Item 1195

DESIGN: Dragonfly ~ Rotor - Hub - Blade Generated Forces - Preliminary:

The preliminary rough calculations of the blade generated forces at the tri-teetering rotor.

Mechanical Design and Description ~ Rotor Hub section is the source of this page and the calculations. [Source ~ MDD p.4-48]

B. Rotor Hubs:

2. Design Considerations:

a. Flight Conditions:

a1. Normal Operating Conditions:

  • Blade's CG - Adjusted for Offset: ~ by DBJ
    • This book considers the blade's CG to be 0.45R for a tapered spar and plan or be 0.48R for a constant spar section. The offset on the Dragonfly [a] is 13.37" from the center of the mast and the rotor's radius is 96". Therefor, the Dragonfly's blade's CG will be 13.37 + (0.45 * (96 - 13.37)) = 50.5"; which is 50.5 / 96 = 0.53 R
  • Blade's Weight - Adjusted SynchroLite Blade: ~ by DBJ
    • The blade weight of the SynchroLite's VR-7 is 6.25 lbs. This includes a 1 lb tip weight. It has a 5.25' chord, no taper, and a rotor radius of 8.67 ft. For confirmation, see: OTHER: Helicopter Specification - SynchroLite
    • The SynchroLite's grip weighs 1.5 lbs.
    • The Dragonfly has a 4.58" chord, no taper, a rotor radius of 8.0 ft., and an offset of 13.37"
    • The cross sectional area of the Dragonfly's blade will be (4.58 / 5.25)2 = 0.76 that of the SynchroLite's blade, and the grip and tip weight will be (4.58 / 5.25)3 = 0.66
    • Assuming that the tip weights have the same 0.76 ratio, the weight of one Dragonfly blade will be Wb = ((6.25 - 1) * 0.76) + ((1 + 1.50) * 0.66) = 5.64 lbs = 2.56 Kg.
    • The Dragonfly's blade will be slightly larger than 2/3's that of the SynchroLite therefore 1.05 * 5.64 lbs = 5.9 lbs = 2.96 K

    Rotor rpm = N = 650 [rpm]

    Number of blades per rotor = b = 3

    Lift of one rotor = L = 275 [lb].

    Radius to the GG of the blade. From above, the radius to the CG of the blade is 0.53R. Therefor r = (D / 2) * 0.53 = (16.0 / 2) * 0.53 = 4.24 ft = 1.29 M

    Normal centrifugal effect = CF = 0.000341 * Wb * r * N2. = 0.000341 * 5.9 * 4.24 * 422,500 = 3,577 lbs = 15.911 N [Comment ~ this value is not much less then the Initial Rough SynchroLite calculation, and also, the SynchroLite's rough calculation is very close to the more detailed one.]

    Average coning angle = β0 = tan-1 (L / (b * CF)) = tan-1 (275 / (3 * 3,577)) = 1.47 degrees

    For the more detailed calculation see; DESIGN: Dragonfly ~ Rotor - Disk - Coning Angle

    Normal operating torque (1 rotor) = T = 63,000 * hp / 700 = 63,000 * (50 / 2) / 700 = 2250 lbf-in = 188 lbf-ft = 254 N-m

    For the more accurate value see; FORM: Dragonfly ~ Power Train

    Torque per blade = Tb = T / b = 188 / 3 = 63 lbf-ft = 85 N-m

    Lag Offset = e = Tb / CF = 63 / 3,577 = 0.0176 in feet?

    Normal lag angle = sin-1(e/a) + sin-1(e/r1) = sin-1(0.0155/13.37) + sin-1(0.0155/(50.5 - 13.37)) = 0.066 + 0.024 = 0.09 degrees.

    Same Page ~ Different Craft: ~ SynchroLite (CVJ) ~ UniCopter

    For comparable calculations on SynchroLite (2-blade) see; DESIGN: SynchroLite ~ Rotor ~ Initial Rough Calculations for Rotor

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    Last Revised: November 5, 2004