Helicopter Liftoff: How Does the Speed of the Rotor Affect the Amount of Lift?
David B. Whyte, PhD, Science Buddies
AbstractHelicopters are fascinating to watch. The spinning rotor blades on top of the helicopter generate lift, allowing it to take off vertically. They can land vertically, too, allowing them to set down in small spaces, such as hospital helipads or on a ship at sea. In this aerodynamics science fair project, you will fly a remote-controlled helicopter and measure how the rate of the rotor's rotation changes as the helicopter hovers and flies up or down.
The objective is to accurately measure the rate of blade rotation on a remote-controlled helicopter and determine how this rate affects the vertical motion of the helicopter.
Remote-controlled helicopters come in a variety of shapes and designs. The least-expensive ones are available for less than $20 and can be flown right out of the box. More-expensive versions fly for longer periods of time and have more sophisticated controls, which require time to master. They are all, however, miniature technical wonders.
The blades on top of a helicopter form the main rotor. The blades are like narrow "wings" and are shaped like airfoils. An airfoil has a specific shape that allows the helicopter to cut through the air in a way that efficiently produces lift. The part of the blade that passes through the air first is called the leading edge. By "tipping" the leading edge of the blade up as it moves, the blade will have a positive angle of attack. As the angle of attack increases, more air is forced down by the blades. This forces the blades up, and the rotor provides the lift needed to make the helicopter fly.
In this aerodynamics science fair project, you will measure the speed of a helicopter's rotor using a digital tachometer. The output is measured in revolutions per minute (rpm), so note that you might need to factor in the number of blades in order to get rpms. For example, if the rotor has two blades and the tachometer is counting both blades as the rotor spins, you will need to divide the tachometer reading by two to get the rpms. The goal is to measure how the rotation rate of the rotor is related to the vertical motion—ascending, descending or hovering—of the helicopter. Since the procedure involves some creative problem solving on your part, this should be regarded as a "do-it-yourself" kind of science fair project.
Note: Real helicopters lift off by changing the blades' angle of attack at a constant rpm. Some simple toy helicopters only adjust rpm, and not angle of attack. In this project, you will just be testing a simple toy helicopter.
Terms and Concepts
- Leading edge
- Angle of attack
- Digital tachometer
- Revolutions per minute (rpm)
- Based on your research, what are the main parts of a helicopter?
- What is the purpose of the rotor on the tail of a helicopter?
- How does air flow over an airfoil?
- HowStuffWorks, Inc. (2009). How Helicopters Work. Retrieved July 20, 2009.
- Wikipedia Contributors. (2009, July 16). Helicopter. Wikipedia: The Free Encyclopedia. Retrieved July 20, 2009.
- Wikipedia Contributors. (2009, July 14). Airfoil. Wikipedia: The Free Encyclopedia. Retrieved July 20, 2009.
- NASA. (n.d.). Four Forces on an Airplane. Retrieved August 13, 2013.
Materials and Equipment
Remote-controlled helicopter; available at toy stores, large department stores and online
- It should be able to fly for at least 10 minutes between charges.
- It should also be safe to fly near people, since there is a good chance that the helicopter could hit your helper while he or she is taking tachometer readings.
- Digital tachometer, available from Amazon.com
- Tape measure
- Optional: Digital scale accurate to 0.1 g or less
- Safety goggles
- Optional: Gloves, for protection from the rotor blades
- Helpers (3, in addition to you)
- Lab notebook
- Optional: Video camera
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Important Notes Before You Begin: In order to get good data, you will need to be able to fly the helicopter in a controlled way. In particular, you will need to be able to make it move up and down at different rates. You will also need to work out the best way to obtain readings from the tachometer and the rate of ascent and descent as the helicopter is flying. Carrying out the experiments will involve some creative problem solving.
Preparing for Testing
- Assemble your remote-controlled helicopter.
- Practice flying the helicopter so that you can make it hover, move vertically up, and move vertically down.
- Place the helicopter on the ground in a flat spot.
- Place the tape measure near the helicopter.
- Pull the tape out to 2 meters (m).
Lock the tape measure open so that it makes a 2-m vertical measuring standard.
- You may need to stabilize the tape measure; for example, by placing it near a wall and attaching it to the wall with adhesive tape.
- Bring the helicopter to a hovering position at about 0.5 m above the ground.
With the helicopter hovering about 0.5 m off of the ground, have a helper record the rate of rotation, using the digital tachometer, in your lab notebook.
- Some tachometers are sensitive to the 60 cycles per second "flicker" in artificial lights. If the tachometer is affected by artificial light, take your readings outdoors, or indoors in an area with sunlight.
As an option, weigh the helicopter.
- Record the weight of the helicopter in your lab notebook.
- The lift provided when the helicopter is hovering equals the weight of the helicopter.
Measuring Rate of Ascent and Rotor Rotation
- Start with the helicopter on the ground near the tape measure.
- Bring the helicopter to a hovering position at about 1 m above the ground.
- Have a helper read the revolutions per minute using the tachometer from below the helicopter.
Increase the rotor speed so that the helicopter flies vertically upward.
- The rate of vertical ascent should be slow and steady.
The helper with the tachometer should record the rpm's as the helicopter flies upward.
- It will probably be easier to take the readings from below.
Have another helper use a stopwatch to measure how long it takes for the helicopter to fly from 1 m to 2 m.
- You might want to increase the length of the tape measure to 3 m or higher in order to get more-accurate readings.
- Record the rate of ascent in meters per second (m/s) and the corresponding rate of rotor rotation.
- The rate of ascent is the distance the helicopter moved in meters, divided by the time in seconds it took to move that distance.
- The third helper can record the data.
- As an option, video record the flight of the helicopter to help collect the data.
- Repeat steps 1–6 two more times, recording the rate of ascent and the rotor rpms each time. Repeating the tests will ensure your results are accurate and repeatable. Try to keep the rate of ascent consistent.
- Repeat steps 1–7 using a different rate of ascent.
Measuring Rate of Descent and Rotor Rotation
- Start with the helicopter hovering at a height of 2 m.
- Have a helper measure the rpm's of the rotor from below.
- Have another helper use a stopwatch to measure how long it takes for the helicopter to descend.
Start the helicopter's descent.
- Descend from 2 m to 1 m.
- The descent should be slow and steady so that the helper can obtain a reading on the tachometer.
- Calculate the rate of descent in meters per second.
- Record the rate of descent and the corresponding rotor rpm's in your lab notebook.
- Repeat steps 1–6 to obtain data for two more rates of descent. Try to keep the rate of descent consistent.
- Repeat steps 1–7 using a different rate of descent.
Analyzing Your Results
- For each rate of ascent or descent, average the rpm's you measured.
Graph the rate of ascent and descent on the on the x-axis and the average rate of rotation of the rotor in rpms on the y-axis.
- On the x-axis, use 0 (zero) for the hovering helicopter, negative numbers for the descending helicopter, and positive numbers for the ascending helicopter. For example, a helicopter that descended at 1 m in 5 s will have a value of -0.2 m/s.
Ask an Expert
- Try adding small weights to the helicopter and measuring how this increased load changes the rpm's of a hovering helicopter. Graph the load, in grams, on the x-axis and the rpm's on the y-axis.
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