RC components - Propeller

I wanted to write a little bit about some of the interesting components of the RC hobby. A lot of the people are actually not so technical as to understand the actual functioning mechanism of those components. I will try  to explain them in a simple way and also quote some examples or analogies so as to make our mind to think it in a simple manner that we could relate, visualize and understand it better, though it might not be technically correct to be so.

We always have a broken propeller resulting from RC plane crashing nose down. Even for a light crash, normally the propeller would break into pieces or bend the shaft of the motor. Therefore it is advisable to have a "prop saver", which is actually a device that one attaches it near to the tip of the propeller shaft and ties the propeller onto it with a rubber band. The propeller shaft is cut and shortened, protruding less to the front so that the impact of the fall would not be transmitted  in full force to bend the shaft, instead the propeller will break loose when the rubber band gives way. It is simple as that!

We are always confused with the various sizes of propellers, and their sizes are denoted by two numbers. E.g.  7x3.5, 8x4.3 etc. The first digit signifies that diameter of the propeller while the second number is the pitch, in imperial unit of inches. Now comes the interesting analogy, imagine that the propeller is a screw, while the air is now the piece of wood. The bigger the diameter of the propeller then you will drill a bigger hole into the air to make way for the plane to pass through. The plane will fly more stable in a bigger tunnel, and more volume of air has to be cut out in order to make that hole bigger. Because of the bigger hole in the sky then the plane will fly at higher speed IF the torque of the motor could handle and displace the bigger volume of air.  Please note the "IF" here because we are talking about a more than enough powerful motor and a bigger diameter propeller will fly a plane faster than a smaller propeller of the SAME pitch.

Look at it in a similar way that if a screw has a fine thread, then the effort to screw it into the wood would be less compared to one with a bigger or wider thread. The pitch is defined as distance traveled per revolution which  is quite similar to the "thread pitch" of the screw. A bigger pitch of thread will require a higher torque from the motor or a screw driver to turn it against the piece of wood because each turn of it will require it to go comparatively deeper into the wood. In the case of RC plane now the air is giving the resistance to the propeller to move forward!  What I am trying to drive at here is that when you break a propeller and could not find the exact size to replace it,  as a rule of thumb, you could find one which is one size bigger in diameter but one size smaller in pitch. In a similar manner if you choose a smaller diameter, you could opt to have a one size bigger pitch propeller. In this way, the motor is not strained as per the original design intended. If we choose our replacing propeller this way, the volume of air cut out roughly remains the same. As the rated output power of a particular motor is a constant, you could not get high speed and high torque at the same time by attempting to play with the selection of various sizes of propellers . So if you want high torque then you have to sacrifice the speed as far as the motor is concerned, and vice versa. Remember the formula of  the power of a motor is equal to Torque x RPM!

Generally speaking when the plane takes off, higher torque is more desirable to climb the plane upwards against the gravity; once it is up the sky, less torque is required and higher speed of horizontal flying could be achieved. Most of the time the combo set we purchase will come with a cheap brushless motor and you will not get both high speed and torque at the same time. If you want both speed and torque, you probably have to upgrade and change to a higher power output motor, this is similar to how we change to bigger cc engines to our cars! We might have to imagine that the MAXIMUM power of a motor is equivalent to the capability of cutting out certain volume of air to allow a plane to fly through the hole, at maximum power output (full throttle), then a smaller diameter propeller should cut a deeper distance into the air and therefore travel faster. Please note it here that the variables to consider are: power of the motor, the diameter and the pitch. To understand them, one has to keep one or two of the variables constant in order to visualize the effect of varying the sizes of the others otherwise we could get very confused over what had been written here.

There is always a maximum size of the propeller that a plane could physically take on. This is because if the propeller is too big, it could hit the ground during take off or landing. While for some pusher prop planes, the propeller is mounted facing the back of the plane, the fuselage, the wing or the space will limit the size that could be mounted. In this case, a higher speed or KVA motor has to be used. For example, with a front mounted propeller RC kite, a 1000 KVA motor with bigger propeller would do the job, while a glider with a back mounted propeller, it needs 2200 KVA together with a 5 x 5 small size propeller.

Please note that KVA rating of a motor denotes the rpm per volt of battery applied to it. If you use a 2S Lipo battery, the voltage is 7.4V while using a 3S Lipo battery the voltage will become 11.1V. You will get a higher rpm for the same motor if a 3S battery is being used instead of a 2S. However one has to bear in mind that with the increase of voltage, we must ensure that the ESC and the motor ampere ratings could take on the extra loading without causing a burn in the electric components or over heating problem. The 3S battery is heavier and bigger size than that of 2S, and for many practical reasons where the weight and CG of a RC plane are to be taken into consideration, therefore an attempt to make such an upgrade of a bigger motor or higher voltage application becomes not possible.