Hover Blade

**Hover Blades** are a type of movements in **Robocraft**.

## Overview

Hover blades, an advanced ducted fan assembly, are used to create a robot that hovers above the ground. Robots that use Hover blades are generally called 'hovercraft' or 'hover'. Hovercraft are less restricted by terrain, such as ice, steep inclines, and harsh pits. While hover blades are ideal for creating highly mobile robot designs, they can be challenging to design and pilot successfully.

Real world hovercraft lift themselves off the ground by inflating a skirt or plenum underneath their chassis with air, while Robocraft hovercraft use hover blades to provide lift and turning forces more like the ducted fans used in many RC drones and small UAVs.

The hover height of Hovercraft is variable and controllable. Hovercraft can ascend up about 30 feet from the ground (enough for most ground based robots) and descend to the ground however they will still move slightly unless a flat base is provided.

## Operational Ceiling

An ascending hovercraft will reach a vertical distance from the surface they are travelling over where hover blades no longer produce any thrust at all, this is the **operational ceiling**.

When a hovercraft exceeds its operational ceiling the loss of thrust means that the hover blades not only stop providing any thrust, they also stop providing lift, attitude control, and are unable to keep the hovercraft level. This causes the hovercraft to shake, shimmy, roll, and/or drift and generally become unresponsive. Having secondary movement systems can help maintain control and stability.

This loss of thrust can be observed when hovering quickly down a steep slope, dropping over a cliff or ledge, skimming over deep dips in the terrain, or when sledding up a steep slope and becoming airborne.

The operational ceiling does nothing to prevent the hover blades from interpreting the *terrain* far below them; the hover blades will continue to stabilize based upon the ground's dips and rises though it will only create instability. That is to say, any *Aircraft*, *Helicopter*, or *Drone* that attempts to use hover blades will spin wildly when it goes over an uneven surface as the hover blades will attempt to stabilize themselves against the truly non-existent fall or rise. This is especially notable when climbing cliffs. Only flying craft that are designed around the hover blades as *landing gear* should attempt to use them.

### Design Solutions

* Use large rear thrusters and power through it.
* Use a couple of lower tier fans which seem to have a lower ceiling, which stops a hovercraft ascending over it's operational ceiling.
* Put the fans into an under and over configuration, so when the top fans cut out, the bottom fans still work.
* Use a few Helium blocks above the centre of gravity of your robot for better resistance to flips and overall stability.
* Place fans in a 'T' shape (total minimum of 3 fans with 2 at the front).

## Advantages and disadvantages

Pros
* Independent of terrain (ice, slopes)
* Fast and highly manoeuvrable (capable of doing swift turns in place).
* Excellent for *Plasma Launcher* *tanks* and *speeders*.
* Adjustable flight height. Can "pop up" from behind low cover to attack/spot enemies, then drop back down to avoid return fire.
* Will fly even if flipped (albeit with left/right controls reversed).
* Redundant blades retain mobility even if some blades are destroyed (depending on placement).
* High speed and hovering above the ground makes them difficult to hit with plasma launchers and avoids its AoE damage.

Cons
* Like *helicopters*, Hovercraft inertially drift when idle. This instability makes them a poor choice for railgun platforms unless a secondary mobility system (e.g. Wheels or Legs) is used as an anchor for the craft.
* Loses pitch/yaw/roll control once blades are above operating altitude; hovercraft can very easily roll and become inverted when making jumps.
* Oversteering: hover blades do not automatically dampen turning forces, especially when stationary.
* Losing a blade can cause balance loss, depending on the build.
* Requires practice to use effectively.
* Limited to weight, no "overloading".
* Hover blades like to flip when they can, they think they are stabilizing you, however this can cause them to think upside down really is right side up. This lead to them flipping
	+ When they are flipped their controls for steering are inverted, making them awkward

## Placement

Hover blades need to be balanced around the true Center Of Mass and to provide balanced lift. Different placements on the same blocks can lead to bots with quite different balance and control, so some experimentation is required to make hovercraft usable.

It is often recommended that hover blades be placed as high as possible to prevent flipping over. However, the Robocraft physics model is sufficiently realistic that it **does not** suffer from the ***pendulum rocket fallacy***, which means that putting hover blades at the top of tall bots could make them more stable.

The wider the placement of hover blades around a robot the more resistant a craft will be to rolling sideways, but this can also make a hovercraft more difficult to right again. Pitching forwards and backwards in a hovercraft has less to do with hover blade placement (they actively try and keep a hovercraft on an even keel) and more to do with where the centre of mass is in relation to the placement of the hover blades. Ballast and floats (Helium cubes) can be used to redistribute weight and tune the balance of a hovercraft.

All hover blades are 1 cube high, and there is no restrictions on cubes placed above and below them, provided they don't extend into the hover blade.

### Notes on Placing Hover blades

* Hover blades that are placed below the centre line (below the level of the centre of mass) can make a hovercraft that unstable and is difficult to right when flipped.
* Hover blades on the centre line (at the level of the centre of mass) are moderately stable, but easily flipped and easily righted.
* Hover blades placed above the centre line make a hovercraft stable and difficult to flip, but are not always easily righted.
* A mix of hover blades at, above, and below the centre line can have a significant effect on a bot's stability, manoeuvrability, and ability to right itself.
* Hover blades placed further away from from the main body can provide significant turning force, like the tail rotor of a helicopter.
* Hover blades placed further away from from the main body can improve stability, but may make righting a flipped hovercraft more difficult.
* Placing your hover blades require a bit of trial and error, it changes for every bot!

### Design Notes on Hovercraft

* Due to the hovercrafts' nature to flip when moving over rough terrain you may find yourself upside down more often than other bots.
* This becomes potent when a hovercraft has most of the mass or weight above the hover blades.
* Building the craft from the top down, starting with the hover blades is one way to make sure that you are 'bottom heavy' and not 'top heavy'.
* Also if you build a craft that is easy to flip try adding 10 or 20 additional chassis cubes on the bottom, or couple of Helium at the top. This will help your bot not flip and let you drive it off cliffs or over uneven terrain.

## Movement Combinations

Hoverblades work in combination with other propulsion systems.

* Wheels provide more speed and some traction while in contact with the ground, and can stabilize a hovercraft when aiming down sights, however they are dead mass while hovering.
* Thrusters provide significant acceleration with Hover blades and make good *speeders*.
* Using Aerofoils and Rudders to try and fly do not work well with hover blades. The hover blades counteract pitch and yaw controls, reducing manoeuvrability at both high and low speeds, once an aircraft exceeds the operational ceiling of the hover blades, the hover blades provide no thrust or attitude control. Horizontal aerofoils and rudders (wings, canards, etc.) can also cause the craft to roll excessively when making low-speed turns.
* A Rudder mounted on the back and facing up or down will cause the hovercraft to straighten up when coming out of a turn, resulting in much better handling. When not used the craft will continue to spin after the turning button has been released, when still moving forward with Thrusters this can be difficult to control. The rudder will provide air resistance and slightly reduce your speed and overall turning.

## Maneuvering

### Righting Maneuvers

While piloting an inverted hovercraft is possible, the inverted steering is a challenge. The following maneuvers may help:

* Sled up a steep hill and let the hovercraft flip front over end
* At speed clip an obstacle (terrain or a bot) with one side of the hovercraft to cause the bot to roll
* When a bot is rolling or flipping apply downward thrust to stabilize the hovercraft

## Statistics

### Additional Statistics

* All hoverblades have a 472.5 Base Health/CPU relationship, except for Hurricane, which has 756
* All hoverblades have a 2.5 Mass/CPU relationship, except for Hurricane, which has 5

## Trivia

* Hoverblades work by pushing air down, however in the game you do not need a clear space above/under the hoverblade.
* In early game versions, it was possible to achieve high speeds by placing hoverblades vertically. This phenomenon is called warping, which no longer exists for hover blades since they can't be placed in this manner anymore.

## External Links

* Aethylred has a YouTube video tutorial for constructing the base chassis of a Whale classed *speeder* that also demonstrates how to correct oversteer by increasing the *mass* of a hovercraft.