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Paul, I use a 3" dia SCAT tube which undoubtedly
causes some loss of pressure, but much better than sucking in hot air from under
the cowling. I used to know a rule of thumb formula that told you how much
power you lost for each 10F temp rise, I think it was 1% power loss for each
10F. If correct then with an OAT of say 60F , your under-the-cowl air
temp could easily be 120F. A 60 deg rise of temp = 6% loss in
power. So say you are making 162HP at 5600 on a 60F day. You put the
cowl on and the temps go to 120F, 6% of 162 = 9.72 HP loss or down to
150.28HP.
.
My spreadsheet shows that a 9.72 HP drop to
150.28HP would give you an rpm of around 5150 - 5200. This sounds fairly
consistent with what you are seeing, I believe.
With my cowl off my static might be 5300 rpm
- with my cowling on with SCAT attached to the throttle body I get at least
5600 rpm and perhaps 5800 on a cooler day. So I think say 7-9 HP is too much to
give up to hot air. Duct that sucker.
Ed A
----- Original Message -----
Sent: Wednesday, February 16, 2005 8:54
PM
Subject: [FlyRotary] Re: I found the
power
Thanks, Ed....I think the results of my
on-again-off-again cowl experiments pretty well proved this. Do you
think that SCAT or SCEET tubing (the one with the wire reinforcement,
whichever that one is) would create too much drag on the incoming air?
Paul Conner
----- Original Message -----
Sent: Wednesday, February 16, 2005 6:07
PM
Subject: [FlyRotary] Re: I found the
power
Paul,
If you are not routing outside air to your
engine intake, you are giving up power. The less dense, hot air
underneath the cowling will take 200-400 rpm away from your rotary.
The NACA ducts should work well for this application.
Ed A
----- Original Message -----
Sent: Wednesday, February 16, 2005
6:26 PM
Subject: [FlyRotary] I found the
power
Today I finished installing the firesleeve on
the remaing fuel lines, made some stand-offs so that the fuel lines would
not be right up against the engine, and finished hooking up and securing a
few other items. With the top cowling off, I ran the pumps for
a minute, then checked for leaks...none found. Then I started the
engine and ran it for 20 or so seconds and shut it down and reinspected
for fuel leaks. None found. I restarted and taxiied away from the
hangar a bit so as to not be as loud for others in and near the hangar and
did a full power runup. I hit 5600 static rpm's. In the past, the
best I could hope for was 5200, most of the time settling for 5100
rpm's. I taxiied back in and installed the top cowling....max rpm's
5100. Took the top cowling back off....max rpm's 5600. One
more try....5100 with the cowling back on. It is beginning to become
more clear to me, that one of two things are affecting rpm's. Option
one is that with the top cowling on the engine is ingesting heated air and
just cannot develop the same power as when it ingests outside unheated
air. Option two is the muffler bearing may be worn. I'm kinda
leaning toward option one.
I was thinking of
installing two small NACA ducts on the bottom of my cowling with two hoses
coming up to feed air to my TWM throttlebody. The NACA ducts I
purchased from Van's RV (made for cowling ventilation) already have a male
outlet already molded in the assembly, so you just slide a hose over it
and secure with a clamp. Very simple installation.(Works well for my cabin
ventillation) If I recall correctly, they are about the same size as
my throttle body bore. I wonder if I will loose much efficiency as the air
goes through the corregated hose, and if the Van's RV ventillation NACA
ducts will provide a sufficient volume of air. Of course while flying, I
would have the advantage of a little bit of ram effect as well as the
ability to suck in fresh outside air that has not been heated inside the
cowling during taxi/takeoff run. Any opinions/suggestions would of
course be much appreciated. I will discuss the vapor lock issue in
the next post, so as to not complicate responses, etc. Thanks to all
who reply. Paul Conner
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