Direct Lift Control

[DontSink]

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

TKS a lot!

[John Farley]

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.

[cwatters]

> 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.

[DontSink]

Thanks a lot! Now I got it!

[John Farley]

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

[Intruder]

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!

[Cornish Jack]

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.

[Bre901]

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 :)

[scanscanscan]

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.

[777AV8R]

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.

[Max Angle]

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?.

[Obi Offiah]

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

Obi

[411A]

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.

[GlueBall]

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

[Obi Offiah]

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

Obi

[Rumet]

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.

411A,

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

thx

Rumet

[411A]

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.

[Obi Offiah]

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

[NWT]

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

[glhcarl]

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.

[411A]

The well known secret to reliable TriStar maintenance (in my personal opinion, anyway) was the mandated used (by the respective airline concerned) of the FIRM manual.

With this method of fault isolation and reporting, snags could well be easily fixed, with minimal ground time.

I worked for one small carrier whose director of maintenance thought the FIRM idea was hogwash...and quite predictably, they had a huge dispatch reliability problem.
They also had missing serviceable tags in many cases, and obtained quite critical inspection by aviation authorities as a result.

Obi, the flare maneuver in the TriStar resulted in a deck angle change of about three degrees.
The approach however, was quite a constant angle, thanks to DLC.
It really does work as advertised.

[Obi Offiah]

Obi, the flare maneuver in the TriStar resulted in a deck angle change of about three degrees.
The approach however, was quite a constant angle, thanks to DLC.
It really does work as advertised.

Cheers :ok:
Obi

[uniuniunium]

.
The approach however, was quite a constant angle, thanks to DLC.
It really does work as advertised.
What controls the actuation of DLC during non-autoland/approach ops? I'd assume it's the control column, but how is it mixed in with pitch control after landing flaps are selected?

[411A]

Yes, it is the control column, uniuniunium, and the exact details I can't remember at the moment.
But I'm sure glhcarl will chime in here as he has the technical details, and has probably forgotten more than I ever knew about the inner workings of the 'ole Lockheed tri-motor.

[glhcarl]

Thanks for the kind words 411A, the problem is that I forget more everyday, However:

Activation of DLC is fully automatic and controled by logic circuitry in the Flight Control Electronics System (FCES) computer. The FCES computer looks for the following:

1. DLC servo is pressuried
2. 2 of 3 Throttle Handles in less then full power.
3. Flap position greater than 30 degrees.
4. Stall warning not detected.
5. Go around not commanded

If the above conditions are met Spoilers 1 - 4 are activated to a new null point of +7 degrees (+9 degrees on -500's). 777AV8R's description a few posts earlier provided an excellent discription of what happens after DLC activation.

The above critera is followed in Autoland and Manual approches.

[777fly]

To answer a previous question: yes, the L1011 did have a conventional flare, you did not 'fly it on' like a carrier landing. DLC could not sense ground proximity, it was purely a pilot input.
The rearward movement of the control column at flare initiation stowed the DLC spoilers from the null postion and gave extra lift into the flare. The secret of a smooth landing was to keep the back pressure on, right through to touchdown. Any relaxation could result in a very firm jolt at touchdown as the DLC redeployed back to null ( or more ) and reduced the lift.

[ZQA297/30]

L1011.
Agree with 411A, with minor mod.
At 50' radalt start gradual flare for additional 3 degrees noseup. The rate of radalt decrease to be proportional to the flare rate. If radalt moving rapidly, a bit of an undignified tug required to avoid a thud.
Great aeroplane, wish I could still fly one.

[Obi Offiah]

:confused: Hmmmm.
Considering the pilots pitch inputs control spoiler deflection, I wonder what occurs during the flare that causes an increased deck angle, as this change in angle doesn't present itself during approach.
Perhaps the usual input to execute a flare, results in a greater change of vertical velocity that during approach, or perhaps ground effect plays a part?.

Obi

[411A]

Well, as usual, it depends.

Large pitch (control column) inputs during approach will definitely change the pitch (deck) angle, small ones will not.

During the flare maneuver, larger than normal column inputs reduce the spoiler extension, while at the same time increases the pitch angle.

Also contributing to the outstanding handling of the TriStar is the all-flying horizontal stabilizer.

Relaxing the backpressure during the flare maneuver however (as has been pointed out already) many times results in a 'thumper'.

Hmmm, a few Boeing guys try 'the push' and then promptly find out it ain't a good idea.
'Tis a good thing the folks at Palmdale installed the wing bolts really tight.:E

[skiesfull]

Oh b*gg*r! I thought the flare on the TriStar always occured when I closed my eyes (the screams were from the other seat - should have closed his eyes too)!!

[glhcarl]

Well, as usual, it depends.

Large pitch (control column) inputs during approach will definitely change the pitch (deck) angle, small ones will not.

During the flare maneuver, larger than normal column inputs reduce the spoiler extension, while at the same time increases the pitch angle.

Also contributing to the outstanding handling of the TriStar is the all-flying horizontal stabilizer.

Relaxing the backpressure during the flare maneuver however (as has been pointed out already) many times results in a 'thumper'.

Hmmm, a few Boeing guys try 'the push' and then promptly find out it ain't a good idea.
'Tis a good thing the folks at Palmdale installed the wing bolts really tight.:E
When you pull aft on the column the FCES thinks the aircarft is below the glideslope and stowes the spoilers, and the stabilizer nose drops which make the nose of the aircraft pitch up. If you push forward on the column the FCES thinks the aircraft is above the glideslope, the spoilers are going to extend further and the stabilizer nose is going to move up. Causing the aircraft to lose altitude rather quickly.

[Obi Offiah]

Ok, so moderate longitudinal control inputs with DLC active, would result in a constant pitch angle but a change in vertical velocity, as the spoilers control the velocity vector. Here the pilot has direct control of the spoilers, but the stabilators are positioned to maintain a constant pitch angle and their positioning is accomplish completely through FCS feedback logic.

As well as direct spoiler control, longitudinal control inputs with DLC active above an FCES defined threshold, would also add or subtract pilot commanded stabilator inputs to blend with the FCS commanded stabilator input.
Is this how the system works?.

Thanks
Obi

[glhcarl]

Obi Offiah,

I am not a pilot (I was a Tech Rep) but the way I have been told the DLC should work is: The stabilizer is used to establish the angle of attack, approxmately 9 degrees nose up. The spoilers, which are controlled by the FCES, and the throttles control the sink rate. If all works, the pitch angle should not change during approch.

[411A]

All correct, except that the normal deck (pitch) angle on approach is approximately 7.5 degrees.
Upon landing, the pitch angle is nine to ten degrees.
IF ten degrees is exceeded during landing, care needs to be exercised, as the tail skid (long body aircraft) will contact the runway surface at approximately 13.5 degrees.
Does this happen?
Yes, IF the aircraft is mishandled.
On a slightly different slant, the geometry of the L1011 (standard, long body model) is such that during crosswind manual landings, the wing tip will contact the runway surface, before the engine pod does.
A very good, and reliable design.
Appreciated by those that have extensive experience in same.:)

[idg]

Was told of an interesting technique used by Pan-Am drivers when they were flying the -500s. Not sure if it was offically approved though!

In the flare they would push the GA button thus stowing the DLC and creating a nice load of lift just when it was needed.

Seemed to me that this was OK as long as there wasn't a very large back stick input which would have stowed them in the first place!!