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Old May 29, 2011, 10:53 AM
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Estimating Sail Plane Flight Times

I had a Helicopter and used my DX6i to time 4-5 minutes of flight time. This method works okay for Helis but is it OK for a Sailplanes?

I was looking at buying a Parkzone Radian Pro with a 1300 mAh Lipo.
My question is how do you determine when you reached a reasonable flight time because so many variable parameters (throttle, wind, gliding ..etc)?
Is there electronics that give you a Low Level Battery cut off and is it a good idea to go that far anyway?

How do you estimate battery levels?
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Old May 29, 2011, 11:19 AM
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Simple answer is soaring is very easy on batteries. There are always exceptions but in general depending on your actual setup and battery size 5 or 6 climbs is not out of the question. The ESC's have a LVCO that will cut the motor voltage but you still have tons to fly back. I've had flights of way over an hour on one launch and barely touched the battery capacity. It will not be like a helo where if you lose power your returning to earth as you will be unpowered most of the time while soaring. To be safe just check the battery with a watt/voltage meter in between flights, that way you know for sure.
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Old May 29, 2011, 01:51 PM
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Originally Posted by msdumo View Post
I had a Helicopter and used my DX6i to time 4-5 minutes of flight time. This method works okay for Helis but is it OK for a Sailplanes?

I was looking at buying a Parkzone Radian Pro with a 1300 mAh Lipo.
My question is how do you determine when you reached a reasonable flight time because so many variable parameters (throttle, wind, gliding ..etc)?
Is there electronics that give you a Low Level Battery cut off and is it a good idea to go that far anyway?

How do you estimate battery levels?
Im still trying to figure that out... I do about 4 climbs at roughly 20 sec each and checked the voltage on the batt. and it was down to 3.88V per cell about 11.4 V combined voltage... Still plenty of juice... So I will figure roughly 6-7 safe climbs at 20 sec bursts... This should still leave me with a couple emergency motor bursts if needed... Note this is with the 1300 mAh lipo that comes with the original radian... I've done 15-20 min at about 3-4 climbs and gliding... Thats at about 150 feet .... haven't really chased thermals as I'm new to sail planes and am getting bugs worked out... But I believe I can reach 1 hour with no problem as I fly over my neighbors farm fields... And its starting to get very warm here
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Old May 29, 2011, 10:50 PM
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I have a Futaba 8FG 2.4 transmitter. With my throttle on a switch, I just set up one of the timers to display cumulative run time. Timer runs when switch is on; motor runs at WOT.

For instance, today with my 3.2 M Pulsar, 19 seconds of total motor run time equaled 424 mAh out of a 2250mAh Lipo. You have to keep track of mAh that go back into your LiPo when you recharge it.
So 19 seconds = about one fifth of total LiPo capacity.
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Last edited by 320pilot; May 29, 2011 at 10:55 PM.
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Old May 30, 2011, 09:59 AM
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Don't forget that you should not discharge your LiPo lower than 20% remaining. On a 2250, you should figure you can safely use 1800 mah so in this case you could figure on 4 launches of 19 seconds each.
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Old May 30, 2011, 12:55 PM
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Some math and physics:

Energy in battery = nominal voltage x capacity x 3600

Energy in battery divided by model weight and acceleration of gravity gives teoretical height the model could climb. There is also efficiency of energy conversion which could be guessed as 30%.

h = 3.6*C*V/(10m)*eta

Example: battery 3S1300, nominal voltage 11.1, model weight 1 kg
h = 3.6*1300*11.1/10/1*0.3 = 1560 m

If a single climb takes 200 m, then 7 climbs can be accomplished with ease.

Hope this makes sense and helps,
Jan
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Old May 30, 2011, 07:34 PM
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Thanks guys. I'll assume 5 climbs of about 200-250m would probably be a good target.
I'll check no load battery voltage after 3 climbs to get a better idea.
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Old May 31, 2011, 11:27 AM
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Thanks guys. I'll assume 5 climbs of about 200-250m would probably be a good target.
I'll check no load battery voltage after 3 climbs to get a better idea.
thats a very good idea... 5-6 climbs on the 1300 mAh battery is about what I do when I do go out...

I usually check my batteries voltage after a set of 3-4 climbs anyway... Habit...

I usually get between 3.8-3.9 Volts/cell at 11.1-11.2 Volts total. I believe you don't want to go lower then 7.4 Volts total for a 3s??? Someone want to chime in
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Old May 31, 2011, 11:39 AM
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If the voltage is 3.8-3.9/cell shouldn't the total voltage be sum of the 3 cells or more like 11.4-11.7 volts?

I was always told that if you go below 3 volts/cell that a battery is no longer any good. If you follow the 80% rule, shouldn't you stop when the total voltage of a 3s Lipo is 11.25 volts?
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Old May 31, 2011, 12:30 PM
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If the voltage is 3.8-3.9/cell shouldn't the total voltage be sum of the 3 cells or more like 11.4-11.7 volts?

I was always told that if you go below 3 volts/cell that a battery is no longer any good. If you follow the 80% rule, shouldn't you stop when the total voltage of a 3s Lipo is 11.25 volts?
Ya at 3.88V/cell its 11.6V total sry... All I know is that at 80% you have used 1040 mAh out of the total 1300 mAh available. The LVC on the radian is 74%...
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Old May 31, 2011, 04:12 PM
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You don't want to confuse battery capacity (mah) with voltage to determine when you should stop. Also a very bad idea to use a percentage setting for LVC. Here's why - a 3 cell LiPo fully charged will measure 12.6 volts. If you draw sufficient current to trigger the LVC, the voltage will fall to 9.3 volts, so no harm to the battery. Now lets say that you have done a launch or two and the voltage is down to 11.6. Now when you apply enough load to trigger the 74% LVC, your voltage has dropped all the way to 8.6. You may not kill the battery, but you will certainly shorten it's life. I'd recommend that you change the LVC to 3 volts/cell, then it will always cut out at 9 volts and won't hurt the battery.

You need to figure how many launches you can get on a charge by measuring the capacity used (mah). If you have a battery which has lets say an 80% use available of 1000 mah and you use say 400 mah per launch, obviously, you can only get 2 launches. Simple to figure out....don't use voltage to try to figure if you are OK or not, it won't work!
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Old May 31, 2011, 04:26 PM
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I've got a very basic charger. Do you get the mah used from the Lipo charger or is this something you can measure with a multimeter?
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Old May 31, 2011, 04:32 PM
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Originally Posted by rcglider View Post
You don't want to confuse battery capacity (mah) with voltage to determine when you should stop. Also a very bad idea to use a percentage setting for LVC. Here's why - a 3 cell LiPo fully charged will measure 12.6 volts. If you draw sufficient current to trigger the LVC, the voltage will fall to 9.3 volts, so no harm to the battery. Now lets say that you have done a launch or two and the voltage is down to 11.6. Now when you apply enough load to trigger the 74% LVC, your voltage has dropped all the way to 8.6. You may not kill the battery, but you will certainly shorten it's life. I'd recommend that you change the LVC to 3 volts/cell, then it will always cut out at 9 volts and won't hurt the battery.

You need to figure how many launches you can get on a charge by measuring the capacity used (mah). If you have a battery which has lets say an 80% use available of 1000 mah and you use say 400 mah per launch, obviously, you can only get 2 launches. Simple to figure out....don't use voltage to try to figure if you are OK or not, it won't work!
Alright so you will need to use a charger like the icharger 106b+ and determine the amount of mA it puts back into the pack... This will then give me an estimate of how many climbs I can have on a pack...

Unfortunately I don't have a charger capable of determining this, yet (not in the budget atm), so I will be safe and say 3 20 sec half throttle bursts to get to about 200m. I will also see if I can change the LVC on the stock radian... Would that be hard to do???
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Old May 31, 2011, 04:54 PM
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Originally Posted by rcglider View Post
Here's why - a 3 cell LiPo fully charged will measure 12.6 volts. If you draw sufficient current to trigger the LVC, the voltage will fall to 9.3 volts, so no harm to the battery. Now lets say that you have done a launch or two and the voltage is down to 11.6. Now when you apply enough load to trigger the 74% LVC, your voltage has dropped all the way to 8.6.
Taken from the 30-Amp Pro Switch-Mode BEC Brushless ESC Instructions

"Programmable LVC with settings for 3-Cell Lipo (9.2V), 4-Cell Lipo (12V), or 74% of the battery starting voltage."

So if I'm reading that right then if the starting voltage of the 3s lipo is say 12.4V then the LVC will be 9.2V through-out the flight... As long as you don't disconnect the battery from the plane during this time... But if you start with a non fully charged lipo or disconnect the battery, say 11.6V, then the LVC will be set to 8.6V which is trouble...

So the 74% setting is fine if you charge fully before flight... At least thats what I've gained

Either way its either the 74% of starting value, or 9.2V constantly...
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Old May 31, 2011, 05:05 PM
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I've got a very basic charger. Do you get the mah used from the Lipo charger or is this something you can measure with a multimeter?
early on, when I was flying RC Helicopters, I bought a Thunder Power 1010C LiPo charger. With a separate cell balancer, it gives me a very accurate picture of LiPo "health" as I charge cells. I can see the voltage in each, individual cell, and if any one cell isn't charging properly, it stops the charging process. It also checks a minimum "safe" residual charge, before it will even start charging. Finally it stops the charge process, when all cells are charged, and displays how many mAh went BACK into the Lipo.

So, it was an expensive charger/balancer but it has certainly paid for itself, and made LiPo charging at home much safer. Thunder Power have a new charger, that's cheaper and doesn't require the separate balancer.
http://www.aero-model.com/Thunder-Po...p-Charger.aspx
more info on charger, here. This version includes a built-in AC-DC converter.
http://www.horizonhobby.com/Products...ID=THP610CACDC

As mentioned above, the important number, is how many mAh go back into the LiPo when you charge it after a flight. Keep track of minutes you run the motor , and you will keep a safe residual capacity, in your LiPos.
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Last edited by 320pilot; May 31, 2011 at 05:34 PM.
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