Braking Systems in Microlight Air Planes

Table of Contents

1. General requirements

2. Design of braking systems
2.1 Inducing the force into the system
2.2 Power transmission
2.3 Producing the braking force
2.3.1 Drum brake
2.3.2 Disc brake

3. Describing a braking process
3.1 Necessary deceleration:
3.2 Braking forces
3.3 Forces on the brake
3.4 Necessary friction between plane and runway

Bibliography

Braking Systems in Microlight Air Planes

One approach for an improvement to microlight aircraft could be a change in the braking systems that are used. In order to understand where improvements can be made or what restrictions actually exist, it is necessary to have a closer look at the general requirements for all systems that could be used in microlight air planes.

1. General requirements

The main problem in every system used in aerospace industry is the reliability. Every system relevant to safety requires in-built redundancies. In other words, when one system fails, another one ensuring no risk to safety appears. Microlight regulations are not as strict as they are for other types of planes. This means that this redundancy cannot be found in all applications. It seems that keeping the weight of the microlight down, is sometimes considered to be more important than the risk of a failure. On the other hand, this means that a one circuit system must be designed very carefully so that a system failure is highly unlikely.

As the weight of a microlight is limited to 300 kg for a single seater and 450 kg for a two seater plane, weight reduction is a very serious matter for every change in the design. A significant weight reduction would increase the range of the plane and would eventually allow for the use of other applications in the plane for possible commercial use in the future.

The current design of the cockpit in an ultralight trike, the special kind of microlight that we try to improve, as it is now, does not allow for any seat adjustment that would allow people with a body height above or below average to use it comfortably. As almost every existing braking system uses foot pedals, it is impossible for some disabled people who do not have use of their legs to fly this type of plane. One aim of our research work is to find a better way to integrate braking applications into the cockpit of a microlight.

2. Design of braking systems

To understand how braking systems work and to see where the biggest improvements can be made, it is helpful to split the braking system into three major parts. The way the braking force is induced into the system, the way the power is transmitted and the brake itself.

2.1 Inducing the force into the system

Two major ways to actuate a brake exist. The first and most common way is the use of a pedal which is moved by the foot. The big advantage of actuating a braking system in this way is that a large force can be generated by the leg. According to ECE R13 which contains most of the regulations for car brakes, the maximum braking force generated by the foot must not be higher than 500 N. The average braking force should not be higher than 150 N.

The second way to actuate a brake is by hand. A common way is using a handbrake similar to the ones found on bicycles. Design-wise, this system is more flexible than braking systems that use feet, but the maximum braking forces that can be generated by hand are of course less. The DIN 79100 defines, that the maximum braking force for bikes must not exceed 180 N. This value should only be used for an emergency brake. In normal use, forces of about 50 – 80 N should be suitable.

The values for both systems are of course taken out of standards for different applications, but should be very similar to the forces that are used in microlight airplanes.

2.2 Power transmission

The most common way to transmit the braking force to the brake is probably by a bowden wire. This system is particularly robust and easy to repair. The necessary forces needed to use the brake are caused by friction between the inner wire and the coating and will be much higher than they would be without friction losses. As the friction increases with the number of bends in the wire, the flexibility of this technique is quite limited. Furthermore, the friction is dependent on the operating time which makes regular maintenance necessary.

Another problem of using a bowden wire to transmit the braking force is that this system is not very precise. On one hand, the large amount of friction does not allow good feedback of the brake; and on the other hand, the changing temperature has a large influence on the length of the wire.

One way to reduce some of the problems above might be the use of different materials. Using a nylon wire instead of a steel wire for example could bring a significant reduction of the friction coefficient. However, most polymer materials have a very low Y-Module, which would cause a huge elongation when braking, resulting in reduced accuracy. Compound may might be a solution for this problem, but there are still very few manufacturers that make bowden wires out of advanced materials.

Another approach to transmitting the forces needed to activate the brake is by a hydraulic system. This way is more flexible than mechanical transmission as bends in the pipe cause no additional friction. Even sharp bends of the conduit can be handled without any problems. As only a small amount of friction appears in a hydraulic braking system, less braking forces have to be generated than in a mechanical system, and the pilot has a much better feedback from the brake.

One big advantage of using a hydraulic system in connection with a disc brake is that it is very easy to bring the brake back into its home position and do the necessary readjustment caused by the abrasion of the brake callipers.

The standard and most easy way to solve these problems in a mechanical braking system is by using a spring that brings the brake back into its home position and a saw tooth profile with an elongated hole that does the readjustment (Picture 1, left drawing).

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Picture 1: Adjustment of a brake

By using a hydraulic disc brake both problems can be solved easily by using a rectangular ring for the sealing of the braking cylinder. If the ring is set under camber by choosing a smaller inner diameter than the outer diameter of the braking cylinder, it will act like a proportional spring which moves the cylinder back into its home position. Of course there are also a few disadvantages of using hydraulic transmission. The components that have to be used for hydraulic transmission are much more expensive than those used in mechanical transmission. To avoid corrosion within the system most braking systems use mineral oil as hydraulic fluid. If the system leaks, these fluids may cause environmental damage. Furthermore, a regular change of the fluid is necessary resulting in higher running costs and disposal problems.

To avoid these problems pneumatic systems can be used. These use compressed air instead of liquid. As pressures in the pneumatic systems are less, the pneumatic components are much cheaper and lighter than for hydraulics. This seems to be perfect for the use in microlight aircraft. However, when comparing the complete system, it becomes obvious that the pneumatic system is more complicated than a hydraulic system.

A pneumatic braking system would usually consist of the following parts:

- Compressor (1)
- Air dryer (2)
- Relief valve (3)
- Compressed air sore (4)
- Control valve (5)
- Actuator (6)

illustration not visible in this excerpt

Picture 2: Pneumatic braking system

It should be obvious that this system may be useful for some applications, but it is far to complex and heavy to be installed in a microlight airplane.

An electronic braking system would probably give the largest amount of flexibility in cockpit design. One large advantage is that only signals need to be transmitted and the actual braking force is produced where it is needed. It is possible to reach almost every lever ratio between input and output force which would bring great advantages for disabled pilots as they will be able to activate the brakes easily. Furthermore, no environmentally hazardous fluids, rather heavy hydraulic pipes or mechanical components are needed.

At first glance, these advantages make an electrical braking system look like the perfect solution. However, the reason why electronic braking systems are not used in more applications, is that the brake itself is particularly heavy. Today, electronic brakes are realized by using an electrical motor that drives the brake callipers by using a gear box. This makes the whole brake a lot heavier than a hydraulic brake. However, it is not only the brake that is heavy. To produce a sufficient force and to make the brake respond quickly enough, a 42 V energy supply is needed. Today, almost every standard on board supply system works on 12 / 14 V. It would be a lot heavier and more expensive to use a 42 V system in a microlight as a bigger battery would be needed and less standard components could be used.

Electrical braking systems also have problems reaching the standards of demanded reliability request in aerospace applications. A larger effort is needed to obtain the same reliability that can be reached with less difficulty with the use of hydraulic systems.

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