Why does elevator input move the turn coordinator ball in steep turns?

Question

When practicing steep turns, I'm running into an unusual phenomenon. If the airplane is losing altitude in coordinated flight and I apply aft stick pressure to pitch up and correct, I notice that the ball swings to the outside of the turn indicating a skidding condition. Conversely if the airplane is gaining altitude and I release back pressure to the stick to descent, the ball always swings to the inside of the turn, indicating a slip. This seems counter intuitive as one would think that an increase in AoA to gain altitude would result in increased adverse yaw, requiring more rudder pressure in the direction of the turn and less rudder pressure when the nose is lowered resulting in a lower AoA and adverse yaw.

I suspect the reason here has to do with the direction the nose is being forced in by the elevator input during the turn. At a steep bank angle the nose is inadvertently being pulled inside the turning flightpath which results in a skidding condition, which should, therefore, be countered by less rudder pressure in the direction of the turn. Conversely when elevator pressure is reduced, the nose would tend to stray outside of the tangential flightpath, resulting in a slipping condition. The end byproduct of this would be reduced or even cross control inputs needed to maintain coordinated flight. Can anyone else confirm this?

Answer 1

In a steep turn you are making power changes, pitch changes and also making constant corrections with aileron to hold the bank angle, without even realizing it. Once established in the turn some airplanes require a bit of in-turn aileron to hold the bank angle, some hold the bank with neutral aileron, and some require top aileron to keep from overbanking. The top and down aileron inputs are inducing constantly changing adverse yaw forces which come and go, changing torque effects from the engine with power adjustments are producing changing yaw forces, and gyro precession from the prop from pitch motions are inducing yaw forces which come and go, and there are bumps, and you start running into your own wake.

In other words, you are in a machine pulled along by a big torque producing gyro that is slithering and sliding around in a gas with half a dozen forces and moments interacting on it simultaneously. With all the subtle forces and inputs happening during a turn, I don't think you can identify and act on a single phenomenon like that. What you have to do is, well, just do whatever it takes to hold the bank angle, altitude and center the ball and don't overthink it.

If you fly gliders, which use a yaw string that is more sensitive than a ball, even without the torque and gyro effects of an engine, the yaw string drifts this way and that while in a turn, seemingly independent of the aileron position some of the time. You generally make stabs of rudder in concert with aileron, but sometimes the yaw string seems to have a mind of its own and you just do what you have to do with your feet.

Answer 2

It is important to realize the "ball" is simply rolling back and forth in a curved glass tube to indicate the direction of net G forces. It is also known as an inclinometer. What is happening in your steep turn is the elevator/wing orientation is now at an angle to gravity, so pulling "up" also tightens your turn radius, forcing the "ball" to the outside. In a steep turn the elevator becomes more "rudder-like". If you roll to 90 degrees, the elevator IS your rudder, and the rudder will pitch your nose up or down.

Your thought to "step on the ball" is correct, as rudder input will bring the nose back into the line of coordinated flight, helping hold altitude. Try adding a bit more power too.

Pulling alot more elevator to hold altitude in a steep turn is not good technique as it can lead to a stall or spiral dive. Sometimes just rolling to a slightly lower bank angle will do the trick.

I would review this with an instructor, but a little "rudder to the sky" may help here. Get coordinated and see how much throttle you need to stay level.

Answer 3

If the airplane is losing altitude in coordinated flight and I apply aft stick pressure to pitch up and correct...Conversely if the airplane is gaining altitude and I release back pressure to the stick to descent

What you really want to do is to change your technique.

Your goal (assuming for example a steady 60 degree AOB) is to roll into the turn, set your G (2G = what you need for 60 degree AOB) and then lock in the back pressure while varying your AOB to set your nose position relative to the horizon. Climbing? Over bank a little to let your nose slice down, and then reset the bank so it holds fixed to the horizon. See what that gives you. Descending? Do the opposite. But don't pump the nose to go up or down. Particularly if you are near a stall AOA, it can cross you over and you can lose control.

This will smooth out your flying and result in constant radius turns.

Answer 4

When practicing steep turns, I'm running into an unusual phenomenon. If the airplane is losing altitude in coordinated flight and I apply aft stick pressure to pitch up and correct, I notice that the ball swings to the outside of the turn indicating a skidding condition. Conversely if the airplane is gaining altitude and I release back pressure to the stick to descent, the ball always swings to the inside of the turn, indicating a slip.

This question surprised me. Your observations are in fact consistent with a statement by Wolfgang Langewiesche in his aviation classic "Stick and Rudder". He claimed that while turning¹, a sideslip is often caused by "insufficient lift" and should often be fixed by increasing elevator back pressure rather than adding inside rudder. But I've never found his claim to be consistently true and I've long believed that it is one of the few significant errors in his otherwise excellent book.²

I have found that very pronounced elevator inputs while turning that create very significant changes in G-loading do produce brief, small excursions of the slip-skid ball in the direction predicted by Langewiesche's idea. I believe these are produced by complex three-dimensional changes in the flight path that are beyond the scope of simple analyses. Or to take a somewhat related situation, in a wingover involving a full 90 degrees of bank, if the aircraft is allowed to "float" over the top at a rather low G-loading (involving little or no aft pressure on the control stick), with no bottom rudder applied, the slip/skid ball and or yaw string will indeed show quite a pronounced slip as the aircraft "falls" earthward.³

But in simple turning maneuvers involving only moderate changes in the direction of the flight path in the vertical plane (i.e. only moderate changes in climb or descent rate), I've not found elevator inputs to play a role in turn coordination (in the sense of avoiding slips and skids) in any consistent manner. (In single-engine prop planes, there definitely can be an effect due to p-factor, but the direction of the effect will depend on the direction of turn.) I've done rather extensive tests to look at this in several single-engine airplanes and ultralights (including some with a high-mounted pusher engine), and several different sailplanes.

The topic has long been of special interest to me, because in the hang gliding community, at least in the US, there's long been an idea that the "correct" pitch inputs while entering a turn will keep the flight path "coordinated", while inadequate pitch inputs will cause the glider to "sideslip". Along the same lines, an intentional nose-down pitch input while the glider is entering a turn, or is turning, is thought to produce a very pronounced "sideslip". (If true, this would be compatible with Langewiesche's idea as noted above.) Many pages have been devoted in training manuals and magazine articles to "explanations" of these supposed phenomena. While it makes some sense to use the word "coordinated" to describe the harmonized use of all available flight controls to produce the desired results-- and the only available control inputs in hang gliding are weight-shift pitch and roll inputs-- I've found the idea that pitch inputs have something to do with causing or preventing sideslip in hang gliders to be a complete "red herring". Some hang glider pilots appear to be mistaking the sensations of a diving, accelerating turn for the sensations of a true "slipping" turn. Extensive experiments with yaw strings and slip-skid balls on hang gliders suggest that sideslip in hang gliders is related almost entirely to roll rate, with a slight residual sideslip remaining when the bank angle is constant-- and this slight slip seen in a constant-banked turn tends to be greater when the airspeed (and sink rate) are low than when the airspeed (and sink rate) are high-- exactly opposite to what the community "conventional wisdom" (and Langewiesche's idea) would predict. Part of the confusion may be due to the fact that in some hang gliders, especially those with pronounced anhedral, "pulling in" the control bar while rolling from wings-level into a turn can produced a marked increase in roll rate, thus indeed producing a rather pronounced sideslip, which ends very soon after the roll rate is brought back to zero. Anyway, the long and short of it is that sideslip in hang gliders is produced primarily by adverse yaw while rolling, and for a given roll rate, this slip due to adverse yaw is not reduced by moving the control bar forward to "coordinate" the turn and reduce the gain in airspeed and loss of altitude. Pulling the bar aft while holding the glider in a constant bank doesn't make the glider sideslip as it dives and accelerates. (End of tangent-- but may help the reader understand why I've spent quite a bit of time exploring these relationships in more "conventional" aircraft as well. And the basic results were the same in all the aircraft tested.)

I'm assuming you are flying a single-engine prop plane with a nose-mounted engine turning in the "conventional" direction (clockwise as seen by pilot). Is this true? Have you demonstrated the phenomena to be consistently true while turning in both directions?

At the risk of contradicting both the original OP of the question and the esteemed Wolfgang Langewiesche, I'd suggest that this is not an effect that can be consistently replicated in both directions in a wide variety of aircraft. Would be most interested to be directed to video footage or other verification that suggests otherwise-- ideally with pilots feet clearly visible off the rudder pedals!

Can anyone else confirm this?

I'm not sure whether the OP meant (by "this") his theory was to what was happening, or simply the observed phenomena, but I'd sure like to see answers posted (or chat replies) by other ASE members who are able (or unable) to confirm the observed phenomena-- with specifics as to bank angle, and whether the effect is transitory (connected to changes elevator back pressure and airspeed) or lasting (enduring as long as aircraft is climbing or descending, even with pitch attitude constant), and confirmation that the effect was observed in both directions of turn, etc-- regardless of what the underlying mechanism is believed to be.⁴

Footnotes:

1. Langewiesche didn't specify that the turn must be steep in order for this phenomena to be observed. OP mentions "steep" turns. It's possible that my own experiments described in this answer were mainly focused around turns of 45 degrees bank or less-- need to check notes for more details to refresh memory.

2. Similar content-- perhaps inspired by the content in "Stick and Rudder"?-- appeared in some (circa 1969?) editions of a book called "Modern Airmanship", edited by Neil D. Van Sickle. In later editions, such as the 1999 8th edition edited by John F. Welch, Lewis Bjork, and Linda Bjork, this content has been removed.

3. We can think of slips as fundamentally being caused by either of two phenomena: (one), an aerodynamic asymmetry creating a yaw torque that is only brought into balance when the aircraft is flying in a slipping condition, with the nose yawed to point to the "outside" or "high side" of the actual direction of the flight path, or (two) a sudden demand for an increased yaw rotation rate (if the nose is to remain aligned with the flight path), which the aircraft's yaw rotational inertia tends to resist, until a large enough slip angle is achieved to create a significant yaw torque to overcome the yaw rotational inertia and increase the yaw rotation rate. As the airspeed reaches a minimum at the top of a wingover, and aerodynamic forces thus are low, while the rate of earthward curvature of the flight path (due to gravity) reaches a maximum, the latter effect probably best describes the reason for the sideslip seen at this point. The steeper the bank angle, the more the earthward curvature of the flight path demands a yaw rotation rather than a pitch rotation, if the nose is to remain aligned with the flight path.

4. A few more words about the possible "underlying mechanism", or lack thereof, for increased back pressure to tend to cause a skid, and decreased back pressure to tend to cause a slip, as OP has reported -- the immediate effect of increased back pressure on the stick or yoke would seem to be an increased aerodynamic force that acts "straight up" in the reference frame of the inclinometer (slip-skid ball) tube, and so should not influence the deflection of the slip-skid ball. (Keep in mind that the slip-skid ball does not respond to some sort of "balance" between aerodynamic forces and gravity-- rather it responds to aerodynamic forces alone.) The longer term effect of increased back pressure on the stick or yoke is a lower airspeed, and thus (for same bank angle) a lower turn radius, a higher turn rate, and a higher required yaw rotation rate. It would seem that when extra back pressure is applied on the stick or yoke and the airspeed drops and the radius of curvature of the flight path decreases, and the turn rate increases, and the required yaw rotation rate also increases, the aircraft's yaw rotational inertia would have some tendency to keep the nose yawed too far toward the "outside" or "high side" of the flight path (basically a "lagging" effect) which should produce some temporary sideslip (not skid). Yet OP's observation is that increased back pressure tends to cause a skid. I'm at a loss to reconcile this discrepancy. Of course, the steeper the bank angle, the less changes in turn rate involve changes in yaw rotation rotation rate at all-- at steep bank angles the dynamics at play mainly involve the pitch axis.

Answer 5

Imagine a banked plane in four scenarios. In each scenario the plane maintains the same vertical speed (it does not accelerate up or down).

1. The plane is banked but not turning. The aircraft creates 1G of lift by angling through the sky in an uncoordinated manner and does not turn at all. The direction of lift is pointed straight up, and the ball moves to a position opposite it, pointed strait to the ground.

2. The plane is banked, but in a slip. The aircraft does turn but not as much as it should given the angel of bank. The aircraft is generating 1G of lift in the vertical direction, but not enough lift in the horizontal to put the combined vector perpendicular to the wings. The ball falls partway towards the ground.

3. The plane is in a coordinated turn. The aircraft is generating 1G of lift in the vertical direction and just enough lift in the horizontal direction to put the combined vector perpendicular to the wings. The ball is centred in the gauge.

4. The plane is in a skidding turn, it is turning faster than it should given the angle of bank. The aircraft is still generating 1G of lift in the vertical direction, but now it is generating more than the required amount in the horizontal direction, and the combined vector is past perpendicular. The ball moves to the outside of the turn.

The essential part here is that unless the aircraft is entering a climb or coming out of a dive, it is producing 1G of lift in the vertical direction. If the plane increases or decreases the amount of total lift produced, the direction of the vector will change accordingly. If the direction of the vector does not match the angle of bank you will be uncoordinated. Too much bank, or not enough lift gives a slip, too little bank or too much lift gives a skid.