could this be of any help? found it on AMYA.
Is this what you want? :lol:
Easy and safe to make and handle.
A brass tube filled with lead, turned wooden ends.
With the right shaped ends it fits the theory beautifully!
If you have a stock of brass tube, let me know the diameter and the effective density of the ballast: we can calculate the shape of the ends quite readily.
Nigel: that seems to be working on an aerofoil theory. IMHO this is wrong. The best evidence for this is aurliner fuselages, which are very difficult to optimise for the various possible angles of attack (remember that a surprisingly high proprtion of the fuel on a flight is consumed during ascent and decent, when the angle of attack is not the cruise optimum). Their solution is to let the wings do the lifting and simply minimise the drag of the fuselage.
Hope this helps!
Although Marco’s program seems to be limited to a 1kg weight you can get round this limitation.
Go to File - Set Bulb Parameters - and input the TC ratio you want and just leave the weight at 1kg.
Then go to the - View Bulb Data - section. It will show a table of dimensions for a length which gives a weight of 1kg. You can then select a different length where highlighted and click O.K . It will then show weights for different lenghts. Just iterate until you get what you want.
If you then go to the Print section and ensure that you select Print Preview you can print off a full size diagram. If you adjust the Number of Sections appropriately you can get easily usable increment lines on the diagram.
I’m just about to turn up my first attempt prior to casting.
Cheers,
Trevor
i thought the main goal was drag redution, so when i saw the chart, i naturally though about this shape. i was thinking more about the shapes of fish, and othe water creatures, not so much the light water (air) shapes.
forgot to mention that my theories are based on guesses, and not years of experience like what most of you all have. (i learn so much here)
I,m always a bit suspicious about analogies with fish. Why nature made them that way is often not pure speed. For example, angel fish are very tall and thin so that they are inconspicuous to their victims when seen end on and can easily hide in certain types of weed.
Similarly, fish of the roach and bream type fool predators by swimming in shoals ACROSS the line of attack or predators. Their deep bodies and (frequently) silvery colour make the shoal as a whole look like a moving curtain and the would-be attacker gets confused.
However, if you look at free water predators like hake (judge by the size of mouth and pointiness of teeth on the fishmonger’s slab), the tendency is to have a cylindrical body that is pretty much parallel over its centre section. However, it would be well to remember that this may have as much to do with where muscle is positioned as purely hydrodynamic benefit.
Is drag the most important thing to minimize in a Footy bulb? The really long bulbs make me think about angular momentum and the forces involved when making turns. Is long better than short? Would a long bulb aid stability even if it takes more force to start a turn? Hmm. I guess I’m just thinking out loud. I’ll have to try different shapes since I made my last keel removable.
This is getting really ferocious - beyond my maths but probably not yours. My guess is that the bigger gyradius of a longer bulb tends to damp piching hydynamically and (probably) in terms of pure mechanics. Whether this outweighs the additional drag caused by the greater change in position of the extremities of the longer bulb on pitching is anybody’s guess. Someone like Grumman make have software that will eaily cope with the problem. I don’t.
My intuitive guess is that a longer, slimmer bulb will work better overall at higher speeds (which tend to involve grreater pitch excursions), but this is really no more than wishful thinking. Empirical rsults from IOMs suggest that the difference is marginal.
That looks like a good place to start. Would you please givethe measurements for the length and diameter of your tube?
thx
I brewed up this little spreadsheet to just give the rough dimensions for a keel weight. The calculations are for a basic cylinder, so for a keel ballast with two pointy ends, you’ll have to make it slightly longer, but you’ll get an idea of the relationship between the diameter and length.
BTW- it’s an image of a spreadsheet.
HTH