Weight and Balance Aircraft attitude

[Battson]

In terms of accuracy and feedback you're soliciting, I would say the level of accuracy is dictated by what you want to achieve with the number. In other words, how you use the CG data should drive the accuracy of measurement arm and weight measurements.

My personal opinion, we don't need engineering precision as the results aren't being used for engineering. Hence, the principle here is look for the highest sources of error, and then determine whether they are material and need to be addressed.

In this case, the largest source of error is probably what we put into the plane (and where we put it). The exact loading condition of the aircraft will likely vary within 3 to 5% at a guess - perhaps more - due to a wide range of errors. Just to name a few for instance, we know the cargo inside the plane can move around during flight, people move around, fuel sloshes around, and we don't weight and determine the CG exactly for everything we put in the plane in the first place.

This is no problem, because the CG limits are far from an exact science in themselves, and can certainly be adjusted by 5 or 10% without any material changes in handling.

So overall, I suggest we should be happy enough with measurements which are within a five percent (arms and weights), as this is likely to be consistent with the other sources of error and accurate enough for the end use.

[jaredyates]

One way we've gotten this far with such measurement vulnerabilities is that we are all (hopefully all) are testing with actual ballast during the Phase 1 and non-US equivalents. The ease of measurement error certainly explains our numerical variations with comfortable CG ranges achieved during testing. Measure as well as you can, but validate with actual flight tests and when it comes to carrying important cargo, employ the data collected in the latter.

[Battson]

I do agree, and would add - we know many people have inadvertently flown with CG locations well outside of the approved envelope (hint: don't do this! ). The plane remains controllable and safe throughout normal flight regimes, albeit unpleasant to fly. This is not a recommendation to push the limits - they are there for your safety, more of a comfort factor that it is unlikely to bite you if you make an error. You'll get a clear message that you got it wrong, and should have time to correct.

[Nev]

It's difficult to know if these people were indeed flying outside the approved envelope, or whether a measurement error during the initial CG calculations caused them to THINK they were, while experiencing fairly normal handling because they were actually inside the envelope but didn't realize it. My own experience is that the Bearhawk gets very pitch sensitive at the aft limit, and I'm quietly understating this.

In a way it's a similar scenario to comparing approach airspeeds that haven't been calibrated for position error. If we tell others that some have flown outside the limits without knowing accurately where those limits are set, there's a risk that they might think it's ok to do the same on their aircraft on which the CG envelope has been measured differently.

I've run a few scenarios on the spreadsheet using various measurements from our weight and balance threads that indicate perhaps this has happened. Hence Jared highlighting the importance of "measure as well as you can, but validate with actual flight tests". Excellent advice.

[Battson]

Oh - we know they were well out....

[Bissetg]

While I agree with the sentiments expressed, keeping it simple and consistent has a lot going for it.

The consequences of getting this stuff wrong admittedly shouldn’t happen if we do our phase 1 testing correctly and incrementally. However, I think for most of us having a standard set of measurements that we know are correct will give us some confidence when doing our own setup.

Thanks for your efforts here Nev.

[nborer]

Certified aircraft have specified tolerances used for testing at weight and CG limits. For FAA-certified airplanes, it used to be covered in 14 CFR 23.21. In the amendment 64, the performance-based requirement is noted in 14 CFR 23.2100 which can be complied with via ASTM F3082 which says basically the same thing as 23.21. Basically, it states that, unless shown for other reasons to be critical, during flight tests, weight can be +5% / - 10%, "critical items affected by weight" are +5%, -1% (I think things like structural strength falls into this bucket), and C.G. is +/- 7% of total travel.

I know we're not "certified" airplanes in terms of having a type certificate, but a lot of these airworthiness requirements are good to know for the experimental world.