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.

Views: 4212

Comment by Jason Franciosa on September 10, 2016 at 5:19pm

@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.

Comment by frederic reblewski on September 10, 2016 at 5:56pm

@ 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

Comment by frederic reblewski on September 10, 2016 at 6:25pm

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...

Comment by Damian on September 11, 2016 at 12:29am

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.


Developer
Comment by John Arne Birkeland on September 11, 2016 at 2:33am

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.

Comment by Damian on September 11, 2016 at 3:24am

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).

Comment by Damian on September 11, 2016 at 3:33am

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 ... 

Comment by Damian on September 11, 2016 at 4:01am

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 ... :)


Developer
Comment by John Arne Birkeland on September 11, 2016 at 4:20am

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.

Comment by Jason Franciosa on September 11, 2016 at 5:35am

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.

Comment

You need to be a member of DIY Drones to add comments!

Join DIY Drones

© 2019   Created by Chris Anderson.   Powered by

Badges  |  Report an Issue  |  Terms of Service