Q-114: I heard you say that you shouldn’t run an electric motor on a test stand in your shop because of the lack of natural air flow for cooling purposes. The motor could easily overheat! But I still would like to be able to estimate the approximate run time of my electric motor, knowing the rated capacity of my battery as well as the motor current in amps. Is there some kind of formula that I can use?
A-114: You should be concerned about running your electric motor for
long periods of time indoors. I really don’t recommend you do that.
As far as a formula goes for calculating run time, I have one that I have used
for many years. All you need is the capacity of your battery in ampere/hours (aH)
and the motor current in amperes (amps). You can either use the capacity stamped
on your battery pack or measure the capacity with a discharge-type tester. The
motor current is best determined with the help of an Astro Flight Whatt-Meter.
With these two parameters, first multiply 60 x the capacity in aH. Let's say I have a battery with a capacity of 700 mAh (which is the same as saying 0.7 aH) and a motor current of 4.6 amps. Take 60 x 0.7 = 42 and then divide that by the motor current of 4.6 amps = 9.1 minutes.
This is only an approximation. Under actual flying conditions your propeller
unwinds in the air, which causes the motor current to reduce and therefore run
for a longer time. During any regular flight you are likely to throttle back the
motor and again the current will be reduced, but you can get a rough guide using
this formula.
Just about when I was answering this question I found an interesting Web site
that allows you to compute flight time. The URL is www.andersonparkflyers.org/calculator2.htm.
Insert the capacity of your battery pack in the first box. Then insert your
motor current in the second box and click on “calculate.” Instantly you have
your motor-run time expressed in minutes. This is an impressive Web site with
much information for the electric-power enthusiast. I urge you to look around
this site and check out what is available.