Discuss "closely cowled"

Let'sgoflying!

Let'sgoflying!

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Dave Taylor
First, define.
Reading from time to time how it is an evil trait, causes problems.
Then I read about how plenum cooling is better. (which seems like the ultimate in a 'close cowl')
So, is having a cowling immediately adjacent the hot bits better or not? Seems like you'd get faster flow (which could be good), if less volume (which could be bad).
Chapter 1293 in my aviation training is somehow blank, on 'Closely Cowled".
 
Closely cowed...

DairyCowsEating640.jpg
 
Closely cowled to me has always meant moving the mass of air needed for cooling with the least amount of parasitic drag. Don't understand why that would be an evil trait? As for "plenum", isnt that what the upper part of the cowling and the engine baffling create on all of our horizontally opposed air cooled engines? Maybe I'm missing something here?

One thing I have noticed is a remarkable number of cowlings have outlets with less cross-section than their inlets.
 
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GRG55 said:
One thing I have noticed is a remarkable number of cowlings have outlets with less cross-section than their inlets.
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Have noticed that too. Assumed it was either an illusion, or I was not understanding something..
 
Just depends on the design. To define "closely cowled" I'd point you to look at the Cessna 195 - so tightly cowled that the rocker arms are imprinted on the shape.

Old designs have not traditionally made good efficient use of air that runs through the cowl for cooling. So, many companies shrunk the cowl to make the plane faster, but this did not make for adequate cooling of the engine. Many Grumman owners will jump down my throat... but they're the best example of this, A significant amount of baffling work was necessary to get adequate cooling in these aircraft, but they are fast.

On the other hand there's the old Piper Comanche cowlings which just gobble obscene amounts of air in.

Now with advancements in fluid modeling and a better understanding as a whole, companies like LoPresti can make very sleek and aerodynamically efficient cowlings while still providing efficient cooling. It's less about how much air and more about being able to force it through the cylinder cooling fins as quickly as possible.

It's a heat exchanger equation Q(Energy) = M(Mass flow rate)Cp(Specific heat)(Delta T)

Cp and Delta T are going to be fixed, so to change the energy transferred to air, you need to increase mass flow rate. Mass flow rate = velocity * cross sectional area * density. Cross sectional area and density are fixed because there are a set number of cooling fins. So in order to increase energy transfer you need to increase the velocity of the air moving over the fins.

There is a smaller cross section for the outlet than the inlet to control the amount of air flow. It's much easier to control outlet size than inlet size.
 
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I have wondered why inlets were not pointed downwards slightly, to allow more air flow in climb, less in cruise. Seems like more heat is generated during the higher power of climb and cruise already has the advantage of air velocity. Directional inlets might solve this? Or is 'most of the air' coming off the prop blast, and angle of attack is irrelevant?
 
Let'sgoflying! said:
I have wondered why inlets were not pointed downwards slightly, to allow more air flow in climb, less in cruise. Seems like more heat is generated during the higher power of climb and cruise already has the advantage of air velocity. Directional inlets might solve this? Or is 'most of the air' coming off the prop blast, and angle of attack is irrelevant?
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The surrounding air that is hitting the aircraft is what creates your angle of attack. The air coming off the prop is so close to the inlets and is by definition parallel to the airframe so engine inlets that point downward wouldn't do anything. That being said, when in a climb, due to P-factor there is going to be a difference in the air flow on the left vs right side of the engine. Just like for left turning tendencies, the descending blade will have a higher relative airspeed and push more air into the engine than the ascending blade. How significant this is, I can't really say.
 
Let'sgoflying! said:
First, define.
Reading from time to time how it is an evil trait, causes problems.
Then I read about how plenum cooling is better. (which seems like the ultimate in a 'close cowl')
So, is having a cowling immediately adjacent the hot bits better or not? Seems like you'd get faster flow (which could be good), if less volume (which could be bad).
Chapter 1293 in my aviation training is somehow blank, on 'Closely Cowled".
Click to expand...

Read this! http://www.n91cz.net/Interesting_Technical_Reports/106-111_BuildingBasics.pdf
 
It's all a matter of velocity and pressure. The inlets are usually at high pressure because of either ram air or the prop blast or both. As the air moves through the engine compartment it loses pressure and expands and needs a larger hole to exit through.
 
Mtns2Skies said:
Just depends on the design. To define "closely cowled" I'd point you to look at the Cessna 195 - so tightly cowled that the rocker arms are imprinted on the shape.
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What you are seeing in the 195 (‘bump cowl’) is more a factor of the design shape of the Jacobs engine than tightly cowled. Almost all cowled airplanes with Jacobs engines have bump cowls, even the first Beech 18s.
 
GlennAB1 said:
Air velocity on the fuselage exterior at the outlet may help draw the air out.
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And the shape of the openings can increase/decrease the pressure as needed (NACA did a lot of studies back in the day) with minimal drag.

Lower pressure at the outlet just further reinforces the need for large area holes on the exit.
 
Fearless Tower said:
What you are seeing in the 195 (‘bump cowl’) is more a factor of the design shape of the Jacobs engine than tightly cowled. Almost all cowled airplanes with Jacobs engines have bump cowls, even the first Beech 18s.
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T-50 didn't.
 
I may be mistaken, but I also believe there was a Waco with a Jake and smooth cowl.
 
bluerooster said:
I may be mistaken, but I also believe there was a Waco with a Jake and smooth cowl.
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Which one?

There are smooth cowled Waco’s, but all the smooth cowls I can think of have Continental W670s. All the Jake powered Waco’s (both cabin and open ship) I’m aware of had bump cowls.

Now that you mention it, seems like I remember hearing about a Waco that used a T-50 cowl, but I can’t recall what specific airplane or engine.
 
Closely Cowled ... is that Simon Cowell's brother?
 
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