Direct Lift Control
 

Is there anyone hear that can explain to me what is Direct Lift control? What airplanes use it?

TKS a lot!

Relax I am here....

Direct lift control is about giving the pilot (or autopilot) the ability to vary the lift at constant pitch attitude (or constant angle of attack if you prefer)

It is often only deployed during the approach phase.

When done aerodynamically it uses spoilers that are set half out at the start of the approach then moved in and out from that mean position to vary the lift.

It removes the effect of aircraft pitch inertia from the handling qualities issues.

There has been a stack of stuff about DLC before. Try a search.

The Tristar was one type that used DLC.

> Is there anyone hear that can explain to me what is Direct Lift control? What airplanes use it?

You will find lots at your local gliding club! They are spoilers that typically project above/below the wing. They destroy a variable amount of lift and at the same time add some drag to steepen the approach without increasing speed. In a perfect langing you make your final turn, pop the spoilers out to 50% and they stay like that all the way down to the ground. That way you have the max amount of throw available in both directions.

Thanks a lot! Now I got it!

cwatters

Sorry my friend what you speak off is not what is known as direct lift control.

For spoilers to qualify for DLC they have to be operated by the pilot using the control column in the normal way. A very different matter.

Of course you may have had you tongue firmly in your cheek - in which case my apolgies

The US Navy F-14 and S-3 use DLC as described, including the button on the control stick to actuate the DLC.

In some installations, arming DLC after landing gear and flaps are extended raises the spoilers on the upper wing surface a small amount. That becomes the basic landing configuration. DLC can then be used to increase lift (reduce rate of descent) by retracting the spoilers, as well as reduce lift (increase rate of descent) by extending the spoilers further with little or no pitch or power correction.

JF:

I'd vote for the use of spoilers in gliders in the landing pattern as a valid implementation of DLC. I was taught to fly the landing approach in the glider with spoilers set half open as a baseline, then using the spoiler lever as a "throttle." The control stick is still used "in the normal way"...

Since lift IS "controlled directly" by means of the same type of control surface (spoilers on the wing), there is no reason to disclaim the glider implementation. The lower-surface spoiler serves as a balancing device, reducing actuation loads as well as required pitch changes.

It makes no dofference to me whether the actuation device is a button or switch on the control stick or throttle lever, or a separate lever in place of the nonexistent throttle lever!

Intruder
I go with JF on this one. The phrase is DIRECT lift control with the direct bit coming from the interaction of pitch input and spoilers. It isn't a two-handed arrangement, which is the glider case or as would be available on any aircraft with spoilers NOT interlinked. The system as implemented on the Tristar was excellent and the resultant pitch stability plus its benign handling characteristics made the technical complication worthwhile.

Intruder

About gliders (a bit off topic) : there have been intensive discussions some years ago, in the french gliding instructors community, about the use of the spoiler lever as a "throttle".

The before last version of the student's handbook had turned the procedure upside down, using the stick to control the glide angle and the spoiler lever to control velocity. Although this can work, it has been canceled in the last version, mainly because that put an extra stress on the student in a rather critical phase, making him/her switch to another control mode for the stick.

The analogy with the throttle has also been dropped, because glider spoilers (at least Schemp-Hirt type ones) act as much on lift and drag, whereas the throttle acts only on traction (aka anti-drag - No need to start dicussing about second order effects as propeller axis offset and flow changes on the wing induced by power changes). Another reason to drop the analogy with the throttle is probably related to some difficulties glider pilots have experienced with go-arounds :)

DLC on the L1011 was primarily required to achieve the necessary consistent accuracy of the Autoland touchdown point for autoland certification
This whilst maintaining an acceptably constant flight deck pitch up angle for low level runway sighting by the pilot at Decision height in conjunction with the correct pilot eye height indicator for autoland decision or man land in event of a late malfunction.
That is what the L1011 test pilots and flight engineers told me, but Hell what do they know?
Heard a rumour the L1011 autoland system was designed by the old Trident autoland team hired enmass and moved to the states.

On the L-1011, DLC became operational on the selection of landing flap. The spoilers automatically set themselves to a 7 degree position and were operable automatically from 14 degrees to zero (stowed).

When on glidepath, the flight deck was in a constant position at all times, relative to the runway environment.

If the aircraft was slightly below glidepath, the spoilers would move towards the 'stowed' position, thus creating more lift, the aircraft would gently regain the glidepath without any pitch change, similarly, if one was above glidepath, the spoilers would move towards the extended position.

The autoland on the tristar was superbe. One wonders what kind of aircraft would be available today for commercial use if Lockheed had stuck to making passenger aircraft. It was a dream to fly.

DLC (and lots of other clever stuff like blown flaps) is used today on the C17 Globemaster so at least someone has carried on the idea, sounds like a good one to me, anyone know why it was not taken up by other companies?.

With DLC is it necessary for the pilot to flare prior to touch down?.

Obi

Oh yes, the flare during the landing maneuver is certainly required....HOWEVER, if one tries the 'Boeing push' it ain't likely to result is a landing you would like to talk about later...:} :E

Having just passed 14,000 hours Command in the TriStar, I would have to say, it truly is one remarkable aeroplane....bar none.

Yes, truly a ladies aeroplane, if ever there was to be one.

I've looking through earlier posts in the Tech Log and practically all of them regarding the Tristar have been very positive.

Obi

411A,

I don't fully understand this, could you pls elaborate just a bit, how should one flare a Tristar ...

thx

Rumet

Everyone who flies the 'ole Lockheed tri-motor has their particular technique, Rumet, but the following works for me.

At the fifty foot radio height call...think about the flare.
At thirty feet, start a nice steady pull on the poll, so as to reach 10 feet with fair amount of backpressure, and it squeeks on nearly every time.

OTOH, a few Boeing guys try the Boeing 'push', whereby they relax backpressure just before touchdown, and just nuddge the pole forward just a bit.
This does indeed work in long body Boeing designs, but with the TriStar, if tried, the spoilers come up (DLC panels) and the aeroplane rather poorly...thumps on.


During automatic approach/land ops, the flare automatically starts at 50 feet, with steady backpressure, autothrust disconnects at 5 feet, and the aeroplane rolls on nicely every time.
Spoilers automatically deploy, the nose is automatically gently lowered, and the aeroplane tracks the localizer down the runway.
A superb design...well ahead of its time.

411A

So the flare in the Tristar wasn't a typical flare i.e slight pitch up before landing, it was done via the DLC at a constant pitch angle, abit like a carrier landing, but without the crash?.

Obi

Yes a lot posts say the Tristar was a nice plan to fly but to work on......

Nice "PLAN" to fly? Why were (actually are as some are still flying) they hard to work on? I worked on them, around them and with them for over 30 years and found them no harder to work on than any other aircraft.