Low disc loading by incorporating large number of electric driven propellers

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karoliina.t.salminen

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I was watching a video about some EADS concept jet where the jet engines were replaced with electric ducted fans. And actually several such ducted fans instead of like two. And I was also looking some unducted fan information at the same time. And what we know is:
- it is beneficial to move a large amount of air a little compared to moving a small amount very fast, so low disc loading is good
- propeller size is limited by physical dimensions of the aircraft typically, e.g. ground clearance considerations
- what if we have one gasoline engine in a hybrid configuration where the gasoline engine turns generator and there are several electric motors turning props which have pitch speed at very high speed, in other words, they produce not so good thrust at low speed, however, because the penalty of adding more of them is smaller with electric technology, what if we had many props that have too high pitch speed and they would together have enough thrust for takeoff and at the same time the optimum cruise speed would be fairly high as the pitch speed of the prop would not be a limiting factor and no complicated mechanics would be needed (for adjusting prop pitch).

So this is the idea that I got today:
- very low disc loading by using multiple props to divide the load to large air mass
- high pitch speed (for high speed cruise)
- and one (or two if redundancy is desired) gasoline engine turning the generator, i.e. motorcycle or car engine. Motorcycle engine would be more ideal because the continuous high rpm (~e.g. 8000 rpm cruise) will allow using smaller and lighter weight generator as the generator can suffice with higher KV.
 

Jay Kempf

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I was watching a video about some EADS concept jet where the jet engines were replaced with electric ducted fans. And actually several such ducted fans instead of like two. And I was also looking some unducted fan information at the same time. And what we know is:
- it is beneficial to move a large amount of air a little compared to moving a small amount very fast, so low disc loading is good
- propeller size is limited by physical dimensions of the aircraft typically, e.g. ground clearance considerations
- what if we have one gasoline engine in a hybrid configuration where the gasoline engine turns generator and there are several electric motors turning props which have pitch speed at very high speed, in other words, they produce not so good thrust at low speed, however, because the penalty of adding more of them is smaller with electric technology, what if we had many props that have too high pitch speed and they would together have enough thrust for takeoff and at the same time the optimum cruise speed would be fairly high as the pitch speed of the prop would not be a limiting factor and no complicated mechanics would be needed (for adjusting prop pitch).

So this is the idea that I got today:
- very low disc loading by using multiple props to divide the load to large air mass
- high pitch speed (for high speed cruise)
- and one (or two if redundancy is desired) gasoline engine turning the generator, i.e. motorcycle or car engine. Motorcycle engine would be more ideal because the continuous high rpm (~e.g. 8000 rpm cruise) will allow using smaller and lighter weight generator as the generator can suffice with higher KV.
It works but the efficiency of the drive system is key. But IC engines are traditionally low in efficiency so the idea of being able to run the generator at peak efficiency at all times is maybe a place to look for as you have already surmised. Over powering high pitched props has been done for a long time and low disk loading will help to optimize that so that is a good thought. Smaller electric motors are available right now although a bit pricey and they have good data for converting continuous Watts to thrust. Multiple smaller electric motors can help a lot of design issues except asymmetrical thrust procedures which is probably not much of a show stopper. So yeah, if you sum up the overall efficiency of the completely installed system including some sort of power to weight factor and cost you should be able to compare directly to a common IC installation. One thing that I think could be a benefit is to have some larger diameter props for climb that could be shut down and folded for cruise. Another way to look at that is to idle say two of 4 motors so they aren't producing drag but also aren't consuming much power. You'll probably still need some batteries to optimize your generator operation. Multiple generators means being able to shut one or more down but then you are carrying the weight penalty at cruise.
 

dino

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Another advantage of multiple props would be blowing large lengths of the wing's the upper surface perhaps in conjunction with flaps for utilizing Coanda effect.

Dino
 

Dan Thomas

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One of the big efficiency losses in propellers is the tip vortex. Lots of propellers offer lots of tip vortices.

A propeller half the diameter of a larger prop has a disc only one quarter the area of the larger prop.

Helicopters use large-span rotors rather than short-span propellers to get that big disc area. A 150 hp engine turning a 30-foot rotor can lift a loaded two-place helicopter, but if it was spinning a six-foot rotor it wouldn't lift the helicopter, even unloaded. It probably wouldn't lift much more than the engine and transmission.

Dan
 

Head in the clouds

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One of the big efficiency losses in propellers is the tip vortex. Lots of propellers offer lots of tip vortices.

A propeller half the diameter of a larger prop has a disc only one quarter the area of the larger prop.

Helicopters use large-span rotors rather than short-span propellers to get that big disc area. A 150 hp engine turning a 30-foot rotor can lift a loaded two-place helicopter, but if it was spinning a six-foot rotor it wouldn't lift the helicopter, even unloaded. It probably wouldn't lift much more than the engine and transmission.

Dan
True enough about the one 6ft rotor, but if you took the same 150hp engine and used it to drive 4x 15ft rotors (by your example approximately equal to the disc area of the one 30ft rotor) it would lift the helicopter just fine. The efficiency would be down a bit but perhaps not as much as might at first be expected. Certainly there would be greater losses from the power transmission through 4x gearboxes/driveshafts but in theory the tip vortex loss of each of the four 15ft rotors would be about a quarter as much as the tip vortex loss from one 30ft rotor.
 

BBerson

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For high speed flight, high disc loading is needed to enable the high speed. Race planes have small props, slow planes have large props.
Low disc loading is efficient for low speed, but at high speed the extra blade area would create excess drag.
 

Aircar

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I took it that Karoliina was not referring to VTOL (helicopter) but to multiple horizontal axis props versus only one or two as usual -- the Aerovironment Pathfinder and Helios have already demonstrated this approach -using electric motors as well . I referred to the idea of a fairly large number of small propellers/motors as being safer than a twin because of the much smaller percentage loss of power and the less asymmetry compared to the WORST possible case -the wing mounted twin . A recent thread but can't recall the title of it - got no response.
 

Hot Wings

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safer than a twin because of the much smaller percentage loss of power and the less asymmetry compared to the WORST possible case -the wing mounted twin .
And since all of those electric motors each have their own controller it would be a fairly simple next step to monitor the watts going to each side and incorporate a smart master controller that automatically balances each side if one motor were to fail. It's not quite that simple since there are several possible failure modes, but still might be worth implementing if you have several small motors.
 

DangerZone

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I took it that Karoliina was not referring to VTOL (helicopter) but to multiple horizontal axis props versus only one or two as usual -- the Aerovironment Pathfinder and Helios have already demonstrated this approach -using electric motors as well . I referred to the idea of a fairly large number of small propellers/motors as being safer than a twin because of the much smaller percentage loss of power and the less asymmetry compared to the WORST possible case -the wing mounted twin . A recent thread but can't recall the title of it - got no response.
It seems Dan has pointed in the right direction when it concerns multiple horizontal axis props, the elecric Cri Cri has shown that there is more efficiency in having two bigger and stronger electric motors than four or more smaller ones. It's just an example of existing aircraft, this is the 4 electric motor Cri Cri aircraft concept:
cri-cri-interview.jpg
...and this is the similar electric Cri Cri with only 2 motors:
cricri283.png

Even though the efficiency in theory should be better on the side of the 4 motor version due to the counter rotating props, the twin motor Cri Cri flew faster and better.

The multiple electric motor concept has indeed some advantage in vertical flight where speed or efficiency might not be needed. For example, the E-Volo team has had very interesting projects in the past few years with vertical and sustained flight.
81d21a98-b61f-48a9-abe5-2d99356ae40e.Full.jpg
[video=youtube;l-00ExudUA4]http://www.youtube.com/watch?v=l-00ExudUA4[/video]
e-volo
This is a true low disc loading example, as the 'wing loading' is spread over a wide surface to allow good and safe vertical flying. The downside is tremendous drag which slows the aircraft for any fuel efficient travel. What I mean by fuel efficient is the fact that most electric airplanes use a range extender in the form of a small gasoline/gas/hydrogen electric generator to travel long distances.

Then there's the nicely designed EADS electric ducted fan or shrouded prop airplane which hasn't shown signs of exploiting the advantages of electric propulsion, neither for efficient flying nor for vertical take-off.
eads_e-fan_2013Paris_480x320.jpg
It is a well made electric flying airplane but with the money they spent, a true vertical take off and efficient electric airplane could have been built. Some five years ago I designed an ultralight twin tandem electric amphibian which had better aerodynamics, weight and balance and efficiency than this multi million project. The only obstacles in such projects is money and EASA legislation which is holding back many good projects, so I guess they figured how to get rid of the red tape burden and finish building the airplane. I admire them and their project for that, I wish we small homebuilders had laws for homebuilt airplanes in Europe as you guys across the pond in the USA. Without having the support of many EU institutions, a couple of universities and lots of other big shots who only wanna join in just for the ride, or having to file 17 kilos of paperwork during the building process.

Karolina has a point in having multiple smaller electric motors IF weight saving is needed. As Hot Wings pointed it out, the good thing about spreading the disc loading is the weight of the smaller motor controllers. Every controller for a 5kW electric motor could be built much lighter that a single big boy for a larger electric motor. For example, voltage can be raised to have higher rpm turns and less heat losses which means lighter wires than those needed for a motor that spins at half the rpm and has a stronger moment (torque). We had to 'beef up' most of the wires and connections when more current was needed, more 'beef' meaning more copper, more copper meaning more weight. My 5kW continuos BLDC controller weighs a bit less than 3 kilos with the wires. A controller for an Emrax weighs about 20kg for their 30kW motor. Add that to the equation and more smaller motors start to make sense. The batteries are heavy themselves so counting on less than an above average person's weight in the battery compartment shows that there is a reason why many of them opt for lighter, simpler and less efficient electric motors. The controller of a Plettenberg Predator 37 which can produce sustained flight at around 5kW (15kW peak for a few seconds) is the size of a pack of cigarettes and weighs less than a kilo with the wires, the motor itself is around 2kg with the carbon prop. The advantage of electric motors is that they are simple and efficient. Head in the clouds already wrote that the benefits of no shafts, PSRUs, and other heavy stuff reduce the weight of the aircraft some. Electric motors could run almost forever if well designed and done, there's no maintenance cause there is no friction except the bearings, and there are lots of good quality bearings today. In the end, it all comes down to what you want to achieve, what kind of flight you have in mind. The technology is there, the knowledge also, all you need is no administration limits and quite an amount of money to achieve whatever you desire.
 
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karoliina.t.salminen

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And since all of those electric motors each have their own controller it would be a fairly simple next step to monitor the watts going to each side and incorporate a smart master controller that automatically balances each side if one motor were to fail. It's not quite that simple since there are several possible failure modes, but still might be worth implementing if you have several small motors.
This is what I have been thinking.
There is no need to balance the motors out manually in case of motor out.

Yes, and blowing could be possible with such a line thruster thing.
 

karoliina.t.salminen

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True enough about the one 6ft rotor, but if you took the same 150hp engine and used it to drive 4x 15ft rotors (by your example approximately equal to the disc area of the one 30ft rotor) it would lift the helicopter just fine. The efficiency would be down a bit but perhaps not as much as might at first be expected. Certainly there would be greater losses from the power transmission through 4x gearboxes/driveshafts but in theory the tip vortex loss of each of the four 15ft rotors would be about a quarter as much as the tip vortex loss from one 30ft rotor.
It is possible that I have misunderstood what you meant, but interpreting you the way I understood your explanation this answer is valid: I don't think you can add up the losses from the 4 gearboxes together.
Lets take a simplified example which is easy to calculate in head.
If your claim was true, this would be true also:
If you would have 25% losses in each gearbox and you would add them up together like this, and you would have 4 props (e.g. a quadrocopter) and a gearbox for each. You would sum them up: 25%+25%+25+25% = 100%. Therefore with your theory being correct the quadrocopter would not lift in the air at all.

I think the total losses are the losses combined per 4 props. If each of these would top to 100%, and would have losses of 25%, it would be
100% loss from 400%. That is 1/4 and also same as 25%. Now we end up with little thinking that the losses combined is the average of the losses per multicopter. And the multicopter lifts to the air happily as a single rotor copter would.

Then the question of tip losses, I think it has nothing to do with number of propellers/motors. I think the tip losses have everything to do with number of blade endings per a single disc. However, combining multiple discs again will have tip losses averaging all props together, not adding them up together because if each thruster loses 15% energy for tip losses, you can't end up with 0% or negative efficiency by using 7 props. Actually the negative efficiency would be interesting because if you would go then backwards :).

I this is the same thing as the speed gains from speed modifications to a certain airframe. You can not add them up together and end up with a system that has 0% drag, instead normal percent math applies.

For high speed flight, high disc loading is needed to enable the high speed. Race planes have small props, slow planes have large props.
Low disc loading is efficient for low speed, but at high speed the extra blade area


And have you thought why exactly is that so? What causes the more drag in case of larger prop exactly.

Because I now have a theory that with multiple small props with total system low disc loading, you could combine these two:
- high speed flight and low disc loading

This is exactly what I was after in my original post.
How to make a prop driven plane which cruises both fast speed and and still takes off with reasonable amount of power without needing e.g. 10000 horsepower.


 

Head in the clouds

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Responding to Karoliina's post#13 above, yes I think that is how the efficiency has to be considered. The losses, whether through gearboxes or tip vortices are a percentage of the power being transmitted though each of those units, not a percentage of the total power in the system. So I think that 4 rotors/propellors, each of 1/4 the area of one larger rotor/prop, if transmitting the same total power will have a combined tip vortex loss quite similar to the one larger rotor/prop, assuming similar disc area loading of the smaller and larger discs. If that is the case then it shouldn't matter whether you have one or fifty rotors/props, as far as tip losses are concerned, and again assuming that the loading is the same on the one or the fifty. And by using electric motors we avoid the need for geared transmissions of course.

Autoreply provided some very interesting input on this very subject in the IC engine to electric motor/Series hybrid thread, I'll see if I can dig it out. I do recall that his theory suggested there was great advantage to be had with electric, in being able to use cruise pitched props over-powered for short periods during takeoff and climb, and thus avoid the need for pitch-adjustable or constant speed props.
 

Aviator168

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I was watching a video about some EADS concept jet where the jet engines were replaced with electric ducted fans. And actually several such ducted fans instead of like two. And I was also looking some unducted fan information at the same time. And what we know is:
- it is beneficial to move a large amount of air a little compared to moving a small amount very fast, so low disc loading is good
- propeller size is limited by physical dimensions of the aircraft typically, e.g. ground clearance considerations
- what if we have one gasoline engine in a hybrid configuration where the gasoline engine turns generator and there are several electric motors turning props which have pitch speed at very high speed, in other words, they produce not so good thrust at low speed, however, because the penalty of adding more of them is smaller with electric technology, what if we had many props that have too high pitch speed and they would together have enough thrust for takeoff and at the same time the optimum cruise speed would be fairly high as the pitch speed of the prop would not be a limiting factor and no complicated mechanics would be needed (for adjusting prop pitch).

So this is the idea that I got today:
- very low disc loading by using multiple props to divide the load to large air mass
- high pitch speed (for high speed cruise)
- and one (or two if redundancy is desired) gasoline engine turning the generator, i.e. motorcycle or car engine. Motorcycle engine would be more ideal because the continuous high rpm (~e.g. 8000 rpm cruise) will allow using smaller and lighter weight generator as the generator can suffice with higher KV.
Why not use a very large "prop" like the fanwing.
 

Jay Kempf

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For high speed flight, high disc loading is needed to enable the high speed. Race planes have small props, slow planes have large props.
Low disc loading is efficient for low speed, but at high speed the extra blade area would create excess drag.
Yes, but the key to what she was saying was optimizing for high speed cruise by sacrificing low speed thrust efficiency. In that case if you have adequate power on tap to drive the cruise prop lightly loaded to takeoff and climb then you have an optimized small cruise prop. If you can shut down some of the props you then have a highly loaded cruise prop that is near optimum. I do think that that works if carefully thought out. In cruise with the right wing the weight penalty is not as much of a factor RE: carrying the extra HP just like any motor that is oversized for cruise. So let's say you could put a generator inline with one IC motor and one prop that wasn't optimized for takeoff. And then you had two boost electric motors and props used for takeoff that had folding props optimized for takeoff and climb thrust. At cruise you only use the IC motor and you store energy for another climb or takeoff. It isn't much of a leap from there to get rid of the IC driven prop and just have a generator and three electric motors, one for cruise and two to augment climb and takeoff.

And just a further thought: if you combine this thinking with the kind of asymmetric thinking that went into the Rutan Boomerang you end up with many many configuration possibilities that can enhance safety of operations not hurt them.
 

autoreply

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Autoreply provided some very interesting input on this very subject in the IC engine to electric motor/Series hybrid thread, I'll see if I can dig it out. I do recall that his theory suggested there was great advantage to be had with electric, in being able to use cruise pitched props over-powered for short periods during takeoff and climb, and thus avoid the need for pitch-adjustable or constant speed props.
Yep.

Ignore the numbers, because they're just for the sake of the argument. If you're climbing @ 60 kts and then cruising @ 120 kts with 20 hp, it's not too hard to work out how much power you need to reach cruise rpm in climb. Since power density for electric motors is so high, there's barely a penalty in just shoving in enough power to reach the same climb rpm as cruise rpm. Even power density of batteries is quickly becoming a non-issue; the Saft V5(LM?) already does a few kW/kg.

Then you can simply optimize a cruise prop and still have a very efficient climb prop, without any of the complexities of variable pitch props.
Some of the electric sailplanes already do this (Binder EB29 and I think also the Antares)
 

Hot Wings

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And have you thought why exactly is that so? What causes the more drag in case of larger prop exactly.
Props and wings work the same way.

You have the F=MA2 part of the problem identified. When we talk about props for some reason the mass flow seems to be the parameter of choice. When we talk about wings the lift to drag ratio seems to be the dominant parameter. Both need to be considered. If mass flow were the only factor to be considered then the formula indicates that longer is better because more mass gets moved less. Large diameter props and TINSFS glider wings are the result.

But when you factor in the lift to drag curve for your performance curve at some point the span/diameter become a disadvantage because the L/D ratio tends to favor larger lift coefficients. A typical L/D curve looks more like negative parabola with the right leg having a limit of Y=0.
 

BBerson

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And have you thought why exactly is that so? What causes the more drag in case of larger prop exactly.

Because I now have a theory that with multiple small props with total system low disc loading, you could combine these two:
- high speed flight and low disc loading

This is exactly what I was after in my original post.
How to make a prop driven plane which cruises both fast speed and and still takes off with reasonable amount of power without needing e.g. 10000 horsepower.


[/COLOR]
I guess I don't understand your questions?
High speed flight requires a high disc loading prop that accelerates the flow somewhat faster than the cruise speed.
Low speed flight is better with low disc loading.
But a prop can't be both without changing the diameter.

The disc loading is defined as pounds of thrust per square foot of disc area.
High speed flight became possible when very high disc loading turbine engines were invented. Later the disc loading was lowered some with the turbofan, but still very high disc loading.
Now if you want to replace the turbine engine with electric, the propulsor (call it prop, fan, whatever) still needs to be the same size.
 

BBerson

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Yes, but the key to what she was saying was optimizing for high speed cruise by sacrificing low speed thrust efficiency. In that case if you have adequate power on tap to drive the cruise prop lightly loaded to takeoff and climb then you have an optimized small cruise prop. If you can shut down some of the props you then have a highly loaded cruise prop that is near optimum. I do think that that works if carefully thought out. In cruise with the right wing the weight penalty is not as much of a factor RE: carrying the extra HP just like any motor that is oversized for cruise. So let's say you could put a generator inline with one IC motor and one prop that wasn't optimized for takeoff. And then you had two boost electric motors and props used for takeoff that had folding props optimized for takeoff and climb thrust. At cruise you only use the IC motor and you store energy for another climb or takeoff. It isn't much of a leap from there to get rid of the IC driven prop and just have a generator and three electric motors, one for cruise and two to augment climb and takeoff.

And just a further thought: if you combine this thinking with the kind of asymmetric thinking that went into the Rutan Boomerang you end up with many many configuration possibilities that can enhance safety of operations not hurt them.
But Karoliina didn't propose this.
The proposal as I see it, was to make an electric turbofan.

I am just saying it will be high disc loading for high speed.
 
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