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Old Nov 28, 2012, 12:00 AM
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Question
6 ch 3 axis FBL flight movement on neutral stick

Why does a 6 ch FBL continue to move when the cyclic is neutral and the swash is level?

An example to clarify what I'm asking.

Example:

You give a touch of forward cyclic. The swash tilts forwards, the heli moves forwards and (maintains altitude?).

You let the control stick come back to neutral/center stick position. The swash levels out (or does it?), but the heli continues to move forward, till you give a touch of reverse cyclic to (bring it back to) a stable hover?

Please note this is a very theoretical question since I crashed my Wasp X3V within a few (albeit exciting) seconds of its only flight!
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Old Nov 28, 2012, 12:18 AM
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It's 6 channel with 3 axis gyro, you need a 6 axis gyro to get the heli to stop when you let go.

A 3 axis gyro does not know up from down. All it can track is the helis rotation and makes sure that the heli stays in that position no matter what.

The swash only need to tilt when the heli needs to rotate, once the target angle is achieved, the swash levels out. Leaving the swashplate angled just enough to counteract the helicopters weight. It doesn't even know it's fighting gravity, all the gyro knows is that something is pushing the heli out of the desired angle and it's continiously adding micro adjustments to the servos to bring it back to alignment.

What were you flying before this?


PS: No, the heli does not maintain altitude. Think of it this way. During a hover 100% of your helis thrust is pointed downwards, if you angle the heli forward that thrust gets divided and you end up with something like 80% down, 20% rear. The heli will go forward but with only 80% of the thrust to lift it up, it'll eventually hit the ground. So you end up increasing the throttle to bring the thrust up to 120% to bring the downforce back to 100% while maintaining the 20% to keep the heli going forward. Of course, once you go back into a hover, you'll end up with 120% downforce and the heli will hit the ceiling.
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Last edited by Hajile; Nov 28, 2012 at 12:57 AM.
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Old Nov 28, 2012, 01:59 AM
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What happened to translational lift?

From helicopterflight.net


"Translational lift occurs with any wind or movement of the helicopter in any direction. Translational lift also improves the aerodynamic efficiency of the rotor progressively and continuously as speed increases. While at slow speeds near the ground this can be demonstrated as less power is necessary to accomplish a given altitude over the ground.

When translational lift improves to the point of Effective Translational Lift (ETL) significantly less power is required to maintain a given altitude. Many pilots incorrectly think this is maximum translational lift. The speed at which ETL occurs varies from aircraft to aircraft but usually occurs at about 12-15 knots. It is important to understand that the rotor does not know direction of movement therefore translational lift will result regardless of the direction of flight or wind. The fuselage and/or airframe does on the other hand, care about the direction of flight or wind since it is designed to fly in one direction.

Translational lift continues to improve with speed infinitely however the negative effects of drag overcome any noticeable increase in performance beyond about 45-knots.

The only sensation the pilot will notice relative to translational lift is the tendency of the helicopter to climb as ETL is reached and conversely the tendency of the helicopter to sink as ETL is slowed through on the approach. Translational lift does not cause any vibrations however it is often confused with the vibration caused by Transverse Flow Effect which occurs at an airspeed slightly below ETL.
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Old Nov 28, 2012, 03:36 AM
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I actually understand what Hajile and karlik are trying to explain to me. I also understand that a 3 axis gyro will stabilize yaw, pitch and roll movements just like a flybar. What I don't understand is why the heli will continue forward motion once control stick is neutral following a forward cyclic command?
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Old Nov 28, 2012, 04:14 AM
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Easiest way to demonstrate is to hold the heli firmly in your hand and spin it up. You may notice that no matter which way you tilt it, the swashplate will always stay level.

Now while holding the heli level, push the cyclic forward. The swash is now permanently tilted forward until you tilt the body forward and the swashplate becomes level relative to the body again. From then on, any other way you move the heli, the swash will only level if the heli is in that exact angle. If you try to level the heli in your hand, the swash will go back to tilting forward and force the heli to tilt forward as well.

If you want an even more detailed explanation, i suggest you try learning to fly a heli in WarRock or Battlefield and seeing what the heli is doing from a cockpit view.
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Old Nov 28, 2012, 04:55 AM
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Another way to look at it - the swash-plate "follows" the stick. When you push the stick forward - the pitch of the nose (attitude) drops down. Neutralizing the stick will level the swash-plate but NOT the helicopter - the helicopter will maintain that same attitude (nose down) until you pull the stick back. In other words a level swash-plate is NOT related to level flight unless you are already in level flight. How much stick movement is required to maintain a stable hover? A quick read of Don't Touch the Controls should help you answer that. Have a great training!

captJac
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Old Nov 28, 2012, 10:19 AM
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Quote:
Originally Posted by CaptJac View Post
Neutralizing the stick will level the swash-plate but NOT the helicopter - the helicopter will maintain that same attitude (nose down) until you pull the stick back.

captJac
.........I think I'm coming close to a Eureka! moment.........

Will the swash-plate be level to the horizontal plane of the tilted forwards heli or the true horizontal (x-axis)?

I appreciate all your help!
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Old Nov 28, 2012, 10:49 AM
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Quote:
Originally Posted by Lentar View Post
Will the swash-plate be level to the horizontal plane of the tilted forwards heli or the true horizontal (x-axis)
There is no reference to the true horizon other than your eyeball. The swash-plate level is referenced to the main shaft of the heli. It is perpendicular (90) to the main shaft when it is level.



EUREKA!!! - you donna smella so gooda yourselfa ..
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Old Nov 28, 2012, 11:38 AM
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Quote:
Originally Posted by Lentar View Post
I actually understand what Hajile and karlik are trying to explain to me. I also understand that a 3 axis gyro will stabilize yaw, pitch and roll movements just like a flybar. What I don't understand is why the heli will continue forward motion once control stick is neutral following a forward cyclic command?
Because you're watching it when it's not in flight? The gyro doesn't understand what is happening when it tries to move the helicopter and it doesn't move, so the control inputs appear to have odd results when the helicopter is not in the air.
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Old Nov 28, 2012, 12:04 PM
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Quote:
Originally Posted by CaptJac View Post
There is no reference to the true horizon other than your eyeball. The swash-plate level is referenced to the main shaft of the heli. It is perpendicular (90) to the main shaft when it is level.
So, heli flying level in a hover, you give forward cyclic, the swash tilts forward, heli pitches down and moves forward.

Stick neutral, swash levels (parallel to heli horizontal axis, perpendicular to mainshaft), heli continues to move forwards and maintains downward pitch.

You give reverse cyclic, swash tilts back, nose pitches up and forward movement slows. When the heli is in a horizontal attitude (level to the ground), you move control stick in neutral position, swash is level, and since heli is level as well, you get back into a hover......

EUREKA! EUREKA!

PS

1. If you give forward cyclic, nose pitches down, heli moves forward, will you track into the ground?

2. If you give forward cyclic, nose pitches down, heli moves forward, you then move stick to neutral position, heli continues to move forwards, nose pitched down, will you track into the ground? Or just continue moving forward with the nose pointing down?
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Old Nov 28, 2012, 12:12 PM
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Quote:
Originally Posted by Lentar View Post
So, heli flying level in a hover, you give forward cyclic, the swash tilts forward, heli pitches down and moves forward.
Yes...

Quote:
Stick neutral, swash levels (parallel to heli horizontal axis, perpendicular to mainshaft), heli continues to move forwards and maintains downward pitch.
Not really... this is a fly-by-wire system, so the swash plate might not move at all, or it might. The important thing is this - the swash moves wherever it needs to be to maintain orientation in all three axes, whenever the pilot centers the cyclic stick. So, the swash might level or it might not - you don't care! All you care about is the thing responds to your commands.

Quote:

1. If you give forward cyclic, nose pitches down, heli moves forward, will you track into the ground?
No, because you also apply additional collective to maintain lift. A helicopter works by combining force vectors so that the resulting overall force is in the direction you want. In level forward flight, the rotor disc is producing a force vector which is upward and to the front - but when you add the force of gravity, it cancels the upward portion of that force vector and you are left with only the forward force remaining. Therefore you move forward and maintain altitude.

I think you're trying to understand this too deeply - first you need to learn how to fly, from a non-physics perspective. You need to learn the "my control stick input makes the helicopter do this" and stop thinking (for now) about how the helicopter accomplishes that. Helicopter physics is not for the faint of heart!
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