Question on NIMH for use in glider

[Gliderhead]

As some of you know, I have a 1/4 scale Schweizer I occasionally fly at Salem. We flew that plane at our local field in Wilton, CT but it was kind of tight and the electric tug was at it's operating limits.

As a result, we thought it would be ideal if we had a smaller version to fly locally. A group of us got together and contributed design / build views which resulted in a set of plans drawn up courtesy of Chris Evens. Several months ago I began the build. The build was finished yesterday and we're waiting for good weather to do the maiden. The result is a (slightly modified) 1/5 scale Swhweizer 1-26. Wingspan about 100", flying weight 85 oz, wing loading somewhere around 13 oz.

Here's my question: Present power set-up is using a 6.0 NIMH 1700 pack. The plane has 7 servos, 3 of which see minimal use. The question is how much fly-time we can expect to get out of this pack ? After an initial drop in voltage, the NIMH curve is nearly flat so % readings are not very useful. How can I measure the pack to know how much power is left in this pack such that the plane can be safely flown ?

All views / opinions gratefully welcomed.

Thanks,
Don

[lenb]

Cool project Don,
Based on past usage of NIMHs, I would guess you should get all day out of that pack.
Maybe you can get some guide lines here .

http://www.hangtimes.com/nickel_metal_hydride.html

http://www.hangtimes.com/redsbatteryclinic.html

[Gliderhead]

All day. That's all I ask, Len.

Thanks

[ingrahal]

Don, the maximum discharge level for NiMH batteries should be 0.9 V per cell or 4.5 V for your 5 cell pack. I never discharge a pack that far at the field though. I run 5 cell 2500 mAh packs in almost all of my 1/4 scale and smaller planes and I quit flying when they reach 5.5 V with a 1.0 A load. When I charge these packs back up they usually take about 600 mA. That represents about an hour of actual flight time. I could easily double my flight time and be safe. Of course your mileage may vary depending on the actual load that particular plane is putting on that pack during flight. Assuming a 0.5A-1.0A load you could get 3.4 - 1.7 hours of flight time based on 1700 mAh.

[Mark9]

I fly mostly TD competition ships using LiFePO4 packs. As you may know they have an even flatter curve than NiMh cells. When they are done they "fall off a cliff" making an ESV totally useless. What I have done to determine a safe time to recharge requires some data acquisition, record keeping and multiple flights. History has shown that the usage can vary quite a bit at least with my TD ships.

First go out and fly normally, but don't "push the limits". Keep notes of the time in the air, I do not count "idle time". This adds a small amount of safety factor to the calculated usage factor. My charger when it is done charging gives a readout of the mah put back into the pack after it completes charging the pack. Make a note of this as well. Then divide the mah returned to the pack (mah used) by the minutes flown. The number provided mah/minute can then be used at the field to estimate when you either field charge or park the airplane for the day.

Note that the type of charging algorithm your charger uses can affect the number provided. My charger when balance charging LiFe packs adds additional mah (some discharge the high cell), which adds another safety factor to the estimated usage. That may not be the case using your charger with NiMh cells, but if it does provide the input value one can still derive a time usage number to gauge when to charge.

Mark

[gunny11]

Well said Ingrahal. I just want to add... Check your pack before each and every flight, it only takes a few seconds.

[ingrahal]

Amen to that!!!

[Csessa]

So here is the prototype in flight.

https://youtu.be/hmlnA5JwhjQ

[Steve P]

Congratulations! Those are some tight tow turns. ;-)

I put your video in viewer mode.

[Swiss1]

Nice, although it's funny how the SWISS CROSS on the Cub is obscured in every picture by the glider wing......does it need inspecting

Jeremy and Ben
SCCAAA TT TN

[Csessa]

Nice, although it's funny how the SWISS CROSS on the Cub is obscured in every picture by the glider wing......does it need inspecting

Jeremy and Ben
SCCAAA TT TN

Ya, maybe.

[Csessa]

Those are some tight tow turns. ;-)

I put your video in viewer mode.

Ok, how do you put a video in viewer mode. I do not think I saw that option.

Tight turns, a recurring problem. Cause...gusting wind or more likely pilot skills.

[Steve P]

Click on the "A" in the toolbar right corner to bring up all of the formatting options, click on the filmstrip, insert your YouTube URL into the space and save