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CG for 0012 airfoil?

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  • CG for 0012 airfoil?

    From what I have referenced, the NACA 0012 airfoil has a max thickness of 12% positioned 30% back from the leading edge. using this airfoil for either main blades or tail blades, would it be safe to assume that the pivot point can be placed at the max thickness (30% back from the leading edge) and move the CG of the blade somewhere ahead of that pivot point? Say 25% or 20% of the chord length is where the CG is found? I have a helicopter aerodynamics book and I imagine the answer to this is buried somewhere in it's 1600 pages of calculus based physics, but I am hoping that someone could shed some light for me before I resort to that monster of a volume that would likely take 10 pages to say yes or no.

  • #2
    The aerodynamic center of the NACA 0012 is located at 25% of chord which is also where the CG should be located.

    The location of the feathering axis is less critical; centrifugal force acting on the CG takes care of that; 30% of chord should be OK.

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    • #3
      I opted to use 0012 extrusions for simplicity and strength. The trouble is that they are quite tail heavy with about the last inch of trailing edge being solid aluminum. I could fill the front ends of the extrusions with weights, but with the available moment being so close to the CG, I believe I will end up with some very heavy tail blades. Would the effect be the same if I were to incorporate a small boom on the blade grips that allowed me to hang weights a few inches forward of the leading edge and bring the CG to 25%? If it works, then that would save a lot of weight and also impart less centrifugal force to the spherical bearings that I wish to use for pitch changes.

      Here are the extrusions I plan to use:

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      • #4
        Here are a couple pictures of a trail rotor from a Rotorway. I see that weights were used ahead of the feathering shafts, though I am not quite sure that the point of it was to shift the CG forward.

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        • #5
          A couple thoughts.

          First, for main blades, any noseweight must be located well out toward the tips (not at them, though). Blades that are 10, 12 or more feet long simply aren't stiff enough in torsion for the weight to be inboard while the nose-up twisting force is mostly outboard. Bensen set the pattern for the location of noseweights; get a copy of his plans for an idea of where on the span the weights should go. Roughly 2/3-3/4 span.

          Second, Chuck Beaty of course is correct that the nominal location of the blade CG to minimize twisting effects caused by misalignment of the aerodynamic center and CG of the blade is ... at the aerodynamic center. But as a footnote, at least, it's interesting to note that people have sometimes broken this rule in the past -- intentionally or not. The results have been anywhere from quite scary to mildly useful; break the rule at your own risk, though. Example:

          McCutchen Skywheels are built of fiberglass, but in a pattern much like your extrusion (relatively thick walls near the trailing edge). As a result, they are tail-heavy. The amount of down-force created by the mass of the blade at the CG is a function solely of blade RPM and G-load. (Blade RPM affects it as long as the rotor is coned.) The amount of aerodynamic force at the blade's aerodynamic center, OTOH, is a function of BOTH RRPM and blade angle of attack (AOA). As a result, when blade AOA increases (initially without an increase in RRPM, which takes place over time), an imbalance of forces results that twists the blade nose-up. This, in turn, adds to the blades' initial increase in AOA. The result is a ballooning effect when you pull the stick back (or plow into an updraft). It's hard to say this effect is particularly useful, though it's harmless in small doses. On heavy gyros, it can cause control problems.

          One of the early NACA reports on full-scale wind-tunnel tests of a Pitcairn gyro rotor mentions that a similar aft-CG setup was intentionally built into the test rotor. The narrative states that this arrangement is intended to twist more pitch into the blades when RRPM rises -- IOW, it was supposed to create a constant-RRPM effect. This idea appears to have been an evolutionary dead-end, though.



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          • #6
            So, it sounds like the tail heavy nature of the blades caused some delayed twisting to happen when used with full sized rotor blades. The pictorial I have of the extrusion would only be used for the tail blades. The actual cut length of extrusion would be around 20" as there would be a bolted blade root with spherical bearings to connect up to the hub. The total diameter of the rail rotor would likely be around 48". With minor pitch changes used to swing the tail in one direction or the other. My impression is that the possible amplified twisting effects of tail heavy blades are not as critical as they would be in 10 foot versions. The main thing I am concerned about is an induced pitch flutter. I imagine if that went down, the setup would last perhaps a second or two and then proceed to fly apart.

            Where could I find some nice reading material on topics like this?

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            • #7
              Jake,
              Under-balancing the chord for tail rotor blades is not critical because the stiffness is very large compared to the short length and low thrust.
              The weights on your picture do not have the function of avoid the twist. They reduce efforts in the rods of the pitch control .

              Doug,
              Yes, the aft-CG setup was intentionally built into the PCA 2 rotor. This arrangement is intended to twist less pitch into the blades when RRPM descends with his fix wings wich relieves the rotor. Due to this, Mu limit reaches 0.7. Very judicious.

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              • #8
                Jean, thanks for the explanation and clarification on the weights. I have lots to learn here :).

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                • #9
                  Click image for larger version

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ID:	1136207Click image for larger version

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ID:	1136208 Centrifugal force acting on a rotorblade’s mass fore and aft of the feathering axis tries to force it to flat pitch. The bob weights at the root ends of blades are to relieve collective pressure in the case of the Bell-47 and torque pedal pressure in the case of the BO-105.
                  JC beat me to it.

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                  • #10
                    Funny how I am just now learning about stuff they already figured out 60 years ago .

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