The landing gear, is a structure (or mechanism) attached to the fuselage (or the body) of the aircraft, helps the aircraft during landing, take-off and ground handling operations. The landing gear plays important role absorbing the crush (or thrust) while landing and thus ensure lower crush related injuries and material damages. For achieving this “crush worthiness” require optimum design of the springs of the landing gears.
I have started the process of the “optimum design” of the landing gear mechanism through theoretical hand calculations. After I established a base design through hand calculation; I shifted to the ADAMS tool. The ADAMS tool seemed to be very powerful for achieving the optimum mechanism design solution through number of iterations.
For the sake of simplicity, I have considered non-retractable type of landing gear for this study. Also, I have considered using only helical compression spring and no torsion spring for this design study.
Research on Naval Helicopter Landing Gear
The naval helicopters operate in much severe landing conditions compared to the commercial helicopters. Hence, while designing the naval helicopter landing gear all the necessary landing conditions should be taken care. In this section I am going to discuss about the types of landing gear and few practical examples about the usage of the landing gears.
History and evolution:
The first wheeled landing gear appeared in Santos-Dumont’s “No.14 bis” on 1906 soon after the Wright brother’s famous flight.
Initially, the landing gear used to have bungee as shock absorbing elements.
The Ford trimotor landing gear, which used rubber discs and rebound cables, was the predecessor of the modern day’s shock absorbing landing gear.
During World War-II, the shock absorbing landing gear had developed further. Use of the “spring” and “lever “came into the landing gear system design.
After the world war, the landing gear design matured further to give modern days sophisticated landing gear system.
Types of Landing Gears: All of the landing gear used in helicopters can be broadly classified in three categories:
Tail dragger Landing Gear: Two main gears are placed under the mid of the fuselage and one tail gear is placed under the tail of the helicopter for the tail dragger landing gear arrangement. This type of landing gears are used in older helicopters (e.g. Seahawk)
Tri Cycle Landing Gear: In this configuration, there are one nose wheel and two main gears at the mid of the fuselage. Most of the modern helicopter has this landing gear configuration.
Tandem Landing Gear: Large aircrafts use multiple wheels in line for each of the landing gears and this configuration is known as Tandem.
Examples about the usage of the landing gears in naval helicopters:
Landing Gear for Seahawk S70B: The Seahawk is an US naval aircraft manufactured by Sikorsky Aircraft in Stratford, Connecticut.
Fig.1: Showing a Seahawk in operation (Image source: http://www.naval-technology.com/projects/seahawk/seahawk2.html)
The chopper has energy absorbing two-wheel tail dragger type of landing gear arrangements. The landing gear design is much simpler compared to the other naval helicopters.
Boeing Vertol CH-46 Sea Knight: Sea Knight is a marine transport helicopter, manufactured by Boeing Vertol.
Fig.2: Showing a Sea Knight
(Image source: http://en.wikipedia.org/wiki/File:USMC_CH-46.jpg)
The Sea knight has tricycle type of landing gear system. Each of the landing gear has twin wheels.
Sikorsky SH-3 Sea King: The Sea King is an anti-submarine amphibian helicopter manufactured by Sikorsky. It is fitted with retractable type trail dragger landing gear arrangement.
MH-53E Sea Dragon: This is a three engine powered large navy helicopter designed for heavy lifting and Airborne Mine Countermeasures (AMCM). It is fitted with twin-wheel tricycle configuration of landing gear system.
Development of the Landing Gear Mechanism
The landing gear mechanism should be strong enough to withstand the specified stringent landing conditions of this assignment. I am planning to develop a landing gear mechanism using two double rear landing gears and a nose landing gear. All the landing gear will use helical compression springs only.
Fig.3: Top view of the landing gear arrangements for the concept
As the above figure shows, the concept will have the centre of gravity somewhere in between the front and the rear landing gears.
Selection of the proper compression spring is the key to the success of the mechanism. Hence I have started with the hand calculation to arrive at the preliminary spring design parameters.
Total mass = 5126 kg
Hence, Sprung mass on each spring = 1025.2 kg
For zero initial velocity:
Say, max. Deformation of spring =35 mm
So, spring rate K = 292.9142857 N/mm
For normal landing:
Initial velocity of helicopter = 0.5 m/sec
Spring rate k = 292.9142857 N/mm
Now, using the formulae: 0.5*m*v^2=0.5*k*x^2
Max deformation of the springs =0.935414347 mm
For hard landing:
Initial velocity of helicopter = 3 m/sec
Initial velocity of deck = 3 m/sec
So, Relative velocity between the helicopter and the deck = 6 m/sec
Spring rate k =292.9142857 N/mm
So, Max deformation of the springs = 11.22 mm
For crush landing:
Initial velocity of helicopter = 15 m/sec
Spring rate k = 292.9142857 N/mm
So, Max deformation of the springs =28.06 mm
Since, the deformation values from the hand calculation are well below 30 mm with the spring rate of 292 N/mm. So, I think it is good to go ahead with these values and check the acceleration results and vibration results by creating the ADAMS model.
Developing ADAMS Model
The ADAMS models of the landing gear mechanism are created by the ADAMS/View. I have come out with two ADAMS design based on the already discussed mechanism concept. The following steps are followed to create each of the ADAMS models:
Unit Setting: I choose to use the units as: Length – Millimeters, Mass – Kg, Force – Newton, Time – Second, Angle – Degree, and Frequency – Hertz. Following consistent units are important for getting accurate results.
Gravity Setting: I activated the gravity.
Points: Points are the basic building block of the whole mechanism.
Box: This option was used for creating the deck.
Torus: All the wheels were created using the torus option.
Link: The structure and the axels were created using the link options.
Translational Spring Damper: This option was utilized for creating all the helical compression springs of the designs.
Contact: The contact option was used for simulating the contacts between the deck and the wheels.
Revolute Joint: The joints between the wheels and the axels were created using the revolute joint option of ADAMS.
Translational Joint: For simulating the vertical descent speed of the helicopter and vertical speed of the deck it was required to create transitional joints between the structure and space and between deck and space.
ADAMS Mechanism Design-1:
Fig.4: ADAMS model of the design option-1.
Fig.5: ADAMS point table for the design option-1.
ADAMS Mechanism Design-2:
Fig.6: ADAMS model of the design option-2
Fig.7: ADAMS point table for the design option-2
The basic difference between the design opton-1 and the design option-2 is in the height of the design. After reviewing the initial displacement results (which I will present in the next section) of the option-1, I have decided to increase the height, as for the specified test condition the structure is hitting the deck for design option-1.
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Result Comparison for Option-1 and Option-2:
Fig.8: Deflection plot of the structure for crush landing condition
The above plot is showing the comparison of the deflection of the top frame (structure connected to the fuselage), it shows that the option-1 has much higher deflection. The deflection value for the option-1 is even higher than the clearance between the structure and the deck. Means, for option-1, the structure will hit the ground for extreme condition. So, Option-2 is a better design.
Testing ADAMS model in Various Landing Conditions
Different landing conditions specified for this assignment is simulated in ADAMS for the design option-2.
Normal landing: Here the vertical descent speed of 0.5 m/sec is applied at the translational joint between the structure and space. Result is shown below:
Fig.9: Normal landing acceleration plot
The result for the normal landing test for the design option-2 is showing that: the maximum acceleration is 6.8 m/sec2.
Hard Landing: For the hard landing test, I applied vertical descent speed of 3m/sec at the joint between the structure and space and vertical deck speed of 3m/sec at the joint between the deck and space. Here is the result:
Fig.10: Hard landing acceleration plot
The above plot is showing that the maximum acceleration value for the hard landing test of the design option-2 is 19.3 m/sec2 .
Crush Landing: In order to simulate the crush landing condition, I applied the vertical approach speed of 15 m/sec at the joint between the structure and space, keeping the deck stationary.
The result of the crush landing test is shown below:
Fig.11: Acceleration plot for the crush landing test
The above plot is showing that the maximum acceleration value for the crush landing test is 206.6m/sec2.
Running Vibration Analysis in ADAMS
The naval helicopter will be kept in landed condition over the aircraft carrier. The aircraft carrier will be oscillating continuously under the influence of the sea waves. The purpose of the vibration analysis is to find out the resonating frequency of the landing gear mechanism under the sea oscillation.
For simulating the sea wave oscillation, I created five kinetic actuators placed at the centre of each of the axels and placed one output channel at the centre of gravity of the top structure.
Frequency response analysis: The frequency response analysis (FRA) shows the amplification of acceleration for each frequency values. The FRA plot for the design option-2 is shown below:
Fig.12: Frequency response plot for the design option-2
The FRA plot above is showing a pick at 2.5 Hz. The pick is the resonating frequency of the landing gear mechanism.
Results of the Different ADAMS Analysis
Maximum acceleration for normal landing = 6.8 m/sec2.
Maximum acceleration for hard landing = 19.3 m/sec2.
Maximum acceleration for crush landing = 206.6 m/sec2.
Resonating frequency of the mechanism = 2.5 Hz.
The conceptual design of the naval landing gear is simulated using ADAMS for the specified landing conditions. The results from the simulation are showing that the maximum acceleration values are well below the specified maximum limit for this assignment. The ADAMS vibration simulation is showing the resonating frequency for the mechanism as 2.5 Hz.
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