Learning Byte Summary ▾
Learning Objectives (LOs): How flapping works?
Grade level: Middle school
Duration: 10 min
Key ideas: Flapping flight; Lift and thrust force; Wingbeat

Introduction

Pacific parrotlet foraging flight with visualized net force, lift, and drag during takeoff and landing.M1

What you might notice?

  • A parrotlet launches off its perch and immediately tilts its wings at a steep forward angle. Watch the black arrow — it swings upward and forward, showing the wing is pushing the bird up and accelerating it away from the perch at the same time. Break that black arrow apart and you see why: the red drag arrow is pointing upward — drag is holding the bird up. The blue lift arrow is pointing forward — lift is pushing the bird ahead. Everything is flipped from what you’d expect. As the bird reaches the middle of its flight, the wings level out. The black arrow straightens upward, lift takes over holding the bird up, and drag settles into its usual backward direction. Then, approaching the landing perch, the wings tilt backward. The black arrow swings rearward — the bird is braking. Now both the red and blue arrows point backward together, sharing the job of slowing the bird down before touchdown. The whole flight lasts less than a second. But in that time, the bird quietly reassigned lift and drag to three completely different jobs — launching, cruising, and braking — just by tilting its wings.

How Flapping Wings Generate Lift

First: Your Plane Understanding Is Correct

  • Plane wings generate lift because:

    • The plane moves forward.
    • Air flows over the wing.
    • The wing is tilted at an [angle of attack].
    • The wing pushes air downward.
    • The air pushes the wing upward.
Air pushed down:  ↓
Wing pushed up:   ↑
  • So for a normal airplane:
Plane motion:  →
Drag:          ←
Lift:          ↑
  • This feels simple because the plane mostly moves forward in a straight line.

The Important Difference

  • In an airplane, the wing’s motion is mostly:
Forward →
  • But in a flapping bird, the wing is moving in a more complicated path:
Down
Forward
Backward
Diagonal
Twisting
  • So the direction of [lift] and [drag] changes during the wingbeat.

The Key Rule

  • [Drag] points opposite the wing’s motion through the air.
  • [Lift] points 90° to the wing’s motion through the air.

So:

Wing motion = reference direction
Drag = opposite wing motion
Lift = 90° to wing motion

Example 1: Plane Wing

  • A plane wing moves forward:
Wing motion:  →
  • Drag points opposite:
Drag:         ←
  • Lift points 90° to the motion:
Lift:         ↑

Full picture:

              Lift
               ↑
               |
Drag ←---------•--------→ Wing motion
  • This is why we usually think:
Lift = up
Drag = backward
  • But that is only because the plane’s wing motion is mostly horizontal.

Example 2: Bird Wing During Downstroke

  • A bird wing may move diagonally downward:
Wing motion:
        ↘
  • Drag points opposite that motion:
Drag:
        ↖
  • Lift points 90° to the wing motion:
Lift:
        ↗

Full picture:

          Lift
           ↗
          /
         /
Drag ↖  •
        \
         \
          ↘ Wing motion
  • The dot • is the wing.

Why the Blue Lift Arrow Can Point Right

  • In your screenshot, the wing looks like it is moving downward and diagonally.

  • If the wing motion is something like:

Wing motion:
        ↘
  • Then lift can point:
Lift:
        ↗

  • That means lift has a forward/rightward part.

  • So the blue lift arrow pointing right is not wrong.

  • It just means:

Lift is not always straight up.
Lift is 90° to the wing’s motion.

How This Connects to the Paper

  • The paper’s main idea is that birds can [repurpose lift and drag].

  • During takeoff:

    • The wing stroke is tilted.
    • [Drag] points partly upward.
    • [Lift] points partly forward.
During takeoff:

Lift  ↗  = helps bird accelerate forward
Drag  ↖  = helps support body weight
  • This is surprising because we usually expect:
Lift = support weight
Drag = slow down
  • But in flapping flight, the forces can be redirected.

The Best Mental Model

Do not start with:

Lift = up
Drag = bad/backward

Start with:

What direction is the wing moving through the air?

Then apply:

Drag = opposite that motion
Lift = 90° to that motion

Then ask:

Where do those arrows point in the room?

Final Takeaway

  • Plane wings and bird wings both generate aerodynamic force by moving through air at an angle.
  • The wing redirects air.
  • The air pushes back on the wing.
  • Scientists split that total air force into:
[drag] = force opposite wing motion
[lift] = force 90° to wing motion
  • For planes, lift usually points upward because the wing moves forward.
  • For flapping birds, lift can point forward, backward, or upward because the wing moves diagonally and changes direction during each flap.

Birds can let some air pass through their feathers because the feathers are not one solid sheet. During the upstroke, the long feathers near the wingtip can spread apart a little, like opening your fingers. Air slips through the gaps, so the wing does not push as much air upward. This helps the bird avoid being pushed downward and makes the upstroke easier. During the downstroke, the feathers press together more tightly, acting like a stronger surface to push air downward and lift the bird up.

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References

  1. 1.
    How Do Airplanes Actually Fly?
    Raymond Adkins · TED-Ed · 2023

Media Credits

  1. M1
    Birds repurpose the role of drag and lift to take off and land (Supplementary Movie 1)
    Diana D. Chin & David Lentink · Nature Communications 10, Article 5354 (2019) · 2019 · CC BY 4.0
How to Cite: Sinha, R. (2026). Lift force. Amazing Birds. https://amazing-birds.gitlab.io/
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