loss of altitude at and after turn to new waypoint

[chhun_y...@yahoo.com]

Hi everybody,
I fly the red uav dev with the 2 perpendicular sensor boards.
Yesterdy I had to manually override while at autonomous flight, since
the plane performed a dive, dangerousely picking up airspeed and
loosing altitude.
I had noticed problems with altitude hold before, usually only at the
beginning of flights, usually the following happens when I switch from
stabilized mode to Waypoint mode: I normally do this while the plane
has reached an altitude of about 40m agl, Target altitude set at 60m
sometimes 70m the plane starts to turn towards the first waypoint
(that is overhead), but turns of the throttle. _ Than I am forced to
again turn Waypoint mode off, . this I do a few times, most of the
times things settle after the plane passes overhead and normally
climbs to target alltitude and stays there for the rest of the flight.
Yesterday it was a problem at the beginning (throttle would turn off
when waypoint mode comes on) things got better after it came overhead,
but literally every time it turned for a new course to a new waypoint
the nose dropped and altitude loss resulted quite a bit until the
plane recovered by itself. As I said 2 times I even fed in a little up
elevator.
Oh on previous serial udb extra outputs I noticed the altimeter
readings in m quite inaccurate.

Personally I would really like the possibility to install a pressure
altimeter sensor -

what is your experience with the altitude hold?
btw,
options header is set:
altitudehold_stabilized AHPitch only
altitudehold_waypoint AH full

speed control 0
desired speed 10ms

[crashmatt]

This is a common problem.  There are a few things contributing to this.

First, the target altitude is set as a linear interpolation between one waypoint and the next.  In this case the first waypoint is where you switched on auto mode.

If you are flying away from the target waypoint when you engage autopilot, the distance to the waypoint is increasing.  MatrixPilot sets your target altitude to be the same as the altitude you engaged autopilot.  It does this until the distance to the target waypoint reduced to less than the distance between the two waypoints.

Second, you need to pull up during the turn.  there is a value that adjusts this in the options call ROLL_ELEVATOR_MIX

http://code.google.com/p/gentlenav/source/browse/trunk/MatrixPilot/options.h#452

If this value is too small, the aircraft will reduce pitch and altitude during the turn.  When this happens, the reduced altitude results in increased throttle.  This should result in increased pitch control, but maybe not enough.  The example options files give you an idea of the range of control used.  Your case is quite extreme so I might suggest you double whatever value you used for this gain.  There is an upper limit to this gain which might be at 0.5 (need to sanity check this).

Regards Matt

[John]

Hi,
many thanks, however I changed the settings and things got worse!
I`m flying a foam sailplane Radian, I`d say similar to Gentlenav.
I doubled the values for Roll_Elevator_Mix from 0.05 to 0.1 and the
one for Rudder_Elevator_Mix from 0.20 to 0.4.
Setting them back made it better.

My plane uses the rudder in stead of ailerons, rollrate is only
controlled by rudder.
SO I don?t think I even need Rudder_Elevator_Mix.

I used the switch on the red board to reverse elevator - could that be
the reason?
In order to stay at its designed altitude when turning towards a new
waypoint, should I decrease the value?

A little bit I have the feeling that the longer the flight lasts, the
more stable the altitude behaviour becomes- could that be?

Best regards,
John

On Oct 8, 1:13 pm, crashmatt <uavflightdirec...@gmail.com> wrote:
> This is a common problem.  There are a few things contributing to this.
>
> First, the target altitude is set as a linear interpolation between one
> waypoint and the next.  In this case the first waypoint is where you
> switched on auto mode.
>
> If you are flying away from the target waypoint when you engage autopilot,
> the distance to the waypoint is increasing.  MatrixPilot sets your target
> altitude to be the same as the altitude you engaged autopilot.  It does
> this until the distance to the target waypoint reduced to less than the
> distance between the two waypoints.
>
> Second, you need to pull up during the turn.  there is a value that adjusts

> this in the options call ROLL_ELEVATOR_MIXhttp://code.google.com/p/gentlenav/source/browse/trunk/MatrixPilot/op...

>
> If this value is too small, the aircraft will reduce pitch and altitude
> during the turn.  When this happens, the reduced altitude results in
> increased throttle.  This should result in increased pitch control, but

> maybe not enough.  The example options files<http://code.google.com/p/gentlenav/source/browse/#svn%2Ftrunk%2FMatri...> give

[crashmatt]

John,

The channel reverse switch should not change the direction of these settings.

RUDDER_ELEV is a little different to ROLL_ELEV.  RUDDER_ELEV applies up elevator when you want to turn (manual or auto).  ROLL_ELEV always applies up elevator when you are rolled over so it always makes you turn.

Interesting that things get better with flying time.  Make sure you do a few turns in manual before switching to auto.  It needs this to get the wind estimation settled.

How steep are the turns? The maths is good for compensating reasonably steep turns, maybe to 45 degrees roll.  Steeper than that and we might be in trouble.

Regards Matt