How do I know how long I can stay up

[Garry Pollard]

Hi

 

I have just aquired a power glider, (my first). My question is how do you calculate the lenght of time the lipo will last. I see the flight as follows

! Motor on, climb to hight

2Switch off motor and glide

3 model looses altitude, so motor on again. How long will lipo last, or is this a "How long is a piece of string " question

 

What do the experts do

 

Garry

 

[Former Member]

[This posting has been removed]

[Martin Harris - Moderator]

I wouldn't claim to be an expert but I calculate the approximate duration from the measured power (this gives a slight safety factor due to the prop unloading in flight) dividing the battery capacity by the current - e.g. as P(Watts) = V(Voltage) x I (current) a 100W motor using a 2 cell 1000mA/h pack would give a current of (100/7.4) @ 13.5 Amps (you'll get this directly from a Wattmeter ) which gives 0.074 hours (1/13.5) or more usefully 4.4 minutes (0.074 x 60).

 

I'd then set a timer alarm on my transmitter for 4 minutes based on these calculations, activated by the throttle, and when it goes beep I wouldn't do any more climbs...

 

Or do it Eric's way which is just as valid - I took too long posting this to see his method detailed!

Edited By Martin Harris on 02/07/2011 00:07:33

[Stephen Grigg]

Gary,its a glider just glide it back down,you know you can

[Barrie Dav 2]

As Eric says 'You do have a meter' ? A watt meter is an essential bit of kit and once you have used it a couple of times as described by Eric you will be able to calculate likely motor runs for all the combinations of batteries, motors and props.

 

For example, for any given motor, each time the prop is changed for one of a different size or pitch the 'fuel' supplied by the battery will last longer or less long as the load is changed.

 

Remember, that when the battery voltage drops too low to power the motor, There will be some power left to drive your receiver and servos but you will have a limited amount of time to get back on the ground. No need to panic but just head for a landing asap.

[Bob Cotsford]

Definitely a variation in methods and opinions on this one!

For me, I would use Eric's or Martin's methods to give me a rough estimate, then fly the model UNDER POWER for around 80% of this period to allow a safety margin, land and then see how much charge goes back into the battery. From this you can get a better idea how long the pack will last, eg, if you are using a 2000mAHr battery and after 5 minutes you put 1000 mAHr back in, then you used 50% of the available power, so in theory you could have got 10 minutes running. Using 100% capacity kills batteries so work on 80%, ie 8 minutes. That's your motor run. How you time it depends on your transmitter, stop watch or whatever you use.

 

As for running until the ESC reaches it's low voltage cutoff, this will work but will not do your battery any favours, I know because I do this on the many occasions I forget to set the timer. Remember that when the LVC cuts power to the motor there is still a lot of energy left in the cells (unless you are using some really small cells), certainly enough to drive the receiver and a couple of servos for several minutes, if not hours.

[ken anderson.]

garry - you'll prob get bored before the batt is done for - or a stiff neck....or Both..

 

ken anderson ne.1.

[Lee Morgan]

Personally that sounds like a load of work

I would go on the basis that you have at least 5mins flight time ( power not always going to prop ) I would take 3 batts ( if you have 3 ) then fly my 5min slots with each ,

 

Now once that is done I would re-charge the batts and see what goes back in , This will give you the best idea of average flight times , Do this a few times and fly in a similar way each time and you'll have the max flight time and a safe fly time i.e. If the batt doesn't take much charge = more flying , If they take a load of charge = less flight time / less power use during flight

 

My 2p

 

Lee

[Tim Mackey]

I wouldnt actually recommend running the setup on the bench for that long. Without any unloading effect, and possible overheating etc, its not normal practice to static run things for more than a minute or two.

Its a guesstimation at best - as there are so many variables to consider - flying style, weather, battery temperatures etc, so my suggestion is to do the maths as suggested by Martin, for a guideline, then do your first flight, but keep it in reasonably close, and as soon as you notice a drop in power, land. Check the time, and set your timer for at least 30 seconds less, and use that as your likely flight time.

I never fly to LVC.

[Pete B]

I'd go along with Lee on that - if you have the average set-up, (what is your set-up?) you should get 4 or 5 climbs to height without any problems. Towards the end, you'll sense that it isn't climbing quite so quickly as it was earlier but you'll have plenty of charge left for the radio.

 

That's the time to change the battery, charge it and see what you've taken out.

 

Pete

 

 

[Garry Pollard]

Thanks for the tips everybody. My main concern was should I get to LVC which I try not to do on my other models, and am high, how long before I loose radio link (Speeky DX7)

I do have a watt meter and use it on all setups.

 

Garry

[PatMc]

Lee's method doesn't really work for motor gliders. It very much depends on the climb rate & glide potential of the model, how it's flown as well as the current draw against battery capacity.

I have 2 climb & glide motor gliders & 2 aerobatic ones. They all have full power times of about 4 - 4.5 mins but the first 2 are able to fly for around 45 - 60 mins per charge without thermal assist whereas the aerobatic models' times are around 15 mins per charge.

 

What's the model - or what are the dimensions, weight etc ?

What's the motor & battery capacity & voltage ?

[Tim Mackey]

Posted by Garry Pollard on 02/07/2011 10:29:53:

Thanks for the tips everybody. My main concern was should I get to LVC which I try not to do on my other models, and am high, how long before I loose radio link (Speeky DX7)

I do have a watt meter and use it on all setups.

 

Garry

 

No worries on that score Garry - after you hit LVC ( which I dont advise you do ) then you have probably hours still left for the radio!

Height isnt a factor - that only affects the signal range, and is unconnected with power supply .

[Garry Pollard]

Sorry, maybe didnt explin the "height ". My worry was not the radio link, as I know that is solid to OOS, the worry was the time I had left before losing contact due to flat lipo

 

Garry

Edited By Garry Pollard on 02/07/2011 14:17:48

[Steve W-O]

I work out my theoretical flight time on a full, 100% capacity battery, take 80% of that, and set my TX timer to that figure.

Then I set my TX to control the countdown timer in proportion the the throttle setting, ie throttle 50%, timer runs at half speed.

 

I get long flight times, and always have a respectable reserve left on recharging.

[Isoaritfirst]

I have flown for an hour with an Easy glider after the bec had cut the prop out.

 

Unless you are flying something big with lots of very powerful servos - you won't have any problems.

 

not something to worry about just fly it until the power drops off then glide for as long as you want/can.

[Tim Mackey]

Eer, actually it was the LVC which cut motor power, not the BEC .

[Ed Anderson]

Let's use some real numbers and see if we can make this more meaningful. I don't know what plane you have so I am going to use my Radian as the reference model.

 

I don't meaasure my time in minutes I measure it in climbs.

 

The stock battery is a 1300 mah pack.

Motor draws about 16 amps at full power. A quick estimate would be that I can get about 4 to 4.5 minutes of full power run time before the LVC kicks in.   Actual time will depend somewhat on the quality of the pack.  And since I am not running my packs hard all the time during the flight  I don't worry about hitting LVC on my gliders, though I typically don't hit it. 

 

A typical inital climb is about 30 seconds to 200 meters. I can do about 6-7 of those before LVC.  Typical reclimb is about 20 seconds - I could do about 8-10 of those after the intial climb before I hit LVC.

 

The motor draws about 16 amps but the 2.4 GHz receiver and 2 micro servos only draw about 200 mili amps on average, or 1.25% of what the motor draws. A fresh pack could run the receiver and servos for about 6.5 hours. 

 

While the LVC might kick in on a full power climb or reclimb, meaning the battery voltage has sagged to about 9V, as soon as I power off the voltage will bounce back up to about 10.3 V. It probably has about 300 mah of useful life at that point. ( there are tables for this) So I can glide for another hour, land in a glide and still have capcity left in the pack that could run the radio.

 

Have I been highly rigerous in this? No. But these numbers will probably come out pretty close if you put the calculator to them.

 

Net Net is I  typically get about an hour of flight time on a 1300 mah pack if there is not a lot of thermal activity in the area and land with capcity left in the pack that could run the radio for quite a while. Climb up, get a 5 minute glide, then climb again.

 

If I hit significant lift, a pack can last hours.

 

On my pure gliders, say a R/E Spirit, with a 72 mhz receiver and 2 servos, a 700 mah NiMh 4 cell pack is good for a full day of flying off a hi-start.

 

Others can add some rigor to this if they like, but the net message is that your radio draws so little power compared to your motor that even after LVC you have lots of flight time left.

 

 

 

Edited By Ed Anderson on 11/07/2011 09:21:09

Edited By Ed Anderson on 11/07/2011 09:24:24

Edited By Ed Anderson on 11/07/2011 09:35:05

Edited By Ed Anderson on 11/07/2011 09:36:18

Edited By Ed Anderson on 11/07/2011 09:40:03

[Trex700e]

What I would do would be to fly a five minute flight (or maybe a 3min flight) and then after the flight measure the amount of mAh left in the pack (devices to measure mAhs are very cheap) then you can see how much you used up in that flight and then see how much you have left to use up and how long this will take