The Murphy Builders Site

Hi Ken !

I did say "or similar" - what I meant was - there's virtually
NO CHANCE of installing a Rotax 912, or one of the new, light,
more fuel-efficient engines. I base this on over 12 years of
flying 912 Rebels with the firewall in the standard position,
and the battery up front. (and sometimes 10 lb. of lead too ...)
Moving it back 3" would make it very unpleasant to fly ...

In standard configuration, the C of G is too far back to allow even
1500 lb gross in a normal passenger/baggage setup - although
the 912 Rebel <could> certainly carry 1,650. It will be interesting
to see how the latest Rebel comes out, with the Rotax mount
extended 3" forward .... I suspect even there, it will not be
possible to carry the usual baggage load, but it should be a
lot more versatile.

You are correct, of course - there are lots of choices in
the 'heavy' engine range - the Subarus are great, as are the
Lyc. clones (XP, etc.), and now a few radial contenders -
and I've heard of a new diesel that will appear at Sun 'n Fun ....

--
......bobp
http://www.prosumers.ca
http://bpatterson.qhealthbeauty.com
http://apatterson2.qhealthzone.com

-------------------------------orig.-------------------------
On Friday 26 January 2007 20:15, Ken wrote:

snip

There's nothing to worry about if it wasn't moved - most builders
today do not move it back, regardless of engine. This gives a much
wider range of engine choices .... If it was moved, you are stuck
with an O-320 or similar

snip

Based only on my aircraft, I don't think I'd agree that a moved FW
dictates an 0-320.
300 lbs of engine x 3" = 900 inch lbs. And the seats go back 3" as well
so say another 500 inch lbs when solo.
However keeping a 20+ lb battery on the FW vs back in the tail has as
much or more effect on the W&B.
Even the PC680 battery will be 20+ lbs with mounting brackets and wiring.
The archives have a recurrent theme of more up elevator, ballast, light
prop, light accessories, etc.,etc. The only Rebel owners that I've
heard say they needed weight in the nose had a rotax 912 or similarly
light engine.

And of course one can always make a custom longer engine mount.

Ken

PS. Ignore the following or read it critically including the disclaimer
;)
These comments are with the Mackenzie leading edge cuff and the Fife
wingtips.
The subaru engine has a more rearward engine c of g than a Lyc. but
similar weight and my FW is moved back. I generally cruise with lower
power than an 0-320 would. The Mackenzie leading edge cuff extends the
leading edge forward (and I think the center of lift) and it effectively
reduces the angle of wing incidence by drooping the leading edge. My wee
batteries are under 20 lb but they are on the floor between the rudder
pedals. I have 20+ lbs of fuel pumps and 2 gallon header tank back
behind the float attach points (behind the baggage area).
Disclaimer - the leading edge cuff may be a major factor here so I'd
definately pay more attention to what MAM and other folks are saying
about this and I haven't done any spin testing although it takes a lot
of back pressure to stall my airplane and it doesn't seem inclined to
enter a spin unintentionally. More power than I have would mean a faster
cruise and probably less up elevator in cruise. I followed Wayne's post
and did not raise the nose of the stab as much as the manual said. Think
I set 0.8 degrees rather than the 1 degree in the manual. However if
doing it again I would not even raise the nose of the stab that much
because I still always have some up elevator. Even at gross weight the
tail generally will come up immediately on the takeoff and it certainly
is easier to land 3 point with an aft c of g. An aft c of g does
generally reduce pitch stability in cruise but my Rebel is amazingly
stable in cruise.

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