... I can see how 'diameter' isn't the best, or even a good method for determining appropriate parachute sizing.
Understand, Joe, that 'diameter' is a *fine* method of expressing parachute sizes -- I see nothing wrong with it -- provided...
TBSSJoe said:
... The reason I am sticking to it is because it's apparently the standard.
This is kind of my point -- well, this *is* the point: There doesn't appear to actually BE a 'standard' (in the hobby, at least) -- some measure it 'over the top' (for shaped canopies like the Spherachutes brand, e.g.) - some measure 'across the opening (aperture)' (as I think Fruity and some others). Then we get to polygonal canopies and some want to measure 'across the points' and some want to measure 'across the flats' (and, from what I've seen, NO one will tell you what the canopy fabric area is).
See what I mean?
<sigh> (Please, someone, make up your mind.)
Joe, if you'll look here:
https://www.rocketryforum.com/showt...hute-and-Recovery-Systems&p=295089#post295089
(this is post #1 from the Technical Reference 'sticky' thread at the top of this forum - this I call Knacke-1991 (Knacke-1978 is post #3)):
...the 'classic' terminal velocity equation for a parachute in stable descent is:
v(eo) = SQRT ( (2*Wt) / (S(o) * Cdo * rho))
where: v(eo) = equilibrium descent velocity, FPS
Wt = Total weight of system - load+parachute, lbs
S(o) = canopy surface area, ft^2
Cdo = parachute drag coefficient related to S(o)
rho = air density at target altitude, slugs/ft^3
(you can see this discussion on page 4-16 (or page 71 in the pdf) of the above document)
You will note that any term for 'diameter' does not exist in that equation. It's all referenced to 'canopy surface area' - S(o) and the Cd value is referenced back to that as well (Cdo). When you think about it, it makes sense -- as all the various and sundry canopy designs that have been developed would make something like 'diameter' a nightmare to quantify in the above equation - *specifically because* of the very thing this hobby is struggling with - "nobody can decide what is really MEANT by 'diameter'". Far simpler to simply add up the square footage of fabric used in the canopy (i.e. S(o)) and work the equation from that perspective. By using a drag coefficient related to that area (i.e. Cdo), then the equation is solved rather simply. This will (automatically) account for any special canopy 'features' (ribs, vanes, etc -- like is used in the guide surface canopies) - it's all simplified back to simply the total area of fabric in the canopy. Knowing that, it is a trivial exercise to express that area as a simple circle - the diameter of which is called 'nominal diameter' - D(o).
The benefit of that is that all the various designs of canopies (flat circular, conical, hemispherical, quarter-spherical, ringslot, ringsail, toroidal (or annular), etc) -- can ALL be compared to each other (to determine which design REALLY IS the best for a particular application) because now they are all quantified on a common footing (i.e. S(o) and Cdo).
That's the point (as far as parachute 'diameters') are concerned - as well as the reason that the commercial/military arena has standardized on this mechanism.
Note, also, that any perforations in the canopy (slots, etc - as well as the apex vent) is ignored in figuring S(o) - just add up the areas as though those 'holes' weren't there. The 'Cdo' will account for whatever effect this will have on performance (with the understanding that extremely large openings - like the missing central portion of the annular canopies (or the large vent on the Fruity IRIS ultra) would be subracted (or not counted) in figuring canopy surface area).
You might also like to look over pages 5-3 thru 5-5 (pages 82-84 in the pdf) on the Knacke-1991 text referenced above for a comparative look at the various canopies and their performance.
Maybe this clarifies things a little bit - as noted, I've got no problems with 'diameter', as long as there's some common footing for the definition of the term.
-- john.