More Direct Lift Control nonsense

[tom775257]

I was bored sitting in the back end of a 777 mid-ocean the other day, and I got thinking. Ok, the L1011 has DLC using spoilers and from reading some old threads in ‘Tech Log’ I have learnt that the L1011-500 and A320 can utilize symmetrical aileron movement for gust (load) alleviation.

Why not design FBW aircraft (with both inboard and outboard ailerons) to use symmetrical inboard aileron movement downwards on rotation for take-off to increase lift directly, allowing a lower deck-angle and thus reducing the chance of a tail-strike. Drag would not be increased significantly due to the fact the ailerons would be in the neutral position for most of the take-off run until rotation. Roll control could be handled by the outboard ailerons and spoilers, perhaps at say 200ft RA normal roll control could be returned…. Surely in long aircraft such as the A340-600 this could be useful?
Any comments?

[fritzi]

If you look at the A330/340 or the A320 series, you will see that the ailerons already droop down during takeoff and landing. They automatically droop down when you extend the flaps to a certain position.


Want evidence? (http://www.airliners.net/open.file/412095/L/)

On the B777, the inboard ailerons droop down during landing but I think they are in the neutral position during takeoff but I am unsure of why. Engines maybe?


Feel free to correct me if I screwed up somewhere. Im no professional, Im only a fanatic with a bit of extra knowledge. :p

[tom775257]

Yes I believe you are correct, however they assume a neutral position pretty much in line with the flaps (AFAIK).
I am thinking of deflection further downwards to the point that gives the aerofoil higher coefficient of lift than with the ailerons in line with the flaps - and specifically at the point of rotation; thus DLC.

[safetypee]

Drooped ailerons do not normally provide DLC per se; spoilers are the usual control surface employed. Load alleviation is more of a long-term control strategy rather than a short-term control function.
The UK researched DLC in the late 70’s using a BAC 1-11 (RAE, XX105). In pitch, the control column was connected to the spoilers (FBW) as well as the elevator. In balanced flight, the spoilers were propositioned approx 10 deg out. Back stick moved the spoilers in, thus increasing lift; forward stick moved the spoilers out, decreasing lift. There was a spoiler - elevator interconnect to negate pitching moment for small stick movements, thus when on the glide-slope the aircraft could be moved up and down without a major pitch change.
Several tests evaluated the advantages of DLC during the landing flare from steep approaches (6 deg). The change in 1-11 flare performance was remarkable; normally from a 6 deg approach the flare was commenced at 100 ft, but with DLC this could be as low as 35 ft. The aim was to improve landing performance by reducing the distance travelled in the flare; this was a success. Unfortunately when landing with spoilers out the approach speed had to be increased to maintain the necessary stall margin; this offset any gain in landing performance.

[LOMCEVAK]

Tom775257, what you are proposing is analogous to lowering the flaps at the point of rotation. This will indeed increase lift but may also induce a pitching moment, either nose up or nose down, which could make the aircraft very unpredictable/difficult to fly just as it unsticks. However, you could have control laws which automatically give an elevator input to counter this pitching moment. As safetypee says, an elevator interconnect with the DLC spoiler deflection works on some aircraft on the approach. However, trim changes at rotate and unstick are more critical as you have to allow for a misset trim (incorrect AUW and c.g. on loadsheet!) and the discontinuities due to wings, tailplane etc being in ground effect.