Thread: I have a few questions for you folks into RC Helicopters

No, this was a wing shaped airplane. But if it works for thrust it will work for lift?

Originally Posted by jcdillin

do you want to have it fly like a plane or just act like a helicopter?

Eventualy both

You can buy ducted fan kits, which state the thrust amount for each unit. But you would need some big bastard ducts to lift a plane which is carrying a PC (based on a *light* 1kg laptop!)

My turn:

If you simply add another rotor on top of an existing one you won't get an increase in overall lift as thrust is dependant on power. To increase the thrust of a rotor you change the angle of attack (incidence) and this angle is limited by blade stall but also to keep the rotor angular velocity high you need a power to overcome the drag of the high incidence.

So if you add a second rotor you simply cannot use each blade as much as it can be and you will also have more balancing work.

The dual rotor systems you see used (both in that vertical takeoff plane) and the Russian helicopter (see picture) with no tail rotor are counter-rotating as this cancels out the torque you get from using a single rotor so you don't need to have a tail rotor like a conventional helicopter. In this setup you need both rotors to have the same pitch to you get the same thrust and torque.

This design makes it very compact as there is no dangerous tail rotor to hit things but is not as efficent per rotor as a single or side by side set up as the lower blades are int he wake of the upper blades making them slightly less efficient. The Chinnook system (fore and aft) also cancels out any torque so it doesnt need a tail rotor and also has the advantage of being able to accomodate a large change in the centre of mass along it's length which is why it is so useful but it cannot get certified for civil use because of the reliance of the gearbox inbetween the two rotors that stops them hitting each other which would destroy the aircraft.

Going back to a single rotor system, if you fit a more powerful engine you could add a second set of blades but you would have to couple the control links etc and it is also likely you would have to increase the power of your tail rotor to maintain good control due to the increased thrust and therefore torque of the main rotor. But don'e expect 2x the power from doubling the blades but do expect a lot more vibration and less reliability.