WesWorld

The difference seems to be that planebuilder seems to assume climbed at full throttle whereas most data is for sustained rate of climb at lower power. It does seem a bit high.

Taking a, um, break, at work :

My 4,625lb max / 4273lt D.XIXC was originally based on the D.XIV airframe (printed out and scaled), converted to metal and with a 940hp Hispano-Suiza 12Y-49 slapped in it. The latest version cleaned up the airframe a bit more.

My D.XIXC is listed at a climb of 2,959fpm, and max speed of 359mph- likely sans bombs or half range performance, not light.

Well if I take D.XIXC, and do the following :
Drop user weight to just guns ~ 299lbs
Drop maximum designed airspeed to just 300 knots. Note that the airframe tab indicates in bold that this is the maximum indicated airspeed.
Drop failure G to 10G

Then the plane has a light weight of 3,728lbs, with a maximum speed of 314knots, a maximum indicated speed of 250 knots, and a rate of climb of 3,650 fpm.

Now, if I use the 1940 12Y-51 (which seems to be the engine weight I use for all the 12Ys), the HP climbs to 1,084, max speed raises to 381mph ( 263 indicated), and the rate of climb shoots to 4,265fpm.

Rate of climb in planebuilder seems to be very sensitive to changes in weight and power. I've had changes of about 2% in weight give much higher changes to the rate of climb.

I can get a dramatic difference in performance just between max and light weights on the same plane.

For example, the G-1C I list the rate of climb as 2,852fpm or 2,991fpm sans bombs. That is however with a full gas tank, all ammo, etc.

If I just take "light" weight - which in my planes still includes the ammo because I stick that under userweight, the rate of climb rises to 3,446fpm.

Edit: I find best rates of climb are with low forward speeds and partial throttles. The numbers above are for a forward speed of 138kts.