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  • Rotor disk loading ?

    I've read some where on the forum in years past that rotor blade loading should not be much less ( minimum ) then 1.2 pounds per sq. ft. of the rotor blade disk area. (3.14 X rotor radius squared) Does any one know the max ?
    Jay Gunderson

    "Wise men talk because they have something to say;
    fools talk because they have to say something."

    Plato

  • #2
    Originally posted by j bird View Post
    I've read some where on the forum in years past that rotor blade loading should not be much less ( minimum ) then 1.2 pounds per sq. ft. of the rotor blade disk area. (3.14 X rotor radius squared) Does any one know the max ?
    The disk loading is 1.98 pounds per square foot on The Predator Jay and Jim who made the blades feels that is fine.

    The blade loading is 65.9 pounds per foot.

    I feel it depends on too many things to make definitive statements about the correct blade loading or disk loading.
    Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

    Comment


    • #3
      The induced drag (drag due to lift) of a rotor at any given airspeed is directly proportional to disc loading: double the disc loading and you double the induced drag.

      Induced stag varies inversely as the square of airspeed; double the airspeed and drag is ¼ as much.

      So how much power do you want to burn?

      Comment


      • #4
        My gyro's all up weight with 10 gal fuel weighs 800 lbs., the rotor disk loading on my 26 ft. Dragon Wings cruiser rotor would be 1.51 lbs. per square ft. of the rotor disk area.
        Just wondering if there was a minimum and maximum!
        Last edited by j bird; 03-12-2018, 10:45 PM.
        Jay Gunderson

        "Wise men talk because they have something to say;
        fools talk because they have to say something."

        Plato

        Comment


        • #5
          Disc loading primarily affects low speed flight; if you went ot fly slowly, you need low disc loading. At cruise speed and above, induced drag is minimal.

          Rotor blade profile drag has a major influence on drag at all speeds and is dependent upon rotor tip speed. Tip speed depends upon blade loading and is approximately equal to 66 x square root of blade loading. Profile power* varies as the cube of tip speed; double tip speed and profile power increases by a factor of 8.

          Cierva standardized on blade loading of 35 lbs/square ft which results in a tip speed of ~390 fps, satisfactory for a top speed of ~100 mph.

          Your rotor speed should be ~327 rpm but is very sensitive to trailing edge angle (reflex).

          So how much power do you want to burn?

          *Profile power is the power required to drag the rotor blades around in a circle and is the reason rotorcraft are so ipower hungry as compared to FW aircraft.
          Last edited by C. Beaty; 03-13-2018, 06:40 AM.

          Comment


          • #6
            J-Bird: There is no "wall" at either end of the disk-loading spectrum. The McCulloch J-2 has, I believe, disk loading of over 2 lb./sq. ft. Some Gyrobees (including mine) run slightly less than 1 lb./sq.ft. Gyrobees are slow and low-powered.

            With too low a disk loading, the rotor RPM is so low that the retreating-blade's airspeed, in turn, is too low. This jacks up the flapping (or blowback) angle and allows so much of the retreating blade to stall that the machine will barely go forward. Carrying things to the extreme you could build a gigantic helicopter with wide, 100-foot blades that turned 1 RPM. It would lift up and down very nicely, but couldn't fly forward without a total retreating-blade stall.

            A ratio of aircraft speed to blade tip speed of about 0.35 appears to be the approximate sweet spot.
            Last edited by Doug Riley; 03-13-2018, 11:17 AM.

            Comment


            • #7
              Thank you for your thoughtful post Doug.

              What happens when you get too far past the sweet spot Doug?

              I have seen over 120 knots(138 mph) in a descent in The Predator putting her close to .4.

              I have also seen the rotor rpm slightly increase as my speed rises.

              Does this increase my top speed?
              Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

              Comment


              • #8
                Vance, Chuck Beaty is the reigning expert on such matters.

                I'm sure you have the general picture: the retreating blade's airspeed (speed from rotation MINUS aircraft speed) gets lower and lower as your aircraft's airspeed picks up. To compensate for the increased dissymmetry of lift, the flap/teeter hinge allows more and more rotor "blowback." Blowback has the effect of adding cyclic pitch to each blade as it becomes the retreating blade.

                You can only milk so much extra lift out of a wing by adding to its angle of attack, though -- add too much and it stalls. Consequently, the increasing blowback adds to the inboard stalled area of the retreating blade. This stalled area spreads outward, adding more and more rotor drag.

                Eventually, either the rotor flapping/blowback will reach the limits of the teeter stops (resulting in stick hammering), or the pilot will run out of forward stick.

                I'd take a guess that your increase in rotor RPM might come from a blade design that isn't quite perfectly "zero moment." The advancing blade, facing higher and higher airspeeds, may be twisting nose-down a bit. Essentially, this action would duplicate what the teeter hinge does anyway -- cyclically reduce the advancing blade's pitch. If you've run out of forward stick, this effect might add the equivalent of some more forward stick. Significant negative pitching moment is not a good thing, however, as it can result in rotor RPM "runaway" and an inability to pull out of a dive. Trailing-edge reflex is there to prevent this problem.

                I believe some pilots of McCulloch J-2's and/or Air and Space 18A's coax a little more top speed out of these gyros by pushing down the collective a bit. This will delay some of the retreating-blade stall.

                Comment


                • #9
                  Thank you Doug,

                  The rotor rpm increase with increased airspeed is very slight.

                  I did not feel anything unusual at those speeds.

                  Thank you for your help.
                  Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

                  Comment


                  • #10
                    The increase of the rpm when forward speed increases is a natural phenomenon due to the increase of the stalled area of the blade. In this area, the cL is not proportional to blades A.o.A

                    The μ ratio may exceed 0.4 when the pitch setting is low, or not exceed 0.25 when the pitch setting is high. But a low pitch setting increases the rrpm, so the profile losses, and drag of the rotor is then greater

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                    • #11
                      JC, you should accompany Mike G. on his forthcoming trip to Florida.

                      Comment


                      • #12
                        I spent last summer at the sister of my wife in West Indies, where two cyclones narrowly missed us.
                        The large iron portal was felled away and two trees torn out in the garden.
                        ...But most of all, Chuck, I do not speak English!

                        Comment


                        • #13
                          You’re quite right, JC; travel in the US is difficult for people who do not speak English, but you would have Mike for an interpreter.

                          By contrast, when I was in Europe, English was spoken everywhere except in Yugoslavia.

                          Comment


                          • #14
                            Originally posted by Jean Claude View Post
                            The increase of the rpm when forward speed increases is a natural phenomenon due to the increase of the stalled area of the blade. In this area, the cL is not proportional to blades A.o.A

                            The μ ratio may exceed 0.4 when the pitch setting is low, or not exceed 0.25 when the pitch setting is high. But a low pitch setting increases the rrpm, so the profile losses, and drag of the rotor is then greater
                            It always impressed me how Jay Carter was able to setup his Gyros to "slow down" the rotors while increasing forward velocity, thus reducing drag to almost nothing. He also designed his Gyros to have wings with spans no greater than the rotor diameters. The last model had a wingspan of 34 feet. So the thing flew quite nicely using wings only above 60mph.

                            I'm a big fan of gyros; but I'm also a big fan of engineers who find really good solutions to doing things a little bit differently. For 30 years, my dreams have included private aviation that exceeds anything commercial.

                            I want to go where I want, when I want... live where I want... and play where I want, no matter the vagaries of commercial scheduling.
                            Regards

                            Frank

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                            • #15
                              Its not unreasonable to travel at 200mph, takeoff and land vertically, and still get 20 miles per gallon doing it.
                              Regards

                              Frank

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