FT Saab Viggen 70 mm EDF.

crash happy wrote: Simon. That is some very interesting information on duct theory ! I could add some curved deflectors from the side wall to the mouth of the fan, would that give any significant improvement ?

How about a tube rolled from foamboard to duct air from the intakes to the fan?

Crash
At this stage I doubt an inlet modification would be effective. It would have to be part of the original build which is why I suggested "if you ever want to build a better one".

Just to give an idea of the principle this is the sort of outline of the ducting, both inner and outer, from the inlets to the fan on a foam board Viggen.

Of course using foam board it is hard to get smooth & gradual changes in profile from what ever shape the inlets are to the circular fan inlet.

It is/was a problem with buried jets in full size and one of the reasons Boeing just hung them under the wings. The inlet duct is short, straight and circular.

Bare a bit of tidying up the Viggen is now ready for maiden. I made up an adapter so the afterburner connects to the balance lead on the flight battery, then via the electronic switch to the LED's. For now it is on aux1 & a switch on the TX. A word of warning, you must use the two outside pins when using the balance lead, any other combination may short out the RX ! I have ordered two 2650 mah 4s, which should be here in a few days. I tested everything on 2200 mah 3s. Five minuets at 50% throttle ( plus a couple of blasts) left 17% in it & 20% once it had been left to recover for a few minuets. Apart from painting the bottom matt black, I'm not sure about the paint scheme yet. The better part of valour would be to see if fly's first ! :woohoo: [badass]

crash
That all looks very neat.
Be aware that 5 minutes at 50% throttle on 3s translates to only about 3 minutes at 50% on 4s even taking the extra capacity into account. 3 minutes represents a 20C discharge and that's at half throttle. :woohoo:
So the question is how much throttle will actually need. It is unusual for an EDF to be as low as 50% so you can see you are into the "not very kind to LiPo" that EDFs are well known for. What C rating are the 2650 mAh LiPo?

On the plus side you will likely find 3 minutes is plenty long enough, at least initially.

I think it'll be surprisingly kind to the battery, got a lot of wing, albeit flat plate, bet it's a bit a floater..nice job crash.
Edit: what's the weight?

You don't perhaps want to take a scalpel and cut a 60degree feather on the inside of those ducts? (every little helps)

It needs some colour, so you can tell top from bottom at a distance.

Simon. The batteries I have ordered are 30c constant / 60c burst. They are a bit of a compromise, as I wanted something that could also be used on the Apprentice & Riot. If all goes well with the maiden I will get some 3300 4s 65c for exclusive use on the Viggen.
Klippy. Good idea, but one step ahead of you, already done. I am am impressed with the fact that it doesn't need cheat holes!
Paul. I will weigh it when I have the batteries & got it balanced. If you want to see the flight envelope see the attached video, you may be surprised.

crash
Nice video. Did you notice the radiuses all the leading edges!

Quorneng wrote: crash
Nice video. Did you notice the radiuses all the leading edges!

No I didn't ! I am glad I added the tube to all the leading edges though.

I borrowed a 4s off my mate & did some comparisons between that & a 3s. Both had a 35c rating. The calculated all up weight of the aircraft with the batteries I ordered is 2.5 lbs. The tests for obvious reasons were static. Max throttle on 3s produced 470 watts which = 188 w/p. I forgot to record amps on the 3s. Max throttle on the 4s produced a whopping 1100 watts at 80 amps, the same as the ESC rating! That = an output of 440 w/p. Throttle travel was the reduced to 70%, this produced a more manageable 970 watts at 66 amps = to 388 w/p.
Due to the lower efficiency of a fan, I believe as a rule of thumb that 150 to 200 watts per pound is acceptable ? Without diving into the heady realms of E-flux & duct theory, & assuming the designer has calculated this to some degree, I am wondering whether to to use two 2200 3s in parallel for the initial flights, or just go ahead with 4s & a throttle limit of 70% ?

Go ahead with the 4s, dont foget to take some clean pants with you and video it for the entertainment of the forum :evil:

crash
Remember weight has a influence on how it will fly.
I would suggest that a 3s and 188 W/lb will be more than sufficient to 'fly with purpose' and with the lighter weight it will be that much more docile handling too. There is no point in giving yourself more problems. Very high powers do tend to cause things to break/fail.
400+ Watts per pound is EDF hovering/unlimited vertical territory. Leave that for when you are confident you know how it performs.

THE BLACKBIRD wrote: Go ahead with the 4s, dont foget to take some clean pants with you and video it for the entertainment of the forum :evil:

You are a very bad man BB ! :lol:

I would question limiting the throttle travel, that's what your left hand's for![badass]