### Impossible to get 1 hour flight with current technology?

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?

Thanks,

Roger Larsen

#### Replies

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• RIght now commercial drone like marvic can fly around 30 minutes, i personnally think that, within maybe 2-3 years some rivals product or the DJI itself will release some kind of 1 hour long flight.

I do wonder, is there any techonolgy could utilize some kind of hybrid tech, like when the drone is flying, the solar battery will be charged

• Beyond the purely mechanical problems with quads, can we talk about the Lipo, electrolytes battery problems with quads in the wider range of climates? There are arctic expeditions that we see helicopters with gas fuels routinely fairy personnel. Quad copters are nowhere to be seen with those high-latitude locales because the electrolytes freeze in hibernation, and Quads can't take gas engines without substantial pulley systems, swashplates add-on. Helicopters, on the other hand, routinely change between battery/motor power and gas engines without adding any pulley/swash. This is the nature of helicopters that de-couple power from control.

Quadcopters can not do polar expeditions or perform high latitude missions in Scandinavian countries in the cold days. Quads are toys at its core.

Is everyone is starting to get the idea that the optimal matching of motors to props isn't trivial?  Then this is a great discourse.

In a BLDC motor, your throttle control directly controls RPM based on the KV rating, of course.  Here's what a typical BLDC performance curve set looks like (courtesy of Johnson Motors):

• The large propeller inertia problem can be solved if you use the swash plate of helicopter to change pitch, instead of changing rotor rpm rapidly in fractions of a second. That is why serious military copter drones that routinely have 1 hour missions are helicopters, not quads.

Motors generally are going to be about 75-85% efficient, and the LiPoly technology is what we're basically stuck with.

The two variables which are under your control are propeller FM and disk loading.  In this worked example of a 4-pound quad, the ideal power (FM=100) is about 19 watts per gram.  If ultra efficiency is your goal, off-the-shelf props are going to disappoint you.

DIYD Thread - The Case For Large-Scale Electric Multicopters

As Jack said, if you made the disk loading light enough, it might work, but then rotational inertia would impact the controllability severely.  Ultimately, you're right - the energy density of batteries is still not there yet.

DIYD Thread - Worked Example of Energy Density Solutions

• I have the RC Timer 4215 650kv motors and would like to use the carbon fiber 15"x5.5 propellers mentioned in this thread. But what would be the best way to mount the propellers to the motors? These motors have a three point prop adapter so I can't do direct mount. Is the best way to ream out the middle mounting hole on the propellers and put on the prop adapter?

• Interesting to note the requirement for high RPM. That might explain why the standard purple Arducopter motors seem so efficient with 10x4.7 props. Hope to see their graphs sometime or do tests myself.

Also wanted to mention some links to micro generators. Don't know if they ever reached production stage, but seem promising.

http://www.sciencedaily.com/releases/2004/11/041124154548.htm and the same one 2 years later http://web.mit.edu/newsoffice/2006/microengines.html

Simpler design: MICE generators article

MICE generators.pdf

• Is everyone is starting to get the idea that the optimal matching of motors to props isn't trivial?  Then this is a great discourse.

In a BLDC motor, your throttle control directly controls RPM based on the KV rating, of course.  Here's what a typical BLDC performance curve set looks like (courtesy of Johnson Motors):

The "X" axis is torque and the "Y" axis is, well, everything else.  It's usually impractical "in the field" to measure either RPM or torque directly, but the two things you can measure are voltage (roughly equivalent to throttle position) and current.  It is important to note that the maximum efficiency peaks at about 30% power, and (as Mike said) maximum power efficiency is only about 60%.  This varies from motor to motor and among manufacturers, but the above curve set relationships are virtually axiomatic across all permanent magnet motors.

Most reputable motor manufacturers will supply data curves with real numbers.  The real baseline measurement is current; if you know that, everything else is determinable from the chart, as current and torque are directly and linearly proportional to each other.

It stands to reason that if one wishes the most efficiency, using over-sized motors for the application would be the rule, provided the weight penalty isn't unreasonable.  You can get more payback this way than careful propeller shopping, where almost all the figure of merit (FM) calculations from empirical data I've looked at place most model props in the 50-60% range in static thrust mode.  The important point here is that small props up to about 18-inches or so actually gain FM the faster you twist 'em, due to the Reynolds Number (Re) paradox.

Empirical measurement always rules.  But the basic rule-of-thumb for efficiency is buy a bigger motor than you need, and spin the prop as fast as you can, but not faster than the manufacturer recommends.

• @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.

• 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:

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