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Bill, Without the original document in which the theory
lead to the equations there is no way to be certain. But,
apparently at least the diameter of the exhaust tube is based (in part) on the
amount of gas it is expected to carry - ergo, the need to know whether the gas
volume is based on 40 cid cylinder or a 100 cid cylinder rather than the number
of rotor faces. At least that seems to make sense to me. More
cylinder volume would imply needing a larger diameter tube.
Second, the length of the exhaust tube is apparently
base on the scavenging effect of the reflected "suction" wave and the rate
of opening and closing of the exhaust port - so there could be
that the actual length for the rotary should be 1/2 of what the equation for a 4
stroke piston engine since a 2 stroke engine (which the rotary acts like exhaust
wise) would do just that - just don't know, but that would be my best
guess.
None of this is simply, but my experience is that you can
work it out using either rotor reference (rotor rpm, rotor degs) or e shaft
reference (which seems to be the way the books mostly do it) as long as you stay
consistent. My preference is to work it out rotor reference (makes my head
hurt less) and then convert every thing to e shaft reference.
Ed
Sent: Saturday, November 20, 2010 9:42 PM
Subject: [FlyRotary] Re: Exhaust Calcuations
Here is where I show my
lack of understanding….
Isn’t 40 cu in the size
of one rotor and shouldn’t this calculation be based on the rotor faces instead
of the entire rotor? With a recip engine the exhaust valve on
a cylinder opens once every other revolution of the crankshaft, but with the
rotary, a rotor face exhausts every revolution of the e-shaft. So there
are twice as many pulses in a rotary header pipe as in a recip
header.
So can the calc for a
recip be used for a rotary without some
modifications?
Bill
B
From:
Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Ed Anderson
Sent: Saturday, November 20, 2010 12:21
PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Exhaust
Calcuations
I did some algebraic manipulations
to find HPL based on a selected HPD (tube diameter) and got an
answer. However, I am somewhat skeptical of it although it does tend
to fit with the general theory of larger dia pipes = shorter
lengths.
By arbitrary selection an
diameter - you automatically select the rpm factor in the
equation (but, its hidden in the HPD value). The way you check what your
new rpm sweet spot with the arbitrary diameter is to vary the rpm in the
first set of equations until you get the length determined by manipulated
equation.
So the revised equation for 1.8" dia
tubes gave 27.58" inches for the HPL. You vary the rpm value in the first
equations until you get an HPL of 27.58" and you get an rpm of approx 7900 - so
I agree, probably too high for your
configuration.
Sent: Saturday,
November 20, 2010 11:18 AM
Subject:
[FlyRotary] Re: Exhaust
Calcuations
Ed,
OK, that answers my question. By going with 1-7/8"
OD pipe (.049 wall), it would be tuned for about 7800 rpm. Obviously, this
is too high for our purposes. I'll go with 1-5/8 (.063 wall) to shoot for
6700 rpm peak power. This is typically the rpm I see on ground roll and
early climb before I start adjusting the prop down to a lower rpm to transition
to cruise climb.
Have you used the header designer tool available from
Burns Stainless? You need to know all the timing numbers, rpm, etc.
I may give it a try and see what I come up with. I have a test
p-port housing that I made up to see if the tube could be welded to the inner
liner. So, I can use that with a spare rotor to verify the EVO and EVC
numbers.
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