Happy-flier
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Spoiler-activated Descent Control

Fri Nov 02, 2012 4:52 pm

Sometimes when watching certain aircraft flare - the A343 comes to mind - I wonder why the designers didn't implement a spoiler-activated descent management regimen, as was used in the L1011. That system allowed the aircraft pitch to be maintained, but rate of descent could be varied by "dumping" some lift via simultaneous partial deployment of the spoilers.

Seems like a great feature to have - wonder why the newer FBW airliners don't use it? Or do they?
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Pihero
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RE: Spoiler-activated Descent Control

Fri Nov 02, 2012 5:56 pm

What you're referring to is the DLC - direct lift control - , one of the features of the Tristar.
This is - very simplistically described - how it works :
From all six spoilers on each wing, spoilers 1 to 4 are extended seven degrees out of the stowed position, when one is in the final approach configuration : flaps at setting 30 or more.
On that final approach, any yoke action - either pilot or A/P induced - will have a primary effect on the 1 to 4 spoilers : a demand for a shallower path would partially retract them and a demand for steeper path will extend them further (to a maximum of 14° ). It's only when the demand exceeds the parameters of the DLC spoilers that the tailplane will intervene.

That system was monstruously complex and it shines on the Lockheed engineers to have pulled the whole system up :
- Mechanically, they had to add DLC to the spoiler roll control, the speedbrake system.... the spoiler mixer was something to behold.
- Electronically, itr was even less simple : consider that as a matter of fact, DLC was controlled by EFCS signals, as a stabilizer out-of-trim signal.
And finally consider the whole tree of conditionals : air/ground; no GoAround.....
The myth is that Douglas, Boeing and Airbus tried to emulate the system, but could not (some patents could have helped !)

Two very beneficial effects arose from the DLC :
1/- Approach stability was phenomenal ( we are talking mid-to-late seventies )
2/- In case of a go-Around, the DLC spoilers would retract with a bang, giving the pilot an immediate 5 tons of lift... and added to the fact that DLC caused a slightly higher thrust setting, hence a quicker engine acceleration, no wonder the L-1011 has had the lowest decision height ever at a few airports I've visited : We tested 12 ft at CDG without a hitch and settled for 15.
Why isn't it used any more ?
Mainly because the newer digital A/Ps were accurate and smooth enough to make a final approach as smooth as the Tristar 35 year's ago... Of course, we'd hit the runway during a go-around at decision height, but it's smooth and quite acceptable... plus, the systems are a lot less complex.

But I still miss the elegance of the Tristar approach.
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Happy-flier
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RE: Spoiler-activated Descent Control

Sat Nov 03, 2012 12:04 am

Pihero - thanks so much for that phenomenal reply. So much great info in there.

Cheers.
May the wind be always at your back . . . except during takeoff & landing.
 
tdscanuck
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RE: Spoiler-activated Descent Control

Sat Nov 03, 2012 12:44 am

Quoting Happy-flier (Thread starter):
Seems like a great feature to have - wonder why the newer FBW airliners don't use it? Or do they?

Like pihero said, they don't need it. They do some other very cool stuff with the spoilers, thanks to FBW, but not DLC. Maneuver load alleviation and spoiler gapping are the latest things though. DLC also, by necessity, hikes drag up. That's not a very favorable thing to do these days.

Tom.
 
Mir
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RE: Spoiler-activated Descent Control

Sat Nov 03, 2012 1:41 am

I'd imagine that having the engines spooled up higher on final wasn't very good from a noise abatement standpoint. That's kind of a thing these days.

-Mir
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vikkyvik
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RE: Spoiler-activated Descent Control

Sat Nov 03, 2012 2:44 am

Quoting Pihero (Reply 1):

Was engine thrust at all tied into DLC, or was it a completely separate loop?
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Fabo
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RE: Spoiler-activated Descent Control

Sat Nov 03, 2012 7:18 am

Quoting vikkyvik (Reply 5):
Was engine thrust at all tied into DLC, or was it a completely separate loop?

I dont see why it would be, it would screw up with the principle of operation.
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vikkyvik
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RE: Spoiler-activated Descent Control

Sat Nov 03, 2012 8:08 am

Quoting Fabo (Reply 6):
I dont see why it would be, it would screw up with the principle of operation.

I ask because raising and lowering the spoilers will increase/decrease drag. Was just wondering if the system was designed to compensate for that and maintain speed while changing approach angle.

Though I suppose that would just be autothrust.
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Fabo
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RE: Spoiler-activated Descent Control

Sat Nov 03, 2012 9:56 am

The same can be told basically about any airplane, when you extend flaps, drag goes up, you need to add a bit of power. Maybe a bit more in Tristar, but after you are set in approach descent, the same pitch&power principle applies. If any system changed engine settings then, it would mean that handflying would feel very different than any other airplane.
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vikkyvik
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RE: Spoiler-activated Descent Control

Sat Nov 03, 2012 7:37 pm

Quoting Fabo (Reply 8):
The same can be told basically about any airplane, when you extend flaps, drag goes up, you need to add a bit of power.

I'm aware, thank you.

Quoting Fabo (Reply 8):
If any system changed engine settings then, it would mean that handflying would feel very different than any other airplane.

I'd assume it already does, given that you're adding or subtracting lift rather than pitching up or pitching down....
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Pihero
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RE: Spoiler-activated Descent Control

Sat Nov 03, 2012 10:32 pm

Quoting Mir (Reply 4):
I'd imagine that having the engines spooled up higher on final wasn't very good from a noise abatement standpoint. That's kind of a thing these days.

I seem to remember that the T* wasn't louder thatn the DC-10, and that was IIRC due to the #2 engine position. But you're right, that sort of set-up would be hard to certify nowadays noise abate-wise.

Quoting vikkyvik (Reply 5):
Was engine thrust at all tied into DLC, or was it a completely separate loop?

There was a component of the AFCS which was called "SCS" for "Speed Control System"' which then had control / monitoring of the ATS aka Auto Throttle System. Those different systems were quite independent of one another (that architecture was unique among all OEMs).

Quoting vikkyvik (Reply 7):
ask because raising and lowering the spoilers will increase/decrease drag. Was just wondering if the system was designed to compensate for that and maintain speed while changing approach angle

As a matter of fact, the Tristar on final was an incredibly steady bird for two reasons :
1/- Inherent inertia of a draggy, heavy body
2/- Energy conservation : If you're high over the glide, you' d push the yoke, causing the DLC spoilers to extend a bit more, causing some drag... in similar conditions, you'd remove some thrust on another aircraft ; here, no need, it seems as if potential energy due to the steeper descent is matched bhy the one caused by the increased drag ; result : one doesn't chase IAS on a Tristar.

[Edited 2012-11-03 15:34:14]
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