Cowl Flap Setting

[MBDiagMan]

I haven’t read all the responses to the OP, so mine may be redundant.

We just finished MooneyMAX yesterday.  In one of the presentations regarding speed improvements, one of the tips was to leave the cowl flaps open one inch to relieve pressure for a slight speed increase as well as better cooling.  Sounds like someone might have been using that trick.

[Joe Linnebur]

I noticed our cowl flaps were flush with the cowl last year and a friend recommended what the service manual states (adjusted to 1/4” to 1/2” open when closed) and I’ve seen a positive difference for sure. Speed-wise it responds better versus before when it felt like it would get “hung up” as the speed increased, and cht has dropped as well. 
 

I’m definitely a believer in the “slightly open cowls” recommendation

[A64Pilot]

Actually, open even partially is more drag. The reason is the drag comes from airflow through the engine compt, I believe up to 20% of an average piston airplane is from engine cooling drag, no airflow, no drag.

There is however an argument that if properly designed there is thrust to be had simply from heating and therefore expanding the air, for example the radiator on the P-51 added speed, not decreased it. Called the Meredith effect, but it’s not applicable here https://en.wikipedia.org/wiki/Meredith_effect.

Best plan I think is to operate the cowl flaps in whatever position required to keep the temps in the middle of the green, and don’t worry about any speed increase or penalty.

[Pinecone]

Not exactly.

Think of the cowling as a straw.  If you blow through the wide open straw, it is pretty each.  Pinch it closed and it is harder.

This is why the smaller cooling openings in the J makes it faster than an F.  Too much air trying to get into the cowling.

At Mooney Max, it was presented (I think by Bob Kormer, you can argue with him if you want) that 1" open is worth about 2 knots.

[Bigdaddie]

I've been having problems with the right bank cylinders running hot.  The right cowl flap was tightly closed and took about 1" of ajustment on the rod to get 1/2" open.  I adjusted the cowl flaps to 1/2" open and well see what I get.  I'll report back after flying.

[A64Pilot]

On 6/29/2023 at 10:55 AM, Pinecone said:

Not exactly.

Think of the cowling as a straw.  If you blow through the wide open straw, it is pretty each.  Pinch it closed and it is harder.

This is why the smaller cooling openings in the J makes it faster than an F.  Too much air trying to get into the cowling.

At Mooney Max, it was presented (I think by Bob Kormer, you can argue with him if you want) that 1" open is worth about 2 knots.

You’re right it is similar to a straw, pinch the end off and you decrease airflow through the straw and therefore decrease the drag. Drag is proportional to airflow through the straw

‘Your assumption is that the cu ft of airflow must remain the same open or closed and if it did then your theory would be correct, but airflow decrease with the flaps closed. Pressure in the straw remains constant only the flow changes

‘It’s very similar to the belief that a ram air intake inlet should be funnel shaped to scoop more air into the intake, except that is completely incorrect, the inlet needs to be a divergent cone to build pressure a convergent cone would build velocity and decrease pressure. The greater the amount of airflow around the engine, the greater the cooling drag

Edited June 30, 2023 by A64Pilot

[hubcap]

On 6/30/2023 at 9:58 AM, A64Pilot said:

You’re right it is similar to a straw, pinch the end off and you decrease airflow through the straw and therefore decrease the drag. Drag is proportional to airflow through the straw

‘Your assumption is that the cu ft of airflow must remain the same open or closed and if it did then your theory would be correct, but airflow decrease with the flaps closed. Pressure in the straw remains constant only the flow changes

‘It’s very similar to the belief that a ram air intake inlet should be funnel shaped to scoop more air into the intake, except that is completely incorrect, the inlet needs to be a divergent cone to build pressure a convergent cone would build velocity and decrease pressure. The greater the amount of airflow around the engine, the greater the cooling drag

Except with the cowl flaps fully closed, the drag is increased because of reduced air flow THROUGH the engine compartment. The air must enter and exit through the same opening. If you open the cowl flaps 1" it decreases the total drag by allowing some of the air to escape through the cowl flaps. 

The Mooney engineer explained it quite succinctly. There is less drag with the cowl flaps 1" open.

[Pinecone]

And he had the flight test data to prove that.

[PT20J]

13 hours ago, hubcap said:

Except with the cowl flaps fully closed, the drag is increased because of reduced air flow THROUGH the engine compartment. The air must enter and exit through the same opening. If you open the cowl flaps 1" it decreases the total drag by allowing some of the air to escape through the cowl flaps. 

The Mooney engineer explained it quite succinctly. There is less drag with the cowl flaps 1" open.

It must be right since somewhere along the way Mooney changed the design of the left cowl flap from flat to curved like the right one is curved for the exhaust. So, the later Js get the same effect with the cowl flaps fully closed.

[ArtVandelay]

It must be right since somewhere along the way Mooney changed the design of the left cowl flap from flat to curved like the right one is curved for the exhaust. So, the later Js get the same effect with the cowl flaps fully closed.

They changed it starting #1616, although earlier years may be retrofitted, my 78 has this and the various engine drain tubes are mounted there. I wonder where they put the drain tubes with the flat cowl flaps?

[EricJ]

9 hours ago, ArtVandelay said:

They changed it starting #1616, although earlier years may be retrofitted, my 78 has this and the various engine drain tubes are mounted there. I wonder where they put the drain tubes with the flat cowl flaps?

I have a flat cowl flap, and there's still the pseudo-NACA gap in the bottom of the airframe that air goes out when the flap is fully closed, and the tubes go out there.   I think there's a little cutout on one side, too, iirc, but mostly just out the gap.

Mine is rigged essentially flush with the bottom of the airplane when closed, since I think the gap is designed to match the intakes when the flap is closed.   I rigged the other, exhaust side, open a little more just to get the temperatures to balance better side to side.   My temperatures are very good this way.   On warm days I may have to leave the cowl flaps open when climbing, and once in a while during a climb on a warm day I'll have to open them halfway if the oil gets hot, but usually all the temps are good this way.

For the engineers, sizing the cowl inlets and outlets is essentially an impedance matching problem.   If the inlet is too big or too small you lose efficiency.   If you want it really efficient in a particular phase of flight (e.g., cruise for a Mooney), then design it for that (e.g., the fully closed position), and allow adjustments for other modes (i.e., open them sufficiently to adjust).

I helped a buddy do a signficant engine upgrade on an experimental that has no cowl flaps.   The system was well designed in that with the old engine the temps stayed pretty good nearly all of the time.   I was worried that with the bigger engine and higher power output and two new turbos it might be a problem, but so far it's not.   When the cowl and baffle system are efficient, they do work very well.    I think the J model and later cowls seem to be quite good.

[Pinecone]

Again, the info that the airplane is faster with the cowl flaps rigged to 1 inch open comes from a Mooney FACTORY TEST PILOT.

And based on TESTING.

Think what you want, but this is NOT a bunch of uninformed internet hype.

Another thing that was mentioned was waxing your wing back to the main spar, top and bottom.  Again FACTORY TESTING.  Past the rear spar, waxing does not help speed, but it does protect the paint.