Vtol design: 1 vs 6 engines

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

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  1. Oct 14, 2008 #1

    leviterande

    leviterande

    leviterande

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    Hi, everyone

    I just became a member and this forum seems to be a wonderful place to talk about homebuilt.:)

    I have been planing to make a vtol craft. I know many who has the same aim and I am not saying I am going to be better or the best but my goals are that the craft is going to be simple cheap design, low maintenance, lo cost, medium efficieny although maybe low..

    the dielmma is what design is the most appropriate:

    Nr1: an engine (rotax 582?) driving a say 87inch prop on the top of a capsule with fixed anti torque stators and vanes. this should eliminate prop torque without the need for complex of a tailrotor or coaxials

    Nr2 : 6 small 200cc engines situated around a cockpit in a circular manner, thus we dont need control surfaces or vanes..

    these engines could be:
    SV-210cc Twin R/C Airplane Engine



    both designs have disadvatages but since I lack the experience of building a real size aircraft I cannot be aware of all the troubles.. however I know for example that I need to find a way to make the 6 engines work at around the same rpm..(hard). I have experience though in model vtols

    the problem with NR1 is to find an apropriate engine for the craft.. for example : the rotax 582 65hp is around 50kg which is heavy.. the maximum thrust I found was to be 475lbs which is not enough .. so what are your opinions about the two designs

    I know there is alot I didnt say but I cant have everything in one post lol

    In the end I want to go for the cheapest way


    estimated specs:

    Empty weight: 120kg
    G.Weight : 220kg


    regards
    Kalle
     

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  2. Oct 14, 2008 #2

    BBerson

    BBerson

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    To design a full size VTOL requires a complete understanding of the propeller "disc loading". If you can get disc loading low enough, then the piston engines might work. For high disc loading you would need a gas turbine engine.

    Next you need a plan for inflight system failure. The military has used seat ejection but that is probably impractical.

    BB
     
  3. Oct 14, 2008 #3

    Dana

    Dana

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    VTOL and "simple cheap" don't generally go together...

    -Dana

    I don't trust a government I can't shoot back at.
     
  4. Oct 14, 2008 #4

    leviterande

    leviterande

    leviterande

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    you are right , it will not be cheap but I am trying to keep it as cheap as possible ..

    so if we assume we use use a rotax 582 to drive a single prop via gearing , how much should the gearing be to get thrust over 200kg? 6:1?



    most aircraft, trikes with 582 dont produce ore then 150kg of thrust. with around 3:1 gearing


    thanx
    Kalle
     
  5. Oct 17, 2008 #5

    leviterande

    leviterande

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    Does any body know or have actuall experience of what is the maximum static thrust from a rotax 582 or a similar one rotating as big propeller as possible "87 inches?" and not a "heli rotor"

    I have read that an X shaped Ivoprop produced 430lb in a 582, but even that is not enough counting the weight of the rotax its mountings, the fram and pilot.. so everything here is about power/thrust to weight ratio..

    one of the reasons I want to use smaller 19hp 6kg engines is just becouse of the high power to weight ratio.. but I also very aware of the fact that making a "four poster" design stable is hard. how can one make the engines work together.. is a problem too


    Kalle
     
  6. Oct 17, 2008 #6

    djschwartz

    djschwartz

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  7. Oct 17, 2008 #7

    Dan Thomas

    Dan Thomas

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    That 432 lb figure I would doubt very much. You can expect around three lbs static thrust per HP from a propeller, so that 65 hp would give you maybe 200 lbs. If it was 432 lb we'd see lots of bare-bones "helicopters" powered by such means.
    Helicopter rotors are long for good reason. Long and slow is far more efficient than short and fast. A guy named Moller has spent 30 years or more, and vast volumes of money, learning this the hard way.


    Dan
     
  8. Oct 17, 2008 #8

    Dana

    Dana

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    Actually I suspect Moller knows it by now... it's his investors who are learning it the hard way! :shock:

    -Dana

    Be wary of strong drink. It can make you shoot at tax collectors-- and miss.
     
  9. Oct 17, 2008 #9

    AFAdrenaline

    AFAdrenaline

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    just throwing in my two cents here...

    obviously the more props you have the greater the chance of a mechanical failure... that being said a single rotor design means that a mechanical failure of a part would be critical.

    while traditionally the powerplant and the props have been connected in a relatively direct manner you might want to consider another approach. you stated that a single rotax engine would not provide enough power. what if you used two and connected them to a common gearbox. that gearbox in turn would then run to your six rotors (via a shaft or belt). in the event you have an engine fail on you, you could still direct power to the fan sections. it might not provide enough power to keep you aloft but perhaps it would provide enough to slow the decent. lastly this would also ensure that all of the props are spinning at the same. differential thrust, for maneuvering, could be achieved if you made several (or all) of the props variable pitch. i'm sure you could also come up with a gearing system to change the speed coming off of the common gearbox to an individual fan but i would imagine that this would be far more complex.

    anyways, just an idea.

    oh, also with your single rotor idea... i saw an article a few years back of a VTOL RC aircraft similar to that. when the flow of air was directed down through the ducting and over the conical shape of the body it the lift was increased in addition to the downward force of the fan. this was caused b/c as the airfoil was accelerated over the top of the body it decreased the static pressure, while the pressure on the bottom of the body remained that of the ambient air.
     
  10. Oct 17, 2008 #10

    leviterande

    leviterande

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    thanx everybody for your interest, i asked IVOPROP and ultralightnews.com what the max St.Thr. from 582 and ultralightnews said 365 ft lbs, didnt they mean torque?

    Dan Thomas,
    here is were they say 430lb from a 582 rotax usign ivoprop:
    Resultat av Google:s bildsökning efter http://www.ivoprop.com/images/ivoehub.jpg
    I doubt that too.. but, when you said 3lbs from 1 hp!?, did you really mean a geared prop?, cuz i am talking about an atleast 4:1 geared 582. Mollers, concept is too unefficient

    I want to have a mix between prop and heli rotors


    AFAdrenaline,
    I totally agree with you , the more engines the more risks of failure.
    I have considered before the idea of using two engines driving a single axle connected to 2 props.. it is really a great idea and efficient safe one.. but the problem is complexisty and cost for the drivetrains and gears


    djschwartz
    thanx for the links, I also have by rule of thumb that a heli requires 150watts to lift 1kg

    Kalle
     
  11. Oct 17, 2008 #11

    orion

    orion

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    The Ivo prop's reputation is relatively mixed with some folks who'll swear by the performance while other say that it's rather overstated at times. I know a few years back there were a few in-flight failures but as the referenced site indicates, that may have been due to improper installation.

    Regarding thrust, for static operation a rough ballpark figure that one can use is about three pounds of thrust for each horsepower unit. But this varies from prop to prop, sometimes significantly and is usually applicable to conventional configurations. Climb props might generate a bit more than that while cruise props will do a bit less.

    Since the most common application of Ivo's products is on ultralights and powered parachutes, the blades tend to be optimized for much lower speeds, so the possibility of generating about four pounds per horse may not be unreasonable. Personally I probably wouldn't count on it but at the same time I wouldn't dismiss it outright.

    For a rough approximation of feasibility you can look at existing examples and extrapolate downward, accounting for the smaller disk area of the prop you're looking to use. It's not a good correlation (the relationship is not linear) but it may give you an indication of whether you're on the right track or not. For instance, most well designed light helicopters will lift about ten pounds for each horsepower. But as the static thrust numbers indicate, the smaller props will lift maybe only three to four pounds per horsepower, although with installation losses, that number can be lower.
     
  12. Oct 17, 2008 #12

    leviterande

    leviterande

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    Hi Orion, I am not an expert but from all the data I have seen from peoble over the net the rotax 582 seem to pull way over 200lbs..

    it is more likely around the 300lbs range with a 3,47:1 gearing that I have seen.. .. 3lbs per 1hp seems to be too little but perhaps for very large airplanes with very high horsepower rating driving small props 3lbs per hp.. as the efficieny decreases with scale

    Kalle
     
  13. Oct 17, 2008 #13

    BBerson

    BBerson

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    A stock prop is designed for cruise. A VTOL lift rotor is designed for static lift.
    The optimal VTOL prop/rotor would need to be custom designed for static lift. (low disc loading and almost flat pitch for static thrust)
    BB
     
  14. Oct 17, 2008 #14

    orion

    orion

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    As I indicated, props designed for slower speeds may deliver better performance than the first cut estimate of 3#/hp. And of course, the slower you turn the prop the more efficient it becomes. The same is true for lowering the number of blades - the most efficient prop is a counterbalanced single blade unit - not too practical though.
     
  15. Oct 17, 2008 #15

    leviterande

    leviterande

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    using small engines the efficieny is actually high, that 19hp I linked to weighs 6kg and produce 128lbs of thrust (6,7lbs per hp) direct drive as i asked the guy but I a still doubt if that is true..but it is possible that it can produce that much cuz the efficieny decrease as you put in more and more hp..

    my small brushless aircraft motors are around 180watt and can lift 1kg verticaly! without gearing. going up in the motor size cuts your lift per hp

    so if we look at the efficieny alone , doesnt the 6 engines(19hp each) over a larger engine as rotax 912

    Kalle
     
  16. Oct 17, 2008 #16

    BBerson

    BBerson

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    I measured 5.5 lbs of static thrust per hp on a 10hp lawnmower with a handmade prop.

    The problem with six engines is getting them all started.
    I think six similar sized electric motors might be the answer. The endurance would be shorter, of course.
    BB
     
  17. Oct 17, 2008 #17

    leviterande

    leviterande

    leviterande

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    BBberson, 5,5 sounds much more logical then only 3 per hp but maybe becouz it is only 10hp



    this is very strange or am I having real bad luck, I just posted a full detailed post on a general idea of my concept craft and when I clicked post reply " didnt find the page came up" internet works but not here it seems .. hope this will show up, I am just going to write a short verson

    so if we forget about big detials or safety factors and focus on the main idea:

    6x19hp engines: 40kg
    airframe: 40
    accesories:10
    me: 80

    total: 170kg

    thrust availabe:360kg
    thrust available to be able to hover and climb without using full throttle or at 85% :300kg
    thrust needed to hover 170kg
    thrust needed to climb 200kg

    so we have 300 kg available with 85 and teh craft requires say 200kg to fly

    if one or two engines die we can make a slow decent

    the challenge is in to find a way to make the engines work together and that means that we have to controll the rpms and that means that we have to install electronic actuators connected to som gyro connected resistor kind of control

    pitch, roll, lift are controlled by thrust differntial but what concerns me the most right now is how RPM is sensitive. I mean if the engiens differs by say 10rpm will the craft be very unbalanced?

    I know it reminds about mollers m200x but the disc loading here is lower

    Kalle
     
  18. Oct 17, 2008 #18

    Dana

    Dana

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    FWIW, a 14hp Solo engine typically used on PPG's will give you around 100 lbs static thrust with a 49" prop and 2.5:1 reduction (the engine turns 6500 rpm).

    Don't base anything on what Moller has [not] done; to the best of my knowledge he's never flown anything to any significant altitude or off a tether.

    Direct lift aircraft like Moller's attempts, the recent so-called "jet pack", and others, when they leave the ground at all, are horribly inefficient, LOUD, and of questionable controllability and/or safety.

    -Dana

    The difference between a hero and a fool is the outcome.
     
  19. Oct 17, 2008 #19

    AFAdrenaline

    AFAdrenaline

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    you also need to take into account that when you loose an engine you're not going to loose thrust from that engine but also the symmetric one on the other side. for example if you number engines 1-6 in clockwise order, if engine 2 cuts out you will have to shut down or reduce thrust in engine 5... unless, of course, you think flipping over would be fun... but i suspect not lol. also i saw no consideration in your weight estimates for for fuel... considering how light your craft is, it could make up a significant portion of your weight buildup.
     
  20. Oct 17, 2008 #20

    Midniteoyl

    Midniteoyl

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    Your not the only one having problems with posting...
     

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