REFLECTOR: Exhaust question

[Al Gietzen]

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.

Would this be better than the forward pipes or the rear pointing ones
getting soot on the prop?

 

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.

 

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.

 

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.

 

Draw your own conclusions.

 

Al

 

 

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