Here is an aircraft engine by Hexidyne.
www.hexidyneaviation .com

This engine is rated at 60 hp AT 6000 rpm so a

PSRU

is necessary
to make it a practical aircraft engine.

Note the useless torque peak at 4500 RPM. A

simple

change in the intake
manifold might move the torque peak up to the

rated

6000 RPM speed and
probably result in ten more HP. On the other

hand

the 6000 RPM breathing
may not be limited by the intake manifold at all

but

by the intake
valves that are probably too small.

This indicates a lack of understanding on the

part

of this engine's designer
about the role torque plays in an aircraft

engine.

Paul Lamar

tim tracey wrote:

So, what you're saying is that anything beyond the
venerable 5252rpm threshold is the aircraft

engines'

true useful powerband where HP is higher than the
torque(assuming we don't go into the area of

serious

drop-off of either HP, torque or engine life!)?

I'm

not arguing, just trying to see the light. Thanks!

Tim

I don't follow you. What is the question? An
aircraft engine needs
peak HP. Peak HP is generated by tuning the intake
so  peak torque
occurs at peak RPM. Peak RPM is of course limited by
the diameter
of the prop. Unless of course you want to sacrifice
peak HP for lower fuel burn at cruise RPM.

Paul Lamar

Hmmmm....well, I'm probably still not going to ask
this question clearly, but here goes......let me put
in a scenario and maybe you can explain it in terms I
can relate to. The following are applied to the same
aircraft and, let's say, a 68" 3-blade inflight
adjustable prop system with 30-105 inches of pitch.

A. Peripheral-ported 13B with tuned intake/exhaust at
peak HP mated to a 3:1 PSRU.

B. Bridge-ported 13B with tuned intake/exhaust at peak
HP mated to a 2.5:1 PSRU.

C. Mild street-ported 13B with tuned intake/exhaust at
peak HP mated to a 2:1 PSRU.

Given that the peripheral doesn't really come into
it's own until about +7,000RPM(~250HP), and the
bridged 13B(~200HP)a little further down the RPM band,
and the streeter(~175HP)just above the norm, does it
stand to reason that the peripheral is going to make
more power and relate that better to more available
torque because of the larger PSRU working in concert,
or are one of the others actually better for overall
performance with the given gear reductions; or is
there something else all together....ballparking, I
mean? I realize there are engine life problems, fuel
loads, PSRU life, prop limits, system costs,
ect.....I'm just trying to grasp the jist of the
situation, not the 'devilish' details. Thanks!

Tim

Not true on the peripheral port. A peripheral port engine
will generate 240 HP at 6000 RPM with the right tuning.

Now do you still want to ask the rest of the questions?
Go back and start your thinking over.

Paul Lamar

There is no such thing as a useless torque peak, and engines
only develop max hp at max torque if they are redlined at that rpm.
Any engine will deliver it's max hp at a higher rpm than the torque
peak = even the amazing Le Mans engine. They have probably
put the torque peak as high as practical for the engine design,
What is so useless about that?  see no "lack of understanding
by the engine designer -- his HP per lb of installed engine
weight is pretty good. - Vance

For an aircraft engine if the torque peak does not coincide with
the red line or shortly below it is a useless torque peak. All 
direct drive aircraft engines like the opposed series have a
straight line function of HP verse RPM indicating the torque
peak is at or near the red line. These A/C engines have a 2700 RPM red
line or there abouts so the ratio of intake valve area to piston
area is not beginning to limit the breathing. In piston engines 
used in cars the breathing starts to fall off around 4500 RPM and so too
does the torque. This cause the HP curve to start to flatten out.

The rotary on the other has no such breathing limitations and as proven
by this chart almost any torque peak at almost any RPM can be had by merely 
tuning the intake runners to the proper length.

So just like a large displacement direct drive aircraft engine
the rotary has a straight line HP curve more or less.
 
In the case of this 2 cyl horz. opposed  engine the intake valves are too small
to achieve this end. If the engine is structurally designed to have
a red line at 6000 RPM an improvement in HP can be had by moving
the torque peak up closer to 6000 RPM by improving the intake valve area.
That may entail using two intake valves per cylinder but it will
never breathe as well as rotary and it may be impossible to get 
the torque peak up to 6000 RPM :)

Paul Lamar
 
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