Propeller Efficiency at Wind Speeds from 0–38 mph

By Lauren Nagel

Wind tunnel testing is an essential step in the drone design and development process. Propulsion systems may perform a certain way in static conditions, and completely differently in real world conditions.

Propeller thrust and efficiency, for example, are crucial for mission completion, yet vary greatly with changing wind speed. Characterizing a drone’s performance at various wind speeds is key for any aircraft with operations beyond fair weather flying.

To test how propeller performance changes with wind speed, we put our prop testing station to use, testing drone propellers at wind speeds from 0 to 38 mph (0 - 17 m/s).

Figure 1: Propeller wind tunnel test setup

Table of Contents:

1. Wind tunnel testing procedure

2. Propeller thrust vs. RPM at different wind speeds

3. Propeller efficiency at different wind speeds

4. Propeller thrust vs. power

1. Wind Tunnel Testing Procedure

Our test setup includes three key pieces of measurement equipment:

1. 2x2 Windshaper - a modular wall of fans generating wind speeds of 16+ m/s, replacing the traditional wind tunnel. The speed and turbulence level of each individual fan is controlled using the WindControl software.

2. Series 1585 thrust stand - a propeller thrust stand measuring thrust, torque, rpm, current, and voltage in order to derive power and efficiency. Tests were automated using theFlight Stand software.

3. Airspeed pressure sensor - connected to a pitot tube and the Flight Stand software, airspeed data is synced with thrust stand measurements.

Figure 2: Series 1585 propeller thrust stand with airspeed sensor and pitot tube

We installed a 9” propeller at 210 cm from the Windshaper and ran an automated step test with four throttle steps: 1210, 1290, 1370, and 1450 µs.

Figure 3: Step test plotted in the Flight Stand software

Each test used a constant wind speed and turbulence level across the fans, and was repeated at 6 different speeds: 

• 0 m/s (0 mph)

• 4.2 m/s (9.4 mph)

• 7.5 m/s (16.8 mph)

• 10.7 m/s (24 mph)

• 14 m/s (31.3 mph)

• 17 m/s (38 mph)

Figure 4: Windshaper fan power set to 100% in the WindControl software

Some considerations regarding the test setup:

• It is essential to leave enough space behind the Windshaper to ensure efficient air intake

• We made sure the entrance to the pitot tube was unobstructed to ensure measurement of smooth air

• We aimed to minimize both the distance between the pitot tube and the test propeller and the airflow disturbance. Next time we will visualize and verify the aerodynamic flows with a WindProbe 3D.

Figure 5: 2x2 Windshaper wind wall with 36 individually controlled fans

2. Propeller thrust vs. RPM at Different Wind Speeds

The graph below shows the results for thrust generation vs. RPM in each wind condition. The four data points in each wind condition represent the readings at each throttle step.

Figure 6: Graph of propeller thrust vs. RPM at wind speeds from 0 to 17 m/s

Thrust generation increased as throttle increased and was highest in the no wind condition. As wind speed increased, a higher rotation speed was required to achieve the same thrust.

For a given torque, propeller RPM increased as throttle input and wind speed increased. The wind from the Windshaper is responsible for the higher RPM, pushing on the propeller’s blades and causing it to rotate even more.

Higher RPM is typically associated with higher thrust generation, but that is only true at constant wind speed. Propeller thrust depends on the physical properties of the propeller, i.e. radius, pitch, but also the pressure of the air flowing through the propeller disk area.

The equation below for dynamic propeller thrust (Electric Aircraft Guy) demonstrates how propeller forward airspeed relates to thrust generation. Note that the coefficients are theoretical and will vary due to the propeller profile, Reynolds number, and environmental conditions. Nevertheless, the equation is useful to understand how changing a parameter will affect thrust. As forward airspeed increases, propeller thrust decreases.

This occurs because thrust is a product of the velocity difference of the air entering and exiting the propeller. Propeller thrust can also be expressed as:

When a propeller is stationary, i.e. when an aircraft is at rest, the surrounding air is rapidly accelerated as it enters the propeller. When the aircraft accelerates and the propeller gains forward speed, the relative velocity of the air meeting the propeller blades (V0) increases, while Ve does not increase proportionally, resulting in lower thrust generation.

Additionally, as wind speed increases, the relative airflow vector at the propeller, i.e. the sum of the propeller’s rotational velocity and the incoming wind, migrates from the side to the front, reducing the angle of attack and the thrust generation.

To learn more about the propeller thrust equations, check out our article on How to Calculate Propeller Thrust.

3. Propeller efficiency at different wind speeds

The graph below depicts mechanical power generation vs. RPM at different wind speeds.

Figure 7: Graph of mechanical power vs. RPM at wind speeds from 0 to 17 m/s

As wind speed increases, propeller efficiency decreases, as demonstrated by the diminishing mechanical power generation across wind speeds. The difference is increasingly pronounced as wind speed increases, with higher RPM required to achieve the same mechanical power.

This effect is observed due a combination of factors: 1) as discussed in the previous section, thrust generation decreases as wind speed increases, 2) mechanical power also decreases and does so more rapidly than thrust, so the thrust: power ratio and efficiency both decline.

Recommended for you: 2 Methods for Balancing Drone Propellers

4. Propeller thrust vs. power

In the graph below, we visualize the difference between thrust and power decline, as well as the thrust: to power ratio:

Figure 8: Propeller thrust, power, and thrust: power ratios

The graph demonstrates how thrust declines more rapidly than power. Even though less thrust is generated, a similar amount of power is drawn, resulting in a lower thrust: power ratio at higher wind speeds.

Between 0 and 14 m/s, for example, thrust declines by 82%, whereas power online declines by 38%.

These results demonstrate that propeller efficiency is highest in the no wind condition.

Conclusion

These experiments illustrate the fascinating relationship between wind speed and propeller thrust, power and efficiency. These types of experiments are essential for understanding how a propulsion system works in real flight conditions.

In this study, we used a 2x2 Windshaper wind tunnel to generate constant speed winds, but these machines are capable of much more. Click here to learn how Windshapers can help you improve your drone's performance.