Tapered exhaust ducts

[Steve Leach]

Hi Gentlemen,

I am just drawing up plans for a twin electric ducted fan model. The best comparison I can give is that of the F86 sabre where the air inlet is almost the entire nose area and the exhaust is through the fuselage exiting at the tail under the tailplane. I am looking for the exhaust area to be 10% less than the intake. I addition there will be two fans occupying this space - side by side. The main reason is to achieve a taper towards the tail for looks.

has anybody had any experience of tapered exhausts? - the reduction of 10% will be achieved over a distance of 20 inches.

Regards Steve

[Phil Clark]

Thinking back 15+ yrs when I was playing about with on own design .45 size I/C DF model, I seem to remember the jet pipe eflux area had to equal the swept area of the fan unit......in my case a 5" dia Thorjet .45 unit........so I had to calculate the area of the 5" fan then deduct the area taken by the centre hub/engine mount......the resulting area = the eflux area. This produced a tapered exhaust duct which in effect increases pressure so increasing eflux speed. (I think.....it's a long time since I was doing this)

Phil

[Steve Leach]

Thanks Phil,

That seems to make good sense .

Regards Steve

[Andy Boylett]

Hi,
I have a sabre that I have converted to EDF. Pretty much the whole of the inside of the fus is now a duct. I had to cut down each fus former until there was only a 5mm hoop left of each one!. The batteries are located in the cockpit and the mini servos lie flat on their sides set into the fus wall - which is fully planked.

In the centre of this model is a Wemotec 90mm midi fan with a 1500 watt (peak) motor. The exhaust duct is tapered down 12% in cross-sectional area from the fan exit to the exhaust exist. The amount of taper required depends on the length of the duct and your EDF - there is a formulae for calculating the optimum for your set-up. The tapering down of the duct causes a decrease in pressure at the exhuast (compared to at the fan exit) and hence the velocity of the outlet gas is higher. This drop in pressure also helps increase the fan efficiency and can have a massive effect on the overall set-up. It is not unusual to get 30 or 40% difference in thrust from gettting this duct ratio correct. The important thing with EDF's is that you need as much inlet area as possible so that you can get the flow into the fan and then you need the optimum reduction incross-sectional area after the fan. Hence, you don't really want to be comparing the exhaust area to the inlet area eg if the fan area is 100 and the exhaust area 90, the inlet could be anywhere from 110 to 150. A lot of models have additional (non scale) inlet holes on the uderside of the fus in front of the EDF to help get the air in.

I am not expert in EDF's and have got most of my help from one of our local club members and from some of the EDF forums where they tell you all about this.

Cheers, Andy

[Steve Leach]

Thanks Andy,

That's very helpful.

Regards Steve