Re: What is angle of sweep and does it affect angle of attack?

Hello Jay: Very good question! The effect of wing sweep on angle of 
attack, especially in a transonic wing, causes loss of roll control during 
a stall. Ideally, the inner wing should stall before the outer wing to 
maintain roll control. In full span stalls either one or the other 
wing “falls” creating an uncontrollable situation. Several things can be 
done mechanically to alleviate this characteristic. Leading edge devices 
usually referred to as “vortilons” can be mounted at the leading edge of 
the inner wing. These devices are essentially “transparent” during normal 
flight. At lower speeds and high angles of attack, they create vortices 
spoiling the laminar flow of the inner wings producing the expected 
warning buffet. See this site on vortilons: http://washingtonpost.com/wp-dyn/articles/A58540-2000Jun3.html

In most cases, stall limiters are also necessary to prevent inadvertent 
stalls. Stall limiters consist of angle of attack sensors that cause the 
control column to mechanically shake at the onset of a stall, and push the 
controls forward near stall. As you can imagine, one must heed the shaker 
at low altitude. I found an Internet site that elaborates on this subject: http://comm.db.erau.edu/esser/wp3.html

The angle of attack (alpha) is defined as the incidence angle of the wing 
with respect to the relative wind. Both angle of attack and the Bernoulli 
effect of the wing generate the lift force that allows the aircraft to 
fly. One of the dramatic effects of the force generated by the angle of 
attack is seen as the aircraft “rotates” on take-off and gain altitude 
quickly (noise abating). This rotation has direct effect on the aircraft 
speed because the forces of drag build-up rapidly in addition to the 
effect of the pitch angle vector (force needed to climb). This is why the 
aircraft pitch angle is soon reduced after lift-off to prevent loss of 
airspeed. Typically the aircraft should never fly at speeds lower than 1.2 
x stall speed. I haven’t addressed the dynamics of the longitudinal mode. 
If you are interested, get back to me through MAD.SCI and I will be happy 
to discuss it.

The following Internet sites also explain with text and graphics how the 
angle of attack is measured: http://www.yesmag.bc.ca/focus/flight/flight_science.html http://www.monmouth.com/~unkfred/buzzards/why.htm

These additional Internet sites are helpful in understanding how an 
airplane works: http://www.gleim.com/Aviation/IntroAirplanes.html http://www.monmouth.com/~jsd/how/htm/intro.html http://www.lerc.nasa.gov/WWW/K-12/airplane/geom.html http://www.monmouth.com/~jsd/how/htm/contents.html

A note of history: during the 1950’s the “Bureau of Naval Weapons” 
contracted Northrop Aircraft Company to define a standard set of symbols, 
angles and vectors, by which an airplane could be modeled (or simulated in 
flight). These standards have been carried throughout industry to this 
day. For this reason Greek letters denote angles and small alpha 
characters denote vectors. This site contains some sample data obtained 
from wind tunnel tests for stability and control analysis: http://naca.larc.nasa.gov/reports/1945/naca-report-825/

Let us explore wing sweep. The faster a plane goes, the easier it is for 
the plane's wings to generate enough lift to support it, but the more 
likelihood there is that some portions of the airflow around the plane 
will exceed the speed of sound and produce shock waves. Since a transonic 
or supersonic plane needs only relatively small wings to support it, 
transport plane design concentrate on shock wave control. Sweeping the 
wings back allows them to avoid some of their own shock waves, increasing 
their energy efficiencies and avoiding shock wave-induced surface damage 
to the wings. Slower planes can't use swept wings because they don't 
generate enough lift at low speeds. 
Wing sweep has its drawbacks because it introduces a lateral-directional 
dynamic instability known as “Dutch roll”. Dutch roll can be explained in 
the simple terms as a “swimming wobble” caused by one of the wings 
developing a bit more lift than the other. As that lift increases, the 
airplane responds by rolling in the opposite direction. The extra drag 
force acting on that wing slows it down. As it slows down, the opposite 
wing speeds-up in similar fashion. The result causes the airplane to roll 
back in the opposite direction. In response to roll, the airplane changes 
course (yaw). At certain points in the “flight envelope” this oscillation 
can become so severe that if the pilot tries to compensate, his lag (or 
delay in reaction) adds to the instability making it much worse. To 
overcome this problem, electronic “black boxes” detect the wobble at the 
onset and correct it with control surfaces working independent of the 
pilot. These “black boxes” are part of the avionics systems and are 
called “yaw dampers” or more commonly “Stability Augmentation Systems” 
(SAS). For clarification on what is meant by “flight envelope” visit this 
Internet site: http://www.elmendorf.af.mil/Units/90FS/aero101.htm

I hope this answers your question. Your MAD.SCI Micro.