Multiple rotor vs single rotor efficiency
Joined: Nov 2004
Posts: 1,334
Likes: 0
From: Cambridgeshire, UK
Dave, refering to post #4: The only reason you can take your hand off a fixed wing stick is that there is a good trim system built into the elevator, and ailerons, which centres the stick. A helicopter with a similar trim system could also fly hands free - this is the point of SCAS.
Your design, like any counterotator, does allow good handling with a very high effective hinge offset. Again SCAS can compensate for any assymetry for a single rotor. Your design may well be better suited to fast rolls though.
Your design, like any counterotator, does allow good handling with a very high effective hinge offset. Again SCAS can compensate for any assymetry for a single rotor. Your design may well be better suited to fast rolls though.
Joined: Apr 2003
Posts: 1,635
Likes: 1
From: Vancouver, BC, Canada
Graviman
OK. A question.
This single rotor helicopter was proposed by Sikorsky at the AHS International 58th Annual Forum in 2002. It is also shown as a candidate for future rotorcraft in Leishman's new book.
How can SCAS compensate for asymmetry on this single rotor helicopter during a power-out transition from cruise to autorotation?
Dave
Again SCAS can compensate for any assymetry for a single rotor.
OK. A question.This single rotor helicopter was proposed by Sikorsky at the AHS International 58th Annual Forum in 2002. It is also shown as a candidate for future rotorcraft in Leishman's new book.
How can SCAS compensate for asymmetry on this single rotor helicopter during a power-out transition from cruise to autorotation?
Dave
Joined: Nov 2004
Posts: 1,334
Likes: 0
From: Cambridgeshire, UK
Dave, the limitation is that the rotor will likely break off if Nr drops too much. Unless you are throwing lots of would-be-payload mass at the rotor, how is your design any different? Most collectives only go to flat pitch.
If you're refering to the right yaw when the single rotor helicopter lost power, then you include a gyro (laser most likely) so that pedals directly control yaw rate. There is no reason the collective can't be included in that SCAS for Nr control.
What i was really getting at is that current light helicopters do not have a centreing trim mechanism on the cyclic. The fact that most helis aren't designed for hands free flight does not mean that they can't be.
I think many of the control limitations are historical. Pilots are used to the heli being difficult to fly, so this becomes the market expectation. There is no need to fundamentally redesign the machine when some clever control engineering will give you what fixed wing pilots take for granted.
If you're refering to the right yaw when the single rotor helicopter lost power, then you include a gyro (laser most likely) so that pedals directly control yaw rate. There is no reason the collective can't be included in that SCAS for Nr control.
What i was really getting at is that current light helicopters do not have a centreing trim mechanism on the cyclic. The fact that most helis aren't designed for hands free flight does not mean that they can't be.
I think many of the control limitations are historical. Pilots are used to the heli being difficult to fly, so this becomes the market expectation. There is no need to fundamentally redesign the machine when some clever control engineering will give you what fixed wing pilots take for granted.
Joined: Apr 2003
Posts: 1,635
Likes: 1
From: Vancouver, BC, Canada
Mart,
It might be considered as a tricky question, however you did say,"ANY assymetry for a single rotor".
A hint.
Reconsider the question, and also take into account that the craft is cruising at 350 knots.
Dave
It might be considered as a tricky question, however you did say,"ANY assymetry for a single rotor".
A hint.
Reconsider the question, and also take into account that the craft is cruising at 350 knots.
Dave
Joined: Nov 2004
Posts: 1,334
Likes: 0
From: Cambridgeshire, UK
Well ok, i was considering more the conventional speed range. For speeds above 200kts clearly some method of counterrotation is the best way avoid all the retreating blade headaches. This could be some of your concepts or X2.
The point is that it is possible to design a control system to give any flying machine good handling characteristics. There is no advantage in increasing complexity unless required by flight envelope.
The point is that it is possible to design a control system to give any flying machine good handling characteristics. There is no advantage in increasing complexity unless required by flight envelope.
Joined: Apr 2003
Posts: 1,635
Likes: 1
From: Vancouver, BC, Canada
To make the transition from cruise to autorotation may be impossible. The retreating blades must make the transition from 100% reverse velocity to 100% conventional velocity. At some point during the transition, the positive lift on the retreating side will be offset by an equal amount of negative lift on the same side. Since there must be bilateral symmetry of lift, all lift must therefore be removed from the advancing side. During this time, only the front and rear quadrants will be contributing to the lift and to the rotation of the rotor.
In addition, the speed or the rotor must be increased from 50% NR to 100% NR. The rotor may well be unable to support the craft from only two quadrants, while also having to increase its rotational speed at the same time.
PS. They added wings on later sketches.
More, if interested.
In addition, the speed or the rotor must be increased from 50% NR to 100% NR. The rotor may well be unable to support the craft from only two quadrants, while also having to increase its rotational speed at the same time.
PS. They added wings on later sketches.
More, if interested.
Joined: Nov 2004
Posts: 1,334
Likes: 0
From: Cambridgeshire, UK
The same fate seems to have fallen all reverse velocity concepts, including stop rotors. I think the dynamics are just too impractical. Ideally you need active blade twist on rotors which are very stiff in bending.
The ideal seems to be counterrotating rotors which allow the retreating blades to aerodynamically disappear. X2 has taken this approach, and stands a realistic chance of being successful.
I can see the logic in Slowrotor's fixed wing based VTOL approach, with high disk loaded rotors for landing. Ultimately i think the X2 based designs will gain high enough efficiency that even this approach will be questionable.
The ideal seems to be counterrotating rotors which allow the retreating blades to aerodynamically disappear. X2 has taken this approach, and stands a realistic chance of being successful.
I can see the logic in Slowrotor's fixed wing based VTOL approach, with high disk loaded rotors for landing. Ultimately i think the X2 based designs will gain high enough efficiency that even this approach will be questionable.
Joined: Jun 2006
Posts: 51
Likes: 0
From: Wellington, New Zealand
The point is that it is possible to design a control system to give any flying machine good handling characteristics. There is no advantage in increasing complexity unless required by flight envelope.
Joined: Nov 2004
Posts: 1,334
Likes: 0
From: Cambridgeshire, UK
As developmental complexity goes, no. Systems already exist to do all this, S.I.G.F. There is no need to develope new configurations to get an aircraft with good flying characteristics.
When i came to this excellent forum, i was championing the original Lockheed mechanical system. This was pioneered on the CL475, and via the Lynx gave rise to modern SCAS. Now with reliable electric actuators there is less need for hydraulics, and so another area of complexity can be removed.
The only justification for a new config, like counterrotation, is to improve the flight envelope. Specifically this is to avoid the retreating blade stall at speed.
When i came to this excellent forum, i was championing the original Lockheed mechanical system. This was pioneered on the CL475, and via the Lynx gave rise to modern SCAS. Now with reliable electric actuators there is less need for hydraulics, and so another area of complexity can be removed.
The only justification for a new config, like counterrotation, is to improve the flight envelope. Specifically this is to avoid the retreating blade stall at speed.
Joined: Jun 2006
Posts: 51
Likes: 0
From: Wellington, New Zealand
What if you can't afford real, certified avionics systems? After all, we are allowing experimental/homebuilt aircraft into the discussion, aren't we?
Light airplanes don't necessarily require powered controls, so mechanical can still be reliable, even if it is complex compared to a similar un-powered controls.
When you rely on electronics to make the aircraft handle the way you want, you are compromising the design in terms of the level of redundancy. Of course, this may be fine depending on the application. But there is still plenty of reason to research new designs, as they may suit some applications better.
Light airplanes don't necessarily require powered controls, so mechanical can still be reliable, even if it is complex compared to a similar un-powered controls.
When you rely on electronics to make the aircraft handle the way you want, you are compromising the design in terms of the level of redundancy. Of course, this may be fine depending on the application. But there is still plenty of reason to research new designs, as they may suit some applications better.
Joined: Nov 2004
Posts: 1,334
Likes: 0
From: Cambridgeshire, UK
SIGF, Dave,
Lockheed managed a mechanical system in 1959:
http://en.wikipedia.org/wiki/Lockheed_XH-51
Helicopter control response is caused by a limited effective hinge offset. You could increase hinge offset at the expense of asymetric control. You could fit side-by-side main rotors for twice the cost.
R66 went for a longer rotor mast and SCAS...
Lockheed managed a mechanical system in 1959:
http://en.wikipedia.org/wiki/Lockheed_XH-51
Helicopter control response is caused by a limited effective hinge offset. You could increase hinge offset at the expense of asymetric control. You could fit side-by-side main rotors for twice the cost.
R66 went for a longer rotor mast and SCAS...
Joined: Nov 2004
Posts: 1,334
Likes: 0
From: Cambridgeshire, UK
SIGF, agreed. But if you look at all the really competetive 4x4s they all have ABS, ASC, Traction Control, Active Steering Control, and many more systems are in development. I worked in the auto industry for on and off road vehicles.
There will always be room for experimental/homebuilt aircraft. What i am trying to say is that you cannot use competetive handling as the reason for a new configuration. The OEM will just call in a control specialist who will shrug his shoulders and ask: Why are you going for this expensive solution when i can design a nice cheap control system?
What i believe is that there are many people who are not control experts, and thus try to find mechanical solutions to control problems. I generally count myself as one of them. But a control system does not need to be 10 supercomputers actively simulating the flight dynamics - indeed a simple mechanical gyroscope was all that was required for early rocketry...
There will always be room for experimental/homebuilt aircraft. What i am trying to say is that you cannot use competetive handling as the reason for a new configuration. The OEM will just call in a control specialist who will shrug his shoulders and ask: Why are you going for this expensive solution when i can design a nice cheap control system?
What i believe is that there are many people who are not control experts, and thus try to find mechanical solutions to control problems. I generally count myself as one of them. But a control system does not need to be 10 supercomputers actively simulating the flight dynamics - indeed a simple mechanical gyroscope was all that was required for early rocketry...
Joined: Apr 2003
Posts: 1,635
Likes: 1
From: Vancouver, BC, Canada
Graviman,
The hardware is one component. The software is the other.
Nick has mentioned that the Comanche required 1/2 M (as I recall) lines of code. This cost was amortized over 3-5 helicopters. The code in Microsoft's operating systems is amortized over tens of millions of computers.
Perhaps you would answer my previous question.
and/or tell me what is today's cost per line of coding?
Dave
The hardware is one component. The software is the other.
Nick has mentioned that the Comanche required 1/2 M (as I recall) lines of code. This cost was amortized over 3-5 helicopters. The code in Microsoft's operating systems is amortized over tens of millions of computers.
Perhaps you would answer my previous question.
What language do you use when writing your computer code?
Dave
Joined: Nov 2004
Posts: 1,334
Likes: 0
From: Cambridgeshire, UK
For what application, Dave?
Much of the time i use software packages developed specifically to solve Finite Element problems or CAD problems. If i am trying to model something in more detail i have been known to write iterative code in Excel. I have written powertrain performance simulations in Pascal and test data processing in Fortran. I have even programmed 4 bit computers in machine code.
But my point is that a control system does not need to be complicated. When my old man was a rocket scientist at Hawker Siddely it was often done using gyros and analogue circuits. In fact the only real advantage of digital is very complex circuitry with no noise. As i say Lockheed did it mechanically.
But there are folk who specialise in this. It would be unrealistic to expect an experimental / homebuilt to perform as well as an OEM machine. If you used the config to get the same handling, the payload/machine ratio would suffer for both cost and weight.
Helicopter design is all about extending the margins...
Much of the time i use software packages developed specifically to solve Finite Element problems or CAD problems. If i am trying to model something in more detail i have been known to write iterative code in Excel. I have written powertrain performance simulations in Pascal and test data processing in Fortran. I have even programmed 4 bit computers in machine code.
But my point is that a control system does not need to be complicated. When my old man was a rocket scientist at Hawker Siddely it was often done using gyros and analogue circuits. In fact the only real advantage of digital is very complex circuitry with no noise. As i say Lockheed did it mechanically.
But there are folk who specialise in this. It would be unrealistic to expect an experimental / homebuilt to perform as well as an OEM machine. If you used the config to get the same handling, the payload/machine ratio would suffer for both cost and weight.
Helicopter design is all about extending the margins...
Last edited by Graviman; 15th January 2008 at 22:01.
Joined: Jun 2006
Posts: 51
Likes: 0
From: Wellington, New Zealand
I'm not quite sure where we're are going with this, but I'll just reply to some of the points anyway.
Yes, but they are expensive at the moment. My point was that cars like Subaru aren't that much more expensive than an equivalent 2WD car since the extra components are negligible (in terms of extra weight, expense, maintenance) in the final product. Just like there are plenty of helis with 2 main rotors, despite that those components are more expensive when only compared to components of a single rotor equivalent.
I'd tell that control specialist that he's better off working in the IT industry, where assumptions that things should only be done one way aren't so harmful. 
It's also not just competitive handling, but reliability, and how it fits in with the rest of the design, among other things. Using a certain type of layout may allow for other requirements to be fulfilled. For example: The Ka-32 is well suited as a MedEvac heli.
And then there are people who have a good understanding of many things, and decide to sit back and take a fresh approach to problems. Whether that solution is in the form of something mechanical may not be simply because it's in the person's comfort zone -- sometimes even the opposite.
A control system can be a simple or as complicated as the design requirements dictate.
There is a big difference between writing non-critical software for R&D and writing critical software systems for controlling aircraft. How many lines of code did most of them have?
No, they don't have to be, and it's best to keep them simply, of course. But sometimes it is necessary to archive a goal. Complexity is also relative to the how it fits in with the rest of the system/design; software is complex, but from the perspective of incorporating a FBW system into the aircraft, it can be simpler.
BTW. Reliability is one advantage of digital -- no moving parts. I've seen an old gyro and it's a pretty heavy, bulky, and mechanically complicated compared to modern solid-state sensors. Also, you don't get noise, but you can still get interference -- I've read that fibre-optics have been considered in FBW for that reason, but I'm not sure about the extent of the research -- it might have been intended for military aircraft.
Not necessarily, since a big part of designing an OEM machine is reducing material and manufacturing costs -- this often compromises other design aspects. Some costs may not even be relevant in a home-built/experimental as they may have no or low cost, or the cost is insignificant compared to other costs. And it's not like amateur designers can't find good info and get advice from specialists in the age of the internet. But now we're getting off topic.
SIGF, agreed. But if you look at all the really competetive 4x4s they all have ABS, ASC, Traction Control, Active Steering Control, and many more systems are in development. I worked in the auto industry for on and off road vehicles.
There will always be room for experimental/homebuilt aircraft. What i am trying to say is that you cannot use competetive handling as the reason for a new configuration. The OEM will just call in a control specialist who will shrug his shoulders and ask: Why are you going for this expensive solution when i can design a nice cheap control system?
It's also not just competitive handling, but reliability, and how it fits in with the rest of the design, among other things. Using a certain type of layout may allow for other requirements to be fulfilled. For example: The Ka-32 is well suited as a MedEvac heli.
What i believe is that there are many people who are not control experts, and thus try to find mechanical solutions to control problems. I generally count myself as one of them. But a control system does not need to be 10 supercomputers actively simulating the flight dynamics - indeed a simple mechanical gyroscope was all that was required for early rocketry...
A control system can be a simple or as complicated as the design requirements dictate.
Much of the time i use software packages developed specifically to solve Finite Element problems or CAD problems. If i am trying to model something in more detail i have been known to write iterative code in Excel. I have written powertrain performance simulations in Pascal and test data processing in Fortran. I have even programmed 4 bit computers in machine code.
But my point is that a control system does not need to be complicated. When my old man was a rocket scientist at Hawker Siddely it was often done using gyros and analogue circuits. In fact the only real advantage of digital is very complex circuitry with no noise. As i say Lockheed did it mechanically.
BTW. Reliability is one advantage of digital -- no moving parts. I've seen an old gyro and it's a pretty heavy, bulky, and mechanically complicated compared to modern solid-state sensors. Also, you don't get noise, but you can still get interference -- I've read that fibre-optics have been considered in FBW for that reason, but I'm not sure about the extent of the research -- it might have been intended for military aircraft.
But there are folk who specialise in this. It would be unrealistic to expect an experimental / homebuilt to perform as well as an OEM machine. If you used the config to get the same handling, the payload/machine ratio would suffer for both cost and weight.
Joined: Nov 2004
Posts: 1,334
Likes: 0
From: Cambridgeshire, UK
SIGF, i took you to be one of Dave's evil interleaving cohorts.
I should explain i am an engineer, so any coding i might have done would be either for my own freetime projects or to solve a specific design problem. I don't get paid for code.
The coaxial, exampled by the KA-32, is probably the exception to my statements above. Even so this only really works if the main rotor was to have more than 6 blades - this way there is no mass compromise. Intermesher comes a close second, and in fact there may be aerodynamic advantages. So far there have only been small intermeshers, with few blades, so there is probably no mass advantage.
What i was assuming was that the justification for side-by-side or interleaver was for roll control, and symetry of cyclic inputs. Hopefully my control arguements will make sense in this context.
Anyway, back to the grind...
I should explain i am an engineer, so any coding i might have done would be either for my own freetime projects or to solve a specific design problem. I don't get paid for code.
The coaxial, exampled by the KA-32, is probably the exception to my statements above. Even so this only really works if the main rotor was to have more than 6 blades - this way there is no mass compromise. Intermesher comes a close second, and in fact there may be aerodynamic advantages. So far there have only been small intermeshers, with few blades, so there is probably no mass advantage.
What i was assuming was that the justification for side-by-side or interleaver was for roll control, and symetry of cyclic inputs. Hopefully my control arguements will make sense in this context.
Anyway, back to the grind...
