Attempt of air assist with 4 X 120mm Fan -> Failed
the air leak by the tip of the blades fan. not enough pressure build up.
only 2.1 m/s best reading against around 7m/s with compressor.
I lost my saturday …
Good attempt. Sometimes at work a design may need a higher pressure fan so I double stack fans to achieve that. It does not double the pressure bit it does increase it.
Play around with the fans a bit, you should atleast see 3-5m/s from four decent 120mm fans.
Probably the fans are working against each other in that setup, if not divided on the insides.
I tried two 140mm fans on a metal funnel with a hose on, almost blew the fish out of the fish tank 
My laser arrives next week, but i was curious to test air assist 
@HP_Persson it’s divided inside. and I love to have 5m/s @ hose. just very cheap fan not suitable for that job. the size of the gap between blades circonferance (red) and enclosure (black) is too big compare to the size of the hose output. Lot of pressure is lost here (4x2w energy need to go somewhere)
I would say with that setup you are doing not much more than creating turbulence and not forcing air where it needs to go. It appears that you are pointing 4 fans inwards instead of just using 1 or 2 sufficient fans to blow the air in the “direction” it needs to go. Also using that type of fan and attempting to force the air into a restricted tube you will lose a lot through the blade gap essentially causing it to even slow down… If it were me I would attempt using a turbine for an RC jet plane. Smaller footprint, less reduction steps and more air flow!
Just a thought… Check this out >>> http://www.ebay.com/itm/64mm-Duct-Fan-RC-4500KV-Brushless-Outrunner-Motor-For-RC-Model-EDF-Jet-AirPlane/331034860809?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D33878%26meid%3Db9fcda43305e4688bfbc9adcdb8b0e6c%26pid%3D100005%26rk%3D3%26rkt%3D4%26sd%3D121875127889