Vtol design: 1 vs 6 engines

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by leviterande, Oct 14, 2008.

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  1. Nov 15, 2008 #61

    BBerson

    BBerson

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    A coaxial has two individual rotors operating within the same descending column of air. The downwash of a coaxial is therefor higher, making the coaxial less efficient. The disc loading calculations of a coaxial should only include the area of one disc, not both.
    The coaxial has no tail rotor loss, so that helps.
    BB
     
  2. Nov 15, 2008 #62

    leviterande

    leviterande

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    Actually, the tandem would be the best solution to have best efficieny but the problem is the huge transmission losses to deliver the power to two rotors and adding structure-weight to two shafts and all the parts for two rotors on each side instead of one single coaxial shaft. so offcourse, the tandem gives more thrust per hp but practicaly I think that extra needed structure and complexity weight plus the higher transmission losses is to be accounted. I am no expert off course though.. I am learning and will never stop learning


    Kalle
     
  3. Nov 30, 2008 #63

    Myaircraftathome

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    This is the thread I've been wanting to see.I have two supersonic scram jet engines in the works specially for a disk shaped aircraft project to come out in 2011,but I'm not concerning them to take off and land vertically. To do so you could also put 3 or 4 or 5 engines around the center but you mainly want one fan just like your picture,If your looking into ducted fan propulsion go here
    its pretty cool the Jet Hawk 2 is my current project at the moment


    Mass Flow

    Uses Mazda 12a to 13b t rotory engines, vtol is a must with wankel engine
     
  4. Nov 30, 2008 #64

    leviterande

    leviterande

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    Hi, nice plane you building, is that your page?

    I realized that for making a succesfull vtol or any aircraft, the most important thing is the design. the start of the whole project is the most vital one.. if you start with faulty design, no matter how much time and money you have you will fail.. thats why i have been searcing for a long time for simply how the craft is going to look like. it is far more important than one might think.. the basic simple idea of any aircraft is the core of the project

    my current "I think" the final aim is the coaxial aproach. i would love t start with something simple as that guy with his worlds smallest heli http
    //www.youtube.com/watch?v=E2Pbq6cqY40

    I am not sure if i need articulation point or how much articulation is needed.

    offcourse the first thing concerning building would building an rc prototype. I have build a single non-computerized 10x4 propeller vtol and despite torqe and gyro effects it was more stable then any rc vtol i have seen. for the real size craft it must have a coaxial fixed pitch system and a propeller of 2.5meter.

    I have a question concerning engine out situation

    if we attach a wing to that little coaxial helicopter say of a "circular" wing area of 80sqm .. and it weighs say 200kg fullloaded.. it will mean it will have a disc loading of 2,5kg/m which is pretty low right?

    now if that little heli engine quits at an altitud of 100 meters shouldnt be able to paraglide safetly?


    Kalle
     
  5. Dec 17, 2008 #65

    leviterande

    leviterande

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    I have been looking at the tip driven designs where the rotors are free of torque because of the propellers/jets on the tips of the rotors

    I wonder though about the gyroscopic effects of the spinning rotor, will the gyro. forces and preccesion it be exactly like any ordinary helicopter?

    Kalle
     
  6. Dec 17, 2008 #66

    BBerson

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    Much more. The tip jet weight and heavier blades have more inertia. In fact, the tip jet powered Djinn had such heavy blades that the dead man curve was eliminated.

    Tip jets use a lot of fuel and are unacceptable noisy, usually.
    BB
     
  7. Dec 17, 2008 #67

    leviterande

    leviterande

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    okay , I never flew a heli before but i know that one has to take account for the gyroscopic precession when chinging the cyclic, so if you want to go forward the force must be aplied 90 degrees before the desired direction of travel. I know that coaxial is the ultimate solution to the gyroscopic precession but i though that gyroscopic precession would be less in a single torqueless rotor, specially if it is designed to rotate very slowly and with very low weights : electric motors instead of gas or jet.

    Kalle
     
  8. Dec 17, 2008 #68

    Dan Thomas

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    Any rotating mass has gyroscopic forces and how they're driven has no bearing on it.

    Slower rotors are useless in terms of forward speed. The difference in airspeed between advancing and retreating blades becomes an issue at much lower speeds, with the retreating blade stalling much sooner and over a wider area of the disc. Even with today's higher-speed rotors, no helicopter travels at much over 160 or 180 MPH.

    As I said once before, workable helicopters have been flying since around 1938. That's 70 years ago, and in those 70 years a vast number of experimental machines have been built, but the helicopters we see now are much the same as the first machines. There just isn't a lot of room for improvement on the ideas we have now. We'd need an entirely new mode of flight to create anything really new and useful.

    Dan
     
  9. Dec 17, 2008 #69

    BBerson

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    As Dan said, any rotating mass has the gyroscopic precession issue, even coaxial. It just requires that the cyclic be offset about 90 degrees. Some helicopters are not set at 90 degrees, it depends on the particular design.
    BB
     
  10. Dec 17, 2008 #70

    leviterande

    leviterande

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    there shouldnt be any gyroscopic precession issue on a coaxial:shock:?!
    how do you mean? I am talking about a fixed pitch co-axial, a fixed pitch coaxial has gyroscopic precession issues as well???


    Kalle
     
  11. Dec 17, 2008 #71

    Topaz

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    Each individual rotor in a coaxial arrangement will experience precession, but the two rotors would cancel each other out for the behavior of the entire assembly. In other words, you'll need to account for precession for each individual rotor, even in a coaxial system.
     
  12. Dec 17, 2008 #72

    leviterande

    leviterande

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    yeah that was what I understand.. there will be no effects when you are flying a fixed pitch coaxial. gyro forces will apear first when something breaks or cracks..offcourse the forces are there but they are eliminated

    since we talk about coaxials, they have bad efficiency compared to single rotors but...helicopters got tail rotors so shouldnt a proper coaxial be just as efficient as the helicopters ..practically. I find it hard to find information on that exact matter. the unicopter guy said from his webiste that you will need whole 50% more hp for the same thrust froma coaxial. but that doesnt sound logical in my opinion


    Kalle
     
  13. Dec 17, 2008 #73

    BBerson

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    Helicopters are not that simple.
    The flexible blades of a coaxial helicopter will "flapback" during forward flight due to the increased lift on the advancing blades and the 90 degree delay from precession. This tends to prevent further forward flight. These forces are not cancelled in a coaxial.
    BB
     
  14. Dec 17, 2008 #74

    Topaz

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    Well, that wasn't quite what I said. There may or may not be effects that the pilot can feel (and BBerson's post suggests that there still is), but even if the pilot couldn't feel it, each individual rotor in a coaxial system will still act as if it were all alone - precession is still just as valid as it would be on a single-rotor machine, from the point of view of that rotor. All the control and structural issues are still in effect.

    I don't know about 50%, but it sounds reasonable to expect that a coaxial system, absorbing a given horsepower, may produce less thrust than a single rotor designed to that same powerplant. The single rotor will be of a much larger diameter, and therefore more efficient than the smaller coaxial pair of rotors. The single rotor will have 'losses' from the power used to drive a tail-rotor, but the the more complicated mechanicals for the coaxial system (including the rather massive gearbox) will offset that somewhat. The net effect, IMHO, is that the larger single rotor will be the more efficient lifting device, and so produce more 'thrust'.

    In the end, it's probably picking nits - either system can be developed to a specific requirement, and it's unlikely that you'll need "maximum" performance for a sport helicopter anyway, so whatever small advantage one offers over the other is probably a moot point. Design what you like.

    If I was ever going to design a helicopter, I'd do a synchropter (meshing rotors), just because I think they look cool. :gig:
     
  15. Dec 18, 2008 #75

    Dan Thomas

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    In a coaxial the lower rotor is flying in the turbulent downwash of the top rotor, affecting its efficiency to a serious degree.

    Dan
     
  16. Dec 18, 2008 #76

    leviterande

    leviterande

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    that is my concern

    Topaz, not to say I dont agree with you but how come this platform is very stable. it seems to be absent of any gyroscopical issues. offcourse the gyro forces want to bend the metall structure but it doesnt want to affect the motion of the craft to even any degree

    he stands and shoots
    [video=youtube;aFS6pMvkjo8]http://www.youtube.com/watch?v=aFS6pMvkjo8[/video]

    kalle
     
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  17. Dec 18, 2008 #77

    Topaz

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    Neat video. :)

    You're looking at the overall control feel of the whole aircraft - I'm talking about the individual rotors. Even if the precession of the two individual rotors cancels each other out in terms of the whole aircraft (and from BBerson's post, that still may not be the case), that doesn't mean that they don't still precess. Any loads that this effect imposes on the structure of the individual rotors will still need to be dealt with in the design process. As the airframe pitches and rolls, each rotor will still precess individually, and that will need to be taken into account in terms of structural and aerodynamic effects, even if the net result is that from the pilot's point of view, the effects somehow cancel each other out.

    I'm not saying it can't be done, and obviously it has been. But there is no 'simple' helicopter design. It's not so simple as to put a couple of counter-rotating propellers over an engine and go flying away. The people who have tried seem to always end up with very limited control of the aircraft, and usually abandon it after the first few attempts at flight.

    There was a fixed-counterrotating-rotor helicopter out at Oshkosh some years ago (early '90s?) that apparently flew rather well, according to some independant pilots that flew the aircraft. The entire rotor head tilted as a unit for pitch and roll control with, IIRC, a complicated dual constant-velocity joint articulating the concentric driveshafts from the engine. Vanes in the downwash controlled yaw. The aircraft was described by the builders as a prototype, and that they were going to be developing a more advanced version as a kit. AFAIK, that never happened, and I have never heard any more about the aircraft. A couple of years later there was a report of some other builder creating a similar aircraft, but he crashed during one of the very first flights, apparently lacking even a semblance of control of the aircraft. Apparently even a fixed rotor design takes some serious analysis - there are a lot of factors to take into account.
     
    Last edited: Dec 18, 2008
  18. Dec 18, 2008 #78

    BBerson

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    Sikorsky and Hiller and others started with coaxial, then they switched to some other configuration.
    There must be a reason.


    On the other hand, Sikorsky is now experimenting with a high speed stiff blade coaxial called X-2. It has pusher prop to make it go fast.
    BB
     
  19. Dec 18, 2008 #79

    Topaz

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    Yeah, I understand that they're using a really stiff, non-flapping blade to get around the lift imbalance at high speeds. Once it's going fast enough, the advancing blade on each rotor (on opposite sides) is doing pretty much all of the lifting, and the retreating blades are producing next to no lift - and may even be in reverse flow if it's going fast enough. The rotor hubs must be incredibly strong and stiff to take that kind of asymmetrical loading!

    It'll be interesting to see how that works out.
     
  20. Dec 19, 2008 #80

    leviterande

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    so the coaxial system simply acts like a single rotor except its 45 degrees instead if 90?
     

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