This is the bird cage approach made with a whole bunch of 1/2
diameter .049 wall tubes. It is less complicated than it looks.
Make a 1/4 scale balsa model to prove to yourself that it
will be rigid.
The rubber bushings are on the engine/psru/prop/
alternator/starter CG so the rear mount is trivial.
Little to no load back there.
The red tubes are the motor mount tubes proper. The green tubes
are necessary to insure the integrity of the RV4 landing gear.
If the transverse green tube is not installed the landing gear will
tend to spread the motor mount tubes. The fore and aft green
tubes feed the landing gear loads into the motor mount.
The new tandem RV's are much simpler with a bent aluminum gear.
A lot of this stuff would be unnecessary.
A custom oil pan must be made to fit inside the bird cage.
When you use a pointy cowl like this kind of motor mount that implies
The mufflers are a problem. Perhaps Mike Wills twin
side mufflers will work fine. Lets keep our fingers
crossed. Here is a picture of Mike's mufflers.
Looks like Mike's engine is a bit further forward than shown
on this sketch. His rubber mounts are aft of the engine/psru/prop/
alternator/starter CG. This picture should also help you to visualize
this motor mount design.
Paul Lamar
OPS! I forgot a tube. I also added a bit of detail because now I can :-)
BTW this mount uses the Tracy trick of the oil foam separator sandwich plate
extended and thickened into a motor mount. We got to give credit where credit is
due. A bit heavy as it is about 144 square inches by 1/4 inch thick
at a volume of 36 cubic inches and 3.6 pounds not counting a few
little holes. It is necessary and would be about one pound in any event.
Could be welded in as part of the custom pan of course.
BTW Mike Wills mount would work well and would be simpler than this one
IMHO if another hard point was constructed at the very top of the
firewall to brace the rear motor mount vertically. This one on the
other hand gives a bit more clearance around the water pump plumbing.
The extreme forward PSRU/motor-mount plate type mount would also work
with this scheme. That would give a bit more room for the front
of the pan.
Let me know if anyone wants this ACAD 12 drawing file.
Paul Lamar
Paul,
There are three problems with this layout:
1. It would appear that a side exit exhaust will be required. There is 'almost' room for a tangential muffler right off the exhaust ports, but will reqire a 'bump' in the cowling. I think that most of us would prefer to use a tuned exhaust which requires pipe lengths of 26" to 32" depending on engine RPM (I hate to give up horsepower that is almost free).
I cannot think of a way to get the exhaust to come out the bottom and and
still have a rad under the engine. What did you have in mind? What do you
see wrong with a side exhaust?
2. The radiator cannot extent near this far forward. Figure that you only have a max radiator width of ~ 19" under the front edge of engine. This is to allow the remainder of the taper of the cowling to the spinner (the stock spinner is 13" in diameter). I am going to be placing the oil cooler beside the radiator so this is a vitally important issue.
Well I was thinking of a 12 inch wide rad. How wide is your rad?
3. If you move the front edge of the radiator under the front of the engine you will find that the motor mounts are in the way. This is really hard to see, but I have 'mocked up' a design very close to what you are proposing.
Got any pictures of your mockup?
In conclusion, my mockup shows that this proposal really doesn't work any better than Mike's design. The only way to provide more room for the exhaust and radiator is to cantilever the mount off the back of the engine. This may be much tougher structurally but the room freed up is needed.
Thanks,
Ken Powell
Well I am open to suggestions. What would you use for hard points on the back
of the engine? Mecanair made a whole new front cover to do this.
My next modification of this mount will put the rubber bushing even further
forward onto Tracy's motor plate. I figure I will then have room for a tangential
muffler and a side exhaust. I think people are over estimating the advantages
of a pointy cowl. A bit of a bulge up there will make little difference
in prop efficiency and an un-measurable reduction in over all drag IMHO.
Paul Lamar
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