Originally Posted by Pete B.
Dennis, Don't you know we are all eternal optimists?? In truth, that weight was just a guess but I've gone with a fixed U/C.......I know, I know, the warbird purists are shaking their heads ...but in my defence , I am a newby and the less problems I have thrown at me, the more likely I am to complete and maiden.besides, our strip can be a little "bumpy" despite all the brest efforts to smooth it out.! LOL! The next one is another story however........Pete.
Don't let the so called experts talk you out of or into thing unless your convinced it’s for the better. Nothing wrong with fixed gear on a war bird even the purists have probably had one or two when they first started. In this day and age I'm just pleased you’re building something instead of the quick fix of an ARF.
You’re going to get a lot of experts with all kinds of suggestions for motor/esc and battery selections. Lots of different opinions. My personal experience is that if a similar plane size and weight flies very well I would like to start with the same pitch and diameter prop turning at the same RPM. This should get me close to what I would like to see my model do.
Start by figuring out the number of cells you need to get that rpm. Is it within the limits allowed by that motor? Size or capacity of the battery for testing is not critical at this time. I have on occasion put two packs in series to get the cell count where I wanted it to be. I will size the battery for run time later. When you place packs together the cell capacities must be the same. If not you risk damaging cells even a fire.
This is where a meter to measure the draw in amps is invaluable. I have a Watt Meter only because it was the only one available when I purchased it. Others including Hobby King sell versions that are very reasonable. These go between the battery and esc via plugs you will have to solder in place to match the battery and esc.
If you can build a test stand, or mount the motor to a piece of wood and assemble the receiver, esc, battery, meter, motor and prop. Run it on the test stand and notice the draw and check the rpm with a prop tachometer. If the draw is too high or near the upper end of what is suggested for this motor esc combination you will have to make some compromises. Reduce the diameter of the prop, reduce the pitch of the prop or reduce the cell count of the battery pack. All of these will result in a reduced performance. You have to decide if it will be enough to fly the model. If the internal combustion model spends most of its time flying around at half throttle with no problem you may indeed be able to compromise. If not then you may have to rethink your motor esc combination.
If the draw is low for this combination then you may be able to increase performance by increasing cell count for more rpm, increase in pitch for more speed or increase in diameter for a more scale like prop and higher tractive effort. Change only one thing at a time and retest watching the draw and temperature of the esc and motor.
When you have what you feel is the right combination charge the batteries fully and with the low voltage cut off set run the combination the on the test stand wide open until it either cuts out or the battery pack gets to hot. Note the length of the run Battery capacity is simply the same as size of fuel tank so roughly if a 2300 mA battery gives a 6 minute run and you would like to fly for 12 to 15 then you would expect to need at least a 6000 mA pack if you run near full throttle for the whole flight. Lower throttle settings will extend flight times.
Now you know the battery pack size you can size the area inside the model to accommodate the pack. The more air you allow to pass by the motor/esc and battery the better.
This method works for sizing your equipment if you have a known comparison of the type of model you are trying to build. There are many other ways to calculate the sizing of your electrical components but it can be complicated and intimidating I leave that to the others who are more expert in the theory.
I hope this helps