Chapter 6 illustrates how to arrive at a preliminary aircraft configuration of a new project starting from scratch, with the expectancy of satisfying the market specification. To progress further, the next task is to lay out the undercarriage (also known as the landing gear) position relative to the aircraft CG, which is accurately established in Chapter 8. This chapter addresses the undercarriage quite extensively but not the detailed design; rather, it focuses on those aspects related to undercarriage layout and sizing during the conceptual study phase. More details on undercarriage design are in the cited references.
This chapter first introduces the undercarriage to serve vehicle ground handling, followed by basic definitions, terminologies, and information used in the design process and integration with an aircraft. Finally, methodologies for layout of the undercarriage and tire sizing are presented to complete the aircraft configuration generated thus far. Considerable attention is required to lay out the undercarriage position and to determine tire size and geometric details to avoid hazards during operation. This book limits the topic to the fundamentals to the extent of the requirements for positioning the undercarriage and sizing the wheels and tires. These fundamentals are shown schematically in the three-view aircraft drawings. Relevant information on wheel tires is also presented in this chapter.
The undercarriage is a complex and heavy item and, therefore, expensive to manufacture. It should be made right the first time. Aircraft designers should know the operational basics, leaving the details to those who specialize in the undercarriage as a system that is integrated with an aircraft as a subsystem. Aircraft designers consult with undercarriage specialists during the conceptual stage.
The location of the aircraft CG is important in laying out the undercarriage. Initially, the CG position is guessed from statistics and past experience. Once the basics of the undercarriage are explained, Chapter 8 addresses aircraft weight estimation and CG location. An iterative assessment follows to revise the undercarriage positioning due to the differences, between the guessed and estimated CG location. The final iteration occurs after the aircraft is sized in Chapter 11.
The undercarriage, as a major component, creates a considerable amount of drag in its extended position during flight. Therefore, its retraction within the aircraft mould lines is necessary to minimize drag. Evolution shows that early designs of a tail-dragging type of undercarriage virtually disappeared and have been replaced by the nose-wheel tricycle type. It is interesting that the first nose wheel – design undercarriage appeared in 1908 on a Curtiss aircraft. The blowout of tires during takeoff and landing is dangerous; the Concorde crash due to a tire bursting is extremely rare but designers must learn from that situation.
In the past, aircraft manufacturers handled the undercarriage design in a vertically integrated factory setup. Today, its complexity has created specialized organizations (e. g., Messier of France and Dowty of the United Kingdom) that are dedicated to undercarriage design, thereby making its management and integration more efficient and resulting in better designs. However, for smaller aircraft in the class of club and private use, manufacturers can make their own undercarriages, and most of them are of the fixed type.
7.1.1 What Is to Be Learned?
This chapter covers the following topics:
Introduction to the undercarriage as a system and its functions Types of undercarriage
Undercarriage layout relative to the CG, nomenclature, and definitions
Undercarriage retraction and stowage issues Undercarriage design drivers and considerations Undercarriage performance on the ground – turning of an aircraft
Types of wheel arrangements
Load on wheels, shock absorber, and deflection
Runway pavement types
Tire nomenclature, designation, and types
Tire friction with ground, rolling, and braking coefficients
Undercarriage layout methodologies
Undercarriage and tire data
7.1.2 Coursework Content
Readers will make a comprehensive layout of the nose wheel-type tricycle undercarriage and position it to fit the aircraft configured in Chapter 6. The first task is to ensure that the layout is safe and satisfies all of its functionality. The wheel and tire are then sized to complete the layout. This section requires computational work when the aircraft CG position is still unknown. The author recommends that readers prepare spreadsheets for repeated calculations because iterations will ensue after the CG is established and the aircraft is sized.
Figure 7.1. Antanov 225 (Mriya) main undercarriage
The undercarriage, also known as the landing gear, is an essential aircraft component for the following functions: (1) support the aircraft when in place or towed, (2) taxi and steer on the ground using an aircraft’s own power, (3) the takeoff run, and (4) landing and braking on the runway. For these reasons, the author prefers the term undercarriage rather than landing gear because the functions encompass more than mere landings. Once an aircraft is airborne, the undercarriage becomes redundant – an appendage that causes drag that can be minimized through retraction.
The undercarriage is seen as a subsystem consisting of a strong support spindle (i. e., strut) with a heavy-duty shock absorber to tackle heavy landings due to a rapid descent, whether inadvertently or on the short runway length of an aircraft – carrier ship. The undercarriage has a steering mechanism with shimmy control (i. e., control of dynamic instability; wheel oscillation about the support shaft and strut axis). The wheels have heavy-duty brakes that cause the temperature to reach high levels, resulting in a potential fire hazard. Heavy braking requires heavy-duty tires, which wear out quickly and are frequently replaced with new ones. Most undercarriages are designed to retract; the longer ones have articulated folding kinematics at retraction. The undercarriage retraction mechanism has hydraulic actuation; smaller aircraft may get by with an electrical motor drive.
The undercarriage is a complex system – the main undercarriage of the world’s largest aircraft (i. e., Antanov 225) is shown in Figure 7.1 (note the relative size of the people in the photograph). It is a bogey system (see Section 7.3) carrying 7 struts (i. e., support shafts with shock absorbers) per side, each carrying 2 wheels for a total of 32 wheels when the 4 nose wheels are added (2 x 2 x 7 + 4 = 32).
The undercarriage stowage bay within the aircraft is compactly sized to the extent that articulation allows. The stowage bay is located in the wing and/or the fuselage, or sometimes in the wing-mounted nacelles, depending on the realistic details of the design considered by aircraft designers at the conceptual stage. It is a challenging task for structural designers to establish a satisfactory design that integrates all the relationships and functionality of the undercarriage with the airframe. The author recommends keeping the undercarriage layout design as simple as possible for better reliability and maintainability without using too much of the articulation and/or stowage space in an aircraft. Reference 7.4 provides more details.
2-point 3-point З-point 4-point 5-point
(bicycle) (tailwheel) (tricycle)
Harrier Piper Cub Learjet45 A380
Chart 7.1. Undercarriage types (land-based)
A large aircraft is heavy enough to damage a metal runway; therefore, its weight is distributed over many wheels on a bogey system, which itself has articulation for retraction. The undercarriage mass can encompass as much as 7% (typically 4 to 5%) of the MTOM for large aircraft, it can weigh up to 3 tons with a corresponding cost of up to 5% of the aircraft total price, and the drag can be 10 to 20% of the total aircraft drag, depending on the size – smaller aircraft have a higher percentage of drag. For small, low-speed aircraft with a low-cost fixed undercarriage without a streamlined shroud, the drag could be as high as nearly a third of the total aircraft drag.
The undercarriage design should be based on the most critical configuration in the family of derivative aircraft offered. Generally, it is the longest one and therefore the heaviest, requiring the longest strut to clear the aft fuselage at maximum rotation. For the smaller version of the family, minor modifications assist in weight savings, yet retain a considerable amount of component commonality that reduces cost. In general, tires are the same size for all variants.
Other special types of undercarriages are not addressed herein. Today, all “flying boats” are amphibians with a retractable undercarriage. Undercarriage types are classified in the next section. Section 7.15 provides statistics. The Harrier VTOL/STOL and B52 aircraft have a bicycle-type undercarriage. These are difficult decisions for designers because there are no easier options other than the bicycle type, which requires an outrigger support wheel to prevent the wing from tipping at the sides. Aircraft with skids are intended for application on snow (the skids are mounted on or replace the wheels) or for gliders operating on grass fields. Some “tail-draggers” get by with using a skid instead of a tail wheel. Special designs use takeoff carts to get airborne; however, landing is another matter.