A truly full explanation will still have to wait :-)
But if you are looking for the absolute optimum HP solution, the inlet should match the Throttle
body ID the diffuser should FOLLOW the throttle body. Only when you factor in aero drag at less
than full throttle does the smaller inlet ID start to make sense and only if the configuration is
cleaner than the very draggy 180 degree setup on this one. I'm guessing that all the papers you
read were written in terms of simple engine performance and nothing else.
The RV-8 ram air inlet implementation is a lot better than on the Renesis.
Hint. Calculate the engine air inlet CFM at maximum power and see how many CFM will flow
through an opening at the aircraft's top speed.
Tracy
On Sun, Apr 5, 2009 at 4:38 AM, steve wrote:
Hi Tracy
We are currently investigating RAM Air feed to the throttle body of our Renesis 4 Port.
We have noticed back in 2005 you published the following on your WEB site - I quote - (see
photo of Otter Engine)
"Here is the RAM air scoop which also functions as a diffuser. The I.D. goes from 2 1/8 inches
at the inlet to 2 3/4 inches where is attaches to the throttle body.
I fully expect someone to say "Why in the hell would you restrict the air passage to less than
the throttle body diameter?" There's a good reason but the full explanation will have to wait for
another installment. "
You mention that full explanation will have to wait!!!
We have read various papers on the subject and are still in the dark.
Could you please explain.
Thanks
Steve Izett
Renesis 4 Port EC2, RD1-C
Perth Western Australia
Tracy's replySteve
I think Tracy is really unable to explain why his inlet area is less than the inlet area of his
throttle. I would have thought with his configuration a slight increase in cross sectional area to
equal that of the throttle would have minimal effect on the drag his system already causes.
If as we propose the intake is in the shadow of the front cowl then I don't think the cross
sectional area of the intake within reason should have all that much effect on drag as the volume of
air ingested is dependent on what the engine breaths. Sure we are hoping that by increasing the
pressure at the throttle will mean the engine takes in more air but it will be only marginal. Within
reason I would have thought that by increasing the cross sectional area at the intake would increase
the pressure at the throttle and hence maximise ram effect ! Hence our proposed conical 90 to 80mm
intake should be acceptable.
Any thoughts ?
Dad
Hi Paul
Do you have a comment to make on this ?
Our calcs if we have got them right indicate that at 7500 rpm the 13b intake air velocity is about
80 MPH through its 80mm diam Mazda throttle so flying at say 200 mph there should be quite a
positive pressure differential in the intake !!!!
We have read an article where the inlet cross sectional area was less than the duct feeding the
throttle but there was no explanation as to why this was preferable !!!
Regards
Bob Izett
Sorry for the delay Steve. This messages came to my EEPC after I left DE.
I think 80 MPH is a bit low. What CFM # did you use for the engine?
Never the less you are right. The dynamic pressure at some speed will
over come the intake man pressure and provide an inch or two of Hg boost
if the area of the scoop is larger than the throttle. There may be some old
NACA papers on the subject. Google NACA.
Paul Lamar
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