Consultation of a specialist

(Skirko Oleg,excerption from article for the magazine " General Aviation;)

Question:What should landing gear be like, taking into consideration specifics of use of ultralights?

Answer:Based on the fact that an ultralight is an aircraf:

  • designed for amateur flights, often taking off with no special runway around,
  • often equipped with engines not recommended for use in an aircraft,
  • its landing gear should satisfy higher requirements to taking loads at takeoff and landing, to absorbing shocks, as well as to bouncing stability. It should also be equipped with reliable brakes.

    While designing, building and operating various kinds of aircraft we regularly faced the problem of choosing reliable landing gear elements

    A carriage spring which has taken its firm place in the ultralight design is quite an elegant and aerodynamically clean solution.Among its attractions is its seeming simplicity and apparent cheapness. But is a carriage spring just the element which will help an amateur pilot not to break his aircraft in case of possible mistake at landing or an experienced pilot to land on a limited ground with unknown surface? With no element that can absorb the shock energy, a carriage spring remains only a spring with almost linear dependance of deformation on load. With load increase the spring distorts up to failure, and if an impact was not very high, the accummulated energy will be passed back to the aircraft, hence high bouncing probability.

    A car shock strut as an alternative to a carriage spring looks better in some cases, but taking into consideration that automobile shock absorbers have been originally created for cars with their loads and operating specifics, it is next to impossible to choose a shock abosrber with suitable characteristics , and a spring makes such landing gear quite heavy. An ordinary car or motorbike is not designed to accept impact with the ground with 3 to 4 m/s vertical speed. Hydraulic shock-absorber is to ensure smooth motion in the first place.

    The only way out is to utilize the traditional aircraft solution taking oleo-gas shock absorbers as a basis. It is generally recognized that a hydropneumatic shock absorber has the maximum ability to absorb impact energy at landing, at the same time ensuring the maximum weight efficiency. There's a great variety of designs. Based on this variety we have set the task to create a shock absorber as cheap as possible, with long useful life, with a possibility to use it in normal conditions without any special tools for pumping.

    In big aviation a shock absorber is specially designed for every aircraft. This can be explained by very high airworthiness requirements to the landing gear elements and to the aircraft as a whole.

    As regards ultralights, the situation looks much more simple. The takeoff weight range for an ultralight is around 450 kg, landing gear designs are very similar in what concerns loads towards the shock strut. All this considered, we have worked to design a one-fits-all shock absorber which can be mounted on any ultralight aircraft.

    After making all necessary calculations and checking them on the test rack we have come to the conclusion that changing oil volume and pumping pressure in the same shock absorber, we can get squeezing scheme satisfying a wide range of technical requirements. Performing tests on a specially designed dropstand we have chosen the valve design to ensure impact with the ground surface without jumping, at the same time ensuring a sufficiently quick reversion at backward motion.

    Our next step was to arrange production of ground shock strut pistons and to find reliable long-life sealings. As a result of all this work we learned to make shock absorbers for a customer's technical specifications, complying with specified parameters

    For us the initial data are as follows:

  • parking load on the landing gear
  • squeezing amount at parking load
  • load with full squeeze which can be defined on the basis of maximum landing overload and landing gear kinematics
  • travel
  • Having designed a universal shock absorber for ultralights, we have utilized standard design schemes to arrange production of shock absorbers almost for any purposeThese are extension and compression shocks, with shock strut piston facing up and down, with parking load on the landing gear shock strut ranging from 80 to 1000 kg.

    Pumping pressure generally doesn't exceed 20 atmospheres, which allows pumping the shock absorber with hand pump for mountain bike shocks. Polyurethane sealings and long-life friction pairs ensure longer shock absorber's operating time than that of an aircraft.

    One of the variants of our shock absorbers designed for a motorbike went 5000 km on our Ukrainian roads, which corresponds to 25 000 of flights. Further check showed no signs of wear that could prevent normal operation.

    Now our shock absorbers are used in different parts of the world, they are mounted on the front forks of trikes and the front landing gear of aircraft, on the main landing gear of motorized paragliders, trikes, gyrocopters and planes. Note that use of oleopneumatics is especially justified in aircraft with higher risk of landing at high vertical speed, such as motorized paragliders and gyrocopters. Installation of oleopneumatic shocks is also reasonable at takeoff weight increase due to installation of heavier power units based on utilization of powerful car engines and engines ROTAX-912(914).


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