Why are low-revving, high torque engines preferred?

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?

Thanks in advance for enlightening me,
Oliver

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.

On Nov 25, 9:19*am, Oliver Arend wrote:

But why does the torque have to be produced by the engine in direct
drive?
------------------------------------------------------------------------------------------------------
Hahahahahahaaaa.... (wheez!)

Sorry Oliver, but that's just so funny.... I forget that most
subscribers to this list are not familiar with 'real' aircraft
engines, ALL of which incorporate some form of speed reduction.
'Real' airplane engines are those big round things with all the
jugs... or those sleek narrow things tucked into the nose of a P-51 or
an Me-109.

'Smalll' aircraft engines.... typically those below 550cid or there
abouts are an EXCEPTION and tend to use direct drive. All the rest of
the world uses 'real' engines... or did, until Frank Whittle got
someone to listen to his 'crazy' ideas.

But as to the core of your question, the reason we don't find speed
reduction units on small engines (*) is their weight. The smaller the
engine, the greater the 'overhead' for a PSRU (Prop Speed Reduction
Unit).

(*) But there are exceptions. Continental made a geared A-85... Just
look for a 'G' in the Type Number. I'm not familiar with any others
but Lycoming and Franklyn probably made them.
-----------------------------------------------------------------------------------------------------------------------------
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?
-----------------------------------------------------------------------------------------------------------------------------

That is correct in theory, especially if the engine or 'power unit' is
water-cooled. Rotax has recently earned a 1500 TBO for the 1300cc
power unit of their 912 engines, although the TBO of the gear
reduction unit remains around 850 hours. (Maybe a Rotax mechanic can
jump in here and give us the actual numbers.)

--------------------------------------------
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.
-----------------------------------------

First off, much of the 'issue' is bureaucratic, in that the FAA
requirements for a type certificate will presently cost about a
quarter of a million dollars to satisfy. That is, over and above your
cost of development, the PAPERWORK will add another quarter-mill to
the pot. Given the market, it simply doesn't make good economic sense
to put that kind of money into ANY aspect of 'General Aviation'
today.

But that doesn't mean small, modern engines such as you've described
aren't out there. For some nice examples of modern light-airplane
engines, take a peek behind the propeller of any of the RPV's and tell
me what you see :-) There are also some shoe-box size turbines
slinging props that beg to be bolted to a KR or similar. In the 50
to 70hp range, the hot section is out of a GPU or APU. As the United
States digs it's self deeper into Third World status we'll see more of
these units appear as surplus... if we don't sell them all to
China :-)

-R.S.Hoover

On Tue, 25 Nov 2008 09:19:01 -0800 (PST), Oliver Arend
wrote:

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?

Thanks in advance for enlightening me,
Oliver
It is done often in the ultralite world - but gear drives add
complexity. If a plane doesn't have a particular part it can't fail -
so the large displacement, slow turning torque machines still win.
Lycoming has made several geared engines over the years and none has
been particularly successfull. I believe the Merlin (or one of the
big "V" engines) was also geared.

The most common geared aircraft engine today is the Rotax 912 series.

On Nov 25, 7:47 pm, wrote:

Lycoming has made several geared engines over the years and none has
been particularly successfull. I believe the Merlin (or one of the
big "V" engines) was also geared.

Both the Merlin and Allison V-12s were geared. And most of the
big radials were geared. It was one of the few ways to get more
horsepower out of a given displacement.

R-1830 radial cutaway, with gears in the front of the case:
http://aviatechno.free.fr/vilgenis/i...830_02_730.jpg

Merlin cutaway: http://www.thunderboats.org/history/...tory0324_1.jpg

Common geared Lycs: GO-435 and GO-480. Continental had the GO-300 and
GTSIO-520.

Daimler Benz DB601a: http://www.aviation-history.com/engines/db601a-1.jpg

Geared engines are nothing new at all.

Dan

On Nov 25, 8:29*pm, wrote:
On Nov 25, 7:47 pm, wrote:

Lycoming has made several geared engines over the years and none has
been particularly successfull. I believe the Merlin *(or one of the
big "V" engines) was also geared.

* * * *Both the Merlin and Allison V-12s were geared. And most of the
big radials were geared. It was one of the few ways to get more
horsepower out of a given displacement.

R-1830 radial cutaway, with gears in the front of the case:http://aviatechno.free.fr/vilgenis/i...830_02_730.jpg

Merlin cutaway:http://www.thunderboats.org/history/...tory0324_1.jpg

Common geared Lycs: GO-435 and GO-480. Continental had the GO-300 and
GTSIO-520.

Daimler Benz DB601a:http://www.aviation-history.com/engines/db601a-1.jpg

* * * * Geared engines are nothing new at all.

* * * *Dan

One of the reasons that converting an auto engine by adding a PRSU is
complicated is that auto engine bearings are not designed for thrust
or side loads. The rear bearing of an auto engine in a car just sees
torque loads.

The PRSU has to be coupled to the crank in the same way as an auto
transmission which means that the small gear or pulley has to 'float'
on its own bearing and couple to the engine's flywheel through
something like a flex coupling.

Geared radial and a few in-line engines used a planetary gearsets.
This is easier since the "sun gear" sees no thrust or side loads. The
"ring gear" sees all those loads.

On Tue, 25 Nov 2008 19:29:06 -0800 (PST),
wrote:

Both the Merlin and Allison V-12s were geared. And most of the
big radials were geared. It was one of the few ways to get more
horsepower out of a given displacement.

Gearing an engine doesn't increase the horsepower; it multiplies the
torque and divides the rpm down to a more usable (by the propeller)
level. It does allow you to build a faster turning (and thus higher
horsepower) engine and still be able to use it.

In general, the way to get more horsepower out of a given displacement
is to turn the engine faster, within limits of course. With the
higher rpm's come increased wear and heat. In the "old" days (up to
the mid 1930's or so) the available materials weren't adequate for a
high revving engine, so low rpm's were the norm, and by happy
coincidence the avilable rpm's were pretty well matched to propeller
sizes convenient for the aircraft. As the technology advanced and
higher rpm's became reasonable, reduction drives began to appear,
especially on higher powered military aircraft... with some compromise
(as others have pointed out) in reliability.

Most of the small aircraft engines in use nowadays are derived from
those 1930's engines, with only minor improvements. The A-65, for
example, was redlined at 2300 rpm; the A-75, if I recall correctly,
was the same engine upgraded to turn a little faster, and today's
0-200 turns around 2600 rpm if I'm not mistaken. By contrast, modern
car engines are redlined at up to 8000 rpm.

There two main reasons we're still using the "old style" aircraft
engines. First is cost; not only does the gearing and such cost more
money, but the existing engine designs are long amortized. Developing
a new engine costs a lot of money, especially with the costs of
_certifying_ a new engine. Second is reliability and longevity; a big
slow turning engine is more reliable and lasts longer than a smaller
fast turning engine of the same horsepower.

Where we _are_ seeing reduction drive engines is in the ultralight /
light sport area, where weight is much more critical. The ultralight
movement introduced fast turning 2-stroke engines, mostly snowmobile
derived, to aviation. When your engine turns 6500 rpm you NEED a
reduction drive! For an ultralight, light weight is far more
important than a 2000 hour TBO. In the case of the Rotax 912, a
geared 4-stroke, Rotax was already used to building engines with
redrives, so it made sense for them to take that approach.

-Dana
--
The gene pool could use a little chlorine.