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: 
BLDC motors usually just use permanent magnets on the rotor (be it in-runner or out-runner) and use a set of windings on the stator connected in a three-phase delta or wye configuration. The speed controller just generates a variable-frequency, three phase waveform to power the motor. Since the windings are symmetric, electrically there's no reason you can't turn the motor in either direction.

As for whether it's a good idea to run a prop backwards on landing, that's more of an aeronautics problem than anything inherently electronic. Having flown some r/c planes, it seems to make sense to me that if you reverse the prop on landing, you're basically just applying a braking force along the line of the axis of rotation. If that line passes above the center of gravity (not below it), that should torque things so that the tail will stay down, so you should be stable if that's the case. If the prop axis is below the CG, though, you're looking at forward torques that would drive the nose down, which would result in damage.
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.