Weight and ballast

[Kenn Sebesta]

Hi--

I just joined this group and have very much loved absorbing everything here. Even though it's just 20 some-odd posts, it's already an amazingly useful resource. Reading through all the docs, I have a question regarding empty weight, with a follow-up about how it affects ballast.

It seems to me that the advertised weight (213kg) and the actual logged weight (243.5kg for SN016 and 250.3kg for SN017) disagree dramatically. Of course, what plane ever came in at published spec and L/D? However, this is a 20% difference, which is substantial. Is this accounted for because the 213kg published weight is without instruments, FES, etc? But Table 6-4 doesn't even consider a ballasted weight >240kg, so hmmm...

I'm definitely getting mixed messages. In fact, this high empty weight might run afoul of the maximum non-lifting parts mass, which is 274kg (Sec. 2.5 in the manual). Unless I'm misunderstanding the significance of the max NLP limitation, this would mean that the pilot and gear must weigh less than ~30kg. So this would mean that planes which have been flying and competing quite successfully are, with the lightest of pilots and the calmest of days, statically overloading the wing spar by 10-20%.

What to think about this?

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While on the topic of weight, are there any limitations to flying with both water ballast and FES system installed? Several times throughout, the manual gives the cases of "without ballast" or "at maximum TOW". Since there are several ways to get to MTOW without requiring water ballast, esp. if the useful load is only 100kg, then it seems an odd choice of words. But I might be reading too much into it. 

Or are people flying in excess of MTOW and then dumping water before landing? Or is water ballast simply not useful outside of competitions where the FES has been removed?

[Kenn Sebesta]

P.S. Bit of a facepalm in calculating the empty NLP mass, which obviously would require subtracting the wing weight from the plane's empty weight. From having removed and replaced wings on aluminum certified planes, I'm guessing that each miniLAK wing weighs substantially less than 50kg. So for a max pilot weight of 110kg, so long as the wings are around 40kg/ea (pretty reasonable), this means that in all cases NLP can be honored.

P.S.S. NLP for the miniLAK is defined in  7.3 as "Weight of non-lifting parts of the sailplane includes weight of pilot, fuselage, stabilizer with elevator, rudder, instruments and equipment."