M20 D/C Cowl Flaps Open in flight

[Tcraft938]

This may be a strange question or maybe I'm making too much of it, since for probably the first 5 decades of my 1963's life no one could see outside what the cowl flaps were doing in flight.  Occasionally using a friend's GoPro cam mounted externally and I've noticed the cowl flaps never close even though I've selected them to do so.  In January the cable broke, I got a replacement from McFarlane Aviation (great people), the wire a little thicker and stiffer.  The A&P braced it well all the way to where it exits the sheath, and when the flaps are closed only about an inch of wire is extended from the sheath.  I could feel a noticable and positive difference when operating the system in the hangar.  I was smooth but could also hear the flaps shut.  This weekend my friend brought his camera, and watched real time on a cool day, just before leveling off I did as A&P recommended, close the cowl flaps as you're about to level, so you have less airspeed.  Sure enough, thump I saw them close and kind of felt it in the floor.  20 seconds later I see them starting open a little, but the control had not moved at all.  Based on the video they were almost fully open at high cruise.  Nothing I tried shut them.

I do not have the cowl enclosure, so maybe that huge opening is letting so much air in it has to go somewhere and it forces the cowl flaps open and this is normal?  I'm also wondering if the long thin spring that helps keep them closed has become aged and weak, perhaps something stronger would help?  Maybe it's normal and not a problem, however, it does explain my low CHT's in cruise.  I read about people being careful with leaning because of CHT, yet in the summer all 4 in cruise are low 300's and in winter 255-285.  Even last year with new engine/cylinders, everyone said watch the CHT like a hawk but never anywhere near raising an eyebrow.

Anyone else mess with this issue, or is the answer, "leave the darn camera home"?   :-)   Thanks 

[47U]

44 minutes ago, Tcraft938 said:

I do not have the cowl enclosure, so maybe that huge opening is letting so much air in it has to go somewhere and it forces the cowl flaps open and this is normal?  I'm also wondering if the long thin spring that helps keep them closed has become aged and weak, perhaps something stronger would help?

As the below thread states, the ‘63 C cowl flaps setting is 1.1” (+- 0.1”) in the open position.  I’m not sure what the thin spring is that you mention, got a picture?  

Shooting from the hip, it sounds like the arm is slipping on the control wire.  That’s where I’d go first.  

 

[MIm20c]

The cowling design/airflow is not ideal on the c/e models having the cowl flaps partially open helps with overall airflow. 

[Tcraft938]

40 minutes ago, 47U said:

As the below thread states, the ‘63 C cowl flaps setting is 1.1” (+- 0.1”) in the open position.  I’m not sure what the thin spring is that you mention, got a picture?  

Shooting from the hip, it sounds like the arm is slipping on the control wire.  That’s where I’d go first.  

 

I have a 1963D/C so you bring up a good point regarding the 1.1" gap on the C, but the D is around 2" if I remember correctly.  We were unsure what to set it at since plane was converted to C, so we set it up to 1.1".  The attached diagram is from a 1970 Part Catalog, so mine is slightly different.  I have drawn in red where the spring is connecting lower control arm to the diagonal engine brace where a bracket is for it to attach.  The spring hold tension in the up/flap closed position and stretches when lower the arm/close the flaps.  I will try to get a pic of the actual on the plane this weekend.  Thanks

Cowl Flap.pdf

[47U]

By 1970, the C had fixed cowl flaps, I think.  I’m not sure if the IBP illustration for the E/F is accurate for the C/D.  Someone smart here will probably know.  @PilotCoyote has a C/D.  My C cable attachment sticks out straight aft (edited from forward), not vertical like in the F.  But, I’m not 100% sure if that’s right.

Correct, the D opening is set to 2.4”.  Question... is the larger opening because the fixed gear/prop D model was a little slower, so required more airflow which would be available with more cowl flap travel?  What are the differences between motors in the -A1D in the C and -A2D in the D?  I think it’s reasonable to adopt the C opening when converting from a D to the C.  

I think the addition of the spring is a bandaid and verification of the cable-to-arm attachment for security and friction lock at the knob is the place to start.

 

Edited March 30, 2021 by 47U
Correction

[Prior owner]

If the knob did not move when the cowl flaps came back down, look at these:

1. Control wire sliding in clamp (per 47U above)

2. The clamps on the engine mount are sliding up and down (I had this problem)

If I could adjust the opening to be larger on my D/C I would, but somebody trimmed the wire too short so I’m stuck with 1.1” for now. I don’t think the fact that you do not have a front cowl closure is relevant, but who knows....?

FYI- the cowl flaps are hard to adjust properly if the clamps on the engine mount are in the wrong positions. Also, Check the front pivot bolts on the flaps to make sure the mounting is not loose, broken or elongated.

Where was your camera mounted? Was it only the outboard edge that popped down, or the entire cowl flap?

 

 

 

 

 

Edited March 31, 2021 by PilotCoyote

[carusoam]

If you lubricate the cowl flap control really well...

There is little friction holding it in....

Another thing that can happen...  the control cable gets anchored to the engine mount...   

Check to see that cable mount is stationary...  mine was well oiled and was moving on its own...  just another way the cable could move...

Old PP memories of an M20C...

Best regards,

-a-

[DCarlton]

A bit off topic, but I added a fixed center cowl opening years ago prior to my overhaul.  My oil temps were always too hot particularly on climb out during the summer.  Now I rarely touch my movable cowl flap; I just leave it open unless I'm on a long high cruise.  I'm not sure I can see a measurable difference in temps regardless of where it's positioned.  Now that I have an engine monitor, and a fresh engine, I'll take a closer look.  Is there really a danger of over cooling? 

[carusoam]

Just now, DCarlton said:

.  Is there really a danger of over cooling? 

Yes...

In the winter at altitude... too much cooling doesn’t activate the chemical reaction for the LL to do its thing...  

So if you see CHTs in the low 200s...  that would be in the too much category...

There is an article by Shell that covers this topic...
 

Best regards,

-a-