Cowling Exit Air

[skippydiesel]

TOP - BOTTOM - SIDES

 

What is/are the best locations? for cowling exit air and why? - are there ant negatives to your recommendation?

[old man emu]

Off the top of my head, I'd say "bottom". If the air intake is high at the front and the outlet is low at the rear, then the air has to flow over more of the engine, allowing more of the engine to transfer heat to the air. If the outlet was at the top, the air would only be in contact with the top parts of the engine. Side outlets would be much the same as the upper ones.

 

Although probably insignificant in a small, low speed aeroplane, the heated expelled air can add to the overall thrust produced by the engine. If the exit hole is smaller than the entry hole, then there would be some compression of the incoming air, which would want to expand as it was heated while passing over the engine. This is the principle of the jet engine.

 

I acknowledge the traditional ... whoops, . I acknowledge that what I have said above is simplistic. To determine the real life effects would involve more engineering calculation than I am capable of. Even the effect of the total thrust on the aerodynamic pitch airframe would have to be considered. 

[Thruster88]

The Piper pa32 Saratoga sp turbo is the only piston aircraft that has a non traditional in the front and out the bottom flow that I can think of. Have not seen inside the cowl. Are engineers just lazy or is the traditional way the best?

[old man emu]

ADDENDUM to my first reply:

Radiators for oil coolers are usually placed down low. If you don't have the airflow from over the engine exit at the bottom, then oily crap gets all over the windscreen or sides of the aeroplane, depending on the location of the outlet.

[kgwilson]

Have a look at the NASA study from 1981 available on the web. It is NASA_CR3405 and has a long title "An Experimental Investigation of the Aerodynamics and Cooling of a Horizontally-Opposed Aircraft Engine Installation". I found it invaluable when designing the airflow system for my engine.

 

Conventional wisdom states that the outflow should be a minimum of 3 times the area of the intake and for effective cooling this is the minimum. In at the top and directed by the plenum design to cool the heads evenly with barrels next and some airflow over the whole engine block and then sucked out at the bottom is the normal process. I also installed an oversize 7 row Positech oil cooler at the bottom of the firewall with its own NACA cool air intake duct and air exhaust. I also created a lip on the bottom of the cowl across the entire width of the airframe which is over 1 metre at that point to generate suction.

 

My engine never runs hot even on 38-40 degree days. If the engine has water cooled heads as is the 912 a lot of the issues disappear.

[facthunter]

Curvature near the cowl exit will benefit a low pressure situation existing for a cowl flap.  Nev

[facthunter]

The above applies to the underneath position. Adjustable exit means that. You can determine the limit stops with experience.  Nev

[Thruster88]

The Piper pa32 with the unique up draft cooling. Note the baffles behind the engine at the bottom. Also has exhaust on top of cylinders. Probably just makes it a pita to work on.

 

[facthunter]

It's what would seem logical but the thermal convective forces are miniscule. Any exit above poses the risk of oil mist or fire near the windscreen but would work well on engine shutdown.. That's one of the LATER Continentals where "everything" is on the underside of the cylinders. (Push rods, exhaust and inlet pipes so whatever cool air could be gotten there would help. .  Nev

[RFguy]

SKippy what you nned to be concerned with the exit cowl air , is the exit air exiting TOO SLOW.  Energy will be required to accelerate cowling exit air up to the flying airspeed, and with the Vsquared parameter in there, it will hurt.

 

So, ensure your cowling exits are not TOO broad/large than optimal .

Too large and the velocity (for a given airflow) will be too slow, and that will be drag because the slow cowling exit air needs to be accelerated. 

Too small  - that will reduce  airflow/ limit pressure difference between frontal pressure and inside cowl (which is required for cooling to work )   ,  but may also provide some thrust. 

 

So the name of the game is not too large, not too small.. 

 

[old man emu]

30 minutes ago, RFguy said:

So the name of the game is not too large, not too small.. 

Ah! The Baby Bear response.

[danny_galaga]

The bushcat has a neat design. One oil cooler on the top is in/out and the main exit of the rest of the cooling air is out the sides

 

Edited December 24, 2022 by danny_galaga