Thank you Mr. Flaps, it was starting to get embarrassing.
Discussion mode re-enabled.
Part I:
FW - are we getting attitudes confused here - is that the ´landing attitude´or the ´approach attitude´? If the former, I would expect loads of nosewheel first touchdowns? I would venture to suggest that most a/c land in a nose high attitude - also what is the engine mounting angle?
Well, I suppose if you wanted to be
really pedantic, you would say neither! As I understand it (and am ready to be corrected) you have 'Approach Configuration' which will normally involve a bit of flap but no gear and 'Landing Configuration' which involves the gear and all the flap you are planning on using. 'Landing Attitude', I would suggest, would be the final pitch achieved during the flare, just before touchdown. Subtle differences I know but you were right to point in that direction.
Looking at the manuals again, I see on the 777 that a 'Standard Landing' involves initiating the flare at c.20R and raising the nose by
one to two degrees. At the moment of touchdown the angle is c.
four degs. Looking at the aircraft sitting on the ground and walking round the engines, comparing the jet pipe to the ground, I can't actually see any evidence of them being mounted other than pointing straight back, although I admit I don't know how good I am at judging these sort of things.
Let's do some maths.
Come back everyone!
Now, before we begin, let's list the assumptions:
1) 'Thrust Angle Relative to Ground'. OK, four degrees touchdown attitude rounded up to 5 in total.
2) We use the most powerful variant of the 777 that I fly (Trent 895).
3) The engines spool up
instantaneously to
G/A thrust. (190,000lbs.)
4) Rate of descent at 25R is 700fpm - consistent with a landing weight of c.180T on a 3 deg. approach.
5) The airframe immediately assumes the touchdown pitch attitude when commencing the flare.
I think these are pretty conservative assumptions and all err on the plus side (as far as the effects of thrust on landing go...)
Resolving forces in the vertical plane...
Component of thrust (Kg) = Sin(5 degs) * 190,000 / 2.2
=> 7,500 == 7.5T
From Newton's 2nd Law, F=ma...
75 (KN) = 180,000 (Kg) * a
=> a = 0.42ms^-2 or around 1 & a third feet per second per second.
At that rate it would take a little under four seconds of FULL POWER, just to reduce the descent speed from 700fpm to 400fpm. There is an effect but it's not as great as we might have hoped. (OK, so if it was a 'normal' landing, being at the attitude described, you should have experienced a 'normal' touchdown but I'm just using these figures to show what
real effect the thrust is
actually having on the rate of descent.)
By raising the nose with the elevator, you can probably produce forces an
order of magnitude greater in a much shorter timespan.
In real life, the engines would take some seconds to spool up to that level and the attitude would be somewhat lower to start with. Also, we are describing a twin with a pretty good power-to-weight ratio. Finally, would you be ready (mentally) to use TO/GA power at this point?
Part II to follow after the interval