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

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Replies to This Discussion

Permalink Reply by Roger Larsen on June 22, 2012 at 3:15pm

So, what if you created an octacopter where the you use 4 large props low kv engines and add 4 higher kv, smaller prop engines for stability?


DeveloperPermalink Reply by John Arne Birkeland on June 22, 2012 at 4:24pm

A pentacopter would probably be better, and you could use a gasser for the big propeller.

http://www.youtube.com/watch?v=0h9oYhsuMq4&feature=relmfu

But again I feel we are starting to move away from the core of a multicopter, which is simplicity.

Permalink Reply by Roger Larsen on June 22, 2012 at 6:37pm

Cool idea, except I cannot occupy the center of the copter with a gasser :) 

Permalink Reply by Roger Larsen on June 22, 2012 at 9:01pm

Thanks for all the valuable input. Based upon my research and your input I will set up a test to stop guessing and ensure I can get the most trust per W. Stability and what not will be not measured yet.

This is my test set up so far;

ECU
http://www.hobbyking.com/hobbyking/store/__2163__TURNIGY_Plush_25am...

MOTOR
http://www.rctigermotor.com/show.php?contentid=142

PROPS

A bunch of variations from 12 to 15" in size
Graupner E-prop
ACP SF

LIPOS

http://www.maxamps.com/Lipo-11000-74-Pack.htm
- 2,3 and 4s

The idea is to test the trust with different combinations of volts / amps / props.
I also have some ideas to improve airflow / turbulence around the prop to see if that does anything.
Dimpled props apparently doesn't do anything; so I will not try that.

Before I order all of this, let me know if there are other configurations (motors / props) you would like for me to test. Just keep in mind is that my goals are a stable platform, with large carrying capacity and range. My goal is not to get a super fast and agile platform. I will build the copter using CF parts myself instead of ordering ridiculously priced kits.

Thanks,

Roger Larsen

Permalink Reply by arashi on June 22, 2012 at 10:52pm

@ roger - Those T-Motors look nice, but expensive.  The most efficient I have experienced so far is 13g/w with Rctimer 5010 360kv motors ($30) on a 4s battery

http://www.rctimer.com/index.php?gOo=goods_details.dwt&goodsid=575

with Rctimer 16" carbon fiber props $45 (ouch)

http://www.rctimer.com/index.php?gOo=goods_details.dwt&goodsid=...

You might check out some of the newer, cheaper Rctimer props for the T-Motors

BTW, everyone told me that my quads wouldn't fly in the wind with such large, slow props, but with the low KV, high torque motors, they are very responsive.

Since you are going for an Octo, you might want to try 8 of these with 14x4.7 props to life over 4 kg with a 1S battery.


Rctimer 4215(3507) 650KV - $20

http://www.rctimer.com/index.php?gOo=goods_details.dwt&goodsid=...

Prop

Volts(V)

Amps(A)

Watts(W)

Thrust(g)

Efficiency (g/W)

14×7E

7.4

7

51.8

552

10.66

8.5

8.6

73.1

701

9.59

10

11.1

111

910

8.2

11.1

13

144.3

1083

7.51
Permalink Reply by Brad Hughey on June 23, 2012 at 5:55am

Those RC Timer props are the closest I've yet seen in the hobby market for optimum taper and twist (can't see the twist, but it looks good).  They should outperform the typical airplane propeller, which generally have FM's in the 45-55 range.  If done correctly, a good lifting prop should achieve (at this scale, has achieved) an FM in the upper 70's.

Permalink Reply by Roger Larsen on June 23, 2012 at 11:25pm

Very promising. What was the trust at 13g/w? These props seems to be easily mountable on that motor also. I noticed that alibaba has a very similar one listed, I wonder if you can strike a better deal there.

Permalink Reply by arashi on June 24, 2012 at 11:25am

@ Roger - The total vehicle weight was 1400g, so around 350g per motor with 4s battery, 360kv rctimer motor and 16" rctimer props.

I bought the expensive 16" rcitmer props before the much less expensive 15" ones came out.  I would go with the 15"x5.5 carbon ones.  4 props for $52.

http://www.rctimer.com/index.php?gOo=goods_details.dwt&goodsid=...

Permalink Reply by Roger Larsen on June 24, 2012 at 11:46am

Have you looked at these?

http://www.smartdrones.de/collections/propellers/products/multicopt...

They have 2 different types of props.

In addition their motors;

http://www.smartdrones.de/collections/motors

I'll worry about price later, I just want the best combo. I also noticed that the microdrone props have a slight upward bend at the end and they look to be HUGE (20"+). 


DeveloperPermalink Reply by John Arne Birkeland on June 23, 2012 at 6:27am

Roger, If you already have the Plush ESC's, they are just fine. But if not take a look at the Turnigy TY-P1 25A ESC. Better MOSFETS and in stock. Compatible with the SimonK firrmware, BlueSeries version (bs.hex).

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=...

Permalink Reply by Jerry Normandin on June 23, 2012 at 7:04am

John,

  I have the BlueSeries 30A ESCs on my quad.  I went overkill to prevent overheating.   Is it compatible with the SimonK firmware ?   Also what motors in the $15 -$20 price range do you suggest for a 450mm frame for

a balance of flight longevity and  lift ?   I'll be around 1600g with landing gear and gopro.

I'm runnng 11.1v 1200kv 26-28a motors right now.


DeveloperPermalink Reply by John Arne Birkeland on June 23, 2012 at 7:55pm

http://wiki.openpilot.org/display/Doc/RapidESC+Database

30A BlueSeries is supported. But there is two versions, so you have to compare the pictures to see which one you have and use the corresponding firmware.

1200kv is perhapse a bit high. I would go for something in the 700-900kv range. Haven't tried this one, but looks good on paper. http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=...

Just be aware that if you have large props on a lightweight frame it may become unstable, because it will fly at very low rpm's making stabilization difficult.

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