Sweepback

[uwochris]

I am hoping someone can shed some light into the benefits of sweept wings in terms of high speed flight.

I have read that sweep back enhances directional stability, but also adversely affects stall characteristics (the tips tend to stall first because of the wing taper).

I do not, however, understand the benefits in terms of high speed flight. The majority of jets seem to have swept wings, so I assume there are some benefits to the design that outweigh the negative stall characteristics. I would believe they influence Mcrit, but I just can't find an explanation of how the two are related (i.e. I am assuming Mcrit is higher for a swept wing, but I do not know why it is higher).

Also, can someone explain what makes sweepback so inefficient for low-speed GA aircraft?

Thanks for any responses,

Chris.

 

[typhoonpilot]

Hi Chris:

I'll take a stab at this, but it is hard to do without some drawing so bare with me and try to visualize some of this.

As we know from basic aerodynamics the local speed of the upper surface air of a wing is higher than the free stream speed. Quite a bit higher with high camber wings. Since this is the case, the airflow over the top of an ordinary straight wing will become sonic well before the free stream. At this airspeed local shock waves are formed on the wing and compressibility effects become apparent; the drag increases, buffeting is felt, and changes in lift and the center of pressure occur. The Mach number where this happens is the critical Mach number and on a typical straight wing airplane would generally be at 0.7 Mach.

By sweeping a wing the velocity vector normal to the leading edge is made less than the chordwise resultant. To visulize this draw a swept wing and then draw two lines from the leading edge; one perpendicular to the leading edge and one parallel to the longitudinal axis of the aircraft. The line that is perpendicular to the leading edge will be shorter than the line that is parallel to the longitudinal axis. As the wing is responsive only to the shorter of the two lines, the one normal to the leading edge ( perpendicular ), for a given mach number the effective chordwise velocity is reduced ( in effect the wing is persuaded to believe that it is flying slower than it really is ). This means that the airspeed can be increased before the effective chordwise component becomes sonic and thus the critical mach number is raised.

As for your question on why light GA airplanes don't have sweep it is because the swept wing will produce less lift than a straight wing for a given airspeed.

Hope that helps,


TP

 

[Mr. Cole]

Based on my basic understanding and a quick review of Aerodynamics for Naval Aviators, I think Ty has it right.


Dave

 

[typhoonpilot]

Lets make that TP instead of Ty, people might start thinking I'm related to Ty Webb .


TP

 

[FracCapt]

I am hoping someone can shed some light into the benefits of sweept wings in terms of high speed flight.

Find a copy of a book titled "Aerodynamics for Naval Aviators". Most large pilot shops have it, and I bet you could even find it on Amazon.com or at a local Barnes and Noble. This book does a good job of explaining the benefits and drawbacks to swept wings(as well as lots of other aerodynamic issues), and has diagrams to make it easier to understand.

 

[FracCapt]

Lets make that TP instead of Ty, people might start thinking I'm related to Ty Webb .


TP

So, instead, you want them to think you're.....well, nevermind...