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Is this a reliable test of approximate flight time?

Old 04-26-2012, 05:09 AM
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flyyy
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Default Is this a reliable test of approximate flight time?

Hook up a fully charged 3s 2200 pack to your motor prop combo. The most amps you can use is 1760 mah which would leave you 20%. Watch your mah guage on the watt meter and start the stop watch. You know about how you fly so go from wot to cruising back and forth and when the mah reads 1760 stop and check the pack after it rest. This should give you the approximate time that you would be safe. Anything wrong with that?
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Old 04-26-2012, 05:33 AM
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mclarkson
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Maybe.

You're generally gonna be pulling more power during your static tests than you would during flight, so I think this method will give you a conservative estimate.

I personally don't like to drain my batteries all the way down. I prefer to fly the plane for, say, five minutes and then measure how much juice it's lost and make estimates on that.

Battery response is non-linear, though, so if it uses 1/2 of your battery in five minutes, that doesn't necessarily mean that it will last 10 minutes. There can be a drop-off as the battery drains.
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Old 04-26-2012, 05:49 AM
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You don't want to run you motor without a bunch or airflow if your anywheres near the maximum amps/watts either. A lot of motors are only rated at maximum for 15 to 30 seconds, and will get hot without the extra airflow.

I just fly low and slow doing touch and goes and go straight verticle take off and see how degraded my power is. As soon as the plane has problems going vertice, I know I'm close to lvc, do one or more touch and goes and I'm done. This will give you a good rough idea if you time it. I usually set my timer on the dx6 for 15 minutes, I usually get 20 mins or more, even if I horse it around. Every time it beeps when I go a minute over I take a mental note of how long I am going over. This way if I'm flying low and in ground effect, I can fly to lvc without leaving a bunch of battery left, and I'm reminded I'm running low if I'm trying to show off or hover a lot or something.
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Old 04-26-2012, 06:12 AM
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flyyy
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Thanks. If (taking into consideration the 20% safety margin] you use 50% in 5 minutes and you don't necessarily have 50% left because it drops off faster, why would you consider this half. Sounds like we need some sort of sliding scale for the different voltages. Don't somebody sell a small volt meter that stays connected to the plane where you can see the voltage without taking the hatch off and unhooking the battery to stick it with a voltmeter?

Last edited by flyyy; 04-26-2012 at 06:16 AM. Reason: by the way
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Old 04-26-2012, 06:44 AM
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mclarkson
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I bought one of these for $2.00 from HobbyKing.

http://www.hobbyking.com/hobbycity/s...itor_2_6S.html

It is reasonably accurate and I use it at the field all the time, especially with new planes. Check voltage. Fly for, say, five minutes. Check voltage. If the voltage looks good, fly for another few minutes. Check voltage again.

Now you know how much voltage is left after seven minutes, or whatever, of actually flying in the air.

I personally don't like to go below 3.75v/cell. So when I see I'm at around 3.8, that's where I'll set my timer, knowing that when it goes off I should still have plenty of time to get it on the ground before LVC.
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Old 04-26-2012, 07:50 AM
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JetPlaneFlyer
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Originally Posted by mclarkson View Post
I bought one of these for $2.00 from HobbyKing.
I have ones similar but with a built in alarm that you can program to go off at a pre-set voltage. It's loud enough to hear from the ground.

Steve
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Old 04-26-2012, 08:33 AM
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That one's strictly for ground-based testing. It doesn't continuously update the voltage display, so if you plug it into a full battery, then run that battery down, it will still show 4.2v / cell, until you un-plug / re-plug it.

It's a great addition to anyone's toolbox, though. For what it is, it works great.
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Old 04-26-2012, 08:34 AM
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I find 3.7v is also a good cutoff. At 3.7vs is generally where the performance of the plane really starts to suffer, as well as being safe for the lipo's. A low voltage alarm could save an airplane. My dads escapade hit lvc while doing a low inverted pass, tried to do an outside loop, stalled then barely got it back to the feild. Didn't have enough airspeed to flare, figured out the previous owner glued the landing gear block in with hot glue, and not very well. The wheel pants went through one wing.

This is also why I'm not a fan of a hard voltage cutoff, and part of the reason I fly the way I do. I slowly ramp up throttle until I'm almost in a hover, try to use rudder to and throttle to slide it a almost knife edge. If the battery gets low, I quickly have a good idea of how much battery is left, and basically if I can't do straight verticle, I come in and land unless I am flying low and slow.
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Old 04-26-2012, 01:22 PM
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Worth pointing out that the 3.7-3.75 voltage that people rightly recommend as a safe terminal voltage is measured with the battery 'resting'... ie. not under any load.

Under load the voltage with drop, how much it will drop depends on how good your battery is and how much load you put it under but 0.3v is typical, it could be much more on old batteries that are past their best. So if you set an alarm on one of these little battery monitors at about 3.4V then the resting volatge should recover to about 3.7V, give or take.

It wouldnt be a good idea to set the alarm at 3.7V because if you did it would go off very early.
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Old 04-29-2012, 09:41 AM
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I have 2 methods in addition to flying.

1. On high power low capacity LiPo models such as my Parkjet .. I am more interested in AMPs / RPM / Watts - so I run the model up on the bench with Wattmeter and Tacho connected. Once I have steady AMPS / Volts / Watts ......... I stop and then use a calculator against the stated capacity of the LiPo. I then divide result by 2 and use that as a safe point.

2. This is brutal but so far I haven't had a problem from doing it ... on lower power models - I strap down on bench ........ Wattmeter connected. Run up and keep going with main time on WOT with throttle down for short rest periods ... keep going until a detection of power drop is noted - usually by sound ! Note time. I then use 75% of that as my safe point.

Then onto flying where I use above as benchmark. I then fly with timer set .. when beeps - I land and check LiPo for whats left with Wattmeter ..... using above - I usually have about 30 - 40% LiPo reading.

I think it's a matter that we all arrive at our own ways of solving the question. I know my method will be frowned upon by many - but it works for me.
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Old 05-01-2012, 03:48 PM
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startazz
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I get to know my planes limits and how long they can fly for by using the timer on my radio,first i start off which flying for a couple of mins and then land and see how much mah i left in the battery and then work out how long i flew for,i keep doing that utill i know how far i can push it.

I know it's not the best way but thats what i do and it's always worked fine for me,but i do also use one of the Hobbyking voltage checker with the alarm on it,just in case for any reason i fly for longer than i normally do or for other reasons the alarm will sound and don't worry you will hear it,and you can set it for what volt per cell you want on it which is easy to do.

http://www.hobbyking.com/hobbyking/s...dproduct=18588
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Old 05-01-2012, 04:25 PM
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I tend to think that most factory recc'd set-ups achieve between 6 and 8 mins. I base that on the RTF's and various models set-up and reading their spec's. My Problem is that I rarely install factory spec gear - so I know that if I go with their power pack size - I will be seriously down on run-time.

My 50mm T45 on factory spec will fly for just on 5mins with the small lipo as it's low C rating and just does not deliver amps. Up the C rating and double the pack size and I get 6.5 mins .... seems out of logic - but that's what happens as with 20C pack there is 240gr thrust ... with the 25C we get 350gr .... a significant jump.

I try to calculate from the Wattmeter readings and so far I get more flight than the calcs give ... but that is because in flight we tend to throttle as well.

At 5 mins - I have to admit that unless its a slow floater type plane - I am looking to land. With my adrenaline jobs - I fly maybe 3 .. 3.5 mins ....
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