Cooling and cowling

[FloridaMan]

Just thought I'd share my minimal understanding of cooling and cowling. If I am completely wrong, by all means, correct me. A number of things have been explained to me that are counter-intuitive -- and apparently the mentality of aircraft designers changed in the 70s to reflect some of this.

 

I've heard it as this: 

 

1/3rd of all drag is cooling drag. 

Tight baffling is very important for cooling. Air through your cowling will take the path of least resistance, which means that it will avoid those fins around your cylinders if it can. 

Breaks or bends in baffling can divert air critically causing premature failure due to lack of cooling. Baffling that inverts itself can prevent the needed cooling air from going where it needs to. 

At speeds above 160kts, you start to gain a (very) slight amount of thrust from the heated air that expands while cooling. On aircraft such as the P51, the thrust from the heated air through the radiator aided in efficiency. 

Running your engine without the cowling can lead to overheating very quickly due to lack of airflow where it matters (around the cylinders as opposed to that air being force through the fins. 

The tighter cowling of the modern Mooneys should cool more efficiently than the vintage cowls. 

[Tom]

What is counter-intuitive on your list?

 

For me, the most cryptic issue about cooling is all the issues and dynamics that occur downstream of the hot stuff.  We seem to naturally focus discussions on the intake and high-pressure side...to include making sure that there are no baffling leaks...though much less discussion seems to occur on the nebulous issue of designing and optimizing hot air exit.

 

I wouldn't say that even current production Mooney cooling drag has reached its optimization limit.  Most certainly Mooney engineers worked hard on what is presently produced, but if you look at the RV fleet experience, you see what literally hundreds or even thousands of speed-thirsty owners can come up with in the plenum and cowling arena with years and years of tweaking.  Sam James continues to optimize his designs.  Feast your eyes on what you might be able to attach to your Mooney in the coming years:

http://www.jamesaircraft.com/679Cowls.html

http://www.jamesaircraft.com/Plenums.html

[Skywarrior]

There cannot be a significant Meredith Effect in a Mooney, because there would have to be a rear-facing vent for the heated air. Normally, the cowl flaps are closed.

 

Editorial: The cooling setup in a Mooney (and most other recips other than radials) is horribly inefficient. The incoming air hits many obstacles on its way through the engine compartment. The worst part is that the incoming air must change direction 90 degrees as it flows down past the cylinder fins. The clutter and the direction change greatly elevate that airflow's Reynolds Number. That means that it has very little ability to soak heat from the cylinders, due to the turbulence in the flow. And, the cooling fins are, in my opinion, too close together to allow easy airflow past them. This causes the air to want to flow *around* the fins - not *through* them.

 

Cooling would happen much more efficiently if the air was forced to flow tightly past the cylinders in a front-to-back direction, instead of the current top-to-bottom direction.

 

<Class Dismissed>

[Marauder]

There cannot be a significant Meredith Effect in a Mooney, because there would have to be a rear-facing vent for the heated air. Normally, the cowl flaps are closed.

 

Editorial: The cooling setup in a Mooney (and most other recips other than radials) is horribly inefficient. The incoming air hits many obstacles on its way through the engine compartment. The worst part is that the incoming air must change direction 90 degrees as it flows down past the cylinder fins. The clutter and the direction change greatly elevate that airflow's Reynolds Number. That means that it has very little ability to soak heat from the cylinders, due to the turbulence in the flow. And, the cooling fins are, in my opinion, too close together to allow easy airflow past them. This causes the air to want to flow *around* the fins - not *through* them.

 

Cooling would happen much more efficiently if the air was forced to flow tightly past the cylinders in a front-to-back direction, instead of the current top-to-bottom direction.

 

<Class Dismissed>

 

 

ZZZZZZZZZZZZZZZ Class over?! Lunch time!

[N201MKTurbo]

All that really matters is the pressure above the cylinders and the pressure below the cylinders.

 

If there are leaks between the top and the bottom it will lower the pressure above the cylinders and raise the pressure below the cylinders. This will leave less energy to drive the air through the fins.

 

When the baffling is all put together correctly (rare) there is about a one inch slot below each barrel and the fins around the head. this probably amounts to less then one square foot of area for the air to flow through and this area is further restricted by the drag of the fins. It doesn't take much of a baffling leak to allow nearly as much air to bypass the fins as go through them.

 

Not only does this reduce the cooling available, it causes more drag.

 

So keep all that baffling in good shape!

[Guest]

Tom,

Are you suggesting that we all go back to the Dog house baffles of the early Mooney's?

Clarence

[Hank]

I would love to get rid of my doghouse, if it's possible without breaking the bank.

Know any sheet metal wizards in the SE who can help extend the life of mine?

[carusoam]

Heat transfer at it's highest level.

First determine how much heat is needed to remove.

Then determine how much surface area is available to remove the heat.

Then calculate the heat transfer property of the metals.

Add the heat transfer properties of all the boundary layers.

Calculate the pressure drop from the top of the cowling to the bottom using the non-ideal fluid model.

Re-run the calculus for various conditions from climb to descent and in Miami to Alaska.

Then decide if using an ethylene glycol radiator buys you something. (Think M20L?)

Or save yourself from the mathematics, get an IO550 (N) with the funky modified cooling fins and modify the baffles to match!

Put it in a tight bullet shaped cowling with minimal openings like the R and S have...

There are costs to all choices.

Al Mooney made a living out of minimizing friction.

Snore,

-a-

[carusoam]

Where is our cowling guy lately?

We have one here somewhere...

Best regards,

-a-

[Tom]

Tom,

Are you suggesting that we all go back to the Dog house baffles of the early Mooney's?

Clarence

 

I would go with the system that works best.  By "works best" I mean allows for higher aircraft speed while keeping the engine cool.  By "system" I mean a combination of components working together.  The best systems seem to routinely include a plenum, even if not using the same sheetmetal monstrosities of the past. 

[bumper]

I haven't measured, but I think the ratio of inlet to outlet area on the 201 is reasonable, at least with cowl flaps open (optimal is on the order of 1 to 1.5). The biggest problem I see is a lack of a top front baffle. As is now, a large amount of top deck (high pressure) cooling air moves forward and exits via the gap behind the spinner - aided by the centrifugal fan effect of the spinner back plate. I once had a minor oil leak at the engine lift case bolt. There was only an oil wet spot as evidence of the source, as all the oil got sucked off, and exited through the spinner gap and sprayed on the windshield - - I was amazed, thought it was from the front crankshaft seal.

 

Any cooling air that doesn't go through the cylinder fins or otherwise remove heat is adding to cooling drag without earning its keep.

 

On my Husky, a top front baffle dropped CHT's in the climb by 50 F.

 

bumper

[Jim Peace]

LASAR cowl just installed....I already see cooler temps

 

1964 M20C

[jetdriven]

I haven't measured, but I think the ratio of inlet to outlet area on the 201 is reasonable, at least with cowl flaps open (optimal is on the order of 1 to 1.5). The biggest problem I see is a lack of a top front baffle. As is now, a large amount of top deck (high pressure) cooling air moves forward and exits via the gap behind the spinner - aided by the centrifugal fan effect of the spinner back plate. I once had a minor oil leak at the engine lift case bolt. There was only an oil wet spot as evidence of the source, as all the oil got sucked off, and exited through the spinner gap and sprayed on the windshield - - I was amazed, thought it was from the front crankshaft seal.

 

Any cooling air that doesn't go through the cylinder fins or otherwise remove heat is adding to cooling drag without earning its keep.

 

On my Husky, a top front baffle dropped CHT's in the climb by 50 F.

 

bumper

I have also noticed this and I wonder if there's a way to engineer a baffle to seal the nose of the case, to prevent air escaping behind the spinner

[N601RX]

Kind of like this.

[Tom]

Crazy question:

Who here would drop some money on a Sam James type cowling/plenum setup right now just to purchase and keep in the back of the hangar in the event that Part 23 changes occur that will allow this?  If the cowling design was molded to fit the Mooney, would you drop $2000 now on such a paperweight?

[orionflt]

I have the dog house on my C, when i redid the engine last yr I also fixed the baffling and sealed up all the little bypass openings. currently my cylinders are running around 300 in cruise, in a climb I haven't seen over 400, most of the time it is around 350. as much of a pain as it is to get to the top of the engine I love the design.

 

Brian

[MB65E]

Tom-I would do my own out of carbon. Make it exactly the way I wanted! Spending 2k on a paperweight while waiting for the GOV to make up their minds and rules is a lot of avgas! I have my original doghouse working pretty well I think...just like Brian. Spent some time with it last year and it seals up pretty well. Just look for daylight thru the front and start sealing away! When your finished you shouldn't see any daylight.

-Matt

[drbob]

     Looking to North Texas AC sales --- Mooney N767DP

 This M20E sports the Open top [F style] cooling baffle system,

Mooney changed to it when they eliminated the dog house system, it also shows relocated

oil cooler, something Mooney also did, when they gave up on having the OC mounted to the low pressure side of the baffle system.

 Lowen STC'd the OIl cooler change. Believe Mooney IIRC eventually moved it back as well.

Not sure who did the baffles, possibly just duplicated the OEM ones from latter F(E?)the lower parts appear to be whats common to the dog house!

Earliest M20E's were M20C's with relocated battery and Angle valve head & injection sys & sump

  ND IO360. Possibilities for similar M20C baffle sys to just use slightly different side (head) baffles.

Looking for input from the MS brain trust here,

Any Mooney Spacer's using the aft  battery relocation STC (Mod Squad) ?

                                             just some baffling observations  drbob

[Sabremech]

Where is our cowling guy lately?

We have one here somewhere...

Best regards,

-a-

If I'm the cowling mod guy you're looking for, I'm 99% sure my STC will include a plenum system.

David

[MB65E]

HI 328 Dave! with the top cowl removed fill any daylight areas that you can see by looking in the doghouse frontal area. I've used clear RTV silicone, RED RTV silicone. Depending on how nice you want the application, you could even tape off the area with masking tape then apply, removing the tape just after. For longer runs, alumnium tape works well, but is harder to work with for areas that require removal for MX. Either way, Preventive re-application will be needed after maintenance. It's a constant effort, but hopefully will improve cooling and a bit of speed. There are larger openings near the engine mount that require some attention. Also the area around the starter and ALT can be tricky. I seal the top portion of the dog house to the vertitcal portions with a thin bead of silicone then screw it together. It helps with the vibration, chafing, and cooling drag. It's takes a bit longer to remove but peals off pretty easily.... If it's a THIN bead.

I'd like to think it improves airspeed by a knot or two, and I think it improves cooling. However, I have no actual data, other than the fact it trues 160kts+ and I have no tempature problems. If ones having cooling problems, I'd defiantly work on the baffling. Pretty cheap overall, just takes time.

-Matt