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  • Rotor Blade Length

    I was wondering if anyone would mind elaborating on the benefits and dis-advantages of rotor blade length changes. I'm focused on 2 place machines in general and outside of the obvious lift increase/decrease. I'm wondering why folks (including manufacturers) settle on the length they have and the mission they fly with them or what they like about that size.
    Thanks,
    Paul
    (925) 481-4304

  • #2
    There are probably disadvantages for very long blades, and one of them has little to do with flight mechanics or with aerodynamics, but with the usual size of hangars. Most hangars built in Europe for small planes measure no more than 9 meters from the door to the back wall...

    Comment


    • #3
      Larger diameter creates more lift, because of a greater square footage of the disk area. And also turn less Rotor rpm so the tips do not go super sonic. (tips traveling feet per second)
      You could take your standard 23' rotor on your single place machine and put it on the two place machine. Because of the increased gross weight, the rotor is turning faster to produce the extra lift which is required for the additional weight. Highly likely the increased rpm will take the rotor tips speed super sonic.
      To calculate the wing/rotor loading: Radius squared times Pie divided into all up weight.
      My Dominator weighs in at 750 pounds and has a 27' rotor. Half of 27' is 13.5'
      13.5 squared = 572, 750 divided by 572 = 1.25 for my wing load.
      Last edited by Gyro28866; 08-17-2017, 04:41 AM.
      David McCutchen
      615-390-2228
      Bensen B7m, 90 hp Mac
      Dominator Tandem, 100 hp Hirth
      Kolb Mark III Classic, 80 hp Verner
      Certified - Advanced Master Beef Producer
      EAA Member #0511805
      PRA Member #28866
      PRA Chapter 16 Member
      Secretary & Treasure - PRA Chapter 16
      President / Sylvia - Yellow Creek Volunteer Fire Dept.
      Chairmen - Dickson County Veteran's Day Committee
      Volunteer - Dickson County Airport Aviation Day Committee
      2 busy 2 No!

      Comment


      • #4
        13.5 squared is 182.25. Should we also figure the cord of the rotor?

        I thought wing loading was the weight divided by the wing area.
        Last edited by AirCommandPilot; 08-17-2017, 05:37 AM.
        Bobby Munroe
        Private Pilot (SEL)
        PRA Chapter 62 #42748
        EAA #1160523

        Comment


        • #5
          Originally posted by Gyro28866 View Post
          Larger diameter creates more lift, because of a greater square footage of the disk area. And also turn less Rotor rpm so the tips do not go super sonic. (tips traveling feet per second)
          You could take your standard 23' rotor on your single place machine and put it on the two place machine. Because of the increased gross weight, the rotor is turning faster to produce the extra lift which is required for the additional weight. Highly likely the increased rpm will take the rotor tips speed super sonic.
          To calculate the wing/rotor loading: Radius squared times Pie divided into all up weight.
          My Dominator weighs in at 750 pounds and has a 27' rotor. Half of 27' is 13.5'
          13.5 squared = 572, 750 divided by 572 = 1.25 for my wing load.
          In my opinion you are calculating the disk loading rather than the blade loading.

          For The Predator with her 30 foot blades the disk loading is about 1.98

          I feel the blade loading is calculated by taking the area of the blades and dividing it into the gross weight. For The Predator that comes out to about 21.2 square feet of area and a blade loading of 66 pounds per square foot.

          It appears to me if I shortened the blades on The Predator to 23 feet at gross weight (1,400 pounds) at sea level they would turn about 550 rotor rpm and have a tip speed of about 450 miles per hour. This is well below the speed of sound at standard temperature and pressure of around 761 miles per hour.
          Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

          Comment


          • #6
            The primary influence of rotor diameter and therefore disc loading is its effect on low speed performance. If you want to fly slow, you need low disc loading. The power consumed by lift, the work of imparting downward momentum to the oncoming air mass depends upon airspeed; at low airspeed, the rotor encounters less air mass per second so must fling it downward at a higher velocity than at higher airspeed where the the rotor encounters more air mass per second. Flinging air at high velocity burns power, the effect showing up as drag. This is known as induced* drag.

            Rotor tip speed at any given air density is primarily influenced by blade loading, the AUW of the gyro divided by area of the rotor blades. Dragging the rotor blades around in a circle at high speed also burns power as the result of both skin friction drag and form drag. This is called profile drag and is the dominant consumer of power at cruise speed and higher. Profile drag varies as the cube of tip speed; increasing tip speed by 10% burns up 33% more profile drag power (1.1 x 1.1 x 1.1 = 1.33).

            Rotor RPM by itself isn’t very relevant.

            *Looks like the aerodynamicists borrowed their terminology for Maxwell’s magnetic induction theory.

            Comment


            • #7
              Sorry Bobby:
              I was in a hurry this morning when I was typing the earlier post.
              radius squared time pie equals area
              27' rotor divided by 2 = 13.5
              13.5 x 13.5 = 182.25, 182.25 x 3.14 = 572.265
              Gross weight 740 divided by 572 = 1.29 pounds per square foot of disk load.
              My bad, Sorry


              David McCutchen
              615-390-2228
              Bensen B7m, 90 hp Mac
              Dominator Tandem, 100 hp Hirth
              Kolb Mark III Classic, 80 hp Verner
              Certified - Advanced Master Beef Producer
              EAA Member #0511805
              PRA Member #28866
              PRA Chapter 16 Member
              Secretary & Treasure - PRA Chapter 16
              President / Sylvia - Yellow Creek Volunteer Fire Dept.
              Chairmen - Dickson County Veteran's Day Committee
              Volunteer - Dickson County Airport Aviation Day Committee
              2 busy 2 No!

              Comment


              • #8
                Something did not seem right about these numbers, so I went and refigured them.
                Here is my thinking out loud:
                Empty weight = 472#
                Pilot (me) = 220#
                Fuel 7 gallons= 42#
                Gross weight = 734#
                ----------------------------------------
                Empty weight = 472#
                Pilot (me) = 220#
                Fuel 2 gallons= 12#
                Gross weight = 704#
                ---------------------------------------
                We have already determined my Disk Area is 572 square feet
                ---------------------------------------
                Most of my flights are Solo and flights of less than one hour; so I am calculating the range of my disk loading for an average flight for me in my tandem Dominator.
                ---------------------------------------
                So, 734 # divided by 572 = 1.28 disk load at normal takeoff solo weight
                and 704 # divided by 572 = 1.23 disk load at normal landing solo weight

                This light disk load is why I am only turning 304 RRPM at 65 MPH. Ernie has told me his recommended minimums for his 27' rotor is 310 RRPM. If the Density Altitude is up around 3500'. I see numbers above the recommended RRPM; or if I put passenger in my rear seat. Then I see numbers from 345 to 365.
                having said all this; when I slow fly or even in a vertical decent. I see numbers decay down to the high 280's. When my numbers are below the 300 mark, I do not throw the machine around. I dance with this lady, and it is a Waltz.
                I have had several Pilots comment about how slow my machine will fly, and even had some try to slow down with me; and they fall out of the sky. I can maintain straight and level at 15 mph IAS at about 75% power setting.
                That is the other part of the equation; 100 hp through a 74" three blade propeller.
                Last edited by Gyro28866; 08-17-2017, 03:51 PM.
                David McCutchen
                615-390-2228
                Bensen B7m, 90 hp Mac
                Dominator Tandem, 100 hp Hirth
                Kolb Mark III Classic, 80 hp Verner
                Certified - Advanced Master Beef Producer
                EAA Member #0511805
                PRA Member #28866
                PRA Chapter 16 Member
                Secretary & Treasure - PRA Chapter 16
                President / Sylvia - Yellow Creek Volunteer Fire Dept.
                Chairmen - Dickson County Veteran's Day Committee
                Volunteer - Dickson County Airport Aviation Day Committee
                2 busy 2 No!

                Comment


                • #9
                  Thanks for the several analysis given here. And, being a relative newbie here, I have questions about rotor dynamics, based on what you have said.
                  These questions come with the assumptions of the same rotor blade length.

                  1. What effect does rotor airfoil profile have on rotor speed, and lift? Say for flat lower surface, semi-symmetrical, and full-symmetrical profiles?
                  2. What effect does rotor chord width have on rotor speed and lift?
                  3. And finally, what effect does rotor angle of incidence from the rotor hub have on rotor speed and lift?

                  Frank
                  Regards

                  Frank

                  Comment


                  • #10
                    Hello Frank:

                    Question#1: That sounds like a Chuck Beaty question.

                    Q#2:I would think a larger cord at the same rotor rpm would produce more lift. But, because our rotors self regulate for the load (all up weight) on them at that time; and if you have not increase the load, then the rotor will decrease the rpm so that lift produced is equal to all up weight. Ernie at Rotor Flight Dynamics is producing 7" and 8 1/2" cord blades. I have not heard about RRPM, however the larger cord required less HP to climb and even less for cruise. The reduced HP tells me the larger cord are more efficient. Likely a reduction in Angle of Incidence and RPM both; which combined produces less Induced Drag and therefore the extra HP can produce a better climb rate or reduce power to maintain the same rate of climb. A reduced power setting for cruise would extend the range of the fuel on board. This is definitely an Ernie question; because he can attest to actual numbers.

                    Q#3: Lets work the question backwards. Because our rotors self regulate RPM for the lift required for the Gross Weight at that moment. If the machine weighs 1000#, then the rotor has to produce 1000# of lift to sustain straight and level flight. Rotor Angle of Incidence and the velocity of the inflow of air will dictate the rpm required to produce the lift required. Example: At 35 mph IAS the AOI is greater and the RRPM is less to produce the 1000# of lift, compared to a 70 mph IAS, at 70 mph IAS the AOA will be less and the RRPM will be higher to produce the same 1000# lift.

                    All aircraft contend with the 4 same forces in flight. Thrust to Drag and Lift to Gravity/weight. When an aircraft is in a Straight and Level Un-accelerated steady state condition, all four of these forces at equal. We manage the energy we have about these four forces. Increase only thrust and the craft accelerates; increase only drag and the craft decelerates, increase only lift and the craft gains altitude, and increase only weight and the craft loses altitude.
                    The Pilots skill and knowledge to manage the IMBALANCE of these four forces is what allows him to Waltz around the three dimension environment in which we operate (fly).
                    Now, take a moment and let that last statement soak in.
                    David McCutchen
                    615-390-2228
                    Bensen B7m, 90 hp Mac
                    Dominator Tandem, 100 hp Hirth
                    Kolb Mark III Classic, 80 hp Verner
                    Certified - Advanced Master Beef Producer
                    EAA Member #0511805
                    PRA Member #28866
                    PRA Chapter 16 Member
                    Secretary & Treasure - PRA Chapter 16
                    President / Sylvia - Yellow Creek Volunteer Fire Dept.
                    Chairmen - Dickson County Veteran's Day Committee
                    Volunteer - Dickson County Airport Aviation Day Committee
                    2 busy 2 No!

                    Comment


                    • #11
                      Originally posted by C. Beaty View Post
                      ...This is known as induced* drag.
                      *Looks like the aerodynamicists borrowed their terminology for Maxwell’s magnetic induction theory.
                      I guess it's just the shortcut of " induced drag by the lift"

                      Comment


                      • #12
                        Originally posted by eutrophicated1 View Post
                        1. What effect does rotor airfoil profile have on rotor speed, and lift? Say for flat lower surface, semi-symmetrical, and full-symmetrical profiles?
                        2. What effect does rotor chord width have on rotor speed and lift?
                        3. And finally, what effect does rotor angle of incidence from the rotor hub have on rotor speed and lift?
                        Question#1 : Single camber profiles should be avoided due to the torsional moments generated.
                        The double camber at zero moment may seem better than the symmetric sections because of a higher Cl max, but this advantage exists only near the hub ie in a zone of weak effect.

                        Question#2 : As David said, the rpm and friction loss decreases as the chord increases.
                        However, two phenomena acts in the opposite direction:
                        - decrease the rrpm requires heavier blades to hold the same coning, and therefore more induced power.
                        - Increase the chord increases losses due to tip vortices.
                        In the end, the optimum chord is smaller than that generally used, but it is also required to resist the twisting or also the bending at rest.

                        Question#3: With a given lift and velocity, a lower Cd min of blade section requires a lower angle of attack and consequently produces less rotor drag
                        Last edited by Jean - Claude; 09-13-2017, 11:09 AM.

                        Comment


                        • #13
                          Originally posted by Jean - Claude View Post
                          I guess it's just the shortcut of " induced drag by the lift"
                          The aerodynamicists plagiarize Maxwell even more when they get into ground effect and talk about “images.”
                          Sounds like the levitation of a magnet over a super conductor.
                          Last edited by C. Beaty; 09-13-2017, 06:50 PM.

                          Comment


                          • #14
                            Jean - Claude:
                            Question#2 : As David said, the rpm and friction loss decreases as the chord increases.
                            However, two phenomena acts in the opposite direction:
                            - decrease the rrpm requires heavier blades to hold the same coning, and therefore more induced power.
                            - Increase the chord increases losses due to tip vortices.
                            In the end, the optimum chord is smaller than that generally used, but it is also required to resist the twisting or also the bending at rest.
                            Spank:
                            Bring it back into Laymens' terms:
                            The rotor is sized (diameter) for the AUW (all up weight) for the average mission of a particular gyro. So that that particular rotor will operate in a specific rpm range. That rpm range will produce a predetermined amount of CONING angle. If you draw a line from the tip of one blade to the tip of the other blade (tip plane), and you look at the vertical dimensions at the hub bar and teeter bolt in relation to the tip plane. Then the Teeter bolt should be at the 50% point; and we are flapping the rotor on the teeter at the vertical CG.
                            As I decrease my airspeed, the rotor rpm decreases; primarily because of an increase in Angle of Attack of the rotor. The main point is the slower rotor rpm also reduces the centripetal forces being applied within the rotor. The reduction of centripetal force will cause the rotor to cone up even more. Then the teeter bolt is below the vertical CG; and there is insufficient under-sling (under-sling being a measurement from the Teeter bolt to the hub-bar). This can create a 2-per rev vibration.
                            Keep in mind, also: as the rotor rpm is reduced the Flapping angle increases to compensate for dissymmetry of lift.
                            David McCutchen
                            615-390-2228
                            Bensen B7m, 90 hp Mac
                            Dominator Tandem, 100 hp Hirth
                            Kolb Mark III Classic, 80 hp Verner
                            Certified - Advanced Master Beef Producer
                            EAA Member #0511805
                            PRA Member #28866
                            PRA Chapter 16 Member
                            Secretary & Treasure - PRA Chapter 16
                            President / Sylvia - Yellow Creek Volunteer Fire Dept.
                            Chairmen - Dickson County Veteran's Day Committee
                            Volunteer - Dickson County Airport Aviation Day Committee
                            2 busy 2 No!

                            Comment


                            • #15
                              Using "images" to mock up specific boundary conditions is a common technique used to solve some class of physics and math problems. I wouldn't call it plagiarizing just because it crops up in E&M as well as in fluid dynamics.

                              The term "induced drag" holds nothing in common with "induced voltage/current/charge" used in electrodynamics except for the word "induced". It just signifies that drag (or voltage/current/charge) is a secondary effect to some primary cause. In the case of aerodynamics the primary cause is the lift generated by the airfoil which brings about (= induces) drag.

                              Greetings, -- Chris.
                              Read about my trip across the USA in an MT03 gyro here.

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