K Cowl Flap Correct Rigging

[PeteMc]

Over in FB someone mentioned that the K (231 in my case) Cowl Flaps should be set to be 1" Open when indicating Closed.  I've never had a mechanic rig mine that way and I've never seen any other K that way.  Am I missing something? 

And yes, I do know you can drop the cowl flaps a bit when it is hot out, I drop the right side a bit every summer.  But I'm talking the technically correct adjustment of the cowl flaps per Mooney.

 

[kortopates]

Much as been written here about the importance of K cowl flaps, particularly the 231, need to be about an inch open when fully closed else it causes a significant loss in cruise speed as well as cooling. See your service manual. You can use google to find older threads with more info on this. 

[EricJ]

The SMM says they should be rigged flush when closed, but lots of people use the 1" guidance instead.    Sometimes the tribal knowledge is useful. 

[hubcap]

Bob Cromer has mentioned this several times at MooneyMax. On the K Model, 1” open equals higher TAS and cooler engine. I had Myrtle rigged this way after learning this and it does make a difference.

[jetdriven]

I can see better cooling, but I don't get how it makes better speed. You can't quite get both can you?

[EricJ]

44 minutes ago, jetdriven said:

I can see better cooling, but I don't get how it makes better speed. You can't quite get both can you?

Minimum drag.   Flow through the cowl is essentially an impedance matching problem where you pick the drag source, either a plugged cowl with no flow so all the air has to go around, up to air flowing through with big cowl flaps hanging down in the breeze creating a low pressure area behind them, but a high pressure area in front (i.e., drag again).   Somewhere in the middle will be optimal for minimizing drag.  The optimal low drag spot may also have decent flow for cooling since it'll flow more than the plugged cowl case (completely closed flaps with no ambient flow).  I think normally it's kind of expected that the closed flap case would be minimum drag, but it might not be if it restricts flow more than the minimum drag case.   So if the ambient flow with closed flaps isn't the minimum drag configuration, opening the flaps a little might be.    I suspect it is airspeed dependent, too.

I think it just means that the K cowl design isn't optimized for minimum drag with the flaps closed, so it gets little faster with them bumped open.   Maybe there were cases where the engine/oil got too cold and they wanted the closed configuration to handle that rather than being the lowest drag configuration.   Who knows.

 

[jetdriven]

Does this apply to the J  also or just the K? I have a MSC cowl flap on the left side and I have the flat one  it came with and I’ve ran them both and it seems to run a little cooler with the MSE Cowl flap, but I never could quantify any difference in speed. The GI175 monitor picks up nearly an inch of manifold pressure ram rise on takeoff with the cowl flaps in trail or closed. . And if you open them at Cruise speed, the manifold pressure comes down slightly. About .3”. 

[wombat]

That was me on FB complaining about my cowl flaps.

Here is what they look like fully closed.  Yes, this is closed, not open.  They about double their distance down when open.

The second picture is just a little closer shot of the mechanism.   

In order to keep the engine cool with the cowl flaps in this position I have to be climbing at about 130 KT.   But I think this is caused by the fact that I'm running about 2 to 3 GPH too lean at full rich.   That's about 10%.   

[EricJ]

2 hours ago, jetdriven said:

Does this apply to the J  also or just the K? I have a MSC cowl flap on the left side and I have the flat one  it came with and I’ve ran them both and it seems to run a little cooler with the MSE Cowl flap, but I never could quantify any difference in speed. The GI175 monitor picks up nearly an inch of manifold pressure ram rise on takeoff with the cowl flaps in trail or closed. . And if you open them at Cruise speed, the manifold pressure comes down slightly. About .3”. 

They're completely different cowls with different flaps, baffling and flow inside, so I'd think any comparison would be coincidental.   The general principals apply, but the engineering optimizations were likely different for each.

[PeteMc]

4 hours ago, kortopates said:

See your service manual.

I didn't see it in the Service Manual.  I'll look again.

[EricJ]

1 hour ago, PeteMc said:

I didn't see it in the Service Manual.  I'll look again.

Sections 71-13 and 71-14 of the RevB Man 134.

[DanM20C]

On 6/16/2024 at 8:19 PM, wombat said:

That was me on FB complaining about my cowl flaps.

Here is what they look like fully closed.  Yes, this is closed, not open.  They about double their distance down when open.

The second picture is just a little closer shot of the mechanism.   

In order to keep the engine cool with the cowl flaps in this position I have to be climbing at about 130 KT.   But I think this is caused by the fact that I'm running about 2 to 3 GPH too lean at full rich.   That's about 10%.   

First off, you need to set up your fuel flows.  24.5 to 25.5 gph at take off.  When I fly a 231 that can only get 23 gph or less, I’ll abort take off.  So that is most of your cooling problem. 
 

Second, something is off with your cowl flaps.  When closed the mechanism rotates much further than yours is shown. Is there a chance you have the wrong oil filter?  I tried a 48109 once in hopes to utilize the larger filter.  But it’s too long and interferes with the cowl flaps when trying to close them.  Very similar to what you have shown.  
 

cheers,

 Dan

[Z W]

5 hours ago, DanM20C said:

First off, you need to set up your fuel flows.  24.5 to 25.5 gph at take off.  When I fly a 231 that can only get 23 gph or less, I’ll abort take off.  So that is most of your cooling problem. 

He has a Rocket conversion, with a liquid-cooled 350 HP TSIO-520NB engine.  I'm not sure what his fuel flow should be, but it isn't a normal K situation.

@wombat those cowl flaps do not look like mine.  I have the fully-adjustable 252-style with two arms driven by a motor.  Are they a Rocket conversion part?

[LANCECASPER]

14 hours ago, Z W said:

He has a Rocket conversion, with a liquid-cooled 350 HP TSIO-520NB engine.  I'm not sure what his fuel flow should be, but it isn't a normal K situation.
[mention=12280]wombat[/mention] those cowl flaps do not look like mine.  I have the fully-adjustable 252-style with two arms driven by a motor.  Are they a Rocket conversion part?

The Rocket (M20K conversion) with a TSIO-520 is not liquid cooled. It's a 305 hp air-cooled engine. (Same engine as a Cessna 340)

Rocket Engineering did do a handful of Liquid Rocket conversions on M20L and M20M airplanes. It was a liquid-cooled Continental TSIOL-550-A3 (Voyageur) 350hp engine. No cowl flaps needed on a liquid cooled engine. Nice concept but didn't work out so well. No one wants to work on one of these.

Here’s an example of an M20L (Porsche) converted to a liquid Rocket:

[Z W]

My mistake.  Didn't Google far enough, apparently.

[DanM20C]

4 hours ago, Z W said:

He has a Rocket conversion, with a liquid-cooled 350 HP TSIO-520NB engine.  I'm not sure what his fuel flow should be, but it isn't a normal K situation.

@wombat those cowl flaps do not look like mine.  I have the fully-adjustable 252-style with two arms driven by a motor.  Are they a Rocket conversion part?

I didn't do my homework!  Thanks!

Dan

[wombat]

@Z W I don't know if they are a normal part or a Rocket conversion part.   I have been hoping to find some pictures of another K model and/or a Rocket that has a similar setup.

One of the things I don't really know is if the arm coming off the torque tube  is supposed to be pointing forward or aft.  If it's pointing forward it's got a really acute angle to the cowl flap.

[PeteMc]

On 6/16/2024 at 6:19 PM, wombat said:

Here is what they look like fully closed.  Yes, this is closed, not open. 

As others have said, that way too open.   It looks like the flaps are in the mid position, not closed.  Are you sure your cable is adjusted properly and when the handle is all the way in that the cowl flaps are not just going to the mid point?

[PeteMc]

On 6/16/2024 at 2:09 PM, kortopates said:

Much as been written here about the importance of K cowl flaps, particularly the 231, need to be about an inch open when fully closed else it causes a significant loss in cruise speed as well as cooling. See your service manual.

So I found it in the Service Manual and as I thought, closed IS closed:  "...Maintain .06 - .12 in. "cushion" on cowl flap control at the instrument panel when cowl flaps are closed."  You may have misread the .12 in (3/25") for 12 in.

And I was surprised that even the summer rigging is basically closed:  " For improved cooling during summer months and warm temperature operations, cowl flaps
may be rigged to have a .25 max inch gap at "B" when at the closed position." 

I do run mine about an inch open in the summer time or if I'm going down south.  But it does confirm what I thought about technically correct rigging per Mooney is basically closed.  

[wombat]

1 hour ago, PeteMc said:

As others have said, that way too open.   It looks like the flaps are in the mid position, not closed.  Are you sure your cable is adjusted properly and when the handle is all the way in that the cowl flaps are not just going to the mid point?

I am not sure that the cable is adjusted properly.   It doesn't go all the way to the panel in the cockpit, even when I have everything  near the cowl flaps themselves disconnected.  It moves very easily until it hits a hard stop.

I didn't know that there was a mid position at all, I thought it had only 'open' and 'closed'.

 

A few weeks ago I called Rocket Engineering and that guy said they should be 1.5" open in flight.  Specifically, he said "One and a half", so if someone mis-read the book, it was him.

But mine are very clearly way wrong.

[PeteMc]

24 minutes ago, wombat said:

A few weeks ago I called Rocket Engineering and that guy said they should be 1.5" open in flight.

OH a Rocket.  Then maybe that is correct for the required cooling of that big engine!  My discussion was more the "Mooney" 231 & 252 Ks.  The aftermarket Rocket has it's own requirements.

 

[Pinecone]

On 6/16/2024 at 5:39 PM, jetdriven said:

I can see better cooling, but I don't get how it makes better speed. You can't quite get both can you?

Lower drag from the air moving through the cowling.  Fully closed, the air backs up and comes out the intake, making drag, plus not flowing though to cool

[LANCECASPER]

6 hours ago, PeteMc said:

So I found it in the Service Manual and as I thought, closed IS closed:  "...Maintain .06 - .12 in. "cushion" on cowl flap control at the instrument panel when cowl flaps are closed."  You may have misread the .12 in (3/25") for 12 in.

And I was surprised that even the summer rigging is basically closed:  " For improved cooling during summer months and warm temperature operations, cowl flaps
may be rigged to have a .25 max inch gap at "B" when at the closed position." 

I do run mine about an inch open in the summer time or if I'm going down south.  But it does confirm what I thought about technically correct rigging per Mooney is basically closed.  

Not sure if Mooney ever revised that in the 231 Maintenance Manual, but in the flight testing for the 252 they found that the cruise speed went up when the cowl flaps were 1 inch open. It makes sense, there has to be somewhere for the air coming in to go, otherwise it acts as a brake.

[jetdriven]

But is that net drag higher than having the cowl flaps not close fully?  When my cowl flaps are in trail they're open about 1.5-2" at the trailihng edge.  The plane also slows down 3 knots TAS.

There's no free lunch here.  Air forced through fins is still doing work.

[PeteMc]

9 minutes ago, jetdriven said:

But is that net drag higher than having the cowl flaps not close fully? 

From what I've learned over the years...   With the cowl flaps slightly open you get increased flow through the engine compartment, so less back pressure.  And with the cowl flaps just slightly open they are more in the rough air from the prop and the downward flow from the face of the lower cowling just below the prop.  So minimal drag in that air and they say the net gain is the better flow through the engine compartment.  And there is better cooling.

Now when you go to the in trail/mid position, the cowl flaps actually move into the airstream and cause both flow disruption and drag.  Hence the loss in speed.