The problems with all these radios vary, but there are some very important things that you need to understand BEFORE YOU EVER SWITCH A TRANSMITTER ON.

I want to cover the 3 most common radios here in this article in order Esky stock radio, Spektrum DX6i and Spektrum DX7.

First of all we need need to digress a bit to explain about what throttle curves are, and what they do.

The first thing to realise is that as soon as the motor starts to turn, the main rotor blades will start to turn as well (on all electric helicopters anyway). There is no clutch system on electric helicopters. Having these spin up unexpectedly is dangerous for obvious reasons, so you do not want this to happen until you really want them to spin up (when ready to fly)

For normal flight, most throttles use a fairly linear increase from 0% up to 100%. This means that when you have the throttle stick fully down, the motor does not turn at all, but at 50% throttle stick setting, the motor will be spinning at around 50%-60% of its maximum speed.

THEREFORE, SO WILL THE MAIN ROTOR HEAD AND BLADES BE SPINNING AT 50% OF MAXIMUM REVS.

At 100% throttle stick setting, the motor will be running at it's maximum rpm, and so will the rotor head and blades.

From this we can see that while we have the throttle stick FULLY DOWN, all is well, and the head will not start to spool up on us.  As soon as we start to move the throttle stick, the motor starts and the head starts.

This arrangement is basically OK for those learning to hover helicopters initially, as the more throttle you apply, the faster the head rotates, and the more lift that is created until the helicopter takes off. (Well, actually the collective pitch on the blades also increase and decrease as you adjust the throttle, but we will cover that elsewhere)

However, if you turn the IDLE UP SWITCH ON, the throttle curve is very different indeed, as can be seen clearly in the diagram below..

You can see that when the throttle stick is fully DOWN for STUNT mode (Idle Up), the Motor, and Head, are at FULL SPEED, as they are at full throttle stick.  Only at mid throttle does it reduce slightly to 80% motor rpm.

THIS MEANS THAT IF YOU HAPPEN TO SWITCH THE IDLE UP SWITCH ON WHEN THE THROTTLE STICK IS AT ZERO, YOUR HELICOPTER WILL STILL SPIN UP TO MAXIMUM SPEED IMMEDIATELY.

Probably seems crazy to you, but it is provided to allow you to fly the helicopter inverted as well as the right way up.  

The ONLY WAY TO USE THIS SAFELY is to bring the throttle stick up to 50% in NORMAL MODE, and ONLY THEN, switch to Idle Up mode, for as you can see in the diagram, the two throttle settings are only the same at that one point.

One other thing also changes when you switch to Idle Up mode with the stock Esky Tx, and that is the amount of pitch on the rotor blades, and if you change modes at 50% throttle, you will see the main blades suddenly pulling the helicopter DOWNWARDS due to the additional negative pitch.

The diagram below illustrates this nicely. The normal pitch curve doesn't start at bottom stick, but rather at 40%, whereas the Idle Up curve goes from 0% to 100% totally linearly.  This means that the normal curve gives you a pitch range of approximately -4 to +8 degrees, but the Idle Up curve gives -9 to +9 or thereabouts.  They only cross at 50% throttle setting.

So, the moral here is to understand what the Idle Up switch does
and to ensure that you never switch it on unless you have the throttle at 50% stick setting

If you do happen to get into Idle Up mode, DO NOT SWITCH BACK TO NORMAL MODE until the helicopter is back on the ground, as the sudden change in Throttle and Pitch settings are almost sure to cause you to crash the helicopter.

OK, lets move on to individual radios now.

ESKY 6 channel radio

This is the most basic of radios, and is supplied as part of the RTF kits supplied by Esky, and in particular the Belt CP, which is the helicopter chosen by many new prospective helicopter fliers.

This radio offers very few control options other than the obvious sticks and their respective trimmers, but the later versions also changed the functionality of one of the control knobs.

The earlier Esky radios had to adjustable knobs on the top of the Transmitter (Tx), one either side of the aerial.  The left hand one has always allowed you to vary the Hovering Pitch to an extent,  while on the earlier TX's the right hand knob allowed small variations to be made to the Hovering Throttle. The later Esky Radio now use the right hand knob to change the gain on the gyro to let you adjust HH gyros more easily.

There are also 2 switches on top of the radio, behind the knobs. The right hand (spring) switch is only ever used if you have another radio acting as a buddy box, with both radios wired together.

The LEFT HAND SWITCH is the REALLY IMPORTANT ONE TO KNOW ABOUT 
- as shown by the red arrow above

This is the one called the IDLE UP switch by Esky, but is also known as the STUNT switch, or even the "Devils" Switch.

As you should by now know, this switch is something that you really do need to treat with extreme care. Always check BEFORE switching your Tx on that this switch is pushed away from you (OFF) so it is in NORMAL MODE.  There is a safety system built in to the electronics that "should" prevent the motor spooling up if the Tx is turned on with the Idle Up switch ON - but then - why risk the consequences. ?

SPEKTRUM DX6i & DX7

These are possibly the most used Radio Systems by RC helicopter pilots today

Both are fully computerised systems, and provides a great many ways for you to adjust and setup your helicopter to make it fly optimally to suit your particular preferences.  Because of that, they are also somewhat complex at first glance, and have lots more switches on them, as you can see from the pictures above.

The right hand picture shows you where the Idle Up switch is located on a DX6i.

Many of the switches/functions on the Dx6i/DX7 are not used by helicopter pilots, such as MIX settings, and indeed, many pilots do not use the DUAL RATE settings/switches either, so we will ignore these and assume they are always left UP (away from us)

The Idle Up switch can be as dangerous as on the Esky radio, except for the fact that YOU CAN SET THE CURVE UP YOURSELF.  Unless you want to fly in Idle up mode, it is a good idea to have both the Throttle and Pitch curves (called STUNT by Spektrum) set up exactly the same as your NORMAL curves, so that if you happen to switch the Idle UP switch on accidently, IT WILL BE NO DIFFERENT to the NORMAL curve.

If you do have a true Idle up curve set, then the same warnings apply as with the Esky radio.

The other very useful switch (IF SETUP IN THE RADIO CONFIGURATION) is the Throttle Hold switch, located at the back right on top of the radio.  Most people set this to be 0% or even a small negative value to ensure that when this switch is ON, the motor WILL NOT START UP AT ALL, irrespective of the throttle stick position.  Clearly this is a very useful safety measure.

The only other switch that helicopter pilots use regularly is the Flaps/Gear switch, which  is typically setup in the configuration to let you switch the mode the gyro operates in between Rate mode and HH mode (AVCS).  Most pilots choose to have the switch away from then (0) to be HH mode, but it is a good idea to check this before you take off.

Before you switch on a DX6i, CHECK ALL SWITCHES and ensure that they are ALL set correctly, but in particular make sure that the IDLE UP/STUNT switch is AWAY FROM YOU (OFF).

The DX6i has various safety circuits built in that warn you of incorrect switch settings on start up.  The two most common you will hear will be beeps indicating that either the Idle Up switch is FORWARD (ON), or that the Throttle Hold is forward (ON), or that the throttle stick or throttle trimmer are not fully down.

In all cases, you need to reset these or change switch settings before the Tx will initialise correctly.

To avoid this, it is always best to get into the habit of ensuring that the throttle and trimmer are fully down, and that both the Idle Up & Throttle Hold switches 
are in the OFF position before switching the Tx on.

If the radio continues to beep and not initialize, check all other switches, but they should not effect the Tx from initializing.

SPEKTRUM DX7

This is the bigger brother of the DX6i, with more features and one additional channel.

It is again a fully computerised system, but with with 7 channels, and provides a great many ways for you to adjust and setup your helicopter.

It also moves the Idle Up switch for some strange reason from the top left to the front left, replacing the original Idle Up switch position with the rudder D/R.

The Idle up switch on the DX7 is now the longer switch that is 2nd in from the left on the top front. It is also a 3 way switch, as the DX7 offers you a full 3rd set of curves.  Also, the Dx7 does NOT have a switch marked Throttle Hold, but most people configure it to still be top right right switch that is labelled as Rudder DR.

Other than this, the Dx7 and the DX6i are pretty well identical in terms of safety when powering them up.

I really do hope that this tip helps 
ALL pilots to avoid powering up their radio incorrectly.....