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I'm working on a science fiction racing game that is centered around hovercraft that will race through various environments, both natural and man-made. I'm trying to establish a design language for these vehicles and I figured the best place to start would be figuring out how they hover and design them from there. This was originally going to be a question about how I achieve lift, but I think I got a workable solution and just need help making sure it's plausible.

Clarification: When I say hovercraft, I don't mean the modern skirted hovercraft. The vehicles I envision would look more like aircraft than hovercraft. I just called them hovercraft because they are going to hover.

My plan is to make it so these vehicles only hover about a meter off the ground, both when they're up to speed and when they're stationary. I also wanted to avoid having them rely on vertical thrust too strongly out a concern about them suddenly becoming VTOL aircraft instead of hovercraft.

I wanted these craft to lose their vertical lift once they're above a certain distance from the ground and enter a sort of controlled fall/glide until they are low to the ground again.

My current plan for how they achieve lift is a two-part solution, but it could end up being reworked if problems present themselves. While at low speeds, a set of thrusters/turbines would provide the vehicle vertical lift to keep it about a meter off the ground (maybe lower, still working out exact ride height.) A set of forward thrusters would push the vehicle forward, up to racing speeds. As the vehicle picks up speed, its aerodynamics would allow it to use the ground-effect to give it lift at the desired ride height. As it generates its own lift, the vertical thrusters/turbines would turn off and the extra power would be stored or diverted into the main engines to allow it to go even faster. The thrusters/turbines might also need to provide supplemental lift when over terrain types that might reduce or break the ground effect.

My question is how do I keep my hovercraft from becoming full on aircraft? What can I do to limit their flight capabilities so they can generate enough lift to get about a meter off the ground without given them the ability to go more than a couple meters off the ground under their own power?

Is limiting the power of the vertical lift thrusters/turbines enough to keep them at a set height? The ground-effect would kind of self-regulate itself in terms of height as the lift is generated by moving close to the ground and would become weaker once it's too high up.

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    $\begingroup$ Just make it an actual hovercraft or ekranoplan. For ground effect you need wings or a big rotor like a helicopter. $\endgroup$
    – causative
    Jul 26 at 19:00
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    $\begingroup$ Is there a reason you need a physical restriction instead of a rules-based one? e.g., if you travel more than a certain distance above the ground, or are outside of a particular three-dimensional "corridor", you're disqualified. $\endgroup$
    – Cadence
    Jul 26 at 20:17
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    $\begingroup$ The only reason I wanted it to be limited by physics was to differentiate these racing vehicles from actual aircraft by capability. I was planning to only limit player movement on the track by side-markers that would disqualify them if they stayed off course too long. I don't want to make a height limit as there are sections where you will be using that controlled falling/gliding I mentioned in the question and don't like the idea of having to render a 3D tunnel over the terrain that players have to stay in. $\endgroup$
    – Arvex
    Jul 26 at 20:26
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    $\begingroup$ Before I provide a full-length answer, is it possible to know what kind of speeds are we talking about here? Something equivalent of everyday common vehicles or military breaking-the-speed-of-sound-without-breaking-a-sweat kind of contraptions? $\endgroup$ Jul 26 at 22:17
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    $\begingroup$ ...or F-Zero from the SNES days. Or just reach out to Anakin ... $\endgroup$
    – A C
    Jul 29 at 3:47

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If you're just looking for a way to ensure they can't take off, ground effect is a good starting point. Without the interaction with a surface, at most a couple of meters below the craft, lift is negligible. Think of it as if you were creating a high pressure region of air that your craft sits on top of. Take away the ground and that pressure doesn't build, so your craft falls.

In classic hovercraft the pressure is maintained with a skirt. Other ground effect vehicles primarily use aerodynamics to form and ride the pressure cushion. For those types the aerodynamic profile of the craft doesn't generate lift, it generates a pressure wave that supports the craft. Going faster doesn't lift you up, it increases the density of the pressure cushion... and presses the craft down onto it. Ideally the increase in downforce should match the increase in pressure, keeping your 'altitude' reasonably consistent. And once you get up to speed you're not pushing air downward with your engine, giving your more power for forward motion.

In addition, the aerodynamic shape of your craft is specifically designed to ride on that cushion of air. Take away the air cushion and the craft becomes inherently unstable. Run it over a ramp for instance and just after leaving the top of the ramp it starts to tumble within seconds. You can maybe stabilise it if you have a big flywheel gyroscope to retain orientation, but a racing hovercraft isn't going to want to drag all that extra weight around when it's only useful in extreme situations that you shouldn't be facing in a race anyway.

In summary:

  • Ground effect provides most of your 'lift.'
  • The faster you go, the more solid your connection to the ground.
  • Leaving the ground, at any speed, is catastrophic.

Now... how do we break this?

Your hovercraft are designed to be flexible, adjusting aerodynamic profile to maximize the ground effect at different speeds by small adjustments of the control surfaces and hull geometry. With a little bit of tweaking, adjusting the flex range of some of the surfaces and adding a little extra to the length of the adjusting mechanism (at the cost of a small weight increase) it might be possible to switch from ground effect to fully aerodynamic profile while at speed. If you get it just right you might be able to 'skip' off the ground effect cushion and get a fairly decent glide, enabling you to cross small ravines and so on. Preferably the ones as far away as possible from the cameras and race officials.

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    $\begingroup$ I wanted to sugest GE too, you saved me the trouble - this question and answer contain a few more hints and even a bit of math aviation.stackexchange.com/questions/22776/… $\endgroup$
    – mart
    Jul 27 at 11:50
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    $\begingroup$ I'll add to this: The disincentive for people to just build an airplane is that wing surfaces sufficient to lift while in GE are smaller - and therefore produce less drag and have less weight. In racing, victory is won at the absolute margins for performance. While a proper aircraft could go higher, a specialized GE-racer will always go faster in the straightaways. $\endgroup$ Jul 27 at 13:22
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    $\begingroup$ I was planning for these craft to be able to safely glide if they become separated from that pressure cushion created by the ground effect, not fail catastrophically. I could have a secondary set of deployable wings for gliding or have the airfoils be able to adjust into a gliding configuration to keep the vehicle stable once they're too far off the ground. I actually was intending for gliding to be part of certain "high risk, high reward" races. $\endgroup$
    – Arvex
    Jul 27 at 16:43
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Wings are simply not allowed

Regulations, regulations, REGULATIONS! Sometimes the easiest solution is the best.

Since you stated this is a racing game, every game must have its rules and the most rational solution is to take the first page of every racing sport ever conceived and simply form the proper regulations to which all the racers must abide. The first and main rule of racing hovercrafts should be, naturally, that the racing vehicles comply with hovercraft standards, meaning they must not have wings or any other features that provide them with sustained flight. How you define the difference between sustained flight and hovering is down to you.

After that it's down to fine-tuning the rules.

If you want a pending story in this racing game, you could think outside of the box and have your racers actively seeking out loopholes in the rules to give themselves an upper hand, that's what actual racers do and have done since the beginning of racing and many of them managed to get away with it until the regulations eventually caught up.

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  • $\begingroup$ If I'm basing them on the ground effect, wings would be necessary to generate lift, though they might not be in a configuration that would be optimal for flight. $\endgroup$
    – Arvex
    Jul 27 at 3:30
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    $\begingroup$ @Arvex But the rules can regulate the size of the wings, the amount of lift they're allowed to generate, or just rule flying illegal. See also: literally every racing championship. F1, WRC, Le Mans, they all have regulations on engine power, aerodynamics, weight, or what have you. $\endgroup$ Jul 27 at 7:24
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    $\begingroup$ You need to research hovercraft more thoroughly, especially the part how they generate lift. The only reason for hovercraft having wings (FLAPS to be more specific) is to direct and control their horizontal movement, they have nothing to do with their lift. The amount of lift is directly determined by engine output. . PS: If your hovercraft manage to clear the surface they're hovering above in its entirety, they cease to be hovercraft and become jet powered aircraft, such as the Harrier Jump Jet and then you got a whole different subject to tackle $\endgroup$ Jul 27 at 10:14
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    $\begingroup$ @ArgentHellion It sounds like their goal is to have ekranoplans (with a bit of VTOL capability). They're using "hovercraft" in a colloquial sense, not the technical sense. $\endgroup$ Jul 27 at 13:36
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    $\begingroup$ Yeah. They're going to be closer to small ekranoplans than contemporary hovercraft. I just call them hovercraft out of habit because I was calling them that before I learned about ground effect vehicles (which renewed my interest in this previously shelved project.) I could see regulations keeping the wings small enough that they cannot produce enough lift without relying on the ground effect. $\endgroup$
    – Arvex
    Jul 27 at 16:37
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Perhaps the hovercrafts do not have a powersource of their own, but are instead somehow electromagnetically powered through the racing track/area, similar to our modern induction chargers. They cannot go offtrack, or fly too high, as the range of the power delivery system is pretty limited. They can have some form of power storage, providing limited power for a few seconds, and thus enabling them to do short jumps (think car racing up a hill, and momentum carrying it in the air once it reaches the top). These batteries/capacitors would prevent a full engine shutdown, followed by a crash.

As a little bonus, if you want, you can also add "nitrous oxide" to spice the races up. A separate induction coil slowly trickles power into a capacitor, which when full, can be discharged for a short burst of speed.

And if you are concerned about explaining electricity based propulsion, you can just use this mechanic for the off-the-ground component, think an electric powered helicopter that can't get too far from the ground without the rotor slowing down.

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  • $\begingroup$ Real-world example of an unpowered hoverboard that can carry a human using magnets: insidescience.org/news/how-lexus-hoverboard-gets-ground . $\endgroup$
    – Brian
    Jul 27 at 18:27
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    $\begingroup$ I think this is the best answer. Ofcourse GE, skirts etc are also a solution, but there's nothing new about these technologies. A solution where the energy storage is not onboard is already awesome from a racing point of view - you reduce the mass of your vehicle, that makes it more dynamic and in the spirit of racing. $\endgroup$
    – MishaP
    Jul 28 at 15:03
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Hovercrafts have skirts.

skirt https://www.neoterichovercraft.com/hovercraft-parts/skirt.php

https://morgridge.org/blue-sky/how-do-hovercrafts-work/

A hovercraft has what’s called a skirt to contain the air. When the air blower is turned on, this skirt creates a pocket that traps the pressurized air. That pressurized air is what gives the lift to make a hovercraft work.

The skirt contains a pressure chamber between body and substrate (ground or water). If the hovercraft lifts off the ground it loses pressure because a gap opens between skirt and substrate. It loses pressure. Then the hovercraft will fall back.

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Event rules

There's an obscure Olympic athletics event, the 50km walk, that works as a good example for this. It's a race, but the rules require you to walk every step. Every single participant is of course capable of running, but if caught doing so they're disqualified. The difference between running and walking is simple, if both your feet are off the ground at the same time, you're running.

You can simply make a rule that if the vehicle is more than 2m off the ground it's disqualified from the race. It's not a regulation on the vehicle but rather on the way the vehicle is used.

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Quantum levitation

One way to ensure hovercraft stay near the track is to ensure that the track itself is what allows them to hover. This can be achieved through quantum levitation, by which a cooled superconductor can float above a magnetic track.

The very superconductor material itself will create a force to inhibit any sort of motion in relation to the magnetic field. If you tilt the superconductor, for example, you will "lock" or "trap" it into that position. It'll go around a whole track with the same tilt angle. This process of locking the superconductor in place by height and orientation reduces any undesirable wobble. https://www.thoughtco.com/quantum-levitation-and-how-does-it-work-2699356

You can see some demonstrations in these videos from 2011:

This differs from traditional mag-lev technology in that the levitation is more stable, making it suitable for the precise movements made during racing. The "locking" effect would also permit racers to regulate their height and tilt within the magnetic field of the track, which could have all sorts of implications such as obstacle avoidance or even adding a third dimension to the track.

In real life, we haven't scaled up the technology to have levitating vehicles yet. And I'm not sure how moving laterally above a single magnetic track would work - perhaps you could have multiple parallel tracks, that hovercraft could "jump" between. But you can decide what to explain or omit, since it's your world you're building!

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Well, there are a couple of ways you could go about this. If the thrusters are powerful enough to lift the craft one meter off the ground just by themselves, they are powerful enough to lift the thing higher, making it an airplane. Thus, the main downward thrusters must not be powerful enough to lift the craft off the ground by themselves.

Of course, this leaves the issue that the thing can't float. So, to resolve this issue, there must be a secondary source of lift that works only when the craft is near the ground. A few options:

The track is magnetized

Part of the force keeping the craft aloft is the exhaust from the thrusters hitting the ground at high speeds, rebounding, and hitting the craft again, thus providing additional lift

The craft relies on aerodynamic lift to stay in the air, and thus cannot lift itself under thruster power for more than a few seconds (long enough to get going).

of course, there is another problem. If the craft can generate lift force via aerodynamic lift, then they will generate more lift as they go faster. (Since the craft are able to glide, this must be the case.) this logically means that if they go fast enough, they should be able to fly.

to solve this, there are two possible solutions:

The craft cannot go fast enough to remain aloft entirely by aerodynamic lift

The thrusters power down when sufficient lift is generated

Either way, the ultimate solution is the same: These are racing hovercraft, and they were specifically designed to be unable to fly. They probably have automatic programming in their controls that prevent the pilots from achieving flight.

I imagine the organization running these races would have many rules and regulations regarding the machines used to ensure that the pilots cannot cheat and achieve powered flight mid-race.

You may also wish to incorporate design elements from modern day hovercrafts: https://www.explainthatstuff.com/hovercraft.html

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  • $\begingroup$ I wanted to avoid something where the track needs to be magnetized because that limits the races to man-made locations only, or at least areas that would need to be modified for race use. The kind of race tracks I have in mind are things like you would see in the old Star Wars: Racer game. The vehicle is already relying on the ground-effect for lift, meaning it's utilizing air that is being deflected into the ground (and back up from the ground if I am understand the ground-effect correctly) for lift. I don't see a reason the downward thrusters cannot also be relying on that as well. $\endgroup$
    – Arvex
    Jul 26 at 19:37
  • $\begingroup$ IRL hovercraft use extreme ground effect and have no where near the lift to fly. $\endgroup$ Jul 27 at 1:17
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    $\begingroup$ This isn't true. Thrusters can use ground effect to lift themselves one meter but not enough to fly. $\endgroup$
    – Thorne
    Jul 27 at 11:43
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Gusty, unpredictable winds

Your planet is filled with rocky outcrops, amazing cliffs, and steep hills. Not only does this make for interesting courses, it messes with the wind.

A subtle change in wind direction or strength results in wildly different observed local winds rapidly changing. The air is basically a criss-cross mess of sharp boundaries (known as wind sheer), making flight difficult. The exciting terrain means that the changes result in unpredictable sudden strong gusts.

Throw in a few downdrafts from strong thunderstorms (resulting in a rapid transition between head wind and tail wind) that produce stalls.

Your hovercrafts get blown around a bit at ground level, it just results in a fun ride.

When you start developing aerofoils and get a bit higher up, the sudden gusts results in destabilisation or outright stalls. Every experimental aeroplane gets smashed into the ground.

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No Vectored Thrust

First, you need a rule that hovercraft must only use fixed thrusters, in combination with the usual limits on total fuel capacity or other things. If you can't turn your thrusters, then it becomes important to maximize the amount of thrust you can apply to lateral movement, at the expense of vertical thrusters, by taking advantage of ground effects.

Then you need to design courses that winged craft are no good at -- courses with some squirrely banked turns, etc. That'll make your game more fun, too.

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You need to look at how real hovercraft work, as described in the answer by @Willk.

I'm going to add a bit on why winged craft won't work, except in very special places. In a word, obstacles. Ground effect is negligible when you're at a distance from the ground that's more than your wingspan, and not all that noticable (personal observation) above about 1/2 the wingspan. Trees, buildings, big rocks, even tall sagebrush, all interfere with your ability to get close enough to use ground effect, especially since you have to be moving fairly fast to get airborne at all.

So if you're going to fly any distance in ground effect, you need a flat surface. Either water (like an ekronoplan: ttps://en.wikipedia.org/wiki/Lun-class_ekranoplan ) or a desert playa, like this: https://blackrockdesert.org/about-the-black-rock-desert-playa/

(FWIW, I've flown across long stretches of it in ground effect, in a Piper Cherokee. It's doable, but if you want to turn more than a few degrees, you need to climb a bit first, otherwise you'd be dragging your wingtip in the dirt.)

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  • $\begingroup$ A vehicle which is designed for the purpose of hovering could exploit ground effect at a greater altitude than one designed for open-air flight, but if anything that would increase the ratio of power requirements for hovering one meter above the ground versus ten meters. $\endgroup$
    – supercat
    Jul 27 at 20:39
  • $\begingroup$ @supercat: Not really. If you're actually hovering - that is, staying above the ground with no forward motion - you have no ground effect at all. Ground effect increases with speed: in practice, with light aircraft it's not all that noticable (again, personal observation) until you're at about 3/4 of the speed where you'd start flying without it. And again, ground effect depends on wingspan: it' $\endgroup$
    – jamesqf
    Jul 27 at 22:16
  • $\begingroup$ When a hovercraft is hovering, air will be moving under the edges of the vehicle. The direction of such motion will be opposite under the front and back of the vehicle, and likewise under the left and right. $\endgroup$
    – supercat
    Jul 27 at 22:20
  • $\begingroup$ @supercat: Not really, because the strength of the ground effect depends on the wingspan, and is negligible at more than 1 wingspan above the ground. OTOH really long wings low to the ground will give strong ground effect. Anyone who's flown sailplanes knows how they seem to float endlessly down the runway unless you use the spoilers. $\endgroup$
    – jamesqf
    Jul 27 at 22:20
  • $\begingroup$ I was envisioning hovercraft that were at least 2m by 3m, if not bigger, and the entire bottom surface would effectively be "wing", so an altitude of 1m would be far below the span or chord of the wing. $\endgroup$
    – supercat
    Jul 28 at 1:20
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The point of building hovercraft is that you get the friction reduction of a flying vehicle, but with much lower power requirements due to still being able to push on the ground directly.

If you limit the power plant size to the correct range and have the race go over terrain where wheeled or tracked vehicles would have difficulty ( a mix of water, mud, and landmines maybe?) then hovercraft become the best option.

Note that even a wheeled vehicle starts to have a tendency to become airborne once you pass a certain speed. Racing vehicles have all kinds of design features to keep them on the ground at 150+mph. Your hovercraft will have the same problem. So if you don't want to just make it a rule of your race that the vehicle must be a hovercraft, then you'll need to design a course where a hovercraft is actually the best option. Which could prove challenging... There's kind of a reason they haven't caught on. Well, lots of reasons actually...

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I have a vision of a hovercraft equivalent to the evolution of America's Cup racers into these cool hyroplaning boats that can stay on the plane even when turning.

Say they start as a hovercraft but instead of heavy rubber flexible skirts have retractable, morphing semi-rigid skirts. As they speed up, ground effect with body shape comes into play. Call it air-surfing (because air-planing sounded too confusing.)

If you have to go back into skirted mode you are running a lot slower. There might be a long straightaway where they build up enough speed to get into air-surfing mode and tactics around being a spoiler causing someone to not make it. If not in surfing mode by the first turn, a bigger gap between racers opens up.

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This would be quite similar to Star Wars podracing. From what I've heard, in podracers, the repulsor engines aren't designed strong enough to fly like airplanes, only get off the ground. (i.e. they have a max height limit)

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    $\begingroup$ I'm trying to avoid using something comparable to the repulsorlift technology from Star Wars unless I can find something where it is scientifically plausible for it to exist. Repulsorlifts are essentially space magic when compared to the technology in my setting. $\endgroup$
    – Arvex
    Jul 27 at 3:10
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Especially for racing. I would suggest helium ballast, connected to a helium tank that is provided by the contest runners.

A computer with tamper proof safe guards, you tamper you auto lose, the computer controls how much helium is in the tank, and how much is in the ballast.

You would need a much larger ballast if you were going to try and be an aircraft.

Obviously you would forbid wings and rotors, thus anyone who attempted flight with helium would just be blown around in the wind. A helicopter style device relies on a rotor for direction control.

Therefore everyone would want to be close to the ground as to not be blown off course.

Then you can focus most if not all of your thruster engines on forward motion instead of vertical thrust.

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