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.