Question: Can you run a [BLDC motor](http://en.wikipedia.org/wiki/Brushless_DC_electric_motor) backwards without damage?

Is it OK to drive a model airplane BLDC engine backwards while landing, so it gets a little "reverse thrust" and come to a stop a little quicker on the runway?

Is it OK to drive a model helicopter BLDC motors backwards so it can hover upside-down?

Or do I need to design the hardware so that it never drives the motor backwards, under any circumstances, no matter what the pilot on the ground does at the transmitter?

Answer: 
Yes, you can drive a brushless DC motor in both directions.

See, for example, the [On Semiconductor MC33035](http://www.onsemi.com/PowerSolutions/product.do?id=MC33035PG) brushless DC motor control chip, which has a pin to control direction.

Here's a little explanation from p. 9 of [the datasheet](http://www.onsemi.com/pub/Collateral/MC33035-D.PDF):

> 
> The Forward/Reverse input (Pin 3) is used to change the
>  direction of motor rotation by reversing the voltage across
>  the stator winding. When the input changes state, from high
>  to low with a given sensor input code (for example 100), the
>  enabled top and bottom drive outputs with the same alpha
>  designation are exchanged (AT to AB, BT to BB, CT to CB).
>  In effect, the commutation sequence is reversed and the
>  motor changes directional rotation.
> 
> 
> 

I believe you do have to be careful about "shoot-through"-- if you're trying to switch the direction of current flow in a winding, you have to be sure to turn one set of FETs off completely before you turn on the other set, or you may inadvertently short your power supply.

You might google "adaptive gate drive" or "dead time" for more details.
What you have to worry about most when you reverse direction of a motor, is that you do not put too much current into either the motor or the electronics/switches that control it. 

When you connect a voltage source across a motor that is at rest and either has a large inertia or a locked rotor, you get a large current flowing through it = V / R where R is the stator winding resistance of the motor. This is called the stall current.

If you are running at full speed with a voltage source across a motor, and you immediately reverse the polarity of the voltage source, you can get up to 2x the stall current, because the voltage source is then at the opposite polarity of the motor's back-emf. This can be too much current, and if that's the case then you have to control the rate at which you reverse voltage across the motor, by using PWM or some other way besides a hard voltage reversal.