Why are low-revving, high torque engines preferred?

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.

"Dana M. Hague" wrote in message
...
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.

To expand the above points just a little:

IIRC, snowmobiles are/were derived from chain saws--which required an engine
light enough for a man to carry and use as a hand held tool. That made them
an obvious choice for another application were light weight was the most
critical factor--even at the cost of reliability and maintenance
intervals...

On Thu, 27 Nov 2008 09:37:54 -0500, "Peter Dohm"
wrote:

IIRC, snowmobiles are/were derived from chain saws--which required an engine
light enough for a man to carry and use as a hand held tool. That made them
an obvious choice for another application were light weight was the most
critical factor--even at the cost of reliability and maintenance
intervals...

I've never heard that, seems unlikely... they are really very
different, except for both (along with outboard motors) being
2-strokes. All for the same reason of light weight.

-Dana
--
If you glue a piece of toast, butter side up, to your cat's back, and drop it from a high place, which way will it land?

"Dana M. Hague" wrote in message
...
On Thu, 27 Nov 2008 09:37:54 -0500, "Peter Dohm"
wrote:

IIRC, snowmobiles are/were derived from chain saws--which required an
engine
light enough for a man to carry and use as a hand held tool. That made
them
an obvious choice for another application were light weight was the most
critical factor--even at the cost of reliability and maintenance
intervals...

I've never heard that, seems unlikely... they are really very
different, except for both (along with outboard motors) being
2-strokes. All for the same reason of light weight.

-Dana
--
If you glue a piece of toast, butter side up, to your cat's back, and drop
it from a high place, which way will it land?

Today, thanks to a substantial market with plenty of discretionary money,
engines for snowmobiles and ultralights have improved drastically.
Gardening equipment also runs far more reliably today--and a portion of that
improvement may have come from improvements made first on ultralights and
snowmobiles.

But it was not always so--and I recommend that you ask a few of the old
timers why they used to refer to some ofthe ultralight areas by nicknames
such as "the killing fields".

Peter

"Peter Dohm" wrote in message
news |
| "Dana M. Hague" wrote in message
| ...
| On Thu, 27 Nov 2008 09:37:54 -0500, "Peter Dohm"
| wrote:
|
| IIRC, snowmobiles are/were derived from chain saws--which required an
| engine
| light enough for a man to carry and use as a hand held tool. That made
| them
| an obvious choice for another application were light weight was the most
| critical factor--even at the cost of reliability and maintenance
| intervals...
|
| I've never heard that, seems unlikely... they are really very
| different, except for both (along with outboard motors) being
| 2-strokes. All for the same reason of light weight.
|
| -Dana
| --
| If you glue a piece of toast, butter side up, to your cat's back, and
drop
| it from a high place, which way will it land?
|
| Today, thanks to a substantial market with plenty of discretionary money,
| engines for snowmobiles and ultralights have improved drastically.
| Gardening equipment also runs far more reliably today--and a portion of
that
| improvement may have come from improvements made first on ultralights and
| snowmobiles.
|
| But it was not always so--and I recommend that you ask a few of the old
| timers why they used to refer to some ofthe ultralight areas by nicknames
| such as "the killing fields".
|
| Peter
|
|

I'm an old timer, with a good bit of experience in ultralights, but I don't
recall engine reliability being all the bad in the early days, or having
much to do with the early ultralight fatalities.

On Fri, 28 Nov 2008 14:01:36 -0500, "Peter Dohm"
But it was not always so--and I recommend that you ask a few of the old
timers why they used to refer to some ofthe ultralight areas by nicknames
such as "the killing fields".

Most of the early ultralight fatalities were due to lack of training
and the occasional structural failure, not engine failure... not that
there weren't frequent engine failures, too. And even at that, many
of the engine failures were pilot related... either not knowing
anything about engines (again, lack of training) or pilots who thought
they could treat a 2-stroke just like a Continental 0-200. How many
pilots fried their Cuyuna engines because they didn't have an EGT to
keep track of the temperatures?

-Dana (who first flew an ultralight in the 1980's, and still flies a
Cuyuna powered ultralight today(yes, with EGT and CHT))


--
People in cars cause accidents. Accidents in cars cause people.

On Fri, 28 Nov 2008 08:30:06 -0500, Dana M. Hague
wrote:

On Thu, 27 Nov 2008 09:37:54 -0500, "Peter Dohm"
wrote:

IIRC, snowmobiles are/were derived from chain saws--which required an engine
light enough for a man to carry and use as a hand held tool. That made them
an obvious choice for another application were light weight was the most
critical factor--even at the cost of reliability and maintenance
intervals...

I've never heard that, seems unlikely... they are really very
different, except for both (along with outboard motors) being
2-strokes. All for the same reason of light weight.

-Dana
Some of the first ultralights DID run chainsaw motors - just like the
go-cart crowd. Some of the small aircooled outboard engines were also
used - and some early snowmobiles also used a converted aircooled
outboard. (horizontal twin Johnson Snow Cruiser and OMC)