It tells you that different companies specify and produce their propellers different, with different interpretations of the word "pitch" (although "diameter" is unambiguous enough that I wouldn't expect much variation).

Now if you try a dozen "identical" props from those same three manufacturers, you'll find out about manufacturing variation as well.

Diameter and pitch only tell part of the story.  The area of each blade, the variation in the width of the blade over the diameter, the tip shape, and the distribution of the blade angle all affect the propeller performance.  I wouldn't be surprised if the surface finish didn't play an important part, too, at least for some props.  Add to that the fact that some props will bend and unload (like your Kavan), and you have lots of variables that are all hidden behind those two innocent-looking numbers.

Top Flight used to sell "regular" wood props and "power" wood props which were basically the same only with a narrower blade -- and they performed differently.

Now forget to balance the props and suddenly you've got another whole set of variables (you did balance them, didn't you?).  A poorly balanced prop will sap power from the engine, generally make your whole setup act strange (except, possibly, at startup), and shorten the life of airframe and engine.

Here's one other data-point that may influence the performance you are seeing:
Graupner 9x4   0.9oz
Kavan 9x4      0.55oz

I don't have stats for an APC, when I checked several of them in my LHS the hole was visibly offset.
I imagine the APC is lighter than the Kavan, both the APC and Graupner are fairly stiff, so the figures are not surprising. The Graupner has a higher moment of inertia (flywheel effect) and I personally find it makes manual starting much easier.



 

Thanks guys for the helpful replies.. I will of coarse try different props on the plane (ringmaster) when I've actually built it! That will be a winter project. And err..well no,.. I ermm didn't balance any of the props, Tim! And I know I should but I'm a lazy sillybilly.. Any ideas on the other point about the engine revving up and down before finally going? Many Thanks for the input..
Phil

Damian, the motor has had 1hrs run time as per the instructions, Its fitted with the r/c carb and 4oz clunk tank on bench stand. Last year it ran fine so have checked fuel lines for leaks everything looks ok, I do choke and flip the prop untill I see fuel going into the carb then flip 2-3 more without finger choke, attatch glo clip and flick to start. Just thought, I used to fill the tank for running in, but have only been putting in about 2oz of fuel or less while testing various props! Could this be the problem?... I'll attempt to post a photo of my benchrun setup although this is of a ASP15s blue head I was running in over the winter (as you can see!) Thanks for any advice
Phil

Hi Phil,

I concur with Paul. It sounds like the engine is gasping for fuel.

Just thought, I used to fill the tank for running in, but have only been putting in about 2oz of fuel or less while testing various props! Could this be the problem?...

It's not the problem per se, but it's consistent with the thought that the engine was struggling to get more fuel. If the tank were full it would provide a little more pressure head to get the flow into the carb. Instead the engine has to suck on the fuel a bit harder to get established. I am assuming you didn't change the needle setting since your full-tank run. Responding by temporarily opening the needle a little until it stabilized would have helped also, but I guess you didn't want to touch the needle setting in order to conduct your experiment.

OK I tried that with a full tank and its no different. However, I closed the carb barrel and I mean closed (couldn't see any hole/gap down the venturi) and it started straight away!! When I opened the barrel again the motor died. Did this severel times so I'm guessing the r/c carb low end needle needs to be re-adjusted. Someone please tell me if I'm wrong...
Thanks Phil

Phil,

I still believe the engine is not getting enough fuel and the way to rectify that is to open the main needle (CCW) a few more ticks and try again. When you close the throttle it acts like a choke; the engine is devoting more suction to fuel than to air. When you open the throttle the engine is getting way more oxygen and fuel is burning efficiently, it speeds up but the fuel isn't meeting the demand.


Are you comfortable/experienced adjusting the needle while the engine is running? Do you have the needle fixed at the current setting?


Let us know how it goes.

Damian

  Eureka!!  I found the problem (with a bit of help from the replies!) The fuel inlet nipple was cracked around the thread. I replaced the r/c carb with a control line venturi and it starts first flip every time. I'm so chuffed, its been bugging me for 2 weeks now. Thanks again for the advice..
Phil

True enough Tim, but I was put on the right track about an air leak, which made me take a closer look at all possible areas of a problem. I was prompted in the right direction!
I'll go with the Graupner, not as sharp as the APC.. Phil