If you have a large round cowl in comparison to the overall diameter of the prop does that impact the amount of thrust your getting from that prop? I've seen some pictures of airplanes with lets say a 16 inch prop but the cowl is 13 inches in diameter. It seems like there is barely enough prop to create thrust or does the cowl being there not negatively impact the overall thrust generated by the prop???? Thanks for any insight!

English w-ind tunnel tests were conducted with three different diameter smooth, blunt nosed bodies behind a two bladed propeller having a pitch/diameter ratio of 0.7. The three body diameters were 0.40, 0.60, and 0.75 times the propeller diameter. Propulsive efficiency with the smallest diameter body was only two percent less than that of the propeller alone. Propeller efficiency was reduced by about five percent with the medium sized body, and about seven percent with the largest body. Thrust loss would be about the same as efficiency loss.

The fuselage that you mention is a highly unusual 81 percent of the propeller diameter. No tests were recorded with such a large diameter body, but I would expect thrust loss to be in the order of ten percent, if the body behind the propeller is smoothly curved.

About 5 years ago I ran some tests measuring static thrust of a 6" prop on an electric motor, behind a 4.5" cowling. With just the motor on the mount thrust was 240g, after adding a tachometer beside the motor thrust was 225 g, and adding the 4.5" cowling reduced thrust to 180 g., a loss of 25% compared to no fuselage at all. The cowling area was 56% of the prop disc area.
Alasdair

Your tests offer significantly different results - of course we don't know the pitch/diameter ratio or rough cowling / fuselage shape from #2.

I've got similar issues with a 1/12 scale FW190 I'm building, and hope others will post as well.

EG

Another factor is the distance between the prop and the front edge of the cowl. I don't have any actual numbers to quote but it is better to move the prop away from the cowl.

John W.

Really, providing the front of the cowl is closed (dummy engine, baffle etc) then there will be insignificant practical reduction in overall aircraft performance. Leaving a big hole up front is like a big flat plate, drag will be high. So long as the prop is far enough forward of the cowl (1"+) then there is no interference with the production of thrust.
Evan, W.B.#12

one inch on 16" prop isn't a lot

the radials used on the old WW2 planes were a compromise -just like airfoils - it is all a compromise and they used huge props to make it all work
on our little IC engines, the big props simply are too big to allow the engine to run in the power band
latr on - the benifit of moving the props as far forward the cowl as practical - and blending the spinner to the cowl - improved things
Personally I try to get the prop as far from the cowl as possible -on any model --

Little Kyosho Gee Bee with a OS 46. Prop is an APC 10-8.. Plane "almost" taxies fast. We managed to get it airborn once and it wasn't pretty. The owner lost interest in this one. No thrust to speak of. Might try a 12-4 or something some day[:'(] Buzz

I have a 10 lb SBC4 bipe on which the 13" 3 blade MAS sticks out 1 3/4" beyond the cowl. Flies fine with a 91 FS. Hope this helps.

A 10" diameter really doesn't give a 46 much to work with, no matter what the pitch. Mine seem to prefer 11" and 12" diameter. Almost any pitch 11" and they seem fairly happy. The OS46s work decently with 12" props. But those 12"ers can't have a lot of pitch. I've found that a 12x6 is maybe the limit for pitch. What you want with that Gee Bee is more diameter than that 10". And your OS will turn more diameter with ease.

I've found that my 46s did ok with 12x6 but were sounding a bit overpropped in the air. So I trimmed a couple of 12x6s to 11.5" diameters, rebalanced and tried those out. Bingo.

Hello, I too wonderd about this a while back. Someone then enlightened me by explaining that the propeller is not moving the aircraft by the blowing of air back, but it is more drilling its way into the air. So, the fact that you may only have 2 in of a 10 ft dia prop..(an exageration) extending beyond the cowl, the prop is still screwing its way into the air and pulling the aircraft with it.
Dan

The above is true....


Ronnie-The Toolman

Most of the thrust is produced by the outer third of the prop blades. Those WW2 radials had reduction gears between the prop and the engine to enable a larger prop to turn at a more efficient RPM.

Scale Freak and Flypaper are both right.

A prop is just like a wing - it works by increasing pressure behind the blades
so when the difference is great the plane accelerates when it is low it cruises .
a big cowl -up close --screws up pressure differences --
the air can't move AFT between the blades and the cowl - It stirs around in a circle and flows outward to join the flow around the cowl. That is inefficient.
If you don't mind a simple test (no wind tunnel needed )
set up a table fan - then move a disc (a plate will do) -up close behind the fan -into the air flow.-
what happens to the fan speed ?
does it speed up or slow down?
why?

dick Hanson,
Actually the correct way of describing it is the propeller is that it uses the pressure differences to move forward, not the increased pressure behind it. Exactly like you stated, same principle as of a wing. A wing is having a lower pressure on the top swich sucks it up....
Ofcourse it is the pressure differance that is making the airfoil to work. Messing up the air behind the prop will affect its performance.
I am a professional pilot flying full-scale.

FWIW I did measurements of air pressure in the props slipstream to see if I could learn anything about thrust, cowling, and possible drag. I used an 049 engine mounted on a test stand.

I found that the areas of greatest pressure were almost exactly in the center of the prop's semi-span and extended behind the prop disk about the length of the prop raduis. Pressures fell off sharply to ambient pressure outside this area.

I was surprised to find that I saw negative pressure readings directly behind the prop tips and near the hub. Pressures in front of the prop showed only slight negative readings generally equal along the blade length and then only in very close proximity to the prop.

If air pressure was related to thrust, it seemed that the area generating the greatest amount of thrust was about half way along the blade length. To me that seemed the area with the greatest need for concern when designing a cowl.