About controls.

it has 'Force', 'Deflection' and 'Harmony'. what we are interested is in the force/unit deflection. The force that servo will feel will be force exerted on the control surface. this can be calculated by applying simple formula 1/2 x 1.225 (Air density at sea level) x square of max speed that you will envisage that your model will travel (Keep the units same) x the surface area. this will give the max force that your servo will feel from an unbalanced surface (??!! ) add 1/2 kg for friction and bends, your servo should have twice the the torgue needed. why twice ? reason 1. in its life time the servo will continue to loose its torgue. reason 2. after 75% of its rated torgue, servo can experience something akin to 'Jack Stall'.

Theoretically it has nothing to do with weight. in fact in a non-powered controlled aircraft, there is a protrusion ahead of the hinge line (Refer image) this is called the horn balance (Not the control horn we know of), purpose of this horn balance is to reduce the control force. in an RC aircraft if this horn is long enough even a micro servo would do (In an aircraft called Devon, sometimes the balancing force was more than the opposing force and once you moved the stick the control surface used to travel to it max deflection of his own accord, this is called overbalance). if you are designing an aeroplane and want to knock weight and money, carefully design long enough horn. (on the lighter side, Make it more Horney).

Mathematically.

Oops ! i was dreading this, let me try and keep it as non mathematical as i can.  for example if you have a model which has a surface area of say 1/2 m²(S)  and it travels at say 60 kmph and if the aerofoil's efficiency at max deflection is say 0.2 (Most symmetrical aerofoil has this value), now let us see how do we arrive at a number. 1/2 ρ v²  is the dynamic pressure. Ct 1/2 ρ v² S total force it can generate. This is what we need to see, and is the total force on the control surface. Let us see the number in SI (Système international ). (Thats what i meant by keep the units same).

0.2 ( 1, being a flat plate at 90 degs to the air flow [Aerofoil efficiency]) x 1/2 x 1.225 (ρ air density at Sea level) x 16.66² (60 kmph converted to mtrs/sec) x 1/2(Surface Area) = 17 kgs.

this is for an unbalanced (Without the aerodynamic horn balance) controls, if you aerodynamically balance it, this can be reduced to 3-7 kgs.




Did I make matters worse ?