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All Air Cooling / Ducting / Radiator System Experts !!

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Victor Bravo

Well-Known Member
Supporting Member
Joined
Jul 30, 2014
Messages
13,443
Location
KWHP, Los Angeles CA, USA
Got a question for the folks who are knowledgeable on cooling systems, radiators, ducts, and cooling air flows....

The conventional method(s) of air cooled system design have always focused on "throttling" or regulating the airflow (and thus the engine temperatures) at the duct or plenum outlet. Cowl flaps are always at the bottom or rear of the cowling, and they open and close to control the amount of airflow that is allowed to escape.

I don't think I've ever ever seen a moveable or adjustable opening at the (engine, cylinder, oil cooler) inlet, except on very fast airplanes where they're managing shock waves and trans-sonic compression stuff.

The question is... why is it more efficient or better to throttle the cooling flows at the back instead of the front, or opening/closing the inlet and outlet equally? Or was it just easier that way, they didn't care that there would be an improvement if the flow was throttled at the front?

Background: I'm in the process of figuring out how and where to mount the oil cooler on a STOL ultralight/LSA type project. The oil radiator is pretty large, the engine mfg. says that it's really important to keep the oil cooled, and the airplane will be operating fairly often at slow speeds on hot days. {Kolb Firestar II, STOL takeoff/climb/approach speeds <35kt., local area air temps <40F to >115F, 4-stroke HKS 700E 60HP engine}

My intention is not to reduce drag chasing higher cruise speeds ( :roll: ), but to have an efficient duct system that still provides good airflow through the oil cooler at slow speed when there's not that much air moving. There will also be frequent operations in cooler temperatures. I'm willing to build in an adjustment or throttle into this duct from the start. The obvious plan is to have a little too much flow available, and adjust it as needed.

I already know that the air entering/exiting the duct has to be decelerated and accelerated smoothly, with no sharp corners, etc. I'm already trying to design it with the intake under the wing leading edge at the highest pressure "stagnation" point, and I'm trying to have the air exiting along the upper curved section of the wing (lower local air pressure).

I'd like to know what the underlying reasons are for the "exit" throttle instead of an "inlet" throttle, and whether there would be any significant benefit to having the inlet open and close instead of or in addition to the outlet.
 
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