Saturday, January 7, 2023

IoT Ultimate Light Weight

Swarm Litter Collecting For Reuse

Now this IoT concept changes the following aims
- short endurance travel and crash landings for reuse
- short distance missions and hop to reach targets
- low-frame-rate/resolution video footage
- low altitude surveillance
- low wind resistance
- *high crash resistance 
It will be based on 120-gram helicopters, pictured upper-right here. A sub-80-gram craft, as pictured lower-right here, should not be considered because it was reported in the video that the payload of the sub-80-gram system could not exceed 8 grams while the lightest IoT board with a camera weighs more than 8 grams.
The swarm can perch on top of building roofs, street lights, and telephone poles. And once the video surveillance mission is complete, they can be swept down, or air-blown down, from high places for collection. The missions themselves don't need to plan their landing, instead, they can just drop from the sky with little impact with the low weight. High fuel economy will be needed to carry less battery weight and reduce craft weight.

Naturally Blade 180 CFX parts are good for this concept because we already use the 180CFX rotor hub. The Align 15T or OMP M1 weighs 120-125 grams on takeoff,

, blades are 120mm long.

Motor Selection For The Lift Coefficient

So, according to NASA's lift equation and XFoil's report that Cl is the same for NACA0012 and NACA0015 foils of Oxy2 blades and 180CFX blades, what should the motor KV be to lift the new craft? We used 300KV with 4S battery pack. The cord length of Oxy2 and 180CFX blades are 18mm and 19.5mm respectively, nearly identical. It should not be a factor in calculating the new KV. We don't have a fixed V in the CL equation, so some calculus will be needed. v is proportional to the radius r from the rotor shaft toward the tip. Every infinitesimal sliver of the wing's v^2*A is (omega*r)^2 * 18e-3 dr. Integrating the slivers means that the lift is the third order proportional to the rotor's diameter. When rotor diameter is doubled, with everthing else being equal, lift will be eight times as large. Oxy2's rotor diameter is 477mm; 180CFX's rotor diameter is 360mm. If at equal RPM, the new craft's lift will be 250g*(360/477)^3 * 19.5/18= 116.4g . That is 116.4/150 = 0.776 fraction of the needed lift. Because lift is proportional to the square of velocity, and omega is proportional to cell S count, we need KV 300*sqrt(150/116.4)*4/2 =681. And for large error tolerance similar to using 400KV instead of 300KV in the extreme diving build, 681 * 400 / 300 = 908. Our motor is 1500KV with a delta configuration out of the factory. Modifying it to a Y configuration will reduce it to 1500/1.7=882, which fits in the range of 681 and 908.

The DFC geometry, whether M1, 180CFX, 180Smart, or Oxy2, is ultimately copied from Trex250's 12mm pitch control arm width as investigated in alternatives.

Blade Root
Pitch Arm
Main Shaft
(All in mm unit, all feathering shaft 2.5)

So, the overall build material changes are here in the following spreadsheet.

The identical geometry would mean that all our general IoT's pitch curves will be identical. PIDs will be identical if P/I ratio linear range is wide enough to cover the new craft weight of 125-200g.

Or, will they? Considering the new swash plate's upper arm is half the length of 12mm that is the arm length of existing horn arm, swash lower arm, and blade pitch arm. The existing system's blade pitch obviously rotates identical angle as the servo horn's rotation angle.

  1. When servo rotates 10 degrees for cyclic control the linkage moves sin(10)=0.17 fraction of horn arm. The opposite servos acccomodate with counter rotation to prevent collective pitch from changing.
  2. Horn arm length is 12mm, so linkage moves 0.1737*12=2.08mm vertical.
  3. Swash lower arm moves 2.08mm.
  4. Swash upper arm moves 1.04mm vertical and slightly inward horizontal, say 0.1mm.
  5. Here comes to the decision point of the DFC arm length and the distance between feather shaft and swash. Ultimately, if this distance is zero, it doesn't matter what the swash upper arm lengh is, the blade pitch rotation is always the same as servo horn's rotation. But lets consider that mentioned distance is also 12mm. DFC arm is sqrt(6^2 + 12^2)=13.4mm.
  6. Blade pitch arm moves nearly as much as the swash upper arm's movement of 1.04mm.
  7. Cyclic pitch rotates about half of 10 degrees.

 So, all cyclic control is basically scaled proportionally to the length of the swash upper arm length.

But the difficulty in implementation is extending the 3mm shaft into the motor 2205-2306 with a precision drill hole for the rotor hub retention bolt. The M1's main shaft is only 62-66mm because the servo cage of M1 is only 20mm tall while Emax 9051 is 30mm tall. Remember the M1's full height is only 88mm including ground clearance below the motor. The 230s frame needs a 90mm shaft. The coaxial helicopters likely have very long shafts but they are rare these days. Shaft couplers are usually very heavy and introduce vibration. Gearing appears inevitable. But one last option is to use the main shaft bearing itself as the coupler with CA glue. The problem is that the bearing only has a thickness of 3mm, and each half of the shaft can only use 1.5mm of such a thin coupler. Remember, our main motor's fasteners do not provide any vertical or horizontal anchoring, only rotational anchoring. Instead, the shaft itself provides vertical and horizontal anchoring, which leaves the CA glue to be entirely responsible for suspending the motor vertically. During a crash, the glue may break and the dangling motor can wreak havoc on everything. And the straightness of installing with such a short coupler also is a question. So, using a 3mm shaft with an M3 thread die with motors that have M3 bolting may be the way to go. But the back bolting often is permanently locked out of the factory. 

CG Setting

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