FBL system phenomena - take off and landing heli

[KarAlb]

Hello!
I have a question regarding Flybarless (FBL) helicopter behaviour.
Since 1-2 years I have been flying with FBL helis. Mostly with Align 450
and their FBL system. But also smaller mini Walkera helis. Recently I
have set up a NAZA-H on another 450 Heli.
I have noticed on all these systems, there seem to be a very critical
moment when take off, just before being airborne. Its the moment when you
as a pilot wonder which why the heli is going to take off at when the skids
leave the ground. You must be prepared. Now, with a flybar heli this is
very predictable regardless of any one particular direction the heli
takes. You just correct it nice and slowly and continue the lift off.
However, on FBL helis at this moment, not all times but sometimes, and
especially if you do the lift off extremely slow, just gradually increase
throttle, the FBL system get crazy and makes a LARGE and FAST correction
in any random direction for seemingly no reason at all. You crash on the
ground with rotors hitting the ground. This occurs also when landing, if just very slowly reduce throttle after touchdown of skids.

Have you experience of this or heard of this phenomena?
What is the cause?
Any idea on how to handle it?
Thanks a lot!

[Obijuan Kenobe]

The consensus as far as I am aware is that FBL systems are 'flying' immediately when powered on.

So if you lift off too slowly, the small corrections you make will result in insufficient response in the gyros...so the FBL will try to add servo movement to get the change in movement it expects to be getting...until it gets it.

The best answer is to keep it on the ground at slightly negative pitch...flying it at all times to keep the head level...and popping off the ground a bit more than you might with a FB?

Slow deliberate take offs require you to be flying when the helicopter is still on the ground so that the FBL doesn't try to do something it shouldn't. More or less. It's the small vibes and movements that occur when the skids start to come off the ground that the FBL is interpreting as flight movements it needs to correct. You need to beat the FBL to the punch on these, or just fly away from the ground faster.



obi

[gitbse]

It's the generally accepted rule now that FBL's don't like being on the ground. "Light on the skids" is something you want to avoid. Let the heli spool up with slight negative to hold it down, the get right up in the air. The ground makes the fbl not know what to do, because if you move the sticks it will sense the input, but not the corresponding roll rates because of the ground.

[kalleh]

With an FBL system, your input is ordering a rate of rotation around an axis. The control loop then controls the servos to achieve that rate of rotation. If the heli is on the ground, the heli can't rotate at all, so the control loop will try progressively harder to make the heli rotate. At lift-off/light skids, this can mean a very sudden response once the skids gets off the ground since the FBL system is trying to catch up.

This is of course a simplification, and I'm sure that the software in the controllers have some dirty tricks to in order to try and mitigate this as much as possible.

[ArchmageAU]

Yes FBL units do not like to be on the ground. The head gyro is like a heading hold tail gyro but with decay built in.

Heading hold tries to rigidly keep a direction (in the case of a rotor head a tilt or aileron and elevator) as the heli body attains the requested attitude the swash returns to level. However, this is not the whole story.

The feed forward sets the initial rate of the alteration, then the decay starts to eat it back. Decay is like the drift of a rate mode gyro (just a little slower). In short the FBL unit tries to act like a super accurate flybar, stabilizing you as quick as possible into an attitude, but with some give so if too much resistance it can normalize (like a regular physical flybar).

What does this all mean. for takeoff and landing:

If the heli starts to tip through blade lift, you can (and should) correct, but appropriately. A slight bit of negative collective on spoolup helps prevent lifting on one skid or another and you can see the rotor disk tilt to correct on ground (and prevent the control feedback loop that will cause a blade strike).

Once the rotor has takeoff speed, ideal to get both skids off the ground (no need to rocket it up), but trying to balance on one skid is generally not a good idea (unless you know what you are doing).

Similarly with landing. Negative collective once skid on ground is you friend. I ease to negative to slow rotor blades too (as long as power is off) as once you are on ground you normally want to stop and make heli safe. I usually mini-auto (TH at about 3 feet or less) and gently touch down with no power. This way a lot of blade speed is already gone when I touch down.

[KarAlb]

Wow Thanks a lot for this fast feed back!
Now I see others for sure experienced this, its not just me maltreating my helis.
Think also I understand the cause of it and how to handle it, at least theoretically.
Its just a bit scary and counter intuitive when flying a new heli for first time to 'pop it up' airborne and be confident the FBL system will work.
What you are saying makes sense though - thanks.

[fozz]

I think it really depends on the unit and set up. Even a flybared heli won't take off straight up on its own due to the tail rotor. On a good flybarless set up, there is no reason to pop it up in the air. It is best to limit cyclic inputs during spool up and spool down, however, because there might not be enough energy in the blades to correct. This can lead to rollover.

[FailureToFly]

I regularly take off my FBL 700 without spooling it up that fast; I also land with little HS. Never ever had it want to tip, shoot up, do weird things.

The reason they do odd things is people make odd stick movements when the FBL unit cannot hope to correct correctly (like stated above, the unit will just try harder to perform something it can't do as friction is stopping it).

Chris at my club last Winter used to 'fly' his Goblin along the wet ground at 10 km/h all the time with no scary moments.

[aterren]

I guess I'm the odd one out.

When I take off I always strive to take off vertically (no sliding to the left). To do this I take off slowly and certainly the heli gets light on the skids and often is only resting on the left skid right be fore liftoff. I've never had any problems.

[ArchmageAU]

Originally Posted by FailureToFly

...
Chris at my club last Winter used to 'fly' his Goblin along the wet ground at 10 km/h all the time with no scary moments.

Well none that you saw..

Chris is an advanced pilot and ground feedback to the FBL unit would not be a problem for him.

I too don't pop either my X5 or my 700E off the ground, but when you know what you are looking at it's easy to see if things need correction before takeoff.

[blade450]

If you know what to expect from the FBL system, you can easily steer on the ground.

I personally watch what the disc is doing. If it isn't flat, I give some input until it is.
If you take off with a flat disc, the helicopter will go straight up. If the disc is tilted to one way, it's much more likely that it is gonna tip over.

The only thing is you have to be a bit careful with your inputs, if you overdo it, you might make it roll over.

[aterren]

Originally Posted by blade450

I personally watch what the disc is doing.

Yes, that is key. The disk should give you a good warning.

It is a minor point but the heli must lean ever so slightly to the right to counteract tail thrust if you are to achieve perfectly vertical (plumb) takeoff. That is always my goal.

[helivader]

Would some FBL gyro behave better than others in this issue ?
I have a 3GX on TREX 600N and trashed servo gear sets twice in the past because of this FBL "feature". Now I always lift off quickly and when landing, always do a mini auto.

[blade450]

Originally Posted by helivader

Would some FBL gyro behave better than others in this issue ?
I have a 3GX on TREX 600N and trashed servo gear sets twice in the past because of this FBL "feature". Now I always lift off quickly and when landing, always do a mini auto.

I know I can steer on the ground safely with the BeastX and the V-bar if I'm careful. Never flew 3gx, so can't comment on that.

[KarAlb]

Originally Posted by helivader

Would some FBL gyro behave better than others in this issue ?
I have a 3GX on TREX 600N and trashed servo gear sets twice in the past because of this FBL "feature". Now I always lift off quickly and when landing, always do a mini auto.

Currently I have two FBL 450 helis, one using Align 3GX the other, just recently Naza-H. Both are at the latest firm ware versions. I think the Naza is more smooth on the take off and don't give me surprises. Still nowadays I always lift off using a negative pitch with the curve turning a bit steep toward hover angle and then I just increase throttle with a certain amount of determination. No mini auto rotation at landing yet, just reducing throttle without delays but no particular hurry. Works for both products. Like someone pointed out here, If you know what to look out for its easier to avoid.

[jmtyndall]

Originally Posted by aterren

I guess I'm the odd one out.

When I take off I always strive to take off vertically (no sliding to the left). To do this I take off slowly and certainly the heli gets light on the skids and often is only resting on the left skid right be fore liftoff. I've never had any problems.

My "nice vertical takeoff" is usually either a) lift off into a piro before the soft start is over, or b) spool to full headspeed with a bit of negative pitch and pop up into a flip lol

[RFCXPerT]

If FBL is not easy to deal with when being lightly on ground, how can you track blades safely?

[peteski]

I dunno, my BeastX helis behave themselves on the ground with no cyclic inputs. In fact as a rule I never touch the cyclic stick until the heli is off the ground and the disk always stays perfectly flat while spooling up. I've never had to make a cyclic correction on the ground - to me that would imply something was wrong, such as excessive vibration or a non-level swash. Same on landing, once on the ground I never touch the cyclic again. Never had a tip-over yet using this method.

[Relisys190]

Spool the helicopter up and pop it in the air quickly. I'm usually up before the disk is up to speed on all of my helicopters. I don't give any cyclic inputs ever.

-Markus

[Relisys190]

Originally Posted by RFCXPerT

If FBL is not easy to deal with when being lightly on ground, how can you track blades safely?

At eye level, with tape. About 15 feet out

-Markus