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
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
That's why I don't encourage aerobatics.