A question about the cyclic delay

[leviterande]

Hi, everyone!

I know that the cyclic lag i.e. the time it takes from the control input to the full tilt path depends on many things such as weight, rotorspan, rpm etc.

But! what is the average cyclic delay? I really would love to know from pilots. It seems it is the main diffeculty in flying because I cant imagine just how hard it must be to fly a sensitive bird that responds 2 seconds after your input. So what is the delay time of medium helis as R44?

[Savoia]

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Well, if you're flying with an 'underslung' rotorhead (typical of the early Bells) it can be a matter of .. contol input .. go make a 'cuppa' .. corresponding reaction to airframe occurs.


It seems it is the main diffeculty in flying because I cant imagine just how hard it must be to fly a sensitive bird that responds 2 seconds after your input.

That sounds like another early Bell (2 second delay). :E

But if you want to know what is feels like to fly a truly responsive aircraft can I recommend the 500D. With a new head (ie. everything still tight) the aircraft tends to respond in close correlation to one's thoughts!

You move the cyclic .. and the craft is already there! A brilliant piece of engineering by the original Hughes company. :ok:

Sav

[Gomer Pylot]

There is no 'average' delay time, because of the many designs around. There is an average for each model, but every model is different from every other model. For a teetering system like early Bells and Robinsons, the delay is caused mainly, IMO, by the way the rotor moves independently of the fuselage, and thus it takes time for aerodynamics to move the fuselage after a cyclic input moves the main rotor plane. More blades generally means quicker response, and models like the AS350, BO105, etc, respond much more quickly, with little if any delay.

[rotorfossil]

Even the Bell 47 was quoted as only having a delay time of .47 second before the corresponding rotor reaction to a cyclic input. I know of no modern helicopter which has a delay time of two seconds. What you see in the cockpit is the time between control input and subsequent fuselage response. This is dependent on the equivalent flapping hinge offset which is effectively zero in two blade teetering heads and 100% for a theoretical completely rigid system (I think about 17% is common). In practice most people would have difficulty noticing the delay in for instance the R44, even less so in the R22.

[leviterande]

So the teetering head is the slowest of them all, as I expected.


hingeless rotors, what about them since the flapping is by flexing, the delay is minimum , right?

[[email protected]]

leviterande, it is called control power and is the amount of cyclic deflection required to produce response from the fuselage.

On a teetering head helicopter, the blades move first and then they drag the fuselage around behind them - positive G is required and if that is not there due to pushover manoeuvres or the like, the fuselage is unaffected by cyclic movement. This is why negative G (or less than 1G) is dangerous in teetering head helos as it can lead to mast bumping or worse - in an R22 at very low G, the only thing producing thrust that can affect the fuselage is the TR, that is why they roll in that configuration, often leading to the MR impacting the tail boom.

As the physical distance between the flapping hinge and the rotor mast increases, so does the control power (effectively a lever to move the fuselage) and, when considering semi-rigid or rigid rotors, and effective hinge offset is usually quoted - as rotorfossil says, 17% is what the Lynx with a titanium forging for a flapping hinge gives.

[Adam Nams]

"... and that, Fotherington-Smythe, is why the winch operator is so good at his job; anticipating the pilot-induced swing of the winchman at the end of the cable; anticipating the movement of the pitching and rolling deck; giving the correct patter to ensure that winchman and boat come together with the lightest of touch, whilst taking into account the experience level of his pilot, the size of his/her hands and the 'helicopter control power'."

"A joy to behold sir."






Mind you, 'helicopter control power' does sound like a(nother) poor excuse for the pilots to hang their hat on at the end of a check ride ;)

[leviterande]

There is an unclear point in my head though:

If we take a teeterhead for instance and push the cyclic forward, the rotor disc plan tilts first and two hours:):} later the fuselage follows the rotor disc, right?ok, here is what I dont understand:

when does the helicopter itself move into the desired direction? is it as fast as the rotordisc is tilted or is it when the fuselage starts to be pulled by the rotor:ugh:?

Hope my question is clear, it aint easy to explain this.

[Savoia]

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Levi: I would be interested to know what it is that you are actually trying to understand. There are already a couple of good replies above.


When does the helicopter itself move into the desired direction? Is it as fast as the rotordisc is tilted or is it when the fuselage starts to be pulled by the rotor?

The helicopter is the whole unit, rotor disk and fuselage. While with the 'underslung' teetering systems there is a delay between cyclic input and the corresponding manifestation in the attitude of the fuselage (and of which we are making much fun) the fact is, as Rotorfossil mentioned, we are dealing with a delay which is just fractions of a second.

So to answer your question, in a helicopter equipped with a teetering rotorhead the 'helicopter' will 'move' in response to cyclic input by tilting the rotor disk which, in turn, has a corresponding effect on the attitude of the fuselage albeit delayed. This 'delay' is symptomatic of the responsiveness associated with teetering systems.

Sav

[brett s]

Go fly a Hiller 12 if you want to see control delay - I still my first couple minutes in one ;)

[MileHi480B]

For all practical purposes ... there is no delay. You move cyclic, helicopter responds before you even start thinking "Wonder when it's going to respond". In other words ... I can think of no instance where you'd wonder if the input was enough and start another input before feeling the reaction of the first input.

Just fly and stop over-thinking it. The movement (even with infinitesimal delays) becomes "natural" to you.

[212man]

It seems it is the main diffeculty in flying because I cant imagine just how hard it must be to fly a sensitive bird that responds 2 seconds after your input

That would be almost impossible to fly and would lead to divergent PIOs in very short order! :eek: As most people have said, there is no detectable delay in anything vaguely modern, regardless of head design.

[Arm out the window]

For the teetering head, there's no reason for the fuselage to tilt with the rotor head directly.
This is the way I think of it:

Cyclic input makes disc tilt, and tilted lift vector drags the disc off in the desired direction.

Point of attachment of mast to disc is the rotor head, so as the disc flies off into the sunset, the head comes with it.

Pendulously dangling fuselage, attached to mast, starts to move off in the same direction. However, as speed builds up, drag means the fuselage hangs back with respect to the head, hence the body angle change we see from the cockpit.

In short, push the cyclic forward, and shortly thereafter the deck angle will change too as you fly off in that direction.

With a more rigid head, the control power leverage as described by Crab above will make things happen quicker.

[delta3]

Levi

Two main groups:

1. The rotor reaction time, depends on many parameters, but is in general very fast, typical 1 to 2 revolutions to execute the cyclic command. On a R44 that is approx 1/5th of a second.

2. The body is a different ball park. In general it is much slower.

2.1 First there is the rotor stiffness as per Crab. Theetering has stiffness 0, multiblades (even with free flapping hindges) are more stiff. That means the rotor can transmit cyclic torques at the rotor head. The stiffer the rotor the faster the response of the hull.

2.2 Inertial forces combined with aerodynamic forces (fins) and gravity explain the rest. They are the slowest (multi second, not hours). They are also the only one's for theetering rotors. The precise dynamic parameters are known by the constructor because they explain the flight behaviour (dutch roll etc). Given some more time I could get some precise time constants out of my R44 model, for for instance lateral and forward cyclic.

Hope that helps, d3

[bast0n]

I think that theories and pontification as to what does what and to whom are a bit overdone!

I was always very grateful if the machine followed my inputs as closely as it felt able at the time, bearing in mind all the external factors, and got me home in one piece. A bit of lag? Well we all feel like that on occasion.

I remember very well being asked by a visiting Crab trapper to explain with the aid of a diagram the theories behind a Teefygram. I broke several pieces of chalk, and learned about torque fracture patterns along the way, and remained a B2,(I meant B1!) for the next 20 odd years.

The aircraft nearly always got me home...................

Happy Easter.

[leviterande]

Thanx for the explanations everyone.

So if I understand this correctly(it is the hull movement that is hard to imagine):

The "Whole helicopter as one unit" starts the acceleration into the desired direction once the hull starts to tilt and not any sooner?!

I hope my question is clear now. I understand that the rotor is one thing and the hull is another. The hull responds slower and after the rotorblades. My wondering is at what instance does the helicopter move

[Savoia]

.
Levi: Yours is an excellent post-April 1st post! :D ;)

Sav

[leviterande]

.... April?

[Robbo Jock]

Just bear in mind:

The rotor disk tilteth and the rotor disk moveth away. The fuselage moves in mysterious ways, it's perambulations to perform.

[delta3]

Levi

A very simple example :

R44, 2 POB, 140 liters, steady 50 feet hover, ISA.

Cyclic is at -2.15 degrees during this hover.
Pilot applies a step input to cyclic bringing and holding it 0.5 degree forward, so at -1.62 degrees.

http://www.pks.be/files/public/Forward%20cyclic%20half%20degree.jpg


Results

1. Not in graph: rotor reacts immediately <0.2 sec
2. First two seconds, no alt change, very little speed changes, just a pitching down.
3. As speed picks up, heli starts descending at 500 f/min, but pitching down is halted
4. After passing TL-point a very progressive blow back occurs that makes to heli pitch up brutally, putting nose right up in the sky.

A Pilot (at least I hope) would of course not fly like this and would apply further forward cyclic to "control" the heli. But it gives you a rough idea about time constants. (Graph starts just at the moment of applying the forward cyclic, an auto-pilot feature of the simulator controlled lateral cyclic and tail-rotor, collective was held steady on 9.5 degrees)

d3

[leviterande]

Very useful find, where do you find these?


So about 2 seconds till the thing moves. hmmm

[[email protected]]

leviterande - it says R44 helicopter simulator at the top:ok:

Don't confuse rotor/fuselage response times with the time taken to overcome the inertia of the helicopter and accelerate it. The graph doesn't show fuselage attitude, only cyclic input.

The simulation also doesn't show inflow roll (or transverse flow for US trained pilots) so don't put too much faith in it.

If rotor/fuselage response times were in the order of 2 seconds, no-one would need ASE/SAS/AFCS to reduce pilot workload since they tend to provide rate damping to slow down the rotor response to cyclic inputs:)

[leviterande]

Ahaha o yeah didnt see that ops! hehe simulator it is:)!

"Don't confuse rotor/fuselage response times with the time taken to overcome the inertia of the helicopter and accelerate it. The graph doesn't show fuselage attitude, only cyclic input."

Your absolutely correct, inertia effects can be pretty significant in a 60 ton russian Mil Mi-26 where the tilted rotordisc&fuselage do not impart movement until 40 years later:\

[Stuck_in_an_ATR]

Well, bear in mind that on a 60 ton Mi-26, for a given tilt angle of the disc, the horizontal thrust component is somewhat bigger than in an R-22 :E - hence the movement of the fuselage shouldn't take too long either :rolleyes:

[rotorfossil]

To try to put a final slant on this topic. On all types that I've flown (and that is quite a few), it is very difficult to demonstrate convincingly a perceptible lag between cyclic input and fuselage attitude change, although the rate of attitude change obviously does vary due to inertia differences. The possible exceptions were the Hiller 12b/c where moving the cyclic operated the paddles which then changed the disc attitude and the Bell 47's where the cyclic input goes through the stabiliser bar and induces a tiny delay (the .47 sec). If you can see half a second delay, good luck!
Inertia then affects the rate at which the change of speed occurs viz R22 at one end and MIL 26 at the other.

[delta3]

You are jumping a little fast to conclusions.

It pretend this is an advanced integrated simulator developped 6 years ago. (not really a humble statement, because I made it). It does very detailed aero dynamic calculations about 500 times per second based on pretty advanced aerodynamic models (not just your plain to earth second order model, but full viscous flow, including stall, but that has been explained 5 years ago). It models most aerodynamic parts including rotor blades, hull, fins and implements inertial calculations in full detail of most (ridgid) parts. So it can correctly predict extreme flight paths and explain detailed air flow in the rotor. It cannot predict vibrations. In all experiments I checked to date with real flight data it showed to be more precise then measurable (typically better then 5%)

I have the complete film the simulator generates from different view points (pilot, outside observer) but from where I am, I cannot extract and post it. Perhaps I will post some snap shots if I can find the time.

But I sincerely think the graph answers precisely Levi's question, without having to go in such detail. I'll try to post the pilot view every second, pretty scarry indeed because after 10 second you crash). You will see the first two seconds consist mostly of a forward pitching and that the blow back if very impressive.


d3

[leviterande]

So we all can be happy:ok: and agree :Dthat during these 2 seconds in your diagram, the fuselage is tilting but not yet acquired forward movement:)

[[email protected]]

Delta3 - I am sure it is a very clever simulator but does it show roll towards the advancing side during a transition? If not then your sums (advanced as they may be) are wrong.

In my humble experience, the problem with simulators is that they are just computers and, like any computer, if you put garbage in you get garbage out.

Many full-size sims costing millions still can't replicate the actual handling qualities of a specific helicopter because all the data has to come from an instrumented aircraft of the same sort to be accurate, instead of the data being modelled with clever mathematics.

[delta3]

Crab

Crab, yes it does.

It starts from basic physics, not from modelled behaviour, which makes it quite unique, but a formidably complex mathematical project (even including ground effect, tail fin down-wash interference etc...) This setup was specifically deveopped to have a precise view on full rotor dynamics during all parts of flight.

In the scenario shown here, model was operating in what I call 2-D mode, auto-pilot feature was doing lateral cyclic and tail rotor. That is an approriate set-up to answer Levi's question, not for your question of lateral transients.

The problem is -as discussed years ago with Nick Lappos- in order to have reproducible data, autopilots capable of flying predetermined paths need to be developped, which is again a project, and as it was supposed to run of a PC, not a super-mini, there is a CPU limitation. We found -as Nick predicted- that it is not possible to get reproducible data by hand flying the sim, you cannot fly it that precisely. So a number of path controllers were added to te sim.

Further more the amount of data produced is enormous, rendering, graphing up to generating full 3D sim like films etc, is again a project.


Development stopped 5 years ago, because the goals of simulating advanced auto rotations at many different configurations (W&B, RPM, density etc...), all sorts of low G situations, the said Frank's Wiwa and all sorts of coning/delta3 related rotor dynamics. Where do you think my alias comes from, I assume you should know that delta3's are related to lateral rolling....

It was for instanced used on this forum while discussing good old Lu's big problem = he strongly believed the R22/R44 was fundamentally not well designed, dynamically speaking. The model showed that it is not the rotor but the rotor/body interaction that is the limiting factor (combination of theetering rotor and R22/R44 inertial body behaviour)

You quote a "role to the advancing side" and put it as a absolute given for sim-consistency, where does that come from?
Years ago there was also the 90° precession dogma...

From all the tests I have seen, these coning related transitional rotor dynamics are very specific to a given rotor and riging, which can be very different, the only common factor is coning.

As I stated many years ago, personally I was surprised by the balance in the simple design of Frank's rotor that minimizes that lateral behaviour. Frank called it WiWa, because a simple way of reproducing it, is tracking the betas when just putting cyclic forward/backward and monitoring the rotor at different coning angles, the reason behind is that the tilting motion also creates transvers flows, even skids on the ground. I am sure that by 'over riging' the rotor I could get a different lateral behaviour, perhaps even a role to the other side. I cannot speak in great detail of other rotor systems, but I know that for instance a stiff four blade rotor has quite different dynamic behaviour than a theetering 2 blade.
Currently I do no longer have the time nor the environment to do elaborate experiments, I am lucky that a basic working setup still is available to do some simple check's (see for instance a fairly recent thread on low rpm blow back during autorotion, or delta3 positive versus negative in the 407 TR).

Given a full working system I could extract precise beta graphs of the blades that precisely show you the blade dynamics during the current experiment, which would answer your question in detail.

d3

[[email protected]]

Delta3 - you are clearly a clever chap but not, in fact, a helicopter pilot - if you were you would know that inflow roll (or transverse flow) is a fact of life in a helicopter, even in an R22. And it's nothing to do with precession:)

Just because your basic physics and advanced maths doesn't reveal its existence in your modelling, doesn't mean it doesn't happen - ask why there is a lateral trim facility on the Robinson!

You'll be telling me your simulator doesn't experience inherent sideslip next:)

[delta3]

Again you jump to the wrong conclusion.

Over and out.

[[email protected]]

What??? you mean you are not a clever chap:):):)

[bast0n]

Crab

AHHH the inflight roll.................used to be an "Oggie" down in Cornwall...........:ok:

But you are right and D3 ain't!

[delta3]

Gentlemen,

Perhaps you should take the time and the elementary politeness of reading what people say, before starting to jump on statements that were not there.

See previous post :

"Crab

Crab, yes it does.

It starts from...."

[[email protected]]

Now now Delta 3 - don't take offence, it was unfair of me to give you banter since English is not your first language:ok:

[Arm out the window]

Inflow roll is most definitely a fact of life ... the aerody texts I learnt about it from (probably not dissimilar to what Crab would use) talked about a progressively increasing inflow into the disc from front to back in the early stages of forward flight, with associated lift reduction leading to flapping down on the advancing side.
This, along with flapback, is easily demonstrated (in the real thing) by initiating a transition from the hover and then holding the cyclic steady while watching what happens to the attitude - a pitch-up and a roll towards the advancing blade side.
Obviously any realistic simulator should replicate that.

Regarding the cyclic lag, it's easy to demo (in a teetering head like a Huey at least) by sitting in the hover and fairly rapidly moving the cyclic back and forth a couple of centimetres whilst watching the attitude and the tip of the disc. The disc will wobble back and forth quite a bit without significant fuselage movement, as by the time it would be thinking about moving you've already made the opposite input.

Which reminds me - hands up if you experienced something like this when you learnt to hover: working bloody hard to stay in one place, waving the cyclic around madly with arm muscles tensing up, instructor takes over, puts one finger on top of cyclic and says "See how much I'm moving the stick? Why don't you just do the same...?"

[Dan Reno]

Perhaps the best examples of a "cyclic delay" were expressed very often by old time US Army H-1 drivers after their first flight in a Blackhawk.
Most said: "When you point the cyclic in the H60, you're there NOW whereas in the H1; She'll eventually get there".

In helos with more than two blades, Tech Reps call cyclic delays: hysteresis. Such "Cyclic delays" (as in the H53) can usually be eliminated by changing out marginal control rod bearings from top to bottom.

[Gomer Pylot]

In a Bell with 2 blades, you have to know where to put the cyclic, put it there, and wait. You will eventually get the result you want, you just have to be patient and wait for it. In a 412 it happens a lot quicker, but with a lot more inertia, it still takes some time to start moving. In cruise flight it's a lot quicker. A more accurate comparison is the 206 vs the AS350, since the weights are more comparable. In a 206 you move the cyclic to where it should be and eventually you get a result. In the AS350 you think about moving the cyclic and you have a result, hopefully not too much. Probably about the same in an MD500.