Dampener effects

[npomeroy]

I have read about the lead/lag and flapping behaviour of blades and understand the dampeners (or dampers or whatever - but lets not open that one up) allow some blade movement and absorb shocks.

Is there a rule of thumb for situations favouring hard vs soft dampeners? I fly slow HS scale helis with SK720 stabilisation. My reading suggests on one hand that gentle scale flight is often favoured by soft dampeners, but on the other hand I understand that FBL systems require hard dampeners so the digital control inputs get direct response.

[extrapilot]

Hi Nelson

It would be a stretch (pun intended) to call them dampers in the conventional sense; our blades are highly damped in flap due to large surface area to mass ratio, and we have lead/lag hinges. They can relieve some stress from the head, but mostly they are about head compliance.

Compliance brings tradeoffs. Example, softer dampers permit more rotor tilt relative to the swash (the rotor can tilt more before it spindle-locks and forces the chassis to rotate). That tilt generates corrective cyclic, which is generally stabilizing. But softer dampers further decouple the chassis inertia from the rotor, which makes the rotor more prone to ugly flapback scenarios (boom strikes, bunts in FFF, etc).

Lower compliance means the opposite. So you may gain in FFF and 3D, but the tight coupling in the head means that there is a larger window for control system resonance (i.e. nodding).

I don’t think you will find some simple formula- lots depends on your blades, your headspeed, the control loop speed of your FBL/servo system, the CG/mass of the airframe, your style of flight, the FBL gains, etc.

[R_Lefebvre]

One of my perceptions is that softer dampers result in less vibration in FFF as they allow the disk to flap over further to alleviate disymmetry of lift? Do you agree?

There's actually a really neat thing you can see very well on data logs from multirotors. The guys spend ages trying to balance their blades, but then as soon as they any forward speed they get a wicked vibration, which I believe is due to the rigid rotors causing a cyclical torsional vibration about an axis parallel to the forward speed. This is because as the blades are perpendicular to airflow, they have a disymmetry of lift, but when they are parallel to the airflow, there is not disymmetry.

But it makes me wonder... this summer I was looking at a large (~10 foot) UAV heli which used a completely rigid rotor system. I asked the inventor how it worked, because the disymmetry of lift would create a rolling moment. The flight controller basically tilts the swashplate to compensate the roll. But I wonder if this would also automatically allow the rotor blades to... well not flap... but, their tip plane might end up such that disymmetry of lift is eliminated, and thus the vibration?

Another thing I'm interested to talk more about is how flap/tilt angle results in "corrective cyclic". I think I kinda get it, but if you want to get into it...

[npomeroy]

Thanks for the comments. On reflection, softer dampers shouldn't dampen the actual cyclic pitch inputs (in terms of actual pitch change at the blade hub) significantly, and so should not limit the ability of the FBL commands to make corrections. But as pointed out a more compliant head can tilt about the place without the chassis responding. For gentle flying styles this sounds like a good thing so long as there is not so much delay in chassis response that the FBL commands create an oscillation. I have heard when raising this elsewhere that softer dampers are used with lower head speeds.

[extrapilot]

Rob

I think the term dissymmetry of lift is sometimes misused. Yea, you do get flap to equality on machines that have no cyclic and which can flap. But for us, on the main rotor, counter-cyclic is used. It is complicated, because you have the blade in all kinds of spanwise flow vectors during the orbit, and it may be more efficient at 2 o’clock than at 4 o’oclock due to a reduction in tip vortex or whatever. So you do tend to get weird twists and bends in a blade (see Youtube for some slow motion- scary!).

Mystics on this site will talk endlessly about how their machines are logged with zero vibration, and zero slop, etc. Problem is, that is nonsense.

There are aerodynamic laws in play that cause root shear for us. As smooth as the machine may be in hover, it is hardly that in FF or acro. Just consider the very real dissymmetry of lift that exists on a TR with no flap hinge in FF… Nice vibration. Or lead/lag, where both blades are biased to one side of the main shaft…

Not to suggest we should not balance our systems- just that you are kidding yourself if you believe a mild vibration on the bench is anything like the vibration caused by aero loading in flight. And no- log data from an FBL doesn’t count. People have no idea how much filtering is going on before sensor data even gets to the processor, and even then, they don’t know the sample rate, or the logic used (is it RMS, peak, post DSP, etc).

Look up ‘n-per’, which is just a method of referencing vibration modes for helis. The math on this is very well established.

On cyclic, consider what causes the rotor to change plane. There are two ways to create that cyclic bias on a conventional rotor. Either you change the swash angle relative to the head, or the head angle changes relative to the swash. The net result is the same. So this phenomenon allows, for example, spinning rotors on a ship-borne heli to roll with the heli, which rolls with the deck… The pilot does not generate cyclic input- the rotor is just seeing an error term, and is flying to a new tip-path plane.

Probably best to look up the concept of rotor ‘following rate’, because there are some implications here that matter.

Regards

[R_Lefebvre]

Is this stuff all covered in Leishman's book? I've got a copy, but haven't started reading it in depth yet.

I'm definitely in agreement with you about the gurus who think the can build the perfect, zero vibration machine, by insane amounts of balancing. I've seen the same thing with multirotors, to the point of a guy who is sanding the bottom of the hub of his rotors, to change the angle of the mounting surface trying to achieve perfect tracking on the bench. He's claiming zero vibration in the lab. Yeah, that's great in the lab, and then in real life you get this (graph is fairly self-explanatory, white line is ground speed, a poor approximation for airspeed, but you get the point):



So it sounds like I could conduct a test, testing different head dampers at cruising speed, and what am I going to see? Does the swash plate tip over more or less or the same, with rigid dampers vs. soft? It now seems to me like the swash will need to tip the same amount. The swashplate needs to be at the same angle to achieve equality, regardless of damper stiffness.

Is there any vibration difference at all with soft vs. hard dampers?

If it's true that "equality" is always done at the swashplate anyway, I would think that we'd still see less vibration with softer dampers if only because those softer dampers *create* less vibration as the feathering shaft tilts back and forth in the head. ie: no matter which dampers are used, but all else being equal, the rotor disk needs to move to a certain angle to achieve symmetry or equality of lift. That rotor disk angle relative to the main shaft means the feathering shaft is tilting in the head. Firmer dampers cause the feathering shaft to impart more torsional vibration to the head. Yeah?

So, why not have a free-flapping head?

My thoughts right now are:

If you have a free flapping head, you'd have less vibration, but the only way to generate flight control (ie: aileron and elevator forces) is by having the thrust vector misaligned from the CG. Tilt the disk over, and as long as you have positive thrust, the thrust vector will no longer pass through the CG, and will thus generate a rolling or pitching moment.

However, if you aren't generating thrust, you'd have practically zero moment. So this won't work for zero lift aerobatics at all.

If you have a rigid head, the rotor disk doesn't actually change angle, (well, OK, the tip path plane will still change a bit due to rotor flex, but lets ignore that for now) and thus the thrust vector is pretty much always through the CG. However, when you change the cyclic angle, it causes the rotor to impart a torque directly on the main shaft through the head.

???

[martin_]

I like to get help from my skookum 720 log analyser to tell me where vibration are coming, it's actually possible to built a machine that vibrate much less than another one, but I know that ''in flight'' vibration cannot be eliminated, as soon as you get airborne, even into hovering when you hear the main blade flapping or the tail blade buzzing, even that the log show ''0'' vibration, there is still already some vib.

Generally the guys flying with an sk 720, according to the vib log, know that you get a lot of high vibration spike here and there during a flight, specially during hard manoeuver, vibe free on the bench/hovering is about the best we can do when you build the heli.

Now into FFF at full pitch with a DFC head and crazy head speed I can get into a very high ''side to side'' vibration that is non present if I'm only doing full pitch vertical climb. With a regular FBL head with softer dampening the vibe are also lower in FFF.

[extrapilot]

Rob

Dr Leishman has covered a ton of stuff for conventional configs. And that is a bit of a problem for us- we don’t tend to see much university research on inverted flight, 10G acro, high speed backwards flight, etc. Also, some of the existing data just doesn’t translate well due to scale. Blade dynamics alone are different- just because of the way the mass/volume/stiffness relationships change with scale.

On dampers/unconstrained teeter, a couple of thoughts. The less coupling that exists between the rotor and the head, the less vibration it can transmit- if the rotor’s behavior remains constant. But that is a catch-22, because the behavior of the rotor is affected by the head and the mass of the airframe. You may not feel as much vibration with a floating spindle shaft, and then the rotor departs in FF (flapback, etc), and cuts the boom off the machine…

Another thing is, there are multiple sources of vibration. For example, with free teetering, you can expect to see higher flap amplitude. But flap tends to cause lead/lag, because the CG of the blade moves closer to the axis of rotation when it is not perpendicular to the axis (conservation of angular momentum). This does not have to be symmetrical, since the rotor can be coned from net generation of lift, so you end up with two blades in a single hemisphere.

Yep, the rigid head aspect is what gives us the ability to more tightly couple the rotor tip path plane with the airframe deck attitude. So it just comes down to sorting through the net of a whole lot of variables for a given mission profile.