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cessnapilot5
04-25-2006, 05:27 AM
Hello,
Just finished my GWS corsair today. Man, I love it, this thing flies great. Just had a quick question regarding the ESC, thought maybe someone could help. The instructions that came with my esc say that it has an auto cut-off at (.67*BATT). Batt., being the battery voltage. Now in my corsair, I'm using a thunderpower 2100 pro-lite (11.1 volt 2100 Mah), with an align outrunner motor. I figure that based on the equation in the esc, that makes the cut-off at about 7.4 volts.....isn't that going to ruin my battery by discharging it too low?

My other question is, when this cut-off occurs, does that mean that I no longer have any power to control the airplane? your thoughts would be much appreciated. I attached some pics of my finished corsair too. Tried airbrushing for the first time on this one.
Thanks,
Brian

cessnapilot5
04-25-2006, 05:46 AM
Just one more pic. Sorry for the multi post, this is the first itme I've tried to post pics.

comets8518
04-25-2006, 05:55 AM
you are right, it will cut at 7.4 volts. That pretty much destroys your lipo. You should never discharge it below 9volts. (3v/cell) as you probably already knew, but once that happens your are not supposed to use them anymore.

When a Low voltage cutoff kicks in (regardless of battery used, lipo or nimh) you will still have power to the controls of the plane. Most ESC's even allow you to throttle down and then throttle back up and you will get power for a couple more moments.

But yeah, the best thing to do is calculate the amps your using and make an educated guess on your expected flight times, then Time a flight and check your packs voltage and adjust your flight times from there.

I've read on the forums recently that its actually recommended to keep lipo's voltages above 3.2v/cell under load and around 3.7 w/ no load (a lipo guru can correct me if i'm wrong) But that should keep your batteries usage life to a maximum

kensp
04-25-2006, 08:32 AM
cesnapilot5

Your full charge voltage for 3 cells is 12.6 volts so your cut off Voltage is
0.67*12.6= 8.44 Volts.

On a 3 cell TP 2100 at a load of 12 amps 8.44volts is equal to an at rest voltage of 3.1 volts.

When using a percentage of maximum voltage cut off type of ESC the battery pack must always be fully charged before flight. To have a partialy charged pack will result in too low a cut of voltage.

Ken

Matt Kirsch
04-25-2006, 08:51 AM
comets8518,

Where did you read that? Who made the statement? There is a lot of "personal preference" that gets stated as "fact" on forums like these.

As far as I know, and as of the Toledo show back in early April, the standard recommended cutoff was still at around 3.0 Volts per cell, at least for FMA and Hobby Lobby.

Ken's got the right idea about the cutoff calculation: It starts with whatever voltage is present at the time the battery is plugged in. It's in your best interest to recharge the battery any time you run it for any signifigant length of time. In other words, if you have a flubbed launch with a total run time of about 10 seconds, you don't need to recharge. If you fly for 3-4 minutes, then it will pay dividends to recharge.

8.44 Volts is an okay cutoff under load, because the 3.0 Volts per cell cutoff recommendation is for the cells at rest. At rest voltage will always be a bit higher than under load voltage.

I still recommend that you do NOT fly to cutoff. It's a good way to crash and damage/destroy an expensive plane. Time your flights and land while you still have enough "fuel" left for at least one full-power go-around if the landing approach isn't quite right. You wouldn't fly a full-scale plane until it was out of fuel, then belly-whomp it into the brush wherever it happens to be... Doing the same with a model on a regular basis makes no more sense.

kensp
04-25-2006, 09:58 AM
Check out this page for some info on the life of the cells I am now using. Nothing about 3.7 volts in this recomendation. It is all about discharge rates.
http://www.aircraft-world.com/prod_datasheets/hp/lipo/vx/hp-lvx-lithium.htm (http://www.aircraft-world.com/prod_datasheets/hp/lipo/vx/hp-lvx-lithium.htm)

There is more information on this page as well.
http://www.aircraft-world.com/prod_datasheets/polyquestxp.htm

Ken

johnp793
04-25-2006, 11:17 AM
There's another alternative- FMA makes a piece of electronics that adapts an ESC so that power cuts off at voltages appropriate for LiPO's. Look for the FMA AV1CAIR (http://www.fmadirect.com/detail.htm?item=1897&section=2). I've used a couple quite successfully. It uses the starting voltage of the battery to guess the number of cells, then sets an appropriate cut off voltage. If it does cut off in flight, you can reset it by going to zero throttle, then come back up for a few more moments of power to support the landing.

But a question for Brian (as I'm about to put together a GWS Corsair)- did you use the single aileron servo as in the plans, or did you use a pair of outboard, direct drive, servos as some people have recommended? If the latter, what did you use for servos?

John

kensp
04-25-2006, 12:35 PM
If you are thinking of using four servos on a 3 cell battery pack then this Switch-mode BEC with Lithium Cutoff is worth looking at.

http://www.dimensionengineering.com/SmartBEC.htm

Ken

RW650
04-25-2006, 01:40 PM
10487
John,
I have a couple of the GWS Corsairs. The first one I built per the instructions with the single servo for the ailerons. I was not impressed with it's performance. The second one (pictured) was built using Hitec 55 servos mounted inside the bottom of the wing directly connecting to the ailerons using a "Y" harness. It really made a difference in the responsiveness of the airplane.
Hope this helps,
RW

comets8518
04-25-2006, 03:00 PM
comets8518,

Where did you read that? Who made the statement? There is a lot of "personal preference" that gets stated as "fact" on forums like these.

.
I believe it was from RD Blakeshee or Everydayflyer on rcgroups, but like I said, I could be wrong, could have quoted wrong, (and that could be personal preferences) so please correct me if i'm wrong.

I would listen to the manufacturers as you guys say.