A second tale of resonance testing, with sadly no appropriate SETP banter.
A few years ago, a TP (not me, but working for me) was minding his own business conducting what passed for high speed testing (somewhere around 80kn I think) in a low speed STOL bird. He may or may not have oversped the prop slightly (the debrief was slightly uncertain on the point), but anyhow was rather disconcerted to find the cockpit filling with fumes that smelled rather like burnt plastic.
He took the normal actions you'd expect of a grown up TP, but nonetheless managed to get it back onto the runway without shutting down or diverting. On inspection, the propeller spinning and the root of one blade had combusted in flight. For those who missed it THE PROPELLER SPONTANEOUSLY COMBUSTED IN FLIGHT.
It's worth understanding the characteristics of the powerplant on the machine. It consisted of a direct drive 4-stroke petrol engine with max.RPM of 3,300 (there, that's identified it, but never mind) driving a 3-blade ground-adjustable high inertia composite prop. Unusually for any piston-prop combination, the engine incorporated no torsional shock absorber.
Anyhow, after a great deal of peering through microscopes, doing sums, interrogating operators of anybody who'd ever had to replace a prop fitted to the same engine type, and just plain sitting down over a mug of tea and saying "what the ***** happened ????", we reached the conclusion that an engine resonance had co-incided with some resonance of the prop which had caused microscopic level heating.
So, in the time honoured practice of bewildered and confused engineers, we decided to do some experiments, thrash around with the data, and see what we got. My "lab equipment" (actually it was all done out in the middle of a field where I could minimise noise nuisance to anybody else) was...
- Front end of a ground-test airframe, on tyres, but nonetheless securely bolted down.
- Serviceable engine
- Half a dozen different prop types.
- Ready supplies of AVGAS 100LL, unleaded MOGAS, leaded MOGAS (the latter was still in use at the time).
- A couple of incredibly sensitive, high-rate, low-mass accelerometers bonded to bits of the engine with superglue.
- A very high rate data-logger, feeding a PC.
We then spent a thoroughly entertaining day in our field, wearing ear defenders, running the engine throughout it's available power range (often briefly, since it didn't cool all that well static, and we hadn't the facility to put extra wind through it). This was done at pretty much every combination of propeller type/pitch/fuel that seemed likely.
After a day of testing, was then some weeks of analysis, passing each data set through the ubiquitous FFT analyser, comparing plots, and trying to make sense of it.
The result was that we (I !) identified a combination of conditions (unleaded MOGAS, high tip Mach No., high inertia propeller) that could lead to severe HF spikes in the frequency spot. This led to some RPM restrictions on the engine, which would avoid the problem, and some advice on prop type and fuel selection for maximising engine life.
A different approach and problem to the Tucano bonk test / boom issue. The underlying issue are however.
- Much vibration testing is done after problems have been found.
- Analysis and reporting takes much longer than testing.
- Anybody undertaking the testing MUST be very hot on their analysis tools, and up to speed on the relevant maths.
- Whatever the Bode plots may indicate, real resonances (as measured rather than predicted) tend to occur at disrete frequencies.
G