Ground Effect - FBL vs FB

[Roger Fowler]

Here's a question - why can I hover my FB 450 right to the ground and land, but when I try the same thing with my FBL 450 it starts to drift off at about 2' above the ground. I just can't land without drift in some direction. Not a big deal - I just do an auto from 3' or so. But it does seem peculiar that the FB machine does not show any reaction to ground effect while my FBL machine does. I've been watching this for over a year and curiosity is finally getting the best of me.

Thanks in advance for any thoughts.

Roger

[jhamel]

I would be interested in seeing a video of this phenomenon.

[Felix the cat]

Originally Posted by Roger Fowler

Here's a question - why can I hover my FB 450 right to the ground and land, but when I try the same thing with my FBL 450 it starts to drift off at about 2' above the ground. I just can't land without drift in some direction. Not a big deal - I just do an auto from 3' or so. But it does seem peculiar that the FB machine does not show any reaction to ground effect while my FBL machine does. I've been watching this for over a year and curiosity is finally getting the best of me.

Thanks in advance for any thoughts.

Roger

FBL gyros auto-correct while flybars (obviously) do not - other than the flybar's effect on disc changes. FB's do not apply direct input to the servos.

When you're close to the ground, the rotorwash bounces against the disc somewhat affecting your pitch and/or roll ever so slightly, so the gyro tries to compensate and thus the "drifting / instability".

On FB there's only one "brain" working the inputs and responses. On FBL there's two.

[svensson_bo_l]

Originally Posted by Roger Fowler

Here's a question - why can I hover my FB 450 right to the ground and land, but when I try the same thing with my FBL 450 it starts to drift off at about 2' above the ground. I just can't land without drift in some direction. Not a big deal - I just do an auto from 3' or so. But it does seem peculiar that the FB machine does not show any reaction to ground effect while my FBL machine does. I've been watching this for over a year and curiosity is finally getting the best of me.

Thanks in advance for any thoughts.

Roger

Right now I feel like I'm just making it harder to understand the cause of what you describe. Hopefully we will find the answer, eventually.

I had a FB 450 that was like your FBL 450.
I have several FBL 450's. Some of them are like your FBL 450, and some are not.
Almost all of my 450's have the same kind of gyro (Vbar), the same kind of servos and, close to, the same settings.

It might relate to slop. Some slop adds up to be quite noticeble in some of my 450's. I have not connected sensitivity to ground effect to slop. I have noticed poor precision in connection between pitch stick position and vertical velocity in small vertical movements. That makes sense, IMO, since the gyro not senses that. It senses difference between swash servos and correct for that. However, not e.g. a slop in all swash servos or any slop that effects "pitch movements" but not "AIL-movements" or "ELE-movements".
I am currently taking several actions to eliminate the lack of "pitch precision".
This is what I am trying:
1. More precise servos.
2. Put balls on swash servo arms closer to servo shaft for better precision.
3. Change blade holder bearing order from RRA to RAR, where it is possible. This is to increase the distance between the radial bearings ( R ).
4. Look for other slop, e.g. if main shaft need shims to be better fixed vertically.

In my case, hunting for better pitch precision, I don't think gyro settings matter much. In your case, with hover IGE (in ground effect) instability, turning down agility in the gyro might make a difference.

I will pay attention to IGE hover stability on my "tweeked" 450's next saturday, when I get a chance to test. If not before.

Please remind me (in a pm) if I forget to report back.

/Bo

[svensson_bo_l]

A comment about 450 and sensitivity to ground effect
In addition to the above I would like to add that among the helis I have flied, 450-700, the 450's are the most sensitive to ground effect (GE) when landing.
Especially indoors or outdoors in no wind. Only a little wind tends to move away that air cushion.

With a 450 I usually try to avoid the GE by mowing slowly forward towards the landing place.

That said, I am still under the impression that one or two of my FBL 450's are more sensitive to GE than the others.

/Bo

[extrapilot]

Bo

A couple of things-

There are some errors above (i.e.. ground effect isn’t about the air bouncing back against the rotor). I don’t think this changes anything you have said, but it matters for at least one thing which contributes to the effect the OP mentions, and for people reading this- better to call out errors so that people can do their own reading- and not pick up wives tales here about how things fly.

On the 450 vs larger machines, keep in mind that the disc loading on the 450 is very low- about half that of most larger machines. Example- a basic old 450 is near 0.366 and a 700 is closer to 0.6. Disc loading is directly related to the velocity of air through the rotor- the higher the disc loading, the higher the flow speed through the rotor in hover. This is just because more mass is being supported by a given rotor area.

So, the outflow is slower compared to larger machines, and as a ratio, it is much more sensitive to crosswind of some given velocity. I mean, the flow velocity at the disc of a hovering 450 (stock) averages just 5.8mph. What is tricky is that crosswinds do a couple things. One is that they alter ground effect negatively. But they also alter inflow positively (the reason a heli is more efficient in slow forward flight than in hover). And at the sort of crosswind speeds in question for most of us (0 to 10mph), that is right in this area where you dance between these factors.

Another issue- FBL is chassis referenced, not rotor referenced. The OP was asking about FB vs FBL here. With FB, you have the gyro (the FB) mounted at the rotor head- and any flap of the rotor (the rotor always wants to flap back against the relative wind) is immediately countered by the FB- which is an analog computer with nearly no delay in the process.

FBL is only watching the chassis pitch/roll/yaw. Say you are landing and you have a crosswind gust from the left. The rotor wants to roll to the right, BUT there is also aero load on the fuselage which tends to want to roll it left (CG affects this- but for most…). So, the FBL may not see as much roll to begin with, and regardless, it sees it much later; the fuselage cannot begin to roll until the rotor has already rolled and put force through the dampers on the head, etc. This means larger control inputs, which alteres the ground effect (changes local outflow velocity), and as mentioned below has some impact on stability for another reason.

The last thing Id mention is that normally, ground effect is stabilizing. Not as much with FW (like a low wing Piper), because of phase lag. But still, it tends to be easier to hold altitude- because on net- the effective thrust of the rotor increases as the distance to the ground decreases. But when you add crosswind, the FBL delays lead to large corrections. That means torque, and that means a change in TR thrust. And TR thrust changes have a huge effect on lateral stability in a hover. So, the sooner an error gets fixed, or the less the pilot of an FB overcorrects, the less torque change you have etc
You can see similar things happen by simply starting a piro CW vs CCW in a stable hover in ground effect especially. You will see the machine bobble up or down as if you changed collective, direction depending on which way you piro…

So yea, just a lot of things going on, all at the same time.

[svensson_bo_l]

Originally Posted by extrapilot

Bo

A couple of things-

There are some errors above (i.e.. ground effect isn’t about the air bouncing back against the rotor). I don’t think this changes anything you have said, but it matters for at least one thing which contributes to the effect the OP mentions, and for people reading this- better to call out errors so that people can do their own reading- and not pick up wives tales here about how things fly.

On the 450 vs larger machines, keep in mind that the disc loading on the 450 is very low- about half that of most larger machines. Example- a basic old 450 is near 0.366 and a 700 is closer to 0.6. Disc loading is directly related to the velocity of air through the rotor- the higher the disc loading, the higher the flow speed through the rotor in hover. This is just because more mass is being supported by a given rotor area.

So, the outflow is slower compared to larger machines, and as a ratio, it is much more sensitive to crosswind of some given velocity. I mean, the flow velocity at the disc of a hovering 450 (stock) averages just 5.8mph. What is tricky is that crosswinds do a couple things. One is that they alter ground effect negatively. But they also alter inflow positively (the reason a heli is more efficient in slow forward flight than in hover). And at the sort of crosswind speeds in question for most of us (0 to 10mph), that is right in this area where you dance between these factors.

Another issue- FBL is chassis referenced, not rotor referenced. The OP was asking about FB vs FBL here. With FB, you have the gyro (the FB) mounted at the rotor head- and any flap of the rotor (the rotor always wants to flap back against the relative wind) is immediately countered by the FB- which is an analog computer with nearly no delay in the process.

FBL is only watching the chassis pitch/roll/yaw. Say you are landing and you have a crosswind gust from the left. The rotor wants to roll to the right, BUT there is also aero load on the fuselage which tends to want to roll it left (CG affects this- but for most…). So, the FBL may not see as much roll to begin with, and regardless, it sees it much later; the fuselage cannot begin to roll until the rotor has already rolled and put force through the dampers on the head, etc. This means larger control inputs, which alteres the ground effect (changes local outflow velocity), and as mentioned below has some impact on stability for another reason.

The last thing Id mention is that normally, ground effect is stabilizing. Not as much with FW (like a low wing Piper), because of phase lag. But still, it tends to be easier to hold altitude- because on net- the effective thrust of the rotor increases as the distance to the ground decreases. But when you add crosswind, the FBL delays lead to large corrections. That means torque, and that means a change in TR thrust. And TR thrust changes have a huge effect on lateral stability in a hover. So, the sooner an error gets fixed, or the less the pilot of an FB overcorrects, the less torque change you have etc
You can see similar things happen by simply starting a piro CW vs CCW in a stable hover in ground effect especially. You will see the machine bobble up or down as if you changed collective, direction depending on which way you piro…

So yea, just a lot of things going on, all at the same time.

Thank you for clearifications, extrapilot!

PS. An important difference you point at, that FB is part of the head and that the FBL sensor is fix in the fuselage may be the explanation of what OP asks about. I will look more to see if I can figure out why I see something similar as what OP sees on FBL on one or two FBL 450's but not on all. I have compared indoor, with no wind.

/Bo