The Effect of Rotor Speed on Vertical Motion in Helicopters

MARCUS & THOMAS LIANG

he/him | ages 13 & 18 | Mississauga & London, ON

Platinum Award, School Science Fair

Edited by Tristan Montoya & Imogen Moore

INTRODUCTION

Our project is on aerodynamics, the study of the motion of air and its interactions with a solid object. The blades of a helicopter form the main rotor and are shaped like airfoils. An airfoil has a specific shape that allows the aircraft to deflect air in a manner to produce lift (Morris, 2023). The angle of attack is the angle at which an airfoil passes through the air. As the angle of attack is increased, more air is deflected downwards. This pushes the blades up, which provides the lift that is needed for the helicopter to fly (“helicopter,” n.d.). Helicopters take off by changing the blades’ angle of attack at a constant rate of rotation (i.e., rotational speed) in revolutions per minute (rpm) (How does A helicopter work: Everything you need to know about helicopters, 2019). We will be using a toy helicopter for this experiment, which changes the rpm of the blades, not the angle of attack. A tachometer is an instrument that measures the rotational speed of an object. It operates by emitting light towards the revolving object, and the reflection pattern provides information on how fast the object is spinning (What is a tachometer and how does it work?, n.d.).

The purpose of this experiment is to measure the rate of blade rotation on a remote-controlled helicopter and determine how this rate affects the helicopter’s rate of ascent or descent. We hypothesize that as the rate of the blade rotation on the remote-controlled helicopter increases, the vertical motion of the helicopter will also increase. The independent variable is the rate of the blade rotation on the remote-controlled helicopter in rpms. The dependent variable is the vertical motion of the helicopter in metres. We had four control variables: the type of remote-controlled helicopter, the location of the experiment, the person obtaining readings from the tachometer, and the markings on the wall.

MATERIALS & METHODS

For this experiment, we used a remote-controlled, battery-powered helicopter. We also used a digital tachometer to measure the blade rotation. The tape and tape measures were used to track how high the helicopter went. It is also helpful to have a stopwatch, lab notebook, safety googles, and two helpers nearby to assist with the experiment.

First, make two markings (one and two metres) on the wall using the tape and tape measure (see Figure 1 for the experimental setup). Start with the helicopter near the ground. Then, bring the helicopter up to one metre off the ground. Have a helper read the rpms while the helicopter flies upwards from one metre to two. Have another helper use a stopwatch to measure how long it takes the helicopter to fly from one metre to two. For the second test, start with the helicopter near the two-metre mark. Slowly lower the helicopter down to the one-metre mark. Have a helper read the rpms while the helicopter descends from two metres to one. Have another helper use a stopwatch to measure how long it takes the helicopter to descend from two metres to one. Repeat the procedure for a total of ten trials for each test.

**Figure 1:** Experimental setup of the helicopter and tape measures. The grey rectangle represents the wall, and the helicopter is placed on the ground before starting the experiment.

RESULTS

We recorded our observations for the helicopter’s ascent (Table 1) and descent (Table 2). The maximum, minimum, final, and average rpm values were recorded. We also took note of the time the helicopter took to reach the two-metre mark using a stopwatch. The average rpm value was calculated by adding the maximum and minimum values and dividing by two. The rate of ascent (in metres per second) was calculated by dividing the vertical displacement (one metre) by the time in seconds.

DISCUSSION

In our experiment, we were interested in how the rate of the blade rotation on a remote-controlled helicopter affects the vertical motion of the helicopter. We found out that when the helicopter was ascending and the rpms increased, the rate of ascent would also increase. In contrast, when the helicopter was descending and the rpms decreased, the rate of descent would decrease. Let us compare 4 of our results.

Ascending:

As you can see, as the average rpm increases, the rate of ascent also increases.

Descending:

As you can see, as the average rpm decreases, the rate of descent also decreases.

This was a fair test because we did each trial ten times. There are 4 forces that act upon an aircraft that is in flight: thrust, drag, weight, and lift (Harris & Homer, 2022). Thrust is the force that propels an aircraft to the direction it is going. If an aircraft is flying straight, thrust is the motion that causes the aircraft to go straight. Thrust is created with a propeller or engine. Drag is the force that opposes thrust, meaning it opposes the flight direction. There are many factors that affect drag, such as the shape and velocity of the aircraft. Friction and differences in air pressure also affect the strength of drag. Weight is the force that acts with gravity. Weight is the force that brings an aircraft towards the ground, and it is always directed towards the center of the earth. Weight also varies depending on the mass of all the airplane parts, the amount of fuel, and any payload on board (people, baggage). Lift is the force that opposes weight, meaning this is the force that keeps the airplane in the air. Like the other forces, many factors affect lift, including shape, size, and velocity of the aircraft. Lift is generated by the motion of the aircraft.

Newton’s third law applies to aerodynamics. The law states that for every action in nature, there is an equal and opposite reaction. This law applies to aerodynamics because we have thrust to counter drag, and lift to counter weight (Welcome to how things fly, n.d.). However, weight and drag are natural forces, while lift and thrust are man-made. For example, once an airplane is in the air, drag automatically begins to pull it backwards. That is why we have engines to create thrust, which propels the airplane forwards. Again, once an airplane is in flight, weight and gravity will try to bring it down. That is why we have wings to keep the airplane up and flying. As mentioned, all four forces must be balanced if an airplane is to fly successfully. If the lift was to become greater than the weight, then the airplane would rise upwards. When an airplane is flying straight at a constant speed, all forces are balanced. This balance changes as the airplane rises and descends, speeds up and down, and turns. An interesting fact is that only two forces exist when an aircraft is in orbit or in space; weight and thrust. First, an aircraft would require thrust to propel itself into space. When it is in space, the only force it has is weight.

Finally, there was a limitation when we were performing our experiment. Our rate of ascent and descent were not steady and consistent. This was due to the difficulty of controlling the helicopter. Sometimes the helicopter would descend/ascend at a steady pace, while other times it would start very slow and then suddenly speed up, or vice versa. Our results were skewed because of this.

CONCLUSION

In conclusion, our hypothesis was partly correct. We found out that when the helicopter was ascending, if the rpms increased, the rate of ascent would also increase. However, we found out that when the helicopter was descending, if the rpms decreased, the rate of descent would decrease. One possible reason for this was due to the difficulty in controlling the rate of descent compared to ascent. Another reason is because the helicopter should slowly come to a controlled stop, which requires more precision and time. A slower descent may be useful for landing the helicopter safely.

REFERENCES

Harris, T., & Homer, T. (2022, April 20). How helicopters work. HowStuffWorks. https://science.howstuffworks.com/transport/flight/modern/helicopter6.htm

helicopter. (n.d.). In Encyclopedia Britannica. https://kids.britannica.com/kids/article/helicopter/390246

How does A helicopter work: Everything you need to know about helicopters. (2019, December 26). https://youtu.be/YJBhWVDArLo

Morris, E. (2023, February 27). How do helicopters fly? Sheffield School of Aeronautics. https://www.sheffield.com/2023/how-do-helicopters-fly.html

Welcome to how things fly. (n.d.). Retrieved August 8, 2023, from https://howthingsfly.si.edu/ask-an-explainer/why-do-helicopters-need-tail-rotor-and-what-torque

What is a tachometer and how does it work? (n.d.). Megadepot.com. Retrieved August 8, 2023, from https://megadepot.com/resource/what-is-a-tachometer-and-how-does-it-work