Aircraft Landing Strut

Question 1

List the main functions of an Aircraft Landing Gear Strut.

Answer 1

• Support the weight of the aircraft.
• Withstand and absorb the kinetic energy and vibrations from the impact of landing and dissipate loads to the airframe to reduce airframe stresses.
• Be able to perform under heavy load conditions and endure thousands of landing cycles (long service life with minimum maintenance).
• Withstand temperatures of -50 degrees C.
• Minimise weight and volume.
• Design must account for requirements in strength, stability, stiffness, damping, fatigue, weight and volume.

Question 2

What is the most suited material for this application.

Answer 2

Ti1023 is the best material for this component.

It is a beta phase Ti alloy meaning that it has a body centred cubic structure which gives it superior strength and toughness.

It has good thermal properties (up to 300 degrees C), excellent corrosion resistance however it can be susceptible to salt stress corrosion cracking. It also has excellent fatigue properties.

Question 3

Why is Steel not used for this application?

Answer 3

Steel alloys were previously used. It is much stiffer than titanium; however, it has a much lower strength and cannot withstand the same loads before failure. It also has a much lower specific strength which increases operational costs.

Question 4

Why is Aluminium not used for this application?

Answer 4

Aluminium alloys were also used due to their high specific strength; however, they are susceptible to fatigue failure and they build up of stress concentrations.

Question 5

Describe the first manufacturing process used to make an aircraft landing strut.

Answer 5

Closed Die Forging:

• Material is heated to 900 degrees C. This is within the B phase region so the material keeps its B phase structure (important for retaining its high toughness and fatigue resistance).
• Dies are designed to allow for excess material to be machined after. The moulds repeatedly move towards one another, forcing the material to fill the shape of the mould. They are designed so that the required machining is kept to a minimum, which reduces time, cost and material waste.
• Forging has the additional benefits of maintaining grain continuity. The directional grain flow results in significantly stronger parts.

Question 6

Describe the second manufacturing process used to make an aircraft landing strut.

Answer 6

Machining:

• This is used to achieve the details of the finished parts. It is difficult as the material is extremely hard.
• A combination high precision boring (the inner cylinder), grinding and milling are used to achieve the desired details.
• Cutting tools experience significant wear due to the toughness and thermal conductivity of the material. Steel transfers 15% of heat to the cutting tool, 60% is transferred when cutting Titanium.
• High pressure coolants are used to extend the tool’s life and improve the finish. This results in significant cost.

Question 7

Give two suitable inspection methods.

Answer 7

Visual inspection:

Surface is inspected by highly skilled technicians (slow and costly). It is good for detecting larger defects, but cannot detect smaller defects/internal damage/microstructure weaknesses.

Eddy Current testing is also suitable.