We had some off-line discussion about how to get a good moment of inertia for the dynamics model. Bifilar pendulum is a standard way. The Finn class uses the Lamboley swing method which can be found here ( www.lamboleyetudes.net). I decided to run the model with +/- 50% pitch inertia change from Alan's original estimate of 20 slug-ft-ft to see if it is sensitive to inertia changes.
Result is shown above for the case of elevator control. Although the high inertia case (30 slug-ft-ft) shows the expected slower response, the basic character of the elevator control ( height goes up before it goes down) does not change.
Still, if someone wants to measure inertia (Lamboley seems the easiest method) it would be interesting to know a more accurate value.
I would consider the rig an essential part of the experiment due to its large percentage of overall hull weight, but damping from the sail would be a concern then. Perhaps a rigged boat with weights, simulating the sail weight, positioned along the mast. I don't wanna think about the effect of the crew weight, although it could be estimated.
ReplyDeleteDoes centre of rotation influence these calcs?
what's your height datum?
ReplyDeleteI agree the rig needs to be included but I think a calculation for the sail (assume uniform weight distribution) would be close enough. Likewise for the mast. The hull and racks are a little harder so I am hoping someone with a solid model can take the trouble to enter panel weights and tube weights and get an estimate.
ReplyDeleteThere are lots of papers about monents of inertia of the human body, from sports and from aerospace. I have not extracted any numbers yet.
The center of rotation does make a difference, in this model the moments are taken about the combined center of mass for the boat and crew.
The graphs are showing depth of the main foil.
narkla speaks more about rig dampening.
ReplyDeleteI think this air friction opposing pitching would be more significant than inertia on these lightweight boat and skippers.
Same is true, but less significant hydrodynamic dampening.