One of the questions most frequently asked by people building their first multirotor craft is "How do I work out how long I can stay in the air?" Answering that question is both remarkably easy and frustratingly difficult at the same time.

The answer to the question is very simple in essence.

Let's assume that you have a power source that weighs 1kg and contains 200 Watt hours of power and you have a propulsion system that will produce enough thrust to hover your 1kg power source in the air.

If your propulsion system delivers this thrust at an efficiency of 13 grammes per Watt, then to keep your power source in the air for one hour you will use 77 Watt hours (1000/13) of power.

If your frame and propulsion system weighs another kilogramme, then you will use twice that much power in one hour or 154 Watt hours. If you fly until your power source is empty, that would add up to 79 minutes of flight time.

This can be expressed by the following formula:

flight time in minutes = density / (1000 * R / efficiency) * 60 where:

"density" is the battery's energy density in Watt hours per kilogramme.
"R" is the ratio between the all up weight of the craft (including battery) and the weight of the battery alone.
"efficiency" is the efficiency of the motor and propeller combination at the chosen battery voltage expressed in grammes per Watt.

The efficiency rating used should be the efficiency when producing sufficient power to hover the craft which is when the propulsion system produces thrust that equals the entire weight of the craft and battery. 

The throttle value should be 50% or less to achieve this much thrust and should never be more than 60% for there to be enough power left over to control the craft.

Here is a spreadsheet which will calculate all this for you:
  GenericFlightTime.xlsx (Size: 6.04 KB / Downloads: 334)

Just fill in the yellow cells and the spreadsheet will do the rest.

So far, so simple.

The hard part is the efficiency rating. If you buy your motors from Banggood, you'll be lucky if there's a chart that shows the power consumption and static thrust generated at full throttle with one particular propeller and maybe two different battery voltages.

The efficiency of a propulsion system is not a constant and varies considerably through the throttle range. Without knowing what the efficiency is at 50% thrust, you won't have the data needed to perform the calculation.

Reputable motor manufacturers like T-Motor and Cobra do publish thrust and efficiency ratings for their motors on a variety of appropriate propellers at various throttle levels and for those who don't, sites like the wonderful miniquadtestbench.com are superb resources.
The other option is getting a good (not Hobbyking) thrust stand and running your own testing to generate the data that you need.

There's also the chicken and egg problem where you don't really know your all up weight before you have chosen the propulsion system, frame and battery. Solving that problem is more a matter of successive iteration than anything else though and can be solved easily enough.

This way of looking at flight time calculation comes from the user 'renatoa' on RCGroups. It's based on simple maths that simply evaluates power consumption for a given ratio of power source and all up weight based on the efficiency of the propulsion system and the amount of power available. It has proved to be remarkably accurate which isn't really surprising as there's no black magic involved in working out how long something that uses power will take to use that power!
 
I hope this post helps people in answering the flight time question!

I would say it mostly depends on your flying style. Most of the time I get 2 -2.30 minutes, but if I'm easy on the throttle and just cruising along, I get almost 4 minutes. So, if hovering outside your neighbours window like most people think we do, we can probably get 5 minutes. Just enough...

(28-Apr-2017, 10:38 PM)oyvinla Wrote: I would say it mostly depends on your flying style. Most of the time I get 2 -2.30 minutes, but if I'm easy on the throttle and just cruising along, I get almost 4 minutes. So, if hovering outside your neighbours window like most people think we do, we can probably get 5 minutes. Just enough...

Actual flight time is, of course, dependent on how you fly and your goals for the particular craft. For racing or freestyle quads, we all know you get around 2 to 6 minutes flight time in the 5" class. You don't really need to calculate it. Racing and freestyle are not the only applications for multirotor craft though and if you need longer flight times, it's well worth doing some planning and working out if a particular configuration will deliver what you need before you buy all the parts.

What you will get from this calculation is a fairly accurate estimation of flight time when flying for aerial photography or similar low to medium speed flight. If your intention is to fly your craft at high speed or for acrobatics, you should use an efficiency rating based on higher thrust levels.

I recently found a very useful series of articles by 'RFEngFPV' about motor performance which include some rather interesting comparisons of motor efficiency over the whole throttle range in graphical format like this:



The data used to create the graphs comes from miniquadtestbench.com This particular graph is really interesting as it shows significant differences in performance even between motors with similar stator sizes and nominal kv ratings swinging the same propeller.

The articles start here at the post "Motor Weight/Efficiency Trade-Off". The site is well worth a visit, especially if you have questions like "which motor is best with the KK 6x4 propeller?", "which is the best motor for a 5" freestyle quad?" or "which 5" propellers are best for the Armattan Oomph motors?".

I have been using this method for quite some time. I fly "big" Pixhawk aerial photography rigs. Using data flash log files I can see voltage and current. With that data I can calculate power. Having power I can calculate the actual g/W, and from that I can calculate my 20% reserve flight time down to the second.