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Badmachine
13th Oct 2009, 03:59
In general, would greater airspeed reduce the affect of crosswinds on an aircraft's directional stability or a neutrally positioned rudder's performance?

barit1
13th Oct 2009, 04:02
In flight, do crosswinds, per se, have an effect on an aircraft's directional stability? Tell me all you know on this. :confused:

Badmachine
13th Oct 2009, 05:08
Will affect of lateral force on an aircraft with a neutrally positioned rudder (crosswind) be reduced by greater airspeed?

I'm fairly certain this relationship can be described mathmatically.

STBYRUD
13th Oct 2009, 09:00
If the crosswind remains constant and you have a higher TAS, clearly, the wind correction angle you have to apply decreases - sin(wca)*TAS=WS. Is that what you mean? Also, if you have a higher TAS, then lateral gusts will not affect the flight path and direction as much - so in that sense stability increases :sad:

QJB
13th Oct 2009, 09:09
This has got me interested. I am not sure of the answer, but I might have a guess that the effect of a crosswind is determined the angle of attack that it creates on the tailfin. The angle of attack is due to the direction of the relative airflow, which is a function of the forward speed of the aircraft (TAS) and the strength of the crosswind. Thus higher forward speed = smaller angle of attack change = crosswind has less effect.

Is this reasoning correct?:uhoh:

STBYRUD
13th Oct 2009, 09:56
Yeah, it makes sense to me :}

Daniel_11000
13th Oct 2009, 13:40
The angle of attack is due to the direction of the relative airflow, which is a function of the forward speed of the aircraft (TAS) and the strength of the crosswind. Thus higher forward speed = smaller angle of attack change = crosswind has less effect.

OJB,
If you approach with the 'crabbing' technique(i.e. : no sideslip), there is no 'angle of attack' on the fin, just as when you are flying the airway with a lot of crosswind, as in this case you are just 'pointing' the aircraft into the wind, but you do not have the ball/slip indicator displaced sideways (so, no slip).
You have a sideslip (i.e. : an angle of attack on the fin) only when you are approaching with a sideslip.

barit1
13th Oct 2009, 13:57
It seems to me that several posters are confusing crosswind with yaw. Merely flying steady-state in a crosswind air mass has no effect on directional stability; you merely align the aircraft a bit differently than the ground track; but the airplane doesn't know this - it's still flying in an air mass with no yaw (ie rudder & ailerons neutral). An analogy would be a ferryboat crossing a river; it crabs into the flowing current, but with the rudder centered.

Flying with the rudder displaced is called yaw; if the ailerons are held neutral, the airplane will be in a skidding turn with the skid ball displaced. Crank in a little opposite aileron, and you can stop the turn, but the skid remains. Because this increases drag, and forces you to the side of your seat, you don't ordinarily do it for long. But it can be useful to steepen your approach path for a short-field landing.

STBYRUD
13th Oct 2009, 14:06
The directional stability could be affected when there is a short-term change in crosswind (while the aircraft momentarily maintains its orientation in space) and thus an angle of attack on the fin - if that gust (lets say 5 knots delta) remains constant, the resultant angle of attack is less for a higher TAS. Thats at least what I meant further up. Naturally, said stability will cause the airplane to decrab (if the new x-wind remains).

HarryMann
13th Oct 2009, 22:27
To clarify (a bit)
Yes, of course yaw angle is in relation to a notional reference frame e.g. in stability theory, tells you how far the fuse axis has rotated (in the x-y plane) from a given (initial) direction. Notation = Capital Phi

Sideslip angle (angle to relative wind, freestream directon) = Beta

Yaw originally came from maritime navigation

In maritime navigation only the yaw angle is important. In fact, the word has a nautical (http://en.wikipedia.org/wiki/Navigation) origin, with the meaning of "bending out of the course". Etymologically, it is related with the verb 'to go'[5] (http://en.wikipedia.org/wiki/Yaw_angle#cite_note-4). It is typically assigned the shorthand notation ψ.[6] (http://en.wikipedia.org/wiki/Yaw_angle#cite_note-5)
It is defined as the angle between a vehicle's heading and a reference heading (normally true or magnetic North (http://en.wikipedia.org/wiki/North))*.


* But not necessarily, that just happened to be a convenient reference system for navigational purposes

Wilbur Wright is credited with inventing the yaw string, but there is little doubt it was simply seen to be used on sailing ships and yachts, in fact everywhere for many centuries :)

cwatters
13th Oct 2009, 23:13
>In general, would greater airspeed reduce the affect of crosswinds on an
>aircraft's directional stability or a neutrally positioned rudder's performance?

Does a cross wind actually effect directional stability much? I don't mean how smooth the take off is I mean its stability from a damped system point of view.

Suppose you are half way down the runway and one main wheel goes through some water causing it to yaw a bit. If a cross wind somehow made the aircraft less stable it would yaw a bit more than normal. Does it? or is the fin equally effective in correcting uncommanded yaw ?

HarryMann
14th Oct 2009, 00:11
In general, would greater airspeed reduce the affect of crosswinds on an aircraft's directional stability or a neutrally positioned rudder's performance? As evinced by the scattergun replies so far - there is no such thing as a general case

In fact the whole question is as good as unanswerable, couched in those terms.

In General Terms (normal flight) crosswinds (steady state) have no effect whatsover on directional stability, as mentioned above.
With a step yaw displacement (e.g. side gust or rudder input), then Altitude, Mach No and airspeed all affect directional stability.
For example, dutch-roll interactions between roll, yaw & pitch stability can vary a great deal with speed, and also altitude and Mach No.
Aerolastic effects can also play a part at higher IAS.

At lower speeds, a/c inertial effects are proportionality higher c.f. restoring forces, perhaps that is what is at the root of this question?

Microburst2002
15th Oct 2009, 14:51
It is a shame I don't have my favourite book here to take a look, but I recall that in all cases of stability (longitudinal, lateral and directional) less speed meant less stability, for a variety of reasons and effects.

The crosswind, of course, does not affect directional stability.

The most important thing about airliners' directional stability is to know that the fin is capable to cope very well with gusts. So much so that if, at any speed, we use rudder to arrest a lateral gust we can lose the fin, along with our lives.