TARMAC (Toowoomba Amateur Radio Model Aero Club) (Vale View, Queensland, Australia)

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Sep 08, 2020, 06:12 PM
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Flying Saucers

I am having another go at my flying saucer with Electric Ducted Fans (EDF). I realise that I had made a mistake in not doing my bench tests and sums first (and you call me an engineer - try first then read the instructions). As a result, my first and second efforts did not work too well. The first flying saucer was too heavy at close to 7 kg and I ended up with melted connectors and a lipo battery disintegrating. But it flew - just showing at least my idea of an EDF flying saucer may work. I am now using heavier connectors and silver solder so connections should be OK. (Silver solder is stronger and has a higher melting point and higher conductivity)

Now I have completed a series of bench tests on the EDF fans to determine their static thrust curves against amps. I also tried various size discharge cones and found maximum thrust is with a flat intake at right angles to intake with curved lip and no exit cone with minimum duct length. (For a plane it might be different as a cone may produce higher exit velocity and higher plane speed and inlet needs to be sharp to reduce velocity drag.) The smaller 4s battery 65 mm diameter EDF fans are a dead loss with only about 1/2 kg of thrust each, however the 70mm EDF fans on 6S batteries (25 volt) produce a maximum of 2.2 kg thrust at about 85 amps.

I could always get 4 of these beasts

Only $1,600 for 4 contra rotating 90mm fans with matched 150 Amp Opto ESC's (Electronic Speed Controllers)
Bit out of my price range but would make a lovely model with 11 minute flight time - the bigger the fan the better the efficiency.

Getting back to reality according to my sums with my new smaller lightweight flying saucer weighing in at 1.3 kg bare / 4.7 kg fully loaded with 4 fans, controls, 100Amp ESC's and batteries, it will work with two 6s 25 Volt 5 Amp hour batteries giving a flight time of 4 minutes at minimum lift 1.2 kg/fan ( 35 amp per fan) and 2 minutes at maximum lift 2.2 kg/fan (85 amps per fan).

My 70mm EDF fans are Freewing 70mm 12 Blade Counter Rotating SMF Set 2200Kv For 6S at $127.30 each
On 25 volts that's a screaming 55,000 RPM - sounds like a real jet and I need earmuffs for testing.

Now the question - which Lipo batteries to buy.
In my bench tests I found that a standard Turnigy Nano Tech 5000 mAh 45-70 C discharge at $154 each - one battery will run at 70 amp and the battery rapidly drops to 7% capacity with reduced battery voltage and gets warm after 2 minutes and then once disconnected recovers to 30% capacity (bounce back). Battery internal resistance across all cells is 22 mOhm. That is the capacity/output of the standard lipo battery dips under load and it will not give the desired result. Plus, I would probably destroy the battery in no time due to the sustained high current discharge.

I have chosen to use two of the 6S 25 Volt 5000 mAh batteries as the best fit for the size of craft and the 70mm EDF fans I have. Bigger batteries give a longer duration, but extra weight causes increased motor current for the required extra lift and there are diminishing returns and I wish to stay under 7 kg total aircraft weight or I will be in the heavy model bracket with increased rules and regulations and safety inspections. Similarly increasing the number of fans from 4 to 6 results in higher weight, less lift per fan and less current per fan but roughly the same flight time on the same batteries.

I have been looking at the Turnigy Nano Tech Plus - Discharge rating 70C continuous 150C burst lipo battery at $173.51 each. Thats a whopping 750 Amps maximum current rating per battery x 2 = 1500 Amps instantaneous. Perhaps this Nano Tech Plus rather expensive battery will not dip under load and will do the task. That is 70 amp for 4 minutes or ~ 85 amp for 3 minutes on a sustained repeated basis. Mike Smit who flys the larger Helicopters and knows batteries recommends the Scorpion 5100 mAh 70c at about $220 and the GenAce 5000 mAh 70c batteries also about $220 but problem with Covid is no one has stock. Some of the claims made by the Lipo battery makers are dubious and I think the Graphene ones are expensive and perhaps not as good as they say. I might have to wait a little longer before I can get the right batteries and get the tried and tested Scorpion or GenAce batteries Mike recommends.

Watch out for UFO's flying over Toowoomba !!

Things we do to amuse ourselves eh - all good fun - cheers George Fricker
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