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As usual, you came through with the information I
was looking for, Ed. Thanks again....I will bring outside air to my
throttlebody one way or another. It is such a noticeable difference.
It might be possible that two things are coming into play here. Number
one, of course is the temperature difference of the air. The second is that the
cowling is fairly close to the intake of the throttlebody, and perhaps the
volume of air entering the throttlebody is being somewhat restricted. One
thing I do know for sure is that I need to bring in that outside air. Paul
Conner
----- Original Message -----
Sent: Wednesday, February 16, 2005 10:01
PM
Subject: [FlyRotary] 3" Duct Was:
[FlyRotary] Re: I found the power
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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