The Fox story — gyroscopic loads
Magazine

The following article, written by Ian Shaughnessy, appeared in the May 2000 issue of Australian Ultralights and demonstrates one result of (a) mismatching engine and propeller, and (b) modifying the design.

Gyroscopic moments induced by the rotating propeller (and by excessive 'overhang' — the distance between the propeller cg and the forward bearing) are dependent on the rate of change in aircraft pitch or yaw, the rotational speed and the moment of inertia of the propeller. The moment of inertia depends on propeller mass and diameter. The gyroscopic loads are transferred to the airframe via the engine crankshaft, crankcase and mountings and under some conditions excessive gyroscopic loads may lead to crankshaft or crankcase failures. See 'Matching engine and propeller'.

This story starts when a friend, with exactly the same aircraft as I'd just purchased, had a 'sudden departure'. After a two-hour flight from Wentworth to Truro he lands, then his propeller decides to part from the aircraft.

What the hell has happened? Back in the hangar it was found the crankshaft has broken just at the back of the propeller boss. A quick analysis you may say.

Many theories were put forward, some based on hearsay, some on fact. On further examination of the broken crankshaft in the area of the break (pic 1 and 3) indicates that the fracture has been growing over some period of time. Why not ask the engine's maker — that seemed a good place to start!

After finding the phone number for Aeropower in WA both the aircraft owner and myself asked for information on the engine. Mick, the Aeropower owner, would have to be one of the most helpful people I have ever spoken to. His first question was what size propeller was on the aircraft at the time of the incident.

When he was told it was a factory fitted 60 inch propeller he automatically said "there is one of your problems straight away". He stated that the engine should not have any propeller larger that a 57 inch fitted.

He had many a discussion with the aircraft manufacturer about this. We mentioned the 2.5 inch aluminium extension which was added to the propeller boss (pic 2) so the propeller cleared the cowling. I cannot repeat in a family magazine, what he said next, but he again observed this was another problem that could have caused the crankshaft to break.

He said the gyroscopic effect on the area where the break happened would have been exacerbated by the propeller being larger than recommended and the additional block of aluminium putting the propeller further away from the boss.

All the Fox modifications had been approved by a Reg. 35 engineer according to Skyfox. I contacted a Fox owner who had never had this problem and asked him what size propeller he had, "surprise" — he got rid of the 60 inch propeller and put on a 56 inch made by a chap called Warren Butler in Kingaroy Queensland. "Call him and you will be set."

I rang Warren, his comment was " well lan, what's new'?" and that he had a Lightwing with an Aeropower fitted and had no trouble, so long as the prop was not larger than 57 inches. Three people had thus confirmed an incorrect propeller.

I then told Warren about the extension bolted on the boss and he, like Mick, could not believe it, and backed Mick about the gyroscopic theory.

Copyright © 2000 Ian Shaughnessy

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