Hi there.

I saw a documentary about the tests and all the work that lay ahead the first flight of the B777, and one thing that had to be done was to weigh (sp?) the aircraft. This was done on three different scales, one for each set of landing-gear. They then somehow added the results from each scale and found out how heavy the plane was.

It's that last process I don't understand.

If you put a metal tube across three scales, one scale wouldn't show how much 1/3 of the tube weighs. The weight of the rest of the tube would affect the scale, and you would end up weighing the same area of the tube on all the scales. (Sounds complicated, I know.) This would happen when weighing a plane aswell, right?

Therefore: How can they find out exactly what a plane weighs?

If anyone understood what my question was based upon, I hope you could give me some answers.

Thom@s

I saw a documentary about the tests and all the work that lay ahead the first flight of the B777, and one thing that had to be done was to weigh (sp?) the aircraft. This was done on three different scales, one for each set of landing-gear. They then somehow added the results from each scale and found out how heavy the plane was.

It's that last process I don't understand.

If you put a metal tube across three scales, one scale wouldn't show how much 1/3 of the tube weighs. The weight of the rest of the tube would affect the scale, and you would end up weighing the same area of the tube on all the scales. (Sounds complicated, I know.) This would happen when weighing a plane aswell, right?

Therefore: How can they find out exactly what a plane weighs?

If anyone understood what my question was based upon, I hope you could give me some answers.

Thom@s

"If guns don't kill people, people kill people - does that mean toasters don't toast toast, toast toast toast?"

Actually, you could lay a tube or an object on multiple scales and the readings would add up to the total weight of the object.

A physics text will probably explain this better, but I'll give it a try. A plane (or any object) presses down on the ground with a force equal to its weight. The ground has to press back with equal force to keep the plane stationary. Some surfaces, mud for instance, are unable to push back hard enough, so the plane sinks until the surface can take the pressure. This could either happen by the landing gear sinking to harder ground or the wings/body contacting the mud and spreading the weight enough for the mud to support it.

But I digress ... scales placed under each set of landing gear will measure the forces placed on them. The sum of all these forces is equal to the weight. If the sum of forces did not equal the weight, the plane would be moving.

If you have a hard time believing this, find two scales and stand with one foot on each. The two readings will add up to your weight.

A physics text will probably explain this better, but I'll give it a try. A plane (or any object) presses down on the ground with a force equal to its weight. The ground has to press back with equal force to keep the plane stationary. Some surfaces, mud for instance, are unable to push back hard enough, so the plane sinks until the surface can take the pressure. This could either happen by the landing gear sinking to harder ground or the wings/body contacting the mud and spreading the weight enough for the mud to support it.

But I digress ... scales placed under each set of landing gear will measure the forces placed on them. The sum of all these forces is equal to the weight. If the sum of forces did not equal the weight, the plane would be moving.

If you have a hard time believing this, find two scales and stand with one foot on each. The two readings will add up to your weight.

Aircraft weighing systems use devices called load-cells which are a simple strain guage type device. The strain guage measures the sheer or torsion load on a piece of high strength steel that holds the weight. Multiple load-cells are used and the electrical output of each is summed in the control or display unit. Highway weigh stations work the same way. Large scales can be supported by many load-cells. In any case the combination of all of them gives the total weight of the object being weighed. Naturally in the case of an aircraft like the 777 you need 3 weigh platforms to support 2 main gear trucks and the nose gear. These systems will correctly measure a persons weight if that person climbed on board the aircraft during the weigh process. I bet it was an exciting moment when the first complete 777 was weighed to see if the engineers correctly estimated it!

I have no memory of this place.

By the way: newer aircraft can weigh themselves, right? How new? Not 727s, I assume, but newer 737s? A300? 757/767? 747-200?

Basically, the weight force remains constant (equalling mass*acceleration of the earth) - so if the plane is supported only in three points, the total force exerted on those three points is equal to the weight. Add the forces, get the total. Easy.

About the measurement:

Gearup:

I am not entirely sure of the way the strain gauges are employed - are they measuring the compressive axial deformation of a block /tube of steel, or the bending strains of a transversely loaded bar of steel? How does torsion enter the analysis? I'm eager to know, as I always found our structural mechanics course complicated.

About the measurement:

Gearup:

I am not entirely sure of the way the strain gauges are employed - are they measuring the compressive axial deformation of a block /tube of steel, or the bending strains of a transversely loaded bar of steel? How does torsion enter the analysis? I'm eager to know, as I always found our structural mechanics course complicated.

Ok, I believe you guys. But it still seems strange to me, a bit of a puzzler. Anyway, thanks for clearing that up.

Thom@s

Thom@s

"If guns don't kill people, people kill people - does that mean toasters don't toast toast, toast toast toast?"

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