REFLECTOR: ram air

[Richard Riley]

I agree with the 1.4" theoretical boost, and how to get it.

But it will never result in decreased fuel 
burn.  At 170 kts your drag is determined 
completely by induced and parasite drag.  Your 
fuel burn is your fuel specific times the 
horsepower required to equal that total drag.

Ram air will give you a little more horsepower, 
should you want it.  In other words, at WOT 
you'll be able to cruise a little faster, and 
burn a little more fuel, than at WOT without ram.

At 11:12 PM 7/4/2006, you wrote:

>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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