Tightly Cowled Cirrus Engine???

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MaydayMayday

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When watching Mike Busch do his engine presentations he talks about recommended max CHT temperatures he say Max 400 F or 380 F in the tightly cowled engines like the Cirrus (or something like that).

Does anyone know what is the difference in design between the cooling baffle design of the Cirrus Continental engine verses another airplane that has the same Continental engine?

Thanks for the information.
 

Dan Thomas

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When watching Mike Busch do his engine presentations he talks about recommended max CHT temperatures he say Max 400 F or 380 F in the tightly cowled engines like the Cirrus (or something like that).

Does anyone know what is the difference in design between the cooling baffle design of the Cirrus Continental engine verses another airplane that has the same Continental engine?

Thanks for the information.
"Tightly cowled" usually means smaller cooling air inlets and outlets. Cooling air represents drag and lost thrust; to make the airplane faster, less cooling air is good, but it also lets the engine get hotter while on the ground.

The baffle design is no different.

Look at the Cirrus' cooling air inlets compared to a 172's:

1632010887669.png

1632010942857.png

...and then remember that the Cirrus SR22 has a 310-hp engine while the 172 has 150, 160 or 180 HP, depending on year. The 172 has way more square inches of inlet (and outlet) per horsepower than the Cirrus.

In making the Cirrus simple to operate, they used fix cooling air outlets. No cowl flaps for the pilot to adjust. Other manufacturers for years used cowl flaps, and large cooling air inlets. If the air can't get out it can't get in, and drag is reduced with the flaps closed. Cowl flaps on a 182:

1632011357998.png

Making airplanes simple reduces their utility. I've flown cowl-flapped airplanes many times and never had any overheating even in extended taxiing.
 
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HomeBuilt101

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Ive seen the Lopresti cowling mods and I can see that they move the cooling inlets closer to the propeller and are circular in shape and the design is such that the propeller pushes the air into the cowling with more force.
 

tspear

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Cirrus has a small gap for air to exit around the exhaust pipes, not much else. If your baffles are in good shape, you can have fairly even CHT spread between all cylinders. On our N/A we peak at 300 or so in climb on a hot summer day. While the turbo guys have to be much more careful as they maintain power much higher (more power = more heat, more altitude = less air for cooling). On COPA, you occasionally read about the Turbo guys doing cruise climbs or even step climbs in Texas in the summer.

Tim
 

Victor Bravo

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In the majority of cases, I believe pushing the cooling air out into a higher-than-ambient pressure region (under the airplane) is wasting efficiency and engine longevity. I understand why the manufacturers made that compromise (sex appeal, sales) but there are other options that I believe are worth making a different compromise.

I also believe Cirrus minimized this problem as much as they could, compared to the "legacy" airplane cooling.

An airplane sales company like Cirrus optimizes the airplane to make the initial sale, and everything else is secondary. An airplane owner/user/maintainer would design it so that is functions better and longer and more reliably.
 

Dan Thomas

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In the majority of cases, I believe pushing the cooling air out into a higher-than-ambient pressure region (under the airplane) is wasting efficiency and engine longevity. I understand why the manufacturers made that compromise (sex appeal, sales) but there are other options that I believe are worth making a different compromise.

I also believe Cirrus minimized this problem as much as they could, compared to the "legacy" airplane cooling.

An airplane sales company like Cirrus optimizes the airplane to make the initial sale, and everything else is secondary. An airplane owner/user/maintainer would design it so that is functions better and longer and more reliably.
The air moving under the airplane actually has a lower-than-ambient pressure. Because it has dynamic pressure (ram pressure) it MUST have lower static pressure. There is no free lunch anywhere in physics. Even under a wing one can measure a lower-than-ambient pressure at lower AoAs. At higher AoA the pressure will rise as the air slows. This sort of thing is what makes getting a decent static pressure reading for your instruments so difficult. It's also the physics behind every stage of a turbine engine.

The cooling system is carefully designed to exchange velocities for pressures. As the air enters the inlets at speed, it enters a large plenum above the engine that has a much larger cross-section area than the inlets, causing it to slow down, raising its pressure. The air leaves the cowl underneath, into a low-pressure zone created by the exit design. In the picture below, a cowl flap deflects the air downward a bit to do that. Sometimes it's just a small lip bent downward. In some, no lip is necessary, just lots of airspeed past a hole in the bottom of the cowling, lower than the fuselage belly. Causes suction. You'll see that on some older 172s, and the Cirrus uses it. The differential in pressures above and below the engine cause the airflow past the cylinders and heads and case, all guided by baffling and seals.

1632868247343.png
 

Victor Bravo

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I am saying that when you really need the cooling system to work, in a climb, the pressure at A is highest, the pressure at B is lowest, the pressure at C is second lowest, and the pressure at D is the third lowest and closest to A.

For the absolute maximum cooling, let the air in at A and out at B. Since the sales department doesn't want that, the next best thing is to let the air out at C, which is still much lower than A and lower than D. Letting the air out at D is the worst of these options for cooling but makes the sales people happy. But it's not as good for the engine as B or C.


150 Climbing.jpg
 

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Victor Bravo

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NO, D has to be lower than A to have flow through the cowling, but exiting the air at C or B instead would improve the flow more than the "worst of the bunch" exit at D.

The air impacting the bottom of the fuselage at D, because of dynamic pressure, is at a higher pressure than the air at the side of the cowl (which should be ambient) and a little more higher than the pressure on the top of the cowl.

Just like the pressure being higher on the bottom of a wing and lower on top.
 

Toobuilder

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Actually, the top of the cowl is pretty high in pressure - influenced greatly by the base of the windscreen. Not a great place to dump air "out".

Also, high alpha, high RPM slow flight can be overcome with a little physics... Augmentor tubes on the exhaust. Lots of exhaust velocity can move a lot of cooling mass flow through the magic of the jet pump.
 

Mad MAC

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Do the Lopresti cowling mods eat engine baffles, as closer to the prop the greater the pulsing in the air intake, which i seem to recall is often a problem with Augmentor tubes. There is no free lunch anywhere!
 

Voidhawk9

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The air impacting the bottom of the fuselage at D, because of dynamic pressure...
Actually, at subsonic speeds (which is probably the case here ;)) flow will probably be essentially parallel to the cowl surface here, perhaps more tending up around the sides than at lower AoA. The dynamic pressure rise is probably negligible, and not significant compared to the low-pressure effect of tripping the flow (drag!) with the cowl lip.
 

BJC

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VB, I thought that your picture meant that B and C were inside the cowling. If that is not your meaning, I retract my comment.


BJC
 

Victor Bravo

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By B, C and D I meant the location of the exit ramp on the surface of the cowling.

B is where the outlets are in Peter Garrison's Melmoth 2, which I think is the best cooling system I have seen on an air cooled opposed "airplane" engine.

C is where airplanes like the Sea Fury, FW-190 and others have "side exits" on the cowl.

D is the classic certified airplane "lip" on the bottom of the cowl, which I am convinced allows air to come in to the cowling rather than go out.
 
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Bill-Higdon

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By B, C and D I meant the location of the exit ramp on the surface of the cowling.

B is where the outlets are in Peter Garrison's Melmoth 2, which I think is the best cooling system I have seen on an air cooled opposed "airplane" engine.

C is where airplanes like the Sea Fury, FW-190 and others have "side exits" on the cowl.

D is the classic certified airplane "lip" on the bottom of the cowl, which I am convinced allows air to come in to the cowling rather than go out.
I'm probably wrong here but I believe C works for low wing aircraft better than high wing aircraft.
 

TFF

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The one thing to say about the Cirrus cowl is it is optimized. It works well. I need to measure it some time.

I would describe the bottom of the cowl opening as something like a half open cowl flap. Because it doesn’t have a moving one, it probably gives up on each end but average is good. For whatever reason if you flew around at 100 kts for an hour, it’s going to get hot. At speed it probably gives up a kt or two. For not having to mess with it, it’s good.

A Cirrus is decently fast, so it’s cowl deals with air different than a Cessna. The air on the Cessna is slow entering, so it needs as much area as possible to get it in and out. There is lots of volume below the Cirrus engine, and there is not a ton of space behind the engine between the firewall. The air probably doesn’t eddie much in there. Oil cooler air must follow out without it damming up pressure.

The Cirrus is not meant to fly low and slow. It’s cowl openings are probably optimized to be good at 10,000 ft. The density at that altitude and the air density change once it has cooled the engine taken into account. Manageable other times.
 
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