Auto conversions & gear boxes

On Fri, 20 Feb 2004 16:27:38 GMT, "Dave Covert"
wrote:

I notice that most auto engine conversions use a gear box between the engine
and the prop. Why is that? Is it because an auto engine's peak HP is too
high for a prop to swing? Is it because auto engines weren't designed to be
pulled around by their crankshafts and don't have proper thrust bearings?
Both?

Are there any auto/motorcycle conversions that don't require gear boxes?

Dave

Dave, this is kindof the Auto Conversion 101 question no. 1. The
answer is pretty much all of the above.

Auto crankshafts aren't designed for bending loads, so if you bolt a
prop to one, you are taking a big chance that the prop loads
transferred directly to the hub of the crankshaft you bolted the prop
to, could fail the crankshaft, just behind the prop hub.

Many of the early VW direct drive engines did fail in this manner.
This resulted in a lot of changes to the VW engine when used as an
airplane engine, one of the changes was to redesign the crankshaft.
Not all VW engines get so modified though, even today.

Props have an rpm range in which they are most effective. There are
an incredible number of variables, but diameter, shape, cord, pitch,
thickness all play a part in prop design. A bitty prop turning very
fast just isn't as effective as a large prop turning slower. The
smaller prop, with it's smaller diameter has less thrust because much
of the prop is blowing air against the nose of the airplane. It's a
lot more complicated than that but that's the gist.

Auto engines are tiny when compared to direct drive airplane engines.
Take a 180 hp Lycoming. It's cubic inch displacement is 360. They
turn the prop at around 2600 to 2700. The Ford V-6 in airplane trim,
puts out 180 hp also. It displaces 232 inches and makes it's power at
4800 rpm. No prop will work at that rpm. To harness the power, it
needs to be turned slower. Enter the prop speed reduction unit.

The psru takes care of keeping the bending loads off the crankshaft
and reduces prop speed to a more useable rpm.

Yes there are motorcycles that are being used in airplanes, the BMW
comes to mind.

Corky Scott