wlbragg wrote in Fri Mar 10, 2017 8:27 pm:I was looking through all the reference material I could find about this and I am unclear as to what these properties are and where their values come from.
hydro/float-beam-ft
hydro/float-beam-ft2
hydro/float-beam-ft3
hydro/float-beam-ft4
These are just scaling parameters to redimensionalize the coefficients - just like wing span, cord and wing area are used for aerodynamic coefficients. In this case I probably only used one which is then squared and cubed for those coefficients where length^2 and length^3 is needed to get the right unit of the force or moment.
The beam is just the beam (I guess typically at the step) of a single float in ft, beam squared, cubed and so on is just that number cubed, squared and so on.
The crucial part is, of course, that the coefficient (if it came from any sort of real data...) was made dimensionless with the same measure.
wlbragg wrote in Fri Mar 10, 2017 8:27 pm:I have been tuning the J3Cub floats and have made some great progress as far as I am concerned. They are behaving much better getting up on step and settling. But the changes I had to make are probably a bit unorthodox.
By changing the metrics/hydro-rp-x-in to -100 I am able to get a nice step effect and if very careful able to land without a bunch of bouncing. Even in the 65hp. Although the 65 is definitely underpowered. Is there another, better way to achieve this other than adjusting the rp-x in such an extreme fashion?
Also I am wondering how to change the shape and size of the float surface and its buoyancy. To an extreme! I've looked at and experimented here and there where I thought it might be but still don't have a handle on it. I imagine the hydro/float-beam-ft is part of my puzzle.
That's pretty unorthodox as it has left even the tiniest connection with the physics of the real system, more or less... :)
The size and shape of the float in terms of buoyancy is expressed in the tables (buoyancy and the moments due to buoyancy). These tables need to be changed (read: recomputed) to reflect the actual shape of the float in question.
wlbragg wrote in Fri Mar 10, 2017 8:27 pm:Trying to adjust metrics/hydro-length-ft and hydro/hull-length-ft didn't seam to do what I expected.
For example, I am trying to model the tundra tires flotation values, or at least get something similar.
Changing both metrics/hydro-length-ft, hydro/hull-length-ft to a value like .9" didn't seem to let the tail settle on the water like I might expect if there was no longer a long float but only a tire with basically the same beam as the floats.
And the last piece is the flotation values, which ones are they, or are they calculated using a combination of
hydro/float-beam-ft
hydro/float-beam-ft2
hydro/float-beam-ft3
hydro/float-beam-ft4
metrics/hydro-length-ft
metrics/hydro-beam-ft
hydro/hull-length-ft
The coefficients are dimensionless and are redimensionalized using float-beam-ft and so on (it should be obvious which is used when looking in the file).
Due to the in-development state of the hydrodynamics system there are a bit too many different dimensions in use there but by searching in the file you should be able to figure out where they are used. The ones in metrics/ and hydro/hull-length-ft are used in the generic hydrodynamics system for computing wave interactions.
IIRC, the aircraft specific hydrodynamics file uses only float-beam-ft (and its powers) to redimensionalize the coefficients there.
The shape of your tundra tires is quite far from the shape of the floats so you need new coefficient tables.
wlbragg wrote in Fri Mar 10, 2017 8:27 pm:I also found hydro/drag-tweak-factor extremely useful if I interpolate it with say speed to help adjust the drag after you come off the step but still allow the under power 65 to get to step.
Not so much for hydro/lift-tweak-factor, it didn't seem to do anything noticeable.
Shedding some light on these would likewise be extremely helpful.
They are just multipliers for the respective function. Now, I think you use the unphysical planing model that we used on the c172p - which is just reusing the hydrostatic tables but multiplied by the dynamic pressure so I think there is no nice interpretation, unfortunately.
Using the hydrodynamic-planing/hydrodynamic-planing-floats system takes the planing part closer to the real physics but has other drawbacks.
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