D. Structures and Mechanisms · Flight

Balanced and Unbalanced Forces: Controlling Flight

D2.3 A1.5 A2.1About 50 minutes

Learning goals

I can explain how balanced and unbalanced forces let a plane take off, cruise, turn and land.
I can name the control surfaces of a plane and what each one does.
I can describe how birds, insects and other living things use balanced and unbalanced forces to control their flight.
I can write simple code that uses input to decide what a flying object will do.

Words to know

balanced forces
Forces that are equal in size and opposite in direction. They cancel out, so the motion does not change.

unbalanced forces
Forces that do not cancel out. They make an object start, stop, speed up, slow down or change direction.

control surface
A movable part on a plane's wings or tail that the pilot tilts to change the forces and steer.

aileron
A hinged part near the tip of each wing. Ailerons roll the plane (tip one wing up and the other down).

elevator
A hinged part on the back edge of the tail's small horizontal wings. It tips the nose up or down.

rudder
A hinged part on the back of the tall tail fin. It swings the nose left or right.

hover
To stay in one place in the air.

How does a pilot steer through the sky?

A car has a steering wheel, a gas pedal and brakes. A plane has no road to push against. So how does a pilot take off, turn and land?

The answer: by making the four forces balanced or unbalanced on purpose. Birds and insects do exactly the same thing with their wings and tails.

Balanced or unbalanced?

Balanced forces cancel out. The object keeps doing what it was doing: staying still, or moving at the same speed in the same direction. A plane cruising at a steady height and speed and a hummingbird hovering in one spot both have balanced forces.
Unbalanced forces change the motion. The object speeds up, slows down, rises, sinks or turns.

To change what a plane is doing, the pilot makes the forces unbalanced for a while. To keep it steady, the pilot balances them again.

A ruby-throated hummingbird with a green back and red throat hovering beside a red flower. — When a hummingbird hovers in one spot, lift balances its weight and thrust balances drag, so its forces are balanced. AI-generated image

Force diagrams for a plane flying to the right. Left: balanced forces (equal arrows in each pair), so the plane keeps the same speed and height. Right: unbalanced forces. Lift is longer than weight and thrust is longer than drag, so the plane starts to rise and speeds up.

Part of the flight	What the forces are doing	Result
Takeoff run	Engines at full power: thrust bigger than drag	The plane speeds up along the runway.
Liftoff	Pilot raises the nose; lift becomes bigger than weight	The plane leaves the ground and starts to climb.
Cruising	Lift equals weight; thrust equals drag (balanced)	Steady speed and height.
Starting the descent	Pilot reduces thrust and lowers the nose a little; lift becomes a bit smaller than weight	The plane starts to come down.
Landing	On the ground: spoilers, wheel brakes and engines blowing air forward make the backward forces much bigger than thrust	The plane slows down and stops.

Forces during a flight

Control surfaces: tilting parts that change the forces

A plane's wings and tail have hinged parts that tilt up or down. Each one changes how the air pushes on that part of the plane, which makes the forces unbalanced in a controlled way.

Elevator (on the tail): tilting it changes the push of air on the tail, which tips the plane's nose up or down. Nose up gives the wings more angle of attack and more lift.
Ailerons (near the wing tips): when one goes up, the other goes down. The wing with its aileron down gets more lift and rises; the other wing drops. The plane rolls (tilts sideways).
Rudder (on the tail fin): swings the nose left or right, like a boat's rudder.
Flaps (on the back of the wings near the body): slide out and down for takeoff and landing. They give more lift at slow speeds (and more drag).
Spoilers (on top of the wings): flip up to spoil (reduce) lift and add drag, for example right after landing.

Turning: a plane turns by banking (rolling) with its ailerons. The lift now tilts sideways a little, and that sideways part of the lift is an unbalanced force that pulls the plane around the curve. The rudder helps keep the turn smooth.

A plane seen from above. The shaded ailerons, flaps and elevators hinge on the back edges of the wings and tail. The spoilers lie on top of the wings, and the rudder is on the back of the tall tail fin.

Control surface	Where it is	What it does
elevator	back edge of the tail's small horizontal wings	tips the nose up or down
ailerons	back edge of the wings, near the tips	rolls the plane to one side (to bank and turn)
rudder	back edge of the tall tail fin	swings the nose left or right
flaps	back edge of the wings, near the body	more lift (and drag) for slow takeoffs and landings
spoilers	top of the wings	less lift and more drag (to slow down and stay on the ground)

A plane's control surfaces

Living things control their flight too

Birds flap harder or faster to make more thrust and lift (to take off or climb). They twist and tilt their wings and tail to turn. To land, a bird tips its body up, spreads its wings and tail wide, and pushes its feet forward. This makes lots of drag, an unbalanced backward force, so the bird slows down.
Hummingbirds (like the ruby-throated hummingbird found in Ontario in summer) can hover. Their fast-beating wings make lift that exactly equals their weight: balanced forces. They can even fly backward.
Dragonflies move their front and back wings separately, so they can hover, dart sideways and change direction very quickly.
Gliders like the northern flying squirrel do not make thrust. They steer by moving their legs to change the shape of the furry skin flaps they glide on, and tip up just before landing to slow down.
Maple keys spin as they fall. Their weight quickly gets balanced by the upward push of the air, so they fall at a slow, steady speed and the wind can carry them away from the parent tree.

Quick check

Balanced = no change: steady speed in a straight line, or staying still (hovering, resting). Unbalanced = change: speeding up, slowing down, going up or down more, or turning.

Worked example

Let's analyse a goose landing on a pond

A Canada goose glides toward a pond at a steady speed. Then it tips its body up, spreads its wings wide and pushes its feet forward. Describe the forces before and after.

Before: gliding at a steady speed means the forces are balanced (it is gliding gently downward, with gravity helping to keep it moving forward while drag pushes back).
After: tipping up and spreading its wings and tail makes much more drag.
Now the backward force is bigger. The forces are unbalanced, so the goose slows down.
Its feet touch the water and the water's drag slows it even more, until it stops.

Balanced while gliding steadily; then unbalanced (drag bigger) so the goose slows down to land.

Worked example

Let's explain how a plane turns right

A pilot wants to turn right. Which control surfaces move, and which forces become unbalanced?

The pilot moves the ailerons: the right aileron goes up and the left aileron goes down.
The left wing gets more lift and rises; the right wing drops. The plane banks (rolls) to the right.
Lift now tilts to the right. Part of the lift pulls the plane sideways to the right. This sideways force is unbalanced, so the plane's direction changes.
The pilot uses the rudder to keep the nose pointing smoothly around the turn and the elevator (nose up a little) to keep from losing height.

Ailerons bank the plane right, the tilted lift becomes an unbalanced sideways force, and the plane curves to the right.

Just for fun

Funny but true: pilots can "wave" with a whole airplane! Rocking the wings from side to side with the ailerons is a friendly way to say hello or "message received".

when green flag clicked

ask [How big is the lift? (0 to 10)] and wait

set [lift] to (answer)

ask [How big is the weight? (0 to 10)] and wait

set [weight] to (answer)

if <(lift) > (weight)> then

say [Unbalanced: the plane rises!] and change y by 50

else if <(lift) < (weight)> then

say [Unbalanced: the plane sinks!] and change y by -50

else

say [Balanced: steady, level flight.]

A block-code plan (like Scratch) that takes typed input and decides what a plane sprite does. You can also get input in other ways: a slider for thrust, the arrow keys, or the tilt sensor on a micro-controller board to steer.

Hands-on activity

Make a paper helicopter (and code a flight)

Part A: paper helicopter. You need: a strip of paper about 5 cm wide and 20 cm long, scissors, a ruler, a paper clip and a stopwatch (a phone timer works).

Safety: use scissors carefully, with a grown-up nearby. Drop helicopters from a stable chair or step only with a grown-up holding it, or just stand and drop from as high as you can reach.

Cut down the middle of the strip from one short end, 8 cm long, to make two blades.
Fold one blade forward and the other back.
Fold the bottom part of the strip in half the long way and clip a paper clip to the bottom.
Drop it from as high as you safely can. Time how long it takes to land. Do 3 drops.
Add a second paper clip (more weight) and do 3 more drops.

Explain: after the first moment, does it fall at a steady speed? (Yes: the upward push of the air balances its weight, like a maple key.) With 2 clips, the weight is bigger, so it must fall faster before the air's push grows enough to balance it again.

Part B (optional coding): in a free block-coding tool such as Scratch, build the program shown above. Then add a second input: a slider or the arrow keys for thrust and drag to make the plane speed up or slow down.

What to look for: fall times are shorter with 2 clips; the child explains using the words balanced and unbalanced. In the code, the sprite rises when lift > weight, sinks when lift < weight and stays level when they are equal.

Your turn

What do balanced forces do to a flying object's motion?

During the takeoff run, which force is bigger, thrust or drag?

Which control surface tips the plane's nose up or down?

Which control surfaces does a pilot use to bank (roll) the plane?

How does a hummingbird hover?

How does a bird slow down to land?

What do spoilers do?

More practice: warm-up

Balanced or unbalanced: a plane cruising at a steady speed and height?

Balanced or unbalanced: a plane speeding down the runway?

Which control surface swings the nose left or right?

True or false: a flying squirrel makes thrust by flapping.

Why do flaps come out for landing?

More practice: on your own

A maple key falls at a slow, steady speed. Are the forces on it balanced or unbalanced? Explain.

Explain how banking helps a plane turn.

A pilot pulls back so the elevator tips the nose up a little while flying level. What happens to lift, and what will the plane start to do?

In the paper helicopter test, Leo's times were 3.1 s with one clip and 2.2 s with two clips. Explain the difference using forces.

Why might a goose flap hard when taking off from a lake, but glide when coming in to land?

Challenge

A plane flies level at a steady speed. The pilot banks into a turn but changes nothing else. The plane starts to lose height. Why? How can the pilot fix it?

In the code above, what will the plane do if the user types lift = 6 and weight = 6? What if lift = 7 and weight = 4?

A dragonfly hovers, then darts sideways to catch a mosquito. Describe the forces in both parts.

Check your understanding

Quiz

A plane is flying at a steady speed and height. The forces are:

Which control surface rolls (banks) a plane?

A plane speeds up on the runway because:

A hummingbird hovering in one spot has:

How does a bird slow down before landing?

What does the elevator do?

In the block code, the plane says "the plane sinks!" when:

For parents and teachers

Big idea (D2.3): flying machines and living things control flight by making forces balanced (steady motion or hovering) or unbalanced (speeding up, slowing down, climbing, sinking, turning). Planes do this with engines and control surfaces; birds and insects do it by changing how they flap and how they hold their wings and tails.

Coding (A2.1): the optional code uses input (typed numbers, sliders, keys or a tilt sensor) to decide what happens. Scratch (scratch.mit.edu) is free; no account is needed to try it.

How to help: watch birds landing at a feeder or on a pond, or watch a video of a plane landing, and ask "Balanced or unbalanced right now? How do you know?" Ask your child to explain using full sentences and the names of the forces (A1.5).

Common difficulties:

Thinking balanced means stopped. Balanced means no change in motion.
Thinking a turning plane at a steady speed has balanced forces. Changing direction needs an unbalanced force.
Safety: scissors with supervision; no climbing on wobbly chairs to drop helicopters.