In QHOVER mode, QuadPlane maintains a consistent altitude while allowing roll, pitch, and yaw to be controlled normally. This page contains important information about using and tuning alt hold.
When altitude hold mode (aka QHOVER) is selected, the throttle position is used to control the climb or descent rate, maintaining current altitude when at mid-stick. Roll, Pitch and Yaw operate the same as in QSTABILIZE mode meaning that the pilot directly controls the roll and pitch lean angles and the heading.
Automatic altitude hold is a feature of many other flight modes (QLOITER, etc.) so the information here pertains to those modes as well.
The autopilot uses a barometer which measures air pressure as the primary means for determining altitude (“Pressure Altitude”) and if the air pressure is changing in your flight area due to extreme weather, the QuadPlane will follow the air pressure change rather than actual altitude.
The pilot can control the climb or descent rate of the vehicle with the throttle stick.
- If the throttle stick is in the middle deadzone set by
RCn_DZ(where n is the channel mapped to throttle input) the vehicle will maintain the current altitude. The default for throttle channel deadzone is 60 (+/- 6%).
- Outside of the mid-throttle deadzone the vehicle will descend or climb depending upon the deflection of the stick. When the stick is completely down the QuadPlane will descend at Q_WP_SPEED_DN and if at the very top it will climb by Q_WP_SPEED_UP.
The Q_P_POSZ_P is used to convert the altitude error (the difference between the desired altitude and the actual altitude) to a desired climb or descent rate. A higher rate will make it more aggressively attempt to maintain it’s altitude but if set too high leads to a jerky throttle response.
The Q_P_VELZ_P (which normally requires no tuning) converts the desired climb or descent rate into a desired acceleration up or down.
The Q_P_ACCZ_P, Q_P_ACCZ_I, Q_P_ACCZ_D gains convert the acceleration error (i.e the difference between the desired acceleration and the actual acceleration) into a motor output. The 1:2 ratio of P to I (i.e. I is twice the size of P) should be maintained if you modify these parameters. These values should never be increased but for very powerful QuadPlanes you may get better response by reducing both by 50% (i.e P to 0.5, I to 1.0).
Verifying altitude hold performance with dataflash logs¶
Viewing the altitude hold performance is best done by downloading a dataflash log from your flight, then open it with the mission planner and graph the barometer altitude, desired altitude and inertial navigation based altitude estimate: CTUN’s BarAlt (baro alt), DAlt (desired alt) and Alt (inertial nav alt estimate)
The three should track well as shown below.
- High vibrations can lead to the QuadPlane rapidly climbing as soon as altitude hold is engaged. Check the Measuring Vibration and Vibration Dampening wiki pages for details on how to measure and reduce vibrations.
- The motors seem to stop for a moment just as an altitude hold mode is engaged but then it soon recovers. This normally occurs when the pilot enters altitude hold modes while climbing rapidly. The target altitude is set at the moment the pilot switches into alt hold but because the vehicle is rising quickly it flies above the target. The aggressive altitude hold controller then responds by momentarily reducing the motors to near minimum until the QuadPlane begins falling back to the target altitude. The workaround is to enter these modes while the QuadPlane is flying at a stable altitude.
- Air pressure changes cause the vehicle to drift up or down by a couple of meters over longer period of time or for the altitude shown on the GCS to be inaccurate by a couple of meters including occasional negative altitudes (meaning altitudes below the home altitude).
- Momentary altitude loss of 1m ~ 2m when the QuadPlane levels out after a
high speed forward flight. This is caused by an aerodynamic effect
which leads to a momentary low pressure bubble forming on the top of
the QuadPlane where the autopilot is mounted which leads the
QHOVER controller to believe it is climbing so it responds by
descending. There is no cure for this behaviour at the moment
although increasing the
EK2_ALT_M_NSEparameter reduces the effect but increases the change of Common Problem #1 listed above. The
EK2_ALT_M_NSEparameter has a range from 0.1 to 10.0 and allows increments of 0.1.
- Altitude hold becomes erratic when the vehicle is close to the ground or during landing. This can be caused by the barometer being affected by pressure changes created by prop-wash. The solution is to move the autopilot out of the prop wash effect or shield it within an appropriately ventilated enclosure.
- Sudden altitude changes caused by light striking the barometer. Assuring sunlight cannot hit the baro will cure this.
- QuadPlane slowly descends or climbs until the pilot retakes control in stabilize. Normally this is caused by not having the throttle stick in the mid position. This commonly happens when the pilot is switching into an altitude holding mode from a manual flight mode (like QSTABILIZE) on a QuadPlane that does not hover at mid throttle. Usually it is desired to hover in any mode at mid-stick on throttle, so that transitions between modes is easily accomplished without throttle position changes. This can be adjusted using the Q_M_THST_HOVER parameter, or automatically learned in QHOVER or QLOITER modes by enabling Q_M_HOVER_LEARN.
It is very important that the vehicle has enough power available. Without this the altitude hold and attitude controllers can require more power than is available from one or more motors and will be forced to sacrifice some control which could lead to a loss of attitude or altitude.
Ideally the vehicle should be able to hover at about 50% throttle (mid stick) and anything higher than 70% is dangerous.
If you incorporate expo on your transmitter, that directly increases the effective size of the throttle dead band.