BS auto-pilot/trimming question

[erjdriver]

Still having some issues/misunderstanding with this.

 

1. On t/o, i pitch down around -10, release the trimmer and then center the joystick - shouldn't the aircraft hold that pitch? it doesn't - pitches up 4-5.

 

2. does the auto-pilot ever "move" the collective by itself - e.g. when maintaining altitude.

 

3. if I have the switch on the right panel set to "RA" (as opposed to altimeter) - and am 100 RA when i release the trimmer (and have ALT HOLD off) - shouldn't the autopilot maintain 100 RA over varying terrain? Getting back to #2 - does it adjust the collective as needed.

[sLYFa]

2./3. The AP has very limited authority over the collective, AFAIK it does not move the collective handle itself but uses servo motors somewhere down the control line to adjust pitch. It does hold altitude but ONLY with ALT HOLD on. The RA/Baro switch determines which data the AP uses to maintain altitude, however with ALT HOLD off it will have no effect.

Remember that the Shark has no terrain following radar like f-16 or tornado. Even small changes in terrain can exceed the AP authority to hold altitude and you will end up in the ground. So when flying very low, you should always maintain altitude by hand. The ALT HOlD function is rather to be used in cruise flight or over flat terrain.

[Catastrophy]

The AP doesn't affect the collective and if you don't have Alt Hold active the AP won't hold the altitude.

[Flagrum]

3. if I have the switch on the right panel set to "RA" (as opposed to altimeter) - and am 100 RA when i release the trimmer (and have ALT HOLD off) - shouldn't the autopilot maintain 100 RA over varying terrain? Getting back to #2 - does it adjust the collective as needed.

 

The RA/Baro switch determines which data the AP uses to maintain altitude, however with ALT HOLD off it will have no effect.

Remember that the Shark has no terrain following radar like f-16 or tornado. Even small changes in terrain can exceed the AP authority to hold altitude and you will end up in the ground. So when flying very low, you should always maintain altitude by hand. The ALT HOlD function is rather to be used in cruise flight or over flat terrain.

 

The AP doesn't affect the collective and if you don't have Alt Hold active the AP won't hold the altitude.

To tell the AP which altitude to hold, you need to enable ALT HOLD first, ofc. Then use the collective brake (instead of cyclic trim) to set the new desired altitude. (Search the forums for more detail on the collective brake - its function and use. )

 

And then, if your altimeter source is set to Radar Alt, the helo actually respects elevation changes of the terrain. But be careful, the warning sLYFa mentioned still applies: that is NOT an actual terrain following feature. The helo only knows its current altitude and has no idea about terrain elevation changes along the flight path. So if you are very low and fast, the helo might not have time enough to react .... ;o)

[Home Fries]

To answer 1, the simplest way to look at it is that the helicopter will try to use the pitch channel to maintain the speed when you trim. It's not exactly that simple (a lot depends on which autopilot channels are enabled, and the Alt channel does affect this), but it's a good enough way to look at it. Set the speed that you want and put it in route mode and it will cruse to the waypoint at that speed. If you have alt hold set, be sure to adjust your collective so that the VSI is level at 0.

[erjdriver]

To answer 1, the simplest way to look at it is that the helicopter will try to use the pitch channel to maintain the speed when you trim. It's not exactly that simple (a lot depends on which autopilot channels are enabled, and the Alt channel does affect this), but it's a good enough way to look at it. Set the speed that you want and put it in route mode and it will cruse to the waypoint at that speed. If you have alt hold set, be sure to adjust your collective so that the VSI is level at 0.

 

ok - then makes sense.

 

I was using thinking that the AP would maintain pitch - but it maintains speed you say.

 

I was setting the trim right after t/o as i was accelerating...so makes sense that if i've set the trim at 50 and accelerating - it'll immediately pitch up to maintain 50.

 

Thanks.

[Fishbreath]

The pitch channel only attempts to hold speed in route mode. Otherwise it does attempt to hold pitch.

 

The reason you might get a bounce while accelerating from a stop, especially past 50-60kmh, is twofold:

 

1) While accelerating from a hover, the pitch forces on the aircraft are rapidly changing. If you release the trimmer before you've found the rough point-of-trim--that is, where the stick needs to be to hold the given pitch--the AP will quickly run out of authority.

2) As you pass 50-60kmh, you start to enter the effective translational lift regime, where airflow from the aircraft's forward motion is a significant component of the airflow over the blades, and therefore the lifting force. The aircraft will want to climb, and so it'll also want to pitch up around that speed.

[buzz1955]

Hi those two pages help me a lot with the trim and the autopilot;

 

http://forums.eagle.ru/showthread.php?t=36246

 

[erjdriver]

 

1) While accelerating from a hover, the pitch forces on the aircraft are rapidly changing. If you release the trimmer before you've found the rough point-of-trim--that is, where the stick needs to be to hold the given pitch--the AP will quickly run out of authority.

 

2) As you pass 50-60kmh, you start to enter the effective translational lift regime, where airflow from the aircraft's forward motion is a significant component of the airflow over the blades, and therefore the lifting force. The aircraft will want to climb, and so it'll also want to pitch up around that speed.

 

1. I understand this - need to hold the pitch steady before release trimmer.

 

 

2. Not sure about this - I understand translational lift and see that there'll be an increase in the rate of climb - but pitch up? I'm talking about pitching up 15-20 deg where I want.

 

Isn't the gain in lift equal (or close to equal) thru out the blade area.

 

 

On further testing and input from users - I think I found the main reason for the major pitch up I was experiencing.

 

Users have told me that normally the AP will maintain pitch mode unless in route mode - in which case speed is maintained (or attempted)

 

As I was accelerating after t/o - I used to enable route mode.

 

As I did this soon after t/o and accelerating - the AP was trying to maintain the speed - ergo the major pitch up.

 

So now, as I accelerate and climbing - I do NOT enable route mode - fly manually and try to set the trimmer at -5 - and it stays close to that.

 

Once I've leveled off and achieved my route speed, I enable route mode and then release the trimmer - and that sets the speed and pitch close to where I released it.

 

Hope this helps the next poor soul.

[zaelu]

To OP.

 

1. You need to constantly trim. As the helicopter accelerates or changes direction you need to retrim. All you see is the effect of the aircraft being trimmed for other speed or direction... from the past.

 

2. It doesn't move the collective but when you put AP in Autodescent or Alt Hold it does play with pitch. Otherwise, when flying with some forward speed it climbs or descends by pitching up or down.