tessmar

I have a Hobbico Superstar EP. My instructor suggested switching to LiPo batteries as the surest way to increase performance and flight times. The stock motor is rated for 7.2-9.6 volts and came with an 8.4v 2100MaH NiCd. I have read much information about changing motors but wonder if simply changing to a LiPo will increase flight time. The LiPos seem to have either 7.4 volts (2 cell) or 11.4 (3 cell). One seems marginally low in voltage for the motor and the other much too high.

jdetray

One reason a lipo may give you better performance is that for the same capacity, it is much lighter than an NiMH -- half the weight or less.

The voltage of a fully-charged 2-cell lipo pack is 8.4V. That will decrease under load, of course, but it may be closer to your NiMH pack than you might think.

- Jeff

tessmar

Thanks, Jeff

I'm obviously just starting up the learning curve here. To switch to LiPo means a fairly high outlay for two batteries and a charge (the one I have now will not handle them). Do you have any thoughts on this versus the much less expensive alternative of switching to a gearbox using essentially the stock motor but enabling it to swing a bigger and more efficient prop?

jdetray

I'm not really qualified (i.e., smart enough) to compare the two types of power systems. You might get some good advice if you post a message in the [link=http://www.rcuniverse.com/forum/forumid_123/tt.htm]Brushed/Brushless motors, speed controls, gear drives[/link] section of this forum.

When you add a gearbox, you'll be adding weight, so that's a consideration. And you'll always benefit from lighter batteries, whether you're running direct drive or geared. Maybe a geared system AND lipos would be optimal!

One point that sometimes gets overlooked is that the lighter weight of lipos may require some relocation of components to maintain the correct CG.

- Jeff

coolbean

It looks like you are wanting to extend your flight times. There are two ways to do that.
1. Get higher capacity batteries
2. Lower amp draw.

The first is obvious. The second is not.
There are several ways to lower your amps.
User a smaller prop, lower your voltage, be conservative with throttle. But of course all of theese things lower the performance of your plane. The best way to figure this out is with motocalc. If you havn't already used it up. I suggest downloading it and using the trial time to play around with different setups. Or you can use this calculator (if it has your motor in it) [link=http://brantuas.com/ezcalc/dma1.asp]http://brantuas.com/ezcalc/dma1.asp[/link]
What motor do you have BTW?

As stated by jeff, the first thing that popps into my mind to increase your times without sacrificing performance is to go to a similar capacity, 2 cell lipo. It is a little less volts and a lot less weight, so performance will be about the same but since you are running less volts, flight time is longer. Reducing your weight does have a couple side effects. First is your plane can fly slower without stalling. Second is that it will be less stable in wind.... allways a trade off.

Chocks_Away

If you want to go to lipos, then I would trade the 8.4v pack for a 2 cell lipo, but of about one and a half times or twice the capacity of the nimh pack if they will physically fit.

The reason being, for a given capacity, you can at least double the capacity of a lipo for the same weight as a nimh or nicad.
eg a 2000mah nimh will weigh more than a 4000 mah lipo

If you double the capacity, you can safely say that you will double the flight time.

I converted a 8.4v 650mah nimh plane that flew for 4 minutes, to a 2 cell 1800mah lipo, and now get a 20 minute flight with no performance loss.
my original 650 nimh weighed 95g
my 2 cell 1800 mah lipo also only weighs 95g ! [X(]

jdetray

Here are some other online calculators. I don't have much experience with these -- just passing them along.

[link=http://www.peakeff.com/Default.aspx]Peak Efficiency motor database[/link]
[link=http://www.badcock.net/cgi-bin/powertrain/propconst.cgi?C_prop=12&RPM=19000&Volts=10.60&Current=11.00&Watts=169.60&Height=+200&Temp=20.0]Prop Power, Thrust and Efficiency Calculations[/link]

- Jeff

tessmar

Thanks for the info and help. Ichecked out the brantuas link Coolbean provided and matched up as close as I could to what I have (Electrifly T601 with an unnnamed but likely Great Planes 2100MaH NiCd). The specs I found from Electrifly look much like the Graupner #6309 and the battery is rather like the Sanyo 2000SCR, at least in weight. This predicts a 262% increase in flight time (4 to 10 1/2 minutes at WOT) by going to a 2.5:1 gearbox and 10x8 prop with the same battery. While I still need to do some more investigation this is a lot of potential gain for a very low cost (around $30). The switch to LiPo, with a new charger and, from what I gather, a higher capacity ESC, would be well up over $125.

coolbean

Your esc should be fine. With a 2 cell lipo you should lower volts and keep amps about the same, but again, use that calculator to make sure.

tessmar

Thanks, Coolbean
I used the brantuas.com/ezcalc calcultor. It does not have my exact motor but from the Electrifly site the Graupner #6309 seems close. Likewise the closest battery to mine seemed to be the Sanyo 2000SCR. This baseline does indeed match pretty well to the flight times experienced with the plane (4 minutes or so).

Changing the calc to a Kokam 2000 Li-Batt with 2 cells gained 20+% to 4:53. But the prop thrust and motor revs drop and I don't really know what that means. However, leaving the stock motor and battery and changing from the stock 9x5.5 prop to the GD600 gearbox at 2.5:1 and a 10x8 prop increase the time by over 150% to 10:35. Again, prop thrust is lower (as are prop revs, but of course not motor revs). Even bigger gains appear with a 3.8:1 ratio and 10x8 prop (with even lower prop revs and thrust). This winds up at 12:56.

Unless I am not understanding the calculator all this says I should try the gearbox and prop change (at well under $50) first before investing $150 or more in LiPOs and a new charger.

I'd really appreciate your thoughts as well as saying thanks for the help you have provided thus far.

Matt Kirsch

My Feedback: (21)

In practice, a 7-cell NiMH will perform similarly to a 2-cell LiPoly.

Look at the Amps figure for each calculation. As you worked through those four iterations, the Amps steadily decreased. That is why you're seeing increased flight times.

What you're doing is decreasing the amount of power that's going to the propeller. That can be good, in that it increases flying time, but it can also be bad in the plane does not have enough power to get out of its own way, and may not fly at all. Every plane has a minimum power requirement.

By using a gearbox and larger prop, you can do more with less (the propeller is tremendously more efficient), so you should be looking for a combination that provides comparable or better thrust at lower Amp draws. I've had success with a 3.0:1 ratio and a 12x8 prop on a similar power system, myself.

tessmar

Thanks, Matt
Your comments mirror what my theory was. Not sure the plane will handle a 12 inch prop unless the gearbox offset exceeds one inch, but I can work that out.

When you say that the thrust should be at least equal to the stock setup I have a question. There are no specs I have found on my motor (supposedly a Great Planes T-601, but that designation is not one of their standard motors). The thrust figures I see on various mnanufacturer sites may or may not be consistenat. Some (Like electrifly) quote thrust with a specific size prop...which in this example is not the one the plane came with. Some, like the Graupner figures on Hobby-Lobby's site, are inconsistent in that some are quoted with a particular prop while others do not list any prop but just show a thrust figture.

From other sites I have used one of the Graupner motors at what is said to be a close match to the T601, ran the calculator, and then started fooling around with other combinations of props, gearboxes, and brushed Graupner motors that P Calc does include to at least match that baseline.

Is this a reasonable approach?