With Reno Biplane Top Qualifier Tom Aberle's prop problems' getting a lot of air time, we thought there would be merit in having a look at what might have happened. The best man to explain that, we figured, would be prop maker to the stars, Craig Catto, whose propellers, wings, and composite tail groups have been used by any number of champions, NASA programs, Rutan and Macreedy projects. They're on the factory Velocity airplanes (the fixed-pitch ones); Dave Lind's Long-EZ (below) has run 235 mph, with a Catto on the back end of an O-320; Formula 1 guys use 'em -- they're everywhere. Craig's props were also on the front of Tom Aberle's racer, Phantom (right); and they didn't last there. Why?

Craig said that there hadn't been a lot of high-speed testing, prior to Reno: "Tom really hadn't gotten a lot of time on that airplane," he said. "We didn't have a lot of test time on it."

Besides that, Aberle was fast. "He was running just under 3600 rpm. The props were redlined at 3600," Mr. Catto noted. (Design rpm is 3200-3400.) "He was turning 3560, straight and level. 242 miles an hour, straight and level."

What does that do to a prop?

"Straight and level is no problem, it's when you roll into the turn around the pylon --It goes into cyclic bending -- we're basically talking P-factor. When you speed that cycle up, that really amplifies bending loads on these propellers; and he was screaming."

Here's one of Tom's props, at Reno '03:

How much screaming? "[The prop tips were going] Mach .93, in a straigt and level condition," Craig said, "Then you add the Gs, or pulling around the pylon -- add the cyclic bending -- it's creating a supersonic shock wave during a portion of the rotation." Is that bad? "An airfoil at Mach .93, angle of attack 5 degrees, is, from an efficiency standpoint -- that's a big no-no," he explained.

Once a shock wave forms, the center of pressure shifts, adding torsional loads as well as the bending loads to the prop.

Catto continued, "This can induce hi-frequency torsional bending on the propeller. It's changing pitch all the time, back and forth -- 0 to 4 degrees, back and forth, twice per revolution. That's over 7000 times a minute (116Hz). Those bending nodes -- at that frequency, a tremendous amount of heat builds up, just like it does when you're bending a piece of metal -- to the point where the epoxy matrix itself breaks down."

It didn't happen to everybody, you know.

[Gold winner] "David Rose -- he was running our prop, the same prop for the past three, four years. He had no problem." What was different with Aberle's prop, or setup? "The longer propeller [on Aberle's biplane] did [have a problem]... there's a possibility it had a peculiar harmonic."

The props are on their way back to Catto's shop, where a detailed analysis will yield some more-definitive data. For now, though, Craig notes, "I'd never recommend running a prop that size [64" diameter (with 70" nominal pitch)] at such high rpm. Aberle ran 3560 [rpm] straight-and-level. In testing, it looked like it would be OK." As your mother would ask, 'Did you learn something?' "I would never recommend it now," he said.

"No one has run a prop that long [that diameter], at that speed, for that long -- Tom's the first to have done it."

So, Craig said, "We're going to have a look at the props when they come back in."

Note: Other Catto props did well at Reno: in addition to bipe winner David Rose's, there was Jason Somes's prop, that ran 187 and change, on an Pitts. Scotty Crandlemire's F1 racer Outrageous (right) took a 5th in the Gold this year; and Frank Jurant's Mong, Bada Bing! was 10 mph faster, with Catto's new design. [Note: Frank averaged 185, taking second in Silver, by 2/10 of a second. Frank's old Catto design -- the one he loaned to Tom Aberle -- was two years old --ed.] Other biplane props were Steve Brown, Mike Farb and another fast Mong (the yellow job, above).

FMI: www.cattoprops.com