Why are low-revving, high torque engines preferred?

Merlin in 51 had a 2-1 gear reduction. 3000 rpm gave 1500 rpm prop
speed as I recall.

Big John



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

Tech Support wrote in message
...
Merlin in 51 had a 2-1 gear reduction. 3000 rpm gave 1500 rpm prop
speed as I recall.

Many of the old big "V" and inline engines were 2:1, and were using the
front end of a very strong camshaft as the prop drive, weren't they?
--
Jim in NC

"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...

Jim

Gear boxes had bearings designed to handle prop loads. Engine only saw
torque and no thrust or side loads, etc.

Only engine I can remember was either a Lyc or Con that drove prop off
cam shaft. Was not very popular for some reason and only lasted a few
years????

Someone with experience with those engines might want to chime in and
comment.

Big John
**************************************************
On Thu, 27 Nov 2008 07:15:24 -0500, "Morgans"
wrote:


Tech Support wrote in message
.. .
Merlin in 51 had a 2-1 gear reduction. 3000 rpm gave 1500 rpm prop
speed as I recall.

Many of the old big "V" and inline engines were 2:1, and were using the
front end of a very strong camshaft as the prop drive, weren't they?

Morgans a écrit :

Many of the old big "V" and inline engines were 2:1, and were using the
front end of a very strong camshaft as the prop drive, weren't they?

Rolls Royce and Allison V engines were (are) overhead camshafts.
The prop reduction gear was driven from the nose of the crankshaft.
According to Rolls Royce, the Merlin XX drive ratio was 0.42.

Best regards,
--
Gilles
http://contrails.free.fr

Tech Support wrote:
Jim

Gear boxes had bearings designed to handle prop loads. Engine only saw
torque and no thrust or side loads, etc.

Only engine I can remember was either a Lyc or Con that drove prop off
cam shaft. Was not very popular for some reason and only lasted a few
years????

Someone with experience with those engines might want to chime in and
comment.

You don't often see that because any torsional resonance problems get
amplified at an even-multiple ratio, like the cam's 2:1 ratio. Note the
final drive ratio of almost any reduction gear train & it will be some
odd number like 2.17:1, 2.85:1, etc. (Those are the options available on
a common reduction drive for rotaries, & the planetary gear set comes
from a heavy duty Ford automatic trans.

BTW, someone mentioned 'no thrust or side loads' with planetary gears.
There can actually be very high thrust loads if the gear set uses
helical cut gears instead of spur gears to get more contact area for a
given gear thickness. The drive mentioned above had to have a thrust
bearing added to its input shaft after early testing because it
destroyed the light duty thrust bearing on the rotary's E-shaft.

Charlie

On Nov 27, 11:58*am, Tech Support wrote:

Only engine I can remember was either a Lyc or Con that drove prop off
cam shaft. Was not very popular for some reason and only lasted a few
years????

The Continental Tiara 6-285. Kept breaking that shaft, IIRC.
There's still a TCDS on it so there's a few out there yet.

Dan

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?

"Charlie" wrote

You don't often see that because any torsional resonance problems get
amplified at an even-multiple ratio, like the cam's 2:1 ratio. Note the
final drive ratio of almost any reduction gear train & it will be some odd
number like 2.17:1, 2.85:1, etc.

Perhaps an even larger factor is to get different teeth meshing together in
a cycle, each time around. it is better to not have the same mesh, time
after time, although what you say about harmonics is also a factor.

In the really big "warbird"engines, the gearboxes had to be built so
hell-for-stout to deal with the HP levels, the harmonics were not as much of
an issue because the resonance was hard to achieve with the components being
so stiff.
--
Jim in NC

wrote

The Continental Tiara 6-285. Kept breaking that shaft, IIRC.
There's still a TCDS on it so there's a few out there yet.

TCDS??
--
Jim in NC