| 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. |
