Why did the L1011's direct lift control help make good landings?

Question

In landing configuration the 1011 had what was called “Direct Lift Control” or DLC. That feature was actually pretty cool. Once the airplane was in landing configuration the spoilers would come up to a neutral position of 7 degrees. If you moved the yoke forward, asking for an increased rate of descent, the spoilers would come up to a maximum of 14 deg but the pitch would not change. If you pulled the yoke back the spoilers would move toward 0 deg or stowed, thereby increasing lift and slowing descent. The DLC worked really well and made for super stable approaches with rock solid pitch attitude and rewarded you with “greaser” landings more often than other big jets I have flown.

Source

Why does altering sink rate without changing pitch make for a super stable approach and "greaser" landing? The explanation I can think of is that moving spoilers is faster than pitching, allowing faster reaction times to errors.

EDIT: B787 has something similar

Answer 1

John's fine answer is correct, but there is a bit more to it:

When increasing lift by pulling on the yoke, in most airplanes the elevator moves trailing edge up, decreasing lift initially. To look at the process in detail:

1. The yoke is pulled, the elevator moves trailing edge up
2. Downforce on the horizontal tail is increased without any lift change on the wing, so total lift is diminished. The sink rate increases slightly.
3. The lift imbalance adds a pitching moment, so the nose of the airplane rises.
4. Now the angle of attack on the wing increases with the pitch angle and lift increases. Only now will the desired reduction of the descent rate kick in.

This effect is especially pronounced in flying wings, but it also is noticeable in regular airplanes. Now compare that with partially deflected spoilers:

1. The yoke is pulled, the spoilers close.
2. Lift increases without a change in the elevator angle or the angle of attack. The desired reduction of the descent rate is immediate.

The effectiveness of spoilers is limited, so this technique will not allow you to fly aerobatic maneuvers, but for landing it is perfect. Drag is increased, so the engines will have to produce a bit more thrust than with spoilers fully closed, giving you more immediate engine response, too.

Answer 2

Direct lift control reduces the need to change pitch attitude to change total lift to change descent rate and therefore fine tune glideslope.

Changing pitch attitude introduces inertial forces in the pitch axis; you have to get this long body with a lot of inertial mass rotating, then you have to stop it. You are doing this with elevator, and there is a lag between input and result when starting and stopping a movement.

On a big airplane like the 1011, with a lot of mass in the fuselage (tail mounted engine and compensating fuselage mass forward), trying to find tune descent rate with small pitch attitude changes can become a game of chasing input/result because of all the inertial mass and lag.

Using spoilers in that way, and seamlessly integrating it into the elevator control system in landing configuration, allows this sort of fine tuning of descent rate to be done without having to get body moving about the pitch axis, allowing extremely precise and near instantaneous adjustments to glide slope that would normally be impossible with all that mass to start, and stop, rotating.

The characteristic would feel a bit odd at first when flying the 1011, when you make a pitch input and the attitude doesn't change, but you would get used to it pretty quickly and the instant sink rate response would be a great feature in a big airplane like that.