Question: In Star Trek Beyond, they fix up the old Federation ship, USS Franklin. It wasn't made to fly in atmosphere, so in order to take off they need to propel it off a cliff and reach terminal velocity before the impulse engines can fire. (From what I can recall from a single viewing - if any of those assumptions are wrong please correct.) 

What I don't understand is *how* does terminal velocity allow impulse engines to just "start working"?

Answer: 
Spaceships in atmosphere are usually depicted as flying due to downward force from an engine, like a hovering harrier. Since the Franklin wasn't designed to fly in atmosphere, it may not be able to direct sufficient force in a downward direction to effect a vertical takeoff.

So, if it can't just take off straight up, it must need to generate lift a different way. Assuming that the ship is producing lift via [Bernoulli's Principle](https://en.wikipedia.org/wiki/Bernoulli's_principle) and an [airfoil](https://en.wikipedia.org/wiki/Airfoil), needing a jump start makes perfect sense.

The lift generated by an airfoil is dependent upon the speed at which the craft is moving through the air. This means that the craft needs to get going fast before it can actually fly. This is why commercial aircraft require long runways, so they can get up to speed. As you may have noticed, there was no runway in front of the Franklin.

Therefore, before the ship could fly under its own power, it needed to get going fast enough that the stabilizers could keep it flying, despite providing minimal lift on their own. This required falling off a cliff.

In other words, it's just what Chekov said:

> 
> We have to achieve terminal velocity in order for the stabilizers to provide lift.
> 
> 
>
They specifically get the inspiration from the idea of the jumpstart. Then they basically "push started" the starship. So I'm assuming the older starships have a manual transmission, and they popped the clutch after getting up to speed. On a car, the wheels transmit the motion to the engine and engaging the engine suddenly can get the engine running without a battery. So, I guess there's something that rotates or moves on the basis of the air (or solar wind?) traveling on the outside of the hull, and engaging the engines suddenly allowed them to start without the power cells working.

But honestly, this part of the movie bothers me more than anything else they've done. It just makes no sense at all!