Center of Mass: Add Hints/Bumps on wings for correct location

[StephenCarlson]

Issue title says it all. Most R/C planes have a hint of some kind for where the aircraft should balance. On a plank, this is even more needed.

Lumped into this issue is the option to add custom datum points on the aircraft, and a corresponding jig or holder bracket, such that the aircraft can be weighed on a kitchen scale with one end off-scale, such that precision Center-of-Mass measurements can be made for the vehicle.

[StephenCarlson]

Not quite solved by ede11e9, but a superior solution is implemented. I've added "jacking points" to the nacelles and tail which allow for a Weight-and-Balance Stand to hold the vehicle while measuring the force from exact datum offsets. Specifically, we want to measure the total weight of the vehicle, and then place the vehicle on the two forward jacking points with the motors in the forward position, and then position the tail jacking point on top of the scale, and then conduct the force contribution of the third (tail) jacking point through the scale. As shown below:

Having the total force and tail force, we can calculate the exact center-of-mass location. The equation is:

CoM_LeadingEdge_Offset = (TailJackingPointMass * 220mm + (TotalVehicleMass - TailJackingPointMass) * (-29.3mm)) / TotalVehicleMass

Remember to remove (tare) the mass of the tail jacking point stand; we only want the actual tail force contribution.

We want the Center-of-Mass Leading-Edge Offset to be, at the absolute maximum, 30mm. 26mm to 28mm is more stable but the further toward the nose the value drifts, the more nose-up trim required. Going past 30mm is hazardous, as the neutral point exists between 34mm and 36mm. The static margin must be sufficient that the vehicle is pitch-stable.

Again, this does not add "balance bumps" to the wing, and I discourage trying to balance this vehicle by hand due to painful experience when I have casually balanced the aircraft in the field, only to later discover that merely balancing on your fingers is very error-prone.

The STLs for the Weight-and-Balance Stand are included in the STLs folder. Constructing it is straight-forward; it uses the same 1.8mm steel rod pieces as are used for the nacelle hinge pin and servo linkages.

[StephenCarlson]

Reopening this issue, as there will inevitably be folks out there that don't have a digital scale and will insist on the finger-balance-bump method. The solution I've got feels pretty good from the test prints I've made of the "bullseye" as shown here:

[StephenCarlson]

This photo shows the real-world weight-and-balance set, with the digital scale positioned under the tail support, and tared to zero to avoid including the mass of the tail support in the calculation. Again, aim for 26mm to 28mm back from the leading edge, or 2mm to 4mm nose-ward of the flight controller mounting hole (these are 30mm back from the leading edge).

[StephenCarlson]

The new CofG bump design is present in the Solar wing variant, pushed at 81abe56c47fa66ecf6d3cfdf3e897b6ba21a7ef4. Will add to the "Vanilla" wing when I get the chance.

[StephenCarlson]

The Plain/Vanilla wing has been pushed as of ce00e7db2625b679fa5b0644c8a87afdc569e009 . Therefore, now that all variants of the wing have been published, and all variants have the C-of-G bump, this task is resolved. I need to scrape all the text and images from this issue and put it in the regular README, since this explains much about how the balancing method works for both the jackstand digital scale method, and the finger-balancing C-of-G bumps.

[StephenCarlson]

Quick note: With the new nacelle design, which looks like this:

Please use the following equation, which is the same as above but with 55mm instead of 29.3mm.

CoM_LeadingEdge_Offset = (TailMassMeasurementNotIncludingTheJackItself * 220mm + (TotalVehicleMass - TailMassMeasurementNotIncludingTheJackItself ) * (-55mm)) / TotalVehicleMass

And as usual, the output of this equation CoM_LeadingEdge_Offset should be equal to 28mm, as close as you can get it or slightly forward up to 26mm or so. Don't let this get back behind 30mm!

[StephenCarlson]

The following table ONLY applies for the new nacelle with the conical cusps located at 55mm forward of the leading edge. The Total Aircraft Weight when placed on the scale alone is shown in the left column. The corresponding right value is what the tail apparent weight should show on the digital scale, with the tail support jack tared to zero such that it is not included:

28mm Center of Gravity - Required Measurements Table with "New" 55mm cusp nacelle design	Total Aircraft Weight (g)	Measured Tail Weight (g)
1000	302
1020	308
1040	314
1060	320
1080	326
1100	332
1120	338
1140	344
1160	350
1180	356
1200	362
1220	368
1240	374
1260	380
1280	386
1300	392
1320	398
1340	404
1360	410
1380	417
1400	423