Originally Posted by
Recycled Flyer
Ok, I am learning here but there seems to be the power absorption coefficient of the propeller missing, as in BHP=P X RPM3.
Where P is that coefficient and running on the assumption that the recommended prop for a Cox 049 diesel is indeed a 6x3 the power increase between 14000 rpm and 16000 could be as low as 1 ounce/inch (going from 0.45 to 0.55) bearing in mind that small diameter props have a far flatter power increase curve than larger ones.
Or as high as 0.75 to 1.5 when using a 7X3 APC prop.
Based on -
http://adriansmodelaeroengines.com/c....php?cat_id=57
But either way 50% is a massive power increase from just a 3% fuel additive.
Thanks.
Gordon Connel's book has a good section on this. It's the traditional Mathematical Modelling Technique for expressing model engine performance used in engine tests since the early 1950's. For various reasons It's only a good estimation.
This is how I understand it, but it's off the top of my head without consulting my notes which are somewhere else..
Your equation is that for a Cubic Polynomial, which you may remember from Algebra at school.
It's the same as the general equation Y = M.X^3
where: Y = Power, M = the P Coefficient, a Constant and a number characteristic of a particular prop, determined (rather crudely) from it's ability to absorb engine power, and X = engine RPM on that prop.
Model engine power curves are assumed to be that close to a cubic polynomial curve that any difference doesn't matter.,
The same prop is used in both cases, with and without the DII in the fuel, so the K factor in both cases will be the same..
Guess a number for the K Factor, and use it in both places in the calculation. Since the general form of the equation has the same Constant in both Numerator and Denominator it cancels out.
So change in Power (really ratio of power) = (K x (rpm2)^3) / (K x(rpm1)^3)
Let's call K =1, rpm1 = 12000, rpm2 = 14000
Substituting, Ratio of Power = (1 x 14000^3) / (1 x 12000^3) = 1.587 or 1 to 1.587
So change in power is 59% which is what our Russian friend got.
The power increase is simply the difference between the Cox running very poorly without any DII and very well on sufficient DII.
Sounds fair enough to me.