Aircraft lateral stability???

[flame_bringer]

Lately iv been reviewing some aerodynamics, and i came across an obscurity in the lateral stability topic .
Here is my problem :
If the aircraft is disturbed about the longitudinal axes , the down going wing experiences an increase in the angle of attack as the relative airflow is moving down from a position forward of and below the wing,this will increase the lift on that wing , conversly , the up going wing expriences a decease in the angle of attack due to the fact that the relative airflow is moving down from a position forward of and above the wing and due to diffrential lift the aircraft ultimately comes to a state of equilibrium.

Ok what i dont understand is , hows the down going wing experiencing an increase in lift and hows air moving upwards from below the wing and what does that even got to do with the angle of attack ( same with the up going wing ) , for some reason that doesnt make sense to me.
So can anyone clarify for me this vagueness?
Thanks in advance

[TURIN]

Are we talking about a swept or straight wing?

Either way, with a disturbance about the longitudenal axis, IE a roll, it is generally assumed that the a/c will begin to sideslip immediately. The relative airflow will therefore be altered as it will be coming from 'below' the down wing.
It's not that straight forward as one also has to consider whether or not the 'up' wing is now parially 'in shadow' from the fuselage.

A swept wing is even more naturally stable as the 'down' wing becomes automatically more perpendicular to the relative airflow whereas the 'up' wing will be more 'swept' to the relative airflow.

(There will now be a chorus of Bo****ks from the dutch roll fanatics!)

Hope this helps and my clumsy attempt at an explanation doesn't confuse even more. :ok:

[flame_bringer]

Its just difficult to imagine , hows it altering the airflow if it sideslips :S and if so , why does a turbo fan engine stop spooling when we feather its blades , why does it not keep rotating as per this law, it should have an increased angle of attack for the lower blades and a decreased angle of attack for the higher blades so it must keep turning as far as i comprehend this theory.

[TURIN]

Er, are you having me on now?

Turbofan fan blades are not feathered. They are fixed. :confused:

[flame_bringer]

Sorry that was a mistake i mean propeller engines ,

[HuntandFish]

flame_bringer

I think I understand lateral stability when dihedral is present but I dont follow the bit you quote . Is it saying that without dihedral if you roll say 10 degrees the wing then holds that attitude and trying to explain why .

nb I only fly models so dont pick on my ignorance

[TURIN]

That's better.

When a prop is feathered the 'lift' forces on both sides of the blade are in balance IE both sides of the blade create lift but because the angle of attack is neutral they cancel each other out.
There is no correlation to a sideslip in a fixed wing a/c.

Try this. imagine an a/c in straight and level flight. all forces in balance.
A disturbance causes the left wing to drop, this action in itself causes the relative angle of attack of the left wing to briefly increase as the wing is moving down towards the airflow, the a/c starts to sideslip to the left. The relative airflow of the down wing is now coming from slightly to the left instead of straight on.
Dihedral, if present, will also increase the relative angle of the airflow compared to the up wing.

Without diagrams it's difficult to explain. I'm sure there must be some listed on the web somewhere, have a google. :ok:

[flame_bringer]

Ahh i get it now so basically it all got to do with the sideslip, so if it sideslips it moves downwards and this downward movement of the aircraft creates a components of the airflow that flows from below the wing to above .
In this case it makes sense !!
Thanks alot, its just that the notes that im reading are **** , they dont have enough info it didnt even mention anything about a bloody sideslip :\

But i'd substantially appreciate it if you can link me to some diagram or something so i can have a clearer visualization of it, I checked through google for almost half an hour before and couldnt find any drawing .
Thanks again now i can carry on my aerodynamics revision comfortably :D

[flame_bringer]

Huntandfish
It says regardless of wether its dihedral or not , the dihedral angle solely enhances this effect

[doppleganger]

Er... no!

You are all confusing static stability with dynamic stability.

What flame_bringer described in his first post was a perfect description of Dynamic Stability (lateral). Everyone then proceeded to explain the dihedral's effect of Static Stability (lateral).

Here's the explanation:

Dynamic Stability (lateral): Stick your arms out like a kid imitating an aeroplane (palms downwards). Now roll to right - right arm goes down and left arm goes up. Whilst you are rolling you experience an updraft on your right hand, and a down draft on your left hand - but ONLY whilst rolling (hence it's called "dynamic" stability). Any updraft from under the wing is effectively an increase in angle of attack (air hitting underside of wing). Any downdraft from above the wing is effectively a decrease in angle of attack (air hitting top surface of wing). This increases the lift on downgoing wing and decreases lift on upgoing wing. This will stop the rolling movement, but once it has stopped rolling, the imbalance in lift is gone, so it cannot restore the aircraft to the level.

Static Stability (lateral): Now the aircraft is listing to right, but no longer rolling (hence the name "Static" stability). NOW the aircraft starts to sideslip. The dihedral NOW causes a greater aoa on lower wing than on upper wing, and this imbalance restores the aircraft to wings level.

Hope that made sense.

Remember, whatever axis the disturbance is in, dynamic stability halts the deviation, static stability returns the aircraft to neutral.

[TURIN]

That's the chap doppleganger.

I did elude to that here..

Try this. imagine an a/c in straight and level flight. all forces in balance.
A disturbance causes the left wing to drop, this action in itself causes the relative angle of attack of the left wing to briefly increase as the wing is moving down towards the airflow, ....

..in a very clumsy roundabout way.

You are right of course, I just couldn't remember the correct terminology, and then went off on a tangent into Static Stability :ok:

[flame_bringer]

Thank you all for all your helps, i owe you a big time
Now i fully understand lateral stability
regards

[purgatore75]

...try to review also the mathematic topic of vectorial summing (velocity is a vector remember!!!). Acting on the wind we have only the relative wind velocity vector, always parallel to the a/c trajectory, or flight path (called asinthotic wind) ;when the a/c starts rolling on its longitudinal axis, this causes one wing moving upwards and the other one moving opposite direction. For example, try to jump theorically upon the wing going down: is acting a relative wing vector from up to down modifying the previous relative wind velocity, thus causes changing in the direction of the new relative wind.....
Try to immagine the scenario; sitting on the wing you could feel 2 different airstreems: the main stream due to thrust (strongest) and the other one due to the roll movement downward (smallest but strong enough to change direction to the asinthotic wind in such a way to reduce a little bit the angle of attack of the wing).This is calculated upon the theory of summing vectors!!!
:ok: