RE: effect of prop pitch on efficiency
 

The prop does not blast thu the cowl -
not how it works
but proper cowl and spinner does make a BIG difference.

RE: effect of prop pitch on efficiency
 

Prop blast is a term that is not adequate. But it does go through the engine, enough to make a differance anyway, that is how it is cooled when sitting on the ground. The second part of that is that the air that does not go though must make its way over the curved outer ring, that is also important. The spinner is also important but the most efficient spinners tend to restrict cooling air on the ground.

RE: effect of prop pitch on efficiency
 

The in cowl cooling - is best done from the airflow over the OUTSIDE of the cowl--
this airflow , over the cowl flaps or the outlet hole (whatever) -creates a low pressure in the cowl -- this is what does the cooling on a cowled engine
try as yo might - you can not blow air thru the cowl The air behind the prop is stagnant simply whirlig around as it is rebuffed by the obstruction of the cowl and engine.
However :
Air movement in the cowl is a product of low pressure aft (and below in some cases) the engine , creating a draft to move the air at rear of prop, into and thru th fins .

RE: effect of prop pitch on efficiency
 

"Pressure cowling" is a design that blocks the airflow that would go through the cowling but not near what needs to be cooled, the cylinder fins etc. The baffling fills what would have been open areas. The baffling comes right up to the cylinders but stops short, leaving space near and through the fins for the air to flow. THEN the negative pressure area behind the engine that is created by the low pressure zones behind the cowl flaps etc attracts the air that's coming into the front of the cowl. That air has only one way to get to the negative pressure, and that's very close by the hot spots.

There's a positive pressure in front, that wouldn't do spit if allowed to blast through the cowling. Hot air is harder to move, and the hot air around the hot engine parts would simply give the cooler air something to avoid and to go around.

There's a negative pressure behind. And there are some paths from the front, right by the hot engine parts that need cooling.

Next time you have the opportunity, walk up to the front of a P47, Corsair, Hellcat, T-6, or any big engined bird and look into that big hole in the cowling. Most of it is blocked off. Now step back just behind the cowl flaps. When some of them are open, you can see daylight inside that cowl. Not much. But lots of air can make it through there when that big old bird is thunderin' along.

RE: effect of prop pitch on efficiency
 

or waiting to take off-
the basic concept of cowl air flow , is not obvious..
Once the idea that air only (ONLY) flows from higher to lower pressure areas, the whole thing falls into place.

RE: effect of prop pitch on efficiency
 

You're probably right, this has turned into an electric power system discussion. Your also right that an electric motor will draw what it wants to draw given a certain battery prop combination WITHOUT limitations. However, I've measured the amps the system is drawing with a fluke clamp meter and that is how I knew where to set the throttle endpoint so as not to burn up the motor. My next toy will be a wattmeter to see what it's doing in the air. So my system, without endpoint limitations will certainly draw more than 35 amps, I've just chosen not to go beyond 35 amps to play it safe with my motor. My goal here is to stay in the air as long as possible between charges, which is why I wanted to know if a 13x8 prop would be more efficient at doing this with my lazy high wing floater than the 13x6.5.

RE: effect of prop pitch on efficiency
 

Are you trying to win the all-up-last-down competitions that are sometimes held at electric fly-ins?

My guess is that the 13x8 will prove to be more efficient but only flying will tell. Your goal is to fly at the airplane's minimum sink rate speed (unpowered) with a prop and motor that are both in their efficiency sweet spots. Sometimes that's best done by using the motor's power to gain altitude and then shutting it off and gliding for a while.

RE: effect of prop pitch on efficiency
 

Both you and Dick are right. However, to say it is negative pressure pulling the air though is not seeing the whole picture, there is both a positive and negative pressure so the air is both pushed and pulled through. I used prop blast because that is what the air flow from the prop is called, I did not mean to say that it blasts through. In fact this area of the prop does not produce a huge amount of air flow. Still, many early radials did not have baffling, either way a well designed cowl allows air to go both through and around. Take a look at the Gee Bee racer, the cowl is almost as big as the prop, but if you look closely you see that the opening is actually much smaller and most of the air is flowing aound the cowl.

RE: effect of prop pitch on efficiency
 

theZ, R1 etc., all had a large opening at rear of cowl.
theair flow e.over the cowl, produced a low pressure in th cowl--
th wascommon method ofcooling the big radials then
later, the pressure cowl, cowl flaps, etc; were added to reduce drag
the current radials, also extend the prop as far forward the cowl aspossible--to minimize-cowl-prop interferrence.

RE: effect of prop pitch on efficiency
 

I would also look at a 12*9

If I have a chance I'll explain why in the future but a 12*9 should be your best prop and it will have a power load somewhere in the range of a 13*6.5 to 13*8.


Steve

RE: effect of prop pitch on efficiency
 

Should note that suction is an urban legend. Movement due to pressure differences is always and only push from higher pressure to lower pressure. Any other concept sucks. :D

RE: effect of prop pitch on efficiency
 

And your idea blows.................. ;)

(and is how it works in RL.
It's true with carburation too.)