Where are the low pressure areas on a plane?

[eflightray]

I have been model building for many, (many), years, but have never really bothered with the 'technical' side of model design, I'm more a suck-it-and-see man.

But for once I would like some technical info regarding low pressure areas on a model. My reason, I occasionally point out to newbies on the forums to not forget about cooling, not just for the motor, but the ESC and battery.
But to get the air traveling through the model requires not just the intake hole(s) but the exit hole(s) as well.

Now the intake hole is likely to be in a fairly high pressure area, probably in the 'firewall', but recently I've been thinking is there a better place for the exit hole, that will aid extraction such as a low pressure area.
Also is there an area that the the exit hole shouldn't really be in.

Sorry, but that was a rather long winded way of asking is there a best place for cooling air exit holes. (Perhaps ones that scale modeler may even accept).

[biber]

Found this pic on the net and it resembles what I've seen elsewhere (e.g. in the aerodynamicists' bible Schlichting/Truckenbrod).

http://www.eol.ucar.edu/raf/Bulletins/Figs/b23_f3-3.png

It's a multihole probe but close enough to common fuselage shapes, you get the idea anyway.

That's for simple holes that are not too big, if they get bigger or odd shaped or have protrusions the pressure you get will differ from static surface pressure at that location.

Right after the spinner cone you will find about the lowest pressure.

biber

[nmasters]

The pressure gradient on a fuselage is the same as that on a wing. The maximum pressure is at the stagnation point at the front and the minimum pressure is near the thickest part. Software like profili or xfoil can calculate it and display it either as vectors or in coefficient form like that drawing biber showed.

[BMatthews]

I'm sorry nmasters but I can't agree with your idea unless the fuselage in question is very closely related to a airfoil shape and the overall cross section is round or nicely oval. Our typical square'ish "tubes" just do not compare to any sort of airfoil found in Profili. Even a flat plate wing has the advantage of having a realistic aspect ratio compared to a typical fuselage. The flow around our typical fuselage is just not going to in any way like that of a wing airfoil.

Some options are no brainers such as a big scoop up near the nose but other points of higher and lower than normal pressure can be very, very decieving.

For example back in the day one of the car companies had a muscle car where the ram air duct wasn't pointed forward. Instead it was opened to the rear with the mouth just at the base of the windshield. Turns out that FOR THAT CAR'S SHAPE this spot was a high pressure area that provided more boost to the intake than even a forward facing scoop would give. Also note the caps and underlining. The next car's front end, hood and windshield shape may or may not provide the same high pressure point. It's all determined by the angles and sizes.

In aircraft have you ever noticed how jet fighter intakes are spaced out from the side with a plate separating the intake from the fuselage side? If you look between there's a wedge in there to split the air that flows between the two. Why do all this? For two reasons. One is to split away the turbulent boundry layer that flows along the skin of the fuselage due in the wake of the radome and crew area. The other is to form a shockwave in front of the intake for when flying supersonic which reduces the local airspeed to subsonic before it flows into the opening. Now I know our stuff isn't supersonic but it points out a few of the issues with trying to find the best spot for locating intakes and vents.

On a glider of mine I located two small scoops just under and behind the leading edge of the wing to duct air around the battery pack. Originally I just used two slanted forward holes with the reasoning that this SHOULD be a relatively high pressure spot. But it wasn't and the pack used to get fairly warm. I formed two shallow scoops from plastic spoons and from there the pack temp went down quite a bit.

Unfortunetly there's no really good way to determine where a high pressure spot will be. At the nose for sure. But a short ways back from the nose? It'll depend on the shape of the nose's taper and how much more widening occurs behind the spot in question. It'll also depend on the angle of attack the model is experiencing at any given moment as well.

[Brandano]

I think that 25% to 40% of the wing chord above the wing would be a safe bet for a low pressure area, as well as above and behind the canopy. I agree with Bruce that Your Mileage May Vary, but usually referring to how other planes have solved the issue is a safe course of actions

[eflightray]

Usually I have been putting exit hole in the bottom of the fuselage, just behind the wing T.E. on low wing models, and a similar location for high or mid wing models.

On scale(ish) models it means they are out of sight, but I was never sure if that was a fairly good place.

My 'logic' said the air leaving the wing T.E. and the fuselage area just there could be a little turbulent but should extract Ok.

Probably having the intake around the motor area, (I'm talking mainly electric here), the pressure should be high enough to overcome just about any exit hole location. But it got me wondering.

Thank you all for your replies.

[HugePanic]

if you have a scale plane, then search for the static-port at the original plane.
this probes are located at the curve (on the fuselage) that has zero-dynamic-pressure in normal flight. easy to find at most airliners....