Ram air

On Jun 1, 4:35 am, Tina wrote:
The Mooney 201 has a ram air port, a half a foot under the prop
spinner. The POH tells us it can be opened at altitude for a very
modest increase in MP and we find maybe a half inch increase in
pressure. The idea of the thing is, if the port is looking right at
the air being thrust toward it by the prop (it can't be more than 6
inches or so behind it) as well as the air impact from the airplane's
motion the air being 'rammed' into it should effectively lower the
altitude the engine thinks it's at. Well, a half inch of Hg is about
500 feet or so. The question is, though, wouldn't you think there
would be a way to capture a great deal more of the ram air effect and
really boost the engine performance? Who wouldn't like to fly at 24
square at 12000 feet without a turbo charger?

What makes me wonder about it is, even at 60 mph holding your hand out
of the window of a car subjects it to a significant backward pressure,
so the energy must be there.

The energy is there but it's no bigger than what Mooney claims.
Flat-plate drag at 100 knots is 29 pounds; dicide that by 144 square
inches and get around 0.2 psi, or about 0.4" Hg. Not much. AT 200
knots it will be four times that, which still isn't a lot.
In the 1970's Ford sold some cars with "Ram-Air Induction"
systems. A scoop mounted on the carb that stuck out above the hood, to
ram vast volumes of air into the carb and get way more horsepower.
That's what they wanted you to believe. At 60 mph the pressure
recovery would have been laughably tiny, but Ford's profits were
impressive.
On airplanes like the Cessna singles, the air intake faces
forward but it doesn't get much ram advantage. The airflow striking
the cowling is deflected around it, which means that the airflow in
the vicinity of the intake is across that intake, not ramming directly
against it. Since Mr. Bernoulli told us that pressure drops with
velocity, the pressure at the face of the air filter is likely lower
than ambient. Homebuilders can tackle that to some degree and get some
improvements in manifold pressure, but those improvements will come
mostly as a result of airflow control, not ram recovery.
And a funnel, contrary to popular belief, does not increase
the pressure within it when facing the airflow. It increases velocity,
which must decrease pressure. It's a convergent duct. A divergent
duct, on the other hand, slows the airflow and increases pressure, and
we find such shapes on jet engine intake ducts, where the cross-
section increases just ahead of the fan or first compressor stage.
See http://www.aoxj32.dsl.pipex.com/NewF...TWPhysics.html
and http://www.thaitechnics.com/engine/e...struction.html


Dan