Herb Sanders wrote:
Paul & Ed, As one who experienced a prop failure on first flight, please
allow me to make a
comment. About 20 years ago at Sun & Fun I met a young man who had built
a Long EZ and was a
graduate aeronautical engineer and was interensted in props Ron Lueck's
dad had built the Air
Shark and Ron has gone on to develop the Comp Monster line of kitplanes.
He offered to custom build me a prop he claimed would go several
knots faster, I agreed to buy
one, and he built one and sent it to me in a couple of months. The
improved efficiency was from
thin blades covered in kevlar for strength. On the first flight, I
lifted off, made no unusual
pitch or yaw moves, climbed up to 1000 ft and stayed in the pattern. On
the downwind at about 140
mph I developed a vibration and it got worse as I turned base, final,
and slowed for landing.
I landed and taxied to the ramp and a friend ran out and said my
prop was smoking. I shut
down, got out and looked over the prop. About a foot out from the
center, both blades were broken
chordwise and you could take the tip and move the blades fore and aft
about a foot. They were
retained only by the kevlar fabric and the smoke was from the
heat/friction of the wood rubbing
together. (remember the Boy Scout trick of starting a fire rubbing wood
together) My engine was
not retained by a cable and either of the blades departing would have
been fatal according to Mike
Melville.
When I was 20 years younger, I flew first flight on eight canards
and did some flight testing
on new products for Sensenich Props and for Ben Ellison on the throttle
body injector. I had a
total engine out from a mixture system failure in flight and the engine
went to idle cutoff - I
was fortunate to land on a paved highway in Arkansas with no damage or
injury.
The point of telling you this is that in testing, some things pass
the test and some fail. I
would really hate for you to look back the next day and think, was it
really necessary to do in
flight testing on the new design, complex, high risk piece of equipment
on the very first day it
was ever spun up?
I have heard that God protects fools and drunks. Since my friend Ed
is no fool, please get him
drunk if he flys this prop on the first day :-).
I love and appreciate all that you do - just be careful.
Regards, Herb Sanders
Hi Herb,
Good to hear from you, its been a while. I value your experience with
the failed prop and I understand the wisdom of your suggestion (well,
perhaps with the exception of fueling the pilot with alcohol {:>)).
You did not mention what prior experience your prop builder had with
building props. Sounds like a fundamental design flaw (perhaps too thin to
structurally support the thrust loads). However, I now can say I've heard of
one composite prop failure - are there more out there?
I know Perry Mick has quite a bit of experience with building composite
props and feel very comfortable with his having done the prop construction.
I could be wrong of course, but I do not have much concern about the
structure integrity of the composite structure of the blade itself. My gut
feeling is that should there be a problem, it would more likely to be at the
blade/journal junction. It appears that the junction of composite and metal
at the "S" of the metal journal would handle the radial loads (it would
require either the metal or composite material to shear before losing a
blade). I am not as certain that it would handle "twisting" loads of the
blade equally well, but then the twisting loads are probably a lot lower.
The blade retention (a row of ball bearings inside a steel race - one half
of the race in the blade journal and the other half in the hub) mechanism
appears to be effective and simple. I would be very surprised if it failed
to retain the blade journal. Butch, is a guy who certainly knows his way
around a mechanical design, metal characteristics and strengths, and the
machining of aircraft quality work.
Not trying to rationalize a decision, but, when you consider all the design
element and components that have to be put together right for a rotary
engine to fly, you have to decide what degree of risk is acceptable and
prudent. We may well decide that after ground run-ups and perhaps some high
speed taxi test, that all involved will have an sufficient sense of
satisfaction to then wait for a test stand and engine to "wring" it out
before eventually flying with the prop.
While I have certainly not made a final decision to fly, I am willing to do
the ground tests we feel will stress the prop to some degree. Certainly, if
any problems show up on the ground there will be no consideration of a
flight. In any case, there would be no flight the "first day".
I appreciate you concern and advise. Its great to have friends who will not
hesitate to express that concern for a friend.
Thanks again, Herb
Ed Anderson
The AirCraft Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site. http://home.earthlink.net/~rotaryeng/
Copyright 1998-2002 All world wide rights reserved.