I have heard that the exit should be twice the area as the inlet.

But what I don't know is ... how do you determine the minimum inlet area.

Any information would be appreciated.

Thanks !

THe only thing that is important is teh air that is flying over the cylinders. All the rest is defeating the purpose.
I would recommend that you baffle the cowl to ram as much air though the cooling fins as possible.
If you have 1000CFM going in and only 10CFM is touching the cylinders, then the 2000CFM exist does you little good in cooling your engine.

Mustang, exhaust should be 3 (at least) times the inlet area, but this is just a theoretical figure and other factors like baffling and vlowme in the cowl all come in to play
trying to get close to that figure is desirable though

Thanks Guys !

I have a completely cowled-in Revolution 52 engine in a Mustang I'm trying to keep cool without cutting up the cowl too much.
I will make sure the air path is headed toward the cylinder. My exit is approx. 2.5 times the inlet ... I am trying to use the Mustangs front Air scoop as the only inlet enlarged as much as I could ... time will tell if it works.

Good luck, Mustangs are a hard one to get right without it looking naff, make sure any ducting you use is very tight fitting around the cylinder(nearly touching the fins, dont use metal)
and make sure their is not an alternative easier route for the airflow to divert(it will always take the path of least resistance)

Probably no one here has heard of pressure cowls -but anyway they were common on speed control line-back when the earth was still flat --
The idea is a small inlet that feeds ONLY into the finned (usually cut way down) section of the cylinder -which fits very closely to the sides of cyl
the outlet is large and situated such that air is extracted .
One can not ram air into the opening - simply can't do it.
the prop in this area is only a spinning log - actually worse than it would be without a prop.

Dick, you would probably be surprised then.

I have tried such a setup on a normal sports CL. There was some noticable ram effect in the front part of the duct, because I had fuel mixture problems, which only were solved by shielding the carb bell from the airflow. The duct was close fitting for about 90 degrees fin circumference each side, and closer fitting at the rear than at the front, which had a generous radiussed inlet to slow down the air at cooling fin entry.
Upon landing, the engine always was cool to the touch.
The duct inlet on the Veco .20 (aluminium piston version) was 10mm wide, and ran from fin start to the top of the cylinder head, which was completely faired over. Air outlet was generous using side gills, to prevent pressure build-up of the heated air.

"One can not ram air into the opening - simply can't do it." What do you mean?

the prop is simply churning air mostly sideways at that point
If there was no prop -then you could
The air only moves thru -if there is a low pressure behind the engine

ah! I get what you're saying.

Hey guys, think about it a moment before jumping to any conclusions.
The plane moves at a certain speed, relative to the air it is in. When it is in front of the air inlet, the relative speed to the plane gets less. There is only one force that puts the brake on that speed, and that is pressure being built up in front of the engine. That is called the ram effect. Ramjets depend on it to function properly.
The moving propeller does disturb the speed of flow somewhat, but certainly does not nullify it, nor can it prevent the air from entering the opening where it has to slow down.
"nuff said?
Hovering is a different matter.
Because the plane speed is nearly zero, the propwash is needed to provide airflow, and it probably is very benificial to create a low pressure area at the air outlet as well. That prop hub section is not a very good blower, like Dick pointed out allready. Yet, the air near the hub is not stagnant, but moves along with the prop wash at near pitch speed. Thus the situation is not as bad as it seems, or all our engines would fry.

I agree, but dick did acknowledge that the air would flow if there is a lower pressure behind the engine which is true. Whether or not this situation exists in with or without a prop is something I won't argue about, but I know the answer.

Do anyone have any ideas or pictures on how to create a low pressure area at the outlet?

yes,
positive pressure is present in areas where the convex part of the curved flow is towards the surface, like the area between bonnet and windshield in a car.
Negative pressure is there where the concave part of the curved flow is toward the surface, like where the windshield merges into the car's roof section.

By positioning air intake and air outlet in the appropriate regions, air flow between these two is the result. Due to the pressure gradient air will flow from the inlet to the outlet.

To create a low pressure, it is sufficient to angle part of the plane skin outward, like in louvres, and position the opening directly beneath, or behind the angled part. In front (upstream) of the louvre there is a small pressure zone, whereas at the apex of the louvre the pressure is quite low, which draws air through the opening.

Here is a picture of a simple setup,which we did for the pipe tunnel on a YAK
It is made from Depron
Also -look at the location of the louver groups on the cowling -
These keep the big radial cool on the full scale craft.
They also use inlet doors to correct engine temps -as required.
On the Yak -with the huge radial - they place the prop way out in front - to help reduce the prop losses
On a model such as this one -it also helps admit air into th cowl
On my model - all outlet air is via the open "box" under the cowl and thru the pipe tunnel -to a lesser extent.
I measure the temps of my engines and this one -tho it is subject to a lot of pogo stick flying and I crank the engine HARD- it all stays cool - about 190 is the hottest I see at the cyl fins after landing
Look at the aft underside of the fuselage and you can just see the pipe exit baffle .

DHanson & PReivers
Like the Idea of creating a low pressure area at the exit ... I will try doing something like in the picture of the YAK.

Thanks guys !

All thoughts are welcome !

Nice work Dick, I notice a lip at the front of the exit, I understand that this is beneficial to the exit flow, often seen in full size cowl exits..is that the reason for the lip on your yak?

Noticed that the YAK had a louver that seems to have a small angle.

What do you guys think is the min and max angles for the louver.

The lip and it's shape - is pretty much a matter of what looks good and is easy to do -
Iwould guess some eons ago - air tuft tests showed that the accepted grouping and shape of louvers - be it on the hood of a 32 chopped coupe or the YAK- is "close enough"
The lip causes a turbulance at the aft edge - that's all that is needed - it makes a low pressure and Mother Nature tries to rebalance it -causing air to flow faster to that spot.

Dick, I know what it does, just wondered if it was deliberate on your yak

I made openings in my louvers and it cools the engine nicely.
I will improve teh opening then paint it to hide the defects in cutting.
I may paint the entire inside black or red to make it look better.

Dick, that turbulator strip may be doing things completely differently from what your visualization suggest. It may just be shedding vortices in front of your pipe exit. In fact, you might have been better off just sticking the pipe exit in clean airflow and rely on the venturi effect to create some negative pressure.

At work, our dept. has a group that does quite a bit of single- and two-phase CFD simulation with [link=http://www.fluent.com/]FLUENT[/link]. I was surprised at some of their findings. They often have to correlate their result with some empirical data, because one small parameter change can lead to completelly different results.

If I were to worry that much about this subject, I would try a NACA duct instead of a turbulator strip. I am sure you all have seen the distinctive signature shape of a NACA duct on some race cars. There, it rely on volume expansion to create a low-pressure region, which is much more reliable and predictable.

Then again, I tend not to worry THAT much about it. Afterall, it's all a bunch of "hot air" anyway.

Well- looking at the oil splatter on the bottom of the plane - the exhaust just makes it out over the aft lip -but it makes it out . I figgered anything that creates a disturbance forward should make a low pressure aft
Those lovely NACA ducts are quite familiar - just don't see a need for em here -
The box the pipe is in flows quite well -just wanted to lower pressure a bit so stuff tends to fly out the hole ---

NACA Duct ... Interesting ....

I'm not sure how to build that into the bottom of the cowl ... or if there is enough room.

But ... very interesting !

But I proved to myself last weekend .. I will need to do something !