If there is any one question which I continually attempt to answer concerning the Quadra Engines, it has to be on proper RPM and Propeller sizes. I don't know whether we were brain-washed into selecting propeller sizes based on engine displacement, or RPM, or if it just happened but, in any event, I would guess that 80 percent of the modelers using any particular sized engines, all use the same size propellers even if they use the engines to power dozens of different sized models and types.

I have reached a point now that instead of answering the question concerning propeller sizes, I instead ask: "What model are you going to fly, and how much does it weigh?" Having this information, I generally can advise a few close propeller sizes suitable for the Quadra Engine as well as the model.

The simple point here is, there is a RANGE of propellers that a Quadra (or any other brand), can turn effectively and efficiently. The basic range lies in sizes of 16x8 thru 20x8 for the Quadra. And, the base-line prop would have to be an 18x8. I do find it rather difficult to have the modeler accept the fact that it is going to take a bit of experimentation to select exactly the right prop for any particular model. Various brands also enter into the selection process. For an example, just look at the various thrust-output articles which show the static-thrust of brand "X" versus brand "Y" when both pitch and diameter are equal. Look at the RPM variations, these could be several hundred RPM apart also. In most cases the reader goes out and obtains whatever brand/size propeller shown with the highest thrust rating, figuring that it is this particular brand/size propeller that will do the best possible job of powering his model. Let me state right here and now, that this is generally the VERY WORST method of prop selection one could make!

Static thrust readings in my own estimation are totally useless in making the proper selection. Only you and your experimentation will lead towards the best diameter, pitch and brand selection. I know that on a whole, the Quadra engines when powering a "Big" or "Ugly-Stick" performs very well with a 16x10 Zinger Brand Prop. Now note I did say, "Generally". On a slightly heavier-side "Big Stick", the Top Flight brand does a bit better. "Cap-20's" Quadra- powered, if not too heavy, also do well on the 16x10's. Average weight and drag Bi-Planes do well using the 18x8 sizes. The Byron "Pitts" does well on a 20x8 cut down to 19 inches and a bit of thinning down at the tip trailing edges. Even a bit of "wash-out" helps. The "Starduster Too", in most cases, dies well on a 18x8, but can be improved with a reworked 19x8. Of all the propellers within the range I have listed, about the only prop I stay away from is the 18x6 size. I have yet to find a model of mine which requires this size. Here then is a basic approach towards making a prop selection which I find fairly safe. If the model is Quadra powered, you can not be too far off making those first flights using the standard or base-line; 18x8 prop. Get the flight-time bugs worked out, or at least to the point that you'll know what the model will or will not do safely. Next, don't make any size changes but rather make a "brand name" change. Sometimes this can make a big difference in flight-characteristics. No change noted? Move to a 20x6 size, or even a 20x8. You bet! These little engines pack a lot of torque. As far as I am concerned, you need never spend one "thin dime" on a tachometer either. Static RPM is just that "Static". When airborne the engine/prop are in a dynamic state, and that's a whole new realm. It is the only thing that really counts. Forget the torque and brake horsepower curves. In the final analysis it will be the size, type and weight, etc., or the model itself which will lead towards the right sized propeller. Altitude, or density altitude, figure in the selection process also. I live at 600 feet above sea level so I don't have the problems associated with say: 6,000 feet ASL.

So until we get into variable-pitch or even constant-speed propellers, I'm afraid it will have to be the "ole" try, change, cut and alter method of prop selection. I'll close just saying that I have at least 30 to 40 different props ranging from 16x8 thru 24x10, with almost everything in between the standards, that I play around with and I just might spend two or three whole weekends of experimentation with a new model, looking for that ONE just right combination. It does make a very big difference between having a just "so-so" flyer and a true "Performer". Own a good tachometer? Well, sell it or trade it in on a good selection of propellers instead.