I dont know exactly what the formular is either..
Spitfire was : 6417 Lbs
Lancaster : Empty 36,900 lbs, Maximum Takeoff 68,000
Both aircraft used the Merlin engine..
6417 X 5.6 = 36000
I just weighed a little model built for an OS 19 called an Aerobat. Typical 22 " That weighs 20.5 OZ.
Doing the same maths
20.5 x 5.6 = 114 oz
So in theory that is the same power to weigh to lifting ratio.. - I don't have the required aerodynamic knowledge to explain why - once you go to 4 engines you can carry more weight that you think - and Yes your correct ..
4 x 15 = 60 and there is no way a 60 would pull around a 110oz aircraft.
The Lancaster proved the similar concept - running 4 x 19 = 76 and mine was heavier at 118 - 120 oz Interesting enough - Like PW - I found you can still do maneouvers on 3 engines - 8's squares ect..
The model will also fly on 1 engine - well - staying airborne is flying ..
Perhaps some of the more learned folk out there can expain the answer - Both bombers proved what the full sized counterparts found, that you can carry more weight. - I've read reports that the Lancaster was quite nimble in the air - I found the same - quite surprising..
I've never dabbled in twins - so I am also keen to read the expected answer into how do you select the engine size - and is it a similar ratio for twins as it is for 4's??
Does the math's halve when you run a twin..
Instead of 5.6 times - its reduced to 2.8 ?
Loaded weight: 17,700 lb Mosquito
6417 Spitfire
6417 x 2.8 = 17 967
Interesting............
Going on that theory, im sure you can calculate what you would require.