Rear Cooling Development History ?

[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.

[general]

Experience Skymaster Pilots know, that all you normally have to do is to rev up the front engine once in awhile, when slow taxing in hot weather.

[ShinnickMark]

Thank you Gentlemen; This data on ground management and history is helpful.

[B2C2]

I would say my T337D experience has been the same as mentioned below. For the most part its a non issue, but every so often I find myself in a situation where I have to wait on the ground for a while before takeoff. When this happens its important to face into the wind, even if its an odd orientation. If you are sideways to the prevailing wind, and its above around 5-10 knots it can create a kind of dead air situation in the rear engine and it starts to head towards a thermal runaway. I also keep the engines full rich on the ground so I have unused fuel cooling it as well.