Paper Airfoil Aerodynamics

5 - Airfoil Design Considerations and More on the Reynolds Number

I. Home

II. Back to UAF Physics 211

A. Next

B. Back

A number of considerations must be made when designing airfoils. Usually it is cost prohibitive to design and test full sized airfoils in the laboratory.  This is why it is usually desirable to test scale models of the wing in a wind tunnel.

Strangely, we have shown that the drag forces generated by a wing are dependent partly on the length the air must travel to cross it.  In a scale model the width of the wing is much different than in a full scale model.  This means that the drag forces would not be scalar if we simply tested the model at the same airspeed.

A principle of the Reynolds number allows for the testing of scale models in flight to accurately predict their behavior at full size and speed.  It can be said that two wings are dynamically similar if they are the similar in shape and operate at the same Reynolds number.

For instance, assume we have a small, scale model of a Cessna aircraft wing that is 1/10 the width of an ordinary wing.  In order to compensate and achieve the same Reynolds number we must increase the speed of airflow by ten times.  For this particular reason, it is fairly clear that scale models of most airplanes suffer performance degradations as they decrease in size.  Figure 5.1 and 5.2 show another example of the scaling problems encountered during scale model testing.

The next wave of aircraft research aims to produce miniature planes that can be mistaken for birds or insects.
     Several promising ideas were showcased here recently at the First International Conference on Emerging Technologies for Micro Air Vehicles.
     Don't confuse these tiny aircraft with model planes. They have little in common. Dr. Rob Michelson of Georgia Tech Research Institute, for example, has developed a vehicle with flapping wings. Powered by gasoline instead of batteries, it is equipped to both fly over and crawl under obstacles.
     Rotary-wing craft are also candidates for miniaturization. Researchers from the German Institute for Micro Technology showed off a peanut-sized helicopter weighing a tenth of an ounce with rotors of aluminized paper. Alas, it can only fly to an altitude of 6 in., while remaining tethered to a battery.
     Researchers believe that in the future, micro air vehicles will search burning buildings for survivors, scout enemy positions, and monitor chemicals and radiation on battlefields.

-- Popular Mechanics - August 1997

Figure 5.3 shows an example of what such a micro air vehicle could look like in the future.

Additional design problems are involved when construction materials such as paper or canvas are used to construct airfoils.  In the next section, I will discuss some of these design considerations.

I. Home

II. Back to UAF Physics 211

A. Next

B. Back

Figure 5.1 and 5.2 ? Scaling in applications where airflow is involved isn?t as straight forward as it would seem at first glance.

Figure 5.3 -- The future of micro-air vehicles?