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Thread: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

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    The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    Hi all!

    I want to design & build my own gyrocopter and started to read a lot about it. The intention is to make a 2-seater Microlight (450 kilo max take-off weight). My most important concern at the moment are the rotor blades.
    Two main questions here:

    A. Theory regarding gyrocopter rotor blade.
    What is the theoretical optimum shape?
    I see many companies offering many types of rotor blades from different materials. Just from pure theoretical perspective:
    (1) what is the ideal form for a gyrocopter rotor blade (shape, angle of attack twist, etceteras)? Since the outer part of the blade produces the lift and the inner part maintains the autogyro effect (depending on circumstances such as forward speed) I can imagine that theoretical optimal shape is something highly complex for a gyrocopter rotor blade. Does anybody have any idea from practical experience or theoretical background?
    (2) Are there rotor blade manufacturing companies who go for the best shape/ best performance? Who either have the theoretical background or practical test facilities. Any suggestions which companies to contact?
    B. Tilt (or bend) the rotor blade to save space.
    Also from what I read so far I would need rotor blades of about 365 cm each, which will make the vehicle too long to handle (won't fit in my garage for sure).
    My friend has a model helicopter where the rotor blades can be folded together (both blades side-by-side in parallel), which reduces the space problem already.
    Due to the centrifugal forces the blades stretch out by themselves without a permanent fixation (they hinge 'freely' around a single pin).
    (1) Could the same trick be used on a large (real-life) scale for a gyrocopter? Does anyone have any experience with this?
    (2) I know naval helicopters can also bend (or fold) a single rotor blade in half, which conciderably saves a lot of space. Can I use the same technique for a gyrocopter rotor blade? Anyone knows if this has been done before? Does anybody know a rotor blade manufacturer who could produce such a blade?
    Closed (bend/ folded) rotor blade.


    Open rotor blade.

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    Moderator Dana's Avatar
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    Re: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    I can't help with blade design, but as far as folding, it's just a strutural design issue, no reason why it couldn't be done. Although centrifugal force is probably sufficient to hold the blades open once they're spinning, I would think you'd want a secondary lock to hold them open while they're spinning up.

    You might want to purchase the rotor head and blades from an established outfit, and design/build the rest of the aircraft yourself. This is what a friend of mine did (actually he started with a complete Air Command gryo, but after a few "incidents" and rebuilds there was nothing left of the original aircraft except for the rotor & blades).

    -Dana

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    Registered User BBerson's Avatar
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    Re: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    In theory, an ideal blade would have twist and taper. But in practice most blades are straight and without twist because its easier to make and the difference is minor.

    The best blade for autorotation would have negative twist, that is, the reverse of a helicopter blade.
    Helicopter blades sometimes have more pitch at the root like a prop, but most small blades are flat.
    BB

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    Registered User Dan Thomas's Avatar
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    Re: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    Quote Originally Posted by BBerson View Post
    In theory, an ideal blade would have twist and taper. But in practice most blades are straight and without twist because its easier to make and the difference is minor.

    The best blade for autorotation would have negative twist, that is, the reverse of a helicopter blade.
    Helicopter blades sometimes have more pitch at the root like a prop, but most small blades are flat.
    BB
    A negatively-twisted blade loses its autorotative capabilities. If the angle of attack on the inboard area is too high, the lift vector moves back behind the drag vector and the forward component of the lift vanishes. Not good at all.
    A hinged blade would be asking for trouble. I think that if it made sense the carrier helicopters would use it. Instead, they pivot them back on the lead/lag hinges. A hinge has to be very strong, to take the centrifugal forces and the retreating-advancing drag changes. A loose hinge would result in flutter that might lead to rapid failure; a busted hinge would mean instant death.
    Airfoils are usually symmetrical to avoid the CP changes that AOA changes bring to cambered blades. CP changes cause vibration. The AOA of a rotor blade is changing constantly.

    Dan

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    Registered User BBerson's Avatar
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    Re: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    Quote Originally Posted by Dan Thomas View Post
    A negatively-twisted blade loses its autorotative capabilities. If the angle of attack on the inboard area is too high, the lift vector moves back behind the drag vector and the forward component of the lift vanishes. Not good at all.

    Dan
    Yeah, you are right. I should have said positive twist when I referred to a blade with less pitch at the root than at the tip.
    Is positive twist better than flat for autorotation?
    BB

  6. #6
    Registered User Dan Thomas's Avatar
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    Re: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    Quote Originally Posted by BBerson View Post
    Yeah, you are right. I should have said positive twist when I referred to a blade with less pitch at the root than at the tip.
    Is positive twist better than flat for autorotation?
    BB
    I don't know. Some helicopter blades have washout (more incidence at the root than at the tip) to some small degree. You can see a tiny bit on this one:
    http://www.qinetiq.com/home/newsroom...n%20blades.jpg

    See this:
    Aerodynamics of Autorotation

    Dan

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    Registered User BBerson's Avatar
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    Re: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    Looks like the stalled root region would benefit from lower pitch in that area.
    Quote from Ray Prouty's helicopter book: "twist that is beneficial in powered flight is detrimental in autorotation."

    As usual, blade design is a compromise depending on the different flight conditions such as forward flight or vertical flight or powered or unpowered.
    BB

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    Registered User Dan Thomas's Avatar
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    Re: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    Quote Originally Posted by BBerson View Post
    Looks like the stalled root region would benefit from lower pitch in that area.
    Quote from Ray Prouty's helicopter book: "twist that is beneficial in powered flight is detrimental in autorotation."

    As usual, blade design is a compromise depending on the different flight conditions such as forward flight or vertical flight or powered or unpowered.
    BB
    Aircraft of all sorts are full of compromises. This is the reason some of us oldtimers snicker when someone new to aviation suggests that there's no good reason why they can't design an airplane that lands on land or water, has a 1000-mile range, cruises at 150 kts, climbs 1500 fpm and carries four people and 400 lbs of their baggage, has a 100-hp car engine that burns three GPH and costs less than $3500 to build.
    Someday, maybe, someone WILL design something that is truly astonishing, but it will generate lift by some other means than we're familiar with now. It will have to generate that lift (and thrust) without all the usual drag. But until then, the naive claims that some new design is going to change flight as we know it are met with a lot of skepticism by most of us. And, of course, if that airplane is ever built (most aren't), it turns out to have the performance ordinarily expected of an airplane of its weight, horsepower and wingspan. And sometimes, it turns out to be spectacularly dangerous.

    Dan

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    Re: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    Diffcult to know where to start. Firstly if you have a flapping hinge away from the rotation axis you must have a lead/lag hinge.
    A change in the flapping angle at this hinge will change the angular momentum of that part of the blade. Conservation of angular momentum means that when the CofG of a part of the blade gets closer to the axis of rotation itís angular velocity must increase. So changes in flapping angle away from the axis of rotation will cause big bending forces if there is no lead/lag hinge.

    My advice based on the work I have done on "the autogyro problem" is either;
    get a Phd in dynamics.
    or;
    build a gyro with at least 4 if not 6 blades.
    An autogyro itself is not an unstable machine. The trouble comes when you put a pilot in it and close the feedback loop. This feedback loop has a time delay in it and therefore has a frequency at which it is unstable.

    Picture yourself flying your gyro; you pull back on the control stick and change the angle of attack of the rotor disc - just like a conventional aircraft changing the aoa of itís wing. This increase in AoA causes an increase in the rotational speed of the rotor, which itself causes an increase in lift.

    Unfortunatly unlike a conventional aircraft where the increase in lift is instant the rotor disc on your gyro has inertia and takes some time to speed up. The angular inertia of the average two blade gyro means that there is a cycle of pilot input and blade reaction which can lead to disaster. The pilot can get into a divergent dynamic cycle of about 1 hz which is a frequency notoriously difficult for humans to deal with.

    The answer to this problem is - in my opinion - to change the angular interia of the blades and therefore the frequency at which this desructive cycle will occur. Angular momentum is a fuction of the cube of the blade length where lift is a function of the square of the blade length. So 6 short blades is going to be a lot easier to fly than 2 long ones.

    Of course with 6 short blades on a fully articulated hub your gyro will fit in your garage easily without a complicated and heavy mid span hinge.

    Laurie (UK)

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    Registered User BBerson's Avatar
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    Re: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    Quote Originally Posted by lauriehurman View Post


    Of course with 6 short blades on a fully articulated hub your gyro will fit in your garage easily without a complicated and heavy mid span hinge.

    Laurie (UK)
    Welcome to the forum.
    Sorry, but I have to disagree with your conclusion. Six short blades will not provide the required low disc loading needed for an autogyro. Large span is required for low disc loading.
    BB

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    Registered User wally's Avatar
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    Re: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    Hi,
    There is already a company that makes just what you need. I think it is called Dragonwheels or Skywheels or something like that. All fiberglass. It is a central hub about 3 feet long with blade shaped sockets in each end. The 2 blades just slip in each end, a couple of bolts through each and you are ready to "gyro". I helped a friend install them on his gyro a couple of times.

    They are strong too. I remember him telling me about the testing the company did. They mounted a tall controllable gyro type pedestal in the middle of a convertable full size Chevy. With the blades mounted and spinning on top and with the Chevy blasting down the "runway", they could lift the back end of the whole car.

    Sky Wheels - Home

    Well it looks like they aren't making them any more but there is some good info to read.
    Best wishes.
    Wally
    Last edited by wally; October 28th, 2008 at 02:04 PM. Reason: added link

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    Registered User wally's Avatar
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    Re: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    well I found several sources for gyro rotor systems

    Rotors & Rotorheads

    Good luck.
    Wally

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    Super Moderator Midniteoyl's Avatar
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    Re: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    WARNING!

    Carefull on the 'skywheels' web page link... Came up as a 'known exploit site'.
    Jim

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    Re: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    Just to add a couple of points to what has been said already.

    While a 4 or 6 bladed rotor may seem like a good idea, you add a large amount of complexity to the rotor head. Without going into the details of rotor head dyinamics to deeply, a 2 bladed heas offers the simplest solution from a head design point of view. A 2 bladed head allows you to use a simple teetering design, meaning that you dont have to have lead/lag hinges.

    Once you go to more than 2 bladed you have to have a fully articulated head (or a ridgid one). And there is quite a bit to go along with that.

    Realistically a gyro is not a high performance machine, and its unlikely that you will se much in the way of returns chasing an optimum profile/tip shape. There are quite a few companies that you can buy the blades from, whether it be composite, or extruded ally.

    IMHO, you would be better off looking at things like dynamic stability, especially if you are planning on doing a pusher.

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    Registered User BBerson's Avatar
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    Re: The best theoretical gyro rotor blade shape? Can a gyro rotor blade be bend in 2?

    There is someone that has built a "roadable gyro". The blades fold up and the machine can be driven on the highway. I forgot the name, but a search of Aero-News found this:
    Aero-News Network: The Aviation and Aerospace World's Daily/Real-Time News and Information Service

    I think there might another later story from the same designer about going into production.
    BB

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