The Pete Bowers Heresy

Posted July 2017

I was once contacted by a man who wanted to modify the Fly Baby design. "Can you re-run Pete Bowers' calculations and determine if this will be OK?"

I had to break the news that whatever calculations Pete did when designing the Fly Baby probably occupied no more than a single page, and that page (undoubtedly beer-soaked) probably hit the Seattle landfill almost 60 years ago.

As an aircraft designer, Pete Bowers was the world's leading aviation historian. Yes, he had a degree in Aeronautical Engineering. But it was a four-year program compressed to two years to meet wartime needs, and included a complete A&P course. Upon completion, Pete joined the USAAF as a maintenance officer, not a design engineer.

The Fly Baby wasn't engineered. The Fly Baby was *derived*. Pete started with the basic Les Long/Tom Story configuration and met design issues by copying similar features on other small aircraft. The Fly Baby spars are not 3/4" thick because of some careful calculation. They're that size because that's how thick the spars of most small aircraft are.

Pete had an encyclopedic knowledge of how hundreds of aircraft solved design issues...and, when necessary, incorporated those solutions into the Fly Baby design.

What happens? Well, usually, the design ends up overbuilt. The Fly Baby could stay somewhat svelte since there were similar planes to copy, but it STILL weighs a hundred pounds more than a Story Special.

That's what happened to Namu. Since it was such a significant departure from other designs, Pete overbuilt it. No way it "needed" an inverted gull wing. But since Pete had little to go on, things got way beefy.

The other thing that happens is unexpected consequences. The Gee Bee style wing bracing has been well proven on a number of aircraft. But Pete needed the flying wires to join at the wheel axle forward of the wing, not to a hard point at the mid-chord point. So on a Fly Baby, flight loads add stress to the wing internal bracing. That was the root issue that led to EAA Canada's recommendation for beefed-up internals.

So... when Pete waved his hands and said Fly Baby aerobatics are OK, that wasn't really based on a careful structural examination. That was based on what OTHER aerobatic airplanes did. So it's a recommendation for slightly thicker wing spars and slightly larger bolts.

The Fly Baby design has had a number of wing failure accidents...and none of them stemmed from too-thin wing spars or the use of AN3 bolts vs. AN4.

So when we discuss aerobatics in Fly Babies, we have to consider the context of the design. Pete didn't run a set of calculations to ensure that the structure was good for 6Gs and a 1.5x safety margin. The approval was ad hoc, probably, mostly to justify his OWN flying of aerobatics in the prototype.

At this date, there's no real evidence to point out to say that Pete was wrong or Pete was right. But while the causes may range from substandard materials to improper maintenance, the Fly Baby has suffered a lot of wing failure accidents associated with aerobatics.

That's why I don't encourage aerobatics.

Ron Wanttaja

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