Manoeuverability, manoeuver stability and stick force stability

[manucordier]

Good afternoon,

I have been wondering a few things about vocabulary after having seen the following question out of Fly Around database (Principles Of Flight) :

The (1) stick force stability and the (2) manoeuvre stability are positively affected by:

b) (1) forward CG movement (2) forward CG movement
(this is the right answer).

- I wanted to know what does "Manoeuver stability" exactly means.

- Is that right to say an aircraft with high manoeuver stability has low
manoeuverability ? Which would mean that the more stable an aircraft is
the slower it responds to pilot inputs.

- What does "Stick force stability" means ? I do know what "stick force" is
but truely I do not see any sense in adding the word "stability" at the end
of that expression.

Thanks a lot.
Emmanuel Cordier.

[Keith.Williams.]

The term “Static Stability” refers to the manner in which an aircraft initially responds to changes in its attitude in flight. A highly stable aircraft will have a strong tendency to resist changes in its attitude.

The term “Manoeuvrability” refers to the ability of the pilot to change the attitude of an aircraft in flight. A highly statically stable aircraft will oppose such changes, so in this sense stability and manoeuvrability are opposing qualities. So increasing static stability decreases manoeuvrability.

The term “Manoeuvre Stability” refers to the force that must be applied to the stick to achieve any given change in g loading. An aircraft that is highly manoeuvre-stable, will require large stick forces to achieve large changes in g loading.

The term “stick force stability” refers to the stick forces that are required to achieve any given change in airspeed from a trimmed condition.

The principal factor that determines manoeuvre stability and stick force stability is the longitudinal distance between the Centre of Gravity and the Neutral Point. The Neutral Point is a point on the longitudinal axis of the aircraft. If the aircraft C of G is located on this point then the aircraft will be neutrally stable. In this condition the stick forces required to achieve any change in g load or airspeed is zero.

If the C of G is in front of the Neutral Point then the aircraft will be positively stable. The further forward the C of G the greater will be the manoeuvre stability and the greater will be the stick force stability. If the C of G is located behind the Neutral Point then the aircraft will be unstable. So the stick force stability and the manoeuvre stability are positively affected by forward movement of the Centre of Gravity.

The matter is further complicated by the fact that in pull-up manoeuvres, the downward sweeping motion of the stabiliser increases its angle of attack. This produces a nose down moment which oposes the nose-up pitching motion. This has the effect of temporarily increasing the longitudinal stability of the aircraft. So during such a manoeuvre the C of G could be slightly further aft than the Neutral Point without resulting in neutral stability. This further aft point is called the Manoeuvre Point.

[manucordier]

Thank you very much. It couldn't have been clearer !

Emmanuel Cordier.

[manucordier]

I found this question in my Fly Around JAA bank : (Principles of flight)

With increasing altitude and constant IAS the static lateral stability (1) and the dynamic lateral/directional stability (2) of an aeroplane with swept-back wing will:

Right answer: (1)increase (2) decrease

I understand that with increasing altitude the air density decreases thus any control surface is less effective. Therefore it makes sense to say that the vertical tail fin is less effective and thus directional static stability decreases.

However I don't understand those 4 points:

- Why dynamic directional stability decreases ?
- Why lateral dynamic stability decreases ?
- Why static lateral stability increases ?
- Why static lateral stability increases while dinamic- decreases ?

Thank you very much to whom will be able to write a clear explaination to that.

Have a good night,
Emmanuel Cordier.

[B2N2]

It's been a while but let me give it a try.
Given:
Higher altitude same IAS. For the IAS to remain constant with increasing altitude means a higher TAS.
Same IAS, higher TAS.
Aerodynamic dampening is a function of TAS.
Less tail angle of attack means less dampening.
Static stability is the "willingness" to change direction.
With the center of pressure forward of the CG as it should be static stability increases with TAS.
Dynamic stability is the "willingness" to return to the starting point and how.
Is it dampening or overshooting the neutral point.
Since the dampening effect is reduced the dynamic stability is reduced.

Static stability increases, dynamic stability decreases.
I had some help figuring this out:

High-Altitude Handling (http://www.boeing.com/commercial/aeromagazine/aero_02/textonly/fo01txt.html)
http://www.flightlab.net/Flightlab.net/Download_Course_Notes_files/6_%20LongitudinalManeu%232BA152.pdf
Interesting PowerPoint about high altitude stability:
http://flightsafety.org/files/Appendix_3-E_HighAltOperations.pps

[B2N2]

See my reply to your other question.
The links explain this as well.
:ok:

[Halfwayback]

There are well qualified folk in the forum to help with this so I'm moving it there!

Good luck

HWB

[Pugilistic Animus]

just to add to what B2N2 had written for the second series of questions with regard to dynamic stability two factors come into play---

one is reduced aerodynamic damping -caused by the control surfaces working through a smaller effective smaller angle ----if you have a high TAS then your the sideways component of damping forces relative to the forward component is decreased as TAS increases. this best seen in an illustration

also the overall effectiveness of the control surfaces are diminished due to compressibility and flow separation therefore less of the stabilizing surface of a particular control or stabilizing surface--- Vertical stab, chines etc.. is aerodynamically available--

-with respect to static directional and lateral stability... it is also attributable to the higher TAS due to the fact that the amplitude of any directional divergences are lessened for the exact same reason as above A large forward velocity component relative to any sideways or upward velocity components about the vertical or longitudinal axis ...for a compilation of 'acceptable and unacceptable stability' I would recommend Hugh Hurt Jr's text Aerodynamics for Naval Aviators and D.P Davies Handing The Big Jets

I'm sure I could have said it better:)

[WeekendFlyer]

One small addition to the above! As altitude increases, air density decreases; this reduces damping, which leads to the decrease in dynamic lateral and directional stability.

A helpful way (if not strictly correct) to think about it is that as an aircraft oscillates laterally, the wings and tailplane move up and down through the air and this creates drag opposing the motion and damping it. If the air is dense (sea level) the drag force is higher, and if the air is thin (hot and/or high conditions) then the drag is lower because there are less gas molecules getting in the way of the motion. With the directional case the tail is moving side to side, as is the fuselage, so this creates side-to-side drag which damps the directional oscillation.