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Beginning Scratchbuilding and Flying

PREVIOUS PAGE: Understanding Power Systems

Calculating how your plane will fly

When scratchbuilding a plane, it is good to have some idea how much the plane will weigh. This means gathering up all the materials you plane to use - foam, battery, motor, radio system, etc., and estimated the approximate weight of the plane. "What?" you say. "How can I know how much my plane weighs before I build it?"

For beginner planes, most designers are kind enough to give you a range in ounces or grams of how much the plane should weigh. In the future, knowing the approximate area of foam required can provide an estimate of the aiframe. What this will do is help you to know what size motor and propellor combination you'll need, so that you will have sufficient thrust to stay airborne. How is that determined? By looking at the watts per pound ratio. This is one of the most helpful ratios to know as a scratchbuilder, because it will help you determine if your plane will fly with the performance you would like!

Let's assume that the designer has provided you how much the plane should weight when finished, and you use all the same materials he did. The plane weighs 12 ounces all-up weight (or AUW). Motor, radio, battery, foam, pushrods, everything - it's already flight ready at 12 ounces. Having put your motor on a thrust stand previously, you've already calculated the watts that the motor will produce with your battery pack. Let's say it produces 50 watts of power. So here's how the calculation is performed:

First, convert your 12 ounce plane to pounds (yes, there's that conversion thing again!)

12 ounces / 16 ounces (1 lb) = 0.75 lbs

Now, calculate the ratio:

50 watts / 0.75 lbs = 66.67 watts per pound

The obvious question now becomes, "Is that good?" Here's a table that gives a very general idea of relative performance:

Watts per PoundPerformance
30 W/lbBarely Flightworthy, minimal climbing ability
50 W/lbSlowflyers, unsuitable for aerobatics
75 W/lbGood overal performance, average aerobatic capability
100-125 W/lbExcellent aerobatic flyer
150-200 W/lb3D flying or ducted fan performance

Remember, it's not the total watts that is important; it's the watts per pound. So the 50 watts for a 12 ounce plane which works out to 66.67 watts per pound will fly quite nicely at mid-range throttle settings, and perhaps provide a bit of limited aerobatic capability when the skill level is achieved, such as loops from shallow dives, or stall turns.

NEXT: Choosing and building a plane

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