Electric propulsion and Superconductivity

Discussion in 'General Experimental Aviation Questions' started by Ethan Appleton, May 19, 2019.

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  1. May 19, 2019 #1

    Ethan Appleton

    Ethan Appleton

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    Current student for my A&P- love the airframe and such but the engines confuse me. I want to build my own sport plane to fly about with but I am planning on doing the first to be fully electric. First question I really have is how can I not put a CVT tranny of sorts into the linkage of prop to engine. Electric motors in vehicles don't have to use trannys but they do to maximize range. If on take off im using full throttle for power and torque to the propeller why is it not possible to lower the rpm like a car does once at speed. I really don't understand how to ask the question because In my mind if the proper is spinning at 100rpm and the engine at 250 then if I can maintain 100rpm on the prop with lowering the engine rpm to 175 will the plane still fly or not? My vocabulary isn't made up of engineering terms, I get that I should be in that field but I don't want to spend 4 years and listen to people telling me my ideas are crazy because of some formula. plus I suck at math! Second to this question is- Is it possible to use liquid nitrogen to freeze an average 4hp electric engine to the point that the coils and wires will act as a super conductive material thus lowering resistance and increase electron flow making more power and using less energy? From what I gather via the internet it should be possible. way I picture my idea is placing the batteries and all supply lines along with the electric motor in a sealed case filled with liquid nitrogen. (the liquid nitrogen I believe weighs similar to av gas soo) the idea was to use the nitrogen as a super cooler to turn the materials in the motor and the wiers and batteries into a system where no resitance existed thus maximizing my pwer to weight given my batteries are going to weigh a tone and so with the liquied. If I have the idea to use this what are the chances it actually works and can somone maybe help me understand enough about the more engineering side of stuff like this because I want to know why not ust a no it wont work or I fix tha **** thing don't make it? Really do want to make this idea possible and ive been sketching some ideas to build the frame for it but if im going to be lacking in power im going to need a larger wing which may mean I need to add more motors and at some point I may have to add some osrt of fueld engine to provide power to the electric motors.
     
  2. May 19, 2019 #2

    akwrencher

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    I got a little lost in your post, but I can answer one question. Batteries don't like to be cold. Electric cars sometimes have to heat their batteries in cold weather. No batteries that are commonly available on the consumer market, as far as I know, like cold. There are successful electric airplanes, I would start by studying what worked for them. That will give you some ideas of what works, and you can go from there. If you want to design a clean sheet airplane, you will want to learn some math. I don't know it, and will stick to plans built. There are some on this forum who do have the skills and knowledge, and you will be well advised to listen carefully to whay they have to say, even if it's not what you want to hear. The laws of physics work for everyone the same......

    I'll leave the cvt and superconducting for the smart folks.....
     
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  3. May 19, 2019 #3

    dougbush

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    Liquid nitrogen works just like sweat -- it cools as it is evaporating.

    To make batteries last, motors should be operated as efficiently as possible. You may find a motor's most efficient rpm is around 90% of its no load rpm, so lowering motor rpm in cruise could be counter-productive.
     
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  4. May 19, 2019 #4

    Dana

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    The aeronautical equivalent of a CVT is a variable pitch propeller. A "constant speed" prop is the most common type. As for superconducting with liquid nitrogen, that's something that's not practical even in ground based systems. Tanks to hold liquid nitrogen are extremely heavy, and they're constantly venting, the cooling effect of the evaporation helps to keep the rest of the liquid cold.

    As for being the first, a number of all electric planes have flown and I'm pretty sure a couple are commercially available. Batteries are the limiting factor; you'll never be able to pack as much energy in a battery as in a comparable weight or volume of gasoline. Google "energy density".
     
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  5. May 19, 2019 #5

    wsimpso1

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    Ethan, When you are asking a bunch of strangers to help you with difficult topics, learn how to use paragraph breaks.

    If you are going to talk airplane design, propellor theory, and superconductivity, mathematical relationships are needed.

    Props - Torque to turn a fixed pitch prop goes with the square of rotation speed, and the power required goes with the cube of rotation speed. If the torque to turn a prop at 1000 rpm is 48 ft-lb, and you double its speed to 2000 rpm, torque required is 48ft-lb*(2000/1000)^2 = 192 ft-lb.

    The work per second to turn the prop is power. Power is force times speed, torque is rotary force, and rpm is rotary speed. The relationship is HP = Tq (in ft-lb) * rpm/5252. In a more basic sense, power to turn a prop goes with the cube of rpm. That same prop at 48 ft-lb and 1000 rpm is 9 hp, but at 2000 rpm it is 73 hp.

    Now for the big leap, at max engine power point of 2700 rpm, torque is 350 ft-lb and power is 180 hp.

    You takeoff at 100% power and climb, then you level off and go down to a cruise power setting. 75% is pretty common. Where would we be? If you are running a fixed pitch prop, rpm would only need to go to about 91% of max, or about 2453 rpm. Torque would drop to only about 83% of max or about 289 ft-lb.

    Go fly an airplane and you find that as speed comes up (and air inflow speed to the prop increases), the engine will speed up some more, to the point where some airplanes will overspeed the engine if you do not pull back the throttle some, but most will make cruise power at somewhat higher rpm and somewhat lower torque.

    Now if you want to maintain the same prop rpm but reduce the torque and thus the power, you can not do that. To keep the prop rpm, you have to maintain the torque and the power... And to make the kinds of power reductions for cruise that are typical, well, the rpm reductions are pretty small, so not much need for a gear box with ratios.

    As Dana pointed out, we do have an effective device. Constant speed props help us with the way that the prop unloads with increasing inflow velocity. Set the manifold pressure and the prop rpm (yeah, engine rpm goes up and down with the prop rpm) and as you level off and accelerate to cruise speed, the prop adds pitch angle to the blades to keep the rpm where you set it. Want to cruise, pull the throttle back some and then the prop back some to any of the combinations for your desired cruise power... When the prop blades are sized properly, this works really well.

    As to super conductivity, you need something a lot colder than liquid nitrogen. Most wiring needs about 4K, which is way colder than liquid nitorgen which is about 77K. Stuff used at Cern etc. is liquid helium. Then you have to keep it that cold, which requires a cryo plant. Then for it to be of benefit, the additional power you get from your motor has to be enough justify running the cryo plant and carrying it aloft in the airplane. The work to turn the prop still must go into the wires at the generator and come out at the motor, all that superconductivity does for you is reduce you line losses, which is a few percent.

    In short, you asked questions where the answers already exist.

    Billski
     
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  6. May 19, 2019 #6

    TFF

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    I would suggest getting into the RC hobby with some of the bigger airplanes. At least 10 lb size. Pretty big for electric. That is pretty much state of the art electric for consumer parts. Once you understand what is out there and how to manipulate it, then you can see if your ideas can apply.

    Generally with crazy ideas, they needs crazy people to just do it to see if it works or not. Also even if it’s a great idea, can you build it with the money you have and tools and skill you have? Sometimes you just have to strike out on your own to know.
     
  7. May 20, 2019 #7

    Ethan Appleton

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    Sorry! typed it up really did not think much would come? I can modestly understand your points. Didn't know about the liquid helium. I saw a video on youtube where a guy put a little dc motor in liquid nitrogen and it made it run faster.
    Question tho? if I run my motorcycle in 2nd or 3d gear I can modestly get up and go so if an engine was set up like that for a plane what would happen? really just want to know if it would work for something simple like a C172?
    I do fly I have 13 hrs on me uninstructed and I do notice that the prop increases and decreases speed and such with airflow or engine power. tho in the electric planes I saw they all have direct drive to the prop. understandably its because the torque is there. I may make no sense but if there was even just a slight 2 to 1 gear increase would anything change in performance?
     
  8. May 20, 2019 #8

    Ethan Appleton

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    Sorry! typed it up really did not think much would come? I can modestly understand your points. Didn't know about the liquid helium. I saw a video on YouTube where a guy put a little dc motor in liquid nitrogen and it made it run faster.
    Question though? if I run my motorcycle in 2nd or 3d gear I can modestly get up and go so if an engine was set up like that for a plane what would happen? really just want to know if it would work for something simple like a C172?
    I do fly I have 13 hrs. on me uninstructed and I do notice that the prop increases and decreases speed and such with airflow or engine power. though in the electric planes I saw they all have direct drive to the prop. understandably its because the torque is there. I may make no sense but if there was even just a slight 2 to 1 gear increase would anything change in performance?
     
  9. May 20, 2019 #9

    Ethan Appleton

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    I looked into that and I came up with the idea of flushing the system before flight ( great idea given the original was to save money on fuel) chilling all the components and then flying till it warmed up again and needed to be rechilled.
     
  10. May 20, 2019 #10

    Ethan Appleton

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    this may make more sense- if I have an electric motor tied to a CVT tranny then to a prop and have the motor running just to spin the prop at a steady rpm of 100 and increase the CVT pullies what will happen? Will not the prop increase rpm and would it not be the same as if it was directly attached to the motor. (I understand with a gas engine it would stall the engine but electric motors I would just have to apply a bit more power till I believe it would maintain and steady out)
     
  11. May 20, 2019 #11

    TFF

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    One thing you have to know is the prop rpm is picked by the diameter. Shifting gears is not what a prop needs. The rpm is under supersonic speed at the tip. Remember a prop is just a wing attached to an engine. Faster it spins the more lift it makes. That is until it goes supersonic; then probably a whole bunch of bad happens. Props are designed to be at best speed, around 70-80% of Mach 1 at the tip. Shifting gears does not help in the same way. A car or motorcycle cycle engine has a small range of power; notice every shift the engine struggles a little bit more to accelerate. The ratios are getting smaller. An airplane prop is already set to top gear, essentially. Because rpm is maxed out for prop design; changing the prop pitch works well for changing thrust as speed picks up. There are gearbox airplane engines but for the most part they have problems of complexity. The engine is spinning faster than the prop for power, but the physics dictates prop rpm at a certain diameter. Gearbox is to match the two. Years ago a special airplane engined drone did use a gearbox that was shifted. It needed it because it flew so high; the air was too thin for the prop to work. It was flying something like 60,000 ft. Abnormal for props. Right now the technology for electric is at the beginning. Ultra lights are being flown some with electric. Useable regular airplanes are a long way away. You can make horsepower easy. You can’t make endurance with the batteries of today. Let’s use a 172 to the max on a road trip. To fly a 172 4 hours on batteries with today’s technology would require about 3000 lbs of batteries. The heaviest a 172 can be with all the people aboard and full of fuel is about 2500 lbs. 35 gallons of gas is about 210 lbs. Subtract that from 3000 and you need 2790 lbs of magic to get the same performance. That is the ugly truth about batteries. You can make a plane like a 172 fly for about 15 minutes and hope the batteries don’t catch fire. Lighter more efficient airplanes are being designed. Always advances with batteries and motors. But technology is a long way from the same performance. It will get there or we may be flying behind helium 3 reactors. Time will tell.
     
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  12. May 20, 2019 #12

    akwrencher

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    The short answer for the cvt is that it just doesn't work and or is not needed for driving propellers. The useful rpm range for a given propeller is too narrow for it to be useful, and the loads are just different than ground bound vehicles. You cant compare the two, no matter how tempting. Re read post 5 carefully.


    Edit: much better reply above. Cross posted.
     
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  13. May 20, 2019 #13

    Tiger Tim

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    Many answers are beating around the bush on the subject of why you don’t really want to shift gears on an airplane propeller but the actual truth is that the aviation equivalent has existed for a very long time: the variable pitch propeller. Hear me out.

    The circumference of a wheel describes how far it will travel in one revolution and the larger the wheel the faster the vehicle will go at a given RPM, the trade off being that it gets progressively harder to turn the wheel as it gets bigger. Since it’s hard to change wheel sizes to suit every condition, we use a transmission to vary how far the wheel turns per engine revolution.

    The pitch of a propeller describes how far it will carve its way through the air in one revolution and the steeper the angle of the blades the faster the airplane should go at a given RPM, the trade off being that it gets progressively harder to turn for a couple of reasons. Since we can’t change propellers in flight to suit every condition, we change the blade angles to vary how far the plane flies per engine revolution.

    As you can see, both methods accomplish the same task but are tailored to the conditions they operate with. While the wheel ideally can’t slip, the propeller can since it operates in a fluid (that fluid being air). Because of this it’s an awful lot harder to load up an airplane engine to the point where it stalls so in many cases a designer will use a fixed pitch propeller that’s a compromise in performance but saves a bunch of weight and complexity.

    I hope that helps explain how you basically had the right idea, just not quite the right way to achieve it.
     
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  14. May 20, 2019 #14

    wsimpso1

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    Ethan,

    First, I caution you on thinking that engineering a two speed gearbox that is durable, reliable, simple, light, and cheap is easily done. 23 years of doing it for a living here, I can tell you it can be done. Done for production, it is a substantial task. Done for a one-off?

    You would need one gear for full power takeoff and climb, another that is overdriven by about 9%. If you were to need much altitude capability, you might need another ratio too... A step of 9%is really a small difference. In cars and motorcycles, the gear ratio steps are usually on the order of 30% or more. The gears would have to be kind of big but light for their size, and finding COTS gears just is not going to happen, so you will be commissioning gears, shafts, ratio changing hardware, etc. Carrying a gear box capable of two or three ratios, seamlessly changing back and forth between them, capable of carrying the prop would be a substantial weight penalty.

    The constant speed prop allows you to select any engine speed and any manifold pressure that makes the power you want, really reliable compared to the gear box solution, and usually way lighter too.

    Billski
     
  15. May 20, 2019 #15

    wsimpso1

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    Let's make something really clear here. Road vehicles connect directly to the road and apply tractive effort. Force is applied through the tread patch. Props (on airplanes and even on boats) run differently. They change engine power into moving fluid. The amount of air they move times how much they change the speed of the air is the thrust force. Boat propellors move a much higher density fluid and so they can effect a much smaller volume, but it is the same thing.

    Every prop has a relatively narrow range of rpm where it can actually react all (or much) the engine torque into the air. Change torque to the prop and it changes speed until the prop and engine torque are again in equilibrium.

    If you have an engine through prop system that is right for takeoff and climb, allowing the engine to make full power in that speed regime, and then you shifted to a 2:1 ratio, (that's upshifting two or three gears on your motorcycle) and you maintained prop speed by dropping the engine to half speed - one divided by two - the engine will now be at half speed and less torque. If we assumed a pretty flat torque curve and it is 90% torque down there, you are now at 0.90 (engine torque)*0.50(1 over the gear ratio) = 0.45 or 45% torque and power is 45%*50% = 22.5% - in most airplanes, little useful will happen, maybe cruise at best glide speed. but to go further, the prop still requires 100% torque to spin at that speed, but you only have 45% torque available to turn the prop, so the prop has to slow down too, and pretty soon the engine is modestly extending your glide from if the engine were not running at all..

    Switch it around to downshifting 2:1 and try to raise the engine rpm for takeoff? Well, once the engine hits its redline, you will be turning the prop slowly, and it will have to be really big to absorb the torque you are making...

    If ratio change gearboxes on the output were any use, they would have gone to them more than a century ago.

    Billski
     
  16. May 20, 2019 #16

    Derswede

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    I figure that energy densities of batteries will reach usable levels about the time I buy my first Mr. Fusion. I am sticking with IC engines for the moment. Looked at the flexible solar panels for awhile, added too much weight per KW. The idea of freezing an engine for superconductivity is a neat idea, it would last until about the forth or fifth power stroke of the engine, as the generated heat would warm it out of such a state rather quickly. Remember playing with super cooled fluids in college, one guy wanted to replace oil with a super slick super cooled fluid (say that 4 times fast!!). He forgot that IC engines use the oil supply for some cooling as well.

    Derswede
     
  17. May 20, 2019 #17

    Swampyankee

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    Liquid nitrogen temperature superconductors would make a lot people very happy, as electric generators, motors, and transmission lines would become a lot more efficient, and liquid nitrogen is cheap.
     
  18. May 20, 2019 #18

    Derswede

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  19. May 22, 2019 #19

    Ethan Appleton

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    So its primarily the torque that is needed to make the prop work. Is that due to the weight of the aircraft? Does this same info apply to ducted fans or UDF engines from GE, even turbine blades. I really appreciate the more detailed answer it makes sense why it would not be practical to. If I took the same idea and applied it to a jet what would happen there. Do you by chance know anything about contra rotating props? I have been trying to understand those too!
     
  20. May 22, 2019 #20

    BJC

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    It takes both a force (torque) and motion (RPM) to do work (accelerate a mass of air). Accelerating the air mass results in a force (thrust) that acts to propel the airplane.


    BJC
     
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