Petkov

Hi everyone my name is Peter and I am a mechanical engineering student and I have entered in a RC competition to build a plane. It was recommended to me that I come here to ask for help on the electrical components since you guys are pretty much experts!

I am constructing an aircraft which I believe will weigh around 5.5 lbs for a competition. I am trying to get about 70 watts per pound so between 350 and 400 watts total. I have a restriction a restriction of 15 amps for my flights and I can only use NiHM type batteries. Each flight in the competition will run for about 4 minutes, so i figure I would need at least 1100 mAh batteries. Using basic ohm's law, I figure i will need about 22 cells of the 1.2V batteries to proved a total voltage of around 26.

(I am still relatively new to the whole electrical component of RC planes so please correct me if I am making any false assumption)

Do you guys have any recommendations for batteries and where to get the packs? I was reading that 2/3 type batteries have great discharge rates and a much recommended brand were the Elite 1500s.
What would a recommended gauge of wire be at those types of currents? I understand that lower gauge wire has a larger diameter which provides lower resistance but if the system already has a higher resistance than a certain gauge, there is no point of adding extra weight in wiring. What do you guys think? I have been also reading that the banana/bullet connectors are not designed for high currents and it is recommended that I use Dean's type connectors.

Any help will be appreciated!

(original thread: http://www.rcuniverse.com/forum/begi...ject-help.html )

blvdbuzzard

I hate DEANS!!!! Just my opinion. I have used them for hears and years but still hate them. I do like the banana plugs much more. 15 amps is easy enough, I run 70 plus through my banana plugs without any problems.

You will need a motor, ESC and battery.


Are you flying it? Do you have a pilot lined up?

Not really sure what you are looking for so I will toss this out there.

5.5 pounds is pretty heavy. That is around a glow powered size of .46 2 stroke engine. Way more than 70 watts a pound. What plane? any type of design yet? Is the weight based on a design requirement? Size? or just a starting point?

How about looking at building a foam plane? You use dollar tree foam sheets at $1.00 a sheet you can build amazing stuff with it and it weighs very little. There is a thread on RCGroups about a 60 inch motor glider that is less then 2 pounds. The 70 watt limit would not be a problem with that plane. For the plane below, you would use a (5) cell pack, not 22 cells. Here is a pack

http://www.hobbypartz.com/98p-5000-6...set-brick.html

http://www.rcgroups.com/forums/showthread.php?t=1138106


Buzz.

Petkov

Thanks for the suggestions and I looked into the foam plane competition. I could take a lot of advice from some of those guys when building the prototypes and final design.

We are currently thinking of building something close to one of the Piper Cub RC planes you may be familiar with. I like the high wing and large aspect ratio which should work really well with the heavy payloads the missions require. I have compiled data from the reports of winning teams of this competition in the past to get a rough estimate for the sizing of the plane this year. I assume as a team, we will keep rerunning the calculations until we settle down of a final design. As for pilot, we have an AMA pilot on the team and although he has only had his licence for a few years, I believe he will have plenty of test flights by the time of the competition. I am also planning on going to airfields to ask for advice of other pilots once we have a prototype build.

The 5.5 pounds includes some payload needed to transport, where more payload gives a better score in certain missions but I have rerun the calculations for the score and we have decided we will go with a lighter plane to achieve a better overall score. The new target weight of my prototype is now between 3.8 and 4.5 pounds fully loaded (but this number might keep changing during the next few weeks). Rerunning all the calculations, I now believe I we need around 280 watts for the motor and only about 19volts (16 cells).

I have actually heard a lot of recommendations to get the battery packs from hobbypartz and I am considering that as a very good option but I was wondering if I should also consider websites that let me build custom battery packs in order to save on wiring and packaging that comes with packing the batteries separately? Also, would I be able to combine all the batteries into one pack or will that be a bad idea considering the charger that has to handle all that voltage at once?

en example of a custom battery website is: http://cheapbatterypacks.com/customp...stomPacks.aspx

blvdbuzzard

You can build the batteries into a single pack. Yet if you get the 5, 6 or 7 cell packs with a "Y" connector, you can move the weight around to help with CG issues. Plus you can charge a small pack faster than the 16 cell pack.

Look over at RCG for the Big Pink Beaver. It is a foam plane that is about the size you are looking at, build is very light weight, yet should be able to carry the payload.


Buzz.

Petkov

Thanks for the recommendation.

aeajr

My Feedback: (2)

Petkov,

Did you post this on RCDiscuss as well. I am responding to a thread over there and it looks like the same discussion.

Petkov

Yeah Aeajr, you found me on both Forums

I did not know what kind of responses I would get so i broadened my search in the beginning.

(the main thread on RCUniverse is: http://www.rcuniverse.com/forum/begi...ject-help.html)

aeajr

My Feedback: (2)

I would do the same thing.

Petkov

An additional question that has come up. If we fly with 1.2V batteries but the motor draws a higher voltage per battery, should we expect any battery damage? If so, what are the ranges?

Batteries we are currently looking at are the elite 1500 which are rated at 10 for the C-rate.

Also, how would we know if a cell has gone bad inside a pack?

aeajr

My Feedback: (2)

Answered in the other thread.