I've been reading about Micro Drones claim of 1 hour flight and I have been pouring over some numbers trying to figure out if this is possible with LiPo batteries. My answer is simply no.
I've look at different batteries and different motors. Typically the motors are able to thrust 10g per W. This means that for every gram of thrust needed you are going to consume 0.1Wh (watts per hour). Even this number is slightly optimistic, some motors / props have half of that output others go as “high” as 11-12g per watt.
Let’s use 0.1Wh per gram of thrust.
Now, let’s look at batteries (I’ve compared thunder rc and hk batteries) and the best pack for the punch is about 650 grams for a 8400mah battery (3s). With LiPos you are supposed to drain about 80% which gives about 75Ws for this battery. If you divide the weight of the battery with the Whs you find that you get about 0.11Wh per gram of battery weight.
So, to simplify; just to carry the weight of batteries you will burn up all the charge in one hour. It doesn’t matter HOW many batteries you add. The equation is still the same. However, I have seen that typically LiPo batteries packs more punch the bigger they get, but I could not find any reasonably priced batteries larger than 8400mah.
My dream of building a 1 hour flight octo with 4lb carry capacity is just impossible.
The only possibility I see is to tweak and build a really large copter and gain a few % here and there or come up with another source of energy than LiPos.
Can someone correct my math if it’s off please, or direct me to more efficient motors and batteries?
Ooo... Looking forward to the results of your tests!
I think one should plan to use the motor-prop combination at a thrust loading where it is most efficient. Most manufacturers only give a single g/W value for a given voltage-prop combination, but one should consider the variation with loading.
Dr. Stephen Prior (winner of UAVforge competition) was kind enough to send me this test data for the RC-Timer 5010-360kV motors and Rc-Timer 15x5.5 CF props combination:
12g/W is impressive, but 250-300g thrust per motor is sadly not very useful if you plan on any kind of payload.
Yes, but 10g/w is still just about 475g of thrust. Even with an octocopter you can then only lift 3.8kg. Barley enough for the frame and a small battery. For a octocopter with large battery, gimbal and camera you usually need about 6.5-7kg of lift. So my application I am looking for efficiency in the 800-900g thrust range.
Edit: But even so, 8g/W for 850g of thrust is still pretty good.
@John: If you want so much lift as well as efficiency then you could consider going co-axial on your octo. The added weight is not as much as when you add arms and the loss in efficiency of a co-axial configuration (typically 15%) would be more than compensated for by the increased efficiency of the motor-prop combination at 400g/motor (11g/W) vs. 8g/W at 800g thrust per motor?
For my application I want to build a quad with all-up weight of 1600g, so I'm willing to settle for 11g/W instead of going for hexa config to get closer to 12g/W.
It is apparently about 8% more efficient to mount the motors below the arms in pusher config rather than on top where the arms are in the rotor downwash. (Also courtesy of Dr. S.P's tests). This value could vary a bit depending on the profile of you motor arms.
The question is just how the g/W curves look for other combinations of props & motors.
Or as I mentioned earlier you can go with 3 blade props. I think it's less than a hit with coaxial props.
I need stability in gusty wind. That's why I built a heavier quad. I'm flying around the shoreline.
I would be interested in building a balsa made quad too see how long of a flight I can achieve. But I think flying along the shoreline it would not be stable.
3 bladed props are not as efficient as 2. The only reason aircraft go with 3 or more blades is because ground clearance requires it.
Murray: A brushless motor usually has a efficiency around 80%, so the g/W gain must be better then that. If you then consider the added weight and complexity of 16! motors I thing it would not be worth the hassle for marginal improvements in flight times.
But mounting the motor and propellers below the arm makes sense, and should be worth a look.
I agree that 16 motors would start looking a bit like a Christmas tree. The 360-15 motor-prop combination is probably not what you need for efficiency at those weights. Hopefully someone can provide efficiency graphs for stronger motors with larger props.
My point is just to select a motor-prop combination with high efficiency for the chosen application. Going up in motor numbers has a penalty of around 100g per motor added, but you could still end up with significantly higher overall efficiency.
I don't quite follow the 80% motor efficiency argument? If one motor is 80% efficient and the g/W ratio is based on that, doesn't that apply to any number of motors? Otherwise what g/W efficiency would you calculate for a quad with the 360-15 combination?
be cautious with the 80% figure. motors are only 80% efficient across a narrow range of load, below that load, they can be ~65%, and above many go down to ~50%. Even though a motor/prop combo can deliver higher thrust, after that thrust curve passes above the bottom 1/3rd of its range, typically, you begin to drop below 75% for the motor, as a general rule. By the time you get to 50%, most motors are well into the 60's in terms of efficiency.
And the behavior for props efficiently delivering thrust has its own arc.
Murrau: You are of course correct that the g/W scale already takes into account motor efficiency loss.
But the g/W is calculated for a single propeller, so the figures when you have two propellers stacked and down wash will most likely be different.
But again I don't see having 16 or even 12 motors as something realistic for practical applications. Just 8 of them is already a chore. If I wanted 1 hours flights at all cost, I would just get a big fuel helicopter and learn to live with the mechanical complexity, motor noise and smell and vibration issues.