he had experienced a

propeller blade failure while en route to Pahrump. He stated that post
accident examination of the
airplane revealed that 21 inches of a
propeller blade had separated. The propeller is an aluminum two bladed
McCauley fixed pitch.

According to the pilot, the resultant imbalance resulted in substantial
damage to the airframe,
leaving the engine hanging by one 3/8-inch
bolt.

This sort of scene has been played in my mind too many times.  I guess

it's

my physics and engineering background that puts the respect into this

kind

of out of balance situation.  That being said I've got some kind of
solution that may be what the safety cable is about.  I don't know, I've
never seen one.

When a prop sheds a blade or part of one, the prop flange end of the

engine

will want to orbit about the new center of mass of that end of the
engine.  This can be calculated if you have an approximate idea where

the

prop is going to break.  The (in our case) water pump end would only see

a

sort of gimbal like motion.

If the safety cable were wrapped around the prop flange end at about the
motor mount station and had enough slack such that the prop flange end

of

the engine, after throwing a blade, would "orbit" inside the loop of

safety

cable.  Then the safety cable would effectively cage the engine in the

loop

but isolate the shake from the airframe.  This of course after the

engine

broke loose from the front mounts (yeah I know, this is a stretch if MY
imagination).  The engine would still be restrained fore and aft by the
rear mounts provided they had the range of motion both up/down and

sideways

so that the prop flange could do its orbit without pulling the rear

mounts

loose.

All this would maybe give you enough time to shut down the engine before

it

made the airplane unrecoverable.

The engineering questions arising (assuming a bed mount) are:
1. Is the "tear loose" force of the out of balance prop going to be

higher

than the negative-G design load of the front mount?  On a metal prop it
could be, on a composite or wood maybe not.  It would need to be.

2. The safety cable loop diameter would be a function of prop weight,
engine nose weight, and where you thought the prop would break.  I seem

to

recall that the amount of prop lost in the above event is common?

Well, this turned into a bit of a read.  Am I on the right path with

this

safety cable?

Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode

While I agree that a cable is not a bad idea, the root cause of this
accident is what is not being addressed by the NTSB.

I am pretty sure that McCauley does not make a fixed pitch metal prop for
aricraft of the Mustang II or RV speed range.  What builders have done for
many years is have the prop re-pitched beyond manufacturer's limits.  This
has resulted in prop failure litterally hundreds of times.
The only fixed metal prop for this speed range is a Sensenich (Sp?) and it
has a HARD rpm limit of 2600 rpm.

Don't depend on re-pitched metal props.

Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com

 
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