Prof2MDA
OK. I misunderstood you.
I had made it easy for myself by ignoring flare and ground effect.
Stabilised, on the glide slope, A/c and passenger experience 1g.
At flare, a/c pitches up, wing loading increases, g force on a/c and passenger a little over 1. Little change in speed, but the angle of approach to the ground drops below 3 degrees. If the pilot can halve that angle of approach in the last few seconds to touchdown, he will reduce landing forces by a factor of 4, because acceleration is a function of v squared. But he wants to get the main gear firmly planted on the ground in order to trigger the auto speed brakes, and so get rid of lift quickly. So he wants to fly it into the ground, but at something less than an angle of 3 degrees. (m&v quotes an average touchdown 2-300fpm, which implies an approach angle of about 1 degree)
(Andrew, If you get your 737 of 747 on Flight Sim on a good ILS approach at the recommended speed, but with speed brakes disabled, them do your 140fpm touchdown, but at the moment of touchdown haul the yoke back, it will float or even lift. You will at best use a lot of runway, and at worst fall back on your tail. So a moderately firm landing, close to the threshold, is important. Speed brakes cut in quickly, a/c quickly plants itself firmly on the runway, wheel braking becomes possible quickly and you can vacate the runway earlier.
Compare that technique with that of landing a little Cessna. You can approach at a steep angle because the a/c has lots of drag, then arrange to be at the threshold close to stall speed. Pull power and hold it just off the runway until it will no longer fly)
OK Prof2MDA. You have made me think about what does happen to the undercarriage in a conventional landing, with flare, ground effect etc.
It seems to me that with a fly-it -on approach,
On final, a/c experiencing 1 g vertical +/- a bit as a/c pitches up and down.
During flare, a/c experiencing a little more than 1 g. From 50 feet, that will occur over a period of about 3 to 4 seconds. If we say the the vertical speed drops linearly from 13 to 5 ft/sec in the last 40 feet, we have a decelleration of 1.8 ft/sec/sec, or an additional .06g.
a/c flys into the deck at something less than 3 degrees, until (weight of a/c - lift) = oleo force upwards.
Then as lift decays, more a/c weight settles on undercarriage and further deflection occurs.
Wind gusts, ripples in the runway etc are going to add small +/- g forces during rollout.
It seems likely that with a really heavy landing, the oleos will over-compress, then extend again before lift decays and they compress again. But you would need to know the load / deflection characteristics of the oleos, which I do not. I would suspect that they would be progressive shock absorbers, becoming stiffer as they are compressed more.
Have I got it right ?
John Farley,
Think you missed a bit in calculating the g force at 140 ft/sec. Your answer was .66 feet/sec/sec - not .66g.
So .66/32.2 is a very low g loading.
Regards to all