Understanding the autopilot

[Owl]

Hello

I am wondering a little bit about the autopilots logic. I’m not talking about bugs or wrong behaviors, and I understand the speed and altitude hold. 
 

But are there times, where you not have the autopilot master switch turned on?  If I turn one off the other dampers off (pitch, roll and yaw), the master damper also turns off. So I can’t believe there will be times, there you switch the pitch, roll or yaw damper off, or am I miss something?

I can see, when starting on the ground, the master autopilot is turned off, and I can’t turn it on, before in the air. What is the logic in that?

 

so if I have to explain my questions, I just wondering why there are dampers for pitch, roll and yaw, then turning one off them off, it also turns the master autopilot off. If having something, why then not just have the master autopilot switch?

 

I know nobody here are French helicopter designers, but maybe someone can give me an explanation?

 

best regards
 

 

[Kinkkujuustovoileipä]

Simple explanation is that the pitch, roll and yaw switches control SCAS. Autopilot is separate system that requires all SCAS channels enabled to function. If one of the SCAS channels is disabled, autopilot will also disable.

[Owl]

@kinkkujuustovoileip thanks for your reply

 

it makes sense, but why have the opportunity to deselect the pitch, roll and yaw?

 

If I only fly from A to B, will the autopilot do any work, if we are NOT using speed or altitude mode? Or is it only using the trim to stabilize the flight behavior? I don’t think the trim have any to do with the autopilot, or?

[Ramsay]

 

11 hours ago, Owl said:

If I only fly from A to B, will the autopilot do any work, if we are NOT using speed or altitude mode?

AFAIK the primary autopilot/SAS mode will attempt to maintain the set attitude (using the SAS pitch and roll channels) and heading (using the SAS yaw or roll channel).

Speed and Altitude modes are "sub" modes of the primary mode using the pitch channel to maintain the selected parameter by adjusting the helicopter's pitch.

11 hours ago, Owl said:

Or is it only using the trim to stabilize the flight behaviour? I don’t think the trim have any to do with the autopilot, or?

AFAIK trim and autopilot/SCAS are interdependent, for example: heading is maintained with the SAS Yaw channel below 90 km/h IAS and switches to using the Roll channel when above 90 km/h IAS

So all 3 SAS channels are used by the autopilot regardless of it's particular "mode", hence why disabling a single SAS channel, also disables the autopilot.

I posted a diagram of the SA-342's Autopilot/SCAS in this thread, along with a link to a French 1970 SA342 Flight Manual (Section 1.7 describes the Autopilot on page 65).

 

Edited December 17, 2024 by Ramsay

[Ramsay]

On 12/16/2024 at 6:14 PM, Owl said:

... just wondering why there are dampers for pitch, roll and yaw, when turning one of them off also turns the master autopilot off. If having something, why then not just have the master autopilot switch?

I know nobody here are French helicopter designers, but maybe someone can give me an explanation?

The 1970 French SA-342 Manual (page 66) explains it is to prevent the entire loss of stabilisation in the event of a chain [channel ?] failure.

Original text [FR]

Quote

2 POSTE DE COMMANDE (Figure 2)
------------------------------
Le poste de commande permet :
- l'engagement general du PA par un interrupteur auto-maintenu (1)
- la selection axe par axe (tangage, roulis, lacet) par inverseur.
Cette solution permet d'eviter la perte de l'ensemble de la stailistion en cas de panne sur chaine.

Goggle Translate

Quote

2 CONTROL STATION (Figure 2)
------------------------------
The control station allows:
- general engagement of the PA by a self-maintained switch (1)
- axis-by-axis selection (pitch, roll, yaw) by inverter.
This solution prevents the loss of the entire stabilization in the event of a chain failure.

 

Edited December 17, 2024 by Ramsay

[Owl]

Thank you very much. I think it makes sense. 
 

best regards