Thought a few of you might get a surprise from this. I know I did. [X(]

Bench ran my C/L O.S. MAX 20 again yesterday. Different weather, so initially same fuel, same prop to establish any variation from the previous datum.

Datum result was the same as Sunday, Bolly Clubman 8.5 x 4 turned 13,300 RPM peak.

Swapped props fitting an new condition but old square tipped black Master Airscrew 9 x 4 I had to hand. Mk I Eyeball of the respective pitches revealed they looked pretty similar, but of course, the blade shapes are radically different with the Master Airscrew also much wider at the tip. Fired her up. Same fuel, but several clicks richer needle setting was required to tune a peak with the Master 9 x 4 of 11,200 RPM, a loss of a whopping 2100 RPM! Even I was incredulous at the difference ½" diameter, but probably more importantly, the radically different blade shape made!

I'll have an APC 9 x 4 and a Bolly Clubman 9.5 x 4 (assuming they make the size) soon, so the further comparison will be interesting. I'm willing to hedge a bet the larger 9.5" diameter Bolly will actually turn faster than the 9" Master. I confess I was expecting anything up to 1000 RPM drop, but over 2000 RPM for ½" of diameter and larger surface area?!!!! I had heard all the disparaging Master Aiscrew "paint stirrer" remarks and previously put them down to just colourful hyperbole. Admittedly I haven't used anything Master Airscrew myself in an age in R/C, but this empirical evidence which can't logically be denied really did shock me. [sm=spinnyeyes.gif]

Wish I had a thrust gauge now out of sheer curiosity. [:@]

I was flying a Fox 35 with an APC 10-6 once and broke the prop and had to use a 10-6 Master airscrew. I wasn't using a tach but I noticed an immediate decrease in performance with the Master Airscrew.

Quite. - it may be that the Master is actually trying to move more air, hence is working harder, hence the lower RPM ?

I often wanted to mess with a fish weighing scale, attached to the rear of a wheeled (dolly if ness) model, on a flat surface, might give comparitive type thrust measurements ?

My ST G20/15D performed at it's best in the air with a wide blade Master Airscrew 7X6.
The later M.A. 7X6 gave more revs but less airspeed, (more like using a 7X6 APC)

My thoughts exactly. I think somebody makes a strain gage type of test stand ...


Phil

I did some in the air prop testing of several props one day. I found that props I expected to be similar gave very different performance and that props which I though quite different gave similar performances. One of them was a Zinger 11 x 5 wide 'prop kit' which I had made into a beautiful undercambered prop. Very nice. Then I made a mistake.[] I put on an unmodified Zinger 11 x 5 wide prop kit, and it performed as well as my hand-crafted beauty. [&o]

Anyway, I'd suggest start out with an APC you think would work and go from there. No telling where you might end up.

There is so much more to prop design than just what RPM it will do on a bench. The only way to evaluate it properly, as Jim said, is to try it under the conditions you are trying to achieve.

Take for example, CL stunt. One prop will give better accelleration out of corners than another. One will cause less wobble during maneuvers, etc. AND...what works on one plane/engine may not on another. Some flyers are so knowledgeable that they can tell when and how to change props in different weather conditions...unfortunately, that is still outside my skill range. []

George

Sigrun,

To expand a bit on a thought mentioned in this thread...

Static testing of propellors is an artificial condition. We generally assume the prop stalled when the model is not moving throgh the air. How quickly the airstream (i.e., model) velocity rises to a level where the prop 'flies cleanly' again is not easily determined. Perhaps a rig, such as Al Rabe used to check some of his unusual airfoils (Sea Fury article, 1970's) could allow some credible testing.

Al built a trapeze type of thing above his auto, mounted a wing section (or the model, my recall isn't perfect) and drove at approximately the model's intended flying speed. Ingenious, and ingenious methods of varying conditions for test.

Something similar, with merely a relatively clean test stand, suspended in it on links to thrust measuring scales, could be informative. A well calibrated speedometer in the vehicle would help, too.

You would benefit from cooperative support, or at least tolerance, from local traffic authorities. ...and from a dedicated friend to operate and record the measuring devices...

Worth the effort? I've seen considerable differences in most common propellors, even of same make and specifications, that such testing could become a new lifelong hobby, leaving little time to apply the findings to model flight... I fly too little to be in the Bolly, Eather, etc., prop clientele - they may be much more alike from prop to prop...