Safe Landings

Departments - Features

Redesigned landing gear looks to deliver increased safety, and lower the cost of travel.

March 27, 2011
Elizabeth Engler Modic

Figure 1: The Crank Shock Strut is a new concept for aircraft landing gear.Aircrafts take off and land with the lift given by forward speed. This results in the undercarriage receiving two shocks of forward and descent directions upon landing. However, the current shock struts of landing gear – known as an oleo strut – only absorb one shock, which is in the descent direction. In simple terms, this means that when the aircraft wheels touch down on the runway at a high rate of speed, great force from the forward direction suddenly pushes on the tires and landing gear. The oleo strut is unable to buffer this forward shock, which results in smoking and severe wear of the aircraft tires – not to mention those periodic rough landings passengers feel. That severe wear and tear on the tires adds considerably to the maintenance and transportation costs in the airline industry.

Koji Yoshioka, CEO, SUS21 Co. Inc., has been working on an answer to address smooth and safe landings, while reducing tire maintenance expenses.

The new concept is a shock strut, as shown in Figure 1, which aims to solve these challenges. The first thing to note is that a tube replaces the piston-cylinder used in the current systems. This tube design realizes safety improvements, a weight reduction of 40%, and a 50% reduction in the number of tire exchanges required. In addition, it is not necessary to check the leak from high-pressure gas and oil. In this way, a new type of landing gear promises safer and lower-cost travel.

Challenges to Solve
Although current systems on aircrafts can pass smoothly over irregularities in road surfaces, compression, and expansion of the spring generates heat by the hydraulic damper, but this energy goes to waste. Yoshioka works to address the challenges concerning safety, cost reduction, and maximize component utilization. 

He questions, and delivers his answer about whether the aircraft builders should:

  • Get better safety on taking off and landing by an integral control of wings and struts like a bird;
  • Quit using heavy metal in the shock strut of landing gear, as plastic material has reduced the fuselage weight; and
  • Work to reduce the number of tire exchanges.

Adoption of disk brake
The Oleo shock strut is a main device of the present landing gear. The oleo strut is an excellent strut used for about 80 years. However, the response of the oleo shock struts is slower than that of wings, because the damping force of the shock strut depends on oil flow. Cost also increases with the oleo strut, because it undergoes large bending on landing and sudden braking, resulting in weight increase to strengthen the structure.

Two tubes in the oleo strut grind against each other, expanding and contracting under the impact bending. To counter this, shock struts are manufactured from heavy metal, not light carbon fiber reinforced plastic – which increases weight and burns more gas. Most people have seen this action when watching a plane land, seeing the tires smoke while the rubber attaches to the runway. Just after landing, the tires undergo excessive wear again due to sudden braking. Therefore, tires need to be replaced frequently in contrast to cars. The expenditure involved with this work raises maintenance and transportation costs. As a result, the oleo shock strut increases in cost.

When driving a car, the moment we say “Watch out,” a disk brake can respond immediately. Basically, a disk brake and an oil damper have the same functions as energy absorbers. Therefore, Yoshioka decided to take this characteristic of disk brake to work it into shock struts. However, that decision had technological limitations.

Figure 2: Basic model of suspension.The struts of vehicles are made up of suspension systems. The basic model of the present suspension system is shown as a simple structure shown in Figure 2. Highlighting the technological limitation that blocks the adoption of disk brake is as follows:

  • Left: Vehicle body block 1 is supported by wheel 2 through spring 3;
  • Central: This model travels. When traveling over a projection on the road, the spring is compressed to reduce an up thrust; and
  • Right: The compressed spring generates a vibration upon rebounding.

A hydraulic damper, which is not shown, absorbs this vibration energy.

The present suspensions based on this model, include airplanes, cars, trains, and agricultural vehicles etc. As shown in the figure, a disk brake to stop rotation cannot be adopted into this model because the system has linear motion. This is the technological limitation.

Figure 3: Taxiing or rotary type of model.Rotary Type Model
To date, there has not been a rotary type model to adopt a disk brake. Yoshioka worked to create that model, paying attention to the pendulum, which rotates and vibrates left and right.

When hearing pendulum, most people think of a pendulum that swings from side to side around the static pivot. However, Yoshioka saw a pendulum that would become a movable member by using a wheel. In two steps, shown in Figure 3, is a pendulum that is a rotary type model.

The diagram shows, left to right, that a pendulum suspends from a fixed pivot. The restoring force of the pendulum causes it to oscillate about the equilibrium position, swinging back and forth like a pendulum clock or a swing in the park. This action is based on gravity spring action. Next in the figure is that an axle of a wheel replaces the pivot, so the pendulum can travel when the wheel is driven. The pendulum swings counterclockwise when the wheel strikes an obstacle on the road. Finally, the weight of a vehicle body, expressed as a block, is applied to the lower end of the pendulum by its strut and the joint part of the pendulum, allowing the strut to rotate. This structure results in a rotary type model in order to adopt a disk brake from a pendulum.

Landing of Rotary Type
Airplanes take off and land with the lift given by forward speed. The wheels receive two shocks in the forward and descent directions on landing. A look at Figure 4 helps to explain the behavior of the wheel installed in the rotary type model.

Figure 4: Here is the behavior of the wheel installed in the rotary type model, upon landing.

The wheel contacts the runway at point A. While the aircraft descends, the crank element rotates counterclockwise, at the same time making the wheel move backward. The rotation of the crank element involves the descent motion of fuselage and the backward motion of wheel. This results in the absorption of the two shocks in the descent and forward directions. Disk brake absorbs the energy while the crank element rotates. In this way, the rotary model can absorb the two shocks, what was previously an unknown characteristic.

Figure 5: Creation of rotary type model in suspension.Taxiing of Rotary Type
On taxiing, the behavior of the rotary type model is different from the model of the present airplane. Figure 5 demonstrates the type of taxiing seen in this rotary type model. Of importance in understanding the redesign, Yoshioka suggests to take notice of action and return, keeping in mind the deflection and restoring of the spring shown in Figure 2.

As seen on the left of the diagram, when taxiing on a flat runway, the pendulum stands vertically due to the weight of fuselage block. Seen next is the action during taxiing, where the pendulum rotates counterclockwise when taxiing over an obstacle. Comparing with the state where the pendulum is fixed, the displacement of the pendulum in the downward direction is shown as ΔH and the displacement in the backward is shown as ΔL. The ΔH generates spring action in the up and down direction while the ΔL generates spring action in the back and forth direction.

Return to the original status (right image of the diagram), after taxiing over an obstacle, shows that the pendulum returns by the weight of the fuselage block.

The characteristic of the rotary type model absorbs the two shocks in the horizontal and vertical directions. As Yoshioka alludes to earlier, the wheel undergoes two shocks of rolling resistance and vertical displacement when taxiing, so Yoshioka’s answer is with the rotary type model.

Figure 6: As seen in the diagram to the left, the crank element of the Crank Shock Strut design by Koji Yoshioka is a pendulum, where two wheels are supported by the crankshaft.Figure 6: Crank Shock Strut
Coming up with this rotary design, Yoshioka has two kinds of suspension model to compare and contrast. Spring and damper, which are basic components in the system, are compared with the present linear type model and the new rotary type model shown, as shown in Figure 6. 

In the spring design, the linear type model requires an elastic spring, including air spring, coil spring, etc. In contrast, the rotary type model does not require an elastic spring because of the generation of spring due to gravity. The linear type model can only respond to up and down shocks. The rotary type model can respond to both back and forth, and up and down shocks.

For the damper, the response of the linear type model is slow and the amount of force is not always controllable due to the flow of oil. In contrast, the response of the rotary type model is fast and the amount of force is arbitrary with the help of a disk brake. Yoshioka feels that the basic properties of the spring and damper in the rotary model are superior to the present linear model.

Manufacturing of the new suspension device is simple, yet offers elements not seen in current technology. However, those elements are what Yoshioka feels will improve safety and lower cost.

For the crank element, the pendulum in the rotary type model is made of the same strength as that of the crankshaft. Since the crank element plays the role of a pendulum, it needs strength to generate as it generates gravity through its spring action.

Then there is the installation of two disks on the crank element, which supports two wheels, and two calipers at the lower ends of the strut. Brake pads inside the caliper are then pressed on the disks to control the rotation of the crank element – Yoshioka names this the disk brake damper. The entire package, called the Crank Shock Strut, is actually a simple structure when viewed part by part.

A look at the main gear and the nose gear, the main landing gear in the new model airplanes is composed of two kinds of disk brakes. One is a conventional main disk brake to stop the rotation of a main wheel. The other is a sub disk brake that acts as a damper to control the rotation of the crank element. The new nose landing gear only has a disk brake damper.

Challenges Considered
Throughout his research, Yoshioka has proven that the new type of landing gear can contribute to safer and cheaper travel.

Some main challenges Yoshioka was able to solve or address, involved the responses at high speed to the optimum value calculated by computer to deal with turbulence; the reduction of heat by more than 69% on the tires; and the reduction of tire wear due to the wheel movement back and forth against the fuselage.

Yoshioka also determines that due to the rotary type model, the wheel moved in a half circle, causing the disk brake damper to absorb the descending energy, creating a safer, smoother landing. The other question Yoshioka considered was the issue of the movable quantity of the wheel, questioning is it was too small. According to Yoshioka’s measurements, the length of a crank element is equivalent to the radius of a circle. When its radius is 20cm, the diameter is 40cm. The length of a half circle is 62.8cm, which is 1.57 times of the diameter. Because of this, the concept is able to obtain enough quantity.

Leaping Ahead
Eighty years have passed since Mr. Oleo invented the Oleo shock strut. Times have changed during this period and technology improvements have been amazing. Yoshioka feels that his design of the Crank Shock Strut has been long in the making and that the time is right to accept this technology.

Completion of computer simulations on the shock isolating – through the input of standard data – enables Yoshioka to indicate his concept will provide a smoother, safer, and less expensive aircraft, without the traditional smoke-filled landings.

SUS21 Co. Inc.
NARA City, Japan