snips..
Ken Welter wrote:
The coot and any pylon pusher plane has a very narrow CG range, the wing has a wider margin that it can be operated in but with this type of plane in the event of a engine failure the prop drag will pull the nose up and with the engine mounted high it will shift the weight aft when the nose comes up. For this reason you need to operate on the forward edge of the CG window of the wings specs. To complicate this the passenger weight is forward and changes the CG drastically and for this I have a 2.5 gallon fuel tank in the nose and a one gallon tank in the tail cone to shift the weight and keep the CG dead on 25 percent of the cord. Jim did not build his coot he just installed the rotary on a coot that he bought and there was two seven gallon tanks in the forward floor boards that I told him needed to be full to fly solo to keep it in CG and he did not believe me but I don't know if they were full at the time. The lake also has this problem and they have weights that they slide in and out of compartments in the front and rear.
Ken Welter
Very interesting Ken. Awhile ago I thought a configuration like this might
lower the position of the CG. Given what you are saying above a pitch up
would not move the CG aft as it does now with a high mounted engine. Since
you are already using a belt drive all you would need is a longer belt.
BTW how is Don Moncrieff coming along?
Paul Lamar
Old message on the subject.
****************************************************************
Subject: Coot motor mount and turbo issues
Date: Sat, 29 Jan 2000 22:20:03 -0800
From: Paul <rotaryeng@earthlink.net>
Paul wrote:
Don Moncrieff wrote:
Paul L--I'm glad my letter got to you. Thank you for your very
interesting ideas re: a Coot motor mount. You make me wish it were about
three years ago when I started planning and building my mount. I choose
al at that time and now am almost ready to bolt my 13B on top. Given
your vision I now see the advisability of the steel mount. I plan to
finish what I've started, but if I build another mount or build another
Coot I'll certainly take your ideas into account. I'll take what
opportunities come my way to pass your ideas on to other Coot builders
who are not as far along as I am.
What I've done is keep the car fan, put a smaller, thicker rad square
in front of the fan (my rad volume is slightly larger than that of the
car rad). Coots don't go fast anyway so I'll be interested in how my
appoach compares to others. If I can get more than 90 mph (what some
others get) I'll be pleased.
I appreciate your question, Paul, about what PSRU I intend to use. I'm
wrestling with that problem. I want to keep the engine as close to the
main structure as I can. Some Coots apparently can't be pulled out of a
steep bank except by the rudder. I'd like to minimize that problem. As I
figure it, then, the best setup would involve the prop drive shaft being
9 or 10 inches above the excentric shaft. That seems to me to call for a
belt or chain. Am I missing something? Further, one Coot builder and 13B
user had a problem with his Ross redrive (an older one) damaging a
bearing in his engine. He ended up pulling his entire engine apart after
only a few hours of ground testing. I, of course, don't want a RSRU that
might damage my engine. If the group has discussed belt drives I missed
the discussion. Has any knowledge been shared that would help me? It
seems important to keep in mind the fact that a Coot is a pusher.
The issue that challenges me most at the moment is the use of a turbo.
Thank you very much Paul Y for your clear and forceful warnings. I now
understand that the small turbo used by Mazda is totally inadequate and
even dangerous at altitude. Thin air is clearly bad news. Overboosting
the engine and overspinning the turbo are "no-nos". I certainly join
Chris Gentry respecting and appreciating your input. Charles Harbert
(thanks Charles) informs us that overrevving to destruction is possible
10K'. With all this challenging input in mind I turn to my 13B and think:
it's mounting on a Coot! The moment it's turbo will be most needed is
climbing the bow wave. That will be a bit lower altitude than the Rx7
crusing along the shore. Once in the air pumping up engine horsepower
will be futile--Coots just don't go fast. My interest is in putting
around the lakes here in Ontario--not much above sea level. I might drop
down from 500' above ground level to get a photo of a moose or bear. I
might splash in to catch a fish. I'm thinking of nicknaming my Coot
"Sunsurfer" imagining surfing along the sparkles of sun on the water. I
doubt if I'll ever want to get over 5000' or want to go very fast (even
if I could). Other Coots seem to get off the water well with abut 200
bhp. Brian (thanks) tells us that 200 bhp is conservative for a stock
Hitachi turbocharger on a 2nd generation Mazda Rx7 on a nice day near
the ground. I'm aiming for nice days a bit below the ground
(sorta//water level). I'm also thinking of the fact that I have a stock
turbo which seems in good shape judging by the Haynes manuel. I got this
turbo for almost nothing with the engine. I'm having a dream of getting
the bhp I need to get off the water at almost no expense (a good price
for me) then opening the waste gate and putting along with delight,
carrying my turbo for my next takeoff assist. Is my dream foolish?
While I'm here--thanks Chris for the info that the switching valve is
not needed.
Thanks too, Rob. Your three lever system seems like an excellent
starting point to help me design what I need.
Thanks to you, Paul L, I'm thinking of putting Kevelar in the cowling
near the turbo--though I hope my use of the turbo will never over stress
it.
With everyones helpful input I think I'm moving towards a plan that
will work and be a lot of fun. At this point I'm wondering if using a
turbo for my peculiar purpose (a push over a bow wave) will turn out to
be quite simple--just pull off all Mazda's extras, stop the holes and
hook up the waste gate to cockpit control. I think I "must" be
oversymplifying but hope I'm not.
Thanks, Don
Not to worry Don. I will see what I can do with the aluminum mount.
If you have some of it done let me know and send me a picture to the
same address.
Paul Lamar
-jim west wrote:
Paul, send my e-mail on to Don, as I'm at the same point as he is with my engine
instalation.
Part of the problum is in the prop. Must have inflight control of the pitch. I would
like to talk to Don about it.
Don, Have you looked into how long of a cog belt is available?
I have given your idea some thought and it occurs to me the pylon, prop shaft
and the engine must be rigidly fastened together for a long belt drive to
work. I will work up a sketch or two. Given that I see no reason why
mounting the engine low will not work.
In the mean time I suggest you use a bearing on the lower end as per this sketch
to prevent breaking the e-shaft from side loads.
Paul Lamar
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