Automatic Takeoff

Plane can automatically launch a wide range of aircraft types. The instructions below will teach you how to setup your mission to support automatic takeoff. Automatic takeoff can also be accomplished by the TAKEOFF Mode supported in ArduPlane 4.0 and later versions.

Basic Instructions

The basic idea of automatic takeoff is for the autopilot to set the throttle to maximum and climb until a designated altitude is reached. To cause the plane to execute a takeoff, add a NAV_TAKEOFF command to your mission, usually as the first command. There are two parameters to this command - the minimum pitch, and the takeoff altitude. The minimum pitch controls how steeply the aircraft will climb during the takeoff. A value of between 10 and 15 degrees is recommended for most aircraft. The takeoff altitude controls the altitude above home at which the takeoff is considered complete. Make sure that this is high enough that the aircraft can safely turn after takeoff. An altitude of 40 meters is good for a wide range of aircraft.

During takeoff the wings will be held level to within LEVEL_ROLL_LIMIT degrees until 5 meters of altitude is reached, gradually allowing more roll until the normal ROLL_LIMIT_DEG limit is allowed once 15 meters of altitude is reached. This prevents a sharp roll from causing the wings to hit the runway for ground takeoffs.

Note that the takeoff direction is set from the direction the plane is pointing when the automatic takeoff command is started. So you need to point the plane in the right direction, then switch to AUTO mode. During the first stage of the takeoff the autopilot will use the gyroscope as the principal mechanism for keeping the aircraft flying straight, or the compass if equipped. After sufficient speed for a good GPS heading is reached the aircraft will switch to using the GPS ground track (and compass if so equipped) which allows it to account for a cross-wind.

You should try to launch into the wind whenever possible.

Hand Launching

A hand launch is a common method to launch smaller aircraft, such as foam gliders. Plane has a number of parameters that control how a hand launch is done. If you are planning to hand launch your aircraft in AUTO mode then please look through these options carefully.

The keys to a good hand launch are:

• if the propeller is behind your hand during launch then ensuring that the motor does not start until it is past your hand

• ensuring that the aircraft does not try to climb out too steeply

• assuring that the parameters are set to actually start the motor after the vehicle leaves your hand

Note

default values for the following parameters will usually start the motor as soon at takeoff is selected (mode or mission item). This may be fine for certain hand launch situations/vehicles, but adjusting the following parameters to non-default values may be needed for safety in other situations.

The main parameters that control the hand launch motor start are:

• TKOFF_THR_MINACC

• TKOFF_THR_DELAY

• TKOFF_THR_MINSPD

• TECS_PITCH_MAX

When the auto takeoff mission command starts (usually by switching to AUTO mode) or you switch to TAKEOFF mode when armed, the autopilot starts in “throttle suppressed” mode. The throttle will not start until the conditions set by the TKOFF_THR_x parameters are met.

The TKOFF_THR_MINACC parameter controls the minimum forward acceleration of the aircraft before the throttle will engage. The forward acceleration comes from the throwing action of your arm as you launch the aircraft. You need to set this value high enough that the motor won’t start automatically when you are carrying the aircraft normally, but low enough that you can reliably trigger the acceleration with a normal throwing action. A value of around 15 m/s/s is good for most aircraft. The default value is 0, meaning this safeguard is not active.

Sometimes it is desirable to require multiple “shakes” of the vehicle above the TKOFF_THR_MINACC before allowing a motor start. The TKOFF_ACCEL_CNT parameter can be used to require multiple events exceeding this acceleration before proceeding in the motor start.

The TKOFF_THR_DELAY parameter is a delay in 1/10 of a second units to hold off starting the motor after the minimum acceleration or acceleration count is reached. This is meant to ensure that the propeller is past your hand before the motor starts. A value of at least 2 (which is 0.2 seconds and the default for the parameter) is recommended for a hand launch. More may be needed.

The TKOFF_THR_MINSPD parameter is a minimum ground speed (as measured by the GPS) before the motor starts after the above dealy. This is an additional safety measure to ensure the aircraft is out of your hand before the motor starts. A value of 4m/s is a good starting point if you wish this safety feature to be active. The default is 0, which would allow the motor to start immediately after the above delay with most GPSes since their velocity estimate is usually a bit noisey.

Note that if your aircraft is a “tractor” type with the motor at the front then you may want to use the default values of TKOFF_THR_DELAY and TKOFF_THR_MINSPD, or use low values.

The final parameter you should think about is the TECS_PITCH_MAX parameter. That controls the maximum pitch which the autopilot will demand in auto flight. When set to a non-zero value this replaces the PTCH_LIM_MAX_DEG parameter for all auto-throttle flight modes. Setting this parameter to a value which is small enough to ensure the aircraft can climb reliably at full throttle will make takeoff much more reliable. A value of 20 is good for most aircraft.

Note

Hand launching a plane usually requires that the vehicle leaves your hand at zero degrees to 5 degs pitch. Higher pitches could lead to a stall. ArduPilot will not start the motors if the pitch is greater than 40 degrees or roll greater than 30 degrees from level. These limits should never be approached when throwing the plane, but are provided as a safety gate.

Catapult Launching

The main differences between catapult launching and hand launching is that a catapult will usually give the aircraft a greater level of acceleration, and the risk involved is primarily that the propeller will strike the catapult frame instead of hitting your hand.

In most other ways a catapult launch is like a hand launch, and the same 4 key parameters apply. If your catapult is setup so that the motor cannot run until the aircraft is out of the frame of the catapult then you will need to choose the parameters to ensure there is sufficient delay. Often this means a higher value for TKOFF_THR_MINACC (say 20m/s/s) and a longer delay before the GPS ground speed is measured. Some experimentation may be needed, but a value of TKOFF_THR_DELAY of 5 is likely to be good for many catapults.

Bungee Launching

A bungee launch uses a long piece of stretched elastic to launch the aircraft. This can be a cheaper alternative to a catapult and gives good results for a lot of small to medium sized models.

The same 4 parameters that apply to hand launch and catapult launch also apply to a bungee launch, but the values you will need are different. The main risk with a bungee launch (especially with a pusher propeller) is that the propeller will strike the bungee cord, damaging either the propeller or the bungee or both. To prevent this from happening you should have a much higher value of TKOFF_THR_DELAY, making it high enough that the aircraft will have released the bungee before the motor starts. A value of around 50 (giving a 5 second delay) may be a good starting point.

Runway Takeoffs (CTOL)

The final class of takeoff is runway takeoff, also known as wheeled takeoff or CTOL (Conventional Takeoff and Landing). Setting up for a good automatic takeoff from a runway is a bit more complex than the other types of launches with more parameters to set and more tuning required. This type of launch greatly benefits from the use of a compass onboard since initial heading is critical.

One key consideration with runway takeoffs is whether you have a tail dragger (tail wheel steering) or tricycle undercarriage (nose wheel steering). Automatic takeoff is easier with a tricycle undercarriage aircraft, with a tail dragger needing additional parameters.

The key parameters for runway takeoff are:

• TKOFF_TDRAG_ELEV

• TKOFF_TDRAG_SPD1

• TKOFF_THR_SLEW

• TKOFF_ROTATE_SPD

• TECS_PITCH_MAX

• GROUND_STEER_ALT

• TKOFF_GND_PITCH

In addition to those parameters you also need to tune ground steering, so that the ground steering controller is able to reliably steer the aircraft. See the separate page on setting up ground steering. As part of this tuning you will need to setup the GROUND_STEER_ALT parameter.

The first two parameters are primarily for tail dragger aircraft, although they can also be used to hold the nose of a tricycle aircraft down on takeoff.

The TKOFF_TDRAG_ELEV parameter is used to hold the tail of a tail dragger hard on the runway during the initial stages of takeoff, to give it enough grip on the runway to steer. For a tail dragger this is normally set to 100, meaning that 100% up elevator is applied during the initial stages of takeoff. For a tricycle undercarriage plane that needs a bit of extra weight on the nose for good steering you may find that a value of -20 (meaning 20% down elevator) may help.

When the takeoff starts, the autopilot will apply TKOFF_TDRAG_ELEV elevator (as a percentage) until the aircraft reaches a speed of TKOFF_TDRAG_SPD1 meters per second. You need to set TKOFF_TDRAG_SPD1 to a speed below the takeoff speed, but above the speed where the aircraft is able to steer using its rudder. When the aircraft reaches TKOFF_TDRAG_SPD1 it will release the elevator and instead use the normal flight pitch controller to try to hold the pitch level. That will have the effect of raising the tail on a tail dragger aircraft.

The TKOFF_ROTATE_SPD parameter controls when the autopilot will try to raise the nose (pitch up) to leave the ground. This needs to be a speed at which the aircraft can sustain a climb, so it should be at least 2 meters per second above the stall speed of the aircraft, preferably more. A higher value will mean a longer takeoff (and thus need more runway). During the run, use TKOFF_GND_PITCH to keep the aircraft on the runway while below this airspeed.

The TKOFF_THR_SLEW parameter controls the throttle slew rate (as a percentage per second) during takeoff. This is used to allow the throttle to ramp up at a rate appropriate for your aircraft. How high this should be depends on the type of aircraft. It is usually a good idea for a ground takeoff to limit how fast the throttle ramps up to prevent torque from the motor causing large steering changes. A value of 20 (meaning 20% throttle change per second) is good for many tail draggers. A tricycle undercarriage aircraft may be able to handle a larger throttle slew rate.

As with other types of takeoff the TECS_PITCH_MAX parameter controls the maximum pitch used when climbing on takeoff. Make sure that this is limited to a value that the aircraft can use to climb quickly at full throttle. A value of around 20 degrees is good for a wide range of aircraft.

Testing Ground Takeoff in FBWA mode

It is sometimes useful to test the takeoff code using the FBWA flight mode. The way you do this is to set the FBWA_TDRAG_CHAN parameter (versions prior to 4.1) or to an RC input channel on your transmitter for a switch (usually a momentary switch, such as the trainer switch), or the channel’s RCx_OPTION to 95 for versions 4.1 and later. When this RC channel goes high while you are on the runway waiting for takeoff in FBWA mode the autopilot will check if you have configured the TKOFF_TDRAG_ELEV and TKOFF_TDRAG_SPD1 parameters. If they have been set to non-zero values then the elevator will be controlled in FBWA in an identical manner to how it is controlled for an AUTO takeoff. The elevator will go to the TKOFF_TDRAG_ELEV value (usually 100% for a tail dragger) as soon as that RC channel goes high, and will stay there until the aircraft reaches an airspeed (if airspeed sensor is used) or groundspeed (if airspeed sensor is not used) of TKOFF_TDRAG_SPD1 meters per second.

This provides a convenient way to test auto takeoff in FBWA mode, and also is a nice way to get better ground steering in FBWA mode in general.

Speed Scaling Issues with no Airspeed Sensor

Since control effectiveness varies with airspeed, ArduPilot automatically scales the control gains in stabilized modes with airspeed to allow stability at low speeds and to avoid oscillations at high airspeeds. However, when an airspeed sensor is not used, an estimated airspeed based on GPS speed, accelerometer inputs, and position changes is used. During takeoffs into strong head wind, this estimate can be wrong and the gains scaled up, resulting in oscillations during the climb into the wind. Setting FLIGHT_OPTIONS bit 7 to 1, the speed scaling will be limited during the takeoff phase of automatic takeoffs to eliminate oscillations, particularly on tightly tuned vehicles.

Catapult Launch without an Airspeed Sensor

Taking off without an airspeed sensor using a catapult may cause less than maximum throttle to be used due to high initial climb rates. For heavy vehicles, this may result in a stall due to the long time constants used in TECS to adjust throttle after the initial launch. The parameter TKOFF_THR_MAX_T can be used to force maximum throttle for a time, irrespective of climb rates from an initial catapult launch to allow the vehicle to obtain sufficient speed.