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|This Chipmunk has a maximum take off weight of 2,200lbs. Its 145hp (108KW) engine gives it a power loading of 49W/lb. (click image to enlarge, and for more information).||3 The P51 Mustang is also a low wing all metal monoplane. 1,720hp (1,282kW) and a max take off weight of 12,100lbs mean a power loading of 106W/lb (click image to enlarge, and for more information)|
Model Power Loading
Similarly, different types of models need different power loadings. It turns out that models can sucessfully use power loadings which are similar to full size aircraft. However, for more satisfying performance, its usual to provide a model with relatively more power than a full size machine. The table below illustrates:
Worked example - a small sport model
Let’s take the example of a small, fairly fast sport model. We want the model to have a good performance, so this means our target is 100-125 Watts per pound. The model is estimated to weigh 3.0lbs ready to fly, so that means 300-375 Watts of power will be required. Let’s say 350 Watts. Now that we’ve decided on the amount of power required, a suitable battery can be chosen.
The 350 Watts of power that we need can be generated in two ways; we can either use a 3-cell lipo battery (3S battery) at 32.4 Amps (10.8 Volts x 32.4 Amps = 350 Watts), or a 4S battery (say 14.4 Volts) at the lower current of 24.3 Amps (14.4V x 24.3A = 350W).
Either way will work, but we’ll opt for a 3S battery in this case as we have plenty of these in stock. So that’s the cell count taken care of, and for simplicity we can round up the expected current to 33 Amps.
|This 3S lipo is nominally 3.7 volts per cell, and has a maximum charge voltage of 12.6 volts. When working out power system details, an on load voltage of 3.6V per cell can be used.||A good rule of thumb is to aim for a battery with a capacity in mAh up to 100 times the expected current in Amps. For example, if the current was 17 Amps, we might choose a battery up to 1,700mAh.|
But what capacity battery do we need? The higher the capacity, the longer the model will fly for. A good rule of thumb is to aim for a battery with a capacity in mAh up to 100 times the expected current in Amps. For example, if the current was 17 Amps, we might choose a battery up to 1,700mAh. Similarly for a current of 25A, a battery up to 2,500mAh would be fine. If duration is particularly important, then this factor of 100 can be used, while if light weight is particularly important for a particular model then a factor perhaps as low as 70 can be used. The smaller battery trades duration for lightness.
In the case of our example model, the expected current is around 33 Amps, so a battery with a capacity up to 3,300mAh will be suitable. In this case, we want a reasonably light model but one which also has good flight duration, so we’ll choose a battery a little smaller than this. For this model, a battery capacity of 3,000mAh should work well.
Battery C rating
The C rating of a battery is an indication of its ability to provide current. The higher the rating, the more suited that battery is to supplying a high current demand. C ratings from 20 or 25 right up to 50 and beyond are common. Any of these should be capable of providing the required current using the rules of thumb mentioned here, although if your battery is relatively small a high C rating is a better choice.
When buying batteries, its worth bearing in mind the issue of quality.
Good quality batteries will have a lower resistance, meaning they will
run cooler for any given current, providing higher performance. Good quality
batteries will also probably last longer. So, even though they cost more,
higher quality batteries may well offer better value for money than cheaper
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