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Old 08-27-2010 | 06:02 AM
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alasdair
 
Joined: Nov 2002
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From: Scotland, UNITED KINGDOM
Default RE: Horizontel stab dihedral


ORIGINAL: pimmnz
And big airliners don't land xwind with one wing down. No need for anyone to do that.
They do when I land them!

When I started, in the seventies on HS Tridents, we used the kick-off-drift in the flare technique. Timing is critical.
Kick off drift too early (before touchdown) and you start drifting towards the edge of the runway.
Leave it too late and you land with yaw. Some of the recent aircraft are designed so that landing with yaw is acceptable, but not elegant.

On the Lockheed L1011 Tristar the autopilot was programmed to autoland in a crosswind as follows
At 300 feet radio height (AGL) the autopilot used the rudder to line up the aircraft centreline with the runway centreline (rudder away from wind direction).
At the same time the ailerons were used (aileron into wind) to dip the into-wind wing to produce a sideslip into wind to counteract the downwind drift.
At about 30 feet on the radio altimeter the flare commenced and shortly afterwards the throttles closed to idle.

When flying manually we used the same procedure, but often started lower, 200 or 100 feet radio. For example for a wind from the left you squeezed the right rudder to make the aircraft heading equal the runway heading (displaying heading on the INS was useful). Then dip the left wing with left aileron to prevent drift to the right. At the usual height (if I remember correctly the flight engineer called 100 feet, 50, 30, 20, 10) you start the flare, close the throttles, land on the into-wind wheel with crossed controls (left aileron, right rudder) then you let the right wheel land and then lower the nose, but still keep some left aileron. Steer with rudder, and keep into-wind aileron all the way down the rollout.

The beauty of this technique is that timing is not critical. You can do one step at a time and get each right. Try it in the simulator first.

Having learned in the L1011 I used this technique on twin props (HS 748 and ATP) and on Boeing 757 and 767 (but NOT 747) and I can recommend it.

In the B767 Flying Manual there is a chart giving the roll and pitch limitations, the angles and combinations at which various parts scrape the runway, for take off and for landing. I don't have one but maybe somebody online will have, and could look up what combination of pitch and roll could scrape a tailplane tip.

WARNING
Don't try it with 4 engined aeroplanes whose outer nacelles scrape the runway with more than a couple of degrees of drift.

Further to the original enquiry, I was told by a British Aerospace designer that the tailplane dihedral is often chosen to ensure that the tail remains wholly in, or wholly out of the prop wash throughout the range of pitch angles. You never want it to be sometimes in, sometimes out.