Slip vs Spiral Instablilty

[hitman2726]

Hi all, I am now studying the content of flight profile and i find that it is confusing about the adverse effect during rolling.

When an aircraft rolls to left, the left wing goes down and the right one goes up. At the same time, the induced drag at right hand side is greater than that at left hand side. This causes the adverse ailerons yaw and the aircraft is said to be slip~~ This effect could be eliminated by applying appropriate left rudder power. And i understand it.

In case of spiral instability, the aircraft banks to left and same effects are generated as mentioned. However, the centrifugal force will cause the relative air flow towards the fuselage of the aircraft. Without the pilot input and the large fin (high directional stability), the aircraft pitches downward and the turning effect is worsen as outer airflow moves faster than the inner one. Result in spiral dive.

So, I would like to ask and seek your advice on:

Will spiral dive only occur in aircraft with large fin (high directional stability) and no pilot inputs ,otherwise, Dutch roll?

Hitman

[foxmoth]

Keeping it as simple as I can
Adverse aileron yaw is caused by the ailerons being up on one side and down on the other giving a difference in drag due to being in different pressure air, this will only happen when the ailerons are deflected i.e. as you roll in (and is usually compensated for in modern aircraft by deflecting the ailerons different amounts up and down - have a look on your next walkround how much more they go up than down at full deflection).
Spiral instability occurs when you are established in a turn so the two things happen at different stages.

[Andy_RR]

Aerodynamic stability does not include control inputs. It is the natural tendency of the aircraft to converge to straight-and-level without any control inputs.

see Wikipedia for some reasonable descriptions of aerodynamic stability.

[hitman2726]

So, foxmoth, as you says that these two phenonmenons take place at two different stages. Does it mean that the a/c will still experience a sprial dive even the adverse aileons yaw effect is compensated by appropritate rudder power?

Hitman

[foxmoth]

Wrong bit for the spiral dive effect - Adverse aileron roll will act in the opposite direction to the roll, Left aileron up/right down = roll left, left aileron is in lower pressure air than the right so yaw right so with secondary this effect of roll this opposes rolling into the turn.
Once in the bank the aircraft will tend to slip in towards the center of turn if back pressure is not maintained (look at the forces in a turn diagram), this will then give an aerodynamic turning force to the left - i.e. Yaw left, and this will develop into the spiral dive so it is not the adverse aileron yaw that causes the spiral dive.:ok:
Going back to the origional question, spiral dives will occur in nearly all aircraft (there are some that have side areas fairly evenly distributed, but not really in a modern light aircraft) and yes, a large fin will tend to spiral dive a little quicker than a small fin - but generally both will do it.
Dutch roll is a seperate subject again, most light aircraft are not subject to it though it can happen. Dutch roll is a oscillatory condition which can be seen by the aircraft nose gently(normally) rotating about a point on the horizon. This tends to occur on airliners and is compensated for with yaw dampers.

[Shaggy Sheep Driver]

You're talking about 2 different things:

Adverse yaw:
Upgoing aileron produces less drag than downgoing one (more lift = more drag, less lift =less drag). Therefore aeroplane will tend to yaw away from the direction of roll. The competent pilot will prevent this by use of co-ordinated rudder with aileron.

Some aircraft have differential ailerons (they go up further than they go down), and /or frise ailerons (the upgoing one protrudes below the wing as well, creating extra drag to lessen the adverse yaw). Both of these techniques spoil the roll performance of the aeroplane, so older types without theses aids and with consequently better handling are much nicer to fly, but demand correct use of rudder to counteract adverse yaw.

Try this in a Chipmunk: Pick a point on the horizon, and roll the aeroplane rapidly left-right-left-right without use of rudder. Notice the chosen point occilate left - right-left of the nose as you do so and the adverse yaw takes effect. Now do the same thing but using co-ordinated rudder with aileron. the aeroplane will roll more crisply, and more importantly the roll will be axial with no yaw - the chosen point will be nailed to the spinner.

Spiral stability:
Piloted aircraft are spirally unstable. that is, if a wing drops the aeroplane will slip toward the dropped wing, the nose will drop, and the speed will increase. If the pilot does nothing about this, it will develop into a spiral dive with a very nose-low very steep spiral and the speed building until it exceeds Vne and soemthing (usually the tailplane, followed imediately by the wing) breaks.

Free-flight model aeroplanes are designed to be spirally stable. By use of large dihedral angles, the aeroplane has so much roll stability that a wing drop will be self-corrected, and a spiral will not develop. Early WW1 aeroplanes were often sprally stable, which explains how they could fly in IMC without use of gyro instruments.

[foxmoth]

You're talking about 2 different things:
I think that is what I said.:hmm:
so the two things happen at different stages.

[Shaggy Sheep Driver]

I think that is what I said.:hmm:

No. You said they happen at different stages. I said (among lots of other stuff) they are different things.:)

[foxmoth]

But I still said
so the two things happen at different stages.
and then described them differently
:p

[hitman2726]

Thank you mates, you have made me understand these two things~~~

But, i am more curious about the mechanism to generate the Dutch roll. I have read the book and it says Dutch roll, in fact, is a consequence of the victory of wing dihedral over the spiral instability of a/c. Is it true?

Hitman

[foxmoth]

That is basically correct. Dutch roll can be gone into in great depth.
This is quite good for a basic description.
http://en.wikipedia.org/wiki/Dutch_roll
note particularly the following :-
Dutch roll results from relatively weaker positive Directional Stability as opposed to positive Lateral Stability. When aircraft rolls around the longitudinal axis, a sideslip is introduced into the relative wind in the direction of the rolling motion. Strong Lateral Stability begins to restore the aircraft to level flight. At the same time, somewhat weaker Directional Stability attempts to correct the sideslip by aligning the aircraft with the perceived relative wind. Since Directional Stability is weaker than Lateral Stability for the particular aircraft, the restoring yaw motion lags significantly behind the restoring roll motion. As such, the aircraft passes through level flight as the yawing motion is continuing in the direction of the original roll. At that point, the sideslip is introduced in the opposite direction and the process is reversed.
which is pretty much what you were saying but in more depth.
This is not often a feature in light aircraft.

n.b. I would completely ignore the article at the end, about the manouver, apart from where it says "This coordination technique is better referred to as "rolling on a heading",", this is not what most people mean by Dutch Roll.