One big dog house
RV8RIVETER said:
it would seem the best plenum design
is a two piece separate plenum for each row of cylinders,
like the Varieze link I posted above. Does anyone have an opinion on that?
Wade you would think two separate left/right plenums would be better, but if
you read the NASA report that I mentioned you referred to (and linked on the
Lancair web site), you see one large plenum is best.
The flow into the right cowl inlet in general is much more than the left. This
is due to the prop wash (p-factor). This effect is made worse by a standard
cowl (like Van's) verses a "NASA" 0.30 or 0.60 cowl (round inlets). The
decimal numbers refer to ratio of inlet air velocity to the free air stream
velocity (Vi/Vo).
Also the plenum volumn acts as a reservoir, so one big plenum should be
better for volumn and balance. With that said I have seen well done left/right
split separated plenums work fine. Why. Well they are better than soft leaky
seals.
The NASA report made this one basic observation LEAKS are terriable for
efficency. It's air doing nothing and robbing the pressure differential between
the area above to below the cylinders. Air just leaking into the lower cowl is
a double whammy. More air inlet area is not better if its not going where it
should. Reducing leaks means you can reduce inlet area and reduce cooling
drag. The Delta Pressure across the cylinders is the mechanism that cools
the engine.
Soft seals will leak no matter how well they are done. A vary difficult area is
the front where the cowl inlets and engine front near the spinner seals. I
have seen excellent soft seal baffles that are works of art, and some not so
good. Bottom line a sealed solid hard cover DOG HOUSE is as good as it gets.
Also as Dave Anders said it's easier to seal the cowl inlet (round) to the
plenum. Here are some pictures I cut and pasted from the NASA report.
http://img133.imageshack.us/img133/9441/nasaexcerpt5bg.jpg
Notice the coefficient of pressure is higher on the right. Its not bad using
a so called NASA/S.J. Miley/Miss State/Texas A&M cowl, designated as 0.30
or 0.60 in the report. However with the standard "STD" stock Piper cowl (not
much unlike vans cowl) you see a big spread between the right to left inlets.
Here is why widely spaced inlets work better, in addition to the reasons that
Dave Anders gave in his lecture notes:
http://img133.imageshack.us/img133/2219/vanscowl3yy.jpg
The prop beats the air to death near the spinner. Prop blade shanks near the
hub are just blunt awaful things that are just a little better than a 2x4 board.
The pressure in the inlet is much higher on the outboard edge, so air just
goes in the outboard edge and out the inboard edge if the cowl inlet, back
into the breeze. This "SPILL" air re-merging with the free airstream is spill
drag and lost efficiency.
That is why the round inlets wide spaced away from the spinner is best. Look
at a P-51. The Mustang spinner is a huge airflow improvment. We can not
do this on our plane with the engine and cowl we have.
**NASA report on Horz engine cooling, 1981 is the tip of the iceberg. There
was more work done in the mid late 70's, published by Miley and Cross in SAE
and AIAA papers. I have seen adaptations or use of this info with varying
degrees of success. The good news is the basic Barnard Holy Cowl/Sam
James Cowl makes good use of the info, but not perfect. I am using a stock
cowl and stock baffle kit to start, but modifying them to be similar to a Sam
James cowl but with differnt ratios, areas and shapes.
Dave and Tracy both run custom cowls and plenums, not off the shelf or
after market. Nothing wrong w/ a Sam James cowl if you are not prepared to
size and shape your own cowl inlets, diffuser and plenum. This could be a fun
area to play around to get a few Ka-nots.