Under carridge dynamics

[sughew]

I have become strangley interested in aircraft undercarridge systems. Some questions....

What are the general dynamic behaviour of the mechanisms? I would imagine the landing shock/g-forces' to be significant. Are the loads typically fed through the wings structure?

When a large airliner lands, the rear-most wheels of the central boggies touch down first, and then the front wheels. Why don't the boggies touchdown in a horizontal manner?

 

[rb1957]

whilst obviously there is a significant vertical load (typically 2x static), the primary loading is a drag oscillation ... "spin up" and "spring back". the drag as the wheels contact the grounds causes the wheels to "spin up" to the ground speed of the airplane. this drag causes the gear to deflect (elastically) and it then releases this energy, by "springing back".

as for the boggie, different planes have different arrangements (more than one way to skin a cat). B747s (i think) have the boggie trimmed nose up (so the aft wheels hit first). this causes a pitch moment on the boggie, intended to absorb some of the energy of the airplane impact, i think be extending the time of the impact event. if you look at the gear arrangment, big planes B747, A340, DC10) have secondary main gear mounted on the fuselage; these are positioned at different fuselage stations, again i think to extend the time of impact.

no-one would have a horizontal boggie, 'cause i dont think you want all the wheels impacting at the same time (much like you generally don't want a three point landing) and it would be way to complicated to arrange (given the airplane's pitch on landing).

try reading a general airplane design book, Niu, Torenbeek, Raymer, or a specialist undercarriage book.

 

[davidjh]

There is a good book available thru AIAA