A question about the cyclic delay
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A question about the cyclic delay
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?
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?
.
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.
That sounds like another early Bell (2 second delay). 
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.
Sav
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.
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.
Sav
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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.
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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.
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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?
hingeless rotors, what about them since the flapping is by flexing, the delay is minimum , right?
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.
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.
"... 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
"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
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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
?
Hope my question is clear, it aint easy to explain this.
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
?Hope my question is clear, it aint easy to explain this.
.
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.
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
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?
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
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Bottom line - over-thinking-it
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.
Just fly and stop over-thinking it. The movement (even with infinitesimal delays) becomes "natural" to you.
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
As most people have said, there is no detectable delay in anything vaguely modern, regardless of head design.
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.
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.
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Time lags cyclic control
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
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
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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.
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.
Last edited by bast0n; 26th Apr 2011 at 17:23. Reason: inacuraccy
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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
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
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Example of R44 dynamics
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.
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
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.
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