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

Pusher beasts are tough to analyze for cooling, at least on the ground.  In
the air, as I think about it for a minute or two, inlets on the leading edges
of the wings might be OK as they provide good ram inlet without adding to
frontal area.  Need good ductwork to the engine and you have to do it so as
to not screw up the flow over the wing and create a stall due to flow
separation because you monkeyed with the leading edge.  I need to think about
it.

With regard to exit area, it is defined by the amount of cooling air that is
needed to cool the engine and the maximum possible exit velocity that
accelerates the flow aft.  This depends on the pressure in the lower portion
of the cowl compared to ambient.

  If the pressure loss over the engine is small compared to the ram pressure
at the inlet, then there is enough pressure left over to accelerate the flow
substantially, the exit velocity is higher (particularly if it is hot so the
air density is lower), the area can be reduced and drag is reduced.

However, if there is NO surplus pressure left over after passing over the
engine (slow climb or maybe on the ground at idle), then there is zero exit
velocity and you need lots of exit area to let the exhaust air drift out as
best it can.

So it depends on 1) how much you heat the air, 2) how much surplus pressure
there is available, and this depends on 2a) the inlet ram pressure which
depends on speed, and 2b) the pressure drop over the engine or radiator.

Confused yet?  Optimum exit area varies depending on lots of variables.  That
is why one should use cowl flaps so exit area can be adjusted in accordance
to need: large at low speed, smaller and high speed. Larger at high power
settings, smaller at lower power settings.  Larger on hot days, smaller on
cold days.

I wish it were simple.  It's not.  I just got back from presenting a talk on
cooling drag to our EAA Chapter 62 monthly meeting this evening.  Took 50
charts on the overhead projector and 90 minutes, but the crowd said they
understood it much better, and I think they did since I got a lot of gook
intelligent questions.  But it takes at least this long to cover the basics.
There are just a lot of variables one needs to consider.  Over the next few
months I am going to try to develop some simplified rules of thumb.  Long
term, I will try to put it into a book.  Does anyone know if EAA might act as
a publisher?

Fred

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