what is bec? is it necessary to hv bec in our  electric model planes?

Ash.. B E C, stands for battery Eliminating Circuit.. yes its a must.. because, 99% of our hobby radio receiver sets are designed to work on 6 Volts Dc maximum.. and yes the additional servos also.. and for a maximum we can feed it to a 7.2 volts, but will be inviting disaster.. and yes have some extra ordinary sets, that can be operate in 12 volts DC.. and yes with appropriate servos.. but again 99% of our hobby radio sets are designed to cope with 6 Volts DC maximum.. and feasible range is about 4.8 to 5 volts DC.. that is why, it is an essntial to feed the RX with an appropriate voltage.. all we know, our battery packs starts from 7.4 volts DC.. so, if you are connecting a RX to the 7.4 volts directly, the RX will be under extreme stress of high voltage to it..

and it goes extreme when we connect a 11.1 or 12 volts or more voltage battery packs.. if so, the RX will destroyed in seconds.. secondly, if you feed the RX with same battery packs intended to drive the motor, affects the voltage to the RX while in high loads, with motor, and also a chance of disturbance with SAWs.. so, in a B E C circuit, we reduces the high battery voltage, to a feasible level for the RX.. is about 4.8 to 5 volts.. so, if you are using a 12 volts battery pack, the B E C reduces the voltage to 4.8 or 5 volts.. and can see a 6 volts in some heavy duty B E C s.. B E C s are coming in two forms .. some may built in ESC itself.. and some as external.. and some are built around simple electronic devices like low drop 7805 regulators, up maximum capacity of 2 to 3 Amps, while some may in to Switched mode, is converting the battery voltage to a high frequency current, then rectified back to DC, then regulated to 5 or 6 volts.. these Switched B E C s are more efficient than that of simple B E Cs..

I always thought BEC stood for baked electronic circuit  

all we know, an airplane model must be kept light as possible by weight.. so, each and every components used to make an airplane model must be lighter as possible.. that is why, manufactures adds the BEC circuit with the ESC it self, to save space and weight.. so, we can find most of the conventional aero modelling ESCs are equipped with a BEC as built in.. but when we adds some heavy duty/ extra heavy duty/ standard servos with the simple 2 or 3 Amps BEC, the circuit starts to heat up, and loses its capabilty to deliver needed current for the servos in a need.. so, while using heavy duty stuffs with BEC, we must need a heavy duty BEC with ESCs.. in these cases we can use an additional BECs known as switched BECs.. some of the heavy duty ESCs are coming with a built in switched BECs are known as U B E Cs...

usually, a microservo drops about 120/160/250 mA/H and may vary in loads.. but when a 4 micro servos goes under extreme load, can calculate the droping current.. will be high.. so, a simple RC plane with some micro servos must not be a problem for its own BECs.. but when this connected to 4 standard servos.. things come matter.. a standard servo can drop about 600 to 700 ma at high loads in some situations.. so the BEC will be in full stress and may be far beyod its capacity to hold.. then it may fails.. to avoid this we can add a simple switched mode or extra BECs to our crafts.. by keeping the lightness, manufatures are making UBECs in very light weigh and size.. so a BEC is the Battery Eliminator Circuit.. that regulates a high battery voltage to 4.8 or 5 /6 Volts DC maximum..

     
do this esc hv a built-in bec    http://www.hobbyking.com/hobbyking/store/__11615__Turnigy_AE_20A_Brushless_ESC.html

Ash.. to determine that the ESC is equipped with BEC, you need not to look in to specifications.. instead you can see a three core flat cable, with a flat connector, one appropriate to the slot of the receiver.. the colour code of this flat cable will be Black ( battery -), Red, (Battery + 5 Volts), and White, ( signal), and in some others as Brown (battery-), Red (Battery +), and Yellow (signal).. so, if your ESC is equipped with this flat cable, its sure the ESC is with the BEC built in.. the ESC in above link is equipped with BEC, as you can read as linear 2Amps @5 Volts.. its a nice one.. for lighter applications.. 

k thanxxs sir got  it 

nothing is amazing.. nothing is extra ordinary.. its all simple and esay to understand..