Free Flight, Free Speech

https://www.flightglobal.com/FlightPDFA ... 200659.PDF

A datum point is a made up point, arbitary point, upon the aircraft from which all other points are measure from.... it can be anywhere as long as every other object is measured from it, it doesn't matter..

that said...

The above image is an example of how a beagle pup got a bigger engine put in her...

If you use the front of the prop on the 100hp beagle as your datum point then that's fine we can all adjust to it... If however you want to then add a bigger engine like the 150hp which stretches the nose then all the dimension have to be adjusted.

Now if you'd used the fire wall or engine mounting wall (same thing) then you only have to work out how much bigger your engine is and where your prop now sits... everything else is the same.

This is how I solved the same problem with the Spitfire which goes through considerably more variations than two.

BTW it's also the place where the RAF puts it's datum point.

Simon

Well, yeah, that makes perfect sense as long as you know, for example the firewall you use as datum point doesn't change between the variants.
I once used for such things a point in the centerline and the middle of where the wings sit. Till I found, Boeing sometimes changed wings between variants the shape of wings a little and my datum would have wandered around by some inches. So, firewall is probably the better choice.

On a side note: My brain still jumps happily back to metric whenever I am a bit tired or sidetracked. So yeah, I can work imperial as long as I am under enough caffeine, and since I really approach fast the point, where I try to let the problem kill by a computer, I would let the box do the units anyway. So, bottom line is, we go yith your used imperial measures, but if something form my end looks weird, may have a look whether there is a mm in it or a Kg.