Forward-Swept Wings

Since sweep produces effects that vary with cos(sweep), we might expect that either forward or aft sweep would yield the same results. To a first approximation, this is true; but, many other considerations can be important in comparing designs with forward and aft sweep. Historically these have led designers to adopt aft-swept wings for most aircraft, but this was not universally true. The Hansa Jet was a forward-swept wing business jet designed in the 1960's. Its forward swept wing permitted a larger cabin without a wing spar interrupting the floor. Some sailplanes have slight forward sweep to provide better visibility. Recently there has been renewed interest in the forward swept wing concept for aerodynamic reasons and a demonstrator / research aircraft, the X-29 was built by Grumman for NASA, DARPA, and the Air Force.



Several aerodynamic advantages of the forward swept wing have been suggested. One of the more interesting of these is illustrated below. The claim is that the lower surface of a swept forward wing contributes a larger share of the total lift than the lower surface of an aft-swept wing.

Although exaggerated in this figure, this effect is predicted and observed. It is due in part to perturbation velocities induced by the 3-D thickness distribution and in part to the velocities induced by streamwise vorticity.

Some of the advantages and disadvantages of forward sweep:

Advantages

• Better off-design span loading (but with less taper: C_l advantage, weight penalty)
  
• Aeroelastically enhanced maneuverability
  
• Smaller basic lift distribution
  
• Reduced leading edge sweep for given structural sweep
  
• Increased trailing edge sweep for given structural sweep - lower C_Dc
  
• Unobstructed cabin
  
• Easy gear placement
  
• Good for turboprop placement
  
• Laminar flow advantages?
  

Disadvantages

• Aeroelastic divergence or penalty to avoid it
  
• Lower |C_lβ| (effective dihedral)
  
• Lower C_nβ (yaw stability)
  
• Bad for winglets
  
• Stall location (more difficult)
  
• Large C_m0 with flaps
  
• Reduced pitch stability due to additional lift and fuse interference
  
• Smaller tail length???