Vtol design: 1 vs 6 engines

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

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  1. Dec 28, 2009 #81

    Swashplatebob

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    I don't have much experience building or flying VTOLS but I do know the theory behind them. To overcome the relatively large effort it takes to change the angular momentum and the rotor plane (ie pitch or roll), you have to lessen the radius of the props. This will sometimes give an advantage to coaxials. They can produce much higher thrust while not requiring so much effort or proplength. However, if you lessen the radius, the props will have to spin a lot faster, and thus less efficiently. This is why 4 coaxial props instead of 2 is acceptable. Preccesing is not a problem in these events, because if it happens in one coaxial, it's canceled out by the counter-rotating blade.

    The problems with vtols is, once they flip over, theres no turnin back, no matter what you do. BTW I don't know what I'm talking about... so don't try.:ban:
     
  2. Jan 2, 2010 #82

    mikemill757

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  3. Jan 7, 2010 #83

    autoreply

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    Having read the thread started me thinking.

    We want vertical take-off and the advantages of a normal aircraft (cheap, simple, decent cruise etc)

    There're two approaches, separate VTOL and horizontal flight and combined (all current solutions). Obviously going separate has a lot of disadvantages (twice the engines, twice a prop/rotor/ twice a gearbox), but it also has advantages.

    Now imagine a normal light aircraft, say a Lancair 360.
    Put a coaxial rotor hub just behind the pilot and drive it by two electric engines since they're incredibly light, maybe 20 lbs each. Except for the weight there's another advantage, electric motors can easily be hollow and thus your inner "shaft" can be non-turning till the top rotor, making the controls much easier. An alternate approach can be servo's in the blades that pitch the blades individually, eliminating that whole control-nightmare and replacing it with a good computer.

    For several reasons, you really need flapping blades. Since it's a VTOL with wings, there's no need for the rotor to cruise over let's say 80 kts or so.
    Take-off, accelerate to 65 kts and stop your rotor. Since you have flapping blades, they'll fold backwards. Open your fuselage and hide them, including the whole assembly.

    But now the clue.
    Up till now the above was a pretty impossible job, batteries were huge, an extra engine (or the transmission to drive the rotor from the other one) is simply too heavy and the servo-steered blades (already in use in some helo's) weren't feasible till recently.
    But now there're high-capacity batteries, powerful enough to put you up and allow for 5 or 10 minutes of hover. Since you don't need that rotor for cruise you can have a pretty big one so it's very efficient.

    To get back to the Lancair, let's assume you have a 30" double rotor and your rotor hub including engines is a good 75 lbs. Add another 50 or so for the blades and another 100 lbs for the batteries. Than you'd probably get away with around 60/70 hp during hover with a max power of around 100 hp for a payload shrinked by 225 lbs.
    Doesn't sound that bad, how do you guys think about that?
     
  4. Jan 7, 2010 #84

    BBerson

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    What is the calculated thrust (vertical lift) of the proposed electric lift package?

    I suspect you will find the lift to weight ratio is rather less than needed.
     
  5. Jan 7, 2010 #85

    autoreply

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    30' diameter is 900*PI()/4 or 700 sqft or 66 m2

    For a total thrust of 1500 lbs or 7500 N that's


    7500^1,5/(SQRT(2*1,225*66)) or 51 kw or 68 HP. A realistic figure of Merit is 0.7 or 0.8 so 100 HP might be more realistic.
     
  6. Jan 8, 2010 #86

    BBerson

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    That makes a bit more sense... your post number 83 proposed a 30" rotor.:ponder:

    A 30 foot rotor might provide the lift. But a 30 foot rotor will most likely not be capable of stopping in flight without destruction. It would also be very heavy.
     
  7. Jan 8, 2010 #87

    autoreply

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    That would indeed make a lousy rotor :speechles

    Why do you think the rotor would be very heavy? According to this site you're looking at 25-60 lbs per rotor and for a normal solidity you're around 35 (so for the dual rotor that's 70 lbs)

    Why do you think stopping is impossible? With a small rotor brake (or reversed engine power) stopping shouldn't be a problem and then folding can be done with an adapted flapping rotor:
    [​IMG]
     
  8. Jan 8, 2010 #88

    BBerson

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    Rotors cannot be slowed or stopped in flight without wild flapping from the wind.

    I would not trust anything from the company you have linked.
     
  9. Jan 8, 2010 #89

    autoreply

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    You made me do some google-research and it seems that you're completely right, folding a rotor in flight is close to impossible, so there goes another daydream...


    So how about strapping a single FJ-33 onto the middle of your fuselage and make it possible to cant it 90 degrees?
    Steering can be done by the same way as thrust vectoring is achieved and the "death zone" is pretty minimal given the huge acceleration of the engine once airborne, combined with a crash-resistant landing gear. Fuel burn is a major consideration of course, any other overseen problems?
     
  10. Jan 8, 2010 #90

    BBerson

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    VTOL has many options.
    A good start for your research is the book "VTOL" by Mike Rogers.

    Take a look at "lift jets". Lift jets are jet engines designed for static lift.

    I am trying to design " lift props". Engines and props for short duration static lift.
    Jet engines work well for lift, but are beyond my price range. A homebuilt VTOL must cost less than a typical military craft where low cost is no object.
     
  11. Jan 9, 2010 #91

    bmcj

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    What about something along the curving wing design concept of the Roberson VTOL or the Ryan Vertiplane?


    Robertson VTOL
    Robertson Aircraft Corporation was formed in October 1956 to build a four seat vertical take-off and landing (VTOL) aircraft powered by two supercharged 340 hp Lycoming GSO-480 engines. The wing had a sliding flap system with a double-slotted full span trailing edge flap providing all control. The flaps were retracted into the low aspect ratio wing for horizontal flight. All fuel and oil were carried in wing tip tanks which also acted as endplates. This capped the wing "buckets" and should have improved cruise efficiency. The aircraft made a tethered flight on 8 January 1957 but was not pursued.

    [​IMG]



    Ryan 92 VZ-3 Vertiplane
    The Ryan 92, designated VZ-3 by the Army in June 1956, was intended to be a reconnaissance and liaison aircraft able to operate from unprepared surfaces. It had a 28 ft metal fuselage and was powered by a 1,000 shp Lycoming T53-L-1 turboshaft engine driving a metal three-blade Harzell propeller on each side. The propellers were situated ahead of and below the wing, so the majority of the propeller slipstream flowed directly into the bucket formed by the extended double flaps and were turned downward for vertical lift. Differential propeller pitch was used for roll control. Engine exhaust was used at the tail for pitch and yaw before aerodynamic controls were effective. Ryan began taxiing trials on 7 February 1958. After extensive wind tunnel tests and aircraft modifications the first flight was made on 21 January 1959. The engines were unable to provide sufficient power to hover without a head wind. An accident the next month grounded it for repairs until its first test by NASA in February 1960; unfortunately the pilot ejected after an unplanned maneuver. It was again rebuilt to flying status: modifications after the crash led to a fabric nose section, an open cockpit, and a different landing gear. It continued flying in 1961, testing low-speed V/STOL handling characteristics.

    [​IMG]


    V/STOL: The First Half-Century - Part 2

    Bruce :)
     

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  12. Jan 9, 2010 #92

    kbarr

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    Not to mention that driving anything that has resistance (propeller vs air) will cause torque equal to that amount of resistance. Good ol' Newton.
     
  13. Jan 9, 2010 #93

    kbarr

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    Blade flex due to precession has to have clearance between rotors sufficient to prevent contact. This is a factor even in single rotor design, where control inputs when the rotor is unloaded cause more precession than the clearance can accommodate. All two blade semi-articulated or teetering rotors (Bell, Robinson, etc) have this issue. Fully articulated systems are less prone.
    Co-axial systems have these transient blade paths occurring in opposition for the same control inputs (running opposite direction, precession is opposite for same desired response), this is why they need even more clearance. Physical constraints outweigh the design benefit. Kaman & some early military designs where the counter-rotating rotors were used vs. coaxial achieved some success, but also less practical for passenger use due to angle of rotation bringing blade path too low.
     
  14. Nov 10, 2010 #94

    leviterande

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    While considering to start a new thread on gyroscopic precession. I thought i could just go ahead and write here instead.


    If we just disregard, noise, and inefficiency in a proposed single propeller vtol for a moment:
    To what length or should I say to "what discloading" can we control a single propeller without dealing with great gyroscopic precession.

    Having a very low discloading is out of question here since we need cyclic control in that case. What I am meaning here is to have a higher discloading without going to small ducted fans which are just too inefficient.

    So the question is, what is the limiting disc loading, where if we go lower, we no longer can control the "rigid prop". In order to control a rigid single propeller, it will be by means of propwash deflectors, and/or air jets at the sides.


    The closest I found about "practical problems" in gyroscopic precession, was in the paramotors area. A typical paramotor is a 1,35 meter diameter propeller geared to a 20-30 hp engine.

    For those who fly paramotors, they certainly feel the precession at several points of flight(maybe, the soft parawing makes the precession even easier).

    Anyway, if we flip this whole paramotor configuration to a horizontal plane as in a proposed VTOL configuration,(so that the propwash is facing directly down, control is made by vanes in the propwash and 4 small airjets/propellers located at the outside four corners far from the C.G.), can we in this case even think about controlling this 20-30hp 1,35 meter propeller in a hover? or will the precession be way too much to handle (unless ofcourse very expensive ultra high end sensors are used?)


    Gyroscopic precession is affected in the end by two things:

    Rotational mass
    Rotational Speed

    How much does precession influence the control on a certain propeller size/loading is hard to know and predict

    Kalle
    [SIZE=-3][/SIZE]
     
  15. Nov 10, 2010 #95

    BBerson

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    The gyroscopic precession force just depends on how quick you want to tilt the disc.
    The torque needs to be neutral as well.
    Why not use four lift props instead of one?
     
  16. Nov 11, 2010 #96

    Dan Thomas

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    Precession means that you have to apply tilting force to the disc at 90°, in the direction of rotation, to get the desired tilt. So, if we have a propeller that spins counterclockwise as seen from above (like an American helicopter) and we want to tilt forward, we have to lift the disc on the left side so it will rise at the back.

    But if we want to bank left we have to apply lift to the disc at the back so that the right side will rise. A helicopter has to do all this.

    If the disc tilts due to some uncommanded force, like a crosswind or turbulence, or assymetric lift due to forward flight, it will also tilt in some direction we don't want it to tilt. And it may become unstable. The typical kid's top, spinning on a tabletop, will spin nice and erect if it's started perfectly vertical, but if it's off-perpendicular even a little it will wobble around its axis. Precession caused by the heaviness on one side, and the wobble caused by the precession chasing itself. The complexity of helicopter rotors is necessary to control and counter all this, and it will be hard to come up with anything much simpler that will be of practical use. Helicopter manufacturers try to keep their costs down to stay competitive, and if they could create a simple system to let them discard all that expensive, heavy, complicated rotor-control machinery, I'm sure they would.

    Dan
     
  17. Nov 13, 2010 #97

    leviterande

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    Hi Dan I wasnt arguing for not using a heli system or arguing for only using a prop system. I was just wondering how one could "predict" the impact of the gyro forces on a particular discloading. In general ofcourse, a lower disc loading will make gyro forces more apparent and dominating as seen in helicopters


    BBerson, believe I am thinking and have been considering 4 lifting props. They must be gasoline engines. Starting each of them is hard but can be done. but what about an engine failure!
     
  18. Nov 13, 2010 #98

    Dana

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    If you have direct lift aircraft with 4 props and 4 engines and one fails, you're screwed. Some have used two engines per prop with overrunning clutches, but that adds weight and complexity.

    Another option would be to use one engine with belt or shaft drive to the props, and variable pitch on the props for control.

    -Dana

    Politicians are those who deal with the problems which would not exist if they didn't exist.
     
  19. Nov 13, 2010 #99

    BBerson

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    Six motors is best. With six, one can fail and the others can compensate automatically.
    I know starting six gas engines is a problem. The weight of six starter motors is a problem. Perhaps they could be started with a belt drive that links all six but gets engaged one at time. (belt slips to engage).

    Or wait for electric motors and batteries to get cheaper.
     
  20. Nov 13, 2010 #100

    TFF

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    Hiller Aviation & Museum | Exhibits


    Cool stuff you have already seen for sure.
    The big problem to solve is translational lift. Hovering is easy but when you start outflying your air cushion all the aerodynamics change.
     
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