Alan Tolle wrote:
Brian
The .049 wall 321 simply will not live in the first bend out of the
block. If you weld on patches cracks will develop right at the weld.
To my knowledge ceramic coatings have never been approved for
certified aircraft application. Exhaust systems for certified
aircraft are engineered to dissipate as much heat as possible (hence
the use of thin wall stainless tubing) into the cowl. Wrapping or
coating is prohibited because it would oppose heat dissipation. I
really don't think that we rotary engine enthusiasts should ignore
proven practice in this regard.
That said, the much higher exhaust gas temperatures of the rotary
engine must be accommodated. The simple solution is heavier wall
tubing and slip joints.
For what it is worth I used 0.125 wall 304 stainless with 1.5 inch
inside diameter for the first foot out of the engine. I made a 0.375
thick steel plate to bolt to the engine, then welded a 1 inch long
stub of straight tube to that. Then came a 90 degree bend followed by
10 inch piece of straight tube. Slip joints were used between each
piece. After that came a slip joint to as much .035 wall 321
stainless of 1.75 inch outside diameter as was necessary to get out of
the cowl.
The slip joints sound like a lot of work but if you don't put them in
now the system will break at those points and you will end up putting
them in later. The slip joints in the heavy wall stuff were made by
welding a one inch collar of 1.75 i.d. pipe to the down stream member
in the joint. Three tabs were welded above and below each joint and
laced together with .035 stainless safety wire.
The entire system turned out to be 32 inches long. Though this is
sometimes mentioned on this news letter as a tuned length for a 13B
exhaust system, such was not the case. It was dyno tested and as
noted earlier, it hurt the power, but less than anything else. It was
flown for 400 plus hours with no repairs.
Note that I am simply giving here an accurate account of history and
saying "this is what worked for me." No attempt is made to advocate
anything and the opinions of others may vary.
Hope this helps.
Alan Tolle
By definition if 32 inch hurt the power less than anything else it
obviously was tuned. Coincidentally Racing Beat sells two into one
headers with 32 inch pipes. The trick is knowing the speed of sound in
the hot exhaust gas. Smith MSAE and Morison Ph.D. in ME in their book
the "Scientific Design of Exhaust & Intake Systems" seem to think it is
1700 feet per second while as I recall the speed of sound in intake air
is around 1100 FPS. Also coincidentally they came up with a tuned
exhaust pipe length for 6000 RPM of 2 ft 10 inches or 34 inches. Page
83. BTW the brits are very good at building Formula One racing engines.
Paul Lamar
Matt Prather wrote:
snip
Being involved with the canard aviators, I have heard lots of discussion
about the relative merits of exhaust coatings on Lycontals... One point
I heard that seems to make quite a bit of sense is that a wrapped pipe
may lower under-cowl temps some, but at the expense of increased
CHT - the externally wrapped pipe is hotter and conducts heat back into
the cylinder right where you can't afford it - the exhaust port and valve.
I wonder whether a coating on the inside of the pipe would cause this
same kind of problem. You certainly would have to get a coating that
survived at EGT temps.
I wonder if this effect is relevant on a rotary - with water cooling, and
no exhaust valve to cook. Whatever, I wouldn't wrap the pipes, but
might consider an internal coating to keep the pipe from picking up any
heat.
Regards,
Matt-
VE N34RD, C150 N714BK
Assume for the moment the rotary engine exhaust pipe would live.
It of course has no valves. In theory any retention of heat within
the engine would improve BSFC by reducing the rate of heat transfer
from the expansion chamber into the coolant. Heat transfer being
directly proportional to the difference in temp between the metal
and the combustion gases.
Continental explored this very carefully in the design of the
Voyager aircraft engines. They attempted to run the engines at
a coolant temp of 250 F. I would say they were rather successful
as a liquid cooled Continental Voyager engine powered Dick and
Jeana around the world non stop. The claimed BSFC was below .4.
Paul Lamar
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