Weighing Aircraft (was Sonex High Wing!)
Posted: Sat Dec 16, 2023 3:02 am
by 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.
Re: Sonex High Wing!
Posted: Sat Dec 16, 2023 3:11 am
by Kai
Murray- look no further!
You know by now that you should avoid expert opinions by anyone (me included) on the internet like the plague!
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
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.
Re: Sonex High Wing!
Posted: Sat Dec 16, 2023 3:16 am
by 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.
Re: Sonex High Wing!
Posted: Sat Dec 16, 2023 3:31 am
by 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?
Re: Sonex High Wing!
Posted: Tue Dec 19, 2023 5:51 pm
by Murray Parr
Kai wrote:Murray- look no further!
You know by now that you should avoid expert opinions by anyone (me included) on the internet like the plague!
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
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.
Re: Sonex High Wing!
Posted: Tue Dec 19, 2023 6:19 pm
by Skippydiesel
Kai wrote: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.
Re: Sonex High Wing!
Posted: Wed Dec 20, 2023 12:49 pm
by Sonerai13
Murray Parr wrote: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.
Re: Sonex High Wing!
Posted: Wed Dec 20, 2023 3:49 pm
by 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.
Re: Sonex High Wing!
Posted: Wed Dec 20, 2023 4:08 pm
by Sonerai13
BRS wrote: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.
Re: Sonex High Wing!
Posted: Thu Dec 21, 2023 11:44 am
by 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!
Re: Sonex High Wing!
Posted: Thu Dec 21, 2023 11:49 am
by Bryan Cotton
I was lucky that there is a guy in my neighborhood who has a real set of aircraft scales. He rents them out for $50 and uses the money to maintain and annually calibrate them.
Re: Sonex High Wing!
Posted: Thu Dec 21, 2023 12:03 pm
by Sonerai13
Bryan Cotton wrote:I was lucky that there is a guy in my neighborhood who has a real set of aircraft scales. He rents them out for $50 and uses the money to maintain and annually calibrate them.
Bryan, what does he use to calibrate his scales, and what makes them “aviation” scales? Not being a smarta**. Just wanting to know what criteria he is using to call them “aviation” scales.
Re: Sonex High Wing!
Posted: Thu Dec 21, 2023 12:20 pm
by Bryan Cotton
Sonerai13 wrote:Bryan, what does he use to calibrate his scales, and what makes them “aviation” scales? Not being a smarta**. Just wanting to know what criteria he is using to call them “aviation” scales.
Hey Joe! He uses a service to calibrate them. I don’t know which one but for our scales at Sikorsky we would call in somebody with a truck that had a bunch of calibrated weights. Basically everything had to be calibrated to a NIST standard. We needed the guy with the truck as the scales for a UH60 were pretty beefy. So the key thing is the paper trail, like a lot of things in aviation.
The scales I borrowed were a kit made by somebody and I’m sure in another life they could have been used to weigh potatoes or something. But this was a set of 3 scales, all plugged into one display so you could read all 3 at once. In particular the shimming on these scales were set up for an A36 Bonanza. On a level floor, drop the Bonanza on these scales and it was level. I had to do my own leveling of course.
Re: Sonex High Wing!
Posted: Fri Dec 22, 2023 10:03 am
by Sonerai13
Bryan, thanks for the additional info. I asked because I have found that the same outfits that make the “aircraft scales” that Aircraft Spruce sells also make identical units that are sold through high performance auto outlets. The only difference is, the “aircraft” scales have only three load pads, and the “auto” sets have 4. Price is about the same either way, so a person is smarter to buy the auto set! (Besides, the extra load pad comes in handy if you need to weight a plane on floats or a helicopter!)