Hi all, is there any rule of thumb on exhaust size and lenght for a given fan? Is there any correlation between the exhaust outlet size and thrust? In the WeMoTec instruction sheet for the Mini Fan 480, the instructions say the exhaust tube should be 50-60mm long with the diameter being reduced from 69 mm down to about 60 mm. This tube is going in a Super Sniper and this short tube leaves a lot of fuselage between the tube end and the fuselage end. The end of the fuselage has an opening about 47 mm. I can increase the fuselage exit size by cutting back on the fuselage end. this would get me another 3 to 5 mm. Will this help? This may seem anal, but I try to optimize all the planes I build or assemble. Thanks, Rich

I think the mini fan advice is about right.

The problem with small scale jet pipes is that the wall friction and viscosity effects sap power seriously compared with full size, where a tapering exhaust is used to increase gas velocity. Also we are talking about just a fan, whereas full size gets energy from hot gas.

So the rule of thumb is to build an inlet as smooth as possible, keeping the area of the fan, and an exhaust should be not much longer than the fan diameter and only slightly tapered. Picking a 'fat' subject with a short tail pipe, like many of the early jets with non axial engines seems a good idea, otherwise you could stretch the fuselage diameter or move the fan towards the rear.

With modern combat jets at small scale perhaps the best solution is to use a pusher propeller, not too visible in flight and more efficient than a fan.

By the way there has been some discussion of tandem fans, but it seems the consensus is that the extra weight does not give any gain because of even worse efficiency - has anyone tried it or is it just a theory?

good luck

Jerry

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!