Electric and Counter Rotation

Discussion in 'Electric Propulsion' started by RonL, Apr 2, 2019.

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  1. May 22, 2019 #41

    BJC

    BJC

    BJC

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    Compare that with my Honda truck. Maximum acceleration uses 280 HP at about 6,800 RPM, but once I am cruising on the interstate highway at 80 MPH, the 6 cylinder engine has shut down 3 of cylinders and is operating at 2,200 RPM.


    BJC
     
  2. May 22, 2019 #42

    RonL

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    No particular relevance I guess, just something I think most pilots seem to face no matter which way they fly. :)
     
  3. May 22, 2019 #43

    pictsidhe

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    Compare that with my first car, with an amazing 29hp. Acceleration wasn't great. It didn't need full throttle to cruise at 70, though. My next car had 50hp and went pretty well, 95 flat out IIRC.
     
  4. May 23, 2019 #44

    daveklingler

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    Pity this thread got bogged down. Contra-rotating props are interesting, contra-rotating props on electric motors even more-so and kind of a killer app for electric motors on airplanes.
     
  5. May 23, 2019 #45

    proppastie

    proppastie

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    Yes but somewhat impractical for the average homebuilder. Probally no one here is doing it or planning to.
     
  6. May 23, 2019 #46

    RonL

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    Thanks for your comment, I have to take credit for not keeping control of my own thread.
    The thing I see with motors and generators, which can be in most cases the same physical unit, is that while motors are generally designed to perform in a continuous operating mode in ambient conditions, they can be (overclocked) for smaller time periods to USE or PRODUCE insane amounts of power without harmful effect to the unit.
    The inability on my part to convey what I know and what I see as possible, without violating the laws of physics, tends to turn people away.

    A generator at speed can be tapped to produce an energy spike that could be compared to a lightning discharge, without damage to the unit, a controlled capture of the energy and lower power distribution is what needs to be developed.
    In my mind, I see the vision of a breaker bar dropped on the terminals of an 8-D battery, I can say from experience, it's a fast method for melting lead. :eek:
     
  7. May 24, 2019 #47

    proppastie

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    I must be missing something, but I admit I have not studied or even listened to the video that close....I do see an advantage as to counterbalance of torque, but really would it not make more sense to add blades or ducted fan.....is the contention that the contra-rotating prop more efficient?
     
  8. May 24, 2019 #48

    henryk

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  9. May 24, 2019 #49

    Victor Bravo

    Victor Bravo

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    Quartzsite !?! Heavens to Murgatroyd Snagglepuss!

    The worst unexpected turbulence I had in recent memory was over Quartzsite, I think it was 1998, ferrying a Taylorcraft from Michigan to Los Angeles. I got hit hard from underneath and then immediately hit again just as hard from above, such that I was thrown up out of the seat and my head pushed the fabric up off of the top of the fuselage (2 wood stringers on the T-craft), and it ripped the glue joint apart.

    There was a private strip in Quartzsite that used to have the carcass of a Twin Beech sitting there frozen in time. I was tempted to land and clean out my shorts. That was now 20+ years ago. When I landed in Blythe (the next town after Quartzsite) I just threw in the towel for the day and went to a motel, even though I could have made it back to LA before dark.

    To answer your question, yes it is possible that there will be days when you can fly that kind of light multi-copter in Quartzsite. I do not believe those days will be common.
     
  10. May 24, 2019 #50

    pictsidhe

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    The problem is that you describe schemes that amount to perpetual motion. You show no maths or experiments to back things up. If it looks like a duck and quacks like a duck, we tend to assume that it is a duck.
    If you did the maths or experiment, you'd be disappointed...

    When I have a particularly fantastic idea, I stop and ask myself a very important question: If it is so easy, why has nobody done it before? Unfortunately, I have usually made an error somewhere. My idea turns out to be a dud.
     
  11. May 24, 2019 #51

    proppastie

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  12. May 24, 2019 #52

    proppastie

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    what about a 4 blade prop or ducted fan....what was the difference between 2 blade to 3 blade ......lots of questions .......the weight,complexity and gearbox friction of CR is not insignificant.
     
  13. May 24, 2019 #53

    wsimpso1

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    Maybe static thrust can be improved this way, but a bigger simple prop could do it too. Now static thrust and dynamic thrust are two different animals.

    You can do all sorts of things to make more thrust statically, even hover. Look at the human powered helicopters out there. Huge rotors, one human making about 1 hp can get them to just barely hover in ground effect for a few seconds.

    Just because you can make big thrust statically does not mean that that this thrust increase will hold up as you pick up some speed. Once you are moving at flight speeds, thrust is absolutely limited to the inverse of airspeed, and the following equation fits the world really well:

    HP*eff*550 ft-lb/sec = F*V

    which solves to

    Thrust possible = HP*eff*(550 ft-lb/s)/V

    Where HP is your horsepower at the prop flange,
    eff is your propellor efficiency which varies but will be around 85% and never reaches unity,
    550 ft-lb/s is the work of 1 hp, so turns hp in to terms of thrust and speed,
    V is velocity in Ft/s;

    Once you are going flight level speeds, the thrust you can make with the prop goes down inversely with speed. Yeah, if you make V small, the equation says you can make awesome thrust statically, but another relationship (with its own equations limits you too:

    mdot*dV = Thrust

    1/2*mdot*dV^2 = HP*eff*550 ft-lb/s

    where mdot is the mass flow of air per second through the prop, and dV is the change in velocity imparted. Statically, dV is the speed of the air through the prop and mdot is the mass density of air. If your prop got you the same dV everywhere through the disc (it does not so you can not do this well, but it gives us an upper bound), mdot = rho*dV*PI()*Dp^2/4 where rho is the mass density of air, dV is the same as befor and Dp is prop diameter. Substituting, we find that:

    (rho*dV^3*PI()*Dp^2)/8 = HP*eff*550 ft-lb/s

    Solving for V and D...

    dV^3*Dp^2 = (8*HP*eff*550)/(rho*PI())

    Then assuming a Dp to get the V limit

    dV^3 = (8*HP*eff*550)/(rho*PI()*Dp^2)

    Take the cube root of that last equation and you can compute thrust based upon prop diameter and horsepower... Once you have done that, there are practical limits on prop blade size and how much prop 1 hp can drag around in a circle, which then drives gearing, etc.

    I ran you guys a quick spreadsheet. All of this is off the top of my head, so I may have missed something in computation of the exact numbers, but all of the concepts are there. Big thing is that statically, you can get huge thrust by using big props (or rotors), but to fly anywhere, you also need dynamic propulsion. Pursuit of static thrust can get in the way of dynamic propulsion. Props end up being compromises based upon what you are doing. If you are running slalom courses or drag races from a standstill in an airboat, by all means, grab all the thrust the rules will allow. But if you want your airplane to go places, you might find other stuff to be important.

    Billski
     

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  14. May 24, 2019 #54

    pictsidhe

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    Contra rotating props main benefit is that they produce axial air flow. A conventional prop produces a helical flow. The tangential component usually being a pure loss. Thus a contra prop can run more efficiently than a regular prop, particularly at coarse pitches, where the tangential loss is higher in a regular prop. A conventional prop with a stator can also remove the tangential component. Shrouded props and ducted fans often have stators, they often double as structural supports.
     
    Last edited: May 25, 2019
  15. May 24, 2019 #55

    proppastie

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    I love it when I get a spread sheet to play with....Looking at my model engine for the Aluminum Dragon I have modified the inputs....My engine is rated at 20 hp..... with the RPM I was getting I estimate 15 hp static with my 32 inch prop.I changed the eff to .6 for the small high rpm prop.

    Now the estimates of static thrust go from "it will hover and climb a 45 lb model"......(50 lb?)....to 75 lb.. various other references.

    You spread sheet gives 495 lb (rule 1... 10 f/sec) and 109 lb rule 2 ....2.66 Dp....the second is in the ball park but still a ways off from others.

    I loved this spread sheet and "off the top of my head" is amazing.......I would love to have 495 lb of thrust.

    I may have an even less than .6 eff in which case your rule 2 may not be very far off at all.
     

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  16. May 24, 2019 #56

    pictsidhe

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    Javaprop is a great tool. It gives realistic numbers, too.
     
  17. May 24, 2019 #57

    henryk

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    =yes, ATLAS get circa 200 kG/HP !

    -old russian heli constructors formula (for hoover comparation purporse)=

    K= Specific Thrust [kG/HP] x sqr Specific Disc Load [kG/m^2]

    f.e. K=230/70 * sqr 230/2.27=3.3*10= 33 (CR system 1.7 m)

    K=26 for 1.7 m single propeller...
     
  18. May 24, 2019 #58

    henryk

    henryk

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    =100 HP differential gearbox have 2.5 kg mass (circa 0.5 l volume)

    and work at temperatur 40 deg.Celsie ...
     
  19. May 24, 2019 #59

    wsimpso1

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    Maybe. It sounds great - "I don't have all that energy in spiraling wash" - and that has its appeal to the point where a bunch of Roll Royce Griffons were built with contra rotating props and a Royal Navy airplane was produced with two turbines each running a prop on one shaft.

    We start somewhere around 85% efficiency with just a prop, then we run a second prop with the attendant losses in the gear train and in a second prop thrashing the air in order to get back some of 15% we were losing. Generally you do not make enough extra efficiency to carry the extra weight. Some folks have improved the net efficiency but only at specific points in the operating range, while the rest of the operating range breaks even or loses.

    If you want to do it, knock yourself out. A gear geek's party. But unless you scheme out something special or really new, it will likely be no more efficient as turning one good prop.

    Billski
     
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  20. May 24, 2019 #60

    wsimpso1

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    The first is the simple power limit, the second is what you power can get statically. At no point will your static thrust exceed the amount from the static estimate, and once your dynamic thrust is less than the static, it dominates. So, you the best you will be able to do is start your takeoff roll around 109 lb (second set of equations with your 2.66 ft prop), and once you get to 40 ft/s, it will commence dropping. Now if you do poorly on prop design, you won't even do that well...

    Billski
     
    Last edited: May 24, 2019
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