Bob.R

My Feedback: (2)

Can any of the professional or amateur aerodynamicists on this forum guide me to a technical source that addresses the interaction of spinner, prop, and cowl/fuselage. I've searched this forum, and I've reviewed my library, but I have not found this subject specifically discussed. I am designing/building a high speed aerobatic plane - structural (engine mounting) and aesthetic issues are driving the design towards a fairly large spinner diameter, specifically 2 3/4". First iteration prop choice to match airframe and engine characteristics is a 10x10. The cowl/fuse smoothly transitions to an elliptic cross section approximately 3.5" x 5" at the widest point, which is about 15" behind the spinner plate. As a percentage of prop diameter, this spinner size is somewhat larger than I have used in past designs, and I am out of my TLAR zone. There is obviously a percentage of prop diameter where the spinner will begin to significantly affect prop efficiency, but where that transition occurs is not intuitively obvious to me. Recommendations for technical sources or discussion of personal experience are much appreciated.

taylorcraft1947

The author of "The Design of the Aeroplane" recommends that the diameter of a spinner should be close to 25% of the propeller diameter. This is because the centers of most propellers are very inefficient. They spin at much lower speeds with less efficient airfoils than the rest of the propeller: therefore, they contribute little to thrust. So, it sounds like your spinner is about right.

taylorcraft1947

acropilot_ty

Big spinners are Good when considering the aerodynamics of the prop, and the aerodynamics of the cowl/fuselage, but they make cooling difficult. If you look at modern full size planes with large spinners they either have cooling inlets that are very far out to the sides like a lancair... or they have an anular inlet around the bottom half of the spinner like Sean Tuckers Pitts, or the new aerobatic airplane called the Chanute.
Ty

rmh

If'n you are building a model airplane to fly - not just looking for wind tunnel guestimates -- be forwarned.
The airflow just behind the prop and whre the cowlis close,is a real problem
You have either a stagnant flow OR the flow is winding and air into the cowl will cool better on one side then the other .
How do I know?
I measured the heads and plugs with a pyrometer , then cut holes in the cowl to change airflow .
Our models engines do not have the benefit of good ductwork to force air thru the fins -so overheating a twin is a reall issue - even a single needs good flow into and if possible forced to flow thru the fins.
On 100-200 CC gasoline engines this can spell life or death of the engine.
The size you are working with suggests you are using a small glow engine of perhaps .40/.60 cu in.
Look at control line speed models or FAI RC speed models and get some feedback on "pressure cowls ".
done right - you can make a very small inlet with an extractor outlet and on the very rich oil/alky mix not cook the engine
Get er dun!