Tell me about cowl flaps...please

[bcg]

Ok, I have a 63 C and need some knowledge about the cowl flaps.  The checklists say to close them in cruise, others have told me to leave them open for good cooling, what do we really do in practice?  Also, it says not to open them above 150MPH on the placard, does that mean make sure they are closed if you're doing 150 or more or does that mean that if they're closed and you're over 150 you should not open them?  How do you decide when you should close or open them?  I assume I always want them open on takeoff and climb out for cooling, correct?  Is there ever a time I would want them open during cruise or should they always be closed as part of the cruise checklist?

 

This is the first plane I've flown with cowl flaps so I'm not sure how I should be using them.

 

Thanks for tolerating my continued questions.  I still love this plane.

[Fly Boomer]

Do whatever necessary to keep all cylinders below 400.  If you don’t know the temperatures of all cylinders, get an engine monitor.  If cylinder temperatures are too high, slow down, open cowl flaps (partially, if that’s an option), enrichen mixture, or any combination that keeps all cylinders below 400.  You will find that your favorite combination of manifold pressure, RPM, and mixture on one day doesn’t work the next day — it’s a dance.

[carusoam]

The M20C started with movable flaps…

Ended with fixed flaps about 1” open….

 

The only answer that Works with your wallet….

Keep the CHTs under 380°F… as often as possible.

Run more ROP if needed…

open cowl flaps more if needed…

Fix the dog house if needed…

use a cowl closure if needed…

 

It all starts with good data…

Best regards,

-a-

Edited September 2, 2022 by carusoam

[Ragsf15e]

One other thing… for optimal cooling you’ll find faster is better and thin air doesn’t cool as well.  So what do I mean by that?  Well on climb out you definitely want the cowl flaps open to give max airflow over the engine, but most people climb out faster than Vy once clear of obstacles.  Try ~115-120mph.  You’ll get better cooling than at Vy.  When you fly up above about 10,000’, you’ll notice that it might try to get real warm.  Your Calibrated airspeed is less and it’s real thin air.  You’ll want to keep a close eye while climbing and setting the mixture to keep the CHTs down. 

Climb with them open, cruise with them closed will work for most if your baffles are good and you have good techniques.  If you need them open in cruise, do it but there’s probably something else (mixture, baffles, rpm) that could keep your temps in check.

[Skates97]

If the doghouse is tight or not makes a huge difference. Also look at the gaps around the starter and generator/alternator. If it hasn't been done fairly recently then replace it with new felt. It is a long job but worth it. Also the rubber baffle around the cowl opening tends to get ignored. Also check the fuel flow at full throttle and 2700rpm on take off. There are a couple different carb options and one offers higher fuel flow than the other. I have gotten mine buttoned up really well, but it is still difficult to keep all four under 400 on climb out. Cowl flaps open on TO or go around and closed in cruise works well, except in the summer out west when it's still warm up at 10k feet...

 

[bcg]

Thanks for the feedback so far.

 

Now I have a couple of other dumb questions...what is the dog house?

 

Does anyone have a picture of the area where the felt is?  I haven't taken the cowl off mine so I'm having a hard time visualizing some of this.

[Hank]

26 minutes ago, bcg said:

Thanks for the feedback so far.

Now I have a couple of other dumb questions...what is the dog house?

Does anyone have a picture of the area where the felt is?  I haven't taken the cowl off mine so I'm having a hard time visualizing some of this.

The doghouse is under the cowl, that wonderful pile of sheet metal that covers the top of your engine.

This is one side of mine at the beginning of its resurrection. Can't find an "after" picture right now.

[Guest]

43 minutes ago, bcg said:

Thanks for the feedback so far.

 

Now I have a couple of other dumb questions...what is the dog house?

 

Does anyone have a picture of the area where the felt is?  I haven't taken the cowl off mine so I'm having a hard time visualizing some of this.

Under your engine cowl your engine will have another complete enclosure, something like this.  There are felt seals everywhere that the metal dog house touches the engine, around the starter the generator etc.  Also there is a rubber lip seal that engages in a slot in the front of the cowl.  The condition of all of these seal greatly effects engine cooling, most have been neglected over the decades.

[bcg]

Thank you Hank and M20Doc!

The A&P mentioned to me that my baffle would need some attention soon but didn't say anything about the dog house.  I'm putting 5 - 6 hours a week on the plane right now so I'll need an oil change soon, I'll give all this a good look over when the oil change is done and figure out a plan of action for it.

[carusoam]

 

Was the felt question answered?

There are a few pieces of felt at the front of the dog house that is used to block airflow around the starter and alternator…

In some cases… the felt is old or cut for a different starter or generator that has been swapped out…

There is a supplier… of silicone seals that may make a better solution…. For the more modern starters and alternators…

 

If Snoopy’s doghouse has bullet holes in it… the air flow starts to escape instead of flowing through the cooling fins….

Having air leaks at the bottom of the house…. Works against its design… the bottom half is supposed to be low pressure…. And no felt or seal allows high pressure into the low pressure area…

PP thoguts only, not a mechanic…

Best regards,

-a-