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Old Mar 07, 2013, 10:29 AM
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BIG cowl....small prop.

I'm building a 1/4 scale Monocoupe,with a 7-1/2" diameter cowl. The recommended engines are .60 size, with 11" or 12" props..

I've installed an OS .91 four stroke,with a 14" x 6" prop.Only because I want a closer to scale (20") prop diameter.

I am NOT afraid to fly it with that engine/prop combo....But the first question everybody asks-(even before "How much does it weigh?") is "is that prop big enough to fly it?"

Keeping in mind the Monocoupe won many races,as did the Barrel shaped Gee-Bees,with those fat cowls leading the way,I have a question.

Why were they faster ? just because of the Gianormous engines,or did the airflow around the cowls just not cause the amount of drag we think they do?
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Old Mar 07, 2013, 11:06 AM
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It was because of the horse power of the engines that they were so fast. For us modelers the last few inches of the prop is where we get or thrust.
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Old Mar 07, 2013, 11:10 AM
Zor
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epoxyearl,

For whatever the following comments are worth.
Others may have other evaluations.

No doubt that a large circular fuselage has a fair amount of drag but they were necessary to mount radial engines.

In the years of these races I think that all ;powerful engines were radial. Engines like the Merlins came later.

If we consider that most of the thrust created by propellers takes place in the outer half of the blades, from half of the prop radius to the tips, then as long as this outer half is further out then the radius of the cowling, we have a reasonably efficient prop.

Just some thinking .

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Old Mar 07, 2013, 11:58 AM
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Yes the propeller is fine. I have flown radial engine cowled planes before where the prop looked way too small, and the plane flew fine like that. I have flown a Hyabusa and a Zero with .60 engines and 11x8 props on them and there was maybe 1 to 2 extra inches of prop extending out past the cowl diameter. The planes flew fine like that.
Granted there is the aerodynamic drag associated with the cowl. but it doesn't really affect the propeller thrust and or speed much, if any.
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Old Mar 07, 2013, 12:43 PM
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Quote:
Originally Posted by earlwb View Post

Yes the propeller is fine. I have flown radial engine cowled planes before where the prop looked way too small, and the plane flew fine like that. I have flown a Hyabusa and a Zero with .60 engines and 11x8 props on them and there was maybe 1 to 2 extra inches of prop extending out past the cowl diameter. The planes flew fine like that.
Granted there is the aerodynamic drag associated with the cowl. but it doesn't really affect the propeller thrust and or speed much, if any.
Note that the cowl around a radial engine not only improves the engine cooling by guiding the airflow on the cylinder fins but may also reduce the total drag as compared to a flat firewall behind the engine.

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Old Mar 07, 2013, 01:39 PM
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Yes-I agree with that..The "ring" cowls cooled the radials more efficiently,but the extended length (back to the firewall) reduced drag, increasing top speed.

This is my point-The power generated is the same,with a given prop,(in this case,11lbs of thrust, whether passing a slim nose or a radial cowl...The difference would be the drag factor.....there's more frontal area, with a round cowl.
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Old Mar 07, 2013, 02:16 PM
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Quote:
Originally Posted by epoxyearl View Post

Yes-I agree with that..The "ring" cowls cooled the radials more efficiently,but the extended length (back to the firewall) reduced drag, increasing top speed.

This is my point-The power generated is the same,with a given prop,(in this case,11lbs of thrust, whether passing a slim nose or a radial cowl...The difference would be the drag factor.....there's more frontal area, with a round cowl.
I happen to see this somewhat different than you seem to.
i do not know how or where you pick up this 11 lbs of thrust.
We cannot measure the thrust in flight or at least I cannot.

I can see that a cowling around a radial engine does some streamlining and smooth out the airflow around the fuselage thus reduced total drag.

I remember the days when Trans Canada Airlines ordered some DC4 and wanted them equipped with Merlin engines instead of radials.

It was renamed "Canadair North Star" which were more flight efficient on cruise speed and fuel consumption.

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Old Mar 07, 2013, 03:13 PM
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Thrust measured on the surface,tied to a fishing scale, full throttle,14 x 6 ,OS .91.

At the same Rpms, flying thrust should remain the same-that's plenty for a 12 lb airplane.
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Old Mar 07, 2013, 03:16 PM
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the cowl over a radial also added a measurable amount of thrust along with the reduced drag.

with the rotaries that came before widespread use of the radial the cowl also decreased the amount of power wasted in spinning the engine:
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Old Mar 07, 2013, 03:47 PM
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I'll bet that without a cowl over a rotary,and all that castor oil spraying about, those pilots didn't have any problem keeping their hair slicked back, either.

I'm familiar with "windage" in racing engines,where the crankshaft whipped the oil out of the oil pan, aerated it, and made it useless...Windage 'trays' or dry sump oiling changed all that.

It's surprising how air reacts,at times.
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Old Mar 07, 2013, 04:53 PM
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Oiled planes are for real men.
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Old Mar 08, 2013, 10:08 PM
Zor
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Originally Posted by Taurus Flyer View Post

Oiled planes are for real men.

That is a lot of oil.
It makes me wonder what the reason(s) could be _ _ _

Many hours of flying without cleaning the model ?
A muffler not setup to divert the exhaust away from the fuselage ?
A fuel leak in which the fuel evaporates while the oil stiks.

Makes me wonder if the inside is similar.

Is this one of your models ?

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Old Mar 09, 2013, 06:46 AM
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Zor-no, that is a picture of a Full size Fokker,with a rotary engine !

The engine used Castor oil as a lubricant, and large amounts of oil were dispersed from the engine during flight.

Most aircraft with rotary engines had the bottom of the cowl cut away,directing the oil downward,hopefully away from the pilot.

Can you imagine the sticky,gooey, mess of castor oil at a COLD 5,000 feet?
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Old Mar 09, 2013, 10:56 AM
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look at a yak. huge cowl eating a little prop. they fly good
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Old Mar 09, 2013, 11:07 AM
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I said to another modeler...How full can you fill a bucket with water ? A;-only until it runs over..

Point being,once the cowl is full of air,the propwash / slipstream does it's intended job of propelling the aircraft.
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