HarryMann;
If only a nosewheel is on the ground then it's not hard to picture this as the pont where rotation might be centred due to it's lateral friction, regardless of the c.g.
Regarding "inertial" vs "aero" forces, were you thinking solely of the above case, "only a nosewheel on the ground"? While I'm trying to picture the dynamics you describe for an aircraft in the air, I can't imagine where any transport would be only on the nosewheel, "about which the airplane would pivot", so I'm sure I'm misunderstanding what you mean.
fdr;
This may be "off-thread" but the thread has wandered anyway so I will risk it in favour of understanding.
While your comments were a good read, I'm still puzzling over the dynamics that you describe thus:
Originally Posted by fdr
The B777-300 in a strong crosswind, above 35kts component, when doing a decrab, the cockpit (pilot eye) lateral position relative to the runway does not move downwind, the gear position shifts upwind. A B747/744 or B767 does the opposite, as do long body Airbus', the pilot eye position moves downwind, and the gear position remains fairly constant in relationship to the runway centerline. -200 simulator doesn't do this, nor does the plane, -300 simulator unknown.
Landing the DC8-61 and 63 series aircraft on US runways (150' wide, Canada's and the ones used in Europe were 200' wide), in a heavy crosswind meant that one placed the cockpit slightly on the upwind side of the centerline so that the gear was on the centerline at touchdown.
A de-crabbing of a long fuselage meant either the cockpit was on centerline and the gear was off the centerline on the downwind side, or the cockpit was flown slightly upwind of the extended centerline to place the gear on the centerline. I would have thought this was well understood on even longer aircraft such as the B777-300 and A340-600 but you say these two aircraft, for example, behave exactly opposite to one another in very high crosswinds. One decrabs "at the cockpit" and the other "at the center of gravity", roughly. I'm trying to understand how.
I can see, given the mass involved, that the pivot may shift slightly due to the effects of the crosswind upon the fuselage's length but it seems that this would apply uniformly over all types.
I'm also still trying to understand the following comment from
fdr and hope perhaps a more detailed explanation might be offered. Not doubting, yet, but trying to imagine the dynamics and "why?" For me, it makes no sense at all but I'm suspending judgement in favour of curiosity:
Both 200 & 300 in a strong crosswind will (without rudder input) will initially diverge track towards the downwind side... up to approximately 60kts, thereafter the aircraft will start to diverge track towards the windward side of the runway. The B747/744/757/767, MD11, A300, A320, A330 and A340 don't do this... The -200 simulator does replicate this characteristic.
Regarding the vertical 'g' parameter for the B777 and your comment:
A similar issue occurs in the measurement of landing g, if the data analysis is merely based on peak recorded values; the accelerometer is not at the cg, or at the centroid of all forces, and records both vertical acceleration against the fixed body axis, and also the rotation of the body. So a late flare will result in additive accelerations being measured, that of the pitch rate and the gear rebound. (simplistically). The -300 accelerometer is... almost exactly the fuselage plug size forward of the -200's (surprising as the location is determined normally by analysis of the natural fuselage harmonic node locations, and is located at or near a node...) and the screening value for hard landings where applied is about 0.2g higher as a result to give rational data. Same issue apples for the lateral g sensing.
A UDRI, (University of Dayton Research Institute) paper for the FAA entitled,
Statistical Loads Data for the Boeing 777-200ER Aircraft in Commercial Operations, discusses these factors at length. The paper also discusses, as do your comments, the variability of the vertical loads accelerometer. In fact we noticed early on that the B777-300 and -200 series vertical acceleration parameter was unreliable, tending to be "spikey", triggering many false "hard landing" events. Here are the comments from the FAA document.
The installation of acceleration sensors in the cockpit rather than near the center of gravity is a deviation, albeit approved by the appropriate airworthiness authorities, from the Federal Aviation Regulation specifications. The transfer functions and filtering techniques used in the transfer of the sensed accelerations at the cockpit to those expected at the center of gravity are oriented towards flight conditions. As a result, the aircraft dynamics experienced during landing conditions are not properly accounted for and the recorded accelerations for these conditions will include unknown errors. These errors were clearly manifested in the vertical and lateral load factors recorded for the touchdown condition when comparing the B-777-200ER load factors with those of other aircraft. This is of particular concern if vertical load factors recorded during touchdown were to be used by the airlines for routine hard landings identification or by the National Transportation Safety Board in landing accident investigations. In such cases, the use of the recorded values would be very questionable. It is recommended that a task be considered that will install a center of gravity accelerometer in one airplane and record both the converted and direct measured accelerations to determine the extent of the differences that may exist. Perhaps some airline might be willing to consider such a task on one airplane for a few hundred hours to at the very least resolve any hard landing concerns.
The B777 AMM states quite clearly at the beginning of the hard landing check, that unless the pilot reports the landing as hard,
it didn't happen and no maintenance inspection will take place. The inspection procedure then goes on to outline the 'g' limits requiring different inspection levels. The 'g' limits change according to roll angle and samples per second, (8x or 16x - ours were 10x).
My main question is about the de-crabbing phenomenon you describe and how it happens. Thanks
fdr.