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Every stroke as the boat goes faster has a lower drag factor than the previous stroke. Same power, faster handle = lower drag factor. Once the boat is at it's steady state, it's the speed of the boat that determines the drag factor. The faster the boat (for a given oar setup) the lower the drag factor.
C1 is the drag coefficient, units [W s³] However, 1 W = 1 J/s = 1 N m/s = 1 kg m²/s³ . Hence the units of the drag coefficient C1 can also be written as [kg m²], same as for J. The value of C1 for a drag factor setting of 120 is 0.000120 kg m² . For our convenience Concept2 has multiplied the drag coefficient by 10^6 and calls it the drag ...
This is in line with how C2 determines the drag factor: in real-time during the recovery, which lasts about 1 sec. The results : - drag factor 80 : drag coefficient C1 = 0.000078 kg m². - drag factor 100 : drag coefficient C1 = 0.000100. - drag factor 125 : drag coefficient C1 = 0.000124.
Only if the air drag coefficient would change with the rotation speed, could there be a power calibration issue. I am possibly the only one outside the C2 engineers who has monitored the flywheel speed during the recovery phase of high power rowing strokes (800W average ; 2000W peak), but I didn't observe a change in drag coefficient with speed.
The PM5 computes the actual force every stroke by measuring how fast the blades slow. If you do the same work, the PM5 measure the same energy and the finish time is the same. Different drag settings give different starting and ending fan speed on each stroke, but same energy is measured. Higher drag factor = slower fan.
For a BikeErg at constant cadence you may assume that the variation in angular speed during a pedal turn is very small. The equation then simplifies to : P = C * ω³. The drag coefficient C [in kg m²] multiplied by 10^6 equals the drag factor displayed by the PM5. E.g. for a drag factor 120, C equals 120 * 10^-6.
Re: Building my own PM. by Nomath » May 1st, 2023, 12:56 pm. There is no simple proportionality between flywheel revolutions and distance. The distance also depends on the drag factor setting and on the inertia of the flywheel. The formulas you should use are : - power [W] : P = C * ω³ + ω * J * dω/dt ; C is the drag coefficient, in [kg ...
- drag coefficient C1 = 0.000107 (corresponding to a drag factor of 125) ; - the force curve is parabolic with a maximum of 650N and a width of about 0.7 sec ; The results are displayed below in a set of graphs Top-left : the force curve Top-right : the angular speed of the flywheel resulting from this force curve and the momentary handle speed.
This drag coefficient accounts for the actual temperature and pressure, and for air flow inside and outside the flywheel cage, including the damper setting. Regarding your last post: the calibration accounts for the temperature and pressure that existed during the calibration, not during the exercise.
The drag coefficient is related to the drag factor. For a drag factor of 120, the drag coefficient Cd = 0.000120 kg m² Using these two characteristics and a very strong last stroke with a handle velocity of 3 m/s (this corresponds roughly to a 600W stroke!), I calculated the residual energy of the flywheel over time.