Landing vehicle dynamics question
Discussion
is the undercarriage of a large jet always aligned exactly parallel to the plane's axis?
Reason for asking is seeing big jets landing heavily at what appear to be large yaw angles, yet not seeming to be adversely affected by wheels pointing in a direction other than that of travel?
Or is there a canny suspension system that reacts to lateral inputs and prevents the kind of tank-slapper my malformed head is suggesting might be a problem, caused by the aft wheels being behind the plane's CoG?
Reason for asking is seeing big jets landing heavily at what appear to be large yaw angles, yet not seeming to be adversely affected by wheels pointing in a direction other than that of travel?
Or is there a canny suspension system that reacts to lateral inputs and prevents the kind of tank-slapper my malformed head is suggesting might be a problem, caused by the aft wheels being behind the plane's CoG?
Some aircraft like the B52 have moveable bogies, but most aircraft don't and so the wheels are aligned with the body.
They don't enter a tankslapper, as with tricycle gear aircraft the CofG is generally ahead of the wheels and so helps damp out any "tankslapper" motion. With tailwheel aircraft though, the CofG is usually behind the wheels and this then makes them directionally unstable and far trickier to handle.
They don't enter a tankslapper, as with tricycle gear aircraft the CofG is generally ahead of the wheels and so helps damp out any "tankslapper" motion. With tailwheel aircraft though, the CofG is usually behind the wheels and this then makes them directionally unstable and far trickier to handle.
gotcha, had another think after what you said and now I agree - CoG in front of wheels is better.
I'm still surprised there isn't more visible sideways reaction to a wonky landing though. When other vehicles land askew (cars, boats, motorbikes, pushbikes) there is a marked and instant response.
I'm still surprised there isn't more visible sideways reaction to a wonky landing though. When other vehicles land askew (cars, boats, motorbikes, pushbikes) there is a marked and instant response.
Watching a large aircraft like a C-5 Galaxy land in a crosswind is amazing. The Galaxy, like the B-52, can angle its main gear to match the direction of the runway.
I remember watching C-5s coming in to land at Dublin in 1978 - they were taking an Irish Army contingent to the Lebanon for UN duies.
I remember watching C-5s coming in to land at Dublin in 1978 - they were taking an Irish Army contingent to the Lebanon for UN duies.
BarnatosGhost said:
is the undercarriage of a large jet always aligned exactly parallel to the plane's axis?
Reason for asking is seeing big jets landing heavily at what appear to be large yaw angles, yet not seeming to be adversely affected by wheels pointing in a direction other than that of travel?
Or is there a canny suspension system that reacts to lateral inputs and prevents the kind of tank-slapper my malformed head is suggesting might be a problem, caused by the aft wheels being behind the plane's CoG?
The inertia of a large aircraft will easily overcome the tyre grip on a sidewind landing, so there's not much point in making the whole undercarriage heavier and more complex to counter this. There are also issues of what would happen when the nosewheel touches down or if the aircraft bounced and then touched town at a slightly different angle: castoring gear would add many other variables, all of which would need to be controlled. Reason for asking is seeing big jets landing heavily at what appear to be large yaw angles, yet not seeming to be adversely affected by wheels pointing in a direction other than that of travel?
Or is there a canny suspension system that reacts to lateral inputs and prevents the kind of tank-slapper my malformed head is suggesting might be a problem, caused by the aft wheels being behind the plane's CoG?
Undercarriage legs have to be extremely strong and, in general, are one of the few items to be made of a high strength forged steel alloy rather than a non-ferrous material (although there is current research into alternative materials). Interestingly, some of the highest loads on an undercarriage are generated while riding bumps while taxying for take off.
Some aircraft have steerable wheels on their bogies for easier groung handling, like the Boeing 777 which has an unusually long wheel bogie system.
IforB said:
They don't have enormous vertical stabilisers though or weigh tens/hundreds of tonnes.
Exactly, the inertial and aerodynamic forces are much larger than the tyre friction forces. Obviously there's a limit to how sideways you can go though Edited by IforB on Thursday 10th September 11:35
Similar question occured to me the other day but with regards to aircraft-carrier landings. If the ship is pitching up and down by several metres per wave, the landing spot is obviously ocillating up and down so does the pilot aim at the 'up' deck position or the 'down' deck position? If 'up' he would be in danger of missing if the deck dropped, but if 'down' there would be a hell of a hard landing if the deck rose.
Ayahuasca said:
Similar question occured to me the other day but with regards to aircraft-carrier landings. If the ship is pitching up and down by several metres per wave, the landing spot is obviously ocillating up and down so does the pilot aim at the 'up' deck position or the 'down' deck position? If 'up' he would be in danger of missing if the deck dropped, but if 'down' there would be a hell of a hard landing if the deck rose.
Don't forget that the landing target is a few tens of metres in from the stern of the ship, so the intersection of the correct glidepath and the deck probably isn't going to change enough to miss the deck altogether, and there are usually four arrester wires to allow for slightly different intersection points. Also, even if pitching caused a change in glidepath angle w.r.t. the deck, the heave of the deck isn't going to be that fast w.r.t. the vertical speed of the aircraft. Not saying it's going to be easy, but...they do it.Gassing Station | Boats, Planes & Trains | Top of Page | What's New | My Stuff