Thursday, August 16, 2012

The Breakdown of a Spiral Turn Before the Breakup

The Breakdown:

Something disturbs the plane in straight and level flight at cruising speed. The right wing drops slightly. The plane enters a gentle sideslip to the right. In a slip a crossflow of air initiates a dihedral response to pick up the right wing.

The vertical tail fin responds, simultaneously, by yawing the plane slightly to the right. If the plane is spirally unstable the rudder (tail fin) forces the plane around before the dihedral had time to pick the right wing up.

The rudder produces a yaw which introduces an over-banking tendency while the plane turns. In a turn to the right the left wing is moving, at the moment, faster through the air and generates more lift. At the same time the right wing is slowed a bit and generates less lift. The net result is the left wing goes up slightly and the right wing drops. The over-banking tendency cancels out the attempt of the dihedral to the right wing to return the plane to level flight. Net result the right wing stays down.

The slight sideslip to the right continues and the dihedral tries again to right the right wing to stop the sideslip. The vertical tail again over-rides the dihedral and the plane enters a new over-banking effect. This process will repeat itself unless stopped.

If this isn't stopped the plane enters a steeper bank and a tighter turn. This continues if you don't interfere. 

The g-load increases due to the centrifugal force build-up. The increase in load forces the plane to drop its nose and pick up speed. The plane has a built-in tendency to keep itself at a constant Angle of Attack. The extra load, created by the centrifugal force build-up, creates a situation where the plane can only maintain a constant Angle of Attack by picking up additional speed. The dive combines with the spiral turn to create a spiral dive.

When an increase in the bank reaches a certain point another effect forces the nose of the plane downward toward the earth. In this deep bank the earth is on the right side and the sky is on the left side. The rudder (vertical fin) continues to push the plane around to the right. It now is pushing the plane into a dive toward the earth.

In summary, this is what a plane "wants to do" once it is in a turn. The plane wants to do this even when the pilot is on the controls. 

This, in previous posts, is the anatomy of the "death spiral" that causes many lost lives where instrument flight skills are lacking and/or the pilot tries to fly an airplane so it makes him "feel good or more 'natural'."

Flight Controllers have testified about their inability to talk an untrained pilot out of a "death spiral." They try to put the aircraft into a flight attitude where their sensations are "more natural." In effect, they don't believe what the plane's instruments are telling the pilot about the actual attitude of the plane (a deep spiral dive).

This is a safety lesson on proper instrument pilot training. Knowing the normal tendency of a plane, in a turn, can set you up for a vestibular illusion that may end your and your passengers lives in a needless spiral dive accident.

Wednesday, August 15, 2012

No Controls Touched - Will a Plane Fly Straight?

Will a plane fly straight or turn away from it's cruising straight ahead attitude of flight , if you release the controls?

What does an airplane really want to do concerning the direction of its flight? 

Unfortunately, aircraft built to American specifications don't have the stability to remain straight and level with the controls released.

A pilot knows this if he experiments at a safe altitude. Release the controls and the plane will enter a turn. Pilots say the reason a plane enters a turn is because of:

  • Torque
  • Stiff controls
  • Wing heaviness
Planes will enter a turn. Once into the turn they will keep increasing the angle of bank, the rate of turn, the speed and the rate of descent. This results in a true spiral which is an ever tightening turn combined with an ever steepening dive.

The end result, if the plane is left to its own fate, is the plane will break up in this tight corkscrew because of excessive g forces that exceed the aircrafts maximum load limits.

The spiral looks like a spin except the plane's controls will function in a normal manner and no stall is involved. You can recover from a spiral dive any time you elect to use the controls.

The entrance into a spiral begins when something disturbs the plane's right-left sense. The aircraft will respond to the disturbance in two different ways simultaneously. One is a planes vertical tail will yaw the plane around to point itself into the direction it was actually moving. At the same time its tendency to refuse to sideslip, due to the dihedral angle of its wings, will cause the plane to lift one wing and drop the other to regain its lateral balance. Both events occur together

The turn causes the relative wind, to blow slightly crosswise at the plane to affect both the vertical tail and the wings dihedral at the same time. A plane will respond to a disturbance depending on weather the yaw effect or the wing-righting response is quicker and more forceful.

Think about a plane that has a very small tail and a pronounced dihedral. If a gust throws the ship into a slight sideslip the right wing will pick itself up before the small tail has a chance to yaw the plane around to the right. After the disturbance the plane will resume its flight in the origiinal direction.

Now think of a plane with no dihedral and a large vertical tail fin. The gust above will cause the plane to turn entirely from the yawing. If it does recover, it will head in a different direction. 

Students think the vertical tail fin is there to keep the plane flying straight. Instead, it actually makes the plane turn.

All of the above also depends on tail length and wing span. The density of the air plays a role in the dampening effect. A gust may cause a quick response but the air density will damp the quickness. A short tail and short wings will react quickly. Slower for a long tail-long wingspan aircraft.

Designing a stable non-spiraling plane is not worth the time for aircraft designers. They design a plane to actually help a pilot in controlling the aircraft. They contend that a spirally stable airplane is hard to fly in rough air. It will tire the pilot out over a long period of time.

To a pilot, in a steep spiral turn, is that tendencies that exist in a plane with controls released will be noticeable when the pilot flies the turn.

Plane design should make stability a priority.