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so when an aircraft flies in cruise, it burns weight and so what impact does this have in lift requirements?

Would there be a need to also deflect flaps to stay in cruise as fuel is burned?

— I’m aware there is a similar question but it didn’t answer my question, it was asking about how pilots react to this, I’m more interested in the aerodynamics side of it

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so when an aircraft flies in cruise, it burns weight and so what impact does this have in lift requirements?

In straight and level flight, total lift equals total weight. As the fuel burns, two things can happen: the balance of the aircraft shifts as weight is taken from one part, and the total weight of the aircraft decreases.

In many planes, the change of balance dominates and you need to correct for that with some frequency, so you'd never notice the weight change.

If your plane is trimmed for straight and level flight and the balance doesn't shift, you might find that it will start to climb a bit as fuel burns off. If so, you could reduce power slightly to maintain level flight.

For a particular airplane, he lighter it is, the less power will be required to fly at a given altitude and speed.

Would there be a need to also deflect flaps to stay in cruise as fuel is burned?

Flaps are not needed or used in cruise. The cruising airplane is controlled through power settings and elevator/trim position.

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    $\begingroup$ Hi so, the elevators are the flaps at the end right? What would they do with them, so let’s say weight decreased, do they put them up or down please? $\endgroup$ Apr 16, 2023 at 13:30
  • $\begingroup$ "Flaps" have a specific meaning in airplanes. They are only one particular control surface. Calling the elevator "flaps at the end" would be confusing. Those tutorials mentioned in the comments would be a good starting point for some of the basics. $\endgroup$
    – BowlOfRed
    Apr 16, 2023 at 20:00
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Pitch trim creates a force balance that drives the aircraft to seek a specific angle of attack. But as weight is reduced, less angle of attack (AOA) is required for level flight.

So, as fuel is burned, with the airplane trimmed to maintain, say, a 4 degree angle of attack, static stability forces will tend to maintain that angle of attack. The reduction in weight means the lift generated at that AOA is excessive, and the plane will climb as it maintains its trimmed AOA.

To compensate, you have to trim to a lower AOA to regain the original lift/gravity equilibrium.

Retrimming to a lower angle of attack reduces drag, and so you also have to reduce thrust/power to keep the original speed.

What happens in practical terms is, you fly along, and we'll say you're hand flying and you have it trimmed hands off. The pitch attitude shows 4 deg nose up. After a little while you'll notice the plane starting to creep into a climb. You'll pitch the nose down slightly, to a slightly lower pitch attitude, say, 3.8 deg. If you do nothing else, the speed will start to creep up, so at the same time you'll reduce thrust a tiny amount to keep the speed constant.

If you're on an autopilot without autothrottle, you'll be observing and noticing that the airplane very gradually noses down slightly over time as the autopilot retrims to compensate for the weight reduction, and while that's happening you'll be easing the thrust back in tiny adjustments to keep the speed bug at the target IAS or Mach#. Or if there's an autothrottle system, you just sit and watch twiddling your thumbs.

From start to finish, depending on how much all that fuel weighed, you might find that the pitch attitude is maybe a degree lower at the end of the cruise than it was as the start, and thrust is a couple of percent lower as well, to maintain the same constant altitude and speed.

Flaps are not used because they are only for low speed use to reduce minimum flying speeds. Deploying even small angles of flap at cruise speed (airplanes generally aren't structurally designed to operate at cruise with flaps extended anyway) you end up with too much lift and a lot more drag, so efficiency goes to heck; there's no point.

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    $\begingroup$ What does airplane trimmed mean please? And what’s pitch please? $\endgroup$ Apr 16, 2023 at 13:31
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    $\begingroup$ @MintyPythonn They are elementary physical concepts without which not much meaningful discussion about control of an airplane can be had. I recommend you to read the excellent online book See How It Flies by John S. Denker. These terms are discussed in chapter 2, but to get good grasp on the physics of fixed-wing flight I recommend you read at least the first 10 chapters. $\endgroup$
    – Jan Hudec
    Apr 16, 2023 at 17:47

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