Multiple rotor vs single rotor efficiency

[slowrotor]

Suppose I built a VTOL with ten small independent electric motor powered rotors of 4 feet diameter each for a total of 125 sq.ft.
And for comparison a single rotor helicopter with the same 125 sq. ft.

The multi-rotor design could be built very light since the failure of one rotor could be controlled. Sort of like the guy that tied 50 weather balloons to a lawn chair.

Considering the reduced weight, which rotor system would be the best in terms of payload?

Any thoughts appreciated.
slowrotor

[delta3]

Scaling is a difficult engineering aspect as major elements such as lenght, weight and strenght don't scale equally. As a result, different size ranges will require different architectural solution.

I assume you are talking a small model.

In this range I am pretty sure 1 rotor will be aerodynamically optimal.

Next comes the electrical motors, again I think lesser will be better.

You raised a second and in may opinion different design criterium about controlability and redundancy.

I think that the first one can be equally realised by some chip governing say 4 servos or a heli kind of cyclic coupled with a gyro.

Redancy is a whole different matter. I have the feeling that even in the real world not everybody is align on the question single or bi-engine.


m2cents, d3

[slowrotor]

Thanks delta3,

Actually I am evaluating a proposed personal VTOL design in Kitplanes (June 1997) by an aero engineer Bill Welch.
He described a VTOL that looked similar to any small airplane but with six small fans installed inside the wings. A VTOL airplane.
He says that the small fans would not need all the heavy and complex hinges required for an open rotor helicopter and it could fly faster with the wing shutters closed.

It's an intriguing idea for its low cost and redundancy as you mentioned.
I think one rotor is probably always more efficient in pure aero terms. But when structural weight is included that is another matter.
slowrotor

[Dave_Jackson]

slowrotor.

delta3's points are good ones.






A couple of additional comments.Electric motors can deliver 2 to 3 times their operational power for a short period of time. This might be used for safety.In general, bigger motors are more efficient. However, there are small outrunner electric motors that have higher power/weight ratios then bigger ones, but their reliability is less. You might find this Patent Application entertaining; 20060266881 ~ Vertical takeoff and landing aircraft using a redundant array of independent rotors. (http://appft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PG01&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.html&r=1&f=G&l=50&s1=%2220060266881%22.PGNR.&OS=DN/20060266881&RS=DN/20060266881)

This might also be of interest; Electrotor-Simplex (http://www.unicopter.com/ElectrotorSimplex.html)
_____________

http://www.unicopter.com/Typing.gif Now that the fingers are warmed up. Your concern with safety has provoked a thought.

I believe that a rotorcraft that has extremely strong bilateral rotors, should be able to fully stop the rotation of the rotors and then, 'hands-off', allow the stalled craft to pitch nose down, regain forward velocity, regain rotor rotation (w/o engine power), and then be recovered from the dive; just like a fixed wing craft.

Perhaps building this model (http://www.unicopter.com/1508.html), without the rubber-band, and then dropping it from an elevation might prove the belief.


Dave

[lelebebbel]

ten tiny rotors would be aerodynamically inefficient, but a single rotor would need a anti-torque device.

2 rotors could be the optimum.

[delta3]

X2 reinvented ....

[Graviman]

Hey Dave, when in the first flight of the X4 scheduled?
You're not slipping again are you? :}

Must admit i prefer keeping things simple, and one rotor is simpler than six. The effective hinge offset will be higher on smaller rotors, so you could design a snappier machine. The problem here is that the multiple rotors will be bigger than the single rotor, for the same actual lifting area. Also the need to stop disk leakage requires some sort of structure, which will be heavy.

I recommend using Excel to generate a weight table. Compare conventional to six rotor, and look at Dave's concepts. I'm betting that innocent X-shaft + gearboxes in the interleaver eats up a bit of payload. Helicopter design is all about margin, how much more payload can i lift for how much less helicopter. Flights of fancy are always fun, but can they beat the S-64 50% payload at MAUW. Also for a privately owned machine, how often do i need to overhaul the drivetrain and rotors. How often is the machine u/s because of a bearing failure. It all adds up.

That said, it's good to think outside the box, but better if you understand the constraints. :ok:

[Dave_Jackson]

Mart,

The following is a little off-topic but you mentioned "X-shaft + gearboxes in the interleaver eats up a bit of payload" and slowrotor mentioned"small independent electric motor".

All-Electric Intermeshing UAV:
The weight of outrunner electric motors is very light therefore the 1/2-scale UniCopter UAV below is designed with 3 separate <20 hp motors at 4.2 lbs each. The electrical power would be provided by on-board batteries or a central engine+generator. The only gearing is a single 2-gear set at each of the two rotors, and no gearing at the propeller.

http://www.unicopter.com/1474_Small.jpgClick for large view of the above drawing. (http://www.unicopter.com/1474_Interleaving_All_Electric.jpg)
Then click on magnifying glass mouse pointer, for larger view.

Dave

[Dave_Jackson]

slowrotor

This might be of interest since it is smilier to the concept in your posting #3.
http://appft1.uspto.gov/netacgi/nph-Parser?TERM1=20070057113&Sect1=PTO1&Sect2=HITOFF&d=PG01&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.html&r=0&f=S&l=50

Dave

[slowrotor]

Dave,
That patent looks pretty close to where I am going!
If I could just figure what a "plurality of electric fans" meant.

You are good finding patents. I don't put much faith in patents, let us see the actual thing fly first before they ask for a patent.

But my plan of moving from STOL gradually to VTOL may still apply.
And it looks like the military futurists are looking at electric fans also.
slowrotor

[NickLappos]

slowrotor,
That idea of ten little motors spinning you into the air has little merit, for two reasons:

1) A number of smaller disks never gets the same disk loading as the single large disk of a helicopter, because all those little inscribed circles that you envision cannot equal the area of the single large helo rotor. Try to draw 10 circles inside one big one, and shade in the area that the small rotors waste - the area between the little rotors that is "lost" because you have too many disks. This lost area is about 40%, and demands that your little rotor solution always has higher disk loading and less efficiency and uses more power than a single rotor solution. Don't think of shrouded rotors as a solution, shrouded rotors never gain in efficiency what they lose in weight over open rotors. The few lift systems that used shrouded props all failed.

2) Think hard about why cars do not have 6 small engines - the greater the number of power contributers, the less the overall efficiency of a system. A twin engine aircraft is about 10 to 20% less efficient than a single, and a three engine is 20 to 30% less efficient, and so on. That is why the world is going the opposite direction that you are in seeking fewer engines to solve the performance problem.

[Dave_Jackson]

Nick, I agree with what you are saying, however, for the fun of technical discussions, the following expansion of your two points is raised; in reverse order.


2) It is appearing that future advancements in electromechanical drives will present new opportunities and require some rethinking. This is one example of electric cars that have a separate motor in each of the four wheels (http://www.autoblog.com/2005/08/24/mitsubishi-testing-lancer-evo-with-in-wheel-electric-motors/).

You have noted in the past that Igor waited until the efficiencies of engines reached a favorable level before he went back to helicopters. This same situation is probably repeating today as people try to apply electric drives to the inefficient rotorcraft.


1) 'Disk loading' is a convenient way of evaluating thrust/weight considerations. However, I think that it looses much of it validity when evaluating comparative rotor configurations.
This has to do with the fact that 50% of a rotor's thrust is coming from the outer 25% of it's disk area. IMHO, one of the strong arguments for the Interleaving Configuration is its distribution of the thrust about the total disk area.
A bunch of babbling on this subject. (http://www.unicopter.com/1121.html)


Dave

[Tandemrotor]

As far as effeciency is concerned, a single rotor system always suffers from torque, and the power 'wasted' to counteract it.

In a multirotor device, all the power acts vertically, thereby optimising lift.

Personally, I prefer tandems! :rolleyes:

[slowrotor]

Thanks Nick,
I am trying to design an airplane with powered lift to allow flight at a lower speed only for takeoff and landing. The powered lift would only be used about 5 seconds or so. The efficiency is not important in this case I contend.

This is a lesson I learned when you were discussing the lack of efficiency of the Harrier jump jet. I think a Harrier test pilot said the Harrier only used about 100 lbs of fuel to make a landing. Sure the Harrier is not efficient in hover but that is not the mission. I also watched a C-130 equipped with 8 JATO rockets take off in a short distance and climb briefly very steep. JATO is not efficient for long haul but probably more efficient for just the needed 15 second thrust.

I think the future of personal VTOL aircraft is more likely to be fixed wing with brief powered lift. It's a different mission than a working helicopter that must hover for long periods.

[Dave_Jackson]

OK, it's time to get serious.
Make the wing out of this special fabric (http://www.unicopter.com/1481.html). Thereby creating a low pressure above the wing and a high pressure under the wing.

Sorry for the use of the word 'pressure'. I go now.

[Dave_Jackson]

slowrotor,

How about this, with a ballistic parachute for the craft or a personal parachute for the pilot; just in-case?

http://youtube.com/watch?v=dmSCyy0_OX4

[Graviman]

Slowrotor, if the mission is just to climb fast i have read books that prove that afterburners are as weight efficient as an additional engine (look at the EE Lightning). For long duration supersonic cruise the world had to wait for combustion chamber engineers to figure how to burn all that fuel, and turbine engineers to handle the temperatures (Concorde cheated with 4 engines). My point is that if there already exists a technology to give improved all round performance, why limit the machine to just that mission?

For +250kias i am convinced that X2 represents the next transition in aircraft capability. If you design a multiple rotor machine you are designing in antiquety. Study X2 and figure how you can make that more cost effective for reduced power/weight piston engines would be my starting point.

Actually i quite like Dave's latest interleaver concepts (but don't tell him that). My experience as a design engineer has taught me that the solution always works it's way towards the simplest design. This is because the commercial pressures on a product force the fewest parts to do as much as physically possible. To me one rotor above the other is simpler than X-shafts.

Dave, i think we've established use of the word pressure is acceptable, if not wholly accurate. Incompressible gas is a mathematical convenience, which it seems does not trully apply to the real world. I'm itching to get some CFD time to understand this better...

[slowrotor]

Dave, Mart,

Cool video of the Hiller platform.
Check this video of a C-130 with 30 jato bottles for assisted lift.
This is what am I trying to do, just get extreme STOL with some type of assisted lift but from multiple propfans rather than rockets. They were planning to land and takeoff the C-130 from a soccer field!
http://www.youtube.com/watch?v=8YOtm9UCQEc

Read the story here:http://en.wikipedia.org/wiki/Operation_Credible_Sport


Mart, Afterburners are not in my price range, but maybe more efficient than rockets.
slowrotor

[Graviman]

Slowrotor, stick to hovercraft - they look safer!

[soupisgoodfood]

Don't forget the 10 electronic speed controllers and the extra wiring for all that. Not sure how it would work out, though.

[Graviman]

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.

[Dave_Jackson]

Graviman Again SCAS can compensate for any assymetry for a single rotor. http://www.unicopter.com/Think.gifOK. 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.

http://www.unicopter.com/Sikorsky_RVRC.jpg

How can SCAS compensate for asymmetry on this single rotor helicopter during a power-out transition from cruise to autorotation?


Dave

[Graviman]

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.

[Dave_Jackson]

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

[Graviman]

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.

[Dave_Jackson]

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. :uhoh:


More, if interested. (http://www.unicopter.com/1281.html)

[Graviman]

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.

[soupisgoodfood]

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.

Do FBW and auto-pilot systems count as increasing complexity?

[Graviman]

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.

[Dave_Jackson]

Graviman,As developmental complexity goes, no. What language do you use when writing your computer code?

[soupisgoodfood]

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.

[Graviman]

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...

[soupisgoodfood]

You could say the same about 4WD cars.

[Graviman]

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...

[Dave_Jackson]

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.What language do you use when writing your computer code? and/or tell me what is today's cost per line of coding?


Dave

[Graviman]

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...

[soupisgoodfood]

I'm not quite sure where we're are going with this, but I'll just reply to some of the points anyway.


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.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.


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?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.


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...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.


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.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?


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.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.


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.

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.

[Graviman]

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...