[rebel-builders] Subaru 2.5 motor mount
Posted: Sun Feb 19, 2012 11:04 am
Ken
I've been out of town and I forgot to measure this morning. However with
that redrive the prop will be farther forward than a Lycoming no matter
what you do so tuck it as close to the firewall as you dare. THe
firewall slopes but my crankshaft harmonic damper is no more than about
4" from the firewall at the crankshaft centerline. That was as tight as
I could get with a vacuum pump mounted behind a camshaft. With a moved
back firewall I believe I had 9.06" c of g. with batteries on the
firewall. Some Lyc's are a bit farther forward than that however farther
back would be nicer. The thing is if you move a 300 lb engine 3" that
is 900 inch-lbs of moment. If you move 18 lb of battery back to the tail
it will be something more like 3,000 inch lb (or more). of moment so you
can see that engine placement is not critical. That said I do have a
preference to minimize the yawing moment of the enitire airplane to
minimize ground looping tendancy but again that is a minor concern. A
similar concern is that the farther forward the prop is, the less
aerodynamic yawing stability there is. You can easilly imagine that a
longer cowling puts more area ahead of the c of g. And there are other
minor reasons to keep the engine close to the firewall as well.
I do recommend that you read Wayne's post on using less up angle of
incidence on the stabilizer. I did and think I set 0.8 instead of 1.0
degrees up incidence. If redoing it I would reduce it to perhaps 0.5 as
I always cruise with a bit of up elevator even if I load to 1730 lbs
with 100+ lbs of baggage. Might be worth asking MAM about that as there
could be other implications that I'm not aware of...
For encouragement I'll mention that I averaged about 14 liters per hour
while flying off time and preliminary indications on a 4 hour x-country
today seem to be only about 16 liters per hour at 4400 rpm and about 85
knots. I will gradually raise the speed until the burn starts to spike.
30+ hours and zero oil consumption so far. I am still prety pleased with
the soob ;)
Ken
ken@dcsol.com wrote:
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I've been out of town and I forgot to measure this morning. However with
that redrive the prop will be farther forward than a Lycoming no matter
what you do so tuck it as close to the firewall as you dare. THe
firewall slopes but my crankshaft harmonic damper is no more than about
4" from the firewall at the crankshaft centerline. That was as tight as
I could get with a vacuum pump mounted behind a camshaft. With a moved
back firewall I believe I had 9.06" c of g. with batteries on the
firewall. Some Lyc's are a bit farther forward than that however farther
back would be nicer. The thing is if you move a 300 lb engine 3" that
is 900 inch-lbs of moment. If you move 18 lb of battery back to the tail
it will be something more like 3,000 inch lb (or more). of moment so you
can see that engine placement is not critical. That said I do have a
preference to minimize the yawing moment of the enitire airplane to
minimize ground looping tendancy but again that is a minor concern. A
similar concern is that the farther forward the prop is, the less
aerodynamic yawing stability there is. You can easilly imagine that a
longer cowling puts more area ahead of the c of g. And there are other
minor reasons to keep the engine close to the firewall as well.
I do recommend that you read Wayne's post on using less up angle of
incidence on the stabilizer. I did and think I set 0.8 instead of 1.0
degrees up incidence. If redoing it I would reduce it to perhaps 0.5 as
I always cruise with a bit of up elevator even if I load to 1730 lbs
with 100+ lbs of baggage. Might be worth asking MAM about that as there
could be other implications that I'm not aware of...
For encouragement I'll mention that I averaged about 14 liters per hour
while flying off time and preliminary indications on a 4 hour x-country
today seem to be only about 16 liters per hour at 4400 rpm and about 85
knots. I will gradually raise the speed until the burn starts to spike.
30+ hours and zero oil consumption so far. I am still prety pleased with
the soob ;)
Ken
ken@dcsol.com wrote:
Starting to build the motor mount for the 2.5 and am looking for info about
how far from the firewall should the engine be mounted. I did not cut the
firewall back and the 2.5 weighs approx 30 lbs more than the 2.2 I am using
a Ross redrive which is 16" long and weighs 35 lbs by the bathroom scale!
Any ideas would be greatly appreciated. Thanks Ken and Bob for the help
already - too much of a commute from Comox to make a meeting!
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