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.
And if we learned something in the process, it's a win-win. :)
I'm going to throw my support behind Jason on this. He's got it. You don't automatically get better efficiency using higher voltage. Yes, it breaks the internet. This happens a lot.
Ok Jason; I assumed the same resistance. You are right. Lower kV motors have much higher resistance.
Great I can keep designing my drones on 3S as it is much cheaper .. :)))
ESC Losses are much more difficult to calculate as it will change throughout the switching speed required by the ESC and the quality of FETs used. I don't know how to calculate it accurately as it is quite complex and dependent on many factors, Motors are much simpler.
How are you saying you are reducing losses by 50%?
You do understand that if a Motor is 1000kv and the resistance is .07, an equally wound 500kv motor will be 4x the resistance right? The resistance changes as the KV changes which is why the 3s is less efficient than 6s is completely misunderstood by many DIY and RC enthusiasts. Use the formulas I provided above and do the math out with accounting for the resistance increase when changing the KV.
When you rewind a motor for lower KV, you use half as thick of wire and double the windings.
Your math is way off.
3s vs 6s efficiency. Just picking up some random motor here http://www.hobbyking.co.uk/hobbyking/store/__18968__Turnigy_D2836_9... with stated internal resistance: 0.07Ohm.
So with 100W power, 10Vs for 3S the estimate current is 10A. 10A x 0.07Ohm = 0.7V; 0.7V x 10A = 7W losses.
So potentially with 6S and similar quality motor we could reduce the losses by about 50% from 7W to 3.5W.
If we fly cruise with 50W power only; the losses would be about 1.75W; and the upgrade from 3S to 6S would save us only 0.875W. Not as good deal as above.
But for 500W required power; the losses would be 175W and the upgrade from 3S to 6S would give us extra 87.5W; almost extra 15%.
Any idea how to calculate ESC losses???
My concern with using the larger props for cruise flight is to generate more lift than the drag for a given required speed. It is OK for e.g. 3D flyer to have prop thrust e.g. 200% of the weight of the plane, but it is an overkill for a cruise optimised drone to have more prop thrust than 10% of its weight; for cruise level flight.
E.g. if I want to fly 2kg plane 100km/h what would require about 200g of prop thrust I am getting this data from the prop calculator http://adamone.rchomepage.com/calc_thrust.htm:
APC E 12x12; 4550rpm; 717g static thrust; 98.4W prop power; pitch speed 83km/h; estimated level speed 100km/h
APC E 6x5.5; 10000rpm; 232g static thrust; 38.1 prop absorbed power; pitch speed 84km/h; estimated level speed 101km/h
So for this particular case of 100km/h cruise and 2kg heavy air plane with drag to lift ratio for the given speed 1/10 the smaller prop is 2.5x more efficient than the larger one. Even the smaller prop is less efficient on its own ...
The RC prop calculator above and some RCGroups posts claim that RC planes can fly a bit faster (15%) than the pitch speed (assuming the total drag is less than the prop thrust).
>> So yes a plane can fly faster than pitch speed since the blades are airfoils that produce lift.
@John, This is most certainly a very misunderstood topic. The number of windings does not effect the torque or the magnetic field if the voltage is changed proportionally and the copper fill is remained the same. Here's the math if you are curious and want to try some different configurations of 3s vs. 6s or whatever.
Skin effect is not much of an issue until you get to a greater than 12 gauge wire winding give or take, which, with these size aircraft I don't think anyone has to worry about. It is still a DC Current not AC, even though it is broken up and phased by the ESC.
The main advantage of higher voltage is if you are drawing high currents and using small wires, fortunately when setup for high efficiency, the standard RC Wiring for the batteries, motors, and electronics is more than sufficient to not have significant losses. The other main advantage of higher voltage is you can use a smaller ESC, but again, weight difference between a 30 amp ESC vs. a 60 amp ESC is negligible for these applications.
The main advantage of 3s over 6s is that many electronics for FPV are directly powered off 12v which eliminates the need for a BEC on 12v. BEC's are considerably less efficient than the losses you are getting from a bit more current draw through our fairly heavy gauge wiring on batteries and ESC's.
Absolutely, efficiency for long range flight is for the complete system. I would argue aircraft aerodynamics and prop selection is far more important than voltage choice.
@Jason, I am by no means an expert on motors. So you might very well be correct. My understanding was that more wingdings would lead to a stronger magnetic field i.e. higher torque. And one has to see the whole system as one when measuring efficiency. Large propeller efficiency gain vs. motor efficiency loss/weight etc.
And because the wires are isolated from each other, skin effect might possibly result in better performance when having multiple thin wires. But this is pure speculation.