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Trim with slip indicator always nose left?


MstrCmdr

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25.02.2022 в 01:40, Dannyvandelft сказал:

Only one confusing here is you bud. The rest of us are right. Slip or crabbing is adjusted by yaw. Not roll. Doesnt matter if it's straight flight, or on a turn. If you're heading 180 and your nose is pointing at 170, you need to adjust yaw to get the nose pointed on the right heading. Start flying warbirds, and you'll learn this real quick.

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If you side slip to your left ie your flight vector is directed left relatively to your nose, then you can compensate for it by including a slight right roll. This aligns flight vector with your nose but now you are in a turn. Which you can nullify using slight left rudder(pedal). 

Now your vector is aligned with your nose in a straight flight only problem being that you now have constant roll angle and your cockpit is rolled. 

Which is compensated in a Hind by oppositly tilting cockpit itself and rotor shaft in relation to fuselage


Edited by Sobakopes
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The helicopter will crab in a zero wind condition. The high mounted tail rotor causes a rolling moment that has to be compensated for with lateral cyclic or transmission tilt. The Apache also has a non-symmetrical profile with the EFABs. Additionally you have the vertical fin which is also going to impart a rolling moment on the fuselage as well. The sum of all of these forces cause the helicopter to fly with a slight yaw offset. The big difference here is that the Apache has the flight path vector which illustrates this phenomenon. Winds will increase or decrease this yaw offset depending on the strength and direction of the wind. Bottom line, it’s normal.

 

I wanted to add that the Apache only has forward transmission tilt and not sideward. We get to compensate with cyclic.


Edited by bradmick
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  • 2 weeks later...

.


Edited by Tiger-II

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You're making the mistake of zero-slip relative to the wind, and GROUND TRACK!

If the ball is centered, the aircraft is flying straight. Whether it is flying straight AHEAD relative to the ground is an entirely different matter.

Assume you have a huge 200 kt wind from your left (270 deg.). If you are flying at 100 kts perfectly straight (centered ball) with the nose pointing at 360 deg., then your ground track will be off towards 060 deg. or so.

Clearer?

If you're hovering in zero wind conditions, it is REQUIRED that you roll slightly to one side (usually towards the side the tail rotor is mounted on). This is to offset the thrust of the tail rotor that will otherwise try to accelerate the aircraft sideways. You roll very slightly to counteract this behavior. Given that the aircraft is in unaccelerated flight, the ball will actually be off to one side slightly due to gravity. It will NOT be centered in this condition.


Edited by Tiger-II
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Motorola 68000 | 1 Mb | Debug port

"When performing a forced landing, fly the aircraft as far into the crash as possible." - Bob Hoover.

The JF-17 is not better than the F-16; it's different. It's how you fly that counts.

"An average aircraft with a skilled pilot, will out-perform the superior aircraft with an average pilot."

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You're not required to roll. This is the difference between aerodynamic trim and nose to tail trim. In fixed wing, or a Kamov, they're the same thing (unless flying a twin with one dead engine, then they become different), but in a conventional helo, you need to choose whether you trim for the ball in the center, or for the FPM in the center. Both have their uses in different situations, the ball in the center is more economical to fly, but nose to tail is useful, for example, for landing, where you need to align with your ground track.

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3 часа назад, Dragon1-1 сказал:

You're not required to roll. This is the difference between aerodynamic trim and nose to tail trim. In fixed wing, or a Kamov, they're the same thing (unless flying a twin with one dead engine, then they become different), but in a conventional helo, you need to choose whether you trim for the ball in the center, or for the FPM in the center. Both have their uses in different situations, the ball in the center is more economical to fly, but nose to tail is useful, for example, for landing, where you need to align with your ground track.

Nose to tail is paramount to eliminate lateral miss with rockets. 

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1 hour ago, Sobakopes said:

Nose to tail is paramount to eliminate lateral miss with rockets. 

In forward flight or when diving during rocket attacks, aerodynamic trim is used for improved rocket precision, otherwise you induce additional weathervaning of the rockets.

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Afterburners are for wussies...hang around the battlefield and dodge tracers like a man.
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On 2/23/2022 at 11:08 PM, MstrCmdr said:

You also have more lift in the front 180 degrees of the rotor when airspeed is above ETL which translates to left side lift due to the gyro precession manifesting at 90 degrees later.

This might actually be it. The whole discussion totally missed the point so far. BTW, to further fuel the fire with some oil, I observed on Wags' videos (Getting Airborne and Landing especially), that the ball immediately slips to the left as soon as he lifts off, while still in stationary hover. How come when the whole thing isn't even moving or just neglectably barely moving at all? I take it's not wind causing this, it happens regardless of the direction he's taken and it it was wind blowing over the airframe to cause this, it would also do it when it's still sitting on the ground as opposed to being in a stationary hover. I stand my point, I've got the feeling something's wrong here and noone has yet been able to explain it.

dcsdashie-hb-ed.jpg

 

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2 hours ago, Eldur said:

This might actually be it. The whole discussion totally missed the point so far. BTW, to further fuel the fire with some oil, I observed on Wags' videos (Getting Airborne and Landing especially), that the ball immediately slips to the left as soon as he lifts off, while still in stationary hover. How come when the whole thing isn't even moving or just neglectably barely moving at all? I take it's not wind causing this, it happens regardless of the direction he's taken and it it was wind blowing over the airframe to cause this, it would also do it when it's still sitting on the ground as opposed to being in a stationary hover. I stand my point, I've got the feeling something's wrong here and noone has yet been able to explain it.

So. When you lift off the tail rotor and torque effect make the helicopter want to slide or drift, in this case would be to the right.  To counteract the "translating tendency" which is what it's called you will displace the cyclic to the left which means the helicopter is literally rolling to the left a little bit. The trim ball because of this will be to the left.  For clockwise rotating rotor this effect is opposite. 

The gyroscopic precession applies to directional control and is corrected through rigging, immediately going through ETL you do have an immediate effect but afterword it balances out.  It doesn't correlate to in or out of trim conditions.  It's just something that happens and it's been accounted for 95%ish. 

The difference of lift in forward flight or dissymmetry of lift between advancing and retreating blades is compensated for differently in different helicopters, but blade flapping and cyclic feathering are common. 

In forward flight if the aircraft is aerodynamically trimmed in relation to wind and the tail offset the aircraft in roll is level even though it's crabbing.  Imagine a boat crossing a river, if it wants to go straight across the river it will crab upstream.  Now imagine that the river is wind and the boat is the helicopter or aircraft. This is more efficient and has an effect in RL for the rockets. 

Nose to tail trim as it's been referenced is where the aircraft flies straight.  If no correction for the wind just like a boat pointing straight to where he wants to go the aircraft will drift downwind or down stream.  So a "slip" is put in same thing as at a hover you're rolling the helicopter to maintain that track.  

I've bolded some of the actual terms so you can google them if you'd like and read up on them if you don't fully understand so you can have some pictures.  I believe fixed wing use the term "slip" but I can't guarantee anything will come up with it so it remains unbolded. 


Edited by kgillers3
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@kgillers3 Have you flown the Hind (in DCS)?  I observed the swash acting without gyro precession.  If you push cyclic in tilts forward...left cyclic tilts it left.  Try after the engines are off, rotors are still, and there is still some hydro left in the hoses.  You get a few movements before it bleeds off but I don't see it operating 90 degrees before the anticipated movement (forward cyclic should tilt the swash left which in a CW rotor means the nose dips...right?)

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2 hours ago, MstrCmdr said:

@kgillers3 Have you flown the Hind (in DCS)?  I observed the swash acting without gyro precession.  If you push cyclic in tilts forward...left cyclic tilts it left.  Try after the engines are off, rotors are still, and there is still some hydro left in the hoses.  You get a few movements before it bleeds off but I don't see it operating 90 degrees before the anticipated movement (forward cyclic should tilt the swash left which in a CW rotor means the nose dips...right?)

So don't get it twisted, this is minutia (heavy emphasis on trivial details of something) unless you're the one designing a helicopter, I've debated on answering this for a bit out of fear for the hind developers of people screaming realism over something this trivial. 

If it's a wholesome desire for just knowledge and that's neat to know. then here's a youtube video that breaks it down better I can in words.

 

 

 


Edited by kgillers3
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6 hours ago, MstrCmdr said:

@kgillers3 Have you flown the Hind (in DCS)?  I observed the swash acting without gyro precession.  If you push cyclic in tilts forward...left cyclic tilts it left.  Try after the engines are off, rotors are still, and there is still some hydro left in the hoses.  You get a few movements before it bleeds off but I don't see it operating 90 degrees before the anticipated movement (forward cyclic should tilt the swash left which in a CW rotor means the nose dips...right?)

While gyroscopic precessing is something to be considered, it's not the dominant force at play, the phase lag is highly dependent on many other factors. Which you'll have to ask someone with a lot more knowledge than me about unfortunately. It's rarely exactly 90 degrees though.

You can see on the in game Hind model that the swashplate acts on the blade roughly 35 degrees ahead due to the pitch horn being offset forward in the rotation. On the Ka50 it looks to be around 50-60 degrees. The Apache appears to be around 40-45 degrees.

TL;DR Rotors are magic.

u4dba08.png


Edited by Vladinsky
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14 hours ago, Vladinsky said:

While gyroscopic precessing is something to be considered, it's not the dominant force at play, the phase lag is highly dependent on many other factors. Which you'll have to ask someone with a lot more knowledge than me about unfortunately. It's rarely exactly 90 degrees though.

You can see on the in game Hind model that the swashplate acts on the blade roughly 35 degrees ahead due to the pitch horn being offset forward in the rotation. On the Ka50 it looks to be around 50-60 degrees. The Apache appears to be around 40-45 degrees.

TL;DR Rotors are magic.

u4dba08.png

 

 

17 hours ago, kgillers3 said:

So don't get it twisted, this is minutia (heavy emphasis on trivial details of something) unless you're the one designing a helicopter, I've debated on answering this for a bit out of fear for the hind developers of people screaming realism over something this trivial. 

If it's a wholesome desire for just knowledge and that's neat to know. then here's a youtube video that breaks it down better I can in words.

 

 

 

 

Ahh ok this is where I'm getting confused.  @kgillers3 I actually watched that awhile back and this is what confused me (im easily confused).  Phase lag is not synonymous with Gyro Precession.  I did some more research and found varying phase lags for different models of helicopters.  While Gyro Precession is always 90 degrees it is not the only factor in determining phase lag.  There's countless other factors that affect it even including the lead and lag of the blades themselves.  This was a lot of fun to learn about.  Im solid on GP but phase lag is highly variable and not one size fits all.  I also learned that not all swashplates account for phase lag.  Some do and others have the pitch horns built in for phase lag (assuming these dudes on AOPA know what they're talking about).  I'm gonna throw in the towel on determining the correct pitch angle for any modelled helo in DCS since I do not have enough manufacturer data to show which direction the swash actually tilts based on cyclic input.  I know it exists and I know the helos are designed for it.  Case closed.

And I just now realized I hijacked my own thread.....

Edit: I appreciate all the help and knowledge from you guys.


Edited by MstrCmdr
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On 2/21/2022 at 12:53 PM, MstrCmdr said:

Maybe someone with more helo experience than I can help answer the idea of a yaw-trimmed helo.

In the MI8 and the MI24 I prefer to trim yaw to zero the drift (i have no idea if this is accurate to RL).  This means I'll inevitably have an unlevel helo as it will slightly dip in the direction the wind is coming from.

In the apache rockets video wags put out the centered ball still results in some weather cocking/crabbing.  Is this normal procedure in all helos or just the apache or what?

Asking for a friend......

Alot going on in this thread.  I don't intend to muddy the waters, but they are already pretty muddy.  

So first I'd ask would be what type of rotor system are we dealing with? 

Obviously the AH-64 is a fully articulated rotor.  This makes a difference because what's being alluded to here, that the helo will hang low on one side due to the tail rotor, yes that's true for a semi-rigid system.  It's not true for a fully articulated rotor.  

"The semi-rigid rotor design eliminates many of the bearing components of
the conventional articulated rotor , giving reduced complexity and hence a
reduction in maintenance requirements. In this application the semi-rigid
rotor also gives a substantial increase in the moments transmitted to the rotor
hub by tilt of the rotor. "https://www.rotaryforum.com/threads/semi-rigid-or-fully-articulated.45784/

So in essence, on a fully articulated system the rotor tilts independently of the fuselage.  There may be a small residual tilt, but with a fully articulated rotor system, you shouldn't be hanging significantly fuselage low on one side at a hover, or in forward flight.  

You can see the AH-64 in this clip hangs vertically while the rotor tips slightly left to compensate for translating forces:
https://www.pond5.com/stock-footage/item/137100910-ah-64-apache-helicopter-hovering-over-ground



It's important when having these debates to consider the difference between transient states and equilibrium states.  Yes the tail rotor produces a force pushing the bird, but the pilot compensating with lateral cyclic restores the equilibrium.  With no sideward drift at a hover the ball should be centered, fuselage level or very nearly so.  In cruise flight, the ball should be centered and the fuselage will be level.  Any wind correction is done via heading and crab angle.  

Regarding the original question, crab angle is the method used in forward flight to compensate for wind, i.e. varying the heading while maintaining aircraft in trim.  On a normal VMC takeoff, a typical procedure would be to maintain runway alignment until 50 feet (aircraft will be out of trim if there is any crosswind component), then place the aircraft in trim, and add necessary crab angle to maintain ground track.  Doing the reverse on approach - aircraft in trim until descending below 50 feet, then placing aircraft aligned with the landing direction.   

In a fully articulated rotor system, aircraft in trim, the fuselage should remain level or very near so, and the ball should be centered, regardless of the explanations mentioned, because we are talking equilibrium states. 


Edited by cw4ogden
Typo error - mixed takeoff procedures with approach. Added clarification.
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And what about this video please ?

Look after 08:00, the ball is centered, yet the velocity vector is near the head tracker.
At these speed in DCS my velocity vector is heavily on the right side !

Is the something new on AH-64E or is the video feed misleading ?

 

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38 minutes ago, jojo said:

And what about this video please ?

Look after 08:00, the ball is centered, yet the velocity vector is near the head tracker.
At these speed in DCS my velocity vector is heavily on the right side !

Is the something new on AH-64E or is the video feed misleading ?

In real life you also have winds...

Afterburners are for wussies...hang around the battlefield and dodge tracers like a man.
DCS Rotor-Head

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Hi Guys,

anyone can elaborate on the subject question? For me eighter the ball is centered (and then the FPM is way off to right, actually crabbing I guess?), or the FPM is centered, and then the ball is way off to left (sliding left).

Thanks a lot !

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Flying on cruise - I have problem with perfect trim. Flying steady hands off, but the final touch is needed.

Typical (no wind)

Ball is 1/2 off left.

Nav point OK - straight ahead.

Steer cue slight right.

DP (I call it Point of Impact) is even farther right of heading.

How to manipulate roll and rudder at this situation for flying straight head to nav point with no crab angle?

Thanks

 

 

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The yaw offset (difference between the FPV and headtracker (nose of the helicopter)) is always there, even in a no wind situation. The yaw offset is currently approximately double what it should be for a given airspeed. At 40 knots the VV and FPV should be centered in the head tracker with a centered trim ball, at 70knots the the fligh path vector circle will enclose the right chevron of the head tracker with the trim ball centered, at 90 knots the tip of the 'left wing' of the flight path vector will be touching the right tip of the head tracker. This equates to roughly 0 degrees yaw offset at 40 knots, 5 degrees yaw offset at 70 knots and 6 or so degrees at 90 knots. At 150 knots the yaw offset is around 10 degrees. 

 

All of the above is in a no wind scenario. So the aircraft can absolutely be 'in trim' (aerodynamically, ball centered), but still have a small degree of sideslip. This isn't unique to the apaches, all helicopters fly with some degree of yaw offset. The difference is, you have symbology now to show you that it's happening, where as before it was a subconscious thing in the background you just accounted for.

 

So, bottom line: You will always have a yaw offset even in zero winds.


Edited by bradmick
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13 minutes ago, bradmick said:

The yaw offset (difference between the FPV and headtracker (nose of the helicopter)) is always there, even in a no wind situation. The yaw offset is currently approximately double what it should be for a given airspeed. At 40 knots the VV and FPV should be centered in the head tracker with a centered trim ball, at 70knots the the fligh path vector circle will enclose the right chevron of the head tracker with the trim ball centered, at 90 knots the tip of the 'left wing' of the flight path vector will be touching the right tip of the head tracker. This equates to roughly 0 degrees yaw offset at 40 knots, 5 degrees yaw offset at 70 knots and 6 or so degrees at 90 knots. At 150 knots the yaw offset is around 10 degrees. 

 

All of the above is in a no wind scenario. So the aircraft can absolutely be 'in trim' (aerodynamically, ball centered), but still have a small degree of sideslip. This isn't unique to the apaches, all helicopters fly with some degree of yaw offset. The difference is, you have symbology now to show you that it's happening, where as before it was a subconscious thing in the background you just accounted for.

 

So, bottom line: You will always have a yaw offset even in zero winds.

 

Thanks for the info. Hopefully they adjust this soon.

 

 

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Here's a quick diagram of the major forces and things going on with the aircraft. You have the main rotor torque effect which is counteracted by the tail rotor, not only that but we have a high set tail rotor too. You then have vertical fin lift which is slowly offloading the tail rotor. Both of these combined will work to cause a rolling moment in the aircraft which has to be counteracted by applying left cyclic. The left efab is also more streamlined than the right efab (so the pilots can more easily get into the crewstations). This causes a differential in lift between the two sides of the helicopter. There are gurney flaps on the stabilator that help to offset this a bit. Translating tendency (tendency of the helicopter to drift in the direction of tail rotor thrust, or to the right for the AH-64D) doesn't go away in forward flight. You're always going to be compensating for it. Not only that, but the main rotor torque also mixes into the equation also. 

 

Bottom line: It's complicated. And all of the forces diagramed out combine to cause the aircraft to fly with a slight yaw offset, even in no winds.

 

image.png

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1 hour ago, bradmick said:

Here's a quick diagram of the major forces and things going on with the aircraft. You have the main rotor torque effect which is counteracted by the tail rotor, not only that but we have a high set tail rotor too. You then have vertical fin lift which is slowly offloading the tail rotor. Both of these combined will work to cause a rolling moment in the aircraft which has to be counteracted by applying left cyclic. The left efab is also more streamlined than the right efab (so the pilots can more easily get into the crewstations). This causes a differential in lift between the two sides of the helicopter. There are gurney flaps on the stabilator that help to offset this a bit. Translating tendency (tendency of the helicopter to drift in the direction of tail rotor thrust, or to the right for the AH-64D) doesn't go away in forward flight. You're always going to be compensating for it. Not only that, but the main rotor torque also mixes into the equation also. 

 

Bottom line: It's complicated. And all of the forces diagramed out combine to cause the aircraft to fly with a slight yaw offset, even in no winds.

 

image.png

Your painting skills are on par with the greats like like Leonardo da Vinci, Picasso  and Michelangelo.🤪


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3 minutes ago, RaisedByWolves said:

Your painting skills are on par with the greats like like Leonardo da Vinci, Picasso  and Michelangelo.🤪

 

So you’re saying I should go repaint the Sistine chapel, got it!


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