Gordon Robertson wrote:
Paul,
Thought your list members would be interested in this helpful exchange from
Barry Controls technical advisor, Todd Thomas.
Gordon Robertson
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-----Original Message-----
From: Gordon Robertson
Sent: Saturday, August 03, 2002 2:25 PM
To: TThomas@BCAS.com
Subject: Engine mounts for Experimental Aircraft
Todd,
I read your reply to John on the RV-list, regarding Engine Sag.
That is not my (immediate) problem, but since you are the technical
support manager, perhaps you can help me.
I am mounting a Mazda rotary (RX7) engine into my Vans RV8 aircraft, and
am
looking for a suitable vibration mount. I dont think the standard 94011
series are correct, because the mounting method is very different from
Lycoming engines, so the applied load will be different.
I am looking for technical data from which I can select the correct mount.
Is there a souce of engineering specifications? Perhaps a catalog or some
such you could send me?
Specifically, I need each mount to carry a weight load of 175 lbs,
increasing to 1000 lbs for the case of a 6G pullup. The maximum very
short
term load it might ever see is 1750, for a 10G very hard landing. The
thrust load will be zero for most of the time, but up to 450 lbs when
flying.
I am trying to decide whether the mount should be with the axis vertical,
or
horizontal, or somewhere in between. So I would like the appropriate
specifications and any advice you could provide.
By the way, the other folks using this engine seem to prefer the 22000
series mounts. Any comments?
Gordon Robertson
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From: TThomas@BCAS.com [mailto:TThomas@BCAS.com]
Sent: Monday, August 05, 2002 8:17 AM
To: Gordon Robertson
Subject: RE: Engine mounts for Experimental Aircraft
Good morning,
I can provide you with a copy of the catalog from our Industrial/Defense
division, which has a very good section up front on isolator theory and
selection.
The 22000 series of mounts would be the mount I would first recommend. A
lot of homebuilders with alternative engines use these with good results.
They are a product of our Industrial/Defense division and will be in the
catalog. Since they are industrial mounts, their price is not as high as the
aerospace mounts either.
As for the mounting orientation, the best way to mount an engine isolator
is for the axis with the most cross-sectional area to take up the engine
weight. The greater the cross-sectional area, the greater the spring rate
which would result in less engine sag. On a 22000 mount, this would be the
axis that is parallel to the through-bolt.
When selecting the isolator to use, our recommendation is to design to the
maximum in-flight load you expect to see. The hard landing mode should not
be a isolator selection criteria, as this is a transient load that is not
expected in normal service. Designing the isolator to that load will result
in an overly stiff setup that will not offer much isolation in normal
flight.
Designing to the 6g load is going to result in a pretty stiff setup to
begin with, but if the cowling is tight you are going to need to keep
deflection down to a minimum. If you feel the level of vibration is too
high, you could step down to the next softest isolator and see if the
deflection allowed is going to be acceptable. Like any engineering decision,
a compromise between advantages and disadvantages will have to be made.
I hope this helps, if you can provide me with an address, I will send the
catalog to you.
Thanks,
Todd Thomas
Manager, Technical Support
Barry Controls Aerospace
Phone: (818) 973-2524
FAX: (818) 845-6978
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That is pretty much what Jim Griswold came up with and I agree with.
My suggestion, as you probably have already discerned, is to use one G
for the in-flight loads. This produces the best isolation.
BTW I had lunch with Dave Anders yesterday and the subject of the Questair
Venture came up. Dave confirmed that the Questair with the wide
spaced Barry 22000 mounts is the smoothest airplane, vibration wise, he has
ever flown. This with a great old honking 650 HP race tuned TIO 550.
Dave is getting ready to run his Questair at Reno again.
Last time Dave qualified second (I think) but crashed landed on
the first lap when the prop ran away.
Dave told me Ed McDonald also had a large hand in the structural
design. He thinks Jim Griswold's son designed the landing gear.
The nose gear seems to be a major problem so John Lynch is redesigning
it with more caster in the strut (strut angled forward like a Bonanza)
and a better steering mechanism.
Paul Lamar
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