Chevy LS2 and Trans??? any real issues besides weight

"Charles K. Scott" wrote in message
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On 10 Nov 2005 12:41:25 -0800, "MrV" wrote:

Hey guys i'm a new pilot that really wants to build his own craft. help
me with this one issue.

I want to use a chevy ls2 or ls7 as the power plant in my craft.
now looking at everything including the hp/torque curves i've decided
running the engine around 3100 rpm should give me around 250 hp with
good torque now getting that power to a propeller seems to be an issue.
I'm wondering besides weight would there be any real issue using the
associated transmission locked in gear maybe 3rd/4th/5th gear whichever
is just under 1:1.

it would seem the car tranny has been engineered to convert the engine
motion into the spinning i would need to propel the propeller. plus
running the engine at 3100 rpm it would prob last longer than i will.

the aircraft i want to design is a very cab foward design with a
pusher prop and the engine would be mounted approx mid craft.

i'm new at this and besides having an engineering background i really
have no exp building an aircraft so any opinions would be helpful

Mrv, you should understand that homebuilders have been thinking that
auto engines should work fine for airplane powerplants from the very
beginning of the homebuilt era.

Not only homebuilders, but a number of qualified aeronautical
engineers thought likewise and have tried through the years, with
varying degrees of success, to convert auto engines to spin
propellers.

Toyota actually managed to get a Lexus based V-8 conversion certified
with a Hamilton prop designed specifically for it. But they withdrew
the engine from the market without attempting to put it into any
airframes, other than the test bed.

There is absolutely nothing inherently wrong with almost any auto
engine's ability to run at aircraft flight power settings for a long
time. That fact has been proven for years. What IS a problem is
fabricating a reliable prop speed reduction unit, and managing to
engineer adaquate cooling for the engine.

The litanny goes, it's not the auto engine that fails, it's everything
else. And there is a lot of everything else that can go wrong and
stop the prop from spinning.

From an aviation stand point, using an auto transmission for a PSRU is
not a great idea. For one thing, it's carrying around a bunch of
gears that add to the weight and aren't being used. That's just
crazy. Also, with the transmission in the car, the drive train is
locked solidly in place and does not impose any side loads to the
transmission at all. All it does is transmit torque as it spins.

But the propeller produces ENORMOUS side loads on the prop drive every
time you turn, hit turbulence or climb or dive. The auto
transmission, as it comes from the car manufacturers simply is not
designed to withstand that kind of side loading.

As mentioned previously, the lower gears in the transmission are
designed to be operated for only short periods. They do not have the
heft and thrust bearing support to manage sustained pressure at high
torque loads.

Finally, while belted PSRU's are fairly well understood at this point,
they tend to be marginal for high output engines. The only PSRU I'd
recommend at this point would be the Geschwender type. See:
http://www.alternate-airpower.com/ for details.

Corky Scott

To me, it's interesting to note that in the automotive role, the engine is
isolated from the load to the maximum extent possible. In other words, the
engine is coupled to the wheels with cardan shafts that have U-Joints and
sliding splines such that engine vibrations, other than torque pulses, don't
get transmitted to the wheels and wheel vibrations don't get transmitted to
the engine. The engine just rocks and rolls in it's own rubber mounts and
transmits only torque to the drive line. The engine bearings see neither
thrust or radial loads. The vision is that everything is isolated with
rubber mounts to eliminate all possible vibrations. That seems to be a
successful formula for cars.

I'd suggest this is a good path to take in auto-engine conversions. Don't
just mount the prop to the crank or mount the PSRU rigidly to the engine.
Separate them and let the prop, PSRU and engine each live in their own
isolated vibration environment.

So, how to do this? First, think of a prop attached to the airframe turning
in it's own bearings that carry the thrust and radial loads with the bearing
carrier on elastomer mounts. Drive the prop with belts that absorb some
torque pulses and drive the belt with a pulley block mounted to the airframe
like the prop that is itself driven through a elastomer flex coupling by an
engine riding in isolation mounts. This way the prop won't see engine
vibrations and the engine won't see prop vibrations. The airframe itself
should see neither.

In drive line systems, there's no such thing as 'good vibrations'.

Bill Daniels