Skid Landing Gears

Helicopter landing gear systems

The rotorcraft possesses the unique ability to efficiently hover. However, the complex nature of the rotor’s oscillatory loads and vibration characteristics induced on each sub system make successful rotorcraft design a challenge to accomplish. See structural analysis for our personal helicopter to better understand acting forces.

Rotary winged aircraft are used extensively in both civil and military missions on a regular basis. Rotorcraft have been extensively used in transportation of military personnel and attack missions, as well as civil training, transportation and rescue and reconnaissance missions. The one unique drawback of such an aircraft is the inability of the pilot and crew to eject with a parachute analogous to that from a fixed winged aircraft. While a rotorcraft does possess the equally unique ability to auto-rotate.

Skid Landing Gears

The landing gear is the undercarriage of an aircraft and is used for takeoff or landing.

Some helicopters are very unforgiving in the event of a pilot error and also do not auto-rotate well. Additionally, a tail rotor system strike resulting in loss of yaw control can cause it to crash if appropriate procedures are not correctly and immediately implemented.

Energy absorbing landing gears play a key role in meeting helicopter crashworthiness design goals of reduced crash injuries, fatalities, and material losses.

Main types systems

The oleo-strut landing gear with wheels offers advantages of initial taxi and take-off run capability but at the cost of design complexity. The wheeled type of landing gear is complex and heavy. The design features include, tires, wheels, braking devices, oleo struts and other hydraulic equipment. Energy absorption takes place through brakes and shock absorbers. These types of landing gears also have to accommodate retracting mechanisms which further complicate the design.

Skid landing gears on the other hand this solution offer simplicity in design and reduction in empty weight (WE). Currently skid landing gears are manufactured from metal alloys such as Aluminum 7075. The elasto-plastic properties of such metals offer significant energy dissipation capabilities during plastic bending.

Primary design goals

Rotorcraft should be crashworthy, but also one of the major challenges associated with the rotorcraft design process is minimizing the gross weight (WG) and empty weight (WE). In order to hover efficiently at higher altitudes and temperatures, as well as have increased performance during cruise. Lightweight design, corrosion resistance concerns in metals, as well as fatigue performance can be adequately addressed with usage of composites which offer other advantages such as increased Specific Energy Absorption (SEA) under crushing loads.

Rotorcraft Crashworthiness

The ability of an airframe structure to maintain a protective shell around occupants during a crash and minimize accelerations applied to the occupiable portion of the aircraft during crash impacts. Federal Aviation Regulations (FARs) require a rotorcraft to be designed for crash loads so as to protect the occupant from injury. Much research and technology development work has been done on crashworthiness of rotorcraft and has been focused on landing systems, fuselage and seats for the cabin and crew. The landing gear is the first sub-system that generally hits the impact surface. Hence, this sub-system is critically important for crashworthiness. Current crashworthiness requirements state that the landing gear must dissipate kinetic energy of the entire aircraft which is equivalent to 20 ft/sec of crash velocity, with the vertical impact velocity being 42 ft/sec.

The primary components of typical landing gear are the two skid tubes and the two cross members on which the fuselage rests. Dampers on the landing gear are placed at appropriate locations to account for potential ground resonance instability issues. Wheels can be separately added on if desired. Currently, skid landing gears are fabricated from metal alloys such as Aluminum 7075 and composite materials.

Civil vs Army

The design criteria and performance goals differ significantly between military and civil helicopters. Landing gear for a U.S. Army helicopter designed to meet MIL-STD-1290 criteria which are subject to a wide range of design requirements.

Army: The gear must sustain normal and hard landing loads, absorb large amounts of crash energy, accommodate off-axis crash impacts, prevent roll-over at angles up to 30 degrees, provide a kneeling capability, and, in some cases, retract to reduce drag and radar cross section. Landing gear in a crash-resistant military aircraft provides protection to the fuselage in a hard landing and contributes to the overall energy absorption system for the occupant in a crash. The fuselage protection requirement is the major influence on the design of a landing gear for military helicopters.

Civil: Conversely, this is not a significant design requirement for civil landing gear. Military landing gear must sustain a 20ft/sec impact without fuselage contact, whereas civil rotorcraft must currently comply with a 10.23-ft/sec reserve energy requirement per FAR Part 27 and 8.02-ft/sec per FAR Part 29. The difference in energy absorption capability is significant: The military gear must absorb approximately four times as much energy as a typical civil helicopter landing gear.

Summary

On the attached images you can see current state of our experimental ultralight personal helicopter for civil use. This construction is based on standard skid landing gear solution without wheels. Wheels may appear in the future updates as a easy to add component. Main material used is aluminium 7075, but also it has metal parts and non-slip rubber finishes. The only difference from the standard helicopter landing gear is a flexible single leaf spring “Cross Tube Support” to take over the stresses, and help to better distribute strength during the crash. This innovation is not based on any documentation, and I haven’t done any physical tests to proof it potential. But potentially when subjected to crash loads, metal plastically deforms, absorbing energy and allowing the fuselage underbelly to crash in a controlled crashworthy manner. Cross tube, horseshoe fitting and saddle are made of hard plain carbon steel. It comes with two pair of steps on both sides for more comfortable getting into the vehicle, to rich the fuel tanks and also for better main hub inspection.

The next task is to design the pilot seat and front windshield to give him a protection. This time the pilot seat should be more accurate than previous builds. Design will focus on the pilot’s safety such as safety belts, durable materials and ergonomic. Those are the last main components to prepare before we start our journey to Unreal Engine 4.

If you have any comments, suggestions or questions, I invite you to our forum with a topic fully devoted to this project. If you would like to be notify about progress please consider to hit the red bell in the left bottom corner of your screen.

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