battery weight

[dusty IV]

What % of an electric airplanes total weight can be the power battery?
Is there a rough rule of thumb?

Assumptions are made that this is a typical airplane with a power of about 100 W per pound.

[slipstick]

Any percentage you like provided that airframe is still strong enough....it's only the overall flying weight that's of any real interest.

People used to try to invent rules about that in the dim and distant past, when we were using 1200-1700mAh NiCd batteries.....but they never worked too well then and the same rule certainly wouldn't apply today with modern batteries.

Steve

[dusty IV]

I kinda figured this question was like "how long is a piece of string".

The thing I'm trying to figure out is what size model is best for using 123 cell's.

The mah is only 2300 with these cells. On other cells you can adjust the mah size to fit the application. On the 123's one size has to fit all. So you get 2300 or in a 2p set up 4600mah.

This really confuses me. It appears that below a certain weight airframe the 123's are to heavy. Then an ok weight appears as the plane size increases up to a point where you have to go to 2p cells. The plane then again is to small to carry the weight until it's size is increased.

Just what these magic numbers are escapes me. For estimating number of 123 cells needed for a given airplane I divide the W/A by 90.Maybe that's good enough.

[davidfee]

One of the old rules of thumb was that the empty airframe weight should not exceed the total power system weight. Now, of course, power systems have gotten much lighter and/or more powerful. Basically, you need to size your model such that you get your desired power loading (W/lb) at a reasonable wing loading (oz/sq ft). A lot of guys use wing cube loading ( http://www.ef-uk.net/data/wcl.htm ) to account for scale factor as well.

[ghoti]

What is a 123 cell? Bill

[jkettu]

What is a 123 cell? Bill
http://www.rcgroups.com/forums/showthread.php?t=573927

[vintage1]

What you need to do is spec out s0me power trains, and work out what watts per pound a modern LIPO/brushless set can deliver. Sans airframe.

Last time I did this it looked like 400W/lb was not hard to achieve, and 1Kw/lb was just possible straining cells to the limit and using very expensive brushless motors.

At 400W/lb , to get a net 100W/lb you are looking at 25% of the model being the power train weight.

Even using old can motors I seldom go much above 25% these days for sporty type stuff.