FPV - Trim Ball - Head Tracker

[Caldera]

Hey All,

OK...

Keeping in mind that I realize that the DCS AH-64D is still in early access, but I am trying to make the mental jump to the "Real McCoy" and how it would fly.

I am talking about alignment between the FPV, the Trim Ball and the Head Tracker. 

There just doesn't seem to be much:

• If I fly straight --> With the Trim Ball centered and aligned with the Head Tracker --> The FPV is way off usually to the right.
• If I fly straight --> With the FPV centered and aligned with the Head Tracker         --> The Trim ball is way off usually to the left.

Shouldn't at some point all three of those flight assistance markers be in alignment, say hypothetically the aircraft is flying in perfect trim in the direction of travel.

 

So this boggles my brain, as I can not quite think my way through it.  Especially when the FPV is off to the right with the Trim Ball Centered.  In my mind, there has to be a skid occurring for that to be true.  The Trim Ball is a gyroscopic measurement if I am correct, and I had thought that this should indicate alignment between the aircraft direction and its path movement vector through space.

 

I generally have been aligning the FPV with the Head Tracker, damn the Trim Ball, and getting better results.

I was wondering how the rest of you deal with it?

Thanks in advance,

Caldera

[frostycab]

I think Wags referred to this in one of his videos. It's the difference between being on nose-to-tail trim (FPV centered) and aerodynamic trim (slip ball centered.) 

I don't truly understand helicopter physics, but I think all helis do crab to some degree depending on the current flight regime. However I'm sure I've heard reliable people comment that at certain speeds the crabbing in the Apache seems rather excessive.  I'd love to get some real insight into things.

[Shimmergloom667]

40 minutes ago, Caldera said:

Hey All,

OK...

Keeping in mind that I realize that the DCS AH-64D is still in early access, but I am trying to make the mental jump to the "Real McCoy" and how it would fly.

I am talking about alignment between the FPV, the Trim Ball and the Head Tracker. 

There just doesn't seem to be much:

• If I fly straight --> With the Trim Ball centered and aligned with the Head Tracker --> The FPV is way off usually to the right.
• If I fly straight --> With the FPV centered and aligned with the Head Tracker         --> The Trim ball is way off usually to the left.

Shouldn't at some point all three of those flight assistance markers be in alignment, say hypothetically the aircraft is flying in perfect trim in the direction of travel.

 

So this boggles my brain, as I can not quite think my way through it.  Especially when the FPV is off to the right with the Trim Ball Centered.  In my mind, there has to be a skid occurring for that to be true.  The Trim Ball is a gyroscopic measurement if I am correct, and I had thought that this should indicate alignment between the aircraft direction and its path movement vector through space.

 

I generally have been aligning the FPV with the Head Tracker, damn the Trim Ball, and getting better results.

I was wondering how the rest of you deal with it?

Thanks in advance,

Caldera

I just accept that helicopters crab when aerodynamically trimmed. No big deal. I trim nose-tail when I need it (landing, NOE flight, stuff like that)

[Eagle7907]

I think I read that one of the top Apache YouTubers said that the amount of crab currently simulated is exaggerated to a degree. Who knows, ED may look into it and change it up a bit, however I remember they did say that at speeds above 100 knots, the Apache will crab. Something like in no wind around 100 knots, the FPV would be lined up underneath the right side of the head alignment diamond, 120 knots it would be just under but slightly outside to the right of head alignment diamond and so forth.


Sent from my iPhone using Tapatalk Pro

[Caldera]

Thanks Guys,

I guess my problem is that I do know that helicopters will crab (Skid), but the indications (as in theory, don't lie) confuse me as to what I am seeing.  Again, I am considering Early Access and just wondering for point of discussion.  My only other experience is the DCS Huey and it has no FPV.

 

Quick Start Manual:

Slip/Skid. The bottom scale of this symbol indicates horizontal acceleration.

This clearly says "horizontal acceleration" so does that mean that the Trim Ball will not detect a horizontal movement if that movement is at a constant speed?  The cockpit instrumentation clearly shows otherwise and to me simply indicates a skidding condition. 

If the aircraft is in trim and the horizontal movement is sideways, as indicated by the FPV,  isn't that a skid?

If the Trim Ball is centered this should mean that the aircraft is in trim.  In fact, that is what the Quick Start Manual says.  Where Trim Ball" is mentioned 16 times.  Nines times it references "trim ball centered" and four times it references "center the trim ball".  This is true for many flight conditions including landing, take-off and firing rockets (straight flight).  I would agree to that except the FPV can be way off from the trimmed intentional direction of flight and essentially requires a banked turn at all times.   To align the FPV with the intended direction of level flight, requires the Trim Ball to be not centered.

Um?  No...

 

Quick Start Manual:

It is important to note that the most accurate rocket employment with forward airspeed results when the aircraft is kept within coordinated flight with the trim ball centered.

Pages 282-285

Um?  No...

 

Does the Head Tracker indicate aircraft center line, like a bore sight?

Maybe I should just press the easy button...

Caldera

Edited April 13, 2022 by Caldera

[shagrat]

vor 3 Stunden schrieb Caldera:

This is true for many flight conditions including landing, take-off and firing rockets (straight flight). 

That's the misconception. Centering the ball in an helicopter, unlike an airplane does not mean straight. It shows the torque/yaw is in equilibrium with the actual flight vector, but most of the time they are two different headings.

Get into the Huey, speed up to 110+ kts, center the ball, and go F3 view. You will notice how the direction of flight is to the right from where the nose is pointing.

Now ignore the ball and eyeball the flight path and get the nose pointing to your actual course... You may need to reduce speed to make it work and counter with left cyclic vs. left pedal. That's the same behavior/effect as in the Apache.

If the aircraft is "in trim" (ball centered) it actually flys a little to the right (FPV), but is in "aerodynamic trim", as in optimum airflow and equilibrium of forces (hope I make sense). On take-off, landing,  firing rockets, you want the nose pointing to where the helicopter is going, so you align the nose with the FPV and ignore the ball. That is called "nose to tail trim".

[mcfleck]

I don't have any stick time in the apache, but some hours in another helicopter and from my experience helos do not necessarily crab.
That is primarily depending on the wind conditions. With calm winds our ground track indicator (kind of a 2D FPV) was ALWAYS aligned with the heading bug which is basically the AC nose.
Also very noticeable on an approach where you keep the AC aerodynamically in trim above 50 ft AGL and align with the rwy direction below 50ft. In no wind situation you end up being aligned with the runwayin aerodynamic trim.
With a 20 kt crosswind (or higher) you might end with a huge difference between those two.
I had approaches with roughly 30 degrees differece between aerodynamic trim and rwy alignment...
DCS does not simulate all the effects of helicopter aerodynamics and that might be a reason why it seems a bit off (I am looking at you, transverse flow effect)

TLDR:
In general:
Zero wind: aerodynamic trim and nose-tail trim should be roughly the same, may vary slightly at higher speeds, but that depends on the specific AC
High crosswind: Deviations between those two may be rather large left or right, depending on the wind direction

[Flamin_Squirrel]

1 hour ago, shagrat said:

That's the misconception. Centering the ball in an helicopter, unlike an airplane does not mean straight. It shows the torque/yaw is in equilibrium with the actual flight vector, but most of the time they are two different headings.

Get into the Huey, speed up to 110+ kts, center the ball, and go F3 view. You will notice how the direction of flight is to the right from where the nose is pointing.

Now ignore the ball and eyeball the flight path and get the nose pointing to your actual course... You may need to reduce speed to make it work and counter with left cyclic vs. left pedal. That's the same behavior/effect as in the Apache.

If the aircraft is "in trim" (ball centered) it actually flys a little to the right (FPV), but is in "aerodynamic trim", as in optimum airflow and equilibrium of forces (hope I make sense). On take-off, landing,  firing rockets, you want the nose pointing to where the helicopter is going, so you align the nose with the FPV and ignore the ball. That is called "nose to tail trim".

Shouldn't the FPM (in a zero wind condition) by definition be straight ahead for there to be zero aerodynamic slip? As you point out in the first paragraph, the ball is perfect when you've got symmetric thrust/drag, but helicopters don't. I suspect the reason the ball is still used is because while not perfect, it's a very simple device, and it's close enough (and the FPM can't be used as it's influenced by wind).

Edited April 13, 2022 by Flamin_Squirrel

[shagrat]

vor 5 Minuten schrieb Flamin_Squirrel:

Shouldn't the FPM (in a zero wind condition) by definition be straight ahead for there to be zero aerodynamic slip? As you point out in the first paragraph, the ball is perfect when you've got symmetric thrust/drag, but helicopters don't. I suspect the reason the ball is still used is because while not perfect, it's a very simple device, and it's close enough (and the FPM can't be used as it's influenced by wind).

 

Not necessarily as the torque from the rotor is pushing the nose to the right, while the tail rotor pushes against the tail boom adding a bit of drift. You compensate this with tilt of the rotor disc, but to my understanding the equilibrium of all forces results in a vector that's off center. Of course wind influences this and can either enhance it or even negate it. Also any changes to the controls will change it a tiny bit.

[Flamin_Squirrel]

27 minutes ago, shagrat said:

Not necessarily as the torque from the rotor is pushing the nose to the right, while the tail rotor pushes against the tail boom adding a bit of drift. You compensate this with tilt of the rotor disc, but to my understanding the equilibrium of all forces results in a vector that's off center. Of course wind influences this and can either enhance it or even negate it. Also any changes to the controls will change it a tiny bit.

You might be right. Lots of forces at play. Just doesn't seem intuitively right to me that it can be, level, constant heading, ball in the middle, and be in trim if it's not heading straight ahead.

[nikoel]

10 hours ago, Flamin_Squirrel said:

You might be right. Lots of forces at play. Just doesn't seem intuitively right to me that it can be, level, constant heading, ball in the middle, and be in trim if it's not heading straight ahead.

I am going to explain this as simply as possible and avoid jargon. This is due to something called Tail Rotor Drift (which causes Roll) that the poster above has referenced. This can be offset by either the pilot or the manufacturer (by naturally tilting the mast in the opposite direction). We can go into physics and aerodynamics but in the end what you need to understand is that the aircraft in a anticlockwise rotor system will naturally want to hang left skid low

Because of this you will see the ball offset slightly [to the left of centre]. It's a similar principle to what a builders construction level ruler would do when it's tilted to one side. This can be done by either having a heavier or a lighter ball

Therefore when you're in forward flight if you would put the ball in the middle you will be skids level. But remember what you learned above. If you are skids level when the aircraft wants to naturally hang with the left skid low, you're aerodynamically 'flying sideways' (off balance). This will be reflected by a simple string on the nose of the aircraft which will be offset also

Now I have never flown the apache but I have taught on many helicopters; and what I will say is that this effect, at least to me, seems to be extremely pronounced in the AH-64. In the same way that Vortex ring is simulated. In reality it can be very difficult to enter even on purpose

Having no experience inside the Apache, I am hesitant to call this out as unrealistic and over exaggerated until this is confirmed by a real Apache Pilot. But it's certainly a magnitude higher than what I was expecting 

Edited April 14, 2022 by nikoel

[Flamin_Squirrel]

8 hours ago, nikoel said:

I am going to explain this as simply as possible and avoid jargon. This is due to something called Tail Rotor Drift (which causes Roll) that the poster above has referenced. This can be offset by either the pilot or the manufacturer (by naturally tilting the mast in the opposite direction). We can go into physics and aerodynamics but in the end what you need to understand is that the aircraft in a anticlockwise rotor system will naturally want to hang left skid low

Because of this you will see the ball offset slightly [to the left of centre]. It's a similar principle to what a builders construction level ruler would do when it's tilted to one side. This can be done by either having a heavier or a lighter ball

Therefore when you're in forward flight if you would put the ball in the middle you will be skids level. But remember what you learned above. If you are skids level when the aircraft wants to naturally hang with the left skid low, you're aerodynamically 'flying sideways' (off balance). This will be reflected by a simple string on the nose of the aircraft which will be offset also

Now I have never flown the apache but I have taught on many helicopters; and what I will say is that this effect, at least to me, seems to be extremely pronounced in the AH-64. In the same way that Vortex ring is simulated. In reality it can be very difficult to enter even on purpose

Having no experience inside the Apache, I am hesitant to call this out as unrealistic and over exaggerated until this is confirmed by a real Apache Pilot. But it's certainly a magnitude higher than what I was expecting 

 

I'm aware of all of that. That was my point.

[nikoel]

4 minutes ago, Flamin_Squirrel said:

I'm aware of all of that. That was my point.

Aaaand I was not talking to you per se. Just quoting your comment and expanding on it

Edited April 14, 2022 by nikoel

[Caldera]

Thanks for all the replies!

 

In some respects, maybe because of Early Access, I still think this is not exactly right.  I get the way a helicopter flies for the most part, but to my mind by centering the Trim Ball it should fly straight to the FPV and not crab.  Otherwise, a skid / crab would always be indicated on the Trim Ball.  It is a catch 22.  Currently it is off by noticeable amount. 

And, maybe I am just a rock head...

Could it be the Trim Ball indication that is not quite correct?

 

Quick Start Manual page 112:

FMC Controls. Toggles individual FMC SCAS axes/modes:
• PITCH. The SCAS will dampen longitudinal (pitch) rates and can command longitudinal cyclic in command mode.
• ROLL. The SCAS will dampen lateral (roll) rates and can command lateral cyclic in command mode.
• YAW. The SCAS will dampen directional (yaw) rates and provide heading hold and turn coordination. <--- This
• COLL. The SCAS can command collective application in command mode.
• TRIM. Toggles the force trim magnetic brakes on the cyclic and pedals.
• NOE/A. Activates FMC nap of earth/approach mode. In NOE/approach mode, the horizontal stabilator is commanded to 25° trailing edge down when airspeed is below 80 knots. This provides better over-the-nose visibility for low-altitude flying.

The above is in the game?

I had thought that the AH-64D SCAS was similar to SAS in the A-10C.  True or False?

 

For example, I wish your opinion on two different methods to make a right turn. 

To make a high speed hard right turn and I want the FPV to lead the Head Tracker through the turn (nose of the aircraft).  When starting the bank I must initially first push the left pedal pretty hard.  By doing this, the FPV  will  move to the right of the Head Tracker.  Then I must add right pedal to further coordinate the turn.  Otherwise, the FPV seems to stay out of the IHADSS on the left and I am guessing results in a slower rate of turn.  For this turn, I want to the nose to be on target when I come out of the turn with allot of forward air speed so I need to maintain more coordinated flight.

For a spinning / sliding right turn (rotation) this is not a factor, where I might simply slam the right peddle to the floor.  For this turn the forward air speed is consumed by the maneuver.  The FPV is way to the left and out of vision. 

The dynamics are way different.   Sound correct?

 

I know that I am probably over thinking the whole thing and maybe jumping too soon.  But as I said earlier, just a topic for discussion that I am enjoying to read and respond.

Caldera

Edited April 14, 2022 by Caldera

[shagrat]

vor 13 Stunden schrieb nikoel:

I am going to explain this as simply as possible and avoid jargon. This is due to something called Tail Rotor Drift (which causes Roll) that the poster above has referenced. This can be offset by either the pilot or the manufacturer (by naturally tilting the mast in the opposite direction). We can go into physics and aerodynamics but in the end what you need to understand is that the aircraft in a anticlockwise rotor system will naturally want to hang left skid low

Because of this you will see the ball offset slightly [to the left of centre]. It's a similar principle to what a builders construction level ruler would do when it's tilted to one side. This can be done by either having a heavier or a lighter ball

Therefore when you're in forward flight if you would put the ball in the middle you will be skids level. But remember what you learned above. If you are skids level when the aircraft wants to naturally hang with the left skid low, you're aerodynamically 'flying sideways' (off balance). This will be reflected by a simple string on the nose of the aircraft which will be offset also

Now I have never flown the apache but I have taught on many helicopters; and what I will say is that this effect, at least to me, seems to be extremely pronounced in the AH-64. In the same way that Vortex ring is simulated. In reality it can be very difficult to enter even on purpose

Having no experience inside the Apache, I am hesitant to call this out as unrealistic and over exaggerated until this is confirmed by a real Apache Pilot. But it's certainly a magnitude higher than what I was expecting 

 

The exaggeration of the effect seems to have been confirmed by some real world Apache jockey, if I am not mistaken. Wasn't there a link to the post and details earlier in this thread?

But the effect itself is there and pretty normal.

[Flamin_Squirrel]

24 minutes ago, Caldera said:

Thanks for all the replies!

 

In some respects, maybe because of Early Access, I still think this is not exactly right.  I get the way a helicopter flies for the most part, but to my mind by centering the Trim Ball it should fly straight to the FPV and not crab.  Otherwise, a skid / crab would always be indicated on the Trim Ball.  It is a catch 22.  Currently it is off by noticeable amount. 

And, maybe I am just a rock head...

Could it be the Trim Ball indication that is not quite correct?

To be sure of zero slip you really need an aerodynamic indicator, like a yaw vane or even a bit of string tied to the canopy. For the ball to work perfectly, there has to be symmetrical thrust / drag - which of course helicopters don't have. See example below: string centred in forward flight, ball is deflected left slightly. 

 

[Eagle7907]

The exaggeration of the effect seems to have been confirmed by some real world Apache jockey, if I am not mistaken. Wasn't there a link to the post and details earlier in this thread?
But the effect itself is there and pretty normal.

I don’t have a link, I just remember seeing this in another thread, where someone in that thread mentioned this. Hearsay, basically.

Who knows, I may have completely got it all wrong, but I remember something on the lines that the crabbing tendencies of the Apache are currently excessive. If it gets addressed or not, IDK.


Sent from my iPhone using Tapatalk Pro

[shagrat]

vor 9 Minuten schrieb Eagle7907:

I don’t have a link, I just remember seeing this in another thread, where someone in that thread mentioned this. Hearsay, basically.

Who knows, I may have completely got it all wrong, but I remember something on the lines that the crabbing tendencies of the Apache are currently excessive. If it gets addressed or not, IDK.


Sent from my iPhone using Tapatalk Pro

Found it linked above in this thread:

 

 

 

[Caldera]

Shagrat / Squirrel,

I believe what you write!  Interesting topic for me, and not really all that important, because it is what it just is.

 

What I think I know...

The RL Trim Ball indication uses a gyro system as a reference instrument? (Not a bubble or a gravity detection instrument)

A gyro (usually in two planes of rotation IE two gyros at 90 degrees) can detect any movement or directional vector of motion that is out of its plane of rotation.  This results in gyroscopic progression within the cage.  Gyroscopic progression is the result of relative change in position between the gyro (movable) and its cage (fixed).  This produces an error signal.  The error signal often is used to re-align the gyro within its cage (due to the mechanical restraints of the gymbal) and is used for other control functions and indications.   Because the cage and the aircraft are mechanically fixed to each other the cage is essentially the aircraft itself.  So the magnitude of the error signal is more or less proportional to the magnitude of the relative angle for the out of plane position of the aircraft.  

Gyroscopic instrumentation can be pretty precise?

Indications derived from the error signal could be an Artificial Horizon Indicator, a Vertical Speed Indicator or a Trim Ball Indicator (True Air Speed?).  If the aircraft is level, not changing altitude or not skidding then the gyro is not producing an error signal (discounting that the earth is round).  So, if the Trim Ball is centered then the aircraft is moving in the same plane as the gyro.  If the Trim Ball is not centered then the aircraft and the gyro are not in the same plane. 

All that said, it is not a perfect world, there would be inaccuracies and it might not be as simple as what I would think. 

 

I can only guess.  But the FPV is the result of a calculated value for all of the above and quite a bit more data.  But I would also guess, that the root of those calculations in based upon a reference provided by the gyro.

Caldera

Edited April 14, 2022 by Caldera

[shagrat]

vor 2 Stunden schrieb Caldera:

Shagrat / Squirrel,

I believe what you write!  Interesting topic for me, and not really all that important, because it is what it just is.

 

What I think I know...

The RL Trim Ball indication uses a gyro system as a reference instrument? (Not a bubble or a gravity detection instrument)

A gyro (usually in two planes of rotation IE two gyros at 90 degrees) can detect any movement or directional vector of motion that is out of its plane of rotation.  This results in gyroscopic progression within the cage.  Gyroscopic progression is the result of relative change in position between the gyro (movable) and its cage (fixed).  This produces an error signal.  The error signal often is used to re-align the gyro within its cage (due to the mechanical restraints of the gymbal) and is used for other control functions and indications.   Because the cage and the aircraft are mechanically fixed to each other the cage is essentially the aircraft itself.  So the magnitude of the error signal is more or less proportional to the magnitude of the relative angle for the out of plane position of the aircraft.  

Gyroscopic instrumentation can be pretty precise?

Indications derived from the error signal could be an Artificial Horizon Indicator, a Vertical Speed Indicator or a Trim Ball Indicator (True Air Speed?).  If the aircraft is level, not changing altitude or not skidding then the gyro is not producing an error signal (discounting that the earth is round).  So, if the Trim Ball is centered then the aircraft is moving in the same plane as the gyro.  If the Trim Ball is not centered then the aircraft and the gyro are not in the same plane. 

All that said, it is not a perfect world, there would be inaccuracies and it might not be as simple as what I would think. 

 

I can only guess.  But the FPV is the result of a calculated value for all of the above and quite a bit more data.  But I would also guess, that the root of those calculations in based upon a reference provided by the gyro.

Caldera

 

I guess there is no gyro involved in the digital slip ball.

Anyway, you can use a simple string or the standard "oil tube ball" in the Huey. The result is the same.

When a helicopter with counter-clockwise spinning blades flies forward, the cockpit dangling under the rotor points to the left from where you're actually flying the faster you go. That's how it is. 

Reason is the vector is a result of the different force vectors of the advancing and retreating blades, different lift produced in the disc, torque acting on the mast, tail rotor thrust and tilted rotor discs lift vector. Whereas a plane usually is either pulled by a propeller or pushed by a jet engine along its fuselage towards where the nose is pointing and with no wind the flight path and where that nose points coincide.

That's why the ball in a helicopter flying "in trim" (ball centered) is different from "nose/tail trimmed" (when the nose/tail points to where you are going). 

[Caldera]

Shagrat,

Thanks!

OK...

One thing that I do notice is that for firing rockets using the best aiming solution, something seems a little off.   I had thought that aligning the Rocket Steering Que, the HMD cross hairs and the Head Tracker would give the best results.  This is not what happens, at least most of the time that do it.

With the Rocket Steering Que and the HMD Cross Hairs in alignment the Head Tracker will generally be to the left a small amount.  The rockets will hit the ground on the Rocket Steering Que / HMD Cross Hairs line and not the Head Tracker line.  (I hope that makes sense)

Wouldn't the rockets would always shoot directly down the bore of the aircraft IE the down the line of the Head Tracker?

 

To be most accurate it seems that I need to be in a skid by just a small amount.

Not this:

Why would that be?

Caldera

[Raptor9]

@Caldera, what you are seeing in the manual is a graphical representation of symbology to illustrate that the Rocket Steering Cursor represents the allowable ballistic solution, but simply as an illustration of the Rocket Steering Cursor symbol, not taking into account all the various ballistics that affect rocket flight from a firing helicopter; further, in the graphic you posted you will notice that the aircraft is in a zero-airspeed hover, not forward flight.  The reason that the rockets don't fly straight toward the head tracker when in forward flight is because they will weathervane into the slipstream. If you are flying in a crab, the rockets will curve in the direction of travel, and this ballistic effect is calculated by the aircraft computers, which is why the Rocket Steering Cursor doesn't always align with the Head Tracker, even though the rocket pods are fixed in azimuth.

This is not an Apache-specific behavior.  The rocket aiming reticles in the Ka-50 and Mi-24 will also travel left and right to predict where the rockets will impact when the nose is crabbing left or right in forward flight.

Edited April 16, 2022 by Raptor9

[Caldera]

Raptor,

OK, Thank You!

I would have never thought about:

12 hours ago, Raptor9 said:

The reason that the rockets don't fly straight toward the head tracker when in forward flight is because they will weathervane into the slipstream. If you are flying in a crab, the rockets will curve in the direction of travel, and this ballistic effect is calculated by the aircraft computers, which is why the Rocket Steering Cursor doesn't always align with the Head Tracker, even though the rocket pods are fixed in azimuth.

I find it amazing that the FC computer can do this.  Yet, can not indicate by the Trim Ball when the aircraft is flying in a skid or true to the direction of flight indicated by the FPV.  There must be just too many variables in the mix due to the fact that the aircraft is a helicopter (not a dart) and inherently wants to always fly a bit sideways.

 

86 KNOTS

99 KNOTS

144 KNOTS

Speed has a major effect.

Caldera

Edited April 16, 2022 by Caldera

[Kharrn]

vor 6 Stunden schrieb Caldera:

Raptor,

OK, Thank You!

I would have never thought about:

I find it amazing that the FC computer can do this.  Yet, can not indicate by the Trim Ball when the aircraft is flying in a skid or true to the direction of flight indicated by the FPV.  There must be just too many variables in the mix due to the fact that the aircraft is a helicopter (not a dart) and inherently wants to always fly a bit sideways.

Speed has a major effect.

Caldera

 

AH64D Quick Start Manual,  Page 84

Skid/Slip Indicator (Trim Ball). Indicates the amount of side acceleration
and when the aircraft is in coordinated flight (“aerodynamic trim” or “in trim”).

Let Casmo explain it for me

So align FPV with HeadTracker (think of nose boresight) and you can see you are flying in 'a skid'. Hope it helps!?

 

K

Edited April 16, 2022 by Kharrn

[Swift.]

Maybe a little late to this, but the way I've had to reason it to make it make sense in my mind is that the ball isnt showing centred when the airflow is perfectly inline with the fuselage. But rather centred means it in the optimum aerodynamic condition, which for reasons described above, is slightly angled to the right. So whilst you could have a more streamlined fuselage position, it wouldnt necessarily be a better position for the aerodynamics of the airframe.