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
@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
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.
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.
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.
Hi @John, I have to respectfully disagree here...
"2. Higher voltage. Frederic touched on it. The main cause of energy loss (heat generation) in an electrical system is current draw. Going with higher voltage means you need less current for the same amount of energy ( P = I x V ). In practical turns this means that you get less head from all electrical components, and more importantly can use thinner copper wire and more winding's in the motor (low kv). Leading to less heat and better motor efficiency. Lower KV also leads to more torque so that you can drive a larger propeller more efficiently (see 1.). So high voltage/low kv is a win-win scenario."
If you rewind a motor with smaller thinner wires, you increase the resistance 4 times over (Wire is twice as long and half as thick). Not sure where you are assuming a low kv high voltage motor is going to have lower resistance?
Also, where does this low kv high voltage means more torque myth come from? This has nothing to do with the winding and or voltage, you can wind a motor with half the wire and 2 times as thick winding (Half the turns) and achieve the same kv ratio (say a kv of 600 on 6s vs. 1200 on 3s) in almost any configuration and nearly identical performance and efficiency. Again, as you change the windings, if done properly, the copper fill remains constant and you do not get losses due to resistance, Especially, when you're talking a motor designed for efficiency when you're cruising at 30-40% throttle. Depending on the motor, you can get more copper fill with higher KV due to less space wasted on wire coating.
Also, if you go with 3S, you can power many of your electronics (VTX and Camera's) directly from the battery instead of requiring a BEC. The inefficiency of a BEC will more than make up for any losses due to higher current draw in efficient flights. This is assuming you're not using 24 gauge motor/battery wires, but, if using proper gauge battery and ESC wires, your additional resistance losses of 3-4 amp draw vs. 6-8 amp draw are negligible.
I've personally rewound multiple motors and seen this at play, the major advantage of high voltage over low voltage is when you need a lot of power and a large ESC is not practical.
Also, High voltage batteries typically will weigh more as they require more cells, wires, and plates between each cell for the same watt hour capacity.
I never said that 12" was the correct size. But general wisdom on this topic is pretty clear. The propeller advance ratio (J) is determined by propeller diameter and pitch. And rule of thumb is that you want low rpm high pitch propellers for efficient high speed operation. Here is a good paper on the subject. http://www.esoteric-david.eu/prilohy/HPB_3.pdf
Also the prop thrust calculator you linked does not seem to take actual propeller efficiency into account when calculating trust. I've played with flying wings and pylon racers using small high speed propellers, and efficient flight is not the first tough that comes to mind.
Hmm, APC E 12x6 prop is really a wrong match for Talon and 60km/h cruise speed.
These are the data for APC E 12x12:
240g static thrust
20w prop power; 28w total electric power
51km/h pitch prop speed; 60km/h cruise plane speed
And here an example of the GWS HD 6x3:
202g of static thrust
21.4W of the prop power; about 30W of the total electric power
50km/h prop pitch speed; about 61km/h cruise plane speed
So actually 12x12 would be a great match for the Talon and I was wrong ... :)
Actually just running the numbers again with the http://adamone.rchomepage.com/calc_thrust.htm and APC E 12x6 prop and I get:
853g static thrust
92.4W of required prop power
about 130W of required total electric power (20% ESC losses; 20% motor losses)
static prop pitch speed 50km/h
estimated level cruise speed: 61km/h
So now the power consumption 130W matches the 1h40min flight time with 200Wh capacity.
So in this case the 6x3 prop can provide theoretically about 130/30 4.3x longer flight time.
Please check my numbers if I have not done any mistake ...
Thanks John
The cruise speed is given as the business requirement; in this case at least 60km/h. It makes sense for 100km flight with the flight time of 1h40min. Also the wing aspect ratio and wing loading are constraint with the given model: Talon 1700mm.
Why would you generate and waste 500g of thrust for 2kg heavy plane with large propeller for the 60km/h cruise if you need only 200g or perhaps even less ??? And the prop pitch speed matches (10% lower) the required cruise speed?
Modern real planes have the thrust to weight ratio for cruise even as low as to 1/18. As the fuel efficiency is one of their main concern.
http://aviation.stackexchange.com/questions/12162/what-is-the-minim...
Based on my numbers above the 6x4 prop will consume half the juice over the 12x6 and increases the cruise speed by 15%.
And based on the web based prop calculator above the even better match for the 2kg plane would be 6x3 prop; 11000 rpm, 200g static thrust, prop power 21.4W; total required electric power about 30W (40% higher for motor and esc losses).
1. Propeller size. The main problem is that propellers get aerodynamically less efficient as they get smaller and rpm increase. So a 6x4 blows (haha) when it comes to efficiency. If you look at long distance fliers (full size) the one thing they all have in common is slow speed and large propellers. This is also why helicopters are the most efficient hover design we know of. Simply because they have the largest propeller size of them all.
2. Higher voltage. Frederic touched on it. The main cause of energy loss (heat generation) in an electrical system is current draw. Going with higher voltage means you need less current for the same amount of energy ( P = I x V ). In practical turns this means that you get less head from all electrical components, and more importantly can use thinner copper wire and more winding's in the motor (low kv). Leading to less heat and better motor efficiency. Lower KV also leads to more torque so that you can drive a larger propeller more efficiently (see 1.). So high voltage/low kv is a win-win scenario.
3. Matching is probably the most important factor. Start with the most efficient propeller (that fits your platform) for the air speed and trust you need. Then a good motor that can drive the propeller with peak efficiency in the rpm range you want. Then ESC to match and lastly batteries for how long you need to fly. When all this is complete and you add payload you have your AUW. And knowing the weight you then might have to iterate the system with this in mind.
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