1. Title of the invention

MAIN LANDING GEAR PITCH DAMPER VALVE

2. Field of invention
It is a mechanical engineering design used in Aircraft Landing Gear system to alleviate aircraft pitching (Nose bobbing) phenomenon during taxiing.

3. Use of invention

This invented MLG damper valve finds application in military as well as transport aircrafts

4. Prior art

Aircraft pitching (Nose bobbing) is relatively common phenomenon in both military and transport aircrafts initiated by runway undulations. Landing gears oleos are designed to damp out such disturbances during taxi to maintain pilot comfort. However, if the damping is found inadequate, for the given aircraft configuration it is always been the practice of increasing the damping of Nose landing gear to minimize Nose bobbing effects. Some designs of nose landing gears with differential damping, for taxi and landing are also in use. The prime focus was on improving Nose landing gear damping characteristics to mitigate problems of Nose bobbing.

5. Draw backs of prior art
Improving damping characteristics of an existing Nose landing gear by way of additional damper valve to give adequate damping during taxi /take off roll is not always feasible. The design of damper valve within the geometrical constraints generally limits the maximum improvement in damping achievable. Designs of differential damping landing gears are complex and heavier.

6. Comparison between prior art and present invention
It is generally accepted practice to incorporate pitch damper valve in NLG alone to reduce aircraft Nose bobbing. This invented damper valve is incorporated in the Main landing gear to minimize Nose bobbing (Aircraft pitching) of the aircraft.

7. Aim of the invention
Aim of the invention is to minimize the aircraft pitching (Nose bobbing) effect during taxiing on normal & moderately rough runways that are expected to be used in the operational envelope of the aircraft.

8. Summary of the present invention
Aircraft pitching motion is induced by surface undulations of runway. Hence, occurrence of Nose bobbing is reveled during actual taxi trials of the aircraft. Simulating the aircraft taxi with theoretical runway profiles using software does not reveal such responses that are encountered during actual trials. Hence, often it is necessary to do the modifications in Nose landing gear based on feed back from actual taxi trials of an aircraft. During developmental trials, the aircraft taxi tests are conducted on a test plate to simulate runway undulations and study the aircraft response during the trial.

As part of developmental trials of an aircraft, a typical bobbing plate with a uniform ramp height of 60mm at the centre with overall length of 3 m in fore & aft direction was used for taxi trials up to 150 Kmph to evaluate the aircraft behavior. Landing gears of the aircraft was instrumented to capture key design parameters. The aircraft nose bobbing was observed during taxi trials on the bobbing pate with sustained oscillations for more than 6 cycles. It is observed that, the Main & nose landing gears have pitching as well as heaving motions. Pitching of Nose & Main landing gears was observed to be out of phase by 180 degrees, whereas the heaving was observed to be in phase with each other. It is generally accepted practice to improve damping of Nose landing gear to alleviate the problems of aircraft pitching. However, the extent of the improvement possible is limited by the prevailing geometrical constraints in the already manufactured landing gear. Hence as NLG pitch damper valve was already in place in this aircraft, to further reduce nose bobbing, incorporating a pitch damper in MLG was investigated.

Dynamic analysis of the aircraft taxing on bobbing plate using ADAMS software, with pitch damper in MLG was compared with that of taxi with pitch damper in NLG alone. It is found that the aircraft pitching was substantially reduced by MLG pitch damper valve. Pitching motion induced by the bobbing plate has damped immediately within two oscillations (refer fig 1 & 2) and hence found to be effective in alleviating the Nose bobbing problem.

9. Brief description of drawings

Fig-1 shows the cut sectional view of the Main Landing Gear shock absorber.

Fig-2 shows the cut sectional view of the Main Landing Gear shock absorber with & with out

10. Statement of invention

This invented pitch damper valve incorporated in Main Landing gear of an aircraft will minimize aircraft pitching (Nose Bobbing) which may be induced due to roughness of runway surfaces.

11. Detailed description of invention
Fig-l shows cross sectional view of Main Landing Gear shock absorber. It consists of counter piston (1) which is fitted with main Orifice (14). It may be seen, that introducing a spring loaded pitch damper valve in the existing shock absorber is constrained by the ID of the counter piston. The size of damper valve orifices is required to be adjusted to maintain the same overall damping coefficient that is present in the existing design.

Fig 2 a. shows the cross sectional view of the shock absorber with out pitch damper valve. The main orifice of the shock absorber (14) is housed at the end of the counter piston (1). The piston head (12) carries gland on one side & recoil orifice cup on the other side. Closing of piston (13) exerts pressure on chamber A & hydraulic damping force is generated as oil flows from chamber A to chamber B through orifice (14).

Fig 2 b. shows the cross sectional view of the shock absorber with pitch damper valve incorporated inside the shock absorber. The counter piston (1) is redesigned to accommodate a spring loaded damper valve without changing the OD on which piston head (12) is sliding. A redesigned orifice cup (5) is fitted in the ID of the counter piston (1) at its flanged end by threaded interface. This orifice cup has 4 holes of area equivalent to that of unmodified shock absorber orifice to maintain the same damping coefficient. Holes in the orifice cup are closed by a spring loaded valve (4) till the pressure built in chamber A is sufficient enough to deflect the spring which allows oil to flow through the orifice. The spring preload is adjusted to give the required constant damping for stroke velocities of 0.3 to 0.4 m/sec which is generally encountered during taxiing. For larger stroke velocities during landing the orifice will open and be operational which provides required damping.

Claims

1. The method of minimizing Nose bobbing of an aircraft by incorporating a pitch damper valve in Main Landing gear.