# Crazy Hovercraft Stunt (Physics + Tech)

1. Jul 26, 2014

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:

Code (Text):

\\  \\||//    //
=>  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!

2. Jul 26, 2014

### 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...

3. Jul 26, 2014

### A.T.

Its momentum cannot carry it back. Momentum is conserved as a vector, unless you apply external forces to change it.

Which parts does a car use to brake? Does a hovercraft have these parts?

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.

4. Jul 26, 2014

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.

5. Jul 26, 2014

### BOAS

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.

6. Jul 26, 2014

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?

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.

Thanks. I'll keep an open mind to reworking the scene.

7. Jul 26, 2014

### A.T.

Just like for braking, you need horizontal forces to turn around.

Sure. A ramp.

8. Jul 26, 2014

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.

9. Jul 26, 2014

### A.T.

As I said, make up your mind if they are hovercraft or helicopters.

10. Jul 26, 2014

Err, this article mentioned rotors a few times, so I was going off that. It's possible I misunderstood something.

11. Jul 26, 2014

### 256bits

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.

12. Jul 26, 2014

### DeanNorris

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.

13. Jul 26, 2014

### A.T.

Yep. Always works in sci-fi movies.

14. Jul 26, 2014

### enorbet

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!

15. Jul 26, 2014

### A.T.

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.

16. Jul 26, 2014

### cyklop

Crazy Stunt

There are some things in various posts that may be slightly misleading. First of all, hovercraft do not use ground effect. They use a plenum chamber that is filled with air creating a higher pressure zone trapped in the plenum chamber and upon which the hovercraft rides.

As an experiment, try this: Take a trash bag (empty!) and place some books on it. Now start blowing into the bag as if you were trying to inflate it to pop it (i.e. scrunch the opening down to make a seal with your mouth). It's not that difficult to raise the books. That's what a hovercraft is doing.

Any break in the contact of the plenum chamber "skirt" with the ground starts to let out air and reduces the ability of the hovercraft to ride along. All hovercraft that are correctly designed can tolerate some of this, but they certainly can not fly nor leap. One of these going off the edge of a cliff will do the same as a brick - it will fall. The exact arc it will follow is dependent on forward velocity, but the DOWNWARD portion of the path is independent of FORWARD velocity.

V = at (and if you integrate this with respect to time, you get d = 1/2 at^2 as referenced in an earlier post)

where V is velocity, a is acceleration due to gravity (-9.806 m/s), t is time in seconds. The force of the impellers will reduce that downward acceleration SLIGHTLY (hovercraft have surprisingly weak impellers) but are completely incapable of supporting the craft. It will still glide like a brick. Leaning the craft to the side to try to "bank" it around the turn makes things worse since you're stealing vertical thrust and diverting it to horizontal. The craft falls faster. Of course, a hovercraft has the thrust fan to move it and this can be used to do any or all of the following:

1. It can apply forward thrust to move the hovercraft forward or slow (and eventually cancel) backward motion.
2. It can apply reverse thrust to move the hovercraft backward or slow (and eventually cancel) forward motion.
3. It can rotate the thrust fan or use louvers or additional fans to "yaw" the vehicle about its center of mass. This has the effect of spinning the hovercraft, but does not change the direction that it is moving. To change the direction of a hovercraft, you yaw the craft so that the thrust fan is pointing to the side and start pushing sideways. The course of the hovercraft will slowly bend in the direction the nose is facing, and friction will slowly reduce the original forward motion. By turning the craft even further, you can trade side thrust and braking thrust to slow the craft sooner. None of these kinds of turns will be fast enough to help you with the cliff arc scenario.

An alternate vehicle is known as a "WIG" - a "Wing in Ground Effect" vehicle. This vehicle IS using ground effect and is actually flying. It has thre main drawbacks. 1. It can't stop. If it slows down, it settles back to the ground. 2. It needs a runway or similar take off and landing point. 3. It is really bad for wooded or cluttered or variegated (hill/mountain) terrain - it needs open space because of the speed at which it moves (no slow flight for it). It does, however, have some benefits for your scenario.

First of all, it CAN fly for short distances (not sure the exact distance, but a couple hundred meters does not seem out of the question). 2, it turns by banking, so your edge cliff loop is not out of the question for it. It's also fast - faster than a hovercraft.

It might be possible to do a hybrid Hovercraft/WIG, but I don't think anyone's tried that.

Last edited: Jul 26, 2014
17. Jul 26, 2014

### twoslit

You might try having your craft have emergency booster JATO units which give it a brief (and spectacular) boost upwards from where it can glide back to the landing. That gets rid of the need for any special physics, of course.

18. Jul 26, 2014

### twoslit

BTW, to get a good intuitive feel for what a real hovercraft behaves like, watch or other movies of the hovercraft car ferry service across the English Channel. As you will see, it steers using directed airfoils at the corners, having no wheels or direct contact with the surface, and you can clearly see the 'brush' at the edge of the skirt. Deceleration was done largely by taking it up a specially built sloping ramp. This is *not* a VTOL aircraft :-)

Last edited by a moderator: Sep 25, 2014