Flight Controls in a Turn

[mutt]

When an airplane is rolled into a bank, coordinated
rudder and aileron are applied. Accepted. Once
established in the turn, we'll say (for the sake of
argument) aileron is neutral, of course depending on
bank, excepting the fact that steep banks (may)
require some opposite aileron to maintain bank.

Question:
While established in a turn must you maintain rudder
pressure to keep a turn coordinated?



Mutt.

[Sink Rate]

Great question, and one which unstuck me very recently.

Having researched the subject through the manuals it seems that most books say "once in the turn apply rudder to centre the ball".

Not very useful.

I have come to the conclusion, having discussed it and thrashed it out witha number of people, that once established in the turn the amount AND EVEN DIRECTION of rudder is dependent on aircraft design.

I believe older types (very early a/c) suffered from masses of adverse aileron yaw and required OUT OF TURN rudder in a turn.

My thoughts are that it is to do with A/c keel area in front and behind the CofG and especially the size of the fin. that is discounting imperfect rigging of the aeroplane.

It will, therefore, be dependant on IAS, bank angle, CofG position and A/c type.

Being a simpleton I now agree with the books and say that rudder should be used to centre the ball!

That might help a little.

[GlueBall]

Flying large transport category airplanes, the only time I use rudder is in a crosswind on final approach, for directional control on the runway, or during an engine failure. My feet are on the floor while handflying and making manual turns.

[Wot No Engines]

Looking at this using a far more sensitive slip indicator - the piece of wool on the outside of a glider canopy.

Consider the aircraft turning with neutral rudder. The airflow over the nose will be coming from slightly into turn, making it look like you are slipping. A slight amount of into turn rudder will compensate for this.

As a result, some opposite aileron is also needed to keep the bank constant.

[Jhieminga]

Flying large transport category airplanes, the only time I use rudder is in a crosswind on final approach, for directional control on the runway, or during an engine failure. My feet are on the floor while handflying and making manual turns.

Glueball: In your a/c type there's probably a yaw damper involved somewhere, so although there may be some rudder needed in a turn, with the damper switched on the damper will take care of this.

[quid]

Hi mutt=

I agree with Sink Rate here. I've flown different a/c that have diffferent chrateristics. Also, it depends on airspeed and G loads. In fact, due to adverse yaw, the old USAF F-100 would flip you over on your back at high G's if you used too much aileron.

[Crossunder]

Rudder serves mainly one purpose; to counter adverse yaw. Once ailerons are returned to neutral, there is no need for rudder input. However, on propeller aircraft, if the rate of pitch is large enough,the propeller(s) will precess (pitch up will cause right yaw in props rotating clockwise), hence requiring rudder input!

[Truckmasters]

As you've alluded to, It will vary with aircraft type. Don't forget that the aircraft stability in the three different axes will come into play also and will determine factors such as whether you need pro/ anti turn aileron etc.

[BEagle]

1. Maintain the desired angle of bank with aileron.

2. Maintain balance by using rudder.

....effects of controls!!

However, with older swept wind fighters where roll reversal might be a factor at high alpha, individual techniques are needed.

[Celtic Frog]

Rudder input

Strictly speaking, the slip indicator is not exactly accurate so long as it is displaced from the C of G.
Lets say you're flying an aircraft with a mile long fuselage, in a perfectly co-ordinated tight turn to the left, and of course the cockpit is half a mile forward of the C of G.
There would be a G force in the cockpit squashing you and your slip indicator to the right. ( I Think)
It's fairly negligible in most aircraft, but noticeably when trying to soar a glider in marginal soaring conditions. Rudder shold be used to position the slip indicator to slightly lag behind the direction of turn, and this is confirmed by seeing the piece of string being positioned straight on the canopy front.
This is why I tend to think of rudder as being a tool to counteract adverse aileron yaw, although in most aircraft, the slip indicator is still the most accurate way of measuring the slip caused by such yaw.
So long as one aileron is deflected down more than the other, there will be adverse aileron yaw caused by increased induced drag on that downward aileron, unless the ailerons are accurately designed differential ones which cause the upward
aileron to deflect more to compensate.
In gliders the rule is simple...pretend that the rudder is connected to the stick with an imaginary mechanism.
Left stick+left rudder, right stick+ right rudder, in relevant proportions.
If you do that correctly, the piece of string will tell you so, and that's about as basic as it gets. However once you get into larger aircraft, more complex designs, and changing airflow directions due to power plants etc, other effects take place which may either exaggerate or nullify what I've just said.
All far too technical for me...

[bookworm]

While established in a turn must you maintain rudder
pressure to keep a turn coordinated?

I think this question goes to the fundamental issue of what happens when an aircraft "turns", and I'm always surprised that the process is not made more explicit in material dealing with aircraft stability and control. Of course that may be because I have it all wrong, but here we go...

Start with two preliminaries:

1) There's a difference between turning and yawing, which I'd like to make explicit for the purpose of this discussion. When a moving object turns, its velocity vector (the direction in which the rigid body is moving) changes direction. When an aircraft yaws, its heading (the direction in which the rigid body is pointing) changes direction. Because aircraft are generally very directionally stable, we tend to think of the two as a single action -- but they're not. In principle a body can yaw without turning (picture a spacecraft like the Apollo command module manoeuvring through 180 degrees to dock with something that was behind it while continuing at high velocity towards the moon), or turn without yawing (picture an object bouncing off a brick wall). The difference between the direction an aircraft is moving and the direction in which it is heading is the sideslip angle.

2) What does the fin do? It applies a yawing moment to the aircraft. When? Well two subtly different circumstances, in fact. a) If the aircraft is in a sideslip, the fin gets an angle of attack and applies a yawing moment. The size of the yawing moment is proportional to the sideslip angle. b) If the aircraft yaws, the fin also gets an angle of attack and applies a yawing moment. If you imagine the aircraft yawing left about its centre of mass, the fin moves right. That means that the airflow is no longer aligned with the fin, so there's an angle of attack. The yawing moment comes out as proportional to the rate of yaw, divided by the airspeed and multiplied by the distance of the fin from the centre of mass. The direction of the yawing moment is against the yaw that's causing it.

OK, now the process of turning. Let's go left, starting from a track of 180, heading of 180.

So we bank the wings to the left and tilt the lift vector. We now have an unbalanced force to the left, so the aircraft accelerates left. It turns. But I haven't given it a reason to yaw yet! It now has a track of, say, 175, and a heading still of 180. That means it's in a sideslip to the left. So the fin starts doing its job and applies a yawing moment to the left. And the aircraft starts to yaw to the left. How quickly? Well remember b) above. The fin also applies a yawing moment to the right, because it now has a rate of yaw to the left. The steady state comes where the rate of turn is equal to the rate of yaw and the moments balance each other out. In an equation,

sideslip_angle = rate_of_yaw * fin_moment_arm / airspeed

So although the aircraft does yaw in the direction of its turn, the change in heading lags the change in the velocity vector slightly, by the sideslip_angle above. Instead of slipping, you can of course apply into-turn rudder to balance the right hand side.

How big is the effect? rate_of_yaw = 3 deg/s, fin_moment_arm = 10 m, airspeed = 100 m/s gives a sideslip angle of 0.3 degrees. Not a lot, but sufficient that if you don't add a little rudder, you will slip slightly.

Why do we notice different effects on different aircraft? As you can see from the above, a slow and long aircraft (e.g. a glider) that turns rapidly sees a much greater sideslip angle than a fast, short aircraft turning at even the same rate.

Hope that makes sense.