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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


Cooling of large model engines: Inviolate principles

By Supercool

Historically, radial engine fighter aircraft are considered to have greater drag than for liquid cooled fighters, which have a much reduced frontal area. There are quite a few reasons for believing this consideration to be facile and false. Not the least of these is that the fastest piston engine aircraft in the world is Rare Bear. Also, the finest fighter aircraft of WW 2 was not the Spitfire or Mustang, but the FW 190 D9. This amazing aircraft, while using a liquid cooled in-line Jumo engine, used a radial-configuration cowling for the radiators!

Here is food for thought indeed! The underlying problems in extracting engine power are piston speed, engine cooling and cooling drag. We have little control over piston speed, but engine cooling and cooling drag are often very poorly implemented, due to a comprehensive ignorance of how to optimise them. These two factors are linked, but first let's look at the classical mistakes made by modellers (NB: not by aircraft engineers).

The typical modeller thinks that the faster the air flows through the cowl, the better the cooling. And if the engine is too wide, he shaves off the cooling fins on the engine sides. The air blasts straight in on the front of the cooling fins, races past the sides and on passing the rear, misses the fins at the rear altogether.

Lesson 1: The greater the area of cooling fins available, the greater the amount of power that can be extracted from the engine. More fins, more nitro, more power.

Lesson 2: The faster the air rushes thru the cowl, the more uneven the cooling, the more nefficient the heat transfer, and the higher the cooling drag.

Lesson 3: Uneven cooling causes power loss thru cylinder distortion. The object is to cool the cylinder uniformly, to keep it round and maintain the piston/cylider seal: leakage past the piston loses power and upsets tuning.

Return now to Rare Bear (full-size) and the Sea Fury. If you care to look inside the cowl, you notice some remarkable things.

1. There is a large space in front of the engine! This is the plenum chamber.

2. There is a very narrow annular air inlet, yet sufficient air gets in to cool 3000 HP!

3. The INSIDE of the cowl is streamlined!

4. There is a fairing over the crankshaft not unlike a mirror image of the spinner!

Lesson 4: Air is most effective at cooling a cylinder when it passes over the cylinder at LOW

speed and high pressure

Lesson 5: The idea behind the plenum chamber structure it to slow the air by expansion after it enters the cowl. When the air expands, its pressure increases, just a fact of nature.

Remember, we live in a sea of compressed air, at roughly 14.7 psi. By being clever with

our aerodynamics, we can raise or lower this pressure. We use wings to lower the

pressure, and plenum chambers to increase it.

Lesson 6: When the 250 MPH air over our model enters the flow annulus, it doesn't want to expand nicely into the plenum just because we want it to. The shape of the INNER cowl has to avoid turbulating the airflow and/or permitting stalled flow to exist. The INNER shape of the cowl is probably more important than the OUTER shape. It certainlyis from the point of view of cooling and cooling drag. The latter can be 40% of the total airframe drag!!

Lesson 7: The lip of the cowl inside the cowling has two components: the spinner side and the external cowl side. By placing a mirror-imaged spinner over the crankshaft, the air avoids stalling on that side and expands down toward the crankshaft smoothly. On the externalcowl side, the lip must be rounded and flow smoothly back toward the cylinder head area,again to avoid stalling and allow smooth expansion. The smoother the expansion, the greater the pressure increase in the cooling air. That spells more cooling and less cooling drag.

Notice that nothing has been said about getting the air out of the cowling! While that is important, thehard work in cowling design is getting the air IN. Getting it out again is not nearly so bad. There needs to be a big enough hole for the air to get out, and it needs to be at a point wherethe pressure over the fuselage is low. That point is where the speed of the external flow is large. The scale location is fine, but the size of the hole can affect cooling drag.

OK. So far, we have really been talking about full-size radials, where there are cylinders everywhere

you look. But in a model, such as the Herbrandson 289 powered Rare Bear, there are only 2, and they

are horizontally opposed. Also, there are carbi are intakes stuck in there as well. So how to handle

this situation?

Well here is a fun idea. Have a look inside a full size light aircraft with a horizontally opposed engine.

Chances are you won't see a cooling fin anywhere! Just some baffles and a big hole.

Lesson 8: The space above a horizontally opposed engine is the plenum chamber. Those holes in the front of the cowl don't go to the cylinders, they go to the plenum chamber. The first

thing the hotshot guys do is make those holes as small as they can to reduce excess flow

and hence cooling drag.

Lesson 9: The air from the plenum chamber is then directed DOWNWARDS through the cooling fins, via a cunningly arranged set of baffles. These baffles have the work cut out for them, as we

really want that high pressure air to pass over all of the fin surface, which is probably not

possible, but it is what we aim for.

Back to the model. We have a problem: we need air for the carbies AND air for cooling. That is NOT

the same air. we need different air! So here is the trick. We want two plenum chambers: one

for the carbies, and one for cooling. This is why I like radial cowls. Make the upper

volume of the cowl for the carbies, and the lower cowl volume for the cooling air. This way

the flow into each can be optimised, assuming we are really smart to begin with! (warning: the carbies

may not be set up for high pressure air from a plenum).

Lesson 10: When we look into the annular cooling ring, we should not see the cooling fins at all. All we should see are upper and lower intakes into the two plenums.

Lesson 11: The two plenums must be completely separate.

Lesson 12: The lower plenum must be baffled, so that UPDRAUGHT flow takes place past the front and rear fin surfaces. The front updraught air must then be directed back over the upper fin surfaces.

Lesson 13: There must be no leakage from either plenum, except for air going over the fins and into the

carbie intakes.

Lesson 14: the baffles may be flexible in cases where flow velocity is not a problem. ie upper plenum.

Ok, that is basically it. Now to comment on your existing model cooling systems: here is

what I see.

1. The design is set up for high speed airflow thru the cowl, leading to poor cooling and high cooling drag

2. While the engine may not show signs of overheating, the cylinders will be out-of round while hot, power

will be lost and nitro content limited

3. Some of the fins have been machined off. Start again with new cylinders, you need all the fins you can get.

4. A real attempt has been made to provide cooling flow, which is good, but it is also misconceived.

I regret that to achieve low cooling drag and efficient cooling, most existing designs have to be scrapped and a great deal of work done to implement the above principles and lessons. I am sure a learning curve is involved. If the

existing system does work, then don't bother. With a new design, you can easily run into a fresh set of problems which will need to be worked thru. But you should end up with more power and much less cooling drag.

Finally, radial cowls require a propeller design matched to the cowling shape. Stock propellers can lose a lot of thrust on radial cowls. Read my website on the AT6 prop.


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