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Nov. 30, 2006

## Interactive Quiz #114: Aerodynamically Speaking

When airfoils slam into enough innocent air molecules at just the right speed, lift results. Seems like magic, but there is a little science involved. So let's explore a few basic tenets of aerodynamics.

INSTRUCTIONS: Answer the questions as best you can, then click on the "Score my quiz answers" button to see your score and read the explanations. If you don't like your score the first time around, you can change some of your answers and resubmit. To get the most out of this quiz, we suggest you keep trying until you get a perfect score.

NOTE: When more than one answer is true, only the most complete, correct answer will be scored as correct. The answers are assumed to apply within the United States unless otherwise noted.

1. Glider pilots are smarter than anyone else. They have to be in order to solo at 14 and fly around without engines. They also claim to understand how that's possible, and they like to intimidate their airplane pilot friends by tossing around aerodynamic terms when describing a wing's attributes. "Aspect ratio" is a favorite term, and we know that you know that the higher the aspect ration the greater a wing's lifting ability. So, regardless of what you fly, please define aspect ratio by choosing the best answer:
a. The ratio between wingspan and the wing's mean chord.
b. The product of the wingspan and the wing's mean chord.
c. The area of the wingspan and the aircraft's empty weight.
d. The ratio between the wingspan and the wing's mean weight.
2. Picture a wing's profile. Chord is a line between the wing's leading edge and trailing edge (some say it's an imaginary line, but we believe it). That line forms an angle between itself and the relative wind. This is called the angle of attack (or AOA when text-messaging your friends). Usually as AOA increases, so does lift. Nothing good lasts forever and if you exceed the (_____) AOA, a stall results. Fill in the blank with the best term:
a. Minimum
b. Extreme
c. Excessive
d. Critical
e. Maximum
3. Please fill in more blanks: Imagine you're operating at high altitude with oxygen masks in place. Compared to sea level operations, the aircraft's true airspeed will (_____) for a given indicated airspeed. The aircraft's indicated stall speed will (_____).
a. increase, remain unchanged
b. decrease, remain unchanged
c. increase, increase
d. decrease, decrease
e. increase, decrease
4. From the earliest ground school lessons pilots study the aircraft's center of gravity (CG). Tailwheel flyers are keenly aware of the CG being located aft of a tail dragger's main gear, adding to the tail's demonic propensity to swing itself toward the nose just when you relax, thinking you've made a swell landing in front of thousands of spectators at Sun 'n Fun. Speed hounds know that an aft CG generally makes an airplane fly faster. We're so well-versed on CG that this question has nothing to do with it. Instead, please decide which answer below best describes center of pressure (CP), sometimes called center of lift:
a. Intersection of lift with the mean camber line on an airfoil
b. Intersection of lift with the chord line on an airfoil
c. Intersection of lift with the thrust line on an airfoil
d. Intersection of lift with the dry line on tinfoil
5. Most pilots know that a forward CG generally makes an airplane easier to handle in stall recovery. So, if airplane designers locate the CG in front of the CP, this will provide an adequate restoring moment for flight equilibrium. Don't nit-pick this concept too much or you'll stall, spin, crash and never finish the quiz.
a. True
b. False
6. More blanks in need of filling: The center of pressure usually moves (_____) as AOA increases and moves (_____) as AOA decreases. Think subsonic.
a. slightly, rapidly
b. aft, forward
c. laterally, longitudinally
d. forward, aft
7. Complete the sentence: The acute angle between the wing chord and the aircraft's longitudinal axis is called the angle of (_____).
a. Attack
b. Incidence
c. Lift
d. Repose
8. A pilot can easily change the wing's camber by using flaps. Imagine what happens as you add full flaps in your Cessna 172. The flaps change the wing's camber by decreasing the upper camber and by creating a positive lower camber.
a. True
b. False
9. One for the homebuilders out there in the garage: If you build it, will it fly? More importantly, will it be stable? Think of stability as the airplane's ability to fly hands-off in a straight-and-level flight. An aircraft in a steady state of flight -- not deviating from its path -- exhibits perfect, Zen-like static stability. Dark forces such as wind gusts, however, will disturb the equilibrium. How well you designed, built and maintained your aircraft will determine how well it deals with forces disturbing the equilibrium. The three types of static stability are:
a. Positive, Negative, Neutral
b. Dihedral, Anhedral, Cathedral
c. Longitudinal, Lateral, Vertical
d. Pitch, Roll, Yaw
10. Aerodynamic excellence can be compromised by nature reshaping your airfoil. In-flight icing is a threat any time of year somewhere above the planet. Rime or clear, a thick accumulation of ice on wings not only adds unwanted weight to an aircraft that may be operating near gross, but ice also alters an airfoil's shape. Changing the shape of a wing changes stall characteristics and usually not for the better, so ice avoidance and removal is obviously important. Frost is regularly present in cold-weather preflight operations but, because its coating is thin, it adds little weight to the aircraft and unlike ice, which builds in ugly blobs, frost does not significantly alter airfoil shape and will blow off quickly once an airplane reaches lift-off speed.
a. True.
b. False

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