Why don’t you ever see biplane shaped birds?
Learning Objectives
After completing this lesson, the student will be able to describe how wing geometry can be used to achieve desired stability, strength, and weight characteristics.
Standards
- NGSS HS-ETS1-3
- CCSS.Math.Practice.MP1
- CCSS.Math.Practice.MP2
- CCSS.Math.Practice.MP4
Supplies
- Paper
- Pencil
- Imagination
Units Used
- None – this is a math-free lesson!
A study in pictures…
Consider the following four airplanes. What differences do you see between their wing configurations? Why do you think the differences occur?
The first picture is a Piper Cub. This wing design is the simplest shown, for the most part having a constant chord. These types of wing structures are comparatively easy to build, making them desirable for low-speed, general aviation airplanes. The second picture is a Pitts aerobatic biplane. The two wings of a biplane allow for a smaller total distance across the wing – wing span. This lets the builder increase stiffness and reduce weight, but all those connecting structures along with the air flow interactions between the two wings results in more drag. The third plane is a Boeing 777. In that picture wing dihedral is particular visible. Dihedral is the upward angle of the wing.
When an airplane with wing dihedral starts to roll, the lower wing produces more lift, making the plane return to level flight. To visualize this, consider the sketch of an airplane below as though you are looking at it head on. Draw the lift vector on each wing. In which direction is the net moment on the aircraft due to the lift from each wing?
This natural roll-righting is a desirable trait for a passenger jet, but makes the plane less agile for a fighter jet, which is why you do not see it in the last picture of an F-14A.
A nifty trick that F-14 wings can do is change their wing sweep. A swept back wing has lower drag, and can fly at higher speeds without causing the flow over the wing to go supersonic. Why would the flow over the wing go supersonic? Think back to the airfoil we sketched in Part A. Recall we said that the flow is faster over the top of the foil than the bottom. This means the air is flowing faster over the top of the wing than the airplane is flying. So even if an airplane is flying below the speed of sound, the flow over the wing may go faster than the speed of sound! By sweeping the wing, we’ve reduced the component of flow travelling parallel to the wing chord, and can delay supersonic flow. A nice article with visual aids summarizing this is at: https://www.boldmethod.com/learn-to-fly/aerodynamics/wing-sweep/.
But, when you come in to land, it’s handy to have your wings not swept back. Those swept wings are made for speed, but will lose lift and stall at low speeds, so the F-14 was designed to do both – fly slow with the wings positioned forward, and fly fast with the wings swept back.
That’s some pretty cool engineering, right? And it’s biologically inspired! Have you ever watched an osprey go fishing? What do you see it doing with its wings when it dives? Morphing wings, like a wing that can change its sweep angle in flight, are a way we humans emulate nature with engineering innovation.
Go Fly a Kite
To help visualize the forces on an airfoil, check out this video on kite aerodynamics.
Next Steps
A great collection of lessons and activities to learn more about aerodynamics and wing geometry is available on NASA Glenn Research Center’s website at: https://www.grc.nasa.gov/www/k-12/airplane/short.html. To continue on this path focused on BLIMPs, check out the next lesson sequence on movement.
Last updated: February 27, 2023.
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