Wing Anhedral and effect on roll stability and control

[aeroseek]

Many fighter aircraft have anhedral on their wings - F-104, Dassault Mirage III.

What is the effect on control - is it neutral roll stability through 360 and more important, what is the effect on stability - is there a 'notch' effect if there is no dihedral where the aircraft tends to assume wings level if left to itself?

What is the effect on a free flight model of such an aircraft - will it be unstable if the scale anhedral used in the aircraft is actually used on the model?

What is the effect on performance since a component of lift is directed outwards?

 

[rcgldr]

Swept back wings have a similar affect on roll stability as dihedral. The roll stability is due to yaw to roll coupling. If the aircraft yaws (either rudder input or a slip to one side), then the higher (dihedral) and/or more forward (swept back) wing generates more lift and the other wing generates less lift. There are radio control models that use dihedral without ailerons that use the rudder to induce yaw, which in turn induces roll.

Anhedral in the wings is used to reduce or eliminate the swept wing yaw to roll coupling (little or zero roll stability), depending on the aircraft. The reduced or eliminated roll stability would not work with a free flight model. Since fighters cruise well above the air speed for best lift to drag ratio, the main component of loss is parasitic (form or profile) drag. I'm not sure of the combined effect of swept back wings and anhedral on overall performance, but lift induced drag is a small component of the overall drag at high cruise speeds.

 

[montoyas7940]

The purpose of anhedral on very high performance aircraft is to destabalize. Vertical stabilizers can become too effective at high speeds. When rudder is used during a roll it can oppose the roll forces provided by the wings and ailerons. Anhedral helps make the roll "snappy".

A free flight scale model may not be stable enough at scale speeds and roll upside down.

 

[strive]

As the first answer has pointed out anhedral is primarily used to counter the positive stability effect of swept wings on roll control. Anhedral is also used on cargo planes with high wing configuration, as those planes can be so stabile (by roll axis) that they can barely turn (rudder does little if the plane is unable to roll when you apply it).

As for lift goes: yes lift does slightly increase with anhedral. You should be able to find research about that on the internet.

A model will not be unstable if you counter the anhedral with back sweep… if I remember correctly the rule of thumb is that 1° of anhedral counters 7° of sweepback.

The “scale” speeds play no role in this… unless you are going transonic and beyond…

Whether or not the plane will be positively stabile by roll axis trough entire 360° rotation (although if the plane is symmetric you only have to check first 180°) will depend on several factors, but it is possible to make it stabile.

 

[aeroseek]

The purpose of anhedral on very high performance aircraft is to destabalize. Vertical stabilizers can become too effective at high speeds. When rudder is used during a roll it can oppose the roll forces provided by the wings and ailerons. Anhedral helps make the roll "snappy".

I understand most of the concepts explained, but I need to clarify a few things.

Firstly, the effect of the vertical stabilizer at high speeds. Then an aircraft is rolled, (say the F-104) there is a force opposing the roll from the vertical stabilizer - force due to roll rate if I am correct. Now since this force acts at a point behind the CG won't it cause a yawing moment since the area ahead of the CG has less of a resistance to the roll rate? How is this corrected? By using the rudder - at high speed?

Secondly, by roll stability do you mean the aircraft tends to return to wings level or resists the rolling forces of the ailerons?

Some good info here:

http://books.google.lk/books?id=hBxBdKr0beYC&printsec=frontcover#v=onepage&q&f=false

 

[strive]

The vertical stabilizers produce a yawing moment, but this does not necessarily act to stabilize the airplane by its longitudinal axis, it can actually make it worse as it turns the plane inwards and thus further decreases the velocity of the air over the inward wing in relationship to the velocity of the air over the outward wing, thus increasing roll.
It's about wing geometry not about stabilizers.

Secondly, by roll stability do you mean the aircraft tends to return to wings level or resists the rolling forces of the ailerons?

We mean that the plane levels the wings.

 

[montoyas7940]

Secondly, by roll stability do you mean the aircraft tends to return to wings level or resists the rolling forces of the ailerons?

There are two kinds of stability. Dynamic and static.

http://www.free-online-private-pilot-ground-school.com/Aeronautics.html
http://www.allstar.fiu.edu/aero/axes33.htm

Many factors have an impact on both types. CG, high wing/low wing, dihedral/anhedral, wing sweep, engine location(s) and even gear position. You specifically asked about the F-104 and Mirage. They have some silmilarities and some significant differences. (The Mirage is even tailless.) Because of this the responses to your question are all over the place.

Narrowing the discussion, I can tell you the F-104 wing vs vertical stabilizer size was an issue mitigated by anhedral. The F-104 also has trapezoidal wing with very little sweep.

If I had to speculate, I would expect a scale free-flight F-104 to very quickly enter a steep spiral dive due to a lack of dihedral effect coupled with a strong directional stability and no control inputs. I think, eventually given enough altitude, it will end up nose down and inverted.

 

[rcgldr]

Narrowing the discussion, I can tell you the F-104 wing vs vertical stabilizer size was an issue mitigated by anhedral. The F-104 also has trapezoidal wing with very little sweep.

Wiki article about this:

Because the vertical fin was only slightly shorter than the length of each wing and nearly as aerodynamically effective, it could act as a wing on rudder application, rolling the aircraft in the opposite direction of rudder input. To offset this effect, the wings were canted downward, giving 10° anhedral.

F104_Airframe.htm

Note that the article is including the rudder as part of the vertical fin, so that left rudder application makes the combined vertical fin and rudder behave like a cambered wing, producing a right roll torque that is countered by anhedral (to produce an opposing left roll torque). The same idea would apply if the F104 was in a left slip.