, as most of the stiffness is provided by the material at the surface in skin effect. The flying wires place substantial compressive column loads on the top wing spars, and any fore-aft flexure of the spar will substantially reduce its column strength and resistance to buckling. We simply cannot approve the use of routed spars in any of our airplanes. The modest weight delta is a small price to pay for peace of mind. Remember also that the wing carries it's own weight - the weight of the wing on the wing is somewhat free. Weight saved in the wing and added to fuselage in fancy radios is certainly not.

When we determined these things, we notified everyone who had obtained routed spars or routed spar wings from Hale and told them we didn't consider them safe, and that the spars already in wings should be replaced. Elsewhere in these FAQs is a description of a process we use to replace damaged spars, and its fairly easy once you get the hang of it. The level of effort is a small fraction of building a new wing.

Why are laminated spars stronger than solid wood?

Any natural grain has a weakest bias and will fail along the weakest grain in a catastrophic manner. The weaknesses in the grain structure line up. With several laminations, each plank has its own weakest bias, and they are unrelated to each other. (We deliberately swap ends and sides of each plank). Destructive testing has shown that each plank fails separately, and the failure is incremental, not coincident. One-piece test sections in comparison simply explode at failure. Another way of looking at this is to envision the complex matrix of grain structure in the laminated cross-section. This lack of "grain in alignment" also explains why laminated spars remain absolutely straight over time, temperature, and moisture.

What are the tradeoffs between built-up and routed-ply wing ribs?

Our humble opinion, built-up is the way to go. Built-up ribs are lighter, stronger (not that ply ribs are unsafe), and they are the classic method of construction. Routed ribs use a bunch of expensive plywood, but routing is a quick, relatively low-labor method of producing ribs.

What are the tradeoffs between aluminum and ply leading edges?

We much prefer ply leading edges over aluminum. The expansion coefficient matches the wood wing, so slip-joints under flannel are not necessity to prevent oil-canning and dimpling on a hot day. Aluminum LE's are also very delicate, and a small hanger bump will leave a permanent dent.

Ply leading edges also stiffen and strengthen the wing. If glued-up properly, the ply LE forms a "D" box section along the front spar, which is rigid and very strong.

  Steen Aero Lab      1451 Clearmont Street NE   Palm Bay, FL 32905 USA     
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