A beginner asked me via PM, why any airplane flyer would ever need more than 4 channels ever ?

After all, all a plane needs are throttle, aileron, elevator and rudder !

Let us see who can explain this well 

Some times need more than 4 channel for steerable nose wheel and Landing lights, Bomb Dropping for fighter jet models, Cameras

Steerable nose wheels are normally linked to the rudder servo and do not need an extra channel. Commonly used additional channels are for flaps and retracts.  Others could be smoke pump control, glider release, etc.

Actually what I was looking for was the clear distinction that each channel can be used to control a particular / single servo, and one set of controls for an airplane (especially aileron) can involve two servos. 

By having each half / side of the aileron controlled separately opens up additional options in flying, like the use of "flaperons". Of course normally a Y cable can be used to control both aileron servos driven from a single channel, but using two separate channels opens up more fun vistas  

This is in addition to other uses like flaps, retracts, lights, smoke, bombs, and so on, which have been mentioned earlier.

okay on large model sometimes two servos are used for elevator control, air brake requires extra servo or models with 3 engines require extra channels, 4D flying needs extra servo to control the pitch of prop ! etc

The basic functions are

1) Throttle
2) Elevator
3) Rudder
4) Ailerons
5) Flaps
6) Aux1 - to be used for some extra functionality


Now in some cases (at least in Spektrum Dx6i) if we want to use the Ailerons as Flaps, then instead on one servo two are required - 1 in the aileron and 2 in the Aux1 control. The control is enabled via programming. However, in this case I am not sure what happens to the empty Flap socket - whether it can be used at all

If we have a single control channel for aileron and separate for Flaps - that is if the wing has both these control surfaces, then the Aux1 can be used for something like retracting of undercarriage

The steering of nose control can be achieved with dual servos linked to the rudder channel.

Pankaj

The other Idea I had in my mind was to use gyros on a plane for fun, you can actually control the gain of those gyros using a spare channel proportionally. Also if you WISH to, you can actually use gyros for helping in landing fast planes or even use them to learn hovering planes . If things go well, my first scratch built plane(yet to build), will have a gyro on it.

Using separate channels and servos for elevators can give you the additional function of the elevators acting as ailerons, in tandem with the ailerons proper.

So, if you had 2 + 2  servos for aileron and elevator, you could have :

1. Differential aileron throw
2. Flaperons
3. Ailevator (as described above)

These are for airplanes. There would be other possibilities for helis.

Another advantage of using 2 separate channels for elevators is the ability to reverse either channel to get the same direction of travel. This is useful when the servos are mounted on the fuselage side at the rear. "normal" operation via  a "Y" connector would not work unless the servos are "displaced" mechanically.

Folks,
In a rc plane, the number of basic controls are 1) throttle 2) elevator 3) ailerons 4)rudder 5)flaps
beyond this, some folks use a gear switch for retracts.

Now the question is, do we require more channels just for 'good to have features' or in some case it is necessary? For example, personally I would not consider 'dropping a bomb' a mandatory feature.

In case there are cases where one needs the extra channel, could someone elaborate?

Useful in cases like this (butterfly mixing etc) :

http://www.rcindia.org/electric-planes/setting-up-a-glider-with-dual-aileron-servos-and-dual-flap-servos/

hmm butterfly brakes. That would be for landing right? Or would be required for controlling the flight pattern?

Well another question here would be that since the glider in question did not have provision for retracts, we could use the gear switch - well atleast an 'air brake' concept can be worked out if the mixing is not possible in the radio.