I am trying to understand this technical note, specifically problem 1 on page 4.
http://naca.central.cranfield.ac.uk/reports/1924/naca-tn-194.pdf
I am quite happy with everything up to equation 2 on page 5. I don't understand where the figure .39D 2/3 has come from nor 48D 2/3 in equation 3.
Can anybody else see where these figures have come from? I've been looking at this for sometime. I realise it is quite a long question but I would be grateful for any help.
The complicated expression for $HP$ comes from page 3.
$HP = .39 \; D^{2/3} \quad (2)$ is just the multiplication and division of the numbers in the complicated expression
$D - 48\; D^{2/3} = 42000 \quad (3)$ comes from substituting the result of $(2)$ into $(1)$ which was $15000 +44 \; HP = .357 \; D$ and multiplying through by $1/0.357$ , with some slight rounding.