Reversing the role of scale: building the model before the prototype!

By Clark Salisbury

PHOTOGRAPHY: CLARK SALISBURY B 18

efore I invest time and money in a flying model, I like to think about my motivation for doing that. I have spent a fair amount of time

looking at the full scale LSA’s (Light Sport Aircraft) that are currently on the market. The advantage in flying one of these planes, is that the aircraft can be flown with just a Sport Pilot license, which is much easier, and cheaper to get than a regular license. The planes are also not nearly as expensive. Here are the rules the FAA has set for

LSA aircraft. (1) Max takeoff weight is 1320 pounds. (2) No more than two seats. (3) Top speed not to exceed 140 mph. I looked at dozens of full-scale LSA aircraft on the mar- ket, but was the most impressed with the A22 Valor. It has tremendous visibility for the pilots, and is not too high priced. So, I thought about the ideal plane I would like to fly myself, and decided to design an eighth scale of what I thought would be a fun plane, with plenty of windows like the A22.

Let’s do a little math for a minute; If you

build a model one eighth scale, and want the model to have the same weight per cubic foot (density) as the full scale plane, then you would first multiply 8×8×8, which is 512. Divide 512 into the 1320-pound max weight for an LSA, and you come up with a model weight of 2.57 pounds. That weight should be easy to work with, so in my mind Sky - Sport seemed feasible, and was born in my mind. I actually hoped to build the model to weigh two pounds or less. Most LSAs currently on the market have

100 horsepower or less. This translates to 1 horsepower per 13.2 pounds of aircraft. This seems to be about right to get these aircraft to a max level flight of 140 mph. SkySport, if we want it to look realistic in flight, should have one eighth of that speed, or 17.5 mph. If SkySport ends up weighing two pounds,

it should be powered with (2/1320 × 100 horsepower) which equals .15 horsepower. There are 746 watts per horsepower, so .15

AT A GLANCE Type:

Construction: Wing span: Wing area: Length: Weight:

Motor:

R/C sport

balsa and ply 47 inches 381 sq. in. 33 inches 32 ounces 12 oz./sq.ft. (2) 8–4

Gorilla Bob’s CR 23M

ESC:(2) 20A Gorilla Bob’s Power Series Battery: Radio:

3-cell 1300 mAh Li-Po 4 channel

Servos: (4) Hextronik 9 gram APRIL 2012

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