First Flight Tests¶
First Spool Up¶
Check the CG of helicopter. It should be exactly on the point of the main shaft. If not, adjust battery until it is.
For the first spool up, before trying to lift into hover, you should verify that the main rotor blades are tracking reasonably well. If not, excessive vibration will be created, perhaps high enough to cause attitude control errors or even cause an EKF Failsafe.
- Arm the vehicle. If there is a pre-arm failure, resolve it before proceeding. Usually you will need to wait until the GPS obtains lock and the EKF sets its origin as indicated by the ground control station messages (and, of course, the failure to arm!)
- Before the automatic disarming timing elapses, engage the Motor Interlock switch and/or raise the collective above 1/4 stick (if using passthru RSC mode)to enable Motor Interlock. The rotor should spool up to speed. Usually, one spools up at 0 deg collective pitch setting to avoid stress while on the ground
- At this point, check that the blades are tracking. If not, adjust a blade’s pitch link to the swashplate and re-try until the plane of both blades is close to identical.
See Traditional Helicopter Tips for more on vibration issues.
Once tracking has been verified, you can do your first test hover. In STABILIZE, repeat the above, but once the rotor has finished spooling up, increase collective slowly and lift off. Be prepared to put in small cyclic corrections to hover the vehicle in place. If the vehicle seems too unstable in attitude, you might try setting the PID values to those used for beginning the fine tuning of the attitude control loops, discussed in the following section (Traditional Helicopter – Tuning ).
Trimming the helicopter in pitch and roll axes is an important step to keep the aircraft from drifting in modes like Stabilize and Althold. The trim attitude in the roll axis is affected by the tail rotor thrust. All conventional single- rotor helicopters with a torque-compensating tail rotor hover either right skid low or left skid low, depending on which way the main rotor turns. The ArduCopter software has a parameter, ATC_HOVR_ROL_TRM, to compensate for this phenomenon. Dual rotor helicopters have counter rotating rotor systems which mostly cancel the torque. For these aircraft, the ATC_HOVR_ROL_TRM should be set to zero. Longitudinal CG location will affect the trim attitude in the pitch axis. There is no parameter to tell the autopilot what pitch attitude the aircraft hovers with no drift. It always targets zero deg pitch as measured by the autopilot. Therefore the actual pitch attitude the aircraft hovers may be 5 deg nose high but the autopilot AHRS Trim value is set to make it think the attitude is zero deg.
In order to trim the aircraft, set the ATC_HOVR_ROL_TRM parameter to zero.
During the initial setup of the autopilot, the
AHRS_TRIM_x values are set
during the accelerometer calibration on the last step that has you level the
aircraft. For that step you should have made certain that the shaft was
perfectly straight up in pitch and roll. For this trim procedure, it is
recommended that you check it and using the method below.
Measure the actual frame angle (on a portion of the frame that is perpendicular to the mainshaft) in pitch and roll with your digital pitch gauge. Connected to your ground station software with MavLink, note the pitch and roll angle the autopilot is “seeing”. Adjust the AHRS_TRIM_X and AHRS_TRIM_Y values so the autopilot “sees” the identical frame angle you measured with the digital pitch gauge. You can use the Level Horizon function in your ground station to level the horizon with the helicopter at actual level. That function will make the adjustments to the AHRS_TRIM’s for you.
The above is necessary so we can accurately measure the roll angle to set the ATC_HOVR_ROL_TRM. The autopilot now “knows” when the mainshaft is perfectly vertical.
Load the helicopter with its normal payload, and hover the helicopter in no-wind conditions in Stabilize flight mode. Land it and pull the log, noting the roll angle and pitch angle that you had to hold with the stick to keep the helicopter from drifting. Enter the roll angle value in the ATC_HOVR_ROL_TRM parameter in centidegrees. For a CW turning main rotor if it took 3.5 degrees of right roll to compensate, enter 350. Negative values are for a CCW turning main rotor that requires left roll to compensate. If the pitch angle was not zero to keep the helicopter from drifting, then enter the pitch attitude required for no drift in the AHRS_TRIM_Y in radians (which is the angle in degrees divided by 57.3). If it took 2 degrees nose down (-2 deg) to hover then you would enter -0.0349 radians into AHRS_TRIM_Y.
Do not use the radio trims at all. Make sure they are at the same position as when the Radio Control Calibration was done.
Your helicopter is now trimmed properly. If you hover the aircraft and it still drifts, check that the aircraft is actually holding the hover attitude determined above with the stick centered. If it does not, then consult the Setting ILMI and IMAX section of the Additional Tuning Topics wiki. This trimming procedure makes the difference between a helicopter that is difficult to handle vs one that flies with true scale quality and handling.
Enable Notch Filtering¶
Assuming that first low in-place hover was successful, which it should be if you have followed the wiki, you should setup a dynamic harmonic notch filter. Tuning of the control loops, which is the next step, is greatly enhanced if the vehicle’s generated vibrations are attenuated by the filter.
Follow the instructions in Helicopter Dynamic Notch Filter Setup.
There usually are two rotational vibration sources in Traditional Helicopters, the main rotor and the tail rotor. The tail rotor frequency may appear as if its a harmonic of the main rotor, but not quite at an integer multiple of the main rotor fundamental frequency. For example, if the tail rotor has a 1:3.8 gearing to the main rotor, it will appear near what would be the 4th harmonic of the main rotor, but is slightly lower (ie 3.8x the fundamental of the main rotor). In this case, the harmonic notch setup for the 4th harmonic may still be effective if the bandwidth of the notches, INS_HNTCH_BW,are wide enough.
After the harmonic notch is setup, proceed to Traditional Helicopter – Tuning.