currently in trying to establish a dependable setup/config for a trike gear (nosewheel) plane: takes of perfectly; landing a bit unpredictable yet.
HOWEVER, the plan is to transfer to a taildragger soon.
Pixhack V5, QGC. px4 1.8.2 stable
I have noticed a parameter that is set for pitch angle and mentions a taildragger situation:
RWTO_PSP (FLOAT) Pitch setpoint during taxi / before takeoff airspeed is reached. A taildragger with stearable wheel might need to pitch up a little to keep it’s wheel on the ground before airspeed to takeoff is reached 0.0 > 20.0 (0.5) 0.0 deg
im getting somewhat anxious about this. Has anyone flown a taildragger in RWTO mode yet, or auto landing?
ive flown taildragger planes manually for a few years now, and i’m worried that the parameter doesn’t indicate proper takeoff procedure…
- taildraggers need pitch up while airspeed is LOW to keep pressure on the rear wheel for steering. that includes taxi and BEGINNING takeoff. usually, this pitch is maximum since the propwash is it’s only pressure source, and the wheel needs to steer and battle adverse influence (wind, torque).
- During takeoff, Pitch is relaxed, usually steadily, as takeoff speed is increased. This is because the fin/rudder begins to affect steering AND to avoid premature liftoff and stall that happens if the pitch up remains. The plane gains speed at a level attitude until take off speed is reached, then pitch up is added again for liftoff.
Currently, i see only a single fixed opition for TO pitch. ?
setting this to level results in the wheel immediately lifting off as the propwash hits it, which usually means it goes sideways as the torque/wind affects the plane.
setting this to “pitch up a little” (usually) results in, predictably, the plane lifting off before it has sufficient speed for stable flight and stalls, drops a wing into the runway and crashes. The airframe behaves the same as a nosewheel plane, which is set to a neutral pitch (0deg?) until take off speed is reached and pitch up is applied for lift off.
what to do/think? anyone have experience with setting up PX4 for taildragger TO/landing?
Now for landing…
3 point landing: this is where the plane approaches the landing in a slow, nose-up attitude so that all 3 wheels (2 mains and 1 tail) are at approximately the same level with respect to the ground. the plane slowly sinks into the ground at this attitude or gently stalls about the time all 3 wheels are at ground level.
Wheel/Mains landing: this is where the plane is brought in (usually) at a faster speed than a 3 point landing and flares to a level attitude, then either sinks or slightly adjusts to negative pitch to touch the main wheels on the runway. The result is the plane rolls a bit on the main wheels with the tail wheel in the air, until the tail stops flying or the pilot pulls up elevator and forces it down. This method (wheel/mains landing) means the plane can come in steeper and faster, doesn’t need such a long shallow approach before touchdown.
SO… after touchdown, adding pitch-up to the elevator too soon can result in a bounce/relaunch to stall-crash. 3point landings might seem safe, but they often touch down with enough airspeed to relaunch if the elevator is pulled up too much/too soon or a headwind gust hits.wheel landings generally neutralize the elevator until the tail stops flying (falls to the ground on its own), then apply pitch-up elevator.
the significance of these procedures is because the plane could still lift off after touchdown, but needs to have steering control ASAP (must prevent veering off course and ground looping). The plane uses elevator pitch for 2 different reasons, at 2 different phases of landing.
does px4 have any way to accomodate this?
Would it be good to have 2 separate pitch set points for a tail dragger, depending on airspeed? …
- tail wheel down (full-up pitch under a certain airspeed)
- RWTO_PSP (airspeed after tail wheel up but before take off airspeed)
so, theoretically, i could program takeoff params to be…
‘tail wheel down’ airpseed = FW_AIRSPD_MIN * “FW_TWD_SC” <-- hypothetical param…value 0%-60% ?
this would apply full-up elevator when active. after this airspeed is reached, RWTO_PSP is activated
… which deactivates after takeoff! to prevent a mishap during in flight maneuvers.
Landing, similarly activates full up elevator only when the LANDING ALTITUDE is reached AND the airspeed is below “tail wheel down” airspeed.
ok, i just discovered that ardupilot already has accomodation for taildragger TO/L. maybe px4 can copy that? seems easy enough.
i may switch over to that flight stack.