X-Mozilla-Keys:                                                                                 
Fred wrote:

Opening the cowl flap increases the "suction" downstream of the radiator,
creates a greater delta P across the radiator, and thus a greater flow.  For
a constant aircraft air speed, greater flow means higher inlet velocity at
the inlet, and so less flow deceleration in front of the inlet (less
diffusion) as you suggest.

Rather than using an inlet area as a percentage of the radiator area, I would
urge folks to think about the inlet area in terms of the inlet velocity, and
I suggest that the inlet velocity should be about half the cruise speed.
Thus at 200 knots TAS cruise, figure 100 knots through the inlet, and then
make a diffuser to take the flow to the front of the radiator.

This suggestion is just based on what seems to work with the Lopresti and
Piper low drag cowls, and is an approximation at best.  If you want to make a
smaller inlet because it is pretty, go for it.  But be aware their may be a
price to pay.

While it would be nice to have rubber inlets that are big when going slow and
small when going fast, we are stuck with rigid fiberglas, etc., and so the
inlet is a compromise.  A little inlet may be just fine at cruise and produce
superior cruise speed numbers.  But you may have to make a real flat climb at
90% of cruise speed to keep the thing cool.  Spending half an hour to get to
10,000 feet at 300 feet per minute while swilling engine-cooling fuel would
be frustrating indeed, and a high price to pay for a few extra knots.

So I come down in favor of slightly larger inlets with nice radiused inlet
lips that will spill extra flow over them with minimum turbulence in high
speed flight, and still let in lots of air (in conjunction with an open cowl
flap) for that August day climb out from Palm Springs when the OAT is still
80F at 10,000 feet.

The common mistake is to make the inlet too small, and the outlet too large.
Instead, make the inlet a bit large (and well streamlined, well rounded) and
make the outlet adjustable with a small outlet for cold day cruise, and a
large outlet for ground operation and hot day climb.

If you insist on a small inlet, be prepared to do a lot of experimental work
to optimize the diffuser because you will need a good diffuser to recover all
the velocity energy from the flow screaming through the little inlet.  I
prefer slowing the flow in front of the airplane where the deceleration is
lossless.  Slow to 50% of free stream velocity, and you enter the inlet with
75% of the available ram pressure.  Then a bad diffuser does not hurt so much.

Your research on good diffuser design is the right place to focus your effort

Fred.

 
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