RC India

As the plane size grows or functionality keeps increasing, one uses multiple servos to do various jobs. Say for example some of the larger builds have 2 servos for elevator, may have two servo rudder (in twin vert stab configuration) etc

So the question is how many can be attached to the RX at the same time?

In an electric setup, the BEC output is connected to the throttle/batt pin and all the current required for the servos are channeled through the thin wires of the Rx. Say I have a 10 servo requirement and if I assume that each servo would draw about 200mA of current, I am looking at 2Amp current drawn. Sp primarily the question is at what point we bring in another dedicated BEC to supply the current? Then I want, the Rx to supply only the signal and the separate BEC to supply the actual load, how will this be achieved? any diagram?

From what I looked up earlier, each servo connector/terminal is rated to around 2.5A.  A full size digital servo can cause up to 2.5A draw under load, so it is best to connect only one of them to each terminal.  Two standard full size servos can also be safely connected. 

Across the whole receiver, the safe total current capacity is typically around 6A. For the small receivers, this would be lower.

So if you have lots of servos or even a significant number of digital servos, then it is best to use a power-box.  A power box does exactly what you described, which is take the signal alone from the receiver, and the power to the servos are fed separately.

a power box is different from a stand alone BEC right?

Similar in a very basic way in the fact that they both give you a regulated output.

But the differences are much, much more. Depending on the level they can give you separate outputs for Rx and servos, dual battery redundancy, monitoring of voltage, etc.

hmm.. just checked out something called a duralite powerbox, but price is around 425$ thats huge... need a cheaper option.

Connect Signal & Ground wires of servo to Receiver. Positive & Ground with Separate Rx battery. Then the limit is Fan out of Rx signal buffer.

So, the question become two optional part.
====================================
1) Max Servo without separate Rx Battery - As Anwar Sir said, 1 recomended.

2) With separate Servo Battery (RX +ve & -ve bus may not be capable to withstand that much of current).

Many airplanes in the 50" - 70" span range use 2 servos for ailerons. They are, almost invariably, used with a "Y" connector which means they are in parallel. I have not yet heard of anyone with problems with this set up.

Purely out of curiosity - and I hope this is not misunderstood - what kind of airplanes are you involved with?

Well... I plan to build a twin motor pusher jet with the following controls

Canard, aileron, elevator, flap, slats, retracts, rudder - with canard, aileron and elevator mixed as elevon that is 6 servos, rudder 1 or 2, retracts mechanical or otherwise require an equivalent of 3 servos, flaps + slats is 2 again. So am looking at powering about 11-12 servos of 9gm HTX900 category.

If I can manage I might go in for TVC as well and in that case there would be additional 2 metal gear high torque servos.

:D :D :D wonder if this thing would fly

Hence the question was how to connect all these servos without burning up the Rx

Important question: How would you manage all these functions with a 6 channel radio?

Elevon will require 2 channels this would control elev, aileron and canard, flap & slats on 1 chanel, the retracts on 1 ch, twin motors on 1 ch and rudder on 1 ch, so a six channel would work.

In the first place I think this is looking more like a theoretical than practical excercise.

But, in any case, 9gm. servos would, typically, draw maybe 30% or less current than a standard servo. So you will be OK with the 10-12 you are/were looking at.

BTW, is the design ready? Is it your own? Concept only ?

Well I'm no aeronautical eng, so the design has to be borrowed, but primarily I shall use this as a ref about dimension and CG, then work from there, but this is still about a couple of months before I start. Have a much simpler build in the pipeline

All the best for your project, whenever you get down to it.