This week I flew my RV-4/13BREW from Everett Wa. to Medford Ore. and
back.
Headwinds on the way down were brisk- 30 kts at 8500 feet on the way down,
but 35 kt
tailwind on the way back. The engine ran flawlessly but the plane is slow-
true
airspeed was 110 kts at 19.5 manifold pressure,5600 rpm oil temp 161 and
coolant 170,
fuel flow around 8 gpm. My prop is a three blade Catto, 72 dia x 88 pitch.
For one, the plane is over propped. I'm going to have Craig Catto cut it
down a
little during the winter maintenance. The other thing is that the cooling
scoop is
effective, but way too draggy, so I'm going to re-do it this winter.
Another issue is my left brake line- I"ve melted it twice. It's plastic,
and although
it is shielded from the exhaust pipes and insulated with fiberfrax, I
think that
heated air from the exhaust pipes is being forced out the opening in the
cowl where
the brake line goes to the gear leg. That's where I have found the line
melted. I
want to put a reverse scoop near the exhaust pipes to get the heated air
out of the
cowl, and to replace the brake line with stainless steel. Everything
inside the cowl
except for the brake line looks OK- I wrapped and shielded everything -
wires,
coolant, fuel,and oil lines. There are three NACA ducts bringing air into
the cowl to
cool the alternator, the coils and exhaust pipes, and an exhaust eductor
to suck air
through the cowl and over the muffler.
Comments?
Brian Trubee
Couple of questions.
EGT?
Size of throttle body?
Is the engine ported?
The manifold pressure is very low.
Is the air filter plugged? :)
Are you getting cold air?
8 GPH is 50 pounds per hour at 6.25 pounds per gallon. At a BSFC of .47
leaned that
is only 106 HP. At a fuel burn of .5 that is only 100 HP. If it is really
rich and
the BSFC is .55 and that is only 90 HP.
You need a p-port Brian :)
Paul Lamar
Brian,
As some of you know my Wheeler will be a blend of certified parts and
experimental parts. I noticed the issues with the nylon brake lines long
ago. I went to NAPA and bought (8) 12" standard brake lines that go on
trucks from the hard line to the wheels. I also bought a 25 foot roll of
3/32 brake tubing, 24 double flare fittings, and a double flare tool. Total
for all of this was $170.00. After I got back to my plane, within 2 hours I
had the brake lines made up from the master cylinders to the wheels. Galv
steel line inside the cockpit, 12 inch flex at the master cylinders, at the
turn into the wings (mine are removable) one more at the flex point of the
gear leg, and the last one at the wheel. I don't anticipate melting my
brake lines. This is the way my Viking was done, and I had no issues with
my 1968 Viking until I blew the Continental up.
My fuel and oil system will be similar. The materials are inexpensive, and
the labor is minimal. Cheap insurance.
Kevin Alderman
brian,
This is the blind leading the blind as I have yet to depart
the earth, but tied to a tree I get MP of 22 at 5000 rpm and from memory 24
at 5600 rpm. Where are you reading it from? Neil.
It's been a while since I did any static runs on the ground. I"d just be
guessing at
this point to give some numbers for static running.
Brian Trubee
Brian,
You stated "...my EGT temps- they show up to 2000 at times- and above
1800 most of the time in cruise." I too experienced very high EGT's
in excess of 1800* when I was using a muffler that was too
restrictive. Also, power was way down, but it was quiet. ;-) The
high temps ended up damaging the o-rings and I had to rebuild the
engine. Since replacing the muffler the egt's have dropped down to
1600 - 1650* range and power is much improved. I suggest you examine
and modify or replace your current muffler.
Mark
The best muffler for a rotary is 3 to 8 feet long about 4 to 5 inches in
diameter.
That gives the lowest aero drag for a given silencing. Lowest frontal
area. Mufflers
work by the exhaust gases rubbing on the metal surface losing kinetic
energy to skin
friction and losing heat to the metal surfaces. The cooler the gases in a
pipe the
lower the gas speed. When high speed gas exits a pipe there is a sudden
expansion
that causes the noise. The idea here is to cool the gases before they get
to the end
of the inner pipe.
The inner tube is best done in .045 Inconnel but there is a trade off with
thickness.
If you make the inner tube 2 inches in diameter and use 321 SS make it
.065 thick
with a bunch of holes drilled in it. The outer tube can be .030" thick 321
SS as it
will always run cooler. Also make the front of the muffler look like this.
Don't drill any holes for the first 8 to 10 inches. The slip stream will
blow cooling
air down between the coaxial tubes. At the back end make the inner tube 6
inches
shorter than the outer tube. That will act as an ejector to draw cooling
air between
the two tubes. Patent applied for:)
A 2 inch diameter SS tube .065 thick 72 inches long will weigh about 8.8
pounds.
A 4 inch diameter SS tube .030 thick 72 inches long will weigh about 7.50
pounds.
Here is a dwg.
A 2.5 inch inner tube and a 5 and 1/2 inch outer tube may give a little
less back
pressure for a weight trade off.
Paul Lamar
Mark ,
From what you say and from the performance of my plane, I'm
starting to think that the Hushpower muffler is causing too much
back pressure and that would explain the anemic performance and
high EGTs. What type of o-rings were you using and do you think
that the high EGTs caused O-ring failure and how so?
Right now I'm thinking of how I can replace the Hushpower with
the design that Paul just put forth. Its a bit tricky hanging
that stuff on an RV-4 and working it around the heat exchanger
and scoop.
Brian Trubee
It might be melting inside.
Here is what Mike Wills did on the RV4 although the front of the muffler
outer tube
is not open for cooling air. This is what gave me the idea for the coaxial
muffler in
the first place.
The subtle point is; cooling the inner tube saves weight as there is a
high temp
metal durability trade off with weight.
For an example Perry Mick has been running an early cast iron thermal
reactor for
years. They are very durable but they are heavy.
For anther example Ed Anderson runs SS pipe hand rail with an .109" thick
wall and
it hardly discolors. However you don't need a motor mount as the engine
can be
cantilevered off the fuse with the exhaust tubes :)
Paul Lamar
Brian,
I was running TES o-rings inner and outer. The high EGT's exceeded the
temp limit of the o-ring, allowing it to ooze out between the mating
surfaces. I switched to the SS handrail exhaust with a DNA muffler (no
longer available) and haven't had any more problems, although the DNA
muffler is not much of a muffler (pretty loud). People comment they can
hear me before they can see me. I wear a Lightspeed Zulu headset.
Mark
Got an pictures of those O-rings? Are you sure about the section size?
Heated silicone can crack cast iron if there is not enough room to expand.
http://www.rbwagner.com/pages/Rbwagner.html
Paul Lamar
Mark,
What kind of muffler were you using that caused all the back pressure?
Brian Trubee
Brian,
It was a tangential muffler. I suspect the problem was that I ran the
outlet tube all the way through the body of the muffler to the front cover.
It had holes all along the length of the tube. In retrospect I don't
think the holes were big enough to let all the exhaust out. I may try it
again later with more/larger holes. It was made from 625 inconnel. To
modify it would require much grinding, then re-welding.
A well-respected racer guru has warned us to be very careful with the
exhaust system design else you'll loose much HP. (He didn't mention
damaging the engine.) Rotaries don't like back pressure.
Mark
That is very restrictive looking.
You might try a 3 rotor version of one of these. See the 3D. It was invented by Dr.
Porsche for the air cooled VW. In theory the air whirls around inside rubbing on the
outer skin.
The center tube has air flowing through it for cooling purpose so an air scope on the
front of the cowl is required. In practice a part of the outer skin opposite the
exhaust tubes catches the brunt of the energy so a skin doubler must be welded on in
this area. This Inconnel version has hundreds of hours on it.
Paul Lamar
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