I Think I’ll Just Wing It. Pt.3

With any Project, I like to Establish my goal first so here we go.

My goal is to build an Electric-powered park flyer from both items I already have that are fairly inexpensive to buy or easily found at discount, dollar and hardware stores. My goal weight is 8 ounces  (227 grams) based on the power system I already have.

Let’s start with the gear I have:

I already have a Transmitter capable of multiple model memory from long ago but most TX systems are really inexpensive now. My motor is an Outrunner type meaning the outer body including magnets rotate while the winding of wire stay stationary unlike a typical armature/can style motor. This is also a Brushless motor, yep no brushes and it is also a 3 phase motor. A lot more efficient than brushed, less drag due to no brushes. They produce 2, 3 and many more times the power of an equal sized brushed motor supplied by the same voltage and current.

When looking at motor/thrust/watts consider this; a 2 cell Lipo battery is labeled 7.4 volts but when fully charged measures 8.4 volts. Think of this of a full gas tank. 7 volts is a near empty tank. At around 7 volts the plane is going to be very slow maybe start to be harder to fly as it is putting out less power from less voltage and ability to handle bursts of current draw diminish greatly. Most speed controls have a voltage cutoff to save the batteries. In this way, they behave differently than their fuel powered counterparts. So when looking at the table below this explains why you see both a 7 and 8-volt reading.

The motor is rated at 40 to 60 watts yielding between 220 and 340 grams of thrust. So here is how I determined the goal weight:

I do not know the origin but long ago I was taught the “Watts per pound” rule and whether or not it came from anywhere credible I can vouch that it is proven guideline.

  • 50W/lb or less – very lightweight low wing loading trainer or slow flyer.
  • 50 to 75 W/lb – light powered gliders, basic park flyers, and trainers, classic biplanes and vintage (‘Old Timer’) type planes. Some of these can fly on the minimum if they are “belly landers” with no landing gear.
  • 80 to 125 W/lb – general sports flying and basic/intermediate aerobatics. Many scale (eg warbirds) subjects suit this power band.
  • 120 to 175W/lb – more serious aerobatics, pattern flying, 3D and scale EDF jets.
  • 180 to 200+W/lb – 3-D Planes, fast pattern planes, pylon racers,  faster jets and anything that needs serious non-scale power.

Just to put this in perspective 1 horsepower is equal to 746 watts.

As I stated above my goal weight is 8 ounces/227 Grams, and the motor at a minimum is rated for 220 grams of thrust, this means I should have  1:1 or greater thrust to weight ratio of about 80 watts per pound. Using the list above I should have more than enough power and some wiggle room with what it will weigh in the end.  We call this “AUW” or “all up weight”, essentially everything that is going in the air.

My battery is a 2s1p or 2 cell lipo 7.4 volt 300mah 30C and I will explain later what all that means when it comes time to power up.

I will use 3 servos to control Rudder, Elevator, and Ailerons. Beyond that, there is the wheels I still need to make,  receiver, speed control or ESC (electronic speed control), prop, pushrods and control horns. So far my airframe parts and electronic gear weigh roughly 138 grams/4.8 ounces and I still have hardware and wheels to weigh in so I think I am going to make goal weight.

The airframe is next…

  • Hobby Dude


Categories: Aircraft, Radio Control

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