Hi Steve,
Hmmm ... yes and no.

1)The price to produce a pack with spacers, or for you to buy 2S packs and tie them together with spacers and solder all the bits together would be substantially  higher.
2) With cell resistances coming down as they are, our 7~8 "C" average and 10~11 "C" peak loads are no problem at all, provided the battery does get cooling.
The thermal conductivity of the cells themselves is surprisingly good.
The RC Pattern guys want 17 or 18C peaks, and until recently, I considered the hassle of putting gapped packs together as worthwhile.
This last generation of light-weight and medium high current cells are coming down with maybe 20 or 25 F rises compared to ambient, in hot weather.
It's no longer worth the bother, in either event.
3) In cold weather you actually want to launch with the batteries warmed to at least body temp, and you'd like to land with them at at least 100F. This means battery cooling paths that are segregated from the motor/ESC so that this path can be partially blocked in cool weather.
4) On the other hand, it is traditional to make Stunt fuselages by bolting the rails to the intended engine, and gluing fuse sides on ... no fat planes can ride here! This has to change. There are plenty of examples of poor cooling design, and hopefully as we move away from conversions to purpose-designs we will fix this one.
Motors and ESCs just get happier the cooler they are.
later,
Dean P.

3) In cold weather you actually want to launch with the batteries warmed to at least body temp, and you'd like to land with them at at least 100F. This means battery cooling paths that are segregated from the motor/ESC so that this path can be partially blocked in cool weather.
4) On the other hand, it is traditional to make Stunt fuselages by bolting the rails to the intended engine, and gluing fuse sides on ... no fat planes can ride here! This has to change. There are plenty of examples of poor cooling design, and hopefully as we move away from conversions to purpose-designs we will fix this one.
Motors and ESCs just get happier the cooler they are.

I see no reason not to stack the cooling ductwork over/under, with the motor ducting on top just below the spinner (or around it) and the battery ducting below.  Like the ductwork on the P-47 Thunderbolt, where the top opening cooled the engine and the bottom opening took air back to the intercooler.  They couldn't use the warm engine cooling for the intercooler, because the target temperature is too low.

And if you played your cards right, you could make it a styling element in the front view of your airplane.

I'm all over that styling element aspect!
Turboprop exhaust ducts would be just right for motor air egress.
Dean

I haven't done it yet, but I have been tempted (after hacking yet another ugly hole behind the wing of an RC ship) of designing a ducting system that brings the air in the front, loops it around, and exits somewhere close to the front again.

I hadn't thought of fake turboprop exhaust ducts, but on the right plane they'd certainly look good.  There's no reason you couldn't do something similar with the chin radiator on a P-40 or a Hawker Typhoon.

With a stunt ship there's no reason you couldn't have the duct exit at a "natural" point and still look good -- I'm thinking of the step in the bottom of a Nobler fuselage, which looks like an exit duct already, and only needs the right structural work to actually be one.

Keep in mind that the exit hole needs to larger in area than the entrance. A good rule of thumb is 1.5 to 2 times the area. This is to account for the velocity drop from the internal drag.  Also another thing to remember is that the internal drag of the ducting/machinery is added to the total drag of the airframe.  This is why real airplanes have "nice" ductwork that is carefully sized for the required airflow. Probably overkill for us to use fluid modeling to size the ducting, though Howard might.   

As PT Barnum said, "This way to the egress".

John

Maybe Johnnie Taylor was singing about COOLING:

Shake it up, shake it down
Move it in, move it round, disco lady
Move it in, move it out,
Move it in round about, disco lady
Shake it up, shake it down
Move it in, move it around, disco lady

I have a bone to pick with that rule of thumb too!
 
Dean P.

An easy-to-make windtunnel design that I could use to test cooling would also be -- well -- cool.

I'd use an automobile.

It would be useful to know where the low pressure and high pressure areas are on the surfaces of a "generic" stunt model, at least maybe on the front third where exit holes are likely to be.

John Witt

I had a cooling hole on the bottom of my cowl.  I wanted to replace it with a cooler-looking NACA duct, but I didn't know whether the air went in or out of the hole. 

The NACA duct is made to scoop air into the airframe.  At the very least you'd want to turn it around backwards.  In general you want the outlet to be at a low pressure point.  I couldn't tell you the best places to look, other than it's probably at or behind the fattest part of the fuselage.

The NACA duct is made to scoop air into the airframe.  At the very least you'd want to turn it around backwards.  In general you want the outlet to be at a low pressure point.  I couldn't tell you the best places to look, other than it's probably at or behind the fattest part of the fuselage.

I'm familiar with NACA inlets; we use them where I worked.  My airplane cooled just fine.  I tufted the hole to see which way the air went.  It went in, so I put an NACA inlet on the cowl.  It cooled just fine with it, too.

What I know of NACA boundary layer ducts, from my race car days, is that they are only used for inlets. Also, they have to be located where the boundary layer is mostly laminar and relatively thin, thirdly, there is a fairly specific set of dimensions/rules for their shape.  If done correctly, an NACA duct can actually decrease the drag by thinning the boundary layer downstream of the duct opening.  I haven't read anything much about exhaust ports, though, to know if there is an equivalent.  It is a problem in impedance matching both entry and exhaust to the surrounding atmosphere.

I once had the chance to ask Paul McCready whether the front end shape of a vehicle or the downstream end was more important from a drag viewpoint. He said the entry shape was the most important. FWIW.

Maybe we need automatic speed brakes for the down lines. Howard, Igor...?

John Witt