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

No you got it all wrong!!!!

Its not the mass thats the problem its the aerodynamic drag!

True it will be harder to spin up due to the interia of the heavier rotor system but once it is going it won't matter - infact more mass can be a good thing. Mass=inertia which is only a problem when you want to change the speed of something turning. True the helicopter will weigh more due to added blades but on a hobby helicopter this is nothing compared to the wieght of the fuel and engine.

On a real helicopter the mass of the rotor head is a big problem as it is so substantial but on models the forces are many magnitudes lower than the scaling would suggest.

The more blades you have the more drag you have. For the same lift you can simply use less collective pitch with twice as many blades which gives you less drag per blade but there is still a basic drag of each blade x the number of blades:

now for the science bit (as they say in adverts):

For a basic aerofoil section (we are doing strip theory so neglecting area and all the rotational helicopter crap)

Lift = cl0 + 1/2 x air density x alpha x cla x velocity^2

where cl0 is the zero angle lift, alpha is the angle the section relative to the direction it is moving (incidence/pitch) and cla is the lift/angle value for the section ie the amount of lift change it can produce for a 1 degree of andgle change (incidence or pitch), and velocity is in m/s and is squared so has a bit effect!

Drag = cd0 + 1/2 x air denisty x alpha x cda x velocity^2

where cd0 is the zero angle drag and cda is is the change in drag per degree of incidence change.

As you can see there is therefore a relationship between lift and drag so as you increase the lift you increase the drag.

This drag will slow your engine velocity.

If you have excess power in your engine then you will get some benefit from adding blades but you will equally get some benefit from making them bigger or setting them at a higher incidence. As blades rotate they produce much more lift at the outer edges however the outer parts are also responsible for more drag as the blade is rotating but the drag coming from lift:drag coming from profile (cd0) ratio is much less so larger diameter or faster rotational speed is better.

If you add a second rotor head like the chinook you don't need a tail rotor and a tail rotor would not provide pure rotation response in thsi situation - it would miainly provide translation with a rotation about a point well in front of the helicopter.

If you control your lift by simply making the blades spin faster then more mass will hinder this response time however if you bags of excess power and fixed collective pitch it could work. In this case it may simply be easier to increase the gearing so the blades spin faster. I have never built an RC helicopter so don't know how optimised the blades are, if the pitch is variable, if they can go faster without problems etc byt if you want more lift then you have to increase the power. A chinook has two rotors but this is due to longitudinal stability requirements of having troops moving around inside and being able to stick any load in and not worry about the thing pitching up or down uncontrollably.

The most powerful helicopter in the world is the sky crane and this has a single rotor with large blades.

Going back to your original point....

I suspect it will be very hard to add a second rotor head to the rotor shaft but it will increase the lift as long as you can maintain the rotor speed. YOu will also have a nightmate fitting it and keeping it all straight and balanced. But it is possible. You may have to then fit a more powerful engine but this will then use more fuel! If you have a helicopter see how much it can lift compared to the weight of the complete unit and then measure the weight increase of a more powerful engine and a second rotor system and see how much you will gain. Large blades are better for overall lift - you don't see helicopters with small blades but you probably have a size restiction so all you can try is increasing the blade and angle and engine power and then adding more blades.

Originally Posted by Bugbyte

You are aware that R/C helicopters are extraordinarily difficult to fly, aren't you?

The shop that used to sponsor me when i raced R/C cars used to specialise is planes and helicopters - the guys working there spent more time rebuilding their choppers than flying them! If you want something easy to control and stable build a 4 rotor rig with the rotors out on booms - like those kids toys you see. If you are good with gearing then you could actaully have them slightly overlapping ie two clockwise and two anticlockwise to enable you to get the maximum rotor diameter as small blades is normally the downfall of those things but they are very stable. Use engine speed to to control lift and then use a mass transfer system to control the direction - it wont be the fastest or most responsive craft but it will hover well and be very stable. Should be much easier to control with a PC. all you would need is a simple 2 axis servo system with some gyros etc for measuring movement.

yup. much easier to control. Needs bigger motors and blades though - i bet that thing can only just keep itself up.

Is that speeded up??? that things s awsome! Check out the hovering upside down. I would not stand anywhere near that guy doing that! Can you imagine crazy UAV military helicopters doing that kind of **** - we are all doomed come judgement day! (as in Temrinator...)