there are various sizes for props. there are 3 bladed props (rare) and 2 bladed props. they come in various sizes. What difference can a different sized prop create? and what about 3 bladed props? how come no1 uses them???

what size do u reccomend for a 40fx?

Without knowing what kind of airplane you are putting the 40fx in I would suggest either a 10-6 or 10-8. You have asked a pretty general question that could take pages to answer completely, but I’ll try to limit my response to something practical.

A propeller is a device to convert power into thrust. It is sort of like a screw that screws it’s way through the air (it is sometimes even refer to it as an “airscrew”). The common specifications include diameter and pitch. For instance a 10-6 propeller has a diameter of ten inches and a pitch of six inches. Pitch is the distance the propeller would travel if air were solid, and there was no slip. In the actual world air is not solid and there is a good bit of slip, however pitch does relate well to the speed at which the propeller operates most efficiently. A fast airplane should be equipped with a relatively high pitch propeller whereas a slower airplane flies better with a lower pitched prop.
For instance if your 40fx were being installed in a trainer, the 10-6 might be appropriate, whereas if it were installed in a pylon racer, the 10-8 would be better. If it were going in a 3D fun fly where hovering was a consideration, an 11-4 or even a 12-4 might be the choice.

The little engines will operate satisfactorily with a fairly wide range of propellers. To choose the best propeller, pick a pitch that relates to the speed, then pick a diameter that will enable the engine to develop maximum power. The choice is not particularly critical. Unless you are heavy into competition and must extract the last drop of efficiency, any prop that the engine will turn will fly the plane after a fashion. Make a best guess based on experience (or recommendations) then try several different ones in that range to see which suits your flying better.

The different manufacturers produce different blade shapes and forms and use different material. Some folk swear by APC others prefer Master Airscrew, or Top Flite, or some other. It is largely a matter of personal preference. As long as the pitch and diameter are close to correct, any of the commercially distributed propellers should work OK.

Propellers with three and four (and on some turbines even six) bladed propellers were developed for full scale airplanes. These were mainly to enable the propeller to absorb additional power within some diameter constraint (usually ground clearance). The multi-blade propellers are not as efficient as two bladed ones, but the differences are small and other considerations usually drive the design. Manufacturers of propellers for models provide three bladed ones primarily for scale models, although they are sometimes used on other types (mostly because they look so cool). When going from a two bladed propeller to a three bladed one, on a model, keep the pitch the same and go with a smaller diameter.

10X8 is a heavy load for any .40.... 10X7 is normal for a .46.

For trainers I recommend 11X4 or 11X5 on a .40. The larger dia and lower pitch helps you have the pull at low airspeed to get out of trouble. The low pitch also helps in slowing down for landing

11 X 4 is ideal for a 4*40... they stick on the ground instead of floatng on past the runway with the engine idleing which is a common complaint with a 10X6. Vertical climb is improved with the low pitch prop too. A .46 0n the 4*40 and the APC 12.25X3.75... unlimited vertical. .40 and 11X4... infinite inside-outside vertical 8's (not quite unlimited vertical climb, but close)

3 blade props at high rpm are very inefficient.. the blade are flying in each other's turbulence unles the plane is moving along fast... problem is, you'll have trouble building up the speed. Glow engines just tend to operate at too high an rpm for 3 blade props to work well. (they look cool, and people mostly use them for a more scale appearance)

If you ever try a ducted fan... remember that you have to build up some airspeed to get decent thrust out of the thing. full throttle and nose high at stall speed, you'll NEVER build speed. the fan blades will be stalled (props are just wings moving in circles) gain a bit more airspeed before lift-off and the thing'll fly like a different airplane. Same idea as the 3 blade prop... just a more dramatic difference.