Tuesday, February 3, 2015

Efficiency and Model Aviation

One of the things in life that really gets me excited is when something is efficient; that is the biggest reason I drive a hybrid car, and enjoy model airplanes that stay airborne with minimum power sources. In looking through definitions I found statements like this; “performing or functioning in the best possible manner with the least waste of resources or effort”.  Much of my flying are model airplanes that are powered by a strip of wound elastic rubber a power source that is easily wasted if not used in a resourceful manner.

Simple Rubber Powered Plane Climbing Outdoors

In starting with simple model gliders you can learn about the concept of “lift over drag” or L/D. Gary Hinze created an excellent tutorial for use with simple gliders that will teach concepts such as L/D that can be found on my website. There is another article about building simple gliders from foam plates and straws. To be efficient the airplane needs to produce the lift needed to support the airplane in flight with the least amount of drag. In level flight the lift only needs to be equal to the weight of the airplane, excess will cause the airplane to climb.  

Gliding from Tutorial

Lift Over Drag

For a longer glide it will be important to reduce the amount of drag as much as possible. Drag is created by the lifting surfaces of the airplane and also by air flowing across all the surface of the airplane. Using the correct airfoil and amount of angle to the wind in the wing will help reduce drag. Reducing the weight of the glider means less lift is needed to be created to oppose the weight of the airplane pulling it down to the earth by gravity; it does not change the L/D but does mean it will glide slower down the glide path.

Adding Power to Glider

Now we add thrust to the airplane in the form of a loop of twisted rubber strip. The challenge is to get the airplane to fly as long as possible on the limited number of turns twisted in the rubber.  With a thinner strip of rubber more turns can be wound in the rubber strip before it breaks. The thinner rubber will have less of a twisting force to spin the propeller that provides thrust to fly the airplane. Also related is the size of the propeller, a thin strip of rubber will spin a larger propeller at a slower speed providing less thrust, if the airplane is very lightweight the thrust will be sufficient to keep the airplane airborne for a very long time. Using a larger rubber strip will provide more thrust but use up the rubber turns faster especially with a smaller propeller.  Getting the optimum combination of rubber width and propeller size is part of the challenge in this type of model aviation.

Stop Watch Resting on Simple Rubber Powered Plane Which Flew 59 Seconds 

From a chart, the maximum turns in a loop of rubber 3/32” wide is 129 turns per inch while for 1/8” rubber strip it goes down to 97 turns per inch. For a rubber loop of 10” this would be 1290 turns for 3/32” and 970 turns for the 1/8” rubber. If the airplane was light enough to fly on the 3/32” the duration of the rubber motor run would be considerably longer. High duration indoor rubber powered free flight models are so light that a huge propeller can turn very slowly to power the model for very long flights. The longest flight ever was just over one hour.

Delicate F1D Model Flies for 30 Minutes Indoors

Adjusting the Flight Path

For a model airplane that is flying without control – free flight, the airplane must have some stability built in and be adjusted to gain altitude and then come down slowly as possible for maximum duration. If the flying is indoors the climb to the maximum height available (the ceiling of the building) should be rather slow but if outdoors reaching maximum altitude possible, quickly is normally the goal. Not only is greater height desired outdoors because it takes longer to come down to the ground but the higher altitude improves the chances of riding thermal air currents.

Powered outdoor free flight airplanes normally are adjusted to circle to the opposite direction of the torque which causes the airplane to circle to the left so the airplane is adjusted to the right. Indoor planes normally are adjusted to circle with the torque to the left.

Airplane in Stall Condition

Important for efficient flight in free flight airplanes is to keep the airplane from stalling in flight which is a loss of lift when the angle of attack is too great, usually after a stall the airplane dives and recovers and the cycle starts over again with a big negative affect on efficiency of the flight.  If the airplane is adjusted without enough climb and flies too fast that is generally not efficient for duration. Adjusting for the optimum flight path throughout a flight is a big part of the fun and challenge to free flight model airplanes.

No doubt I have oversimplified aspects of my explanation, I do hope people check out some of my resources and build some model airplanes.

Bill Kuhl

Important Concepts to the Efficiency of Flight for Free Flight Model Airplane

*  The airplane should fly through the air with just enough lift with a minimum amount of drag, many factors relate to this but important ones relate to size and shape of the wing.

*  Powering the airplane with elastic rubber strip it is important to match the parameters of the rubber motor and the propeller to power the airplane in an efficient flight path.

*  Adjustments to get the proper flight path and avoid stalls or dives are critical for maximum flight times.

Additional Resources

http://ideas-inspire.com/simple-glider-curriculum/  - Gary Hinze Tutorial




Additional Video That Might be Used in Float
Float Documentary Trailer an Upcoming Movie About F1D Indoor Model Airplanes

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