NASA’s Mars 2020 Mission, featuring the Perseverance rover, landed on Mars on February 18, 2021. The rover carried a small helicopter named Ingenuity. On April 19, 2021, Ingenuity became the first aircraft to fly under its own power on a planet other than Earth.

This page presents two options for a lesson about Ingenuity’s historic flight. The first option, immediately below, provides a self-paced, online lesson that allows you or your students to explore the design of this remarkable aircraft. The second option, which spans the remainder of this page, describes an in-class version of this lesson that teachers can use to guide their students through the design process for building a helicopter to fly on another world.

In each version of the lesson, students consider how the conditions on Mars might affect helicopter flight, and how the design of a helicopter might need to be changed to work under those conditions. The lesson also touches upon the structure of Earth’s atmosphere, the physics of flight, aeronautics terminology, the basics of helicopter flight and design, forces and vectors related to aircraft, and the history of flight on other planets.

• Teacher asks: What is different about Mars (as compared to Earth) that would affect the design of aircraft, specifically helicopters?
  • Background info: Gravity is less on Mars, so objects (including aircraft) weigh less. Martian surface gravity is about 38% of Earth’s gravity, so objects weigh less than half as much on Mars.
  • Background info: The atmosphere of Mars is MUCH thinner. At sea level on Earth, atmospheric pressure is around 1 bar or 1,013 millibars (14.7 pounds per square inch or 101,325 pascals). Average surface pressure on Mars is around 6 millibars (0.09 psi, 610 pascals). Air pressure on Earth is at least 100 times greater than on Mars!
  • Key points: Objects weigh less than half as much on Mars, so it is easier to get them airborne. However, the air is 100 times thinner, so it is much harder for aircraft to generate lift. With half the weight BUT only 1% of Earthly air pressure, we might guess that it will be about 50 times harder to fly on Mars than on Earth.
  • Background info: Earth’s atmosphere primarily consists of nitrogen and oxygen. Mars’ atmosphere is mainly carbon dioxide. Carbon dioxide gas has a higher density than nitrogen gas or oxygen gas at the same pressure. Even accounting for this, the density of the Martian atmosphere is still less than 1% of Earth’s atmospheric density at sea level.
  • Background info: Air pressure decreases with altitude. That’s why climbers on high peaks carry bottled oxygen, and why commercial airlines have masks for breathing in emergencies. The surface air pressure on Mars is like the air pressure in Earth’s atmosphere more than 50 kilometers up (about 30 miles or 160,000 feet). By comparison, Mount Everest is a bit under 10 kilometers tall and commercial jetliners fly just above that height. The famous SR-71 Blackbird spy plane flew up to 26 km high.
  • Background info: This question naturally raises issues about the structure and layers of Earth’s atmosphere, which is typically taught in Earth science classes. You can use this opportunity to teach about the troposphere, stratosphere, mesosphere and ozone layer.

• Teacher asks: Based on the differences between Earth and Mars, how would you change the design of a helicopter to allow it to fly in the atmosphere of Mars?
  • Background info: Helicopters are complex machines. The aerodynamics of some aspects of helicopter flight are very complicated, and not an appropriate topic for an introductory lesson. In this lesson, we will focus on several variables in helicopter designs that are easy for novices to grasp. We will NOT cover everything one could do to change a helicopter’s performance.
  • Background info: This lesson provides background information about 5 helicopter design variables: rotor blade length, rotor blade width, number of rotor blades per rotor, number of rotors, and rotation rate of the rotor(s). There are plenty of other variables one could change; consideration of those is beyond the scope of this lesson plan.
  • Teaching Tip: Use photos and diagrams of aeronautics concepts and examples of helicopter configurations to show students concrete examples of helicopter design options.
  • Teaching Tip: Consider bringing one or more household fans into class to help students observe a physical object while they brainstorm ideas about changes to helicopter designs. Fans often have varying numbers of blades, just as helicopters have different numbers of rotor blades. Most fans have different speed settings, which brings up the issue of rotation rate of helicopter rotors. Fan blades come in a variety of sizes and shapes, as is also true for helicopter rotors.
  • Background info – Basics of Flight: You may want to explain some basic concepts of flight to your students to start this section. You might show them a vector diagram of the forces on an aircraft: lift, weight, thrust and drag. Helicopter rotor blades are airfoils, like airplane wings. Explain how airfoils generate lift: partially via the pressure difference described by Bernoulli’s principle, and partly via the tilt (angle of attack) of the airfoil which deflects air downward.
  • Background info – Rotor Blade Area and Lift: In general, the larger the surface area of a wing, the more lifting force it can supply. The same holds true for helicopter rotor blades. A longer rotor blade has more area than a shorter blade, and a wider blade has more area than a narrow blade. A rotor blade that is long and wide has even more surface area.
  • Background info – Rotor Blade Length: A longer rotor blade has more surface area, and thus should generate more lift than a shorter blade. However, because the rotor must spin rapidly to generate lift, the speed of the rotor blade tips can cause problems. If the speed of the rotor blade tips reaches or exceeds the speed of sound, air flow around the rotor blade becomes very turbulent and noisy and the lifting force is reduced. This issue limits the practical length of rotor blades. To increase total rotor blade area, and thus the amount of lifting force, without making the blades longer, helicopter designers can instead: make the blades wider, use more blades per rotor, or add more rotors. Long rotor blades that are spinning rapidly also put a lot of mechanical stress on the rotor hub.
  • Background info – Rotor Blade Width: In aeronautics, the width of a wing (or rotor blade) is called its “chord length“. Increasing the chord length of a rotor blade would also increase its area, like increasing its length does. However, a rotor blade with a high chord length creates a VERY bumpy ride, which limits how large of a chord length can practically be used. A large chord length also produces a lot of stress on the rotor hub and is quite noisy. In general, it isn’t practical to increase the chord length very much.
  • Background info – Blades per Rotor: It is possible to increase the total rotor blade area, and thus lift, by simply increasing the number of rotor blades. Helicopters commonly have between two and six rotor blades. Helicopter designers often use more rotor blades, instead of blades with a high chord length, to provide more lift while maintaining a smooth, quiet ride. The wake of disturbed air behind one rotor blade can interfere with the aerodynamics of the blade that follows it, reducing lift. For this reason, designers limit the number of blades to two unless other design considerations call for more blades.
  • Background info – Number of Rotors: Another way to increase total rotor blade area, and thus overall lift, is to use more than one rotor. Many large helicopters have two rotors, while most small helicopters have just one. Some drone aircraft have 8 or more rotors. Adding a second rotor can increase the number of rotor blades, and thus increase lift, without making the rotor longer or wider or requiring it to spin faster. However, increasing the number of rotors adds weight and complexity to the helicopter design. A second rotor requires at least another driveshaft and more gears, and might require a second motor. Besides adding weight, the extra moving parts means there are more things that could fail in-flight, raising safety concerns.
  • Background info – Offsetting Rotor Torque: Newton’s Third Law of Motion is often stated as “for every action, there is an equal and opposite reaction”. The force from the engine that makes a helicopter’s rotor spin also pushes in the opposite direction on the helicopter’s body, and would gradually cause the aircraft’s body to spin if nothing was done to prevent that. Small helicopters with a single rotor generally employ a tail rotor, a smaller propeller mounted on the tail so it pushes sideways, to counteract this torque. This is not ideal, since the tail rotor requires a significant amount of the engine’s power that would otherwise be used by the main rotor to produce more lift.
  • Background info – Multirotor Configurations: Some helicopter designs use two main rotors, spinning in opposite directions, to avoid the need for a tail rotor. The most common layout, with one rotor in the front and another in the rear of the helicopter, is called a tandem design. The rotors can also be placed side-by-side, in a transverse design, which was used occasionally during the history of helicopter evolution and is now mainly employed on tiltrotor aircraft. Some modern helicopters also have coaxial rotors, one mounted above the other, that use concentric drive shafts to power the rotors. Quadcopters are one of the most common configurations of drone aircraft, though some drones have as many as six or eight rotors. Note that these drone configurations all have even numbers of rotors, with half spinning in each direction, which cancels out the torque effect.
  • Background info – Rotation Rate of Rotors: Typical rotation rates for helicopter rotors are around 400 to 500 rpm (revolutions per minute). Some large, multirotor helicopters have slower rates, closer to 200 rpm. As mentioned earlier, helicopter designers must balance this rotation rate agains the rotor blade length to make certain the rotor blade tips don’t exceed the speed of sound. Higher rotation rates produce more lift, just like a house fan on the high speed setting blows more air. Small, radio-controlled (RC) helicopters used by model airplane hobbyists have much higher rotor speeds over 2,000 rpm to allow them to perform high-speed tricks. The shorter blades on RC helicopters can spin faster without breaking the sound barrier.

For each aspect of helicopter design that students identify as a variable to change to enable flight on Mars, ask students to hypothesize whether more or less of the quantity is needed. In each case, ask students to explain “why” they think increasing or decreasing the value for the variable under consideration would help.

• Rotor Blade Length
  • Longer rotor blades have a greater surface area, producing more lift. It is reasonable to suggest that increasing rotor blade length might help provide the extra lift needed in the thin Martian atmosphere.
• Rotor Blade Width
  • Wider rotor blades also have more surface area and produce more lift. It is reasonable to suggest that increasing rotor blade chord length might also help provide extra lift.
• Number of Rotor Blades per Rotor
  • More rotor blades also have more total surface area than fewer blades. It is reasonable to suggest that increasing the number of rotor blades might also help provide more lift.
• Number of Rotors
  • More rotors can support more rotor blades, producing a net increase in the total rotor blade surface area. It is reasonable to suggest that increasing the number of rotors might also help produce greater lift. Students should realize that this might also add weight, which is counterproductive. Longer, wider, and more rotor blades would also tend to add some weight.
• Rotation Rate of the Rotor(s)
  • Up to a point, a faster rotation rate generates more lift. It is reasonable to suggest that increasing the rotation rate might help the Mars helicopter get airborne. The analogy to the speed settings on house fans might be helpful. At high rotation rates, the issue of rotor blade tips breaking the speed of sound becomes a problem.