REFLECTOR: ram air

[Al Gietzen]

Subject: REFLECTOR: ram air

 

To any of you engine guys out there, how much of a performance boost will I
get from a normally aspirated, fuel injected Lycoming if I provide ram air
for the intake? 

 

Dynamic pressure is ½ x density x V2.  At a cruise speed of 170 KIAS that
translates to a ‘theoretical’ pressure 1.4” Hg.  The key then is to design
the scoop to garner as much of that as possible.  In order to do that you
have to determine, via the rpm and displacement, the amount of air the
engine wants to breathe, then size the scoop to give just a bit more then
that at the speed you are going (area x velocity gives volume) in order to
give the dynamic pressure advantage without adding drag.

 

If things work out right you can gain about 3-4% performance increase
(decrease fuel burn) at 170 knots. For other cruise speeds remember it
varies as the square of the velocity, 4-5% at 190 Kts.  Seemed to me worth
going after; and in my case the intake manifold is up on the side, so I
could get a pretty straight shot in; and have the scoop above the strake.
(Photo).

 

Some complicating issues are:  Since you size it for cruise rpm/cruise
speed, it is a bit too small for when you are at low speed, like
accelerating on the runway, so the spring-loaded door someone else mentioned
can help it be less restrictive for that condition, and since you are
drawing air in rather than it being forced in, you need to make a nice
rounded lip at the entrance. (Photo).

 

Al

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