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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


The Prop Doctor diagnoses: propeller failures


Carbon-fiber/epoxy is such a magic material that one can, at times, forget its limitations. With, of course, in the case of propellers, drastic results. Propeller failures are bad. In a perfect world, they would never occur. In the real world, they continue to occur, for reasons we will examine now.
* Case study 1: F3D pylon. Both blades departed the airplane in flight, each departing through its spinner slot and leaving nothing behind on the hub. The departing hub sections damaged the spinner, since they could not fit through the slot otherwise. No damage to the airplane. Cause of failure: the spinner back-plate was smooth, allowing the propeller to slip against the rotation torque. Use of a 12" ring spanner solved the slippage, but crushed the hub, leading to the afore mentioned catastrophic failure. Moral: propellers are driven by friction between the hub and the prop driver. No other method is acceptable, in any circumstance at all. 

* Case study 2: F2C T/R. Hub damage to prop. Cause of failure. The propeller was driven by a steel pin, which protruded from the perfectly smooth prop driver. Moral: drive pins are never acceptable for transferring torque to the propeller, as the engine impulses work the pin back and forth in the drive hole. In the case of engines with multiple propeller retention bolts, the function of these bolts is to produce uniform friction against the prop driver: they must not be used to transmit torque. 

* Case study 3: 21QM pylon. Blade broke off when the engine was flicked over prior to starting. Cause of failure. Faulty manufacture of composite prop. The hub had been filled with short lengths of glass fiber and an excess of resin: closure of the mould pushed the long fibers into the flash, fatally weakening the blades in the region of the hub. 

* Case study 4: Bendix T/R. Blade departed airplane in flight, instantaneously re-kitting the airplane and packaging it in a plastic bag. Cause of failure. Glass filled nylon prop insufficiently strong for high RPM racing engine. Moral: if using this type of prop, test it on someone else's airplane first. 

* Case study 5: F3A aerobatics. Blade tip departed airplane at idle RPM in workshop, puncturing metal ceiling. Cause of failure. Faulty manufacture of glass filled nylon prop: strength of these props is affected by the injection-moulding conditions of pressure and temperature, which may vary according to position on the production run. Moral: you have been warned. 

* Case study 6: Unlimited pylon race. Blades experienced compression failure near the tips after only one minute of ground run. Condition noted before catastrophic failure. Cause of failure. Carbon fiber prop insufficiently rigid to resist 50 lb thrust bending load on 220cc racing engine at 9500 RPM. Moral: not all carbon props are equal. This propeller had a high amount of resin compared to fiber, greatly reducing the compressive strength on the front face. 

Some of these cases are indicative of problems that may occur in the future. To gain more power from a given capacity engine, it is necessary to run it at higher RPM than ever used previously. We are looking at a rev range from 20000 to 38000 on modern racing engines.

Furthermore, to allow the engines to reach these values, the propellers must be smaller in size, with a resultant reduction in strength. The requirement for thin airfoil sections to yield the best propulsive efficiency exacerbates this problem. In most cases, glass-filled nylon cannot be used with safety.

Carbon props of sizes used in F2A, 2cc speed and 1/2A pylon are so small now that if the RPM is pushed up much further they will disappear altogether. The resin to carbon ratios in these props is critical now. In the 60's, racing 40's used props like 8X8 at 14000: now they use 7X7 at 29000, quite a shock to relics like me.

The same problem applies even to very large engines. In the US, 220cc engines are turning narrow-bladed 19X27 propellers at 10000 RPM, for airspeeds over 200 MPH. That is a relatively tiny prop. 

The current nostalgia trend presents a new set of worries. B team race in 1957 could safely use wood props. But now Schneurle engines are permitted, yet safe propellers capable of handling their RPM are banned. We all have a legal duty of care in this area: such rules may in time prove to be real liabilities if an accident occurs. 

I have myself been struck by a propeller which departed the airplane. Interestingly, it still had half the crankshaft bolted to it! You can't win them all.

Fly safely.

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