Mody, with respect to multi-blade propellers -- other than scale appearance (your Mustang, for example), the technical applications for multi-blade propellers are: to increase aero-braking (your Mustang again), reduce prop noise (many aerobatic competitions now have noise limits), or to enable the full output of an engine to be used in a situation where the diameter of the propeller is constrained by some factor & a two-blade cannot deliver the full output. However, the use of multi-blade props incurs performance penalties -- such a loss of performance is the price for meeting the other relevant criteria.

Regarding efficiency --- staying with the same manufacturer, the same material of construction, the same prop blade geometry (aspect ratio, shape, etc), a 3 blade prop for the same power load will always be less efficient that the 2 blade -- it isn't debatable -- it is simple fact & physics. There is more mass, more wetted area, more frontal area, one more root-transition zone, and for a prop of the same pitch -- less diameter. All of which simply means more parasitic power losses from drag, plus the loss of disc area in the case of smaller diameter. The engine makes the same HP (same load), but it just produces less usefull work.

However, as soon as you start mixing & matching prop performance factors, you can have some 3B props that actually "outperform" some 2B props -- but you are comparing apples to oranges in those cases & this is where much of the confusion & endless debate arrises.

The more efficient the 2B prop design, the smaller the performance differences will be between the particular manufacturer's 2B & 3B props. Conversely, the poorer the 2B performs, the greater will be the disparity in performance (poorer) of the equivalent 3B.


Guidelines for 2B - 3B conversion are simply that -- guidelines. In the prop size-range that is appropriate for your Mustang, for a particular prop manufacturer, using the same material of construction & with props of the same blade geometry (shape, aspect ratio, etc) --- the approximate equivalency is: from 2B to 3B, reduce the diameter 1" for the same pitch, or reduce the pitch 2" for the same diameter.

In the case of a your EVO engine, an 11-5 2B, 11-6 2B, or 10-7 2B is about right for general flying. All of those props will let the engine pull acceptable RPM's & generate reasonable thrust. Staying with the same prop manufacturer etc, an appropriate 3B would be a 10-5, 10-6, or 9-7. If you could find them, you could also use an 11-3 3B, or an 11-4 3B, but airspeed may begin to suffer unacceptably.

If you mix manufacturers, materials, blade shapes & aspect ratios, it becomes very difficult to use anything other than suck-it-and-see testing to determine which 3B prop is equivalent to your favorite 2B, or vice-versa. The guidelines become almost useless, other than for vague ball-park guessing.