Static Aeroelasticity: Swept Wing

Question 1

What are the reasons to use a sweep angle? (and the reason to not use a swept forward wing).

Answer 1

Swept back wings:
1. To improve longitudinal stability by changing distance btween the AC and CG of the wing.
2. To delay transonic drag rise (compressibility). The critical mach number is increased, transforming normal to obilique shockwaves.

Swept forwards wings:
1. Aeroelastic problems, although the design is lighter because the attachment is lighter.

Question 2

What happens to the torsion and bending loads for a swept wing?

Answer 2

• The structural reference frame is rotated by the angle Λ with respect to the aerodynamic reference frame
  *The aerodynamic forces developed by the airfoils are aligned with the asymptotic speed
• Torsion and bending are about the axis of the structural reference frame

1) Only a portion of the torsion of the structure results in a change of AoA. The rest is change of direction of lift. It produces two angles, Δα and φ.
2) When a swept back wing bends up, the AoA of streamwise sections will reduce.

Question 3

Discuss the divergence of a swept back typical section.

Answer 3

A rigid model is introduced with two springs at the root representing the torsional and bending stiffness.

qD: The higher is Λ, the higher qD is.

1) If Λ>0 and e > 0, it is possible to identify a critical sweet angle Λcrit > 0 above which no divergence exists. Specifically, qD grows until it reaches infinite at Λcrit and then starts becoming negative with a modulus that reduces the higher is Λ.
2) If Λ<0 (swept back wing), the higher is |Λ|, the lower is qD. There is a fast drop of the divergence speed for negative sweep angles (below -30 degrees, the speed is close to zero).

Question 4

How does the swept back wing affect lift effectiveness?

Answer 4

For Λ=Λcrit, the elastic lift is equal to the rigid one. For sweep back angles below this one (including negative ones), the lift efficiency is increased. For large positive sweep angles, the divergence disappears but there is a reduction of the lift effectiveness.

Question 5

How does the swept back wing affect control reversal?

Answer 5

• Increasing Λ, especially above Λcrit, qRΛ is increased, but the divergence dynamic pressure qD <0 becomes more negative decreasing significantly the lift effectiveness.
  *Reversal speed may grow, but due to elastic deformations (also bending causes a reduction of lift) the control effectiveness is rapidly reduced.

Question 6

What is aeroelastic tailoring?

Answer 6

It is the coupling of torsion and bending stiffness. It can be accomplished by using composite materials with appropriate directions for the deposition of the fibers. Due to this coupling, it is also possible to obtain a negative critical sweep angle if k (coupling stiffness) is larger than 0 and large enough in modulus. It means to implement a wash-out effect on the forward swept wing.

Question 7

Summarize the effects of swept wing to the divergence speed, lift effectiveness and controllability.

Answer 7

Positive sweep
*Increases divergence speed, that often dissapears
*Decreases lift effectiveness
*Decreases controllability

Negative sweep
*Decreases divergence speed, that often dissapears
*Increases lift effectiveness
*Increases controllability

Aeroelastic tailoring can be used to change this behavior.