Save the Mentor!- page 3 | History | Air & Space Magazine

Save the Mentor!

T-34 owners are the latest to prove the value of good old-fashioned American ingenuity.

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(Continued from page 2)

Other Beech aircraft-Queen Airs, King Airs, and Beech 99 Airliners-experienced fatigue cracking, and Saunders adapted his straps to all of them. When it became apparent that T-34s were going to need spar work, Saunders lost no time designing a strap for them; it is also suitable for Barons and Bonanzas.

Saunders Strap (Richard Fleck)
The Saunders Strap costs only $12,000 installed, and the down time, assuming that the required eddy-current inspection doesn't turn up cracks in the spar, is only two to three weeks. While no one questions the structural effectiveness of the strap-no airplane equipped with one has ever suffered a wing failure-some T-34 owners object to the ridge it produces along the underside of the wing; purists don't like the idea of tacking a conspicuous structural Band-Aid to the outside. And, though Saunders denies it, some say that the strap saps performance.

Earle Parks' Amarillo, Texas shop is equipped to rebuild T-34s from any condition. In addition to a huge inventory of spare parts, Parks has enough tooling to build an entire airframe from scratch-if it were legal to do so.

Parks Jig (Peter Garrison)
Parks had his own ideas about the T-34 spar. He had seen enough disassembled T-34 wings to know that there was some random variation in the size and shape of the many small parts, some of them shims and spacers to bring larger members into alignment with one another. Beech had eliminated the buildup of small parts when it designed a new spar that it has installed in new Bonanzas and Barons since 1973, and Parks decided that he could do the same. He replaced the inboard section of the shear web-the thin vertical element of the I-shaped main spar-with a sheet of heavier stock, eliminating the joggled lap joint that coincided with the location of the fatigue failure in the Sky Warriors airplane. He replaced the complicated buildup of small parts in the lower spar cap-only the lower cap is subject to significant fatigue-with a single long part machined from a solid piece of aluminum.

Replacing a spar sounds like a huge job. In most wings, skins and ribs are riveted directly to the spar, so removing the spar entails drilling out nearly every rivet in the wing. The Beech wing, however, is an unusual design. It consists of three separate assemblies: the D-shape leading edge, the main spar, and the main torque box, a sheet metal structure between the main and rear spars. The three assemblies are neither riveted nor bolted together; instead, they are joined by stainless steel wires, about the thickness of a wire hanger, that run the full length of the wing through interlocking piano-style hinges. To separate the spar from the rest of the wing, all you do is pull out the wires. In building the first post-World War II high-performance personal airplane, Beech seems to have anticipated that periodic spar inspections might be needed someday.

The FAA required that Parks perform a stress test to the 9 G ultimate load. Parks built a heavy steel fixture, put a wing into it, and pushed on the wing with a hydraulic ram. The steel fixture deformed, but the wing did not. Parks got his Supplemental Type Certificate.

The most technically economical response to the spar situation emerged from GAMI, George Braly and Tim Roehl's company. GAMI first did a computer survey of the T-34 spar, using the now-universal method called finite element analysis, and found a hot spot of concentrated stress at the exact point where the Sky Warriors wing had failed. Then, using electronic strain gauges affixed to wing and spar surfaces, the engineers recorded the structure's reactions to G-loads applied in flight. The results confirmed the computer's diagnosis.

GAMI gusset (Luke Kerr)
FAA Designated Engineering Representative Victor Juarez then designed a small, artfully tapered gusset that bridges the critical area, eliminating the stress concentration. The GAMI team hardened the perimeters of rivet and bolt holes in the affected area, increasing their lives several-fold. The FAA approved the modification without testing a wing, solely on the basis of extensive analytical documentation the company supplied.

GAMI intended to turn over the rights to the AMOC to the T-34 Association, but to Braly and Roehl's surprise, the association's board chose not to involve itself in the airframe repair business or to endorse any particular AMOC. So Braly and Roehl formed the T-34 Spar Corporation, which provides the required inspections and modifications at a number of sites for $14,000.

The costliest repair is offered by Nogle & Black Aviation of Tuscola, Illinois. Charlie Nogle and his son Jud are, like Earle Parks, longtime eminences in the T-34 community. The Nogles scrap the existing spars and replace them with one more massive and better made than the original. The new spar also provides a shear web that runs all the way out to the wingtip (the original T-34 web stops a little outboard of the landing gear). The full-length web looks stronger, but its real function is to provide support for supplemental rubber fuel bladders in the leading edges outboard of the standard tanks, whose 50-gallon capacity may not be enough for the bigger-engine airplanes.

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