Weighing a Flying Boat

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Chris In Marshfield

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In another thread this week, the topic of the Spencer Aircar came up. We discussed various aspects of the airplane's design, engine choices, and CG. The idea of building the Aircar as a pure flying boat (no landing gear), comparable to flying straight floats on regular plane, was mentioned. Combine that with picking and engine and what folks have had to do to get the CG right, it makes me wonder: how do you weigh a flying boat? I presume you'd have to put together some sort of fixture that the craft could rest on having scale locations with known distance from the datum. I'm presuming that a hanging weight wouldn't be an ideal situation.
 

BBerson

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Probably jack it up at the lower strut points and tail or any point near the front if it leans forward.
 

Riggerrob

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Build cradles that allow you to position scales exactly under where wheels would normally touch the ground. Weigh as per manual for the amphibious version.
 

Turd Ferguson

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In another thread this week, the topic of the Spencer Aircar came up. We discussed various aspects of the airplane's design, engine choices, and CG. The idea of building the Aircar as a pure flying boat (no landing gear), comparable to flying straight floats on regular plane, was mentioned. Combine that with picking and engine and what folks have had to do to get the CG right, it makes me wonder: how do you weigh a flying boat? I presume you'd have to put together some sort of fixture that the craft could rest on having scale locations with known distance from the datum. I'm presuming that a hanging weight wouldn't be an ideal situation.
Should have known jack points with precise locations. Put the jacks on load cells then get out your paper & crayon, start crunching numbers.

Big planes are weighed on jacks and load cells, you don't put scales under the wheels
 
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cluttonfred

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You take a 15' diameter kiddie swimming pool and put it inside a 20' diameter one. Fill the smaller one with water right to the rim, no water in the bigger one. Lower your plane down gently until it's floating in the water, the water displaced will end up in the bigger pool. Measure the amount of water in the bigger pool in gallons and multiply that times 8.35 lb/gallon. That's how much your plane weighs, easy peasy. ;-)
 

Chris In Marshfield

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You take a 15' diameter kiddie swimming pool and put it inside a 20' diameter one. Fill the smaller one with water right to the rim, no water in the bigger one. Lower your plane down gently until it's floating in the water, the water displaced will end up in the bigger pool. Measure the amount of water in the bigger pool in gallons and multiply that times 8.35 lb/gallon. That's how much your plane weighs, easy peasy. ;-)
Score! :) :roll:
 

Norm Langlois

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If you are weighing for CG its a calculation . If you are weighing for total empty weight it does not need any more than the total of the all points indication as achieved with any good accurate certified tools . Thus the water solution displacement is a good choice, water is calculable,by displacement. A three point total of all indicted loads . If you are measuring for Cg and your pilot is located forward you had better have a pilot in the seat or a substitute . Real Live weight is better than a sand bag. because of the real distribution over the seating and leg room. The calculations are still based from what ever point load you choose relevant to the datum point . If you where to use a less accurate measuring scale + or minus - it does not matter what it actually weighs for a CG as long as you use that same tool for all three points . the calculation will still give the true CG location. It just will not be the true weight of the aircraft. most devices are say accurate within + or - 5 lbs this error will be constant not usually a variable,as you move it around the locations.

Realistically. doing the Cg is a very general number , that is effected by so many variables . The perfect number will elude you. Because every thing that changes is not located directly under the CG. In the end you want a nose heavy rather than tail . 20, 30, 40 lbs forward not a problem but never toward the tail.
 
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