Aerospace How is the moment of inertia of airplanes measured?

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1. Apr 10, 2017

ramzerimar

I know it's possible to use a trifilar pendulum to extract information on moments of inertia by measuring the period of oscillation. I've seen this test being executed on small aircraft, like in this video.

But I wonder how do is the momement of inertia calculated on big aircraft, like a Boeing 707. Do they use the same principle of trifilar pendulum, just on a larger scale, or it's some other method?

2. Apr 11, 2017

Baluncore

It would be difficult to measure since the weigh of fuel and payload would be important under different conditions. The test would need to be repeated under different loads.
For a large aircraft the moment of inertia could be accurately calculated from the known mass distribution. That could be done for many different situations.

3. Apr 11, 2017

Nidum

Most aircraft manufacturers until recently had a weights department responsible for calculating weights of all individual components , subassemblies and complete aircraft .

The weights information was tabulated in such a way that the centre of mass position and moment of inertia could easily be worked out multiple times as the design evolved but without having to do the whole calculation from scratch again .

4. Apr 11, 2017

CWatters

The centre of mass of an aircraft has to be controlled because it affects stability - so it's vital to know where all the mass is going during the design phase. Should be reasonably simple to calculate the moment of inertia from that data.

5. Apr 11, 2017

Nidum

The mass distribution is important as well since this affects the way that g loads are distributed in the structure .

For practical calculations on complete aircraft or larger subsections like fuselage or wings the individual component masses were usually lumped together at nodes distributed along or over the structure .

Incidentally they were always 'Weights' departments . This comes from the earliest days of aviation when only the simple 1g weight of components mattered much for aircraft performance . It was quite a few years later before higher g levels in flight became a problem .

Last edited: Apr 11, 2017