Originally Posted by LMF5000
I take it that implies that removing the duct puts the motor under more strain even though it spins the same fan.
Depends on the particular fan and ducting. Generally, using a restricted tailpipe reduces thrust and fan loading. Good inlet ducting increases static thrust and fan loading. Poor ducting may do the opposite.
But just saying 'increases' or 'reduces' is not very useful without knowing how much
it changes. Here are some real-world numbers (Haoye 7 blade 64mm fan, 3400Kv outrunner, 2S Lipo):-
Bare fan (no ducting): 134W, 22650rpm, 267g
With tailpipe 95% fsa: 132W, 22753rpm, 250g
Inlet ring and tailpipe: 151W, 22130rpm, 390g
In GWS ME262 nacelle: 153W, 22105rpm, 329g
Note that these are all static tests. Though using an inlet ring boosts static thrust significantly, it has little effect on in-flight thrust. A restricted tailpipe increases exhaust velocity, increasing thrust at high airspeed.
If the fan is for a slow model (eg. glider, airliner) then use a fat inlet ring and a short tailpipe opened up to 100% fsa (fan swept area). For a fast jet you can have long ducting, but keep the mouth thin (for lower drag), maintain inlet ducting at 100% fsa throughout its length, and restrict the outlet to 85% fsa.