X-Mozilla-Keys:                                                                                 
Fredmoreno@aol.com wrote:

 A common misperception is that putting hot air exits in low pressure areas
decreases drag.  Not so.  It DOES improve engine cooling because the pressure
difference between the inlet (ram pressure) and discharge is maximized which
improves flow rate through the cowl and thus improves cooling.  But the hot
air flow exiting the cowl is deficient in momentum and so when injected into
a low pressure area (which is a high velocity area in accordance with
Bernoulli) it has insufficient momentum to overcome the rising pressure as
the local flow slows back down to the free stream velocity. Separation
results and thus overall airframe drag is increased.  This was demonstrated
by the Germans in W.W.II with the ME 109 (and reported by Horner) and
confirmed for a modified Piper Aztec in the work of Miley in the 1980s.

So, dumping the flow under the cowl is just fine, except when you need
maximum pressure drop across the engine or radiator, but don't have enough
ram pressure (hot day climb).  In this case, one uses cowl flaps which when
open increase the exit area, and create some suction to improve the pressure
difference available.  But at the cost of considerable drag.  But since this
occurs during climb, the drag is not that great (low speed) but cooling is
much better.

Another common fallacy: small inlets means lower drag.  Maybe so, but not
always.  It depends.

If the airplane is blunt in the front (like a radial or see the front of a
Bonanza with its big chin oriented against the flow) the flow slows in front
of the inlets, and a larger inlet is appropriate to induct the slower moving
flow.  Being blunt, there is lots of room for big inlets.  An overly small
inlet will create extra losses and more drag.

If the airplane is pointy (few or no surfaces inclined at angles approaching
perpendicular to the flow, see jet fighters) then one has to put a small
inlet on to keep the thing from getting too blunt.  But, the flow is now
entering the inlet at high speed, and thus the inlet and diffuser which
follows and slows the flow must be well designed to minimize losses.  See
inlets on jet fighters.

Generally lowest COOLING drag is obtained with the largest possible inlet,
efficient diffusers, zero leakage around engine or radiator, and smallest
possible outlets to accelerate the flow backward and some of he momentum lost
when the flow was captured in the first place.  This exit area depends on
velocity and pressure drop inside the cowl, and varies depending on air
speed.  So that is another reason why cowl flaps are appropriate.  However,
in making inlets large, one must not make the airplane too blunt.  Otherwise
one gets lower cooling drag, but greater airframe form drag. As in all things
associated with aircraft, it is a tradeoff.

Fred Moreno

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