Multiple rotor vs single rotor efficiency
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From: Port Townsend,WA. USA
Multiple rotor vs single rotor efficiency
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
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
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Slowrotor
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
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
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From: Port Townsend,WA. USA
General Aviation VTOL
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
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
Joined: Apr 2003
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From: Vancouver, BC, Canada
slowrotor.
delta3's points are good ones.
A couple of additional comments.
This might also be of interest; Electrotor-Simplex
_____________
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, without the rubber-band, and then dropping it from an elevation might prove the belief.
Dave
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. This might also be of interest; Electrotor-Simplex
_____________
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, without the rubber-band, and then dropping it from an elevation might prove the belief.
Dave
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From: Cambridgeshire, UK
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.
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.
Last edited by Graviman; 16th December 2007 at 11:33.
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From: Vancouver, BC, Canada
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.
Click for large view of the above drawing.
Then click on magnifying glass mouse pointer, for larger view.
Dave
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.
Then click on magnifying glass mouse pointer, for larger view.
Dave
Joined: Apr 2003
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From: Vancouver, BC, Canada
slowrotor
This might be of interest since it is smilier to the concept in your posting #3.
http://appft1.uspto.gov/netacgi/nph-...l&r=0&f=S&l=50
Dave
This might be of interest since it is smilier to the concept in your posting #3.
http://appft1.uspto.gov/netacgi/nph-...l&r=0&f=S&l=50
Dave
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From: Port Townsend,WA. USA
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
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
Joined: Apr 2003
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From: USA
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.
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.
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From: Vancouver, BC, Canada
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.
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.
Dave
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.
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.
Dave
Last edited by Dave_Jackson; 21st December 2007 at 04:13. Reason: Spelling
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From: Nova
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!
In a multirotor device, all the power acts vertically, thereby optimising lift.
Personally, I prefer tandems!
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From: Port Townsend,WA. USA
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.
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.
Joined: Apr 2003
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From: Vancouver, BC, Canada
OK, it's time to get serious.
Make the wing out of this special fabric. 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.
Make the wing out of this special fabric. 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.
Joined: Apr 2003
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From: Vancouver, BC, Canada
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
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
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From: Cambridgeshire, UK
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...
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...
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From: Port Townsend,WA. USA
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
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