REFLECTOR: ram air

Wed Jul 5 12:16:55 CDT 2006

Thanks to everyone for the great info. If I can get a cold air induction 
system that faces aft and it's pretty easy to get a straight shot to some 
ram air, I'll do it. With fuel injection, the cold air probably helps as the 
ram air.

I'm a little confused about the aerodynamics of bringing the air into the 
cowl, though. I don't want to give up the head room in the back seat with 
naca ducts, and I saw some pictures of the air pressure gradients on a 
LongEz that makes me suspect that ducts above the spar and next to the 
fuselage would be an efficient way to get down draft air to the Lycoming 
cylinder heads. But do I mount the intake duct flush with the strake and 
fuselage, or leave an inch or so of space in order to get out of the 
boundary layer. I've attached a crude sketch so I don't have to write so 
much. Is A, B, or C the best?

Doug Holub
----- Original Message ----- 
From: <J.F.Sheehan at larc.nasa.gov>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Sent: Wednesday, July 05, 2006 10:21 AM
Subject: Re: REFLECTOR: ram air

The theoretical is possible with a very good designed intake. The practical 
gain in manifold pressure is somewhere between 1/2inch and 1 inch MP. The 
opening of the ram air scoop should be sized for the volume. The passage to 
the servo should be a little larger to allow the volume to expand thus 
maintaining the same pressure at the servo. I used a air scoop from Velocity 
I don't know if it is on the store web site but you can call. The scoop is 
mounted under the airplane just forward of the firewall with the end of the 
scoop coming up right behind the firewall. I have the scoop attached as part 
of the lower cowl. If I had it to do over I would make it attached to the 
fuselage so that i did not have to remove the scat hose connecting the scoop 
to the servo each time I removed the cowl. (Hind sight) Thw scat tube (4 
inch dia) is connected to a  airflow performance adapter (made for IO-540). 
(www.airflowperformance.com) It has a butterfly valve which allows me to 
select ram air or filtered air. The filtered air is taken from inside the 
engine compartment through a K&N air filter attached to the adapter. ( race 
car style).
In flight I see a difference of 3/4 inch MP at 10K feet between the filtered 
air and the ram air. Photo of adapter shows the air filter the open end of 
the adapter goes to the scoop. it is important to get the scoop opening at 
lest 3/4 inch away from the fuselage in order to be out of the boundary 
layer to keep the drag a low as possible. scoop needs a nice rounded edge 
not a sharp edge.
I think it was worth the effort and expense

Jack Sheehan
N55XL
Race 55

> 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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