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Under cowl temps?

N729LS

Well Known Member
Patron
Before I go set up the instrumentation, I thought to ask if anyone has done any measurement of temperatures in the cowling volume behind the engine? If so, could you post the values you measured, along with the ground/flight conditions and the location of the sensor(s)?

Reason: I am thinking to implement an automatic "reverse cowl flap", i.e. a flap that would block off some of the cowling exit area until temps come up to some desired level. My -8 is very slow to warm up on colder days. I already have winter inlet restrictors (remove 8 in²/side) in place.

Background Info: Older air-cooled Volkswagens had a cowl flap that was mechanically actuated by a thermostatic bellows. The flap stayed closed until a "warm" under-cowl temp was reached, and then the bellows automatically opened the flap to keep the engine cool. These bellows are available in different opening temp ranges of 70°/80°90° C, and are available in both fail-open and fail-closed models. (thermostatic actuators)

I am also, of course, looking at manual activation via simple Bowden cable, which might end being the better, but less spiffy, option. Might offer more exact control in descents, too?

Andy
 
What is the OAT, how are you trying to warm the engine, and what location and temperature are you targeting?

I will go look for temps, as I have measured that location under many conditions. IIRC, never above 190F.

I preheat, but many years of engine warming would support steady load at 1100-1200 rpm will warm the engine nicely.

There were two VW (air cooled) methods, a collar that blocked flow into the fan (pre '60), and flaps that closed airflow at the exit of the fan housing.
 
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Good comments. :)
When I am at my home field, I also preheat. But sometimes I am away and unable to get an extension cord to the plane. I also go to the mountains! Once I tried to start up at -7°F =-22°C! :eek: Added a small camp stove ducted into the cowling for preheat, but it ran out of fuel and froze to the ramp!

I have found that with temps nearing 0°F, even with a cover over the engine area, sometimes my little 2-pad heater won't get the oil past +40°F. By the time I get loaded and in the plane, it's cooled off more, and then it can take 15 min at 1200 RPM to get the oil to >130°F. I think the cowl flap would reduce this quite a bit.

Also, even with the inlet restrictors in place, there can be an issue with excessive engine cooling during descent, again especially in the mountains. I think reducing the exit area would help there, too.

Andy
 
If your cowl seal and or plenum is good, you should never have an issue with accessory case temperature. Having said that, I like to hedge my bet, always add cooling tubes to the Mags or electronic ignition system, and a blast tube to the alternator. then, as long as the CHT's are OK, the Accessory section is just fine!!

Gary
 
Might be for different reasons, but the radials I’ve flown have all instructed to warm up the engine with the cowl flaps OPEN, to promote airflow and aid equal heating of all components. The T-28 NATOPS we referenced specifically said NOT to speed warmup along by closing the cowl flaps.

Again, might be different, but I suppose those big expensive engines were operated that way for a reason?
 
That might be for the big 1820 Cyclone, but I think it depends on the paticular installation and cowling set-up. The Yaks and CJs with M-14s all warm up with the shutters closed. They do have 7 gallons of oil to heat up!
Andy
 
Returning to the OP's question...

Before I go set up the instrumentation, I thought to ask if anyone has done any measurement of temperatures in the cowling volume behind the engine?

Speaking strictly of air temperatures at the exit, we've had reports ranging from a low of 120F to a high of 228F. The 120F airplane suggests very poor heat transfer, hot engine to air, while 228 suggests good transfer. The drag goal is to carry as much heat as possible with as little air as possible.

Note both the 120 and the 228 examples resulted in acceptable CHT. The 120 example was simply leaky, with lots of cool air bypassing the hot parts. You'll need to establish your own typical range of exit temperatures in order to pick a thermostatic bellows. If you do something to change epsilon (heat transfer efficiency), you may need a bellows with a new temperature rating.
 
Accessory Section Temperatures

I, too, have always been curious about accessory section temperatures. So, when I installed my HDX based panel, I installed a temperature probe on the firewall, just below the midpoint vertically. The accessory section temperature varies with the outside air temperature, but the temperature change from ambient to accessory section (delta) is consistently about 90 degrees. The temperature in the accessory section on a 60-degree day is about 145-150 degrees F.
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Snippet from my files, exit temps at various speeds, at two OAT's.

Note temperature probe is located in the ducted exit stream, not up on the firewall, so it includes air heating from the exhaust pipes. No radiant heating of significance however...the probe is mounted on a reflectored, insulated composite mount.

That's a coaxial pitot static probe on the left.
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