Grade by Grade
D. Structures and Mechanisms · Flight

Changing the Four Forces of Flight

D2.4 A1.2 A3.2About 50 minutes
Learning goals
  • I can describe ways to increase or decrease lift, weight, thrust and drag.
  • I can explain how designers, pilots and living things change the forces of flight.
  • I can plan and carry out a fair test with paper airplanes and use averages to compare results.

Words to know

streamlined
Smooth, rounded and tapered so that air (or water) flows around it easily, with little drag.
variable
Something that can change in an experiment.
fair test
An experiment where you change only one variable and keep everything else the same.
average (mean)
A typical value: add up all the results and divide by how many results there are.
payload
What a flying machine carries: passengers, cargo, mail or equipment.
landing gear
The wheels and legs a plane uses on the ground. Many planes fold them up into the body after takeoff.

Faster, higher, farther!

Why do fast birds like peregrine falcons tuck their wings in to dive? Why do big planes fold up their wheels after takeoff? Why are modern planes made of super-light materials?

Each one is a way of changing one of the four forces of flight. Designers, pilots and animals all do it.

A grey and white peregrine falcon diving head-first with its wings tucked close to its body.
By tucking in its wings, a diving falcon cuts drag and lift, so its weight pulls it down very fast. AI-generated image
ForceWays to make it biggerWays to make it smaller
liftbigger wings; flaps out; more speed; tilt the wings more (but not too much); denser (cooler) airsmaller wings; less speed; less tilt; spoilers up; thinner air (hot or high up)
weightmore payload (passengers, cargo); more fuel; heavier materialslighter materials (aluminium, carbon-fibre composites); less payload; burning fuel during the flight
thrustmore engine power; bigger or more propellers or engines; a bird flapping harderless engine power; gliding with no engine
dragmore speed; flat or bumpy shapes; flaps, spoilers, wheels down; parachutesa streamlined shape; smooth surfaces; folding up the landing gear; tucking in wings (birds)
Ways to change the four forces
flat face: more dragstreamlined: less drag
Air flowing from the left meets two shapes. The flat face blocks the air and leaves swirls behind it, so it has a lot of drag. The smooth, rounded, tapered (streamlined) shape lets air flow around it easily, so it has much less drag.

Some forces are linked

Changing one force often changes another:

  • More speed gives more lift but also more drag.
  • Bigger wings give more lift but add weight and drag.
  • More fuel lets a plane fly farther, but adds weight, so it needs more lift and thrust.
  • Flaps add lift and drag, which is useful when landing slowly.

So engineers are always making trade-offs: choosing what matters most for each job. A glider has long, thin wings for lots of lift with little drag. A fighter jet has short wings and huge engines for speed. A bush plane that lands on short northern lakes has big wings and big flaps so it can fly slowly.

Living things change their forces too

  • A peregrine falcon tucks its wings close to its body to dive. This cuts drag and lets gravity speed it up a lot.
  • A bird taking off flaps hard and fast to make more thrust and lift.
  • A bird or bat spreads its wings wide to glide slowly (more lift and more drag).
  • A flying squirrel spreads its legs to stretch its gliding flaps, which gives more lift and drag so it glides instead of falling.
Worked example

Let's find the averages

Aanya tests a paper airplane with no paper clips and with one paper clip on the nose. She throws each one 3 times from the same spot, with the same strength. No clips: 4.2 m, 3.8 m, 4.6 m. One clip: 5.1 m, 4.7 m, 5.0 m. Which flew farther on average?

  1. No clips: 4.2 + 3.8 + 4.6 = 12.6 m. Divide by 3: 12.6 ÷ 3 = 4.2 m.
  2. One clip: 5.1 + 4.7 + 5.0 = 14.8 m. Divide by 3: 14.8 ÷ 3 ≈ 4.9 m (rounded to one decimal place).
  3. Compare: 4.9 m is more than 4.2 m.
  4. Why do 3 throws? One throw could be lucky or unlucky. An average gives a fairer picture.

With one clip, the plane flew farther on average: about 4.9 m compared to 4.2 m. (A clip on the nose can make a paper plane fly more steadily, but test it, because every plane is different.)

0123456distance (m)average 4.2 mno clipsaverage about 4.9 mone clip= one throw
Aanya's results. Each dot is one throw; each bar shows the average of the three throws. The plane with one paper clip flew farther on average (about 4.9 m compared with 4.2 m).
Worked example

Let's choose changes for a job

A company wants a drone to carry heavier packages to a remote community. The heavier package adds weight. Suggest changes to the four forces so the drone can still fly.

  1. More payload means more weight, so the drone needs more lift.
  2. A drone's spinning propellers make its lift and thrust. Use bigger propellers or more powerful motors to push more air down.
  3. Make the drone's own body lighter (for example with carbon-fibre parts) to cut weight.
  4. Give the package a streamlined case to reduce drag, so the battery lasts longer.

Increase lift and thrust (bigger propellers, stronger motors), reduce weight (lighter materials) and reduce drag (streamlined case).

Just for fun

A peregrine falcon that tucks in its wings to cut drag can dive at more than 300 km/h. That is over three times the speed limit on most Ontario highways, and it never gets a speeding ticket!

Hands-on activity

Paper airplane fair test

You need: 2 sheets of the same paper, 2 paper clips, scissors (only if you choose to cut the wingtips), a measuring tape, masking tape and a notebook.

Safety: throw only in a clear space, never toward people's faces or pets. Pick up paper clips afterward.

  1. Fold two identical paper airplanes (use the same design and the same folds).
  2. Pick one variable to change. Ideas: add a paper clip to the nose of plane B; or fold up a 1 cm flap on the back edge of each wing of plane B; or cut 2 cm off each wingtip of plane B (with a grown-up nearby). Keep plane A unchanged.
  3. Write your question and prediction: "If I add a paper clip to the nose, then the plane will fly ___ because ___."
  4. Put a strip of masking tape on the floor as a start line. Throw each plane 3 times from the line, the same way, with the same strength. Measure where it first touches the floor.
  5. Record your results in a table. Find the average for each plane.
  6. Write a conclusion: was your prediction correct? Which force did your change affect?

What to look for: a clear table with 3 throws and an average for each plane, only one thing changed, and a conclusion that names a force (e.g. "The paper clip added weight at the nose, and the plane flew straighter and farther"). Results vary, and an unexpected result is still a good result if the test was fair.

Your turn

1

Name two ways to increase lift.

2

Name two ways to reduce weight.

3

Name two ways to reduce drag.

4

How can a plane get more thrust?

5

Why do many planes fold up their wheels after takeoff?

6

Why does a falcon tuck its wings in to dive?

7

Find the average of 6 m, 7 m and 8 m.

More practice: warm-up

1

Which force do flaps increase that helps planes fly slowly?

2

What does a streamlined shape reduce?

3

Does carrying more passengers increase or decrease weight?

4

Which force does a glider not have?

5

In a fair test, how many variables do you change?

More practice: on your own

1

A plane's weight goes down during a long flight. Why?

2

Why don't designers just give every plane enormous wings for lots of lift?

3

A bush pilot needs to land on a short frozen lake. Which force changes would help the plane land slowly and stop quickly?

4

Owen threw his plane 3 times: 2.5 m, 3.0 m, 3.5 m. What is the average? Why is the average more useful than one throw?

5

A cyclist crouches low over the handlebars in a race. Which force of flight does the same idea reduce, and how?

Challenge

1

Plane A and plane B are the same design. Plane B has a paper clip on its nose and is also thrown harder. It flies farther. Can you say the paper clip caused the longer flight? Explain.

2

A passenger jet adds 20 tonnes of extra fuel for a longer trip. Describe how this affects at least three of the four forces.

3

Some modern planes have small upturned tips at the ends of their wings (winglets). Engineers say they reduce drag. Why would an airline want less drag?

Check your understanding

Quiz

1.

Which change reduces drag?

2.

Which change increases lift?

3.

Why does a plane get lighter during a flight?

4.

How can a plane get more thrust?

5.

In a fair test with paper airplanes, you should:

6.

What is the average of 3 m, 5 m and 7 m?

For parents and teachers

Big idea (D2.4): each force of flight can be changed: lift (wing size, flaps, speed, tilt, air density), weight (materials, payload, fuel), thrust (engine power, propellers, flapping) and drag (streamlining, landing gear, speed). Changing one force often changes another, so designers make trade-offs.

Math link (A3.2): the fair test uses averages (mean) and decimals, which connects to Grade 6 math. Let your child do the calculations; a calculator is fine for checking.

How to help: do the paper airplane test together and act as the "measurer" while your child records. Ask "What is the one thing you changed?" and "Which force did that change?"

Common difficulties:

  • Changing two things at once. Encourage writing the question first: "If I change ___ ..."
  • Expecting one "right" result. Paper planes vary; a fair test with an unexpected result is still good science.
  • Safety: throw in a clear space away from faces and pets; supervise any cutting.