Bell X-Hawk
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IFMU,
Yes, it would have an extremely poor lift-to-power ratio, as others have mentioned.
IMHO, it is doubtful that any 'self contained' craft with ducted lift fans will ever become a viable VTOL machine. Even the Harrier is said to use uses rolling takeoffs when possible, to conserve fuel and increase the payload.
However, if the electrical energy (batteries, fuel cells, etc.) is kept on the ground vehicle then the craft might be viable for specific applications. Perhaps, fire departments might replace one or more of their ladder trucks with this aerial platform.
Just groping for applications.
Then again, it's Friday night and there are better things to be groping.
Dave
Yes, it would have an extremely poor lift-to-power ratio, as others have mentioned.
IMHO, it is doubtful that any 'self contained' craft with ducted lift fans will ever become a viable VTOL machine. Even the Harrier is said to use uses rolling takeoffs when possible, to conserve fuel and increase the payload.
However, if the electrical energy (batteries, fuel cells, etc.) is kept on the ground vehicle then the craft might be viable for specific applications. Perhaps, fire departments might replace one or more of their ladder trucks with this aerial platform.
Just groping for applications.
Then again, it's Friday night and there are better things to be groping.
Dave
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Dave,
The real point being made here is that no system without mechanical control can achieve the analytical 1E-9 failsafe requirement. I had forgotten FBW has mechanical backup (actually this also highlights why i think the mechanical Lockheed gyro system should be the norm - but that is another thread).
Agreed that a rescue platform is a good objective for rotorcraft engineers, but it has to be safe enough to generate a market. I have similary wondered about a firefighting platform which consisted of a conventional rotorcraft with turboshaft driven feed pump supplying water from the ground, to overcome cavitation. This means the machine could stay on the spot and deliver a huge quantity of water to a given point. Obviously the head feed pump would need to be built into a truck or van, capable of carrying water or connecting to a water supply. Well it's on the web now...
The real question to ask here is how to calculate, and more importantly demonstrate, that an electrical system really meets 1E-9 failsafe requirement. A TP with Nick's experience will justifiably shy away from any development program until that basic criterion can be met - something about fools going where angels fear to tread?
Mart
The real point being made here is that no system without mechanical control can achieve the analytical 1E-9 failsafe requirement. I had forgotten FBW has mechanical backup (actually this also highlights why i think the mechanical Lockheed gyro system should be the norm - but that is another thread).
Agreed that a rescue platform is a good objective for rotorcraft engineers, but it has to be safe enough to generate a market. I have similary wondered about a firefighting platform which consisted of a conventional rotorcraft with turboshaft driven feed pump supplying water from the ground, to overcome cavitation. This means the machine could stay on the spot and deliver a huge quantity of water to a given point. Obviously the head feed pump would need to be built into a truck or van, capable of carrying water or connecting to a water supply. Well it's on the web now...
The real question to ask here is how to calculate, and more importantly demonstrate, that an electrical system really meets 1E-9 failsafe requirement. A TP with Nick's experience will justifiably shy away from any development program until that basic criterion can be met - something about fools going where angels fear to tread?
Mart
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No Graviman, you miss the point, unfortunately. The typical FBW system has NO mechanical backup.
What is at issue is that the Xhawk, like the Harrier, must have engines running to have control at all. In effect, its engines are its FBW system. But unlike modern computer systems, engines are not so reliable as to trust even large numbers of them with the actual survival of the aircraft.
In short, all current passenger aircraft must maintain flight safety after total engine power loss.
If the Xhawk is to be occupied by any form of commercial passenger (if it is to be FAA/JAA certified) it too must show that it can be controlled to a safe landing if the engines quit. That is a double problem for the Xhawk, because it derives both lift and control solely from its engines. It has no autorotation capability, and no control without its fans somehow receiving engine power.
Any aircraft that cannot show fundamental lift and fundamental flight control to a reliability of about 10e9 cannot be certified, and I believe the Xhawk is one of those unfortunates.
What is at issue is that the Xhawk, like the Harrier, must have engines running to have control at all. In effect, its engines are its FBW system. But unlike modern computer systems, engines are not so reliable as to trust even large numbers of them with the actual survival of the aircraft.
In short, all current passenger aircraft must maintain flight safety after total engine power loss.
If the Xhawk is to be occupied by any form of commercial passenger (if it is to be FAA/JAA certified) it too must show that it can be controlled to a safe landing if the engines quit. That is a double problem for the Xhawk, because it derives both lift and control solely from its engines. It has no autorotation capability, and no control without its fans somehow receiving engine power.
Any aircraft that cannot show fundamental lift and fundamental flight control to a reliability of about 10e9 cannot be certified, and I believe the Xhawk is one of those unfortunates.
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This is all understood, Nick. My point is that the prime mover, turboshaft say, could be backed up by a brushless motor design. These really have no parts to wear and are as reliable as FBW, often being critical parts of those systems. The motor could act as a generator/motor in a parallel hybrid or as with a seperate generator in a series hybrid. There would have to be a power store like a battery (or better flywheel motor/gen), with sufficient energy to get the crew down safely from ceiling height.
I accept that this design will never have the appeal of a machine that can autorotate, and i'm not trying to justify that. What i am saying is that for applications where exposed rotors are just to risky, another solution has to be found - albeit less cost effective. I am just suggesting a possible method to generate a wrinkle rather than an outright frown from the FAA/JAA.
Mart
I accept that this design will never have the appeal of a machine that can autorotate, and i'm not trying to justify that. What i am saying is that for applications where exposed rotors are just to risky, another solution has to be found - albeit less cost effective. I am just suggesting a possible method to generate a wrinkle rather than an outright frown from the FAA/JAA.
Mart
Last edited by Graviman; 9th Sep 2006 at 17:23.
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The thread is turning into a technical one.
The participants in this thread, and logic, appear to agree that using ducted fans for lift is verboten, at least for the foreseeable future. However, IMHO, electric drives and control have a very promising near-term future.
As you have said Mart, existing brushless motors have no moving parts, except for a couple of bearings. These bearings could be air or magnetic bearings in the future, thereby creating a motor with no components to wear out.
Additional safety is available now, by simply connecting the windings of a single motor into two or three separate groups (in addition to the 3-phase grouping). Each group will have its own controller and power supply. In a motor with three groupings, if one of these highly reliable electric groups was to fail, the motor can still produce 2/3 of its nominal torque. Also, electrical motors can deliver 3x to 10x their nominal torque for a few seconds without damage.
Axial flux motors produce high torque at slow rotational speeds. An axial flux motor can be directly coupled to a helicopter rotor. The weight of a mechanical transmission, plus its safety concerns, are totally eliminated. The power/weight ratio of electric motors is also improving by the use of existing Halbach magnets and the potential of carbon nanotube wire.
The storage of electrical power is the main obstacle, however billions of dollars will be spent to overcome this problem. Here is one of many current research projects. Initially the benefit will be realized in ground vehicles, then airplanes, and eventually rotorcraft.
In the early days of helicopters, at least one aspirant waited for a sufficiently powerful engine to come along before starting to develop his rotor control concepts. I suggest that this time around electric rotorcraft research and development should be conducted now, so that there is a timely marriage of motor, battery and integrative helicopter.
Anyone want to participate in the development toward man's first electrical rotorcraft flight?
The participants in this thread, and logic, appear to agree that using ducted fans for lift is verboten, at least for the foreseeable future. However, IMHO, electric drives and control have a very promising near-term future.
As you have said Mart, existing brushless motors have no moving parts, except for a couple of bearings. These bearings could be air or magnetic bearings in the future, thereby creating a motor with no components to wear out.
Additional safety is available now, by simply connecting the windings of a single motor into two or three separate groups (in addition to the 3-phase grouping). Each group will have its own controller and power supply. In a motor with three groupings, if one of these highly reliable electric groups was to fail, the motor can still produce 2/3 of its nominal torque. Also, electrical motors can deliver 3x to 10x their nominal torque for a few seconds without damage.
Axial flux motors produce high torque at slow rotational speeds. An axial flux motor can be directly coupled to a helicopter rotor. The weight of a mechanical transmission, plus its safety concerns, are totally eliminated. The power/weight ratio of electric motors is also improving by the use of existing Halbach magnets and the potential of carbon nanotube wire.
The storage of electrical power is the main obstacle, however billions of dollars will be spent to overcome this problem. Here is one of many current research projects. Initially the benefit will be realized in ground vehicles, then airplanes, and eventually rotorcraft.
In the early days of helicopters, at least one aspirant waited for a sufficiently powerful engine to come along before starting to develop his rotor control concepts. I suggest that this time around electric rotorcraft research and development should be conducted now, so that there is a timely marriage of motor, battery and integrative helicopter.
Anyone want to participate in the development toward man's first electrical rotorcraft flight?
Anybody who's into electric r/c can certainly tell you that even brushless motors can fail mechanically (magnets can get thrown among other things) - as can the batteries or motor controller. They aren't a magic bullet of 100% reliability, they're just a lot better than brushed motors or small nitro powered engines
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Folks, stop me if i'm going too far off Rotorheads territory.
Brett,
For passenger carrying the motor and all systems would have to be developed/tested for the 1E-9 loadcases. As with airframes this would not always guarantee 100% failsafe.
Dave,
Motors are certainly getting to the point where direct drive is possible, but i wouldn't reject the humble gearbox until you have done a good concept study of ALL the available systems. Axial flux or "pancake" motors are the best way to minimise the flux return paths (ie steel), but RPM is the best way to achieve power/weight. Besides epicyclic gearboxes are compact and reliable, and package very well in a motor housing.
The big problem is going to be the power source for some time to come. You can couple a turboshaft to a generator (turbogenset had a good direct drive concept before they went bust). The question is why would you want to cost in a generator & motor when a driveshaft is just as effective? In the case of high disk loading ducted fans, i was proposing the use of motors only to overcome the inherent lack of autorotation during engine failure. To my mind the solution has to be fuel cell that will operate on kerosine. This sounds far fetched but think about what it has to replace. Turboshafts are:
1. Cost effective.
2. Reliable.
3. Efficient.
4. Low weight
5. Infrastructure supported.
6. Durable.
7. Improving by development.
This means that the successor has a lot to live up to to be commercially accepted. In the long run i believe this is practical, but you will see it on the road long before you see it in the air. The main reason for this is convenience, since service will be well in excess of the 100+ hours between service that folks now expect from their car.
If you just want to be in Guinness, then stick with LiMH battery packs. For a short duration it will pack a good enough punch to get airborn. Ultracaps are being developed primarily as a means of regenerative braking, but offer no real advantage in a heli.
Mart
Brett,
For passenger carrying the motor and all systems would have to be developed/tested for the 1E-9 loadcases. As with airframes this would not always guarantee 100% failsafe.
Dave,
Motors are certainly getting to the point where direct drive is possible, but i wouldn't reject the humble gearbox until you have done a good concept study of ALL the available systems. Axial flux or "pancake" motors are the best way to minimise the flux return paths (ie steel), but RPM is the best way to achieve power/weight. Besides epicyclic gearboxes are compact and reliable, and package very well in a motor housing.
The big problem is going to be the power source for some time to come. You can couple a turboshaft to a generator (turbogenset had a good direct drive concept before they went bust). The question is why would you want to cost in a generator & motor when a driveshaft is just as effective? In the case of high disk loading ducted fans, i was proposing the use of motors only to overcome the inherent lack of autorotation during engine failure. To my mind the solution has to be fuel cell that will operate on kerosine. This sounds far fetched but think about what it has to replace. Turboshafts are:
1. Cost effective.
2. Reliable.
3. Efficient.
4. Low weight
5. Infrastructure supported.
6. Durable.
7. Improving by development.
This means that the successor has a lot to live up to to be commercially accepted. In the long run i believe this is practical, but you will see it on the road long before you see it in the air. The main reason for this is convenience, since service will be well in excess of the 100+ hours between service that folks now expect from their car.
If you just want to be in Guinness, then stick with LiMH battery packs. For a short duration it will pack a good enough punch to get airborn. Ultracaps are being developed primarily as a means of regenerative braking, but offer no real advantage in a heli.
Mart
Last edited by Graviman; 9th Sep 2006 at 22:57.
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The thread is still in Rotorhead territory but it is sliding off topic. 
Brett,
Thanks for mentioning a couple of potential problems.
Do you happen to know if any of the RC electric motors use a Halbach array? The reason for asking is that, in addition to their performance advantage, all the Halbach array magnets are tightly abutting the adjacent magnets. This may give better battery fixation than the conventional surface mounting or the embedding of the magnet in the rotor's epoxy.
Theoretically, the failure of a controller or battery pack will not be critical. It will only reduce the available continuous-power.
Mart,
From the motors perspective ~ yes. However, in this application I wonder. A gearbox requires a minimum of two reductions and one of them must be a right angle set. Perhaps if Sikorsky gets their face gear patent to work, the proceeding statement may be a lie. 
The use of an axial flux (pancake, disk) motor conjures up the possibility of locating it in the rotor hub. By adding electrical blade pitch to the rotor, the power train and the flight controls, as we now know them, will ceases to exist.
DESIGN OF AXIAL-FLUX PERMANENT-MAGNET LOW-SPEED MACHINES AND PERFORMANCE COMPARISON BETWEEN RADIAL-FLUX AND AXIAL-FLUX MACHINES
I wonder. The ultra capacitor will offer a high power/weight ratio but a low power/volume ratio. These two ratios suggest that the capacitors could be located in the tail boom. The small amount of power that they would hold could be used to supplement the batteries during takeoff and high maneuvering, plus with regeneration they could provided the power for an autorotation landing flare.
Energy Storage in Advanced Vehicle Systems
Dave
Brett,
Thanks for mentioning a couple of potential problems.
Do you happen to know if any of the RC electric motors use a Halbach array? The reason for asking is that, in addition to their performance advantage, all the Halbach array magnets are tightly abutting the adjacent magnets. This may give better battery fixation than the conventional surface mounting or the embedding of the magnet in the rotor's epoxy.
Theoretically, the failure of a controller or battery pack will not be critical. It will only reduce the available continuous-power.
Mart,
"RPM is the best way to achieve power/weight."
The use of an axial flux (pancake, disk) motor conjures up the possibility of locating it in the rotor hub. By adding electrical blade pitch to the rotor, the power train and the flight controls, as we now know them, will ceases to exist.
DESIGN OF AXIAL-FLUX PERMANENT-MAGNET LOW-SPEED MACHINES AND PERFORMANCE COMPARISON BETWEEN RADIAL-FLUX AND AXIAL-FLUX MACHINES
"Ultracaps are being developed primarily as a means of regenerative braking, but offer no real advantage in a heli."
Energy Storage in Advanced Vehicle Systems
Dave
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While electrics seems like a way out of the woods, the weight of a horsepower is too much for any flight system to allow electrics to be used on anything but a solar powered glider. The batteries in a Prius car carry about 40 WH/Kg (see: http://www.peve.panasonic.co.jp/catalog/e_maru.html)
Since the Xhawk carries 2 x T800 engines (which put out about 3000 HP total) one must believe to make a controlled crash, the vehicle needs at least 1000 HP. If the emergency landing takes 6 minutes then it needs about 1800 Kg of batteries alone (no motor yet) to solve the landing problem (1000Hp=724KW, for 6 minutes it is 72.4 KW-H of energy, at .04 WH/Kg, that means 1810Kg of batteries!). Since the Xhawk weighs 3000 Kg, that means it is 60% battery just to be safe for humans!
I would wish someone else would offer to do the heavy lifting around here!
Since the Xhawk carries 2 x T800 engines (which put out about 3000 HP total) one must believe to make a controlled crash, the vehicle needs at least 1000 HP. If the emergency landing takes 6 minutes then it needs about 1800 Kg of batteries alone (no motor yet) to solve the landing problem (1000Hp=724KW, for 6 minutes it is 72.4 KW-H of energy, at .04 WH/Kg, that means 1810Kg of batteries!). Since the Xhawk weighs 3000 Kg, that means it is 60% battery just to be safe for humans!
I would wish someone else would offer to do the heavy lifting around here!
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Nick,
This may lift your spirits a little.
Lithium Technology Corporation is currently producing the 60 Ah HE-602050 Lithium Ion cell battery, with an energy density of 150 WH/Kg.
Moli Energy (Canada) Limited is currently producing the model ICR-18650J Lithium Ion cell battery, with an energy density of 188 Wh/kg
It appears that the Zinc-Air battery is (or will be) 300 Wh/kg
With hundreds of companies spending billions of dollars on batteries, fuel cells and Ultracapacitors, the future should be very enlightening.
Dave
This may lift your spirits a little.
The batteries in a Prius car carry about 40 WH/Kg
Moli Energy (Canada) Limited is currently producing the model ICR-18650J Lithium Ion cell battery, with an energy density of 188 Wh/kg
It appears that the Zinc-Air battery is (or will be) 300 Wh/kg
With hundreds of companies spending billions of dollars on batteries, fuel cells and Ultracapacitors, the future should be very enlightening.
Dave
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Dave,
A 2 or 3 stage epicyclic is still the most compact soln (sum input, planet output - so annulus can be structural). For a given torque requirement I would estimate the weight of a motor/gearbox, then motor direct drive. Motor weight is pretty much linearly dependant on torque, for estimates.
Nick,
Actually the humble Duracell battery offers 144WH/kg, since this is emergency power, and can be trickle charged ~10 times without the manufacturers consent (with the right charger - yes i do have one
). Other technology: Sodium Sulphur offers 150Wh/kg (at 350'C internal temp 
), and flywheels about 130Wh/kg. Dave is right about battery energy density slowly increasing through development, but still only enough for a short hop.
You have hit the nail on the head though that even this "solution" is only suited for specialised applications. Emergency power offers a solution near the ground, but from height the pilot would have the fun of dropping like a stone then using the several minutes of power to generate say 2g (consuming more power) before the final descent. The machine really needs a parachute to carry any useful payload at height - perhaps not ideal for EMS.
Duracell energy density (page 9):
http://www.duracell.com/oem/Pdf/othe...y%20density%22
Sodium-Sulphur:
http://www.powerstream.com/BatteryFAQ.html
Flywheel:
http://en.wikipedia.org/wiki/Flywheel_energy_storage
Actually flywheel power could be designed into a mechanical only solution, as long as switchable gear ratio allowed a sensible RRPM range (more weight
). I admit that this thread has convinced me about low disk loading being the best solution...
Mart
A 2 or 3 stage epicyclic is still the most compact soln (sum input, planet output - so annulus can be structural). For a given torque requirement I would estimate the weight of a motor/gearbox, then motor direct drive. Motor weight is pretty much linearly dependant on torque, for estimates.
Nick,
Actually the humble Duracell battery offers 144WH/kg, since this is emergency power, and can be trickle charged ~10 times without the manufacturers consent (with the right charger - yes i do have one
). Other technology: Sodium Sulphur offers 150Wh/kg (at 350'C internal temp ), and flywheels about 130Wh/kg. Dave is right about battery energy density slowly increasing through development, but still only enough for a short hop.You have hit the nail on the head though that even this "solution" is only suited for specialised applications. Emergency power offers a solution near the ground, but from height the pilot would have the fun of dropping like a stone then using the several minutes of power to generate say 2g (consuming more power) before the final descent. The machine really needs a parachute to carry any useful payload at height - perhaps not ideal for EMS.
Duracell energy density (page 9):
http://www.duracell.com/oem/Pdf/othe...y%20density%22
Sodium-Sulphur:
http://www.powerstream.com/BatteryFAQ.html
Flywheel:
http://en.wikipedia.org/wiki/Flywheel_energy_storage
Actually flywheel power could be designed into a mechanical only solution, as long as switchable gear ratio allowed a sensible RRPM range (more weight
). I admit that this thread has convinced me about low disk loading being the best solution...Mart
Last edited by Graviman; 10th Sep 2006 at 11:11. Reason: Duracell information
The hell with batteries. I'm waiting for Mr. Fusion, like in the Back to the Future movie. That's the real answer.
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Mart,
OK. And the Federation Aeronautique Internationale. 
It can't get any simpler;
Dave
"If you just want to be in Guinness, ...."
It can't get any simpler;
- Side-by-side configuration.
- 2-blade teetering rotors.
- 10 HP axial-flux direct drive motor at both rotors.
- Batteries located under the rotors.
- Blade pairs are interlocked with positive pitches.
- Collective control by RPM of rotors.
- Cyclic control by spider and two Bowden cables, per rotor.
Dave
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Current electric models seem to have more short term power than fuel powered models. I recently watched a model airplane descend vertically in a deep stall and just before ground contact the aircraft pitched up and hovered then climbed out vertical at a high rate.
Power failure is not a problem, the wing works as a parachute when at high angle of attack.
The future helicopter is a vertical take off airplane!
slowrotor
Power failure is not a problem, the wing works as a parachute when at high angle of attack.
The future helicopter is a vertical take off airplane!
slowrotor
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Slowrotor,
In support of what you say, this 30-lb model airplane can fly vertical on its prop, driven by a small 3-lb electric motor.
Kitplanes had three articles in 2002 and 2003 discussing the ongoing work on a Lafayette III 2-seat airplane with UQM electric motor and controller. It is/was intended to have a 100-mile range. Article 1, Article 2, Article 3
Article three stated that they had begun taxi tests, with temporary batteries. The next article was going to discuss the energy source of power. Unfortunately, there has not been a fourth article. Perhaps they are waiting for a viable power source to be developed.
You said;
Maybe yes. Maybe no.
IMHO, two primary requirements must be met to achieve efficient vertical takeoff+landing and efficient fast forward flight. These two requirements are; a power source and an aerodynamic shape that can 'morph' to be efficient in both modes of flight.
The electric motor, with its ability to provide very high short-term torque, may eventually solve the power requirement.
Slowed rotors c/w propulsors may solve the aerodynamic requirement.
Dave
In support of what you say, this 30-lb model airplane can fly vertical on its prop, driven by a small 3-lb electric motor.
Kitplanes had three articles in 2002 and 2003 discussing the ongoing work on a Lafayette III 2-seat airplane with UQM electric motor and controller. It is/was intended to have a 100-mile range. Article 1, Article 2, Article 3
Article three stated that they had begun taxi tests, with temporary batteries. The next article was going to discuss the energy source of power. Unfortunately, there has not been a fourth article. Perhaps they are waiting for a viable power source to be developed.
You said;
The future helicopter is a vertical take off airplane!
IMHO, two primary requirements must be met to achieve efficient vertical takeoff+landing and efficient fast forward flight. These two requirements are; a power source and an aerodynamic shape that can 'morph' to be efficient in both modes of flight.
The electric motor, with its ability to provide very high short-term torque, may eventually solve the power requirement.
Slowed rotors c/w propulsors may solve the aerodynamic requirement.
Dave
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Dave,
Thanks for the interesting links.
I think an electric hybrid might work, using electrics for 30 seconds of direct lift for takeoff and landing. Use fuel for the cruise portion of the flight.
Multiple motors might be an advantage, say ten or more if they are cheap.
One or two motor failures would not be significant, if they each have a separate battery.
The problem now is batteries cannot put out high current in 30 seconds.
When they get that fixed a hybrid will make sense.
Thanks for the interesting links.
I think an electric hybrid might work, using electrics for 30 seconds of direct lift for takeoff and landing. Use fuel for the cruise portion of the flight.
Multiple motors might be an advantage, say ten or more if they are cheap.
One or two motor failures would not be significant, if they each have a separate battery.
The problem now is batteries cannot put out high current in 30 seconds.
When they get that fixed a hybrid will make sense.
