I have been thinking about sending my X-UAV Talon on a 100km flight and despite pondering it for a long time today I hit the milestone in an unplanned way.
During a checkout flight for our spare support aircraft for West Coast UAV's entry in the 2016 Medical Express challenge I noticed that we had an amazingly efficient combination of airframe/motor/prop and the numbers were looking good for a 100km run.
So up the Talon went carrying 20AH of 3S Multistar batteries driving a NTM1100KV motor and APC 12x10 prop. The average speed was kept low and despite encountering some airspeed measurement issues it crossed the 100km mark at just over 1 hour and 40 minutes airborne.
The landing was unfortunately a bit hairy and fast as the voltage had dropped fairly low and the airframe copped a few dents but really happy to have knocked over the 100km milestone.
Comments
Thanks Frederic
1/ I have used this RC prop calculator (http://adamone.rchomepage.com/calc_thrust.htm) to find out required prop power for 12x6 prop with straight cruise of 60km/h. And added 20% ESC losses and 20% motor losses. It roughly matches my test data, although it is just a static thrust test on bench.
2/ Agreed with optimum efficiency speed of 130% of the stall speed. But my understanding was that 60km/h was a business requirement.
3/ For the max efficiency straight cruise your prop pitch speed should be about 10% less than your cruise speed. And you should generate just enough prop thrust to overcame the drag. Normal plane cruise prop thrust to weight ratio is about 1/10 to 1/16 (e.g. Cessnas). The OP thrust to weight ratio for the cruise is 1/4 what is not necessary. This would be required for e.g. 20%/30% climb only.
4/ Based on my data above the OP battery capacity is about 35% higher than it needs to be. But I assume he requires some back up for bad wind / etc ...
5/ The issue with larger props is that they generate/use too much thrust/power for cruise. On the other side they are more efficient for climb/3D. Smaller props other way around.
6/ Thanks for explaining 3s/6s efficiency problem. It makes sense.
as for the 3S vs 6S, I think the only real advantage is to lower resistive loss. very rough evaluation: if during cruise you are @ 100w and your total resistive path ( batteries, contacts, cables, on resistance of the Fets, motor ) is 0.1 Ohm that's around a 8W loss on 3S and 2W on 6S. really second order compared to plane/motor/propeller matching...
@ Damian, I am curious. where does the 70W come from?with 20AH and 3S you have around 800KJ. the flight being 100min that's about 130W average power? if your assumption is true on the L/D ratio the mechanical power needed is DxV so around 33W the difference being the power lost in the electronics and the overall electrical to mechanical yield(cables, esc, motor, propeller)?
I do not know about the efficiency of the relative propellers but if you want to fly far I think you are supposed to fly at the best L/D ratio. as a first estimate it is around 30% over stall speed. @2Kg a talon has most likely a stall speed of around 8m/s ( wing area =0.6m2 Cl max=0.8). so the game would be to optimize the motor/propeller combo to be at peak efficiency @ around 40 Km/h. most likely a reducted large propeller turning slowly. cheers
@John, why would 6s be more efficient than 3s assuming the cabling can handle the current draw without significant losses? If he needs 100 watts to generate thrust, it's 100 watts weather he is on 3S or 6S. And actually, it will be more wasteful for the switching BEC's to drop a higher voltage down to a lower voltage for feeding all the other electronics. Assuming the ESC is rated for the current draws, I don't understand where this obsession that higher voltage is more efficient comes from. Higher voltage is great if you need a ton of watts and don't want to have a huge ESC to handle high amps or huge heavy cables, but, with how low the current draws are for a high efficiency setup, even on 3s, it really shouldn't make much of a difference at all what voltage is chosen assuming the KV of the motor and quality of the windings is equal.
Also a tractor prop can give you extra 10%/15% more efficiency over your pusher prop ...
The 12x10 prop seems a bit of thrust/power efficiency overkill for the Talon straight cruise flight.
Let's assume 2kg flying weight; nothing special lift to drag ratio 10/1; so required prop thrust would be 200g+ for a straight cruise.
In your case 100km in 1h40min equal to about 60km/h speed with zero wind.
The 12x10 prop and 60km/h cruise flight requires about 70W with generated 500g of static thrust.
If you go for e.g. 6x4 prop you would get about 72km/h cruise flight with about 34W consumed with 200g of static thrust; 10000rpm. Obviously you would like to have some extra thrust margin when needed so with 3S (10V) you should go for a 2000kV/3000kV motor, to run it about 30%/50% max power; what is also the peak motor efficiency. And you can go for full throttle to double/triple the thrust when needed.
So with the smaller prop you could almost double the flight time. Theoretically .. :)
Personally I have no idea how much 6s would be more efficient over 3s .Also some people claim that running motor and ESC on e.g. 40% is less efficient than 90%; but other disagree.
congrats! I also fly Talons and love them. In my opinion the best fixed wing platform (flies well, very customizable, able to fly with an AUW of 4,5Kg and still take off from a catapult, able to embark multiple cameras, stable, can be fixed after every crash even hard ones, etc)
Nice one Ben!
Out of interest what can size NTM motor did you use? I can't find any red ones on HK.
congrats! how much range did you get on RFD900.
Thanks Gary - the Talon really is an amazing aircraft. Perfect combination of size (fits in car fully assembled), weight, endurance, speed, handling in air is great and very versatile for different payloads and missions.
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