# Crazy Hovercraft Stunt (Physics + Tech)

Tags: crazy, hovercraft, physics, stunt, tech
 P: 5 Hi guys, I need some help with a sci-fi screenplay featuring very maneuverable hover cars. Past the maneuverability, the details are all subject to change so don't hold back. I've got one of these hovercrafts going off a cliff, over a canyon, at 60-100 MPH... and what I need is for it to get back onto that same cliff, close to where it came from. The best way I can think of is to have the driver turn it sideways so that its momentum carries it back to land. Like so:  \\ \\||// // => v vv v v <= => <= ___=>____________________________<=_______________ => ^ => \\ ==> (X) Starting at (X). Arrows demonstrate the craft's front end facing. Or maybe he performs a rollercoaster-esque corkscrew over thin air which lands him back right before the cliffside? My questions are: 1. Which of these seems more sensible? Or maybe there's another way I'm overlooking. 2. How do you see the scenario playing out? IE, if it's the horizontal turn, would the driver need to raise the car in the air before the turn so that the car wouldn't dip while it's in the air and just crash into the cliff on completing the turn? 3. Is it plausible that the rotors are themselves maneuverable mid-flight? So that by rotating them the hover car could bounce off walls like a bumper car? How many rotors should the car have? Two big ones (front and back) or lots of little ones all over to allow for ultra-precise movement? 4. I read that "you can't brake a hovercraft like you can a car." What's the reasoning behind this? Apologies in advance if my questions seem quaint. :) I want to try to get this within the realm of plausibility if possible. Any suggestions/advice is appreciated!
 P: 273 As soon as the hovercraft leaves the cliff, it's going to start falling. An upwards force must be provided to counteract gravity, or the section of the cliff you intend to land on must be lower than where you started. Is this going to be animated? I hope you don't intend for someone to actually attempt this...
P: 4,209
 Quote by inquistador that its momentum carries it back to land.
Its momentum cannot carry it back. Momentum is conserved as a vector, unless you apply external forces to change it.

 Quote by inquistador I read that "you can't brake a hovercraft like you can a car." What's the reasoning behind this?
Which parts does a car use to brake? Does a hovercraft have these parts?

 Quote by inquistador I want to try to get this within the realm of plausibility if possible.
Make up your mind if they are hovercraft or helicopters. Although helicopters can hover too, the term "hovercraft" usually refers to vehicles that can only hover close to the surface, and not fly and turn around over a deep canyon.

P: 5
Crazy Hovercraft Stunt (Physics + Tech)

 Quote by BOAS As soon as the hovercraft leaves the cliff, it's going to start falling. An upwards force must be provided to counteract gravity, or the section of the cliff you intend to land on must be lower than where you started. Is this going to be animated? I hope you don't intend for someone to actually attempt this...
No hovercraft operators will be harmed in the making of this script. :)

Is there a formula to figure out how fast the car would fall given its forward/sideways momentum of, say, 100 mph? Then he can overcharge the hover mechanism right before taking the plunge to compensate for the incoming drop.
P: 273
 Quote by inquistador No hovercraft operators will be harmed in the making of this script. :) Is there a formula to figure out how fast the car would fall given its forward/sideways momentum of, say, 100 mph? Then he can overcharge the hover mechanism right before taking the plunge to compensate for the incoming drop.
Horizontal velocity will not affect the rate at which the hovercraft falls. Momentum, by the way, is measured in kgms-1, not mph (which is a measurement of speed).

For a very simple approach you could use the kinematics equation, s = ut + 1/2 at2

initial velocity, u (in the 'downwards' direction) is 0, so it becomes s = 1/2at2

a is the acceleration due to gravity (9.81 ms-2) and t is the amount of time the hovercraft will spend in the air.

s is displacement, which is measured in meters. i.e how far the car will fall.
P: 5
 Quote by A.T. Its momentum cannot carry it back. Momentum is conserved as a vector, unless you apply external forces to change it.
Could you be a little bit more specific about which part is impossible? Is it the angle of the turn that's too crazy? I imagine that there must be SOME terrain layout that would result in a brief "freefall" period followed by landing on the other side.

Is there any relevant external force that might be used in this situation?

 Which parts does a car use to brake? Does a hovercraft have these parts?
Right. Brakes. Wheels. So either there's some sort of diagonally-mounted rotor at the front that could help stop this hovervehicle... or it's gonna stop with the precision of a freight train.

 Make up your mind if they are hovercraft or helicopters. Although helicopters can hover too, the term "hovercraft" usually refers to vehicles that can only hover close to the surface, and not fly and turn around over a deep canyon.
Thanks. I'll keep an open mind to reworking the scene.
P: 4,209
 Quote by inquistador Could you be a little bit more specific about which part is impossible? Is it the angle of the turn that's too crazy?
Just like for braking, you need horizontal forces to turn around.

 Quote by inquistador I imagine that there must be SOME terrain layout that would result in a brief "freefall" period followed by landing on the other side.
Sure. A ramp.
P: 5
 Quote by A.T. Just like for braking, you need horizontal forces to turn around.
What about returning boomerangs? They turn around in the air.

Perhaps you were implying that there's no practical way to apply any non-horizontal-force-turning reasoning to a car to achieve an effect similar to what I'm looking for.

Although this car does have rotors in it, and the returning boomerang being similar to a rotor is what makes it work the way it does. I'm guessing they'd have to be much bigger to make a difference for something as heavy as a car, though.
P: 4,209
 Quote by inquistador Although this car does have rotors in it,
As I said, make up your mind if they are hovercraft or helicopters.
P: 5
 Quote by A.T. As I said, make up your mind if they are hovercraft or helicopters.
Err, this article mentioned rotors a few times, so I was going off that. It's possible I misunderstood something.
 P: 1,494 If the vehicle is using surface affect, such as the usual hovercraft with a curtain around it, then as soon as the surface affect is lost, such as going over a cliff, the vehicle will drop. The vehicle you linked to is using surface affect.
 P: 1 Why not increase power to the max just before the cliff in order that upward momentum is established just before the ground ends. Then bank backwards as a surfer might so the extra power reverses direction of the hovercraft before the machine drops too far.
P: 4,209
 Quote by DeanNorris power to the max
Yep. Always works in sci-fi movies.
 P: 190 Greetings True Hovercraft rely on ground effects for sufficient lift to ...well, hover. This is one of the reasons that they employ a flexible shroud, the "skirt", at the base so that they don't bottom out on bumps and grades. The ground effects are employed so that a design incapable of actual flight (ie: insufficiently powerful engine w/ practical fuel requirements) can escape the losses of friction. Absent Sci-Fi Anti-Gravity, they aren't very practical since the power to produce those ground effects is far more expensive than wheels. So, considering this is Sci-Fi for which you wish to build what I can only assume is a danger, tension/release scene, it seems to me you should attempt to allow either for some totally Sci-Fi power source or some emergency device that can briefly cross into the realm of a helicopter or at least a "flying brick" complete with a very bumpy landing. Consider the real so-called Jet Pack, the totally stripped-down (no fuselage) rocket on a backpack, and recall that, to date, an impulse , and flight, of mere seconds is "practical" and those devices cost hundreds of thousands of dollars. For a fun update see http://en.wikipedia.org/wiki/Jet_pack . The below attached concept "photo" might provide an example of a hybrid device concept, though in truth it is 99% helicopter and at best 1% Hovercraft (at the very least due to the lack of a skirt) and in this concept even if it only was expected to carry one individual, in reality it could not carry sufficient fuel to go but a very short distance being limited by the very same physics that make Jet Packs impractical, short term, and highly dangerous. NOTE: For all you TLDRs, the above picture is NOT REAL!
P: 4,209
 Quote by enorbet The below attached concept "photo" might provide an example of a hybrid device concept, though in truth it is 99% helicopter and at best 1% Hovercraft
Looks more like a hybrid of helicopter and plane. The hover part is implied by the helicopter part, because helicopters can hover using the ground effect.