Cowl Flap fine adjustments

[larrynimmo]

I have a 1981 M20J and my cowl flaps are basically either open, 50% at low speeds only, or just closed.  Is there a way to modify, as there are times id like to be open 10 or 20% for improved head cooling and taking advantage of a leaner mixture and yet keep CHT below 400 degrees F.

[INA201]

In my 78J I can set the flaps to slightly open.  I think the friction from the control mechanism holds it open in various settings but isn’t necesarrily designed as such.  I couldn’t tell you what % opened or closed it is however.  My cowl flaps are also set to about 3/4” open when in the closed position.  I don’t currently have any CHT issues (+380) except for extremely hot days. Mine will also do the trailing and full open settings. I hope that helps a little anyway. Others will chime in as well I’m sure.

[CaptRJM]

When I purchase my K from Weber Aircraft they told me to set the cowl flaps “in trail” as it reduces temps.  Mine are also set to be about 3/4” open when fully closed. Nearly never have any temp problems. 

 

[Jeev]

14 hours ago, INA201 said:

In my 78J I can set the flaps to slightly open.  I think the friction from the control mechanism holds it open in various settings but isn’t necesarrily designed as such.  I couldn’t tell you what % opened or closed it is however.  My cowl flaps are also set to about 3/4” open when in the closed position.  I don’t currently have any CHT issues (+380) except for extremely hot days. Mine will also do the trailing and full open settings. I hope that helps a little anyway. Others will chime in as well I’m sure.

This is how I had the cowl flats on my J set up.  Would have them in trail about an inch in the summer and fully closed in the winter and that set up worked very well.  If you reach up in the cowl flap and unhook the spring loaded ball joint you can loosen the lock nut and spin the fitting up and down on the threads to adjust.

[lifendet]

77J, mines are either full open or closed. Tried keeping it 1-2 inch open but air flow pushes it wide open. There is a sweet spot I can get it to stay a little open in flight but no actual designed stops. 

[EricJ]

1 hour ago, lifendet said:

77J, mines are either full open or closed. Tried keeping it 1-2 inch open but air flow pushes it wide open. There is a sweet spot I can get it to stay a little open in flight but no actual designed stops. 

That's my understanding, that there are two styles of the manual cowl flaps, one with two stable states:  closed and fully open, and one with three stable states:  closed, half-open, and fully open.   My 77J has the three-state kind.   I've no idea which airplanes got which styles, and it's interesting that here are two examples of 77J models with the two styles.

 

[markejackson02]

4 hours ago, EricJ said:

That's my understanding, that there are two styles of the manual cowl flaps, one with two stable states:  closed and fully open, and one with three stable states:  closed, half-open, and fully open.   My 77J has the three-state kind.   I've no idea which airplanes got which styles, and it's interesting that here are two examples of 77J models with the two styles.

 

I have an early 77J (actually made in 76) and it is open/closed.

[EricJ]

4 hours ago, markejackson02 said:

I have an early 77J (actually made in 76) and it is open/closed.

My airplane made it's first test flight on Dec 10, 1976, and the 2nd on Jan 8, 1977.   S/N 24-0077.   It has the entry step and a three-position cowl flap.   A hangar neighbor has a J about 25 serial numbers before mine and no step from the factory.   Next time I see him I'll ask whether his cowl flaps are two or three position.

 

[Jeev]

My 78 was SN 24-515 had no factory step and 3 position cowl flaps

[INA201]

I’m 24-0578.....Yep, no step and three position. 

[One Whiskey Hotel]

I recall reading recently somewhere, maybe here, that adjusting cowl flaps to remain partly open can cause damage to the fiberglass lower cowling where the hinge is attached, as the cowl flaps will buffet in the wind. All that motion wears out either the hinge or the attachment to the cowling, the latter of which tends to be an expensive repair. Anyone have further insight into this source of concern, and would trailing them via normal positive control prevent this damage?

[Mooney in Oz]

On 11/5/2018 at 2:03 AM, larrynimmo said:

I have a 1981 M20J and my cowl flaps are basically either open, 50% at low speeds only, or just closed.  Is there a way to modify, as there are times id like to be open 10 or 20% for improved head cooling and taking advantage of a leaner mixture and yet keep CHT below 400 degrees F.

Hi Larry.  On page 2 of the above previous thread you will see a post and photos of my set up.  I have an 80 J and had my mechanic remake my cowl flap mechanism by fitting a vernier system which enables me to select any cowl flap position I want for the exact same reason you mention.  He had to obtain what we call an engineering order here in Australia to make it legal, this would be similar to your 337 system I believe.  If you want to go this route then talk to your mechanic about the legalities.

You'll be able to select any setting you like, just like the electric system of later models except it will be mechanical without having to replace an expensive, electrical motor when it eventually stops working.

9 hours ago, One Whiskey Hotel said:

I recall reading recently somewhere, maybe here, that adjusting cowl flaps to remain partly open can cause damage to the fiberglass lower cowling where the hinge is attached, as the cowl flaps will buffet in the wind

I had mine modified about 9 years ago and have not had this problem and I cannot feel any vibration.  You won't need them open every time you fly and I only use mine to keep the CHTs under 380 degrees.

In addition to the above, about a year before the modification I had to purchase a new cowl flap for the left side (opposite side from the exhaust) from Mooney and was provided one with the shape that fits around the exhaust, so that both cowl flaps look the same and this also improved cooling.  Unless it has changed since, Mooney does not provide the old, flat bottom surface cowl flap that you may still have.

You may also have to look at the condition of your baffles.  If your hottest is the No. 1 cylinder then check if your mechanic can cut away from the top, the horizontal metal baffle by about 1/2 to 3/4 of an inch. You can easily see this across the fins as you look into the cowl air intake on the passenger side. That should keep it that cylinder about 20 to 30 degrees cooler.    

[markejackson02]

15 hours ago, EricJ said:

My airplane made it's first test flight on Dec 10, 1976, and the 2nd on Jan 8, 1977.   S/N 24-0077.   It has the entry step and a three-position cowl flap.   A hangar neighbor has a J about 25 serial numbers before mine and no step from the factory.   Next time I see him I'll ask whether his cowl flaps are two or three position.

 

Yeah mine is 24-0048.  No step and 2 position cowl flaps.  I wonder if they took a bunch of F models and modified them.