Rear Cooling Development History ?

[ShinnickMark]

Hello, I'm a longtime Rutan Defiant guy looking to apply Defiant lessons learned to an upcoming 337 project.

Defiant had its own cooling challenges before I got that figured out by around 1999. That cooling was a patient project with numerous tests and re-shaping's at my base in Mojave before CHT's never got more than the mid-300's. That's one Centerline that really has done pure single engine ops.

I wonder if anyone might know some developmental history before someone eventually decided that 336/337 rear cooling was finally figured-out. Just looking at it...I'm not so sure it ever really was. I'd love to hear where the most typical problems temps usually show up.

Maybe I can also hear about when did the upper inlet scoop appear within Skymaster development in relation to when the rear cowling's annular shape?

Thanks.

[kbecker]

I have a 77 337G, normally aspirated. When I purchased the plane (used) I took it into a dark hangar and used a flashlight to find every leak between the upper section and the lower section. For small openings I used RTV to seal the openings. There was a larger opening when the cowl flap operating rod goes through the metal baffling. I did some testing in moving the cowl flap doors open and closed, and could see how this rod travels in the opening. We then cut a piece of rubber baffle material and RTV'd it over the hole, with a slit for the rod to travel in its normal course. Additionally, as I recall there was a fairly large gap near the accessory case which was easily filled in with rubber baffling material.
Since then I have never had a cooling issue with the rear engine in flight. The only rear cooling issue I have had is when I have landed at an airport for a full stop and taxi back, and have to wait a long time to take off again. If I know or suspect that is going to happen I open the rear cowl flaps fully during the approach to the airport. This cools things down well enough to buy me a few extra minutes of time on the ground.

[ShinnickMark]

Yes, this makes perfect sense maybe not so obvious, where rigorously tracking-down any leaking makes a rather huge difference in protecting precious plenum pressure.

[edasmus]

My experience is identical to "kbecker" above. My plane is a 1973 C337G naturally aspirated. Essentially the same airplane as "kbecker's." The only thing I would add is if I do get stuck with a lengthy (8-10 minutes) wait on the ground (very common at my home airport), any turn into the wind solves the problem. If the breeze is blowing into the air intakes, heating is a non-issue. Tail wind or cross wind and I watch the rear engine heat up to 400 degrees CHT with the engine at idle. During the take-off, by the time I'm reaching for the gear handle, the temps are back down to a more comfortable 350 CHT while initial climb speed is 120 MPH and cowl flaps open.

[kbecker]

Agreed. Always try to park into the wind even if you are somewhat sideways on the taxiway. I watch my CHT's and also the oil temp. On occasion I have increased front prop speed slightly to increase airflow to the rear engine via the top scoop when waiting. A very short term solution.

I have told tower before I have X minutes before I have to go park and cool down (both the engines and my attitude). Sometimes they are accommodating, sometimes not. I have enjoyed more than one burger while waiting for the temps to come down before restarting.

[hharney]

When Cessna introduced the 336 in 1964 it didn't have the upper scoop and relied on a fan driven by the engine to pull heat from the rear cowl. There was one large cowl flap at the top center of the cowling to allow air to be introduced to the cowling mechanically. It worked ok but there were some rear heat issues that has plagued the design ever since. You can always identify a pilot that has never flown a 337 Skymaster because they think and ask about the rear heat problem. There really isn't a rear heating problem on the 337 design with the upper scoop and mechanical cowl flaps on each side of the cowl. When the turbo version was produced the cowl flap size was increased about 25% and that carried on to the pressurized version beginning in 1973 with the G model change. All 500 or so military built 337's had these larger cowl doors even though they were all normal aspirated.

In the 49 years that I have been around Skymasters I have never had an issue with rear engine heat while flying. Although one time at Oshkosh waiting for an IFR departure, I sat on the ground for maybe 30 to 40 minutes and the rear oil temp increased into the red line and I had to shut down. If I would have increased the front engine to provide more air flow I may have been ok but was not able to do that in the line at the time.

The cowl flaps are critical to be operational in hot weather conditions. The electric motors that mechanically provided the operation were a bit troublesome if not correctly calibrated or gotten dirty. If the motors were cared for they were reliable but owners either neglected them or the mechanic was not familiar with them.

The P model engine was much tighter in the cowl and the turbo was producing more heat so those would tend to require more attention even with the big doors.

[n86121]

My one and only rear 'started to thermal runaway' was years ago at Hanscom AFB, when the place had 450+ based aircraft, was DEC helicopters' base, and a wild and wholly place to operate.

One Saturday I was number eight million in line for takeoff, 100+F on the taxiway.

The rear engine started to get hotter and hotter.

"Uh oh," I thought.

I found the trick was to keep about 1,200 RPM or so on the front engine, to drive airflow into the top rear scoop, w flaps open.

My turbo has the big juicy flaps and much more open space under the spinner too.

Since then all fine, never an issue.