Landing+Gear+2012

Questions and issues regarding this page can be addressed to Alfie Tham.

Goal: To provide stability when vehicle is stationary such that pilots can quickly enter and exit the vehicle.

Priorities: - Simple to implement, maintain and manufacture - Reliable and robust - Easy to use - High stability - Adjustable lengths for versatility on slopes - Lightweight - Should not fail or fracture under shock loads

Constraints: - Must not fail under normal static loading - Space constraint within vehicle
 * o Must not hinder other vehicular or maintenance functions (i.e. brakes, wheel removal)
 * o Must not obstruct pilot (vision, seating, etc.)
 * o Must not compromise vehicle’s structures

Brainstormed Ideas Landing gear unit: - Vertical axle – wheels rotate out - Telescoping – conventional slider - Telescoping – slider with turning pair - Single pivoting arm – slightly tilted axle - Four-bar mechanism with coupler point - Bicycle kick-stand Retraction mechanism: - Springs – center spring to reduce bending moment - Geared rope pulleys - Lever system Hatchways: - Latex/rubber openings Locking mechanisms: - Locking mechanisms in foldable tables - Rock-climbing 1-way pulley as locking mechanism - Ratchet system

Other investigations Cannondale Lefty The use of the Cannondale’s Lefty fork was investigated. The Lefty uses linear needle bearings for a smooth slide even if under large sideways forces. It was also built such that relative rotational motion is restricted. This idea was scrapped because the extension length is too short. It is also probably heavy.

Static Stability Analysis Basis: A vehicle is statically stable when its center of mass (CoM) is within the base of the vehicle. This ‘base’ is defined by its points of contact on the ground. In this report, the ‘base’ will be referred to as the ‘stability polygon’. The vehicle is statically unstable when its CoM moves outside of its base. A vehicle is perceived by users as having low stability when its CoM is very sensitive relative to its base size (i.e. volatile CoM, small base or both).

Characterizing Stability: The landing gears were developed for optimized stability by employing an analytical model as well as verification with experimental testing. The goal of optimizing the landing gears’ stability is to provide the required functionality while meeting design objectives, namely spatial and weight constraints. In this project, static stability was characterized by analyzing the smallest distance that the CoM has to shift before the vehicle is unstable; when the CoM moves outside the stability polygon. With CoM shift distance as a measurable metric, the next step is to experimentally determine the CoM shift distance for use in design. The minimum CoM shift distance required by a rider to comfortably enter and exit the vehicle was determined through experimentation and testing.

Rough Initial Test: Our 2011 vehicle, Vortex, was used as a mock vehicle for this experiment and is a geometrically reasonable approximation to the 2012 vehicle. The mock vehicle was constrained to always be vertical and stationary by applying an external force as shown in Figure 1. Applying this constraint mimics the function and behaviour of an ideal landing gear. A rider then enters and exits the vehicle to simulate normal use conditions. The maximum external force exerted was measured and recorded. Figure 1. Location of external force applied on the vehicle Figure 2 shows the free-body diagram of a snapshot in time when the CoM shifted the furthest away from its origin. This is also when the maximum external force is required. Figure 2. Free-body diagram Since the vehicle is static throughout the experiment, summing of moments about point C equals to zero allows for CoM shift distance, ‘d’ to be solved. The CoM shift distance involved was found to be approximately 5cm.

Approximating the center of mass while stationary: The vehicle’s CoM while the pilot is in a stationary sitting mode was approximated by measuring the total weight distribution on the front and back wheels. With the total weight and distance between the front and back wheel known, the neutral location of the CoM can be solved for using a two-dimensional free-body diagram.

Stability Analysis modelling using MATLAB: A MATLAB script [lgStabilityGUI2.m] was written to model the relationships between the landing gear’s location, width and its corresponding stability with respect to the location of the CoM. Script utilizes basic geometrical calculations. Good for giving designer a feel for the different relationships, and a sensitivity analysis. Output is however not useful without a good correlation.

Correlating analytical stability values with user perception and tolerance of stability: The stability model merely describes the stability of a defined geometry in terms of the minimum distance that the CoM has to move before falling. This output is of very limited use until it is correlated with actual user’s perception and tolerance of stability. A correlation was made by carrying out experiments with a sample population of users. It is very important that this correlation is accurate otherwise the analytical stability will be unreliable. A jig was built and rigidly attached on Vortex to simulate contact points of a landing gear with the ground (See Figure LGF1). The jig was designed to allow for varying landing gear widths. 

Figure LGF1. Landing gear stability testing jig A number of users were invited enter and exit the vehicle. From a scale of 1 to 10 each user was asked to rate if they were able to quickly and comfortably operate, enter and exit the vehicle without diverting a lot of attention towards keeping the vehicle stable. The experiment was repeated with different widths.

Structural Analysis COSMOS Works’ Finite element method were used on the shaft and bushing assembly, as well as the landing gear fork separately for sizing purposes. These FEA were mainly applied on static loading cases only. It was thought that designing for shock loads would make the landing gear unnecessarily bulky and shock loads could be avoided if the pilot uses the landing gear only at extremely low speeds. It is a good idea to size for a higher factor of safely when in doubt, sacrificing weight.

Post-mortem comments (26 May 2012) In this section, comments regarding the performance failures of this design after its implementation are recorded.

Actual stability versus stability analysis It was noted that the actual implemented landing gear design failed to meet the stability requirements. The geometry of the landing gear was built as designed, within a good tolerance. One major error is probably in the stability analysis and the associated assumptions. Realistic testing and assumptions are very important before trusting the test results. A loose correlation is highly discouraged. I would recommend collecting actual data on the stability behaviors - possibly expand on and refine the experimental method described in Section 'Rough Initial Test' or develop better methods.

In the analysis and testing, a four-point contact was assumed. After implementation, this four-point contact was discovered to be difficult to achieve with the locking mechanism employed – with the rock climbing ascender units, as they have a ‘spring-back’ of approximately a few millimetres till up to 2 centimetres. The small size of the Bluenose canopy opening also contributed to the more volatile movements in the combined center-of-mass. This was also not appropriately captured in the testing that was carried out on Vortex.

I wonder if Missouri S&T’s Kronos’s landing gear system actually lifts the back wheel a little or does it not have both the landing gear in contact with the ground at the same time at any one moment.

Construction and implementation Despite the loose fitting tolerance for the shaft and bushings, one of the two assemblies does not slide smoothly. This could however be easily fixed by sanding. The fit tolerance is approximately equivalent to a running clearance of 7 (RC 7). It is important to balance the spring forces in a 2-sided landing gear as a large imbalance in the forces will confuse the pilot.

Structural Performance No real data regarding the structural performance is available yet.

Questions and issues regarding this page can be addressed to Alfie Tham.