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Good words, Al. I feel the same as you - "short" is good and easy and works
well. "Tuned" for a desired rpm will be a bit better than un-tuned. As I
learned in the Pentagon, "Better is the worst enemy of good." That was a
different context, though, and I don't apply that to this discussion. I
agree with you.
David
Do not archive (does that work on the FlyRotary list?)
----- Original Message ----- From: "Al Gietzen" <ALVentures@cox.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Saturday, April 17, 2004 2:42 PM
Subject: [FlyRotary] Re: intake ideas?
Subject: [FlyRotary] Re: intake ideas?
Russell, you will be on as solid ground as anyone else to simply use the
Atkins "short intake". I've visited with him extensively, visited his shop
and asked every question I could think of, and here's what Dave says and I
believe to be so on intake manifolds:
We have 2 choices: 1) Long tuned manifolds that have more resistance to
mass flow of air, or 2) the short untuned manifold (used by Mazda Racing??)
which gives higher mass flow.
- They both generate about same torque and HP but 2) has less space
problem.
- Dave is a competive racer and is one of those guys with max
experience with rotaries - and he is quite comfortable with the short
manifold.
I'm going to start off with the Atkins short manifold - or one similar that
fits the RX-8 Renesis engine.
This topic has been hashed and re-hashed over the years, and it's a little
like the old 'blind man describing the elephant' story. When it comes to
hands-on experience and knowledge of the rotaries, Atkins is probably 'da
man'. When it comes to something departing from his experience and into the
realm of analysis or theory, Dave is definitely 'anti'. And from a racing
standpoint, where you may be running 7,000 - 10,000 Rpm; short is good.
On the other hand, both the analysis and dyno results make it conclusive
that there is performance benefit to tuned intake runners, and it is
especially applicable to aircraft application where we will generally
operate over a small rpm range, say 5000-5500 Rpm. This is a different
question than where you put the injectors, and there is some reason to
believe that having the primary close to the port, or in the housing, is
good. Having the secondary a bit further out may have an advantage at
higher air flow allowing more thorough vaporization and mixing of the fuel.
The question of tuned runners is 'how much is the benefit?', and 'what do
you want to give up for it'. Could maybe gain 4-7%. Compactness inside the
cowling can be an important factor. I went with short manifold on my 20B
based on wanting a compact configuration, and Atkins convincing me that
short was just as good. That was before I did much analysis and studied
dyno data. I'm happy with my installation, and the dyno results show the
performance is fine; but if I were starting over I might look a bit longer
at fitting in tuned runners.
My very flat torque curve is great for a car, but a bit more peaking in the
5000-5500 to improve cruise economy in the plane would be nice. What I have
instead is the hp curve still going up in a nearly straight line at 7000
rpm. The short manifold may be great for 2.85 : 1 redrive. More pics and
dyno results at http://members.cox.net/alg3/airplane.htm
FWIW,
Al (obviously not at Fun-n-Sun)
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