# Racer Setup
This page describes how to setup and configure a racer for optimal performance (in particular for Acro mode).
Keep in mind that racers are fast vehicles, specifically designed to be overpowered! You should already have some experience, or let someone with experience help you.
Many things described here can also be applied to improve the flight performance of other types of multicopters.
A racer usually omits some sensors (e.g. GPS). As a result, fewer failsafe options are available.
# Build Options
A racer usually omits some sensors.
The minimal configuration is to use only a gyro and accelerometer sensor.
If the board has an internal magnetometer, it should not be used (small racers are particularly prone to strong electromagnetic interference).
Racers typically do not have a GPS as it adds some weight and is prone to damage during crashes (a GPS + external magnetometer must be placed on a GPS mast away from high currents to avoid magnetic interference, which unfortunately means that it is easy to break).
There are however some benefits in adding GPS, particularly for beginners:
- You can go into position hold and the vehicle will just stay in one place. This is handy if you lose the orientation or need a brake. It can also be used to land safely.
- Return mode can be used, either on a switch or as RC loss/low battery failsafe.
- You will have the last position when it crashes.
- The log contains the flight track, which means you can review the flight (in 3D). This can help to improve your acrobatic flight skills.
During aggressive acrobatic maneuvers the GPS can lose its position fix for a short time. If you switch into position mode during that time, altitude mode will be used instead until the position becomes valid again.
# Hardware Setup
The following paragraphs describe a few important points when building the vehicle. If you need complete build instructions, you can follow the QAV-R 5" KISS ESC Racer build log.
# Vibration Setup
There are various mounting approaches to reduce vibrations. For example, the flight controller can be mounted with vibration dampening foam, or using O-rings.
While there is no single best method, you will typically have fewer problems with vibrations if you use high-quality components (frame, motors, props) as for example used in the QAV-R 5" KISS ESC Racer.
Make sure to use balanced props.
# Center of Gravity
Make sure that the center of gravity is as close as possible to the center of thrust. Left-right balance is usually not a problem, but front-back balance may be. You can move the battery until it is correct and mark it on the frame so you will always place it correctly.
The integral term can account for an imbalanced setup, and a custom mixer can do that even better. However it is best to fix any imbalance as part of the vehicle setup.
# Motor Ordering
If you plan to use a 4-in-1 ESC, such as the Hobbywing XRotor Micro 40A 4in1 (opens new window), you will notice that it uses a motor ordering that is different from the one that PX4 uses. PX4 allows you to change the motor ordering in software via MOT_ORDERING parameter. You can select the Betaflight/Cleanflight motor ordering that is typically used on these 4-in-1 ESCs.
# Software Setup
After having built the racer, you will need to configure the software.
Go through the Basic Configuration Guide. In particular, set the Airframe that most closely matches your frame (typically you will choose the Generic 250 Racer airframe, which sets some racer-specific parameters by default).
These parameters are important:
- Enable One-Shot (set PWM_MAIN_RATE to 0) or DShot (DSHOT_CONFIG).
- Set the maximum roll-, pitch- and yaw rates for Manual/Stabilized mode as desired: MC_ROLLRATE_MAX, MC_PITCHRATE_MAX and MC_YAWRATE_MAX. The maximum tilt angle is configured with MPC_MAN_TILT_MAX.
- The minimum thrust MPC_MANTHR_MIN should be set to 0.
If you use a GPS you can skip this section and use the default estimator. Otherwise you should switch to the Q attitude estimator, which works without a magnetometer or barometer.
To select it, set SYS_MC_EST_GROUP to 1, and change the following parameters:
- Set SYS_HAS_MAG to 0 if the system does not have a magnetometer.
- Set SYS_HAS_BARO to 0 if the system does not have a barometer.
- Configure the Q estimator: set ATT_ACC_COMP to 0, ATT_W_ACC to 0.4 and ATT_W_GYRO_BIAS to 0. You can tune these later if you wish.
Configure RC loss and low battery failsafe. If you do not use a GPS, set the failsafe to Lockdown, which turns off the motors. Test RC loss on the bench without props attached by turning off the remote when the vehicle is armed.
# PID Tuning
Make sure to calibrate the ESCs before doing any tuning.
At this point you should be ready for a first test flight.
Assuming the vehicle is able to fly using the default settings, we then do a first pass of Basic MC PID tuning. The vehicle needs to be undertuned (the P and D gains should be set too low), such that there are no oscillations from the controller that could be interpreted as noise (the default gains might be good enough). This is important for the filter tuning (there will be a second PID tuning round later).
# Control Latency
The control latency is the delay from a physical disturbance of the vehicle until the motors react to the change.
It is crucial to reduce the control latency as much as possible! A lower latency allows you to increase the rate P gains, which means better flight performance. Even one millisecond added to the latency makes a difference.
These are the factors that affect the latency:
- A soft airframe or soft vibration mounting increases latency (they act as a filter).
- Low-pass filters in software and on the sensor chip trade off increased latency for improved noise filtering.
- PX4 software internals: the sensor signals need to be read in the driver and then pass through the controller to the output driver.
- The IO chip (MAIN pins) adds about 5.4 ms latency compared to using the AUX pins (this does not apply to a Pixracer or Omnibus F4, but does apply to a Pixhawk). To avoid the IO delay, disable SYS_USE_IO and attach the motors to the AUX pins instead.
- PWM output signal: enable One-Shot to reduce latency (PWM_MAIN_RATE=0).
# Filter Tuning
Filters trade off control latency and noise filtering, both of which impact performance. For information see: Filter/Control Latency Tuning
# PID Tuning (Second Round)
Now do a second round of PID tuning, this time as tight as possible, and also tuning the thrust curve.
You can use the approach described in Basic MC PID tuning to tune the frame, but you will need to use the Advanced Multicopter PID Tuning Guide (Advanced/Detailed) to understand how to tune the thrust curve.
After you have verified that the vehicle flies well at low and high throttle, you can enable airmode with the MC_AIRMODE parameter. This feature makes sure that the vehicle is still controllable and tracks the rate at low throttle.
Happy flipping 😃