18
Estimating performance factors
If you don’t have a way to directly measure your propulsion system’s electrical parameters, the
Cellpro Multi4 Charger enables you to estimate them using before- and after-ight measure-
ments.
1. Collect data
a. Charge pack.
b. When charging is nished, record Fuel % and total pack voltage (i.e. sum of cell volt-
ages).
c. Fly plane (or test on the ground). Record ight time in minutes.
d. Connect pack to charger. Record Fuel % and total pack voltage.
2. Calculate performance factors
(Fuel % before ight) – (Fuel % after ight)
100
x (Pack capacity, Ah) = Capacity consumed during ight, Ah
(Capacity consumed during ight, Ah) x 60
(Flight time, minutes)
= Average current during ight, A
(Pack voltage before ight, V) + (Pack voltage after ight, V)
2
= Average voltage during ight, V
(Average voltage during ight, A) x (Average current during ight, V) = Average power during ight, Watts
The “LiPo Performance Estimator” spreadsheet, which simplies these calculations, can be
downloaded from www.fmadirect.com/tech_data/techdocs/.
3. Evaluate results
n
Average current during ight gives you a rough idea whether system components—ESC,
motor, connectors and wiring—are operating within their current ratings. Keep in mind that
peak current during ight may greatly exceed the average current you calculated.
n
Watts per pound is an approximate indicator of aircraft performance (other factors inuenc-
ing performance include lift, drag and motor type). Here are some guidelines:
25 to 30 watts per pound: level ight.
40 to 50 watts per pound: take off from smooth surface, climb.
50 to 75 watts per pound: take off from grass, sport aerobatics.
75 to 125 watts per pound: pattern aerobatics.
Over 125 watts per pound: 3D.
Tip: For more direct electrical measurements, consider these FMA products:
n
60A Current Shunt (Model DVM-SHUNT-60)
n
Digital Multimeter (Model DVM-VC890D)
(Average power during ight, Watts)
(Model weight, pounds)
= Watts per pound
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