Ron Wanttaja wrote in message . ..
[Ron throws his oar in]

On Thu, 4 Dec 2003 20:24:00 -0500, "Mark" wrote:

Does a prop turning at 2300 rpm on a 150hp motor produce more thrust than a
prop spinning 2300 rpm on a 100 hp motor ?

Assuming the engines are mounted on a ground test stand, and the propellers
are "normal" (no paddlewheels, etc.):

If the props are identical fixed pitch propellers, the same thrust is
produced. Both engines are producing the same power.

If the engines are NOT producing the same power, it is because their
throttle positions and the propellers. Either may produce more thrust,
depending on the combination of the two parameters. For instance, the 150
HP engine could have controllable-pitch prop at flat pitch....it might turn
2300 with the throttle barely cracked.

If both engines are producing their rated horsepower, the 150 HP engine
produces more thrust. Its propeller must move more air in order to absorb
the higher power at the same RPM.

Ron Wanttaja


Typical static thrust for the engines in light airplanes is around
three pounds per horsepower, IIRC. This figure will vary somewhat with
prop efficiency and rated RPM. A lower rated RPM for a given HP will
produce more thrust, as less of the energy is lost to prop blade drag.
An example of this is the Thielert Centurion diesel now STCd for
the Cessna 172 N and P models, among others. It produces 135 Hp at
2300 instead of the 160 at 2700 from the Lyc it replaces, yet the
performance doesn't suffer at all. Drag is about 28% less on 16% less
power. These are my rough calculations and I'm open to correction. Try
http://www.centurion-engines.com/
Turning an engine much faster to increase HP has been a standard
trick of both auto manufacturers and aircraft engine makers, but so
much of the increased HP just goes into making more noise, especially
when driving a prop.

Dan