I revised the example part of the original post, so I thought I would provide the updated part here.
This should be fun. Let's see where these formulas take us! We will use a
24 ounce, 1.5 pound plane as our example. If we want basic flight you will
need 50 watts per pound or about 75 watts input to your motor for this 1.5
pound plane. That is, 50 watts per pound X 1.5 pounds = 75 watts needed
for basic flying performance. If you want a little more spirited plane, we
could use 75 watts X 1.5 pounds which is about 112.5 watts.
Lets use 100 watts as the total target, just to be simple, shall we? I am
going to use a lot of round numbers here. I hope you can follow.
If we use an 8 cell NiMh battery pack at 9.6 V it will have to deliver 10.4
amps to hit our 100 watts input target ( 100/9.6 = 10.41amps) If my
battery pack cells are NiMh cells that are rated at 10C then I need an 8
cell pack rated at 1100 mah to be able to deliver 11 amps. Sounds about
Now I select a motor that can handle 100 watts or about 10.4 amps at 9.6
Volts. From experience we know this could be a speed 400, a speed 480 or
some kind of a brushless motor.
We now need a propeller that will cause the motor to draw about 100 watts. I
don't know off the top of my head what that would be. I would go to some mfg
chart as a starting point. GWS has good charts!
I see that if I use a direct drive speed 400 with a 5X4.3 prop at 9.6V then
the motor will draw about 12.4 amps or about 119 watts. This would be a
good candidate motor/prop for the plane using a 9.6V pack that can put out
12.4 or more amps. This would be a set-up for a fast plane as that motor
will spin that small prop very fast.
However maybe I don't want such a fast plane but one with a really good
climb and lots of low end pull to help out a new pilot who is in training or
to do more low speed aerobatics
I can also use a speed 400 with a 2.38 gearbox and run it at 9.6V spinning a
9X7 prop and run at about 12.8 amps for 120 watts.
The larger prop will give this plane a strong climb, but since the prop
speed has been reduced by 2.38 times, it won't be as fast. Spinning a
bigger prop gives me more thrust but a lower top speed typically. This is a
common strategy for 3D planes.
Back to battery packs and motors
So if I shop for a 9.6V pack to be able to handle about 15-20 amps, I should
do just fine and not over stress the batteries. In NiMh that would probably
be a 2/3 or 4/5 A pack of about 1000 -1300 mah capacity. Some examples here: http://www.cheapbatterypacks.com/mai...ells&chem=NIMH
We view the battery and motor as a linked unit with a target power profile,
in this case about 100 watts. We use the prop and gearbox, if any, to
produce the manner in which we want to deliver that power to the air to
pull/push the plane.
If this is a pusher, I may not have clearance to spin that big prop so I
may have to go for the smaller but faster prop combo.
If this is a puller, then I can choose my prop by ground clearance or some
other criteria and match a gear box to it.
See, that was easy, right? ( well sorta but ....)
But we are not done! Oh no!
I could try to do it with a 2 cell lithium pack rated 7.4V. To get 100 watts
I now need a pack that can deliver 13.5 amps and a motor/prop combination
that will draw that much. So if I have 10 C rated lithiums, then the pack
better be at least 1350 mah. Probably use a 1500 mah pack to be safe.
Well, when I look at the chart for the geared speed 400 I see that,
regardless of prop, at 7.4V I am not going to have enough voltage (
pressure) to push 13 amps into this motor. So the 2 cell lithium won't meet
my performance goal of 100 watts+ per pound using this gear box.
If I go back to the charts and look at a different gear boxes. I can't hit my
power goals using 7.4V. Maybe we go back to direct drive.
We see that the best I can get this speed 400 to do is a total of 70 watts
at 7.2V ( close enough ) so I can't hit my power goals using a speed 400 at
this voltage. but 70 watts would be about 48 watts per pound so I could have
a flyable plane, but not an aerobatic plane using this two cell pack.
Now, in fact that is NOT how I would do this. I would decide on the watt
target, go to the chart, find a combo that meets my goals, then select a
battery that will meet the demand and see if my weight comes up at the
target I set. A little tuning and I come up with a workable combo.
I often use the MaxxProd combos for reference. If you read the details on each
package they have wonderful information. And, the fact is that I generally go
with brushelss motors these days. Costs are reasonable and their higher efficency
gives me more performance and longer flight times.
Following the example above, the combo 10 on that page would be an excellent
fit for my 1.5 pound plane for sport flying.
The Combo 049 might be a good fit for a slow flyer. Either way the package
has all I need.
If I wanted the plane to have all out performance, the 15A or 19A package would
be my pick. Note that these would need either higher voltage or higher amperage
battery packs. The flyers/PDF for the packages make recommendations.
For those who like to be even more analytical about it, there are packages
like MotoCalc that will allow me to play with all sorts of combinations and
make suggestions on what I should use. There is a link for MotoCalc below.
So, in these few paragraphs you have taken in a basic knowledge of how electric
power systems are sized, the factors that are considered an how to predict
the outcome. Simple, right?
Of course there is a lot more to know and time and experience will teach
you plenty, but with this basic understanding you are better prepared to
begin playing with the power systems you put in your planes.