Tuesday, September 25, 2012

Tail of an Airplane - Misunderstood

Tail of an Airplane

The tail of an airplane is arranged to resist the diving tendency of a plane. It is not meant to hold the lighter end of an airplane up in the air while in flight.

The purpose of the horizontal fin of the tail is to hold it down. It is like a wing but set at a negative angle of attack so the air flow against the horizontal tail fin produces, in normal flight, a downward force.

The reason for this downward force is the tail operates in the downwash of air that flows off the upper surface of the wing.

You recall an airplane creates "lift" by pushing air mass downward that creates a force upward that equals the weight of the plane in level, cruising flight. The downwash flows down on the horizontal fin of the tail and pushes it down more than you realize.

You know, from just looking at a plane, the nose is heavier than the tail. The nose-heavy plane tends to "nose the plane down" while the horizontal tail stabilizer tends to "nose it up."

When you trim a plane for level flight at cruising speed the two forces exactly balance each other. That is exactly what we do when we trim a plane for a certain speed. We adjust the angle of the horizontal tail fin so, at the particular speed we want to achieve, the downward air mass force on the tail will exactly balance the downward pull of the force of gravity on the nose.