Yes - I do have 'O' level Math[s] as well...smithyrc30 wrote:Erm, isn't area directly proportional to diameter (area= Pi*diameter squared/4)
so the discussion about area vs diameter is a moot point?
... and no, it's not a moot point. It's "Logic and Reality"...
Totally agreed that it's easy to work out the area from the diameter using PI.
BUT:
1) Who bothers doing this? No-one, but why don't they?
2) We are looking at the annulus anyway, not the "male" part of the device.
(And yes, annulus and anus are related - "Annulus" means "ring"...).
We can take the simplistic view, or the realistic view!
We should be looking at the "ring"...
Let's start with an imaginary 0.100" needle...
The index point 1 is specified as 0.099" and the jet is 0.100".
We therefore have half a thou gap round the circumference of
the needle, between it, and the jet.
(This is where the fuel comes out...).
Let's now do a scale drawing.
Get a 0.5mm propelling pencil and draw a circle 100mm diameter.
(We are using a hybrid scale of 1mm to 1 thou)
This is an approximation of what the needle would look like in its
working environment. Try to visualise how much pencil lead is on
the paper, we won't do the maths, just take a look at what you see.
(Boring, isn't it?).
Now let's say that another needle is 0.098", only 1 thou different.
We can draw this on the same diagram, by making another pencil line
round the inside of the first one. OK, the line is very slightly shorter,
but to all intents and purposes, there is TWICE AS MUCH lead showing.
Agreed?
Initial Conclusion:
We have reduced the diameter of the needle by about 1% diameter,
The amount of fuelling change is an additional 100% (or thereabouts).
There is a far greater effect at the fat/idle end of the needle than there
is at the thin end. The charts on the "Needles.ods" spreadsheet are far
more representative of the amount of fuel likely to be issuing because
they are area based.
Another example with 90thou needles A5 vs. W3.
A5 is 0.089" at index 1
W3 is 0.088" at index 1 For A5, at index 1, there is an annular area of 141 square thou.
For W3, at index 1, there is an annular area of 280 square thou.
They are within 2 thou of each other along their whole length...
BUT:
Look how much difference percentage-wise there is at the bottom end,
compared to the top end...
Looking specifically at W3:
At index point 2, we are showing about 1000 square thou.
At index point 6, we are showing about 2000 square thou.
... Double the area...
At index point 12, we are showing about 3000 square thou.
... Triple the area...
Now try visualising that, just from the diameter values of
0.088", 0.0745" and 0.065" respectively. It's not going to happen...
More?
OK...
How about the old favourite No7 in 0.090" compared to OA7 in 0.100"?
What would "Minty" show - Utter crap, unable to compute, the lines won't
even be close because they are out of two different sized jets, etc...
Get this:... Well, blow me down...
So if we had a 1&1/2" SU with a No7 in it, if we swapped the jet to a 100 thou
one and used OA7 instead, the results would be broadly similar...
(The Formula Junior engines used 0.125" jets in an HS6 instead of 0.100",
Needles.ods would be able to show valid comparisons.... because it's,
wait for it - AREA BASED).
Many years ago I went to Aldon, those of "Dizzy" fame.
I came away with a needle with a F#~*& great flat down it that they
had made for me. I thought it looked a bit sh!t, so I calculated the
area of what they'd filed off and calculated it back round again.
I worked out that a butchered KK was the equivalent of a round SZ.
See the "Filed AF" feature on "Needles"...
When I went back to Aldon the next time, the op said the needle was
spot on and asked how I came about that profile. I told him that
I had reverse-engineered their filed one using area based calculations
then re-worked it round again.
A giant
appeared above his head...
Ian
The data in the spreadsheet has come from several sources, some procured, some typed manually and