Hello Aireze. Here is a copy from an email I got from Wemotec's owner regarding exhaust size for the MiniFan 480. I was building a foamy F18 and had asked for info on a dual exhaust using a single MF480 unit. I figured he should be a reliable source!

"Best exhaust dia. is 56 mm for a single exhaust.
This compares to 24,63 sqcm
Divide this by two, makes about 12.3 sqcm x 2
SO two times 40 mm dia is a safe choice.
Length of the tubes is not that critical.
To get the right shape, you may check the pictures in the following thread.
They are for an F-18 although for 120 mm fan.

http://www.rc-network.de/cgi-bin/bei...5;t=000517;p=3 "

I also asked about how much length is needed to taper the exhaust and got this info:

"We recoomend a tapering length of 5-6cm, but I do not see this critical."

I ran MotoCalc with a 6" exhaust tube and exhaust diameter of 56mm / 2.2" and 47mm / 1.85". At 56mm you have 30.9oz thrust and 81mph top speed while at 48mm it drops to 21.8oz and 72mph top speed. But I think the data is bogus. For the heck of it I entered a 4" exhaust diameter and it said the plane would have 102oz of thrust and a top speed of 117mph. That is nuts and obviously flawed. [sm=spinnyeyes.gif]

Motor: Mega ACn 16-15-2; 4600rpm/V; 2.97A no-load; 0.012 Ohms.
Battery: PolyQuest; 3 series x 2 parallel cells; 2600mAh @ 3.7V; 0.022 Ohms/cell.
Speed Control: Castle Creations Phoenix 45; 0.0026 Ohms; High rate.
Drive System: WeMoTec Mini Fan 480; 2.7x3.66 (1.3in hub); (Pcoeff=0.506; Tcoeff=0.807).
Airframe: F18 3D Foam; 275sq.in; 34.9oz; 18.3oz/sq.ft; Cd=0.033; Cl=0; Clopt=0.33; Clmax=0.65.
Stats: 209 W/lb in; 180 W/lb out; 28mph stall; 39mph opt @ 53% (37:55, 103°F); 1801ft/min @ 32°; -540ft/min @ -9.1°.

Hope this helps you out!