Wizard61

My Feedback: (7)

I am hoping you can help me, I am new to electrics and have just recently been flying a GP Super Sportsster EP (the larger red one). The plane is OK in its stock set-up but the weight of the 550 motor/gearbox and the 7 cell 8.4V 2100 mAh pack make the plane nose heavy to the point that I have to add weight to the tail to obtain the correct CG. I can't move the battery back any more or it will interfear with the operation of the servos.

Can the stock pack of Ni cad's be replaced with Li-Po's for that motor? What else would I have to replace, i.e., speed controller etc? My hope is to lower the overall weight of the plane and possibly increase it's performance(slightly) and duration with out having to replace the motor and gearbox.

Any input or guidance would be greatly appreciated.

Regards,

James Andrews

jooNorway

If you replace with 2S LiPo you will loose some pounch in the motor due to the lower voltage compared to 7 NiCd`s. No need to replace controller if it has proper cutoff for LiPo`s, but not good for your hope for better performance...

If you step up to 3S LiPo there is a chance you will reduce lifetime of the motor... And you sure will nead to find a matching prop (smaller!!). The current consumption will increase, maybe forcing you to change the regulator. Also check if the esc has the proper cutoff abilities.
The best way is to go for LiPo, new brushless motor and then you also need a brushless esc.

Wizard61

My Feedback: (7)

Thanks for the reply JooNorway:

I have been doing some reading since I posted. A 2s pack will give me only 7.4 volts as opposed to the 8.4 that the stock battery pack will give me. So I will have a 12% reduction in RPM with a cooresponding reduction in weight of only about 7 grams(since the model weighs 52 oz.). This would be a 13% reduction in weight. So, if I was able to make the change without having to add any more weight to the aircraft to obtain the correct CG, then I should at least obtain a similar power to weight ratio. In other words the plane would fly in a very similar manner to the stock set-up, with the possible exception of longer flight times. Am I looking at this the right way?

A 3s pack would provide 11.1 volts which would increase my motor's RPM, and likely burn it out faster.


Thanks for your help.

Regards,

James Andrews

Dyehard

Here are some hopefully helpfull observations. I fly a Mini-sportster on a TP 2100 mill lipo pack. An eight cell sub C nimh pack weighs around 13 ounces. I would think the seven cell pack that comes with the larger Sportster would go around 11 to 12 ounces. I have held one (two friends have the large Sportster, we just haven't flown them yet) and it is pretty heavy. The TP two cell 2100 mil pack weighs 3.5 ounces, according to my triple bean Ohaus balances. That is a saving of around seven to eight ounces. I have an eight cell 800 mill nimh pack. I just ran a check by putting my digital Fluke meter across the motor leads. At full throttle the two cell lipo pack ran from 6.84 to 6.9 volts. This pack was flown last Sunday for ten minutes and read 6.96 volts before the test. I then tried the eight cell nimh pack, which had just been topped on my Triton, and it started at 7.06 volts but dropped within five seconds to 6.8 to 6.9 volts. In other words, the lipo pack, without a full charge, had the same voltage to the motor, under load, that the 9.6 volt nimh pack had. An seven cell sub C pack might have less internal resistance than the eight cell AA pack that I tried and therefore hold it's voltage under load, but I dought there would be a large difference. I think that you will find that a two cell lipl pack will have at lest as much voltage to the motor as your seven cell nimh pack and maybe more, and for the same capacity save you at least seven ounces.

Matt Kirsch

My Feedback: (21)

Dyehard's on the right track. In practice, you would replace a 7-cell NiCd or NiMH pack with a 2S LiPoly (7.4V nominal) because in real-world conditions, the NiCd and NiMH packs suffer greater "voltage depression" under load.

With NiCd or NiMH, you'll be lucky to get 1.1 Volts per cell under load, which translates to 7.7 Volts for a 7-cell NiCd or NiMH pack. Under heavier loads you would be lucky to see 1.0 Volts per cell, and that's only at the beginning of the flight.

On the other hand, a properly sized 2S LiPoly pack will hold at least its nominal voltage during most of the flight (7.4V).

Wizard61

My Feedback: (7)

Thanks alot guys,

I think I am going to order one up and compare. Currently I am "hot charging" the stock packs and getting 8+ mins. of flight time, but for how long?

Thanks again for your help.

James