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Old Jan 27, 2013, 02:03 PM
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It's certainly possible to have adverse yaw so powerful that it creates a turn rate opposite that of the bank angle. The obvious example would be to simply deflect one aileron 90 degrees down. This creates a tremendous drag/yaw force but hardly any lift/roll. The result then being a slight bank in one direction with a sustained turn in the opposite direction. This is a very extreme example however, and very few planes are designed poorly enough to exhibit this behavior, but it is theoretically possible, and it is not dependent on the fuselage or rudder.

The fuselage itself cannot produce a turn as described. It can produce only a side force, not a yaw rate.
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Old Jan 27, 2013, 04:18 PM
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This one has always baffled me... an Henri Farman, from 1909, said to be one of the first with ailerons. From the photo, the ailerons would float up in flight to trail behind the wing, and the only control would be to pull them down, which would be our buddy Adverse yaw in action.
Reading in "Flying in Flanders" by Willy Coppens, a Belgian ace in WWI, he mentions those very early planes were lucky to fly at all, because the principles of control weren't understood. He mentions one of his instructors tried that new-fangled "coordinated turn', and spun in, in one of these stick and canvas planes.
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Old Jan 27, 2013, 08:00 PM
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Haha! I got quite a laugh out of the idea of someone actually trying something as ridiculous as my down-only aileron example but after looking into the Farman, it seems that the ailerons were actuated with a single cable, but only upward not downward.
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Old Jan 28, 2013, 05:26 AM
if you see my flying, run
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aeronaut you are right, allowing the rudder to float would effectively only reduce the yaw damping of the vertical tail but it would barely maintain a side slip unless there was some considerable control hysteresis/friction.
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Old Jan 29, 2013, 11:08 AM
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It's certainly possible to have adverse yaw so powerful that it creates a turn rate opposite that of the bank angle. The obvious example would be to simply deflect one aileron 90 degrees down. This creates a tremendous drag/yaw force but hardly any lift/roll. The result then being a slight bank in one direction with a sustained turn in the opposite direction. This is a very extreme example however, and very few planes are designed poorly enough to exhibit this behavior, but it is theoretically possible, and it is not dependent on the fuselage or rudder.

The fuselage itself cannot produce a turn as described. It can produce only a side force, not a yaw rate.
Vespa you are missing something crucial. Consider the earth's orbit around the sun. The steady pull of the sun's gravity acting perpendicular to the earth's trajectory or "flight path", creates the curvature in the trajectory.

So too will a steady sideforce, say from the fuselage making "sideways lift", cause the flight path to curve,

In a non-curving, constant-heading, sideslip (such as for a crosswind landing or to steepen the glide path), this does not happen because the sideforce is opposed by the horizontal component of the banked wing's lift. The amount of bank required to stop the turn, depends on the amount of sideforce that is being generated by the fuselage, vertical fin, etc.

The banked wing contributes a turning force--this is the horizontal component of the wing's lift-- acting perpendicular to the flight path and tending to curve the flight path.

The sideforce from the fuse, vertical tail, etc also contributes a turning force, acting perpendicular to the flight path and tending to curve the flight path.

Only when these forces are equal and opposite, can there be no turn.

When we realize that sideforce can create a turn (or increase or decrease the turn rate that we would normally see at any given bank angle), we can appreciate its importance re spiral stability / instability and control response. After all, any time the flight path is curving, there is a yaw rotation (unless the slip angle is changing), so the inside and outside wingtips are moving at different airspeeds, which is hugely important to spiral stability/ instability. Likewise when a sideforce prevents a turn which would otherwise normally occur at any given bank angle, the aircraft's tendency to roll left or right will be quite different than if the sideforce were absent and the bank were causing a turn.

Naturally, the detailed step-by-step explanation as to how a sideforce (acting at the CG) can produce a yaw rotation, would involve reference to the aircraft's "directional stability" or "weathervane effect". Initially the sideforce will create an increasing slip angle but no change in heading, but the increasing slip angle will cause the vertical fin, etc to create a yaw torque that overcomes yaw rotational inertia and initiates a yaw rotation. Ultimately we could end up with a situation involving a steady sideforce, a steady yaw rotation rate, a steady turn rate, and a steady slip angle. In some exotic cases (see below) the slip angle may even be zero.

A similar explanation is required to understand why banking the wing creates a turn, rather than just dragging an aircraft sideways through the sky. Fundamentally there is no difference in the way that banking can create a turn, and in the way that sideforce can create a turn.

One interesting example of a turn created by sideforce, is a control-line model airplane flying in a circle with no bank. Here the sideforce is not aerodynamic, but rather created by the pull of the control lines. And the slip angle is zero.

Some other ways to create an aerodynamic sideforce with zero slip (zero yaw string deflection):

1) Attach an auxilliary engine at the CG, thrusting perpendicular to the flight path

2) Attach a moveable rudder at the CG

3) Use a tail-mounted rudder in an unorthodox way: use left rudder to push air to the left and create a sideforce to the right. Normally left rudder would yaw the nose to the left, exposing the right side of the fuselage to the airflow and creating a sideforce to the left that overpowers that smaller sideforce to the right from the rudder itself. We can avoid the left yaw and keep the slip angle at zero by opening an airbrake as needed on the right wingtip. Now left rudder (plus right wingtip airbrake) creates a sideforce to the right. We can fly around in a flat unbanked right turn with zero slip angle (zero yaw string deflection), just as if we had deflected a CG-mounted-rudder to the left. The Northrop YA-9 used this scheme or something similar for improved gun-aiming and/or evasive maneuvers.

4) Same as 3), but increase thrust of engine on left wing rather than open airbrake on right wingtip. We still end up with a rightwards sideforce from the left-deflected rudder, causing a right turn, with no slip (yaw string centered). This will be familiar to anyone familiar with the theory or practice of operation of twin-engined aircraft after losing power on one side (the right side in this example). When exactly enough left rudder is applied to center the yaw string, the aircraft will still tend to turn right, due to the right sideforce caused by the left-deflected rudder shoving air to the left. This "residual" right turn is best prevented by banking to the left (toward the good engine), not by applying additional left rudder. Regardless of whether the turn is stopped by banking, or allowed to continue, the optimal amount of left rudder will cause the yaw string to be centered, and the slip-skid ball to be deflected to the left. There is no net slip (yaw string centered) but there is a net sideforce. If the pilot applies more left rudder to center the slip-skid ball, this will also stop the right turn, now without any need for banking to the left. But now the airflow is hitting the right side of the fuselage (yaw string deflected to left)-- this is not efficient and the sink rate will be greater than if the yaw string is centered and the ball is off-center.

All my previous posts on this thread only make sense when we understand that the presence or absence of sideforce will alter the turn rate that we see at any given bank angle. A corollary is that sideforce can cause a turn even in the absence of bank.

Conversely, in something like a flying-wing aircraft with minimal cross-sectional area as viewed from the side, where sideforce is minimal, even extreme adverse yaw should produce very little slowing of the turn rate for any given instantaneous bank angle. Especially in gliding flight, or descending at low power settings. The yawing of the thrust line could be significant at high power. However adverse yaw still could produce some degradation in roll rate-- during the moments that the slip angle is increasing, the resulting difference in airspeed between the wingtips should slow the roll rate. However once the slip angle is constant (likely true soon after the roll rate becomes constant), even if the nose is severely adverse-yawed off to the side (yaw string deflected), there should be no adverse effect on the roll rate. This wouldn't be true if we had a lot of side area, tending to turn the plane in the opposite direction, and likewise tending to slow the roll rate due to the difference in airspeed between the two wingtips induced by the wrong-way turn, or by the hindering of the right-way turn!

Steve
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Old Jan 29, 2013, 12:11 PM
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Originally Posted by aeronaut999 View Post

sideforce can cause a turn even in the absence of bank.

Steve
For those who don't actually fly models - there are a number of contemporary designs which will do extremely flat, tight turns with no bank angle whatsoever . These designs simply have fuselage lateral area sufficient to the task.
Doing lazy flat 8's is simple with these designs. rudder ONLY is used -
a slight corrective elevator is sometimes required.
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Old Jan 29, 2013, 01:36 PM
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Sorry aero, that's all completely wrong. A turn requires both yaw rate and side force. It is absolutely impossible for a continuous turn to occur with only a single, non-rotating force (F=ma => straight line). Planets orbit because the gravitational force is rotating relative to their path. Planes turn because they continuously yaw and thus the side force is then rotating relative to their path. Planes turn with aileron-only inputs because they slip inward and the vertical tail then causes an inward yaw rate.
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Old Jan 29, 2013, 02:33 PM
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my models will turn in constant flat circles - as I noted - -as will any craft with side area and arranged in same manner The attached model -a simple profile - exhibits this easily
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Old Jan 29, 2013, 03:28 PM
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Any airplane can fly flat circles, even a flying wing with no fuselage or vertical tail whatsoever. All you need is something to create a yaw rate. For example a split aileron/drag rudder. Lateral area greatly improves the effect but it's the yaw rate that makes it possible.

edit: somehow got fixated on yaw rates and forgot what I said two posts up. I should have said that any airplane with any amount of usable side area can perform a flat turn, since a lateral force is still needed in conjunction with the yaw rate. A B-2 Stealth Bomber style aircraft with no usable vertical surface would simply fly straight, but at an oblique angle if yawed.
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Old Jan 29, 2013, 04:25 PM
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Any airplane can fly flat circles, even a flying wing with no fuselage or vertical tail whatsoever. All you need is something to create a yaw rate. For example a split aileron/drag rudder. Lateral area greatly improves the effect but it's the yaw rate that makes it possible.
Yes -
tho some designs do not turn with only banking inputs.
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Old Jan 29, 2013, 09:09 PM
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No, actually if it's a "plane" it will turn whenever banked. By "plane" I mean it's not a missile flying a zero-g ballistic trajectory, and it's not unstable or untrimmed. The reason being that a stable plane in trim must have some decalage or reflex -- by definition. So if you took one of the extreme example planes pictured below and actually trimmed them for stable level flight you would have to have some positive static margin offset by some up elevon trim per the definition of pitch stability. In that case the "wing" will fly at some positive AOA at any bank angle, while the "fuselage" will always try to fly at zero AOA. Hence you'll get a lateral lift component and subsequent sideslip that produces a yaw rate.

If it's unstable in pitch (as many are) this will not be the case. In fact these types of planes are often flown with negative margin and are divergent in pitch and may even require down trim for level flight, in which case they would indeed turn opposite the bank angle -- in theory. I say "in theory" because if it's divergent then there really isn't any point in claiming what it would do in a turn because it can't do anything on it's own and can't fly in any direction without randomly darting in some other direction so that's far outside the realm of applicable theory.

Of course a symmetrical plane like this with exactly zero static margin could be in trim and able to maintain lifting flight at any bank angle without turning, but again this is only theory since it's not actually possible to get a true zero static margin.
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Old Jan 29, 2013, 10:11 PM
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In actual practice the very lightly loaded stuff with lots of lateral area & balanced corrrectly DO simply hold a straight path when the normal level wing flight is altered - weird stuff to fly- I had one of those Hyper Taxi- it was not good at doing this -but other bipes with huge interplanes etc., were very good . George Hicks designed the first one of these I flew - George is a design engineer for an aircraft co and also a very sharp model designer and flier.
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Old Jan 30, 2013, 01:12 AM
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No, actually if it's a "plane" it will turn whenever banked.
Most planes can be made to do a level "steady heading sideslip" (hold a constant bank angle with no turn). In most cases, all that is required is to apply sufficient rudder opposite the bank angle.

An airplane with lots of side area, strong apparent dihedral effect, strong adverse yaw due to aileron deflection, and weak directional stability can even do a steady heading sideslip with no rudder deflection. In extreme cases, an aircraft like this could do a level turn to the right in a bank to the left, with no rudder deflection. This unlikely be a desirable flying quality.
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Old Jan 30, 2013, 10:48 AM
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Sorry aero, that's all completely wrong. A turn requires both yaw rate and side force. It is absolutely impossible for a continuous turn to occur with only a single, non-rotating force (F=ma => straight line). Planets orbit because the gravitational force is rotating relative to their path. Planes turn because they continuously yaw and thus the side force is then rotating relative to their path. Planes turn with aileron-only inputs because they slip inward and the vertical tail then causes an inward yaw rate.
EDIT-- re-reading, the following point seemed lost in the rest of the post so I will say it again up front:

"Once the yaw rotation has increased as needed to keep up with the turn rate (curvature in the flight path), the requirement for a yaw torque vanishes, and depending on various other details at play, sideslip may vanish too."

--END EDIT

What you are missing Vespa is that providing the yaw rate is trivial. It just takes a very small yaw torque and temporary yaw torque, to overcome the plane's yaw rotational inertia and initiate the required yaw rotation. There is no problem with the plane's "weathervane effect" or "directional stability" acting to provide this yaw torque, even if we are using something like a CG-mounted rudder or a sidewise-mounted CG-mounted thruster to drive the turn, which creates no yaw torque in and of itself.

Consider a control line airplane. The control lines connecting near the CG are providing the centripetal force that drives the turn. Is something putting a steady yaw torque on the plane? Is it a requirement, for example, that the plane's rudder be deflected toward the inside of the turn, or that the airflow strike the inside of the vertical fin, to provide the needed yaw torque to keep the turn going? Not at all. At a steady turn rate, net yaw torque is zero.

I stand 100% by all my comments of my previous post. Consider carefully and perhaps you might see things in a different light. Turning by sideforce is fundamentally no different than turning by banking. It is generally much less efficient, but there is no fundamental difference as far as the need for some sort of yaw torque to be imposed upon the aircraft, etc.

It is a misconception that a turn requires a large yaw torque. Whether we are turning by banking the wing or turning by sideforce, the heart of a turn is the centripetal force acting at the CG of the aircraft. Only a small, momentary yaw torque is needed to initiate the yaw rotation, and this yaw torque can easily be provided by the plane's "weathervane effect" or directional stability, after a few brief moments of sideslip create a sideways flow over the aircraft. Once the yaw rotation has increased as needed to keep up with the turn rate (curvature in the flight path), the requirement for a yaw torque vanishes, and depending on various other details at play, sideslip may vanish too.

Even after settling into a constant-bank constant-rate turn, we do usually see some continuous sideslip in a rudderless aircraft. That is not because the slip is "needed" to provide a net yaw torque to power the turn. Rather, that is because the outboard wingtip flies faster and creates more drag than the inboard wingtip, which creates a yawing-out torque, and the only way the net yaw torque can balance out to zero is for the nose to be yawed/displaced a bit toward the outside of the turn (yaw string streams to inside). The resulting sideways airflow interacts with the vertical fin or other equivalent surfaces to provide the yawing-in torque that is equal and opposite to the yawing-out torque from the drag of the outboard wingtip, so that the net yaw torque may be zero.

Of course, steep-bank turns involve more pitch rotation than yaw rotation. That is a different set of dynamics which we could easily spell out step-by-step, but won't, at the moment....

Give it a bit more thought and you will see that the fundamental requirement for a turn, is a centripetal force acting at the CG of the aircraft. We can provide this force by the unorthodox means I spelled out in my previous post, or we can hold lots of inside rudder and do a flat turn while giving whatever roll input is needed to force the wings to stay level, in which case the airflow hitting the side of the fuselage provides the centripetal force that drives the turn, or we can provide the centripetal force in the usual (and usually more efficient) way, by banking. In no case is there a requirement for a sustained net yaw torque. Net yaw torque is zero in a constant-rate turn at a constant slip angle.

Steve
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Old Jan 30, 2013, 10:56 AM
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It is absolutely impossible for a continuous turn to occur with only a single, non-rotating force (F=ma => straight line).
Consider what you mean by "non-rotating". To impose a condition of "non-rotating", you must somehow disable the aircraft's "weathervane effect" or "directional stability" which tends to keep the nose aligned with the flight path or flying at some constant slip or skid angle relative to the flight path. If the flight path is curving and the aircraft is flying at a fixed slip/skid angle relative to the flight path (including a zero slip/skid angle), then the aircraft heading is not constant and the direction of the centripetal force created by our wing, sideways-flying fuselage, CG-mounted-sideways thruster, CG-mounted sideforce rudder, or whatever other device we are using to generate centripetal force, is not constant either.

I'm not saying that a turn involves no net yaw torque. There is a brief net yaw torque applied to overcome yaw rotational inertia and initiate the yaw rotation. Likewise in pitch.

Let's not waste time preaching each other things we already both know. What specifically, exactly, are you saying was wrong with my earlier post re turning by sideforce?

Steve
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