Helicopter is much more agile with AP on?

Greetings to developers!

Thank you for this helicopter, it looks and flies really well! I have however noticed interesting thing: When the "autopilot" pitch/roll axis is turned on (green light is shining), the helicopter is much more agile (reacts on much smaller and shorter pitch/roll inputs) than when this AP is turned off. When it is off, helicopter seems to be much more stable.

As "autopilot" in this helicopter is designed primarily for it's stabilization, I think it should be exactly opposite. I discussed this with my flight instructor, who spend thousands hours on Mi-8 and Mi-17 and he confirmed this - AP on - stable / AP off - unstable.

Vita

AP on - stable / AP off - unstable.

This is how I experience it, too.

Can you post a track where you demonstrate the opposite behaviour that you seem to experience?

Yes. I will do it in the evening my time (UTC+1).

Vita

Greetings to developers!

Thank you for this helicopter, it looks and flies really well! I have however noticed interesting thing: When the "autopilot" pitch/roll axis is turned on (green light is shining), the helicopter is much more agile (reacts on much smaller and shorter pitch/roll inputs) than when this AP is turned off. When it is off, helicopter seems to be much more stable.

 

As "autopilot" in this helicopter is designed primarily for it's stabilization, I think it should be exactly opposite. I discussed this with my flight instructor, who spend thousands hours on Mi-8 and Mi-17 and he confirmed this - AP on - stable / AP off - unstable.

 

Vita

To me happened totally the oposite. When the green light was off, it was REALY HARD to controle it and when got it on, it was really pleasant to fly.

:joystick: :thumbup:

Here is the track - please see that when AP is on (green light shines), helicopter is more agile and harder to control...

Vita

Agileness demonstration.trk

Hello,

For me I meet the same situation that ljuba, if I switch OFF the "Roll & Pitch" channel my Mi-8 is more hard to control, if I switch ON the "Roll & Pitch" channel my Mi-8 is more easy to control...

Like the KA-50 if I switch OFF the 3 channels of the autopilot it's very more hard to fly, it's the same situation for the Mi-8.

Sometime I takeoff and quickly my Mi-8 becomes unstable and very hard to control, I need just 2 secondes for understand that the "Roll & Pitch" channel is OFF and quickly I press the green switch (green light ON) and my Mi-8 is very stable and more easy to control.

Bye, Skull.

From the Mi-8 Manual:

2-34 Automatic Flight Control System

(AFCS)

The helicopter is equipped with the AP-34B

autopilot system. The autopilot stabilizes the helicopter

in pitch, heading, roll, altitude, and airspeed. The

autopilot interfaces with the helicopter navigation

equipment to hold a selected course. The AFCS

includes the four-channel autopilot system and an

airspeed correction unit. The autopilot system

receives 27 VDC power from the Battery and Rectifier

Buses via three AUTOPILOT circuit breakers on

the right cb panel. The autopilot receives 36 VAC

3~phase power via three 5-amp fuses on the main

fuse panel.

2-35 AP-34B Autopilot System

The autopilot system is designed to stabilize

control of the helicopter while taxiing, during takeoff,

while hovering, in flight, and during landing. Under

normal operating conditions, the yaw, pitch, and roll

channels are engaged before beginning to taxi and

remain engaged throughout the flight and landing.

The altitude channel is engaged as needed to maintain

the selected barometric altitude. The autopilot

system includes an integrated control panel for the

yaw, roll, pitch, and barometric altitude channels; a

zero indicator unit; an amplifier/control unit; pitch

and roll compensation transducers; and yaw, pitch,

and roll rate gyros. The control panel and zero

indicator unit are located on the center console. The

hydraulic flight control servos apply autopilot

corrections to the flight control surfaces and provide

feedback signals to the autopilot channels. Autopilot

roll, pitch, and altitude correction signals are limited

to a maximum of 20% of control travel for flight

safety in the event of false signals or system failure.

The pilot may intervene at any time while the autopilot

is engaged to make manual corrections by operating

the flight controls. The autopilot channels are

engaged by pressing the green ON buttons at the top

of the control panel. The roll and pitch channels are

designed to work together continuously, while the

yaw and altitude channels can operate independently.

The yaw and altitude channels can be disengaged

individually using the red OFF buttons on the

control panel. Each channel has a zero indicator

which shows the relative displacement of the flight

control servo spindle. The control panel has centering

knobs for the yaw, pitch, and roll channels which

allow the pilot to introduce small corrections (±10o)

by turning the knob for the channel requiring correction.

2-37 Roll/Pitch Channel Operation

The roll and pitch channels are engaged simultaneously.

The roll channel receives signals proportional

to the angle of roll from the copilot’s attitude

indicating system, rate of roll signals from the roll

rate gyro, and signals equal to the inverse of the

cyclic stick’s lateral deflection from the roll compensation

transducer. The output signals from the roll

channel are sent to the lateral flight control servo. The

pitch channel receives signals proportional to the

angle and rate of pitch from the copilot’s attitude

indicating system, rate of pitch signals from the pitch

rate gyro, and signals equal to the inverse of the

cyclic stick’s longitudinal deflection from the pitch

compensation transducer. The signal from the pitch

compensation transducer serves to reset the channel

to the new pitch setting when the pilot moves the

cyclic stick and to stabilize abrupt movements.

Output signals from the pitch channel are sent to the

longitudinal flight control servo. When the altitude

channel is engaged, the pitch channel receives

correction signals from the KZSP airspeed correction

unit to stabilize the airspeed. When the TRIM DISENGAGE

button on the cyclic is pressed, the output

signals from the roll and pitch channels and the

signals from the roll and pitch compensation transducers

are reset to zero.

Hello,

 

For me I meet the same situation that ljuba, if I switch OFF the "Roll & Pitch" channel my Mi-8 is more hard to control, if I switch ON the "Roll & Pitch" channel my Mi-8 is more easy to control...

Like the KA-50 if I switch OFF the 3 channels of the autopilot it's very more hard to fly, it's the same situation for the Mi-8.

 

Sometime I takeoff and quickly my Mi-8 becomes unstable and very hard to control, I need just 2 secondes for understand that the "Roll & Pitch" channel is OFF and quickly I press the green switch (green light ON) and my Mi-8 is very stable and more easy to control.

 

Bye, Skull.

that

For me the 8 is much easier to control with AP ON. With AP OFF there are more oscillations that are harder to counter - thus it may seem more agile however it is less controllable (you exert an input and you have to counter a fast oscillation with second input). With AP ON, all oscillations are countered, so the control with single inputs is more precise and it may seem more agile, but it really is just more controllable and stable.

Here is the track - please see that when AP is on (green light shines), helicopter is more agile and harder to control...

Vita

I can see what you mean ... somehow.

My thoughts on this, after watching your track and after trying it myself (but I have to say that I am not really experienced on the Mi-8, yet):

• Your inputs when flying without AP were very quick and also short (mostly). The Mi-8 is big and has a lot of inertia - that "dampens" the reactions to such inputs quite a bit, I would assume.
  
• Your inputs when flying with AP where a bit wider? It looked to me that way at least ... but perhaps I should watch the track again...
  
• When I tried it myself, I had the feeling that with or without AP the reactions to sudden, but wide® inputs where about the same. What I and others stated, that the helo (should) feel more agile without AP might be based on the fact, that it is in general more "twitchy". Now I think, there is no big difference between these two variants in terms of agility.
  
• Maybe someone else, more experienced, should investigate a bit deeper here :o)
  

I tried to do roughly the same inputs. With AP off the pitch and roll changes seems to me much slower and with greater delay, while with AP on, they are almost immediate.

I usually do not wiggle joystick this way, it was done mainly for test purposes...

Hi all!

Thank you for your inputs to this discussion. The problem was that I completely misunderstood how to use autopilot and force trim during flight.

Today my instructor visited my place. He has thousands of hours on Mi-8/17. He was really excited about this model and explained to me that the way I control it is completely wrong.

I originally thought that force trim is here just to release force from my cyclick. So I tried to fly it as I am used to in light helos like Schweizer and only when there was excessive force in the cyclic, used the force trim.

He said this machine has to be flown different way and I should use force trim BEFORE I do any cyclic movement, keep it pressed while establishing new pitch and roll, then release it and let the autopilot do it's job and do not try to fight with stick forces - only apply slight pressure when absolutely necessary. When I need to use bigger movement, I should use force trim again. It works really well and Mi-8 is now my favorite machine to fly :-)

We also tried autorotation - myself in controls and under my instructor supervision and he stated it was like in real life.

Overall, he was really impressed.

Vita

Really off-topic and quite a longshot, but...

Is his name Vic H. by any chance?