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so I understand that an elevator is used to maintain cruise (straight level and unaccelerated flight) but I’m curious now about how the flaps and elevator is positioned whilst an aeroplane is trying to maintain cruise?

Anyone know any good books on this sort of things please or a video maybe please?

Also why/how is it that a flap cannot be deflected in cruise and an elevator can? What’s the difference?

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  • $\begingroup$ This NASA's guide should help $\endgroup$
    – sophit
    Oct 20, 2023 at 18:38
  • $\begingroup$ Hi thanks Sophit, I’ll take a look tonight $\endgroup$
    – James
    Oct 20, 2023 at 19:22
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    $\begingroup$ You're welcome. And please wait at least 24 hours before selecting an answer in order to give all the people around the globe a chance to come up with a proper answer 🖖 $\endgroup$
    – sophit
    Oct 20, 2023 at 19:24

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Wing flaps are used to significantly change the wing (size/shape/aerodynamics) so that it flies better at the slower speeds associated with taking off & landing. With rare exceptions (fly-by-wire F/A-18, for example), flaps are set in fixed positions and aren't moved continuously during flight. For cruise flight, including everything from about a minute or two after takeoff until a few minutes before landing, the flaps are in the fixed "up" (i.e. fully retracted) position.

The primary flight controls, the ailerons, elevators, and (to a somewhat lesser extent) rudder, move dynamically throughout the flight, controlled by the pilot or the autopilot, to maintain the desired attitude. If the aircraft starts to climb (when level flight is desired), the pilot applies forward pressure on the yoke, lowering the elevator very slightly, increasing lift on the tail and lowering the nose by the small amount needed to arrest the undesired climb.

These small corrections continue throughout the entire flight to keep, and restore, the aircraft's attitude to the desired attitude. Much like the small corrections required to keep an automobile centered in its lane while driving, even as curves in the road, winds, variable road surfaces, and the like tend to cause the car to drift left & right.

Edit: Flaps have limitations regarding how fast the aircraft is allowed to be flying when they're extended, since putting them out at high speed can impose stress on the flaps & the associated hardware that connects them to the wings. For airliners that cruise high enough that cruise speeds are measured in high Mach numbers (i.e flying around at a large fraction of the speed of sound, where transonic airflows can have effects on the flight controls), there can also be limitations on how high you're allowed to extend flaps. In the 737, those limits are 250 knots and 20,000' for the initial extension, and around 168 knots for full flaps. So in typical cruise flight, you would be out of parameters to extend them.

Happily, at cruise speeds, you don't need to extend them, since you're well outside of the low-speed regime where they are helpful.

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  • $\begingroup$ Not to get overly detailed but I think it's worth also mentioning slats (perhaps this comment is sufficient), which serves the same general purpose as flaps but are located on the leading edge of the wing on most jet aircraft, at least all jet airliners seem to have them. Slats are normally extended at the same time the flaps are. However they are often not noticed because of the much smaller size and shorter movements. $\endgroup$ Oct 20, 2023 at 19:43
  • $\begingroup$ @StevePemberton I think that's a level of detail better suited for a separate question $\endgroup$
    – Ralph J
    Oct 20, 2023 at 19:46
  • $\begingroup$ RalphJ - The OP likely didn't mention slats possibly because they weren't aware of their similar function to flaps, or possibly even their existence, but I think it fits into their question "how is it that a flap cannot be deflected in cruise and an elevator can. Since much of your answer also applies to the slats I think it's worth at least mentioning their existence and general similarity to flaps to inform the OP. No other detail is needed, i.e. the technical differences between slats and flaps, which I agree would be better in a separate question. And maybe the comment was sufficient. $\endgroup$ Oct 20, 2023 at 19:59
  • $\begingroup$ There's likely to be automatic cruise flaps, a.k.a. variable camber, in all new airliners; the 787, 777X, and A350 all have it. $\endgroup$
    – user71659
    Oct 20, 2023 at 21:01
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    $\begingroup$ @James While flaps are extended and retracted, the elevator simply pivots. The motion of the yoke is continuous, from full forward to full aft, corresponding to the full range of elevator travel. It's never retracted, just repositioned. $\endgroup$
    – Ralph J
    Oct 21, 2023 at 14:14

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