Jake Crause wrote:
Hello everyone,
I've heard references to augmenters but don't have a good
understanding
of what they are or how they work. I was hoping someone could
explain
them or point me to where I can find some info. Are any being
used in
light aircraft or are they still in the theoretical stage?
Thanks in
advance.
Jake
There are a bunch of NACA papers on the subject.
You can download them for free from the NASA web site.
http://naca.larc.nasa.gov/
snip<
In addition to the NACA papers there are a number of articles in
Contact
(#39, #40, #47 and #66). Also, there is an article in Sport
Aviation,
March
2002 issue on Charlie Airesman's VariEze Subaru installation.
Exhaust augmentation discharges exhaust gases into a larger duct
that
conducts cooling air and exhaust gases out of the cowling. The high
velocity exhaust pulses act as a jet pump to expel the cooling air.
Charles
Airesman, in his article "Drag Reduction Through Water Cooling and
Exhaust
Augmentation", Contact issue 39, documented ground runup vacuum
pressures at
the augmentor as follows:
RPM Water Column Height in inches
3,000 -0.6
4,000 -0.9
5,000 -1.6
6,000 -2.9
He then blocked the airflow through the radiator and measured vacuum
at the
augmentor. At idle he measured a 1 inch drop in the manometer, at
4,000 RPM
he pegged 5 inches!
One of the points that Daniel Nicoson makes in his "Cooling Systems
with
Exhaust
Augmenters" article in Contact issue #66 is that Cooling airflow and
Cooling
Demand are both a function engine power (when using exhaust
augmentation).
You therefore increase cooling capacity when you need it most.
Paul, If I recall, in past emails on this subject you have
suggested
that
exhaust augmentation may affect high speed performance (hopefully
I'm
not
putting words in your mouth). Can you explain that further. Both
Airesman's and Nicoson's articles suggest that one of the major
benefits
with augmentation is that it can reduce cooling drag which,
according
to
Nicoson, can be 20-35 percent of the total drag (depending on the
aircraft).
Most airplane cooling systems are designed for the worst case
situation
(full power climb-outs at moderate airspeeds). As a result air
intake
ports
are larger then they are required for cruise. Because, with
augmentation,
you achieve maximum augmentation with maximum power you can probably
get by
with smaller intake ducts.
Your comments would be appreciated.
Exhaust augmentors work best at zero airspeed full power. As the
airspeed increases
the effectiveness diminishes. At some speed they do nothing. At real
high cruise speeds they could be allowing too much air to flow through
the rad overcooling it and causing more drag than necessary.
The opposite possibility is too little opening at such a speed were
the augmentor effect is inadequate. Some sort of augmentor
system combined with a cowl flap is probably optimum.
I think the British Seafury had a system like that but I can't find
a picture. I think I have one.
The points where all this happens depends on the exact design.
I highly recommend getting the NACA papers.
Paul Lamar
-Jake Crause wrote:
Thanks Don and Paul for your responses. I've been looking at some of
the papers you mentioned and some others I have found. I think this
is
a fascinating subject. Thanks for the tips on the Contact articles
and
the CD, I'll be getting copies here pretty soon.
I understand that NACA scoops do not work very well in a situation
where
they are trying to force air through heat exchangers. Is it possible
that an augmenter may help make a NACA scoop a more viable option? I
will be building a Cozy in the near future and I would love to be able
to use the stock NACA scoop instead of hanging a scoop out in the
airstream like the VariEze.
Paul wrote: "Some sort of augmentor system combined with a cowl flap
is
probably optimum."
What about a variable geometry scoop? Is this less effective than
cowl
flaps or just more complicated to fabricate?
Thanks.
Jake Crause-
Perhaps on the NACA ducts but why handicap yourself. Nore complicated to
fab. Here is one I saw at Copper State.
Paul Lamar
Jake Crause wrote:
Would the use of a turbo affect the effectiveness of an augmenter? How
much does a turbo reduce the velocity of the exhaust gasses? I
understand
augmenters were used by the Germans in WWII but that was on supercharged
not
turbocharged engines.
Jake Crause
I think it would. You are taking energy out of the exhaust with the turbo.
Paul Lamar
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