# B RC Helicopter in a Train (Forces)

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1. Mar 8, 2017

### BrendanKerrisk

I was talking to my Physics teacher about this at lunch.
If I was to have an rc Helicopter which i had hovering perfecting (if only i had the skill) on a train at rest. What would happen in terms of the position of the helicopter as the train accelerated? Would is stay at the same point in reference to the train (e.g above my table) or would it stay is the same point in the air and the train would move without it? My teacher and i thought that at first the helicopter would go backwards but then just stay hovering slightly behind where it was when at rest. Any insights or thoughts plz.

2. Mar 8, 2017

### rcgldr

Assuming the helicopter is inside a box car of the train, if the train accelerates forwards, then the helicopter would only accelerate forwards at a small fraction of the train's acceleration, due to the drag of the forward accelerating air on the helicopter, and the back wall of the box car would collide into the helicopter.

3. Mar 8, 2017

### Staff: Mentor

...unless in your attempt to maintain the hover you pitch forward and increase lift.

4. Mar 8, 2017

### Staff: Mentor

Is it any different than a passenger in the train? Don't you feel your body press into the seat as the train accelerates? Every free body in the train need a force to accelerate it to match the train's speed.

Even the air in the car must experience higher pressure at the back of the car as the train accelerates.

5. Mar 8, 2017

### A.T.

This could happen without control input, when the helicopter starts moving back through the air.

6. Mar 8, 2017

### Staff: Mentor

Yes, but both in proportion to the slide and neither enough to maintain position and altitude without control input.

7. Mar 8, 2017

### rcgldr

As russ_watters, the pilot could compensate with forward pitch. If the air ends up circulating so that it's moving horizontally backwards as it passes by the helicopter, then the lift (collective / power) input would be decreased. In a hover, a helicopter ends up operating in it's own induced downwash, requiring more power than in forward flight, where the helicopter advances into "fresh" air, from the air's frame of reference. From the helicopters frame of reference, if the air is moving "backwards", then it takes less collective / power to maintain altitude.

There's a limit to this. As forward speed (relative to air) increases, eventually the power / collective will have to be increased, including beyond that needed for a hover.