What’s Propwash in FPV Drone Flying and How to Eliminate It

Propwash is a common issue in FPV drone flying, especially during altitude drops and sharp 180-degree turns. It manifests as oscillations or vibrations due to the turbulent airflow created by a drone’s propellers, significantly impacting flight stability and video quality. This tutorial will explore what propwash is, what causes it, and how you can minimize it through tuning and configuration adjustments.

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What is Propwash?

Propwash occurs when the turbulent air created by a drone’s propellers interferes with the drone’s flight. This turbulence can cause instability, resulting in wobbling or oscillations, especially during maneuvers like sharp turns, rapid descents, or aggressive braking. The effect is often visible in the drone’s video footage as jello-like vibrations, which can be particularly frustrating for pilots aiming for smooth and cinematic shots.

What Causes Propwash?

Propwash is primarily caused by the interaction of the drone’s propellers with turbulent air. When a drone is in motion, especially during quick maneuvers, the air disturbed by the propellers doesn’t have enough time to settle before the propellers pass through it again. This creates a feedback loop of turbulence that destabilizes the drone.

Several factors can make propwash worse:

• Propeller Design: The shape, size, and pitch of the propellers affect how air is moved.
• Flight Maneuvers: Aggressive maneuvers increase the likelihood of encountering propwash.
• Tuning and PID Settings: Poorly tuned filter and PID settings can exacerbate the effects of propwash.
• Weight and Frame Design: The drone’s weight distribution and frame design can influence how it handles turbulent air.

Tips to Reduce Propwash

Reducing or minimizing propwash requires a combination of proper tuning, flight techniques, and hardware adjustments. Here are some effective tips.

Ensure Mechanical Integrity

First, ensure that your quad is mechanically sound:

• Ensure the flight controller is soft-mounted, and mounted securely.
• Check for loose screws on the frame, motors, and other components.
• Verify that no carbon fiber plates in the frame are cracked, and the frame is as stiff as it can be.
• Ensure motors are in good condition, bearings are smooth, and bells are not loose.

Use Low Pitch Propellers

Using high-quality, low-pitch propellers can make a significant difference in handling propwash. While higher-pitch props are great for achieving higher top speeds, lower-pitch props can change RPM faster, making your drone more responsive and quicker to adjust to air turbulence.

Here are my propeller recommendations: https://oscarliang.com/propellers/#Recommendations

Enable RPM Filter

It’s recommended to enable RPM filtering, following this tutorial: https://oscarliang.com/rpm-filter/.

If you have BLHeli_S ESCs, you should flash Bluejay firmware onto them. If you have BLHeli_32 or AM32 ESCs, you’re already set for RPM filtering, follow these guides to configure your ESC for optimal performance:

• Best BLHeli32 settings: https://oscarliang.com/best-blheli-32-settings/
• Best AM32 settings: https://oscarliang.com/best-am32-settings/

Adjust Filter Settings

Once your quad is mechanically sound and running the latest firmware with RPM filtering enabled, you might want to try these filter adjustments:

• Disable Gyro Lowpass 1 Filter: This can usually be safely turned off with RPM filtering enabled on most quads. It reduces filter delay and improves propwash handling. You should do a short test flight to check if motors get hot before commiting.
• Reduce Filtering Further: In Betaflight, go to the PID tuning tab and filter settings. Move the Gyro Lowpass filter sliders to the right (to 1.2 or 1.3) to reduce filtering while ensuring the quad still flies well without hot motors.
• Fine-Tune: If you notice problems at higher values (e.g., 1.5), back the sliders down a bit to find the minimum amount of filtering you can get away with.

Reducing filtering can help push PID gains higher without inducing oscillations, however, this requires a clean and well-maintained quad, and being able to analyse it through Blackbox logs is also strongly recommended.

Proper PID Tuning

Default PIDs in Betaflight are generally very good, but there is always room to fine-tune propwash handling:

• Increase D-Term: The D-term in your PID settings helps counteract rapid changes in motion. Higher D gain can help reduce oscillations caused by propwash. However, increasing the D-term too much can lead to motor overheating and oscillations, so make adjustments gradually through proper tuning methods.
• Adjust P-Term and I-Term: An ideal PID gain requires good ratios between P and D, and between P and I. Therefore, when making changes to D gain, you should also adjust P and I gains to maintain these ratios.

Further Readings:

• If you know how to use Blackbox, check out my indepth PID tuning tutorial: https://oscarliang.com/pid-filter-tuning-blackbox/
• If you don’t know how to use Blackbox, you can still perform some basic yet effective tuning following these simple steps: https://oscarliang.com/fpv-drone-tuning/
• Why excessive D gain can lead to motor overheating: https://oscarliang.com/excessive-d-gain-cause-oscillations-motor-overheat/

Implement Dynamic Idle

Dynamic idle ensures your motors maintain a minimum RPM even in reverse flow situations, minimizing propwash.

When prop wash occurs, the flight controller attempts to stabilize the quadcopter by rapidly speeding up or slowing down the motors. Without Dynamic Idle, the minimum RPM your motors can reach is 5.5% by default. In contrast, when Dynamic Idle is enabled, Betaflight can command the motors to drop to 0% RPM, so the range of throttle the motors can reach when fighting prop wash is much wider. This enhanced braking capability noticeably improves prop wash handling.

I have a whole blog post explaining what Dynamic Idle is and how to configure it: https://oscarliang.com/how-to-enable-and-configure-betaflight-dynamic-idle/

Change the Way You Fly

Aggressive flying often leads to more pronounced propwash. By practicing smoother, more controlled maneuvers, you can reduce the turbulence created by your drone’s propellers. For example, when descending, try to pitch forward slightly to minimize air turbulence directly hitting the propellers, effectively reducing propwash.

Conclusion

Propwash is a challenge that every FPV pilot faces, but with the right combination of tuning, hardware adjustments, and flight techniques, you can significantly reduce its impact. By understanding the causes of propwash and applying these tips, you’ll be able to enjoy a more stable flight and capture smoother video footage. Happy flying!