I dug out the notes I wrote for a failsafe unit we made (called SparkOut) for petrol-powered models. It explains what failsafe is and does - and it will probably scare the hell out of you unless you've already found out about it. The bottom line is that you should ALWAYS know what might happen if the receiver suddenly loses contact with the transmitter i.e. will the motors stop or go to full reverse. You find out very easily by switching the TX off with the Rx still on - but only do this with the model out of the water! Hope it's useful:
Irrespective of the frequency of radio transmission (MHz or GHz) all proportional servos operate the same way. Regular pulses of electricity are sent from the receiver along the signal wire to the servos; this is the wire which is neither black nor white. The duration of these pulses varies in length between about one and two thousandths of a second, or milliseconds (mS), depending on the position of the stick or switch which controls that channel from the transmitter. These pulses control the position of the servo. For example, full speed might be a signal of 1.9mS while slow throttle would be 1.1mS. Moving the trim lever will stretch the pulse length at one end of the scale or the other. Any signal with a pulse length of less than around 0.9mS or more than 2.2mS will be ignored by the servo as it is ‘invalid’ (remember this word for later). OK so far? Jolly good!This is all fine while the transmitter and receiver are in contact with each other, but what happens if the transmitter is suddenly switched off or the model goes out of range? That depends upon whether or not the radio is equipped with a failsafe function and, if it is, how that function works.If the radio has no failsafe function then the receiver becomes open to any rogue signals on the same or nearby radio frequencies. In extreme cases these signals can make the servos twitch or even lock-up at one end of their travel. Generally, however, the servos simply freeze in the position they had when the transmitter signal failed. Naturally if this involves full throttle then the model will become out of control with the engine at full speed – not exactly a safe state of affairs!If the radio has a failsafe then the receiver will briefly send a signal of some sort to the servos when the transmitter signal is lost. That way the servo positions will not be affected by any rogue signals from an outside source. The basic failsafe function will return the servos to a fixed position before reducing the length of the signal pulses to zero. Thus a Planet T5 will momentarily feed a signal of 1.5mS to centre each servo before the signals become 0.0mS – which is an invalid value and will be rejected by the servos, freezing them in the centre position Other radios will go immediately to this zero signal value, thus freezing the servo in its most recent position by (again) sending an invalid signal to the servos. This is typified by the Spektrum DX5e.
More sophisticated radios have computerised functions and the user can program what the throttle servo will do if the radio fails. They can either be programmed to freeze the servos at their last known “good signal” position (which is not a good idea, as we have already decided) or to move the throttle servo to a pre-set position. If you have one of these sets then you MUST select the second option i.e. a preset throttle position. This position should be with the throttle stick and throttle trim both fully down (= dead slow)*. Some radios have a Throttle Cut button which will generate the same “dead slow” signal when pressed. Examples of this type of radio are the Futaba 6EX, Futaba T6J, Hitec Optic 6 and Spektrum DX6i. In all cases the Instruction Manual for the radio will give directions on how to set the failsafe signal. If you have lost the manual then most can be viewed or downloaded from the Internet.
(*Remember this was written for a gas-powered model where fully down stick = engine stopped. For an electric model with forward and reverse speed control the corresponding stick and trim positions are in the centre)
DM