----- Original Message -----
Sent: Monday, October
09, 2006 12:40 PM
Subject: [FlyRotary]
Exhaust Update
Friends and others,
As you may recall, on the way to Osh on July 23 my exhaust system decided to
depart the engine leaving a lot of melted wires and charred cowling. I got it
tacked back together and raced home to Seattle
from western Nebraska.
Once home I began the process of doing a permanent repair of the wires and
developing a new exhaust system. The wires were the easy part. The exhaust
system has been a long arduous process in which I pursued several options.
In the end I mocked up a design using 1 ½” and
2” Sched 40 ABS plumbing pipe; not quite the exact dimensions for
2” and 2.5” ID x .049 stainless– but close enough. I sent the
model off to Aircraft Exhaust in Princeton,
Minnesota and a mere 4-5 weeks
later I got my finished exhaust system. I had them put a couple of “ball
joints” in right after the first bend about half-way to the merge. Also,
the model was designed, and I emphasized with them over the phone, that each
leg was to be pretty close to (exactly?) 21” from the flange to the merge
(per Lynn Hanover’s advice).
I received the exhaust 2 weeks ago. Getting it on the plane
was a breeze. Unfortunately designing and fabricating a support system for it,
so it is firmly braced to the engine, and fabricating a stainless heat shield
took 24 or more hours! Since the airport is 50 miles away that’s a lot of
work and has taken 2 full Saturdays!
Anyway, my efforts were rewarded late Saturday when I
finally got it all together and began testing. I ran the engine on the ground
at low RPM (1200-2000) while the oil warmed up. After the oil was above 120
deg. I did a static test and was rewarded with another 200 RPM.
Oh, let me backup a minute. While the engine was down
– waiting for the new exhaust I decided to re-do my intake manifold. I
have the short Atkins manifold and had a plenum attached directly to the TWM
throttle body – with nice sharp edges butting up to the throttle body
(hey I didn’t know!). So I created some 11” long intake tubes that
flare to a “sort-of” bell mouth inside my air-cleaner plenum. I
tested that briefly with the old muffler and on the ground got another 100-150
RPM static and another 200-300 RPM in the air – up to 5400RPM and another
3-5kts airspeed (162kts). So that was a win.
OK, back to the new exhaust – I was not getting
4400-4500 RPM static and after running for 20 minutes or so and checking to
make sure everything was still secure – I decided to take to the skies
before it got totally dark. There was a very noticeable improvement in takeoff
and climb – VERY NOTICEABLE. So I headed out west of the airport
over Puget Sound and opened it up to see what
would happen. It shot past 162kts and settled in around 179kts and about 5950
RPM!! Which is also pretty close to the design speed of the prop! So, overall
it looks like I have some significant performance gains!!
Around the end of the flight (or actually the cause of the
end), I thought I felt a little change in vibration and it looked like the
EGT’s went down – so I hightailed it back to the airport at reduced
power (of course descending even at reduced power meant I was still moving at
200MPH) and had an uneventful landing and taxi back to the hangar. I
didn’t have time to pull the cowls – but everything I could see
from the outside looked OK. So, I have to see whether the problem was in my
head or in the engine!!
Attached are some pics of the model and the new exhaust. The
“NewExhaust2” picture also shows the new intake in the upper right
corner.
Performance Re-cap:
Static RPM Flying RPM Max
Speed
Original
4100
5150(max) 157kts TAS
New Intake
4250
5400
162kts TAS
New Exhaust
4500
5950
179kts TAS
Obviously the Tangential muffler sucks (actually blows) when
it comes to performance! WAY too much back pressure.
Regards,
Joe Hull
Cozy Mk-IV N31CZ (65 hrs - Rotary 13B NA)
Redmond (Seattle), Washington
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