The airfoil profile is (NASA predecessor)NACA0015. The symmetrical airfoil is generally known to have the center of pressure CP at quarter blade width from the leading edge during cruise flight when the attack angle is smaller than 2-3 degrees in the research paper. As shown in the picture below, the CP point is at 17.5mm/4 = 4.375mm from the leading edge but the bolting is at 4.3mm, so, when the attack angle is very small, it has perpetual pitch suppression. The research measurement also shows that the airfoil has perpetual up-pitching torque at a cruise attack angle of 6 degrees because of the "center of pressure moving forward of the quarter chord".
When the mechanical margin of error is added between the actuator servos and the actual blade position, the blade angle shifts as the following timeline.
This means that a large slop gap can result in an unstable craft. And in the above example depiction, the slop should be improved from 3.2 degrees to 3 degrees to escape the perpetual pitch suppression slump. And to avoid over-correction, the slop needs to be smaller than 2.1 degrees.
Balancing With Feathering Shaft Slop Larger Than 0.2 mm
Going from 2650 to 3150 and higher RPM requires fine-tuning the rotor with hand-tethered hovering. Here the R/D test retained the original decal of the blades. And the tuning took 3 tries in the research process, as pictured below. First, I fixed the tremoring by cutting out a corner of a decal. When aggressively trimming the corner, it was over-corrected, and tremoring reoccurred. The balance was restored by cutting the corner of the other decal. Once the amount of taping was determined, all decals were removed, and the clear tape was put in place for the required balancing. The balancing progress during the research is the 4 pictured below from left to right. The conclusion is that one square centimeter of tape makes all the difference because the rotor is stretched by centrifugal force like a musical instrument wire, and the 3150 RPM is on one of the resonance frequencies of the device. The thickness of tape 0.002-0.003inch = 0.005-0.0075cm, weight (assuming density 1g/cm-square) 0.005-0.0075 grams. This means RPM3150 needs a precision balance of 1/100th of a gram.
However, during multiple test builds, it was discovered that most builds could not achieve smooth balance and that blade swapping was needed. One of the successful blade pairings was pictured above on the right with a green checkmark. The 2 distinct symmetries, rotation and mirror, for balancing the rotors is diagramed here.
The swapping needs to restore the rotational symmetrically of curved blades because our rotor hub does not have a cord-wise balancing bolting like the Bell 222 rotor, as highlighted in the picture on the right. The 2 blades with up-down warpings can be none-mirroring as the right side in the above diagram when subjected to 3250+ RPM's centrifugal force. The solution is to pick and match a pair that dynamically match the warping to each other.A new blade with natural backward curving when spun up (not the same curving at rest) can be matched to a old blade with crash-induced backward bending, as shown in the above example with a green checkmark; the older blade's color fades, but it is the best match picked out from 3 different sets of new purchases. The at-rest warping mismatch shall be disregarded. At rest, one may warp up, and the other down, but the forces may balance out when spun up to 3250+ RPM, depending on the material's internal stress. All the other blades from the 3 sets fail to match the lower blade in the picture. The only way to check the pairing is to spin them up to full speed. And each pairing test needs to be checked with 2 alternative orientations of the grip installation. In the pictured example, only one orientation can produce a smooth spin. The other orientation always has vibrations that can't be tuned out with scotch tape.
Before the pairing process of a purchase, mark each blade with a unique serial number you manage, then weigh each blade individually and note the heaviest blade. The following pictures show an example of the heaviest blade, blade 3, of 7.301 grams. Then use scotch tapes spread evenly on each blade (except the heaviest one) to patch all the blades in the purchase to the heaviest weight. This ensures that the pairing process focuses on matching the the warpings of the blade material.
So, what happens if the rotor is a non-symmetrical mismatch? RPM detection with Engine RPM Android App can not lock and detect the RPM.
Use the Android app Engine RPM to measure the rotor's RPM with the settings in the screenshot below. The app automatically logs the detected RPM every 3 seconds, as seen in the screenshot below. There is a 1.5% discrepancy between the PWM control signal and the actual measured value. The PWM controls are pictured with the respective actual RPM logs below. We use the actual measured RPM as standard.
The ultimate smooth balancing has a lateral (on the side) ballast, in addition to the axial balancing, as follows with 24 AWG copper wires.
The first loop has the crossing beneath the DFC bolting and excess wire trimmed. Then, pinch and turn the wire for a few turns. In the example on the right, the 7 loops on one side are the correct balancing because 14 loops were tried on one side, and zero loops on either side were also tested to be lesser balance. This example ballast weighed 0.2 grams with 7 loops. However, this level of ultra balance is not required for smooth HD video shooting.
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