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