OK, I have no pride... What's the best place to make the intake hole for the "turbo" spinners? That is, where on the fuselage is the pressure usually highest?

You have to remember that pressure is all relative - I'm hoping Don Stackhouse might chime in here to rescue me (you listenin' Don?) but I would suggest that it is right behind the prop while the motor is running. Just when the cooling air benefits the motor most is, I would suggest, debatable - whether with power on or on the glide - and this would affect positioning of intake/exhaust in order to obtain maximum benefit.

From : Don Stackhouse

The flow around spinners is very complex. Depending on the flight condition and the details of the spinner and the surrounding airframe, you could get any number of answers. This is probably why this thread inspired so much debate.

The answer, as is often the case, is simply to break down the complexities of the flow into an analysis of each of the individual factors at play, then look at how these factors add up to create the net result.

First of all, a closed-front body of any sort has a small zone of stagnant air at the front, called the stagnation zone. Any air striking above this zone goes over the top, and any air hitting below the stagnation zone goes under the bottom. The stagnation zone is air that can't make up its mind which way to go, so it just crashes head-on into the front of the object. The pressure in the stagnation zone is generally full ram pressure, the sum of the air's static and dynamic pressure. However, just outside of the stagnation zone (which, depending on the shape of the object, can be quite small), the air is busy accelerating around the object, so the pressure there is quite low, just like on the top-front surface of an airfoil.

Now, if we put a hole in the front of this object and start sucking air in through it, the pressure will depend on how the size of the hole compares to the size of the stagnation zone, and on how fast we're sucking air through the hole. We'll also have a new stagnation zone, a ring-shaped one sitting around the lip of the hole. Any air that hits outside of the stagnation zone will go around the object, and any air striking inside the stagnation ring will go through the hole.

In the case of a spinner things get trickier. Spinners spin. It's what they do. That also means that the air outside of them feels that spin, and may spin some itself. In extreme cases, the flow on the outside of spinners can get really weird. For example, I know of cases where the forward speed of the aircraft, vectorially summed with the rotational speed of the spinner surface, results in a speed that exceeds Mach 1.0, even though the airplane is still most definitely subsonic. In that case, we have a boundary layer that's supersonic, running underneath a subsonic free stream.

However, the effects of spinner rotation on the outside surface of the spinner tend to be fairly minor. The things that are going on inside the spinner are what really complicate things.

First of all, on a non-rotating spinner with no hole in the front, the air inside is stagnant. That means that the pressure inside is usually greater than everywhere on the outer surface with the exception of the stagnation zone itself. This is why spinners that break on full-size aircraft usually blow off to the front, at least initially. The same phenomenon is applicable to the cabins of aircraft, which is why if a door comes open in flight, it may be impossible to get it closed again; it will hang open about 3" or so and refuse to budge outward or inward from there. There was a Beech Bonanza crash near Phoenix a number of years ago where an experienced pilot had the door pop open on takeoff, and got so fixated in her futile attempts to close the door that she lost control of the plane and crashed.

In general, most of the inside of the fuselage is likely to be at a higher pressure than the air outside. This can be reversed if you can find a truly low-pressure place to vent the fuselage. The aft tip of the tail boom is a common vent location, although this isn't necessarily a low-pressure area in all cases!

If you do have a leak in spot where the pressure outside is lower, you can expect to have some separated flow at that spot. An unsealed gap around the wing saddle is a very common location for exactly this problem.

BTW, this is also the same principle at work when you get "grabbed" by a shower curtain. The air outside the shower stall is essentially stagnant, the air inside the shower stall is moving because the rush of water from the shower stirred it up, and therefore the pressure of the air in the shower is lower than the pressure outside. The difference in air pressure pushes the wet shower curtain inward, where it promptly sticks to you! If you leave the curtain open about 3"-6" at the far end from the shower head, the water probably won't splatter outside enough to cause trouble, but the opening will allow enough airflow to reduce the pressure differential and (hopefully) encourage the shower curtain to not be quite so "friendly".

So far, so good. Now, what about the effects of the spinner's rotation on the air inside? Well, if the spinner is rotating, and the prop hub inside is rotating, then the air in there is sooner or later going to rotate as well, and at about the same speed. Even with no propeller inside, the air still spins. This turns the spinner into a centrifugal air pump. There will tend to be an outward flow through the propeller blade cutouts, and the air on the back face of the spinner's main bulkhead (that disk-shaped plate at the aft end of the spinner, often called a "spinner backplate" by modelers) will also be slung outward as friction with the aft face of the bulkhead sets the air spinning. The resulting flow can be surprisingly powerful.

Back in my "previous lifetime" in the propeller industry, I had a few opportunities to do double-centrifugal (141% rpm) spin tests on some spinners. One that I remember in particular was a big carbon fiber/epoxy spinner, about 2 ft. diameter, for a small regional airliner. The spin test rig we used at the time had been originally built for smaller applications, and only had a 5 hp electric motor. I bolted the empty spinner to the test rig's shaft, checked all the fasteners, and started it up, gradually increasing rpm in small, slow steps towards the target speed. Pretty soon there was a strong breeze emanating outward from the spinner and blowing around loose objects clear on the other side of the room. A low moan rose from the air rushing through the spinner's blade cutouts. I had to be careful where I put my notepad if I wanted to avoid having its contents scattered all over the lab. Long before I got anywhere near the target rpm, the power absorbed by all this air that the spinner was pumping was enough to overload the 5 hp motor and trip its thermal protection cutout switch.

I taped over the blade cutouts with what I thought was plenty of very strong, good quality duct tape and (after the motor cooled down and reset) restarted the test. The low moan was gone, but there was still a lot of air being pumped from the aft face of the bulkhead. What was more ominous was the alarming bulging of the duct tape in the blade cutouts, which I could clearly see thanks to the stop-motion effects of our strobe light. A moment later, at about 120% rpm, one of the duct tape patches blew out with a loud CRACK, followed by some moderately severe vibration, some dancing around of the whole test rig, and a loud flapping noise from the loose tape. I was glad to have a very effective emergency stop switch on the end of a long cord!

v I re-taped the cutouts, this time using a lot more tape on the inside of the cutouts, with a lot more overlap onto the inside surface of the spinner dome, and finally succeeded in completing the test.

The spinners passed with flying colors, and went on to perform very well in flight tests.

As the above demonstrates, air has mass, and therefore is affected by centrifugal force. This is also an issue in the design of hollow propeller blades. Back in the 40's, 50's ad 60's, hollow steel blades were fairly common on some types of large aircraft. These blades typically had vents in their tips to help equalize the pressure inside and outside the blade. I heard tales of what happened when Hamilton Standard tried to make some hollow aluminum blades and didn't put any tip vents in them. The air inside all slung out to the tips of the blades, generating enough air pressure inside to blow the tips of the blades apart. Modern composite propeller blades normally have foam cores. The cells of the foam trap the air inside, so the air is not able to all sling out to the blade tips.

So, what's likely to happen with a spinner on the nose of a model? If the spinner is not rotating and does not have a hole in its nose, you're likely to have some air flow from inside the fuselage out through the gap between the spinner's trailing edge and the front edge of the fuselage. If the slope of the fuselage at that point is not sufficiently divergent, there's a very real chance that the flow over the fuselage could become separated by this outward jet of air. It might be a good idea to shape the spinner and fuselage to guide the outward flow through the spinner/fuselage gap smoothly around the corner and back along the fuselage.

If the spinner main bulkhead has any holes in it, you can also expect some outward flow through any gaps between the blade roots and their cutouts in the spinner dome.

If you have the fuselage vented to a low-pressure area and the prop is stopped, the flow from the fuselage could be inward through the blade cutouts.

If the spinner is spinning, the pumping effects of the rotation will be added to the problem. If the fuselage is not properly vented and/or is not completely sealed, the pumping effects of the spinner will probably pull air from the fuselage forward through the fuselage nose and out through the spinner/fuselage gap and the blade cutouts.

If there is a hole in the nose of the spinner, you're probably going to lose some of its flow through the spinner/fuselage gap and the blade cutouts. Depending on how effective your fuselage outlets are, you might still get some flow back into the fuselage from the spinner inlet. Or, you could get all sorts of pumping through the spinner from its nose inlet to the spinner/fuselage gap, while that flow effectively blocks any flow into or out of the fuselage. While the spinner is awash with a flood of cooling air, your motor is trapped in a pocket of stagnant air in the nose, and proceeds to fry itself.

An annular inlet just behind the spinner, such as at the front end of a Rolls Royce Dart engine (as used on Vickers Viscount, Convair 640, Grumman Gulfstream G1, YS-11, BAE HS-748, among others), can be designed to capture the spinner gap flow, but the design of that is still tricky. If you get it wrong, the flow separation from the outward flow at the gap can end up blocking the annular inlet. Also, that inlet is likely to cause a fair amount of extra drag during a power-off glide.

As usual, lots of "if's" and few or no sure bets. You just need to look at the individual case, figure out which of these factors and design priorities dominate the picture, and figure out what design solution makes the most sense.

Don Stackhouse
DJ Aerotech