Channel 4 – Scrapheap Challenge 2004 – Challenges – Flying Cars

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Science

General science

Designs

The Thirsty Knights used the car engine to power their propeller. In both cases, the priority was for a light-weight frame

To traverse the slalom, the vehicles needed to be able to turn quickly. Both teams used a front castor wheel to provide maximum maneuverability

General science

Three forces were acting on the flying cars. The thrust from the propeller pushed forward, the drag from the shape of the car pushed backwards and the weight of the car pushed downwards as friction. For the car to move, the propeller had to produce a big enough thrust to overcome the weight and the drag.

Propellers

A propeller is a spinning wing. As it spins, a pressure difference is created across the blade. The pressure difference causes large amounts of air to be accelerated through the plane of the propeller and the reaction of the vehicle to this motion generates a force called thrust. The thrust pushes the aircraft forward in accordance with Newton's third law of motion: 'for every action, there is an equal and opposite reaction'. That means if you chuck air backwards, you go forwards.

Gearing

A propeller has an optimum 'rpm range' within which it will produce the maximum thrust. In order to have the propeller rotating within this range, it would be necessary for the teams to build a reduction drive between the engine and the propeller.

The teams needed to use gears to match the speed of the engine's rotation to the optimum speed of the propeller's rotation. If the propeller rotated faster than its maximum speed, it would explode!

Steering

The Thirsty Knights steered with a rudder. In an aircraft, there are three terms to describe movement: pitch, roll and yaw.

Pitch is the movement of the nose up and down
Roll is the tipping of the plane from side to side
Yaw is the movement of the nose left and right

The first two are controlled by altering the flaps on the wings and are only needed for a plane. Yaw, the left/right motion, is controlled by a rudder. The rudder acts by directing the flow of air to either side of the rear of the plane, which pushes the nose of the plane in the opposite direction. In this instance, the only thing our team had to worry about was the yaw.

The Fireblades steered with a bow thruster and clamshells. These sit around the ducted fan and when they are fully open, the air flow is straight out, hence the movement of the vehicle is straight ahead. By closing the left shell, the flow of air is diverted to the right and the vehicle will tend to the left. By closing the clamshells, the air is directed to either side of the thruster, back towards the front of the car. This acts as a breaking mechanism and potentially a reverse.

The bow thruster is a small fan on the front of the car which will reinforce the left/right movement by pushing the front end of the car around. The Fireblades made this from a lorry cooling fan which was run forwards or backwards to achieve a left/right motion at the front of the car. The back was moved by the clamshells.

The Fireblades used a rear-fitted, fixed-position propeller with fins to direct the air-flow and steer the vehicle

The Knights rotating fan provided them with excellent maneuverability, which gave them the edge in the slalom

Designs

The Fireblades

The Fireblades' expert, John, is one of the foremost hovercraft experts in the UK. He's spent his entire life building everything from one-man racers to huge passenger and military craft.

The team built an airboat – the sort of thing you would see on the Mississippi delta, but with casters underneath. With such a big airplane propeller, it was nice and fast, but as it was only allowed to steer through air displacement, it found it difficult to weave its way through the Scrapheap course.

The Thirsty Knights

Billy, The Thirsty Knights' expert, is a microlight genius. He and The Knights made a cross between a Robin Reliant and a Harrier Hawker. A large industrial fan was attached to the back, which rotated to steer the madcap machine. It was small and manoeuvrable, but was a bit slow with all those bikers on board.