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RAF Rotor Blades

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  • #16
    You would think in denser air an increase in pitch would slow the rotor speed down because of increased drag and you would have to decrease pitch the to increase rotor speed.

    This is where a constant speed prop works better than a fixed pitch one. there is a trade off between pitch,rotor rpm, and air density. remember the inboard section is also its engine.

    If the rotor was powered increasing the pitch would work if the engine was powerful enough to drive an increase in pitch. the gyro rotor has a narrow window for the self propelled

    rotor to work well in. this is just my layman's opinion,I am not a math expert,and I am sure there is a lot more to having a good efficent rotor than I will ever know about.
    Best Regards,
    Eddie Sigman,Polvadera,nm
    (575) 835-4921

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    • #17
      The designers of the RAF-2000 gyro, like most retail gyro designers, used the ”by guess and by gosh” method and wouldn’t have the foggiest notion of how rotor RPM related to air density. They knew from experience that rotor RPM increased with altitude.

      My guess is that they increased pitch with altitude because they were concerned about the strength of blade/hub attachment and wanted to restrict rotor RPM.

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      • #18
        Thanks for you answer chuck,that makes sense to me.I know absolutely nothing about rotor blade dymanics and how they work.
        Best Regards,
        Eddie Sigman,Polvadera,nm
        (575) 835-4921

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        • #19
          Too much restricting the rpm also decreases the forward speed limit which trigger the blade flapping divergence in restless air.
          My calc said mu max = 0.26 with the "optimum" pitch setting as guessed by RAF for 6000 ft

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          • #20
            JC, during my early days of gyrocoptering, my partner and I wanted to find the limit of pitch setting for rotor blades.

            We set our Bensen type metal blades at the upper limit of their pitch adjustment and with the two of us hand spinning the blades, managed to get them started.

            Top speed was ~20 mph with the stick against the forward stop. More power; climb; less power; descend, all at 20 mph. It felt like riding a screw controlled by the throttle.

            We didn’t have an accurate rotor tachometer; simply a bicycle generator running a voltmeter.

            Other gyro flyers tried the same thing with other metal blades available at the time with similar results.

            I wouldn't recommend trying this with rotor blades that had a negative pitching moment coefficient.
            Last edited by C. Beaty; 12-07-2017, 03:12 AM.

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            • #21
              Originally posted by C. Beaty View Post
              JC, during my early days of gyrocoptering, my partner and I wanted to find the limit of pitch setting for rotor blades.

              We set our Bensen type metal blades at the upper limit of their pitch adjustment and with the two of us hand spinning the blades, managed to get them started.

              Top speed was ~20 mph with the stick against the forward stop. More power; climb; less power; descend, all at 20 mph. It felt like riding a screw controlled by the throttle.

              We didn’t have an accurate rotor tachometer; simply a bicycle generator running a voltmeter.

              Other gyro flyers tried the same thing with other metal blades available at the time with similar results.

              I wouldn't recommend trying this with rotor blades that had a negative pitching moment coefficient.


              Like slow-flying a FW STOL plane with deployed flaps and the nose very high... I understand that both lift and drag increase a lot, but lift generation is still enough. If I'm not wrong, that would mean that the rotor turned more slowly, and that the fuel consumption was very high, thus lowering endurance & range...

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              • #22
                Chuck,
                The high Cmo of Bensen's profile and its low torsional stiffness mades possible a large angular flapping before reaching the excessive stall of the retreating blade: Thus in flight, the stop of the stick was reached before the divergence of a1.
                But with a moment coefficient just zero, the divergence appears before the stop of the stick. In this case the blades hit their stops in flight
                Last edited by Jean Claude; 12-07-2017, 05:57 AM.

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                • #23
                  Chuck thanks for sharing that story with us,it sounds like you were right on the edge,but that's what test pilots do, they go right to the edge.!!
                  Best Regards,
                  Eddie Sigman,Polvadera,nm
                  (575) 835-4921

                  Comment


                  • #24
                    JC, the Bensen rotor blades had 0.032” upper skins and 0.050” lower skins; the other metal blades of that era, Stanzee and Rotordyne. had 0.032” aluminum skins wrapped around an aluminum spar.

                    The stick was against the forward stop because the Bensens of that time were mostly CLT and had no effective horizontal stabilizer, resulting in an extreme nose high attitude of the airframe at ~20 MPH.

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                    • #25
                      Originally posted by C. Beaty View Post
                      The designers of the RAF-2000 gyro, like most retail gyro designers, used the ”by guess and by gosh” method and wouldn’t have the foggiest notion of how rotor RPM related to air density. They knew from experience that rotor RPM increased with altitude.

                      My guess is that they increased pitch with altitude because they were concerned about the strength of blade/hub attachment and wanted to restrict rotor RPM.
                      So most gyroplane designers of commercial gyroplanes have no clue of effect of density altitude on true air speed? Because rotor RPM is nothing more aerodynamically on a blade section than speed. Really Chuck? May be you should restrict this comment to a subset or particular manufacturer(s).
                      Last edited by fara; 12-07-2017, 08:09 PM.

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                      • #26
                        Originally posted by C. Beaty View Post
                        JC, the Bensen rotor blades had 0.032” upper skins and 0.050” lower skins; the other metal blades of that era, Stanzee and Rotordyne. had 0.032” aluminum skins wrapped around an aluminum spar.
                        The stick was against the forward stop because the Bensens of that time were mostly CLT and had no effective horizontal stabilizer, resulting in an extreme nose high attitude of the airframe at ~20 MPH.
                        Chuck,
                        Are you trying to tell me that the Bensen's blades can not be deformed enough by the Cm0, to explain the stick on its front stop? This would only be because of the flapping angle a1?
                        Yet, my simulations shows that the steady flapping angle does not exceed 3 degrees, even with Mu = 0.35
                        And this is confirmed by the measures shown in the NACA report 475. While you say that the control reaches 8 ° less than the tip plane (Stick on the front stop).
                        My opinion is that the supplement is due to the deformation of the blades. What is missing?
                        Thank you for your clarifications
                        Click image for larger version  Name:	Sans titre1.png Views:	1 Size:	4.9 KB ID:	1127796

                        Last edited by Jean Claude; 12-08-2017, 03:03 AM.

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                        • #27
                          I expect you’re correct, JC. I know that Bensen wood rotor blades had considerable positive moment coefficient that caused the stick to reach the forward stop at ~60 MPH.

                          But I still have difficulties understanding how Bensen’s metal blades could have had a strong nose up moment; being derived from an 8H12 airfoil along with sufficient torsional flexibility to produce so much flapping.

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                          • #28
                            I am having some difficulty is trying to figure out how a twisted blade would affect the driven/driving areas of the rotor. It seems to me that a greater relative angle of attack near the hub would increase the area of the stalled region. This would seem to me to make retreating blade stall induced vibrations of the rotor to increase. I may have this backwards in my head though. It would also help if I could find at what incidence untwisted rotors such as on the Mto or AR-1 are set at. Also is the twist of the RAF rotors constant and linear from root to tip, or is more twisted just at the root. Hope this all makes sense.

                            RogerS

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                            • #29
                              Roger,
                              If the twisted blades are set to get the same rpm as with non-twisted blades, then the angle of attack at 0.75 R is almost unchanged, and the
                              twisting only changed the distribution of lift along the blade span. Thus, a negative twist moves the thrust vector of each blade a little closer to the center.
                              The lift component closer to the center gives less cone, which reduces the vibration due to the cone
                              The drag component closer to the center gives less drag vibration
                              The widening of the stalled area on the retreating blade is almost without effect due to low airspeed at this place
                              The lower angle of attack on tip decreases the ratio L/D in this high airspeed area and increases a bit the general drag of rotor.

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                              • #30
                                This helps. I see that if the rrpm is unchanged the overall angle of attack is the same. The summation of the whole blade is the same. I wasn't visualizing it as a negative twist. So if the RAF is set with the tip at 1.5 degrees, this is less than a straight blade would be at the tip? It would still help if I knew what angle the straight blades were normally set at. I understand what you are saying about the coning. So the twist moves the driving section closer the center and at the same time the lift vector is angled forward more, increasing driving force, but over a shorter section of the blade? Thanks for the education.

                                RogerS

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