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