Can yaw develop a spiral dive?

Question

I'm wondering if the 'prolonged' effect of using yaw causes a spiral dive. I understand that a spiral dive is caused by an excessive angle of bank, but can it also be created by an excessive amount of yaw? A spiral dive has the characteristics of a rapid speed increase, rapid loss of altitude and unstalled wings. An aircraft that is yawing too one side for a long time does all of these things, so shouldn't it the prolonged effect of yaw be a spiral dive?

Also, when the aircraft does a skidding turn, why doesn't the aircraft develop a spiral dive? Why is it only when an aircraft does a slipping turn, that the aircraft may only enter a spiral dive?

Thanks.

Answer 1

I'm wondering if the 'prolonged' effect of using yaw causes a spiral dive. I understand that a spiral dive is caused by an excessive angle of bank, but can it also be created by an excessive amount of yaw?

If by "yaw" you mean the aircraft's yaw rate, then we can observe the following--

1. In a low-airspeed constant-altitude turn where the bank angle is rather steep-- say 45 degrees-- the aircraft's yaw rotation rate is high, but the aircraft is not in a spiral dive. (A sailplane circling in a thermal updraft is another example of an aircraft with a high rotation rate, that is not in a spiral dive.) But in this situation, the pilot may be holding some rolling-out input with the ailerons to prevent the bank angle from increasing-- see below for more on this.

2. Even in fully "coordinated" flight (aircraft is aligned with the instantaneous direction of the flight path and therefore is pointing directly into the "relative wind"), a high yaw rotation rate creates a difference in airspeed between the two wingtips which tends to make the bank angle increase. Unless the pilot makes a roll input with the ailerons to counteract this, this can tend to make the aircraft enter a spiral dive.

3. If we are flying along in wings-level flight and then we stomp on one of the rudder pedals to establish a high yaw rotation rate, we'll end up in a skidding turn which can indeed lead to a spiral dive. See below for more.

Also, when the aircraft does a skidding turn, why doesn't the aircraft develop a spiral dive? Why is it only when an aircraft does a slipping turn, that the aircraft may only enter a spiral dive?

This is a misconception. In a typical general aviation plane, if you trim for level flight, take your hands off the control yoke, and then hold full rudder, you'll end up rolling into a rather steep bank in the direction of your rudder input. The roll is driven not only by the difference in airspeed between the two wingtips that we noted above, but also by the sideways airflow interacting with the dihedral geometry of the wing (including related effects such as the dihedral-like effect generated by a high-wing configuration.) You'll find yourself in a spiral dive in the direction of your rudder input, with the slip-skid ball displaced toward the outside or high side of the turn. In other words, you'll be in a skidding spiral dive.

It's true that, in the absence of pilot rudder input, a spiral dive will typically be associated with a small amount of a sideslip-- but this is true of level turns as well. Even if sideslip is present in a spiral dive, the sideslip is not really what is driving the spiral dive. Adding inside rudder to eliminate the sideslip will typically make the bank angle get steeper, which will make the dive angle get steeper.

This answer is built on the concept that a steep bank angle is a key element of a spiral dive. If you are trying to ask if a spiral dive can develop due to an excessive yaw rate while the bank angle stays shallow, the answer would generally be "no". If you enter a severe skidding turn while using the ailerons as needed to keep the bank angle shallow, the extra drag from flying sideways through the air will tend to increase the sink rate, but not to the extent that the maneuver would generally be described as a "spiral dive". But don't forget that a skidding turn can be an invitation to a spin entry! That's the classic "trap" in the landing pattern-- the pilot perceives a need to increase the turn rate but is hesitant to increase the bank angle, so he (perhaps unconsciously) tries to increase the turn rate by using extra rudder instead, but this tends to lower the nose, tempting the pilot to move the stick or yoke excessively aft-- and the plane stalls and spins.

Answer 2

It must be remembered that the wing, at any given moment, is generating more force than any other part of the airplane, including the engine, by far.

Slipping turns are much more prone to create a spiral dive because the critical ingredients are already there: loss of lift from the steeper bank and a stronger force pulling the plane sideways. Because of the bank, the horizontal empennage, including the elevator, will begin to pull the plane into a descending spiral. Pilots, seeking to stop the descent (especially when disoriented by lack of VFR conditions), will tighten the spiral by pulling on the elevator more.

Vertical lift is much less affected by the smaller bank of the skidding turn, so it is less known to create a spiral (but more known to create a deadly tip stall).

The key element of a "skid" input (rudder) is the ability of the rudder to roll the airplane. High wing trainers have a lot of area underneath the CG and must be "helped" with the ailerons to maintain bank angle in a turn. Without aileron input they will have a greater tendency to "skid" (inclinometer ball to the outside).

Low wing aircraft, with more area above the CG, and dihedral, can be "snap rolled" with hard rudder alone.

Gliders will also have a greater tendency to roll with rudder owing to the wing tip speed differential of their long wings.

So, one can see it is possible to design an airplane to do a coordinated roll/yaw turn with rudder alone. (If it rolls enough, it is no longer skidding!).

A skidding turn does create a lot of drag, which can start a downward spiral path due to loss of airspeed, but it is the wing and horizontal surfaces in a bank that are the real culprits to this hair-raising scenario.

Fortunately, one can recover by relaxing elevator, cutting power, and rolling to level before pulling out.