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  • Rotor sweep?

    My interest is in building an ultralight tractor gyro. Designing something that flies well and is a legal US part 103 is hard enough for a fixed wing. To get good performance with part 103 restrictions means optimizing as much of the aircraft as possible - within a defined budget.

    To that end I'm starting with the rotor since it is the defining feature of a gyro. There has been much discussion here regarding 'tail heavy' rotor blades and the related pitch change as a result. A variable collective seems to have some advantages but given the weight limit of part 103, and the complexity added, this doesn't seem to be an option. However with modern composite technology we can, or should be able to, design a rotor blade that has about any desired aeroelastic response we desire. In other words a passive collective should be possible.

    In the various threads where there has been debate about rotor speed not changing because the rotor blades balance aft of the 25% chord, but it being alleged that the rotors do in fact balance at 25% how much of the blade twist might be due to undetected sweep effect?

    The sketch below shows how even if the blade has been strung so that the line between the same point on each tip crosses the rotor axis a small amount of sweep, either for or aft, would go undetected and result in a large torque about the aerodynamic axis. It seems to me that this sweep could be used to our advantage?
    Scale 1"=1'

    Click image for larger version

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  • #2
    Good morning Leon,

    I admire your willingness to take on a challenging project.

    What advantage do you feel you could get from the “sweep”?

    Not all the posters on The Rotary Wing Forum are knowledgeable so I feel it is important to carefully filter the information you find here.

    The book “Aerodynamics of the Helicopter” by Gessow & Myers has lots of information on rotor systems. You may find more reliable information there.
    Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

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    • #3
      I always thought that this type of sweep ( lead, lag ) was only a powered rotor idiosyncrasy
      Life,The leading cause of Death

      Live and Learn--OR--Die and be an example

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      • #4
        Originally posted by Vance View Post
        Not all the posters on The Rotary Wing Forum are knowledgeable so I feel it is important to carefully filter the information you find here.
        True- and there are some pretty knowledgeable individuals here as well. I'm pretty good at sorting out the good bits of grain from the mouse droppings.

        I've done a fair bit of research relating to swept flying wings as part of my fixed wing part 103 project and know how important aeroelastic tailoring can be. It just seems from what I've read that this is an area that hasn't had much interest in the gyro world. The improvement in performance possible is not as inexpensive or simple as just adding power. For a typical aircraft that works most of the time but when dealing with artificially restricted aircraft, such as part 103 and LSA, optimization of the air-frame and aerodynamics becomes more important.

        I understand that I'm reaching for the fruit near the top of the tree.

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        • #5
          I did not know what Aeroelastic Tailoring was so I looked up the NASA report and I am having a hard time seeing how it relates to rotor "sweep".

          https://ntrs.nasa.gov/archive/nasa/c...0140006404.pdf

          The rotors on the gyroplane I fly have a pivot and a soft bushing that allows the two blade semi rigid rotor to better manage lead/lag forces.

          I have not been able to imagine how that could be used to improve rotor efficiency.

          I don't want to distract you from your mission.

          I am simply curious.
          Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

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          • #6
            Originally posted by Vance View Post
            I
            The rotors on the gyroplane I fly have a pivot and a soft bushing that allows the two blade semi rigid rotor to better manage lead/lag forces.
            .
            The rubber bushing on your rotor would pretty well eliminate any possibility of inadvertent sweep.

            Imagine a classic Bensen bolted block attachment but the mating ends got milled 1/2 degree off from 90 and assembled in the most adverse way. An almost imperceptible error with rather large effect. Sketch represents 1 degree error. Even a 1/3 degree total error still means a sin(1/3) x rotor length sweep, or about a 7/8" sweep on a 24 ft rotor.
            Click image for larger version

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            • #7
              I have flown gyroplanes with the blades not straight and it appears to me it makes things shake without an appreciable change in performance.

              How does the sweep change the rotor efficiency?
              Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

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              • #8
                Given a totally rigid teetering rotor, sweep would make no difference. With lead lag pivot, or rubber mounts, there can be no sweep, unless it's designed into the blade similar to the "V" shaped rotor tips

                A straight rotor with forward sweep as it is flexed upward also has it's angle of attack increased at the tip, just like a forward swept wing of a fixed wing aircraft. This is a divergent interaction that prevented using forward sweep on aircraft until we learned how to design wings stiff enough to not flex and still be light enough to fly. Using composites we can move the torsional axis around so that the wing twists in the opposite direction. This is a self correcting interaction. Trying to do this with an isotropic material such as aluminum is pretty hard.

                A rear sweep will tend to do the opposite. Even if the center of gravity of the blade is at, or in front of, the aerodynamic center (25ish%) the center of lift is behind that center and even with reflex will tend to washout the blade if that center of lift is behind the mechanical torsional axis of the blade.

                Why my interest in this? Try to get Mu above .25 or so for a gyro that has a top speed of 60 mph. If i want to fly on low power (more power means weighs) at high altitude efficiency matters.

                How - might - this increase efficiency? Airfoils have a set AOA where the L/D is lowest. The closer the airfoil is to that ideal lift coefficient the the less power needed. Sailplanes can change that ideal spot by reflexing their flaps/ailerons for different cruise speeds. Short of using some kind of computer controlled piezo operated Gurney flaps, twisting the rotor blade for different loads/speed is about all we have the technology to do today? Mechanical collective is just too heavy for part 103.

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                • #9
                  Thank you, I feel confused on a higher level.
                  Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

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                  • #10
                    I don't mean to confuse - but that is the result too often. Keep in mind that I can be quite dyslexic at times.

                    Not all of my fixed wing aero theory knowledge may translate to rotor craft. I'm hoping others here will guide me down the correct path.

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                    • #11
                      There is a wealth of academic research on these matters. Here's one example:

                      https://ntrs.nasa.gov/archive/nasa/c...9950021841.pdf


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                      • #12
                        Originally posted by WaspAir View Post
                        There is a wealth of academic research on these matters.
                        Thanks for the link! Looks very useful.
                        I did not have that paper. 440 pages! It is too bad that NASA is so much more verbose in their reports than the NACA was.

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                        • #13
                          Hi Leon,

                          this is a very interesting thread! Designing a rotor yourselfe means you certainly don't aim low in your endeavours...;-) For some more material on all sorts of stuff related to rotorcraft you might want to have a look at the "Technical Papers Books and Publications" section of the forum:
                          https://www.rotaryforum.com/forum/ro...d-publications

                          I am a mechanical engineer with a fairly decent background in Finite Element Methods and for your task it seems you would need something like this. Please keep us informed about the steps you want to take and the progress of your project!

                          Cheers,

                          Juergen

                          ..Il semble que la perfection soit atteinte..
                          ....non quand il n'y a plus rien à ajouter,...
                          ...mais quand il n'y a plus rien à retrancher...
                          - Antoine de Saint-Exupéry -

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                          • #14
                            Originally posted by Hot Wings View Post
                            Airfoils have a set AOA where the L/D is lowest. The closer the airfoil is to that ideal lift coefficient the the less power needed. Sailplanes can change that ideal spot by reflexing their flaps/ailerons for different cruise speeds..
                            By necessity to balance the teeter, the A.o.A of a section of the blade varies from 2° to 8° during a half-turn . But unlike a glider wing, it is not allowed to change the Cm linked to the best adaptations.

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                            • #15
                              Originally posted by Hot Wings View Post
                              Imagine a classic Bensen bolted block attachment but the mating ends got milled 1/2 degree off from 90 and assembled in the most adverse way. An almost imperceptible error with rather large effect. Sketch represents 1 degree error. Even a 1/3 degree total error still means a sin(1/3) x rotor length sweep, or about a 7/8" sweep on a 24 ft rotor.
                              The relevant reference line should be the one that joins the hub to the application point of the centrifugal force of the blade.
                              Therefore,Leon, the sweep is neglectible

                              Click image for larger version

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