12.3.6 Nose Oil Cooler Control

This entry is part 34 of 50 in the series 12 - Engine / Propeller

I posted earlier about my idea to control the incoming air to the oil cooler by means of a door/flap/diverter.

My initial plan was to mount control arms to the pivots for the flappers and connect the control arms to control cables that would be accessed from the cabin. Once I started thinking about push/pull controls sticking out of the instrument panel, I changed my mind about that approach.

So I went to the local auto salvage yard and picked up a couple of heater control heads from old cars. Two levers (one for each flap) and a fan control. All I would need to do is modify it for my application.

Then I had an epiphany… While buying some switch guards from Perihelion Design (it’s where I got the dimmers for the LED cabin lighting I built), I noticed they had Servo Controllers. It occurred to me that I could control the flaps with servos. Which would mean two small knobs instead of a big honkin’ heater control box out of an old Buick. And it would be MUCH easier to route 6 small wires from the nose to the cabin rather than 2 huge, still control cables.

I swapped some emails with Eric Jones at Perihelion Design about the viability of the concept and purchased a servo controller for testing and a high-torque servo from Servo City.  Initial results were very positive. Good control of the servo and decent torque.

My plan was to still attach an arm to the pivot shaft of the flaps and use linkage to connect the flap arm to the servo arm. But that quickly became cumbersome. It worked, but it wasn’t very… pretty.

Then I decided to try geared drive. The hardest part was determining the correct gear ratio. Fortunately, my son Steve who is now working on a Physics/Electrical Engineering degree at Montana State helped out with the math on that. I picked up a large gear for the flap and a small one for the servo.


Here’s the flapper pivot with the large gear installed.

I tried a couple of different placements for the servo.

Next to the NACA duct.

But I settled on mounting it to the inside skin of the fuselage.

After determining and marking the proper location, I created and bonded a hardpoint to the inside of the fuselage.

Once that was done, it was time to install the servo and test.

There’s lots I don’t know about this endeavor. For example, I have no idea how much force will be applied to this mechanism by the incoming air in flight. I have a fairly high-torque servo (but not the highest available). One of the things I did do was with the door set to different positions, I directed air from the compressed air hose towards the NACA duct. The flap maintained position and the servo controller didn’t overheat. Now that’s not a conclusive test, but it’s the best I can do on the ground without a full scale wind tunnel.

My next task on this assembly is to do the same for the lower flap which diverts air into the cabin.

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