REFLECTOR: Exhaust question

[doug.holub at tx.rr.com]

"One thing to keep in mind is that a plume of exhaust going perpendicular to free stream constitutes drag."

Also lift!
---- Al Gietzen <ALVentures at cox.net> wrote: 
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> Has anyone put straight down exhaust pipes on a Velocity? My measurments
> show that they would be 8 inches or less if cut just outside the cowl.
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> Would this be better than the forward pipes or the rear pointing ones
> getting soot on the prop?
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> One thing to keep in mind is that a plume of exhaust going perpendicular to
> free stream constitutes drag.  Ideally, you’d like to have the exhaust
> exiting straight back.
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> I run the single 2 ¾” exhaust from my 265 hp rotary out the back of the
> cowling through the prop about 12 inches out from center.  The end of the
> pipe is about 6 ½” from the 3-blade Catto prop.  Catto’s opinion was that it
> should not be a problem.  I measured the max temp on the surface of the prop
> with temp sensitive tape, and IIRC it was 130-140F.  Not positive now about
> the number; but I am certain that the conclusion was ‘No issue’.  Over time
> there is just a slight discoloration of the acrylic urethane clearcoat on
> the prop, confirming that it is not damaging.
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> Note that my engine is running just over twice the rpm of the prop, and
> rotary exhaust is hotter than that from a piston LYC.  The exhaust expands
> rapidly from the exit of the pipe, so the expansion cooling makes a big
> difference; and the prop blade is only in the hot region about 10% of it’s
> circuit, so it is seeing a lot more cool air than hot.
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> Draw your own conclusions.
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> Al
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