Why are low-revving, high torque engines preferred?

Oliver Arend schreef:
This may sound like a stupid question; I realize the prop can only
turn at a certain speed to avoid transonic effects at the tips and has
to be turned with a certain torque to transmit the power needed/
produce enough thrust. So far so good.

But why does the torque have to be produced by the engine in direct
drive? Couldn't weight and space be saved by using a high-revving,
small displacement engine (such as a car or even motorcycle engine)
with a reduction gearbox? I'm aware that a reduction gearbox will add
weight (but not that much?), complexity and failure modes, and that
transmitting the forces created by the prop to the airframe could be
an issue. Does it boil down to the price? Is a Lycosaur engine cheaper
than, say, a motorcycle engine of equivalent power plus the gearbox?

At the risk of feeding the trolls:
There are several examples of the setup you describe.
The Rotax 4-stroke engines have a (belt?) reduction.
In Europe one sees more and more BMW-motorcycle engines
driving planes through a reduction, either gear or belt.

I even seem to remember car/bike engines driving a prop through
the original gearbox, fixed in one gear; but never with good results.

The main disadvantage of automotive engines is that they were
never designed for delivering their output power over
a prolonged period of time, like an aircraft engine does.
This also applies to an even greater degree to motorbike engines.

But even in the country of Lycosaurs some people
are flying behind (or before...) car engine conversions,
you might wish to search for the Corvair engine in particular.

PS if you are interested in cheap engines for modest homebuilt planes,
read every page of Bob Hoover's blog, frequently mentioned on these pages.
Good reading both for wisdom and for technical insight!

Hope this helps,

"jan olieslagers" wrote in message
...
Oliver Arend schreef:
At the risk of feeding the trolls:
There are several examples of the setup you describe.
The Rotax 4-stroke engines have a (belt?) reduction.
In Europe one sees more and more BMW-motorcycle engines
driving planes through a reduction, either gear or belt.

There are also several versions of Lycoming / Contintntal / other "brand
name" aircraft engines with reduction units.

Adds cost, complexity, and can result in durability issues (one more thing
to go wrong).

Sometimes it works out better, sometimes not. It all depends on the details
of your objectives.

For extra points - why do Ford V8's have overhead cams and Chevy V8's tend
towards pushrods - you would thing that one would be "better", right? Why
are they different?
































Because Ford management gives the engine designers a "horsepower per
displacement" objective and Chevy magement gives them a "horsepower per
package volume" objective.

(per engine guys who have worked at both shops)

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

"jan olieslagers" wrote in message
...
|
| At the risk of feeding the trolls:
| There are several examples of the setup you describe.
| The Rotax 4-stroke engines have a (belt?) reduction.
| In Europe one sees more and more BMW-motorcycle engines
| driving planes through a reduction, either gear or belt.
|
| I even seem to remember car/bike engines driving a prop through
| the original gearbox, fixed in one gear; but never with good results.
|
| The main disadvantage of automotive engines is that they were
| never designed for delivering their output power over
| a prolonged period of time, like an aircraft engine does.
| This also applies to an even greater degree to motorbike engines.
|
| But even in the country of Lycosaurs some people
| are flying behind (or before...) car engine conversions,
| you might wish to search for the Corvair engine in particular.
|
| PS if you are interested in cheap engines for modest homebuilt planes,
| read every page of Bob Hoover's blog, frequently mentioned on these pages.
| Good reading both for wisdom and for technical insight!
|
| Hope this helps,

Ditto all the above, but also torsional vibration and resonance.

An engines crankshaft is constantly changing speed, slowing during
compression, and accelerating upon combustion. A propeller is a very large
flywheel, that likes to turn the same speed all the time. Direct drive
applications have so little flex and such a fast recovery time after each
combustion event, that the ill effects of torsional vibration and
differences between crankshaft and propeller speed are greatly minimized.
All types of reduction systems have flex or free play that allows this
speed/vibration difference to cause serious problems. Further, the design of
each reduction type causes it's resonance problems to differ greatly from
one design to the next, and one rpm range to the next, and the end results
can be utterly amazing in their ability to destroy the overall system.

Simply put, designing a reduction system is much, much more complicated than
simply choosing the right gears, belts or sheaves. Therefore the reliability
of these systems are still a good bit suspect in most peoples minds, and
properly designing solutions for these issues add more weight than the
uninitiated would suspect.

There is also the issue of in-flight restarts. If an engine looses power in
flight, due to changing fuel tanks, carb ice, etc. Direct drive engines are
much more likely to keep the engine turning until the pilot solves the
problem. Reduction engines are much more likely to stop turning and force
the pilot to rely on the electrical system for a re-start.

In article ,
jan olieslagers wrote:

Oliver Arend schreef:
This may sound like a stupid question; I realize the prop can only
turn at a certain speed to avoid transonic effects at the tips and has
to be turned with a certain torque to transmit the power needed/
produce enough thrust. So far so good.

But why does the torque have to be produced by the engine in direct
drive? Couldn't weight and space be saved by using a high-revving,
small displacement engine (such as a car or even motorcycle engine)
with a reduction gearbox? I'm aware that a reduction gearbox will add
weight (but not that much?), complexity and failure modes, and that
transmitting the forces created by the prop to the airframe could be
an issue. Does it boil down to the price? Is a Lycosaur engine cheaper
than, say, a motorcycle engine of equivalent power plus the gearbox?

At the risk of feeding the trolls:
There are several examples of the setup you describe.
The Rotax 4-stroke engines have a (belt?) reduction.

Geared in the 912 series, at least.

It's the only one I've seen taken apart, so far.