Rudder effectiveness

[XPMorten]

Hi all,

Wondering if anyone has some information on rudder effectiveness.
I've got information that indicates Airliners are in the
region of 55%-66%. Meaning if an airliner has e.g. has
a 30 degress rudder deflection, it will be able to decrab/slip
about 18 degrees of beta.

Can anyone confirm this? Information on other types of acf is
of interest as well,

Cheers,

XPM

[SNS3Guppy]

At what speed and with what systems involved? With a rudder radio system, for example, rudder deflection is a function of airspeed, and the subsequent yawing action is subject to increasing forces on the vertical stab and fuselage. Increase airspeed, experiencing a greater damping effect and reduced rudder travel, with a reduced offset from streamlined, with a full deflection.

It's a very open ended question.

[XPMorten]

The airliner information above is at landing, decrab 1,3 Vs, full flap, full deflection. As far as I know, this is where you will get the highest beta angle.

Airliners have phase out rudder deflection. Above about 165 kts, rudder
deflection is reduced by gearing systems.

Any situations/speeds etc are of interest since there is very little public
info to find on this.

Cheers,

XPM

[Mad (Flt) Scientist]

That configuration and condition (Vref, landing config) is definitely NOT the critical configuration for rudder effectiveness/sideslip capability on any type I know of.

A couple of points.

"Rudder effectiveness" is really only one component of the ability to generate sideslip; just as critical is directional stability. Because the sideslip you can generate is a balance between how much yawing moment the rudder can generate, and how much restoring yawing moment the sideslip generates in return. (I'm assuming that the roll and pitch axes are not significant, which may not be the case for all types; aircraft using a lot of roll control at high sideslip angles, due to a large dihedral effect, can end up generating significant yawing moment through the roll controls, especially iof spoilerons are being used).

Generally speaking, the rudder is more effective (considered in isolation) at lower Mach numbers (it's a trailing edge control, after all), which is usually not an issue since rudder usage at high speed is small. It's also more effective at lower angles of attack, because you will start to get more and more disturbed flow off the fuselage passing over the fin at higher AoAs. That translates into having less rudder power at near stall speeds than in normal flight.

However, to discuss sideslip capability, we need to consider directional stability. Often an airliner will be significantly less stable at higher Mach numbers, to an extent which overpowers the rudder effectiveness losses. Thus it's common to have to restrict rudder travel in some fashion at higher Mach numbers, to maintain a sensible sideslip envelope 9either from a handling/departure concern, or a loads concern, or both). Additionally, loading issues may require rudder restriction with airspeed as well.

At lower speeds, as the flaps are deployed you usually get a little more directional stability; that's why I'd be surprised to find the landing configuration as the critical case. Instead it's usually either the first takeoff flap setting, or the flaps retracted case, which are the least directionally stable. If the aircraft has slats, chances are the slat out/flap retracted case will be the worst, because it will get to lower speeds and higher angles of attack. FAR25.177 requires that directional stability be assessed "at speeds from 1.13 V SR 1 to V FE, V LE,or V FC/ M FC"; usually the 1.13Vsr case becomes the critical one.

To provide some rough numbers as guidance, from an aircraft with rudder travel electronically constrained:

Max rudder at low speed, landing config: ~25 degrees, max beta 15~20
Max rudder at low speed, approach config: ~20 degrees, max beta 15~20
Max rudder at low speed, takeoff config: ~15 degrees, max beta 15~20
Max rudder at low speed, slats only config: ~10 degrees, max beta 10~15
Max rudder at ~250kts, all configs: ~10 degrees, max beta ~10
Max rudder at Vmo/Mmo: 5~10 degrees, max beta 5~10

You'll note that the max beta for all the takeoff and landing configurations is similar; generally you'll want at least 15 degrees sideslip to get a decent crosswind capability, but more than 20 degrees brings all kinds of nastiness on a swept wing, for no real benefit, so we'd tend to consider that a "design limit". The non-field performance configs just need enough rudder for the OEI trim cases, so there's no need to push the envelope in what is often a less stable config anyway. The high speed cases are, as mentioned, constrained from loads concerns (for max rudder) and yaw damper/trim authority concerns from below.

So the landing config does indeed have a rudder/beta ratio of about 0.6-0.8, but it may be close to unity in other configurations, and even above that for some special cases.

[XPMorten]

Thanks Mad for a very informative answer:ok:

Does Radii of Gyration or mass distribution in any way
play a factor? (assuming CG location is constant)

Cheers,

XPM

[Mad (Flt) Scientist]

Radii of gyration (or moments of inertia) play no part in sideslip capability (though cg does; those sideslip numbers above are really for the aft cg end, and would be correspondingly reduced at forward cgs).

Of course, yaw RATE generation ability is affected by moments of inertia.

[XPMorten]

Thanks again Mad, one final question :)

Does one size the ailerons/spoilers so that one always
have enough roll-power to counter the upwing motion in a slip?
Or - will you "flip over" if you crank in enough rudder?

Cheers,

XPM

[Mad (Flt) Scientist]

Usually the roll controls are indeed enough to control the aircraft in roll out to the maximum sideslip the rudder will give you. However, I've seen specific, limited cases where the aircraft was actually roll control limited first. In such a case you have to continue the test to full rudder anyway, but it's not a stable point of course.

[XPMorten]

Excellent, thanks Mad! :)

M

[deagles]

I remember that Tornado was to be able to land drift free in 30 knots cross wind. The only way it could be achieved was using the dynamic overswing; i.e. kick off drift with full rudder and hit the deck as the Beta arrived at max and before it stabilised at a lower level!