Hi Bill, Try this explanation:1. Torque is a direct result
 of the
 combustion
 chamber's volumetric efficiency (rotary or piston
 engine). When this
 efficiency peaks; torque peaks.

 2. The V.E. is mainly governed by the intake system. In
 the intake pipe
 the rush of air resonates, that is each time the
 combustion chamber
 intake closes (whether rotary, 2cycle piston or 4cycle
 piston) the
 air rush stops (sometimes even reverses).

 3. This very brief stop sends an energy pulse back up the
 pipe toward the
 open inlet.

 4. # 2,and 3 change with RPM.

 5. A perfect length intake pipe for a GIVEN RPM will
 contain that energy
 and
 revert it to push the next charge TOWARD the combustion
 chamber.
 This is "resonating."

 6. This is important: Look at #5 again. The pipe length
 that works
 perfectly at XXX RPM is not the correct length for YYY
 rpm. So, what to
 do
 Adjustable length intake pipes have been used on racing
 engines but
 are too cumbersome for aircraft use. So the next best
 choice is to
 pick an RPM that the engine likes and tune the intake to
 operate
 there and accept less V.E. at RPM's above and below
 that engine
 speed.
 The aircraft piston engine (big bore/big stroke = lots of
 mass moving back and forth large distances) is happiest
 at about
 2500 RPM and the intake is designed around that.
 NASCAR engines
 (big bore/ short stroke = lightweight mass moving back
 and forth
 short distances) are happiest around 8000 RPM and their
 intake is
 designed around that (put a NASCAR intake manifold on
 your family
 ride and you will be terribly disappointed)
 The rotary engine
 (lots of mass but NO DIRECTIONAL CHANGE) is happiest
 around
 6000 RPM (rotor speed is around 2000 RPM, bearing loads
 are
 lightest, V.E. is pretty good, vibration is quite low,
 power
 output vs. engine wear is very good).

 So with an engine that
 wants to run forever at 6000 RPM that's where
 aircraft conversions
 are run and the intakes are designed for that speed. To
 get back
 to a proper propeller speed requires a +/- 40% reduction
 box.

 Seems like a hell of a penalty until you look at the
 whole picture.
 Even with the added weight, the power to weight ratio for
 a P-Ported
 rotary easily surpasses today's standard aircraft
 engine.

 If you've
 ever shock cooled your Lycoming, your repair bill
 exceeded the cost
 of TWO COMPLETE rotaries. Can't happen with liquid
 cooling. Have you
 priced a Continental exhaust valve lately Rotaries have
 no exhaust
 valves. This is only some of why we're here. Stay
 with the newsletter,
 there are nuggets of valueable info here.

 Hope this
 affords some more
 understanding.
 Dave M

 I agree with most everything you said Dave - except for
 the bit about
 aircraft engine intakes being tuned for 2500rpm.I
 don't think
 they're tuned at all except for easy packaging.Regardless of the
 engine type, tuning for 2500rpm will require _long_
 runners.

 This
 site is interesting:
 http://www.wallaceracing.com/runnertorquecalc.php
 This page has a model for calculating runner specifics.2700 rpm and
 360 cu-in in four cylinders (a Lycoming) gives runners 31
 inches long.

 A similar length pipe would be required for the exhaust.Most
 aircraft don't have room for such long runners.The
 F1 racers are
 probably the closest - with headers-collectors on some of
 the O-200's.

 They're quite long (24" - a guess), but the
 engines turn ~4000 rpm.
 Headers optimized to work at the stock 2750 rpm red line
 would be
 significantly longer.The F1 rules don't allow
 diddling with the
 intake configuration - otherwise you'd see long intake
 spiders too.

 The model doesn't really appear to be set up for a
 Wankel rotary.A
 160 cu-in two rotor (13b) at 6000 rpm calculates to a
 14" runner,
 which experience has shown is too short.There's
 probably something
 about the multiple rotor faces sharing the same runner,
 and that the
 rotor turns at 1/3 of the e-shaft speed that comes into
 play - I don't
 have that all figured out...It seems like the calculated
 runner
 length is about 1/2 what it should be.


 I would add to #2 above that the reason the intake is so
 important to
 VE is that on a naturally aspirated engine, only
 atmospheric pressure
 is available for cylinder filling.I think things look
 different if
 you have forced induction.

 Regards,
 Matt-

 Matt,
 A tuned intake is a function of velocity which in turn is a
 function of
 intake diameter and length and RPM.
 If you have a set diameter and RPM you can work out the
 rest.
 George (down under)

 Or you can just copy this.
 Paul Lamar


 This discussion is a little over my head (and I currently have the flue)
 but I wounder what effect a device such as a standpipe commonly used in
 plumbing water systems would have on the resonance of the intakeAs the
 intake port is closed, excess air would in part, be diverted into the
 stand pipe.As the port opens, it would add to the air going into the
 chamber.

 For those of you who are not versed in plumbing 101, a stand pipe
 is an additional vertical pipe with one end caped and the other
 is attached to the water line in the proximity of a valve.It prevents
 what is called a hammer noise in the Water line when the valve is
 closed quickly diverting the water flow and compressing the air
 in the standpipe.

 Joel

 I'm taking three aspirin, so call me in the morning.

 Joel,
 The standpipe idea is used on single cylinder dirt bikes. They
 came up with the idea of calling it a"Boost Bottle."
 Laughable name but the idea is the same. Not sure if it
 will help a rotary as the pulses are much closer together,
 but the idea has been used before.
 Bill Jepson

 Its is really a Helmholtz resonator.
 Look it up on google.


http://www.dinamoto.it/DINAMOTO/on-line%20papers/risuonatore/risuonatore.html


 Late model Porsche engines use the principle.

 Paul Lamar

That is a very interesting paper.

My comment refers to one point only, namely that of Matt, which reads:

/"The model doesn't really appear to be set up for a Wankel rotary.
A 160 cu-in two rotor (13b) at 6000 rpm calculates to a 14" runner,
which experience has shown is too short.There's probably something
about the multiple rotor faces sharing the same runner, and that the
rotor turns at 1/3 of the e-shaft speed that comes into play - I don't
have that all figured out...It seems like the calculated runner
length is about 1/2 what it should be. "
/
The speed of 6000 rpm in the Rotary would compare to 4000 rpm in a Recip.
Plugging in 2/3 the speed or here 4000 rpm instead of 6000 would be more
suitable for any calculator designed for the RPE.

In the Recip the crankshaft turns 180 degrees from TDC to BDC, while in
the Rotary the crankshafts turns 270 degrees from TDC to BDC, (so to
speak).

Regards
Rolf


Paul,
On the rotary engine, 6000 rpm makes 6000 pulses per intake tube. On 4
stroke piston engines it is only 3000 pulses at 6000 rpm.
The piston engine intake has a large (relative) time lag from the valve
closing to when the intake valve opens again.
The rotary has a very short time span of the port being closed so we end up
with a very different looking pressure cylce in the intake runner.
This explains why the tuning software for piston engine intake runner length
doesn't work directly on the rotary.
Hope you are enjoying your trip,
Larry