I need learning on cowl flaps

Matthew

Matthew

Touchdown! Greaser!
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Matthew
Cowl flaps:

Never used them before.

My understanding - opened up, they allow more airflow over the engine for cooling.

Are they normally operated on a simple procedure like open during taxi, open during takeoff and climb, closed during cruise? Or is it more complicated than that?
 
Ideally, you look at CHT to determine when to open them. But it often ends up being just like you describe (except they should also be closed for descent and landing, and open for go-around).
 
Read the POH of the appropriate airplane. You may find some occasional differences, but generally: open on startup, taxi, takeoff and climb. Close in cruise (assuming temps come down and stabilize). Keep closed until landing then open once clear of runway.

Occasionally, when running at high power/low altitude in hot environments, you may find that you need to leave the cowl flaps open or half closed in cruise to keep CHTs down.
 
Sounds like it's a simple enough concept. I can understand things better when I know the "why" behind it.

I might be riding along in a 182 soon and wanted to understand the usage.
 
If you have a cylinder temperature probe (ideally for all cylinders), you may sometimes open the cowl flaps partially (there are typically several notches to select from) to keep the cylinder temps where you want them, balancing speed/drag vs. engine longevity. (This may be further modulated using the mixture control in some cases.)
 
Matthew said:
Sounds like it's a simple enough concept. I can understand things better when I know the "why" behind it.
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Your engine's cooling airflow requirements change with the amount of power being generated, i.e., more power requires more cooling flow. OTOH, the cooling flow is proportional to two functions (airspeed and cooling channel cross-sectional area) which are not directly tied to power level. If they designed enough cooling area to allow sufficient cooling for climb (max power, low airspeed), there would be too much cooling at cruise (moderate power, high airspeed) as well as increased cooling drag (costing you speed). Likewise, on the ground, while you're at low power, you're also at zero airspeed, so cooling flow is reduced, and the engine wouldn't have enough through-flow to keep CHT's acceptable without opening up the cooling flow area more.

The solution to this is to create a control to manage the cooling channel area so you can get more flow through at low speeds and less flow through at high speeds. The cowl flaps modulate the cooling outlet area, which accomplishes this goal.

I know there are a couple of oversimplifications in the above discussion, but it's close enough to understand the basic "why?" for cowl flaps.
 
:idea:

Dang, not much to add to either Matthew's already basically good grasp of the matter nor Ron's very well written explanation. Yeah... I know.

About the only thing I would add as additional explanation for Matthew's benefit is that if you are wondering why have them closed during descent and landing, one big reason is to help prevent shock cooling. They are basically an additional powerplant management control. You probably started with two, throttle and mixture then later added a third, prop. Now you have a fourth, cowl flaps. Fun stuff!
 
Do non HP engines just not generate enough power to require them for extra cooling?

I've flown glider tows before, and have always found the need to keep an eye on engine temps because of that. Shock-cooling is always kept in mind when you release a tow then hurry back down to get the next.
 
Matthew said:
Do non HP engines just not generate enough power to require them for extra cooling?

I've flown glider tows before, and have always found the need to keep an eye on engine temps because of that. Shock-cooling is always kept in mind when you release a tow then hurry back down to get the next.
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It is more a function of airflow and cowl design.

There are non-hp airplanes with cowl flaps (Cessna Cardinal) and some HP airplanes without cowl flaps (Cherokee 6).
 
Cowl flaps are pilot controlled heat managing devices. How a guy uses them in 90* temps will be different than in -20* temps. The same may be true of floats vs wheels. One airplane is expected to perform within normal temperature ranges in a variety of conditions and configurations. Some of that requires pilot participation.
 
Fearless Tower said:
There are non-hp airplanes with cowl flaps (Cessna Cardinal) and some HP airplanes without cowl flaps (Cherokee 6).
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Cardinals do and don't -- only 1970 and newer (177B and 177RG) have cowl flaps.

Other high performance airplanes built without cowl flaps -- Beech 33 series Debonairs/Bonanzas with 225 hp IO-470-J/K engines, and the Model B36TC Turbo Bonanza. Movable cowl flaps are a popular retrofit on the B36TC; without them the engine runs hot.
 
On the aircraft I've flown mostly the C182 and Mooney retracts cowl flaps open during taxi and takeoff per the checklist from the poh and closed at cruise then open after landing.
 
Navions got cowl flaps only with the larger engines, but there were 205 and 225 HP without them. Definitely makes quite a speed difference when they are closed. Now that I have the other cooling issues resolved on my aircraft, they get opened at engine start and closed when transitioning to cruise.
 
On bone chilling days I keep them closed after start up until oil temp goes green.
 
If the CHT is cold, or about to be, then close the clown flaps.
If the CHT is HOT, or about to be, then open the clown flaps.


There you go, that'll be $100 dollars..
 
Tom-D said:
Right answer. Simple, do what the book says, Test piloting was done at the factory.
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Good advice!!!!!!!

tn19560402-0-C-1.jpg


tn19560402-0-C-2.jpg
 
Kritchlow said:
If so, why have movable cowl flaps? Why not just have permantly open vents?
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As I alluded to before, cooling drag - and often a bit of form drag as well.

Yet that's essentially what some aircraft like the Cirrus and others named prior in this thread do, with no huge penalty if the cooling system is properly designed and maintained.
 
Kritchlow said:
It's unclear to me... Are you a "no such thing as shock cooking guy"?



If so, why have movable cowl flaps? Why not just have permantly open vents?
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Overheating and the need for cooking are not the same as shock cooling.

His eye roll is in reference to the fact that much of what people refer to as 'shock cooling' is an OWT.

I should add that I believe shock cooling exists, but is a way overused term in aviation.
 
Fearless Tower said:
I should add that I believe shock cooling exists, but is a way overused term in aviation.
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This.

I don't deny it completely, but pulling off 2" of MAP instead of 1" isn't going to crack cylinders. :lol: It takes drastic measures to get even remotely close to "shock cooling" your engine.
 
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The greatest rate of temperature change in my engine happens when I shut it down. The next greatest rate of change happens when I start it up. Temperature change on descent is a non issue for my normally operated airplane. Not warming it properly prior to laying the whip to it is a much bigger threat.
 
There was an extensive analysis of cylinder failures a year or so back. Excessive heat was specifically called out as the usual causal factor in failures. Excessive cooling didn't even get a mention. Better to adjust your cowl flaps, climb rate, etc... to keep the cylinder temps down than to worry about how fast you're descending.

I can tell you that excessive heat causes a wide variety of problems in the engine. It's pretty much what killed mine as determined in the subsequent post mortem. Due to a crappy baffling design, mine used to run at the redlines all the time (and except in very cold temps always with the cowl flaps open). I actually conducted an experiment with some duct tape to prove if the baffling was sealed up it could stay cool, but tape doesn't hold for long.
 
Matthew said:
Cowl flaps:

Never used them before.

My understanding - opened up, they allow more airflow over the engine for cooling.

Are they normally operated on a simple procedure like open during taxi, open during takeoff and climb, closed during cruise? Or is it more complicated than that?
Click to expand...

You open them as necessary to control temps. If it's cold out, I leave them closed until I start getting the temps up where I want them as even the prop slipstream can have a noticeable effect. In cruise I monitor temps and adjust as required, typically I run low enough power settings that I can have them fully closed even on a hot day.
 
I had cowl flaps on my Turbo Arrow, but they were an STC retrofit that came with the intercooler.

I'm not exactly sure why the addition of the intercooler necessitated cowl flaps, other than possibly better cooling flow over the intercooler itself.
 
An intercooler is going to introduce additional heat inside the cowling, both directly and by making the fuel burn hotter. It also takes up some space and will modify airflow because of that.
 
Sac Arrow said:
I had cowl flaps on my Turbo Arrow, but they were an STC retrofit that came with the intercooler.

I'm not exactly sure why the addition of the intercooler necessitated cowl flaps, other than possibly better cooling flow over the intercooler itself.
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That, and the ability to make more power (heat) at higher altitude where it's more difficult to shed.
 
Henning said:
That, and the ability to make more power (heat) at higher altitude where it's more difficult to shed.
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Maybe on a high altitude climb, but the cowl flaps are closed in cruise.
 
It's just temp control, like a manual thermostat in your house, if the engine is getting hot open them up, if it's getting cold close them.

You'll learn the plane and open or close them during stages of flight and based on OAT automatically after a couple flights.
 
Basically, there is no magic. They do a job, and you use them when needed.
 
MAKG1 said:
An intercooler is going to introduce additional heat inside the cowling, both directly and by making the fuel burn hotter. It also takes up some space and will modify airflow because of that.
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Additional heat will be introduced inside the cowling yes, but in this particular case you aren't making any more power, just operating at lower manifold pressures, so as far as I know, EGT's are unaffected by the intercooler at a given power setting.

I'm not sure if there is net loss of space in the cowling area, but it's probably minimal, as you have to lose the air conditioning compressor to accommodate the intercooler.
 
Sac Arrow said:
Additional heat will be introduced inside the cowling yes, but in this particular case you aren't making any more power, just operating at lower manifold pressures, so as far as I know, EGT's are unaffected by the intercooler at a given power setting.

I'm not sure if there is net loss of space in the cowling area, but it's probably minimal, as you have to lose the air conditioning compressor to accommodate the intercooler.
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The energy produced in a burn is a function of pressure and (manifold) temperature. The point of an intercooler is to lower the intake temperature so you can cram some more fuel in there. If you offset that with reduced throttle, there is no point to having an intercooler -- fuel will evaporate better without it.

As a rule of thumb, with combustible mixtures in an internal combustion engine, 1/3 of the energy goes out the exhaust pipe, 1/3 goes out the cooling system (cowling, in this case, including both cylinder cooling and oil cooling), and 1/3 goes out the crankshaft. If you raise the total energy consumed, which is the point of an intercooler, you raise all three roughly in proportion.

The proportions are changed with a turbocharger, as you're grabbing some of the exhaust energy to make the other two larger. But the intercooler doesn't change them.
 
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Henning said:
They're automatic?
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No, they get closed per POH procedure, regardless of altitude.

MAKG1 said:
The energy produced in a burn is a function of pressure and (manifold) temperature. The point of an intercooler is to lower the intake temperature so you can cram some more fuel in there. If you offset that with reduced throttle, there is no point to having an intercooler -- fuel will evaporate better without it.

As a rule of thumb, with combustible mixtures in an internal combustion engine, 1/3 of the energy goes out the exhaust pipe, 1/3 goes out the cooling system (cowling, in this case, including both cylinder cooling and oil cooling), and 1/3 goes out the crankshaft. If you raise the total energy consumed, which is the point of an intercooler, you raise all three roughly in proportion.

The proportions are changed with a turbocharger, as you're grabbing some of the exhaust energy to make the other two larger. But the intercooler doesn't change them.
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The STC claims "equal or better fuel efficiency at a given power setting" with the intercooler installed. I suspect it's closer to "equal". I didn't have it installed, it came with the plane.
 
Sac Arrow said:
No, they get closed per POH procedure, regardless of altitude.



The STC claims "equal or better fuel efficiency at a given power setting" with the intercooler installed. I suspect it's closer to "equal". I didn't have it installed, it came with the plane.
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Efficiency is not the same as fuel consumption, and marketers in this field use that to their advantage.

If you make more power, you burn more fuel and make more heat. If it's in proportion, that's the same efficiency.

The point of an intercooler is to cram more fuel/air in. Why would you use the same power setting?
 
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MAKG1 said:
Efficiency is not the same as fuel consumption, and marketers in this field use that to their advantage.

If you make more power, you burn more fuel and make more heat. If it's in proportion, that's the same efficiency.

The point of an intercooler is to cram more fuel/air in.
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Yep, the only thing it does is increase the air charge density, to take advantage of it, you have to put in more fuel. In aircraft the primary advantage it gives you is to increase the altitude at which you can make 'x' horsepower. Since an airplane uses horsepower more efficiently at higher altitude, the overall efficiency of the aircraft increases; the efficiency of the engine remains the same.
 
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