SetVelocity.Velocity takes an array of 3 numbers, similar to setpos.
example
plane setvelocity [100,100,0]
The numbers say in which direction along each of the axes, and how fast, the plane travels.
The numbers do not depend on the direction the plane is facing. Rather they relate to directions on the map. The first number is the east-to-west component (+100 means going east on the map). the second is the south-to-north component (+100 means north). The third number is the upwards/downwards component. +10 would mean up. -10 would mean down.
How fast the plane travels after this can be calculated by pythagoras' theorem.
You can also calculate the direction of movement using trig. Using the sin and cos functions, you can make the plane move in what ever direction it's facing.
Example
car setvelocity [10*sin(getdir car),10*cos(getdir car),0]