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.

Logfile Link

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.

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Comment by Stephen Gloor on September 11, 2016 at 7:23am

Well done Ben.

Ben did not mention that the whole time it was flying he was helping me get the other half of the team's planes going. 

The Talon is an amazing aircraft and Ben has set it up really well. 

Comment by Ben Dellar on September 11, 2016 at 7:35am

Thanks Steve - 1hr 40min is a long time in the air so was good to have other tasks to think through.

Regarding our config choices - we are working on the conventional basis that a bigger slower turning prop is more efficient. The theory boils down to the longer you can wait before a blade has to re-enter the turbulence behind another blade the more efficient (ie slowest RPM possible and therefore largest propeller possible to ensure enough thrust)

We selected 12 inch is as the maximum size for the Talon because the lower side vertical tailfin only allows enough clearance for a 12" prop without either risking striking the ground on landing or possibly injuring the person throwing it for launch. In theory if we were really chasing uber-max range a larger prop would do the trick with careful flying but the aircraft does exist to perform a defined mission reliably rather then chase records at the risk of unsafe or unreliable take-off and landing.

We selected 10 inch pitch based on needing a sustainable cruise speed of 70-80km/h during the 2016 Outback Challenge. A slightly lower pitch would probably be better for max range at a lower speed but we had a speed requirement for our desired mission and only need a range of 60km + 10 minutes loiter so can accept the efficiency trade off.

To be honest the cell count again was a matter of ease for us - much easier to run ancillary equipment from 3S voltages then 6S but I do agree - if every last km counted the higher voltage would likely be a good call.

The key take away for me remains that the Talon is an amazing air-frame - even with all of the operational compromises we made to achieve the mission (smaller than theoretically maximum prop diameter, higher than optimal pitch, 3S instead of 6S with higher KV motor to offset) it still can tick off 100km flights.

Comment by Gary Mortimer on September 11, 2016 at 10:01am

Do you have the wing extensions? I found up here at altitude they turn it from a fairly sporty machine to a pussycat.It starts to be a ground handling issue though as it is quite a size at 2m span. http://diydrones.com/profiles/blogs/x-uav-talon-wing-extensions Just because of its bigness. I would love to get my hands on the even bigger one. I can't even remember what its called there are so few in the wild.

Comment by frederic reblewski on September 11, 2016 at 10:55am

@Damian, I do not understand. you say that the plane would fly faster that the pitchxrpm speed. I thought it was never the case and that the propeller <100% efficiency was coming from the fact that it was "slipping" and so the pitchxRPM was always lower than plane speed.  the propeller is like a screw and considering that the screw could move more than pitchxturns seems strange physically.

from what I understand the thrust comes from the fact that a mass of air is accelerated when going through the propeller disk. how can you accelerate if the air going through the disk is slower than the incoming air?

the funny thing is that when you closer to 100% efficiency the thrust tends to 0 as incoming air speed and propeller induced air speed are equal.

on the propeller diameter the efficiency of the propeller is inversely proportional to the square root of the propeller load ( more or less thrust/propeller disk area ). that is why everything else being equal it is better to match using a larger propeller with a larger pitch at a lower RPM through a gear box. the reason why you do not see that more in general aviation is that it is not easy to fit a large propeller with a reasonable landing gear and gearboxes induce reliability issues

Comment by John Arne Birkeland on September 11, 2016 at 10:57am

@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.

Comment by Jason Franciosa on September 11, 2016 at 11:21am

@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.

Comment by Damian on September 11, 2016 at 11:57am


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.

Comment by Damian on September 11, 2016 at 12:12pm


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 ...

Comment by Damian on September 11, 2016 at 12:30pm

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???

Comment by Jason Franciosa on September 11, 2016 at 12:51pm


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.


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