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Flight Time ?


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Is there a rule of thumb for flight time.

 

If I have a setup (motor, ESC, prop, battery) I can use something like eCalc or DriveCalc to tell me what the peak amperage is.

 

So say it is 40A

 

If battery capacity is 2200mAh - then I get 3.3 minutes at 40A

 

But first thing - that is completely emptying the battery which I don't think we do - how much is left when you stop flying - 25% ??

 

So let's say it is 25% - then I get 2.5 minutes

 

But second thing - that is flying flat out - and that is not how we usually flying - for the most part I will fly around at half stick with occasional full stick and zero stick thrown in for manoeuvres.

 

Is there a good rule of thumb for average amps and therefore expected flight time?

 

Trying to choose some batteries for some planes - I want to keep them as light as possible but I also want a decent flight time - so trying to find the best compromise.

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If you put all the details into eCalc it will give you an estimated mixed flying time which I have found to be reasonably accurate.

Aside from the ESC rating, what current is actually being drawn?

For example, my Riot originally had a 40A ESC and the motor would draw 28A static, obviously that reduces when the model is in motion. A 3s 2200mAh battery would give a good 5 mins of flying without draining the battery too far. When I replaced the motor with one that draws 38A static a 3000mAh battery would give a little longer as I was not using the full motor power except on take off and for some aerobatics.

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Are you aiming for a specific flight duration?  If not, my approach is to find the largest batteries (from weight and size point of view) that will comfortably fit in the model.  In that quest I also bear in mind that to achieve a certain amps capability (i.e. Ah x C) you can increase the mAh capacity or the C rating, or both, and the weights and dimensions will be almost the same for any specific amps capability. I prefer to stick with 20-30C packs so I can get more mAh.

 

Then it's just a matter of starting with a short flight and gradually increasing duration until your packs come down with about 25% remaining after the flight.

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15 minutes ago, Paul De Tourtoulon said:

Too many variables for me, motor and esc quality, temperature, prop, plane's weight, the way you fly, battery type and condition, I could go on,

put one of these in your plane.

shopping.png

Yep, I use one of those for initial flights if there's room in the fuselage

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The best way to achieve anything like a realistic estimate is measure what your full power amps actually are using a Watt meter.

Not only can you then establish a true "full power" fight time which will represent the worst case duration but it will show how close you are to the ESC's and motor's specification limit and how hard you are working the battery. Note In flight the amp draw is slightly less than when measured static. 

There is nothing stopping you also measuring the amps at the sort of reduced throttle position you might use in a typical flight to give you an idea of how much longer you could fly.

It does take a bit of flight trials to find out what is a practical maximum duration unless of course you have live battery telemetry in operation. 😉

 

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My rule of thumb for leccy is 7 mins for prop, 4 mins for EDF, these are for first flights. After landing check with battery checker (looking for circa 20% left) but record how much is put back in battery when charging. That will tell you, based on your flying habits, how much you have really used and as a result what you can set your timer to. Obviously in flight telemetry is a more accurate way of monitoring what’s actually going on.

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Like Ron, it's bit suck and see and you get to know what works. With my electrics (60cc size) on a 12s set up it's 7 minutes with an after flight cell reading 0f 3.7-3.8v per cell. That's before I measure how much juice goes back when charging. On a windy day when the packs have to work harder my timer is set at 6 1/2 minutes for similar voltage values. 

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I usually fly with telemetry so I get battery voltage, RPM, current, and mAh used. This makes interesting reading when logged.

It tells me that the flight duration is significantly dependant on the use of the throttle stick (!).

Looking at a typical log file for a model using a 12x6 prop and power supplied by two, 4-cell, 3000mAh lipos in parallel (so 6000 mAh) I see at the start take off I hit 59.9A at 15.4V (actually limited in the ESC to 60A, a FrSky neuron) and 10570 RPM. This drops to 53.2A at 10800 RPM 3 seconds later.

I then see, shortly after take off, I reduce the throttle to 65%, and see 28.5A at 9540 RPM and 15.7V, dropping to 23A at 9940 RPM and 16V.

 

So compared to static at the start of takeoff (922 watts) I'm flying with just 600 RPM less, but at a little over a third of the current (368 watts).

I see for much of the flight, I'm at between 55% and 65% throttle and somewhere in the 20-25A, 300-350 watt area and 9000 RPM, with more power for aerobatics.

This particular flight lasted 8 minutes and used 2900 mAh, so just under half the battery capacity.

If I had used full throttle for much of the flight, I would have been running at 45-50A, so the flight would have only been around 4 minutes for the same capacity used.

 

At the end of this flight, the battery showed 15.2V, so 3.8V per cell.

 

For this model I use the capacity used rather than a timer to know when to land!

 

Mike

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I am another who uses telemetry to assist in battery duration.

 

As I do a lot of instructing, my battery estimates would be all over the place as it depends on the lesson at the time. Gentle circuit training uses less power than aerobatics or touch & goes.

 

So as I use FrSky lipo sensors, this sends data to my Tx which can call the details out or post them on screen. Once the battery shows below 10% with load applied then it is time to land. Once the load is removed the battery recovers to about 20% which seems to be fine in my experience.

 

Obviously hooning around at full throttle reduces the capacity much quicker as does the cooler winter conditions.

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I fly F3A aerobatics that uses a 10S 5000 mAh pack.  The motors are usually rated around 3 kW.  I did a trial using both ground and airborne readings when I was unsure about the new 3 blade prop (21x13) I was using on a Hacker Q80 motor.  The Q80 is rated at 2.8 kW.  I had telemetry that gave V, A and W on the read out.  The airframe is a 2 m class that weighs 5 Kg.

 

On a tethered run, full throttle produced a reading of 3.6 kW whereupon I smartly closed the throttle!  However, once airborne, I found that even full throttle on a vertical upline topped out at 2.75 kW.  I decided to really try hard to hit or exceed 2.8 kW so with a new pack and right after take off I did a 180 and then slowed the model right down to try and load the prop as much as possible.  Then while pulling to the vertical applied full power and recorded a max of 2.85 kW.  That was the only time I exceeded 2.8 kW.  That has led me to disbelieve the static figures as they will always be high as that's the nature of an electric motor - load it up and it will keep drawing current.  

 

As regards duration, the schedule I fly has 17 manoeuvres not including take off and landing.  I generally record a time from take off to landing of 7.5 mins.  Technically, the limit is 8 mins but that's only adhered to for international or team selection competitions. The pack normally has a per cell reading of 3.75 - 3.8 v.  The capacity used is between 3,100 to 3,300 mAh per flight.

 

Hope that helps.

Edited by Peter Jenkins
Added capacity used.
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Talking to people in my club, whilst 30% is optimal for storage, flight time is variable. You can fly until power starts to drop off. There isn't really any difference between multiple short flights and a few less long flights, over the long term. Batteries are like fuel for IC planes- once exhausted, discard them. Having said that, I have batteries that should have been discarded, but still give a 6-minute flight (2200 3s). But this may be a minority view.

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50 minutes ago, Arthur Harris said:

Talking to people in my club, whilst 30% is optimal for storage, flight time is variable. You can fly until power starts to drop off. There isn't really any difference between multiple short flights and a few less long flights, over the long term. Batteries are like fuel for IC planes- once exhausted, discard them. Having said that, I have batteries that should have been discarded, but still give a 6-minute flight (2200 3s). But this may be a minority view.

The other way of determining what capacity remains is to use on-board telemetry to find out how much capacity remains in the pack.  Generally, you need to specify how much is in the pack capacity wise and then the system will use current demand and time to determine the charge being used.  I have used a product called Unisens-E by S M Modelbau (weighs about 20 g) and fits between the pack and the ESC and gives Pack V, A and W as well as RPM, and height (there is a vario function as well.  It costs about Euro 66 plus shipping but there are no other sensors needed to give what I have described.  By using the free download program you can set the Unisens-E to speak most of the current Radio Protocols.

 

Capacity remaining is now the only alarm I have set on my Tx and it triggers with 1,000 mAh remaining - from a 5,000 mAh pack.  If you fly multiple flights with the same pack, you will need to re-set the capacity remaining as shown on your Tx screen as otherwise you will start with the original limit you have set in your Tx.  Might be worth a try if you feel that you'd like to know what is going on with capacity.

 

The system gives you real time readouts and there is also a function that gives max and min readings for that flight.  All the data is wiped as soon as you disconnect the flight pack though.  S M Modelbau also do another product called Unilog-2 that has provision for an SD Card that records all this data for every flight for off line review.  

 

If you are limited to 400 ft, you can set that as an alarm so that you can avoid infringing it.

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1 hour ago, Arthur Harris said:

Talking to people in my club, whilst 30% is optimal for storage, flight time is variable. You can fly until power starts to drop off. There isn't really any difference between multiple short flights and a few less long flights, over the long term. Batteries are like fuel for IC planes- once exhausted, discard them. Having said that, I have batteries that should have been discarded, but still give a 6-minute flight (2200 3s). But this may be a minority view.

I get where you're coming from -  the variables asociated with battery use really makes it impossible to quote  a one size fits all rule. Personally, I prefer not to run my batteries down to where the esc begins to reduce power, even with the threshold set to its highest option. I have my timer set and can also judge when the model is starting to lose its zippyness (no pun intended).  I know flyers who squeeze every bit of life out of badly swollen packs that I would have discarded ages ago.

BTW, Arthur - have your clubmates nicknamed you 'Bomber' or 'Butch'?😃

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45 minutes ago, Cuban8 said:

I get where you're coming from -  the variables asociated with battery use really makes it impossible to quote  a one size fits all rule. Personally, I prefer not to run my batteries down to where the esc begins to reduce power, even with the threshold set to its highest option. I have my timer set and can also judge when the model is starting to lose its zippyness (no pun intended).  I know flyers who squeeze every bit of life out of badly swollen packs that I would have discarded ages ago.

BTW, Arthur - have your clubmates nicknamed you 'Bomber' or 'Butch'?😃

I'm really confused about battery care. A youngster (in his 20s, most of us are retired) joined our club a year ago, said he was bored with computer games, progressed from a Carbon Cub till he's now flying like he's at Joe Nall- prop-hanging, slow circuits with rolls, etc- he's become our "go to" to trim our planes. I asked him what time he set on his throttle- he said he just flies until the plane feels a bit soggy. I'm thinking that maybe I'm a bit precious about my batteries.

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3 hours ago, Arthur Harris said:

Talking to people in my club, whilst 30% is optimal for storage, flight time is variable. You can fly until power starts to drop off. There isn't really any difference between multiple short flights and a few less long flights, over the long term. Batteries are like fuel for IC planes- once exhausted, discard them. Having said that, I have batteries that should have been discarded, but still give a 6-minute flight (2200 3s). But this may be a minority view.

Folks talk about percentages all the time, but the pack used capacity percentage estimation is not a good indicator of battery pack health.

 

To get a good idea of battery pack health you need to monitor individual cell voltages and ideally individual cell internal resistance and keep an eye on those over time. Probably the biggest cause of battery pack failure is becoming unbalanced and of one weaker cell dragging the whole pack down, over time, which will increase more if the pack is worked hard by taking the voltage of the cells down too low. I'm fairly conservative with the level to which I'll fly my batteries down to and aim to have landed before they get down to 3.7v/cell and certainly before 3.6v/cell. I can count on one hand the number of times that I have had more than one flight on a charged battery pack in a session - I'm set up for 5-6 minute flights using a high proportion of full throttle, as that's about the length of flight that I like to have with most of my models. Other models I have set up for longer 7-8 minute flights. Flying lipos down to LVC is a certain recipe to damage the packs over time and once the puffiness starts to build up, they are only going to get worse. 

 

That said, for single cell batteries, where balance isn't an issue and transmitters which are equipped with battery telemetry, there doesn't seem to be an issue with puffiness arising - those batteries seem to behave themselves very well, but the current draw is tiny and flight times are typically 10-12 minutes for a 1s 400mah battery.

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1 hour ago, Arthur Harris said:

I'm really confused about battery care. A youngster (in his 20s, most of us are retired) joined our club a year ago, said he was bored with computer games, progressed from a Carbon Cub till he's now flying like he's at Joe Nall- prop-hanging, slow circuits with rolls, etc- he's become our "go to" to trim our planes. I asked him what time he set on his throttle- he said he just flies until the plane feels a bit soggy. I'm thinking that maybe I'm a bit precious about my batteries.

Well, 'flying until the plane feels a bit soggy' is one way determining whether the flight is over, but it's very crude and will almost certainly result in damage to the battery if over done. Not the way to do it and not neccessary if simple precautions that don't delve into complex battery talk are taken.

Most sport flyers, who only have a basic grasp of the theory (and more than likely, little interest in it anyway) are perfectly able to manage their batteries by timing their flights and using one of the inexpensive lipo checkers that are readily available to get an indication of remaining charge. With a few exceptions, that's how folks go about it both of my clubs.

Certainly, one can go into battery management very deeply and the data collected will be of use to those that can use it.

 

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I would just add that percentage capacity in the pack should always be backed up by reading the per cell voltage and degree of voltage imbalance between the cells.  A reading of not less than 3.7 V/cell should be taken as the minimum reading if you wish to get long pack life.

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1 hour ago, Cuban8 said:

Well, 'flying until the plane feels a bit soggy' is one way determining whether the flight is over, but it's very crude and will almost certainly result in damage to the battery if over done. Not the way to do it and not neccessary if simple precautions that don't delve into complex battery talk are taken.

Most sport flyers, who only have a basic grasp of the theory (and more than likely, little interest in it anyway) are perfectly able to manage their batteries by timing their flights and using one of the inexpensive lipo checkers that are readily available to get an indication of remaining charge. With a few exceptions, that's how folks go about it both of my clubs.

Certainly, one can go into battery management very deeply and the data collected will be of use to those that can use it.

 

There's nothing complicated about checking the individual cell voltages and ensuring that they are reasonably well balanced. As I said, that percentage figure that is so often quoted tells you very little about the health of the battery pack as a whole and gives very little warning of  pack degradation over time. The majority of inexpensive lip checkers give the readily available information on individual cell voltage by simply pressing a button. In fact the first ones to be widely available, the Battery Doctor, defaulted to giving that information.

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I do agree with ieccyflier but perhaps I go one step further. I time my flights and always note the cell voltages and how much is taken out by the charger to achieve storage level. If the charger has to put some in to reach storage then I know the flight was too long or too energetic..

With a bit of practise most flights are terminated before storage level. 

I do have some planes with a maximum duration getting on for an hour but I only ever fly that long as an experiment just to it can.     

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