clare at snyder.on.ca wrote:
On Thu, 19 Jan 2006 15:35:13 -0500, "Morgans"
wrote:


"Bret Ludwig" wrote in message
groups.com...

I just think hanging a prop on a crank directly is a non-starter in
the first place...especially on a crank and case not specifically
designed for this in the first palce. Maybe a good redrive and flywheel
would be a better way to go?

That is my opinion, also.


What isn't there can't break. That's my reson for a direct drive 'vair
insted of a geared Soob - same weight - same HP.

Exactly right Clare. The soob has a bulletproof interior but the use of
liquid cooling plus a drive system adds two complete failure modes that
aren't there at all with the Corvair. With the Corvair if you take care
of the systems design aspect, basically by using sound aircraft design
practices for carburation and ignition, you address the vast majority of
the reliability issue and the only open question left is how strong are
the basic mechanicals and that is something that is finally being addressed.

I'm kinda glad that these crank failures have come about because it was
always clear to me that the crank configuration should be considered
"marginal" when subjected to prop gyro loads at higher power outputs,
since simply by looking at it you see that bending loads can't be
absorbed by the 1st bearing and bending is happening. One bit of good
news is that the failure mode contains the end of the crank and does not
result in the prop leaving the aircraft.

I thought from the get go that the guys using extension shafts were
nuts. However they have unwittingly provided a service by finally
uncovering the crank's weak point in what amounted to a severe service
qualification endurance test, ending that uncomfortable sense that
nobody really knew just how strong the crank was or wasn't, or exactly
where its weak point was. It's a great credit to William that he
immediately responded by publicizing the issue and conducting further
testing. As someone with a job that provides a ringside to seat to
qualification, certification and continuing airworthiness of components
on regional jets, I found his approach to be very much like, and
sometimes superior to, the commercial world (in terms of letting it all
hang out and responding to crises).

Anyway, mistakes in calculations or engineering judgment in
certification of commercial airliner components sometimes results in
certified parts that are not up to snuff and fail in service well before
predicted (I see this all the time). Truth is, sometimes the only thing
that keeps commercial jets raining down on peoples' heads is double and
triple redundancy, not the super duper construction of their components.

There is still an unknown though. What I'd personally like to see
William do is send the fracture results and the metallurgical data on
the crank to a metallurgical and dynamic stress specialist who can
calculate the loads/cycles that it took to initiate and propagate the
cracks, then work backwards to establish the gyro forces and torque
forces required to generate those loads, then apply a safety factor and
establish safe propeller weight/length/horsepower limits for the
existing configuration with a nitrided crank (the calculated limits may
make a lot of people unhappy though). Builders need to know just where
the safety boundaries are for the existing config.

Myself I am still a big fan of the Corvair but will probably adopt the
extra bearing mod he's working on if I ever get to that point.

John Kahn
Montreal