Weighing Aircraft (was Sonex High Wing!)

[Kai]

Murray- look no further!

We have already done the tests, several years ago. The short and simple answer is that it depends on the scales you are using.

Put the airplane on high quality electronic load cells like the racing car society is using, and your results should be fairly accurate. Of course, there might be exeptions. I have never tried them, but I suspect that certified aircraft scales would also work just fine. However, if you use three identical bathroom scales, they take none to kindly to side loads- side loads that will occur because of gear leg and tyre deflection. Even worse, if you use one single scale that you move around with a combination of distance blocks to compensate for uneven loading, you will be surprised how much the slightest deviation influences the end result.

Bathroom scales are generally made for exact vertical loads, so you must ensure that the loads imposed on them by the tyres really are vertical. We solved it by tensioning up a thin steel wire between the axle stubs on the titanium rods, effectively preventing any horizontal movement on the scale platforms (jack up the plane, install the wire, tension it so much that nothing can deflect, and lower the plane back on the scale platforms). That way, we got results 3-6% lower than without the wire.

[Kai]

Murray- look no further!

1)
You know by now that you should avoid expert opinions by anyone (me included) on the internet like the plague!

2)
This misconception of the R912 weight must be corrected. When you get in the coolant, the radiator, and the oil thank under the cowling, the installed weight of the motor is very close to the Jab 33A

3)
We have already done the tests, several years ago. The short and simple answer is that it depends on the scales you are using.

Put the airplane on high quality electronic load cells like the racing car society is using, and your results should be fairly accurate. Of course, there might be exeptions. I have never tried them, but I suspect that certified aircraft scales would also work just fine. However, if you use three identical bathroom scales, they take none to kindly to side loads- side loads that will occur because of horizontal gear leg and tyre deflection. Even worse, if you use one single scale that you move around with a combination of distance blocks to compensate for uneven loading, you will be surprised how much the slightest deviation influences the end result.

Bathroom scales are generally made for exact vertical loads, so you must ensure that the loads imposed on them by the tyres really are vertical. We solved it by tensioning up a thin steel wire between the axle stubs on the titanium rods, effectively preventing any horizontal movement on the scale platforms (jack up the plane, install the wire, tension it so much that nothing can deflect, and lower the plane back on the scale platforms). That way, we got results 3-6% lower than without the wire.

[Skippydiesel]

Wow!

3-6% reduction that could take my obese, 340 kg Sonex , down to 320 Kg - exactly what the original builder estimated the empty weight to be.

Definitely get a reweigh with a wire to control suspension spread.

[Kai]

Be it far from it that I´d be the one to mar this moment of rejoice and jubilation, but may I suggest that a reduction from 340 kg to 120 kg is slightly optimistic. May I suggest 320 kg?

[Murray Parr]

Murray- look no further!

1)
You know by now that you should avoid expert opinions by anyone (me included) on the internet like the plague!

2)
This misconception of the R912 weight must be corrected. When you get in the coolant, the radiator, and the oil thank under the cowling, the installed weight of the motor is very close to the Jab 33A

3)
We have already done the tests, several years ago. The short and simple answer is that it depends on the scales you are using.

Put the airplane on high quality electronic load cells like the racing car society is using, and your results should be fairly accurate. Of course, there might be exeptions. I have never tried them, but I suspect that certified aircraft scales would also work just fine. However, if you use three identical bathroom scales, they take none to kindly to side loads- side loads that will occur because of horizontal gear leg and tyre deflection. Even worse, if you use one single scale that you move around with a combination of distance blocks to compensate for uneven loading, you will be surprised how much the slightest deviation influences the end result.

Bathroom scales are generally made for exact vertical loads, so you must ensure that the loads imposed on them by the tyres really are vertical. We solved it by tensioning up a thin steel wire between the axle stubs on the titanium rods, effectively preventing any horizontal movement on the scale platforms (jack up the plane, install the wire, tension it so much that nothing can deflect, and lower the plane back on the scale platforms). That way, we got results 3-6% lower than without the wire.

Thanks for all the advice from all that chimed in.

I would never use anything other than high quality calibrated scales and the ones used the first weighing were certified aircraft scales. I will weigh it again in the future and do all I can to remove the side load forces. Maybe there are a lot of planes out there with falsely high weights due to not being fussy with this. I will be surprised if mine doesn't lose some weight by weighing it more carefully as my current empty weight would still be a good 50 pounds heavier than standard even if I removed the few extras, I put in.

It will be interesting to see if a test can be worth a thousand expert opinions as Bryan pointed out...

On a side note, it seems this thread has become about weight and balance and not really specific to the high wing. Not sure if it would be possible to have the last few pages moved to a new title something like weight and balance.

[Skippydiesel]

Be it far from it that I´d be the one to mar this moment of rejoice and jubilation, but may I suggest that a reduction from 340 kg to 120 kg is slightly optimistic. May I suggest 320 kg?

Thanks Kai - I can be a bit slack when it comes to proofreading - correction made.

[Sonerai13]

I would never use anything other than high quality calibrated scales and the ones used the first weighing were certified aircraft scales.

Just so everyone understands, in the USA there are no such thing as "Certified Aircraft Scales". The FAA does not certify scales, or any other tools for that matter. They stipulate that tools be calibrated/maintained in accordance with manufacturers' instructions, but they do not certify anything.

It is always recommended that you check any scale you are going to use with a known, verified weight so as to establish accuracy at the time of use for aircraft purposes, and document this calibration check. Once you ascertain that the scales you are using are accurate, you can be assured that your aircraft weight is accurate enough to satisfy the FAA.

[BRS]

I was involved in the develepement of an STC years ago where we needed to 'calibrate' our scales. We purchased a 50# bag of play sand which we were going to use later as ballast. That be came our "known weight" since it was clearly labeled as 50#s. Seems that was acceptable to the FAA.

[Sonerai13]

I was involved in the develepement of an STC years ago where we needed to 'calibrate' our scales. We purchased a 50# bag of play sand which we were going to use later as ballast. That be came our "known weight" since it was clearly labeled as 50#s. Seems that was acceptable to the FAA.

Yep, especially if all three scales agree that it is indeed 50 lbs.

[Kai]

Agreed,

Calibration does not have to be more complicated than that.

But 50 lbs seems on the low side: would´nt 300 lbs preferable? Some scales show errors that vary with load, and the majority of planes seem to weigh in at 600-700 lbs EW. IMHO this should eliminate some erroneous readings.

If working with two bathroom scales, look around you, and select the heaviest looking person. Order him up on your two scales. They should both show the same weight: if not, adjust them. Chances are your live calibration weight will protest that the scales are off. Never mind- he is obviously lying out of the back of his teeth: two identical scales can´t be that far off!