Re: REFLECTOR: Need advice on air scoop

["Al Gietzen" <alventures@email.msn.com>]

Dennis;

Sounds like an interesting  installation.  I had pondered also whether using
heater core and fan would sufficiently augment cooling to handle the taxiway
problem.

>I have three questions:
>
>1. Should I make a NACA scoop for this inlet, and if so, how big should it
>be?
>
>2. Would it be a big mistake to make a 4" long rectangular hole, and not
>make a NACA scoop?

I'm sure you know that this is no simple problem, and has a lot more
variables than you've given.  The key questions are what mass flow rate you
need (which you don't know); and what is the pressure drop across the core
as a function of flow rate (which you also don't know).

In rough terms, you want to slow the air sufficiently before the core so the
pressure drop through the core is less than the static head you generate at
the speed your flying.  So for a thin, fairly open core, say 1 - 1 1/2", you
might be able to get by with about 30 - 40 ft/sec or so.  Using a design
airspeed of 150 mph (220 fps) you'd guess an effective inlet flow area of
1/6 or 1/7 the area of your core.  Since your scoop will not be 100%
effective, add about 15% more area.  For a thicker core (and of course it
also depends on core and fin density) of about 3" you might need to slow the
air to 15 fps or less.  Some sort of testing prior to installation would be
helpful. If you're mostly interested in this as a heater and vent inlet, a 1
sq. in. inlet would probably suffice.

As I understand inlet design, a NACA is most effective in converting dynamic
pressure to static pressure under variable flow, whereas a ram scoop will
give better max flow for the same inlet area, but less pressure off its
design point as it spills air.  If you will have flow through this scoop at
all times, you may wish to use a ram scoop.

Good luck.

Al Gietzen