Wind Tunnel : Spin Testing

January 09, 2018

Wind Tunnel : Spin Testing

" I have a fear of tunnels. Going through any tunnel causes me great anxiety "

--- Tyler Perry

INTRODUCTION :

Spin and stall are a problem since the beginning of aviation – for aircraft designers, but also for pilots and flight instructors. These phenomena are much of interest during aircraft design and flight testing process.

Understanding of stall and spin is a crucial part of pilots’ training. Per numbers cited by spin and stall cause 12% of accidents and 25% of disasters in general aviation in the USA.

Although general flight mechanics and aerodynamics are same for all the airplanes, it is obvious that specific aircraft behavior is individual and is a function of design features. It applies to stall and spin characteristics as well.
As said above, airplane spin and stall behavior is an item of interest during design process, but it also must be verified during flight testing, which is costly and could be risky. Wind tunnel spin testing before flight tests seems to be safe, cheap and productive method of initial experimental verification of spin behavior of an aircraft.

Courtesy : NASA

PURPOSE OF THE RESEARCH :

The purpose of the research is to initially evaluate aircraft spin characteristics and steering effectiveness in a spin based on testing of dynamically similar scaled model.

STALL PHENOMENON :

For better understanding of airplane behavior in stall and spin conditions, let’s briefly remind fundamentals of steady-state horizontal flight aerodynamics and balance of forces.
In the steady- state horizontal flight of an airplane weight of the aircraft is balanced by lift, whereas aircraft’s drag is balanced with thrust of the power unit; in this case – piston engine with a propeller.

As known from the basics of a wing section theory, an aerodynamic force appears on a wing as an effect of pressure distribution on the wing section, In analysis of the steady-state horizontal flight the aerodynamic force is usually considered after splitting into two components: lift, normal to the horizontal plane, and drag, which is tangent to the flight path and opposite to the engine thrust.

Courtesy : NASA

Conclusion from the above brief summary of the spin theory is that configuration of spin is a result of airplane aerodynamics and mass geometry. There is one aircraft orientation in spin for certain configuration, in which equilibrium exists. Each aircraft control, such as rudder, elevator, ailerons, flaps, can influence spin characteristics. Rudder is the most influencing control and it is critical for spin recovery.

This article would be incomplete without mentioning spin recovery procedure. There are two general circumstances of spin recovery:
– recovery from incipient spin. The key is to prevent the aircraft from developing spin and equilibrium conditions. Pilot should first of all focus on elimination of flow separations: reduce angle of attack and increase speed,
– recovery from developed spin. In this condition pilot should impact equilibrium using rudder to hamper spinning. After spinning is eliminated, pilot should restore speed sufficient for a horizontal flight.

Spin testing require a wind tunnel with vertical flow. A scale model is hanging in the center of the test space on a thin rope. The rope is attach to the model the way allowing for pitch angle change during the test. In some cases additional rope, attached to the nose of the model, is required for stabilization. Airflow is applied vertically upwards to simulate aircraft descending.

It is worth of saying that more sophisticated tests include remote control of the wind tunnel test model, or even free-flying models.