Jump to content

M20C (fixed) cowl flaps opening


Recommended Posts

Our 1970 M20C has fixed cowl flaps and especially in the summertime keeping temperatures down during climb can be very difficult. This weekend I looked at a friends 1963 M20c with manual operated cowl flaps and was surprised to see how big the difference between our fixed position and the full open position in the 1963 is. Researching older posts I noted fellow M20C with fixed flaps reporting satisfactory cooling. Does anybody know how wide the fixed flaps should be open, and lucky M20c owners with fixed cowl flaps experiencing old cooling could you please provide measurement of your cowl flaps opening.

Thank you all,

Frank 

  • Like 1
Link to comment
Share on other sites

4 minutes ago, N9495V said:

 Does anybody know how wide the fixed flaps should be open, and lucky M20c owners with fixed cowl flaps experiencing old cooling could you please provide measurement of your cowl flaps opening.

My C has fixed cowl flaps and a "doghouse".  Flying in the Atlanta area, I've never had cooling problems, but keeping your doghouse in good repair is very important.  Does your 70 have a doghouse?

If you would tell me exactly where to measure the opening, I'll be glad to tell you the dimension on mine; however, I'm not sure whether all the cowl openings were set to the same size, or if they were factory adjusted to balance the doghouse flow.

Link to comment
Share on other sites

Alex _ when I was using solely the factory CHT gauge I didn't have temperature issues at all. "Mysteriously" the engine started running hot after we installed an EDM700

Mooneymite _ I would greatly appreciate if you could give me your measurements at the end of the cowl flaps as indicated in the picture. Interesting point regarding the factory adjustment. It would be interesting to get data points from multiple owners to see the spread.

Thank you again for your help.

Frank

cowl flap.jpg

Link to comment
Share on other sites

The cowl closure up front has a way of altering airflow as well. 

The fixed cowl flaps didn't change anything else.

the gap is measured in inches.  I believe, probably, guessing, 2".

Without a Monitor, it is a challenge to know how hot the CHTs are really getting.

My 65C's were set to emulate the later C's fixed gap.  

Without a digital speed indicator, it is difficult to know a speed difference.

I should have bought some instruments.

Fuzzy PP memories, not a mechanic.

Best regards,

-a-

Link to comment
Share on other sites

1 hour ago, N9495V said:

Our 1970 M20C has fixed cowl flaps and especially in the summertime keeping temperatures down during climb can be very difficult. This weekend I looked at a friends 1963 M20c with manual operated cowl flaps and was surprised to see how big the difference between our fixed position and the full open position in the 1963 is. Researching older posts I noted fellow M20C with fixed flaps reporting satisfactory cooling. Does anybody know how wide the fixed flaps should be open, and lucky M20c owners with fixed cowl flaps experiencing old cooling could you please provide measurement of your cowl flaps opening.

Thank you all,

Frank 

Good topic for me. I'll send the actual measurement on my '68C's fixed cowl flaps when I get it back from annual, hopefully next week.  2" or so sounds right in my fuzzy memory. How big is your opening?  I'd be curious to know how much difference the movable flaps make from someone with a C model and a good engine monitor- i.e. climbing at the fixed setting versus wide open.  

The prevailing wisdom seems to be that the tightness of the baffling is a bigger factor than the cowl flap position.  Everyone here seems convinced, and the holes I've been able to close have indeed made a difference. Everyone except all the A&Ps I've dealt with- who say my baffle is great and ignore my requests to close the hard to reach holes - particularly in the front on the prop side of the alternator and starter.  I really begged and pleaded this year, offering to sacrifice my retirement savings and future first born, so maybe it will come out of annual in much better shape.  

Of note the C model cowl design in general is lousy too. Super draggy, with poor airflow under best of circumstances relative to modern designs.  See description in this old MAPA article:http://www.mooneypilots.com/mapalog/M20C Evaluation/M20C_Evaluation_Report.html  It's interesting to note that the author doesn't bat an eye at CHTs up to 440 during his test climbs with cowl flaps open.  I definitely bat an eye though - I have 2 cylinders off right now during annual at 900 hrs.

Now where is that nice Sabremech fella?  It looked like he had a kick ass STC in the works to fix all this mess.

 

  • Like 2
Link to comment
Share on other sites

My maintenance manual says:
778b325ce3b1bf9698d134a82bc16ad6.jpg

During the last Mooney summit a Mooney test pilot from that era stated the cowl design for the legacy aircraft was so bad that leaving the cowl flaps partially open resulted in both better cooling and an actual small increase in airspeed because it reduced airstream flow reversal inside the cowling (creates back pressure). I have a 67C and have adjusted the cowl flaps to around 1.5 inch in the closed position and a little over 2.5 inches or so in the fully open position


Sent from my iPad using Tapatalk

  • Like 3
Link to comment
Share on other sites

The cowl flaps are also fixed in my 1970 C. Resurrecting the doghouse took two annuals, the second annual also fixed the joint on the carb heat box that broke off on one side and would default to wide open . . . She barely climbs and flies pretty slow that way!

Two things will improve cooling:  1) get the cowl closure, that huge guppy mouth opening all the way around the prop actually creates high pressure under the cowl, and air flows out the front; 2) fix up your doghouse, repairing/replacing broken metal bits and missing screws, and make it seal up tight. I'll take pictures of mine at annual in a couple of weeks.

Next time I'm at the hangar, I'll measure my cowl flap opening on both sides. I do know it's somewhat tedious threading my oil drain hose out the left flap, and of course the exhaust goes out the right one . . . .

Link to comment
Share on other sites

Mooneymite _ Thank you for posting the pictures with the measurements. Do you have an engine monitor or do you you assess your good cooling with the factory gauge? It looks like yours are even less open than the cowl flaps on our 1970. The 1970 has the doghouse as well and I have tried to keep everything as airtight as possible. Does anybody know if there is a difference in the cowling / cooling system between the early models with manual cowl flaps and the later years with fixed flaps. If not shouldn't be the the measurement at least 1.1" per maintenance manual?  

Also we have a powerflow exhaust and the pipe seems to be bigger more restrictive than the stock and I keep wondering if the cowl flap should be opened more to make up for the reduced area?  

Link to comment
Share on other sites

11 hours ago, Hector said:

My maintenance manual says:
778b325ce3b1bf9698d134a82bc16ad6.jpg

During the last Mooney summit a Mooney test pilot from that era stated the cowl design for the legacy aircraft was so bad that leaving the cowl flaps partially open resulted in both better cooling and an actual small increase in airspeed because it reduced airstream flow reversal inside the cowling (creates back pressure). I have a 67C and have adjusted the cowl flaps to around 1.5 inch in the closed position and a little over 2.5 inches or so in the fully open position


Sent from my iPad using Tapatalk

Hector - How much cooling improvement did you experience after you did the adjustment 

Link to comment
Share on other sites

Hector - How much cooling improvement did you experience after you did the adjustment 


Hard to say. I adjusted them before I installed my JPI so I don't have before/after data. The single CHT gage original to the aircraft I'm afraid is not accurate at all. Presently, my 67C cruises with highest cylinder temperatures in the 360 range, BUT.......I will still go over 400 on takeoff for a short time until I get into a cruise climb setting.


Sent from my iPad using Tapatalk
Link to comment
Share on other sites

2 hours ago, N9495V said:

 Do you have an engine monitor or do you you assess your good cooling with the factory gauge?

I've had my C for almost 19 years.  On my second engine since I've owned it; first engine was running well at 2400+ hours when we exchanged it.  Just the factory gauges....Normally climbs and cruises mid-green, but on really hot Atlanta days, it'll be at the top of the green.  (Pretty much everything on my C is "as it came from the factory", except for the avionics.)  While some people do not like the doghouse, I think it works pretty well for me.  As far as I can determine, the cowl flaps are as set by the factory.

Edited by Mooneymite
  • Like 1
Link to comment
Share on other sites

14 hours ago, Hector said:

My maintenance manual says:
778b325ce3b1bf9698d134a82bc16ad6.jpg

During the last Mooney summit a Mooney test pilot from that era stated the cowl design for the legacy aircraft was so bad that leaving the cowl flaps partially open resulted in both better cooling and an actual small increase in airspeed because it reduced airstream flow reversal inside the cowling (creates back pressure). I have a 67C and have adjusted the cowl flaps to around 1.5 inch in the closed position and a little over 2.5 inches or so in the fully open position


Sent from my iPad using Tapatalk

This is very interesting that partially opening the cowl flaps could have simultaneous cooling and drag benefit.  I wonder if that sweet spot in cruise was used to set the cowl flap opening when they became fixed. 

Link to comment
Share on other sites

Didn't have a ruler or tape but measured with my finger on pilots side and checked at home and it's about 2.2 inches mine are not fixed. To be honest I really don't see much increase in speed when closed perhaps a knot or two at best. I get just barely above 400 in climb and settles down to around 370 in cruise but is just on number three with a factory gauge but was just rebuilt and calibrated. Oil temp seems to show the most difference between open and closed mostly I leave them in trail all the while in cruise and only close them in descent other wise my cylinder will drop below the 300 degree mark.

Link to comment
Share on other sites

Bonal, the only reason I can think of for your engine temps to fall on descent is if you reduce power. All of my descents are power on--all I do is walk throttle back and EGT forward to maintain my cruise MP and EGT values as I come down. Throttle is reduced only to slow down by the IAF or pattern entry. I like to have Takeoff flaps down before midfield on downwind, and I am often around 160 mph when I level off at pattern altitude (generally 170-175 mph at 500 fpm descent).

This works when descending from 9500' to land at 40 msl at the beach, or from 3000' lunch run to 1700 msl pattern, except the former requires several more adjustments. The extra speed is free as I convert potential energy (altitude), but getting it was costly in the slow IAS climb at high fuel burn. It makes up for lost time due to slow climb, and evens out the fuel burn from the high throttle climb (same fuel burn as at cruise, but the higher speed covers more ground in less time).

There may have been a few times I've dropped flaps (125 mph) while crossing over the field and turning downwind . . .  in that awkward stage after leveling off 3-5 nm away to slow down, and figuring out how much buffer is enough to have everything as I want it with minimum low speed drive from level off to downwind. Sometimes it still doesn't work out, but I've figured out how to reroute my approach so that I'm not entering downwind at 140-150 mph when the wind messes up my plans.

  • Like 1
Link to comment
Share on other sites

Hey Hank, how's them time to climb numbers holding up. Believe me my descents are anything but slow. As is common my destination is not far from some pretty high hills and am forced to drop a lot of altitude in a short amount of distance often I need to go 750 to 1000fpm if I don't pull some power I'm getting close to Vne. I don't start to bring the speed down until just a couple miles to the field. Last night I did a nice stable approach from 10 miles out on a straight in at 500fpm 160mph slowed it down to gear speed at one mile then flaps and 75mph over the numbers. I try to keep temps under control and in the winter my CHT will drop below the green arc if I don't close up that 2 inch opening in my cowl flaps a bit. My power reductions are just a couple inches at a time keeping an eye on the ASI. I have another fail safe in winter when I hit 180mph my roof vent will open on its own. The cable control works fine but for some reason the high airspeed overwhelms the grip always lets me know I'm getting close to red line.

Link to comment
Share on other sites

  • 2 weeks later...
On 12/29/2016 at 5:53 PM, Hank said:

My numbers are a tiny bit off from Gus's. Don't know if it's enough to make a difference. 

Exhaust side:

20161229_160742.jpg

Pilot side:

20161229_160720.jpg

I got my plane back and did this measurement. It is identical to Hank's.  I wonder why there is less opening on the side where the exhaust takes up part of the space.  Perhaps the larger opening on the left is to improve airflow over #4, which sits further back in the doghouse?

 

Link to comment
Share on other sites

On 12/28/2016 at 1:41 PM, Raptor05121 said:

What are you using for temperature display? I have adjustable cowl flaps and I've never even seen the needle move on my factory CHT gauge (#3cyl) when full open or full close.

I know you're past the limit for annual spending on airplane maintenance ;-)... but I'd put an engine monitor in at your earliest convenience. I'd be nervous as a cat only knowing the CHT on #3.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.