Auto conversions & gear boxes

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 Covert" wrote in
:

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




The sonex can use a VW conversion attached directly.

http://www.aeroconversions.com/ or www.sonex-ltd.com

--
ET

(Future student pilot and future Sonex Builder)


"A common mistake people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools."---- Douglas Adams

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

"Dave Covert" wrote in message
...
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



Because the RPMs at which the motor is producing the proper amount of power
would be spinning the prop to fast.

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

Some people think aircraft engines are "old fashioned technology"
and have not kept up with developments in auto engine field.

They point out that aircraft engines haven't changed much in
over 50 years.

Some people feel that auto engines can be used to power airplanes.

To some extent, all three of these ideas are true.

Aircraft engine do not run like car motors.

Aircraft engines run at much higher sustained power settings and
constant rpm for long periods of time.

And then there is the propeller...
Turning the propeller is what it's all about.

The propeller converts the engine's power into thrust.
As always, when energy is converted, there are losses.

Moving through the air at very high speeds, the propeller
makes lift (thrust, which is power successfully converted into
forward motion) and drag (pure conversion losses).

So, propeller efficiency is extremely important.

If the propeller is only 50% efficient, half of the
power generated by the engine is wasted in losses.
Yes, literally.

Only one hard rule for propellers - longer is better.

But longer blades mean lower RPM because the tips of the
propeller blades MUST stay below the speed of sound (yep,
Mach 1, really) for any efficiency at all.

Part of the reason for this is the huge increase in drag
as the tip enters the transonic (speed) region.

It takes TORQUE to turn that propeller - not horsepower.

A given propeller needs to turn at a given RPM, which
will require a given amount of torque.

If the engine makes enough torque to turn the propeller at
that RPM, a direct drive set up may be possible.

There are a lot of other minor details that may get in the way -
Harmonic Resonance is a big one.
But, it may be possible to run this combination direct drive.

If the engine needs to turn at a higher RPM to make adequate
power, some kind of gearing would be necessary to reduce engine
RPM to propeller RPM. Notice that reducing RPM will increase
torque proportionally. Seems like a nice trade off.

Now the engine should be running at an RPM near the peak of its'
torque curve. This is for best engine operating economy.

And the (longer) propeller is running at a comfortable (lower)
RPM for good efficiency. Life is wonderful.

Except for the weight.
Auto engines are seldom as light as possible.
Then we add more weight in the form of a gearbox and such.
Radiators full of heavy (hot!) fluids.
External oil sump?
Mounting?
Propeller gyroscopic forces operating on the crankshaft?

Weight is critical to any flying machine.
(Go back and look at how birds are built)

So...
Think of it as evolution in action.

The reason our old antique Lycosourus engines are the way they
are is that they evolved into a very narrow niche.

They turn propellers to pull airplanes.

They make very high torque
at very low RPM,
and are as light as possible.
They are tremendously reliable and fairly efficient.

Prices are high because of limited production and high demand.
Simple economics.

But the economics of engine development (and risk assesment) are
anything but simple.

I have a big bore VW (2180cc) on my parasol.
That's a converted car motor.

There is a weatlh of prior art using VW engines for small airplanes
(if one is inclined to use it).

What works, and what doesn't. (eg: breaking cast crankshafts)

Mine is a very simple conversion, using high quality (GPAS) parts
built by a little German perfectinist.
I trust it - so far.

I also don't push it beyond conservative limits.

All VW engines are 40 hp engine (IMHO).
Some can make more power than that - for a while.
This one is _rated_ at 70 hp.
But will reach thermal limits of the fin area and overheat
if not throttled back (to roughly 40?)

It's a fairly expensive motor.
The jugs and pistons are standard parts, but the crank (!) and
accessories and machine work are all specialty items.
A new 2180 can easily go over $5000 with a few bells and whistles.

But the weight, power, reliability, and operating cost are all within
reason for this particular airplane.

The airplane itself can land slowly, around 35 mph.

The chances of getting down safely if the engine quits are a lot better
at 35 than they are at 53.

To me, it seems like a reasonable risk for the potential rewards.

But...

Your milage may vary.

Richard

http://www.flash.net/~lamb01


PS: I've read of a Curtiss Hawk replica that uses a direct drive Chevy
350.
It's supposed to make roughly 190 hp?
It would obviously be a heavy motor.
Not something you'd hang on a glass slipper.
But on a big old biplane with a looong prop
it seems to be just the ticket.

On Fri, 20 Feb 2004 18:08:35 GMT,
(Corky Scott) wrote:


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.

Speaking of Fords! How's your project coming?

__________________
Note: To reply, replace the word 'spam' embedded in return address with 'mail'.
N38.6 W121.4

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

The Corvair engine operates successfully in direct drive configuration.
See http://www.flycorvair.com/

Nice job Richard. Nice Job.

I might just add one more comment to clear something up a bit. When
you chose a prop, you design it so the tip speeds don't exceed 0.80
Mach, or 80% the speed of sound. As the propeller tips approach
speeds over that, the airflow can become super sonic even though the
prop tip is well below Mach-1. As you mentioned, that takes a lot of
energy that is wasted and is not available for thrust to create a
shock wave and sustain it.

When I was the test pilot on the OMABP RV-6A project, we used the
Chevy Vortec V-6 engine, the PSRU was specifically designed to turn
the prop at tip speed below 80% the speed of sound. Jess Meyers also
used a reduction ratio number that was about equal to the square root
of 2 to eliminate harmonics that could have resulted in reversed
torque pulses reflecting back into the engine. By using a reduction
ratio of 1.41 (or close to it) he eliminated many sympathetic
harmonics that may have occured.

BWB

But with the stock Corvair engine having its peak torque at 3000 (or
4000-4500 for a modified unit) it would still be best to gear it down
for a prop speed around 2500 right?

Dave

Tom Cummings wrote:
Are there any auto/motorcycle conversions that don't require gear boxes?


The Corvair engine operates successfully in direct drive configuration.
See http://www.flycorvair.com/

Badwater Bill wrote:

Nice job Richard. Nice Job.


Thank you sir!

I've heard or read that reasonable limits are about
750 feet per second for wood props
and 800, or a little more for metal props.

However...

I've seen 3000 rpm static on my engine and prop (62x29 Tennessee).
That's 811 fps tip speed! Static?
(And yes, she sings a bit at full throttle)

That could come up another couple hundred rpm at higher speed.
But 3200 rpm = 865 fps?
Holy cow!

Not bad for 40 hp, huh.

Cruise at 2800 rpm gives a stately 757 fps at 75 mph.
Fuel burn looks like just over 3 gallons per hour.


Giving credit where credit is due.
This engine was built by my neighbor, Oscar the Grouch.

Oscar is a retired police officer, and life long racing addict.

Oscar gives good motor.

He has three VW powered airplanes of his own.

His wife's "car" is a 1960's slingshot dragster with a small
block Chevy. Hand formed gloss black and gold(leaf!) aluminum body.

Grandma turns 8.80 and 152 mph in the quarter mile.


And!
Just in case the skeptics in the audience disbelieve?

I believe our (Zuehl) spring fly-in will be in May this year.
Drop in and see for yourself...

Richard