vax said:
How cool would be single seater mini jet fighter, with two hiddent edf-s...
Turnigy Rotomax, for example, with cnc milled impellers.
Something similar to Bede BD-5, bt with twin electric EDF-s
[youtube]Lu81z9RpA5Y[/youtube]
Well the only problem is you would need EDF's that are significantly larger to get the same amount performance out of that jet.
Its all about the exhaust velocity, and the mass flow rate.
Jet engines have a high exhaust velocity, with a moderate mass flow rate.
EDF's tend to have a low(ish) exhaust velocity (granted faster than props), and a high mass flow rate.
Its basically a kinetic energy function, which is KE=(Mass)*(velocity)^2
Jet engines are far superior because the velocity is squared, which is why a 10 lb thrust jet turbine will perform similarly to a 50 lb thrust EDF with respect to top speed. So jet engines will always have a higher top speed than EDF's or props.
Acceleration however is all about total thrust, since thats just the good ole standard F=ma, or what we care about a=F/m (F is total force lbs thrust in this case, m is mass, and a is your acceleration)
The Turnigy rotomax's only have 10kW of power each, I don't think you would make it off the ground with only two of them on a BD-5.
The jet engine in that BD-5 is about 326 lbs thrust and is 12" dia x ~25" long if I remember correctly, so the size of EDF that would needed to equal it would be relatively large, to give you an idea you would need 16 of these EDF fans http://www.schuebeler-jets.de/en/hdt-fans-en/ds-94-dia-hdt.html, and they are some of the most powerful RC EDF fans you can buy.
Next problem is power, each of those fans take 5.7 kW of power, so you would need to be able to supply ~100 kW of power to fly it, it flies for about an hour on the jet engine, but we will aim for a quarter that at 15 minutes. Just long enough to take off, circle around a few times, and land again. So that would be a 25 kWhr battery pack, a 8kWhr pack that I made weighed 125 lbs in batteries alone using 20C turnigy lipoly so that is around 390 lbs for the battery pack.
The BD-5 carries 30 gallons of Jet fuel, which weighs 204 lbs, or 51 lbs to give it the same range as the batteries. So it should still be able to take off with that extra 190 lbs of weight. But between the lower exhaust velocity on the EDF's and the heavier weight its flight characteristics will likely not be nearly as good as with the jet. Certain farm animal names come to mind to describe the likely characteristics. Not to mention you just strapped yourself on top of 390lbs of lithium in a small jet with inherently touchy and unstable flight characteristics.
The probably fatal problem is that while you can likely take off with that 390 lb battery pack, landing might not go so well. Your max landing weight is usually a lot less than your max takeoff weight. So normally if pilots have to land again with a full fuel load they dump some before attempting to land. The same applies for the BD-5, in fact an airshow pilot Chuck Lischer died flying a BD-5 when he attempted to land with the tanks more full than they recommended. Since batteries don't lose very much mass when they discharge (electrons don't weigh all that much :lol
you will either have to land at too high of a speed and break the landing gear and end in a lithium fireball, or stall out and end in a lithium fireball.
So I don't think the day of the electric jet fighter has quite arrived yet. But I definitely would love the idea of just plugging in my jet fighter at night after i am done flying, and not have to pay for the thousands of dollars in fuel to takes to fly them. Some day perhaps.
