M20F Overhead Air Scoop... Max Speed?

[gt1878]

Well, I'm 15 hours into my newly acquired '69 F model. As warmer weather is approaching, I'm interested in keeping that cabin cool. POH says to open cabin vent and overhead air scoop for maximum cabin air flow.

The control knob for the overhead air scoop works, that wire looks awfully thin. Will it actually close that scoop in flight?

So, the operation is a little rough, if the vent is completely closed on top of the plane, the control knob won't open the vent. On the ground, if you get up top and open the vent, say 1/2" then the control knob operates it up and down fine, as long as when you close it, you leave it slightly cracked about 1/2".

Is there not a max speed you can have the scoop open?

[orionflt]

sounds like you may need some lubrication and adjustment to your cable. it should open and close the vent both on the ground and in the air at cruise conditions 

 

Brian

[Andy95W]

Never seen a max speed for the top vent, only the small pilot's window.

A little lubricating oil on the spiral control push wire sheath, as well as the knob mechanism, should loosen up your vent mechanism nicely. It should open and close it fully while in flight, but after 45 years it probably has some surface rust and the mechanism is probably gummy.

[Bob_Belville]

I had to replace that wire on my '66E a few months ago. It was probably original equipment and it had been tough to move but there should not be any airspeed limitations. I replaced the wire that broke off at the hole in the knob shaft. I used "throttle" wire from a lawn equipment dealer. .050" I think. See if graphite lube is not recommended for the shield. 

[Hank]

I open and close mine in cruise. Full open on the ground in warm weather, climb to a cool altitude and close it when I get cold. Reopen on descent after the cockpit gets muggy from heat and humidity. Anything helps, right? Seal in the cold, dry air up nigh and dont let the warm, muggy air in until you have to, then crank her open. I typically descend at 500 fpm, indicating around 170 mph.

[Bob_Belville]

I suspect it costs at least 2 kts cruise speed when open. 

[Marauder]

There is no speed restriction on the operation, but as Bob points out, it does inflict a speed penalty.

[DrBill]

That 2 knots is well worth the cooling when you need it.

[Bob_Belville]

That 2 knots is well worth the cooling when you need it.

Naw, let 'em sweat. 

[Hank]

Naw, let 'em sweat.

That's fine, long as I don't sweat too much. Close the right eyeball vent by their knee?

[bonal]

When I first got mine the control would not function at all it just needed to be cleaned and re set in the right place on the wire. It works fine but if I get close to red line speed it will open on its own and won't close until speed drops. I will have to re tighten on the wire. The first time it did this it was a bit of a surprise and as long as I'm in the yellow it won't close so needs tightening. When open it lets lots of cooling air in.

[StinkBug]

Mine is really hard to move, even after trying to lube it. On my trip home from Texas I made the mistake of exploring my new plane and decided to try opening it.....then couldn't get it to shut. Thankfully I was able to block most of the cold air by closing the ceiling vents, otherwise I would have been making an unplanned stop LOL! 

[Marauder]

Mine is really hard to move, even after trying to lube it. On my trip home from Texas I made the mistake of exploring my new plane and decided to try opening it.....then couldn't get it to shut. Thankfully I was able to block most of the cold air by closing the ceiling vents, otherwise I would have been making an unplanned stop LOL!

Unfortunately, if it was forced at one point, that cable can kink a bit and then all the lubrication in Saudi Arabia won't free it up.

[TTaylor]

I was flying over the great basin a few years ago in late December. We opened the vent to help defog before take-off. As I was climbing out I tried to close it and it was stuck open. Outside air temp as about -20 F and my wife nearly froze in the backseat with full cabin heat on before I crossed the Great Salt Lake and landed at Battle Mountain where duct tape was found to seal it shut. Required cleaning and lubing plus a complete disassembly to get it working correctly again. Moral of the story was don't open it somplace if you really will want it closed in flight.

[RobertGary1]

And when those cables break you know it. Mine went departing out of Vegas. Sounded like a bomb. I'd replaced the wire with one that apparently was slightly smaller gauge. The gauge of the replacement wire is critical to avoid that excitement. Also you may want to replace the original hog's hair with some foam. 

 

-Robert