How to calculate the Dynamic Take-off and Landing Weight

[barnas]

Hi..
Appreciate if anyone can help me to calculate the dynamic weight of a helicopter during take-off and touch down if static weight is given. For example, the MTOW of AS350 B2 is 2250kg (4960 lbs) which is its Static Weight. What is its Dyanmic Weight during Take-off and Touch-down?

[311kph]

Depends... can go from static to A LOT

Second helicopter crash test - YouTube

180 Autorotation accident - Low rotor RPM - YouTube

:):*

[9Aplus]

:* Hm, NASA videos are nice, but Barnas is looking on HOW-TO
for structural design of elevated helipad, somewhere in Nepal.

Best way is to start from ICAO Heliport manual DOC 9261-AN/903
page 6. -> Case of emergency touch down 3,6 m/s -> 2,5 x MTOW

@Barnas in case you may need that doc, feel free to PM me...

[awblain]

On landing, the load depends on the time profile of the touchdown, which depends on the touch-down speed (v), the response of the landing gear, and the response of the structures of the aircraft and deck.

M_dyn . g = M_static . v / delta-t

(M_dyn here is the added component to M_static the structure needs to be able to absorb)

If everything's very stiff (delta-t small), then it could be huge.

Descending at v=0.1 m/s, and stopping dead in delta-t=0.1s would cause an average 10% increase over "static weight" for 0.1s. Descending at 0.5 m/s, and stopping in 0.1s would cause a 50% increase… if descending at 3.6m/s, and stopping in 1s(0.1)s, 30(300)% more.

If there's any horizontal component of motion to arrest on touch down, then that will also need to be considered.

During take off, this definition of "dynamic weight" on the wheels/skids should be negative. However, there's the force from deflecting the rotor downdraft to consider too - the force on the air from the aircraft on departure is greater than the weight of the aircraft, and some of that is transfered to the deck beneath in a broad footprint. However, that all takes place over a few seconds, so the impulsive load is going to be modest.

By carefully designing the response of the springing of the deck, you can impose a minimum delta-t, and cap the peak force as touch down occurs.

Suggest you follow the appropriate building codes from a trustworthy locale?

[Peter-RB]

Good morning Barnas,

To keep things simple, just over engineer the legs and crossmembers, if using wood at leat 12/12 " (305mm Sq) with 2"(50mm) thk planks, that sort of timber will take an 8tonne Fork truck, or if steel use 10"x 5" U/Beams for legs and crossbeams with wooden or steel spars for deck, that will take a 10 tonne Fork truck.

Or build from rocks and concrete, that'll take anything that lands, and is simple, and dont need any engineering skill !!

Merry Christmas

Peter R-B
Lancashire

[JohnDixson]

From FAA AC 150:

707. Design loads. Design and construct the TLOF and any load-bearing surfaces to support the weight of the design helicopter and any ground support vehicles. Loads are applied through the contact area of the tires for wheel-equipped helicopters or the contact area of the skid for skid equipped helicopters. Find lists of Helicopter weights, landing gear configurations, and dimensional data in Appendix B.
a. Static loads. For design purposes, the design static load is equal to the helicopter's maximum takeoff weight applied through the total contact area of the wheels or skids. Contact manufacturers to obtain the contact area for the specific helicopters of interest.
b. Dynamic loads. A dynamic load of 0.2 second or less duration may occur during a hard landing. For design purposes, assume dynamic loads at 150 percent of the takeoff weight of the design helicopter. When specific loading data is not available, assume 75 percent of the weight of the design helicopter to be applied equally through the contact area of the rear two rear wheels (or the pair rear wheels of a dual-wheel configuration) of a wheel-equipped helicopter. For a skid equipped helicopter assume 75 percent of the weight of the design helicopter to be applied equally through the aft contact areas of the two skids of a skid-equipped helicopter. (See Figure 7–2.) Contact manufacturers to obtain the aft contact area for specific helicopters of interest.
c. Rotor loads. Rotor downwash loads are approximately equal to the weight of the helicopter distributed uniformly over the disk area of the rotor. Tests have established that rotor downwash loads are generally less than the loads specified in building codes for snow, rain, or wind loads typically used in structural design calculations.