Airplane propellers generate most of the noise that is the source of complaints (at least those against piston and turboprop airplanes). The noise becomes much worse as propellers reach transonic speeds. If airplane wings can spoof critical Mach numbers by means of their sweep-back, why aren’t the propellers on small airplanes also swept back?
A) Hydrodynamics isn’t intuitive, and neither is the behavior of fluid motion. The fact is that sweep-back totally loses its effectiveness on smaller scales, where airfoil chords are measured in inches, rather than in feet.
B) Although it is true that small airplane propellers could benefit to some extent from even a small degree of ‘back sweep’, generally most low speed airplanes wouldn’t benefit significantly from it.
C) Most people couldn’t afford it. Scimitar-shaped propellers are actually common among racing planes, but at a cost of about $20,000, they are beyond the reach of most.
D) They don’t need to be. Most people don’t realize it, but the very property of increasing tangential velocities at greater and greater distances from the center of rotation effectively gives propellers a forward sweep, which actually accomplishes the same thing.
Answer: There are a few advantages. Although swept-back rotating airfoils experience span-wise stresses on the blades, due to centripetal force not being parallel to their length, propellers with some type of sweep-back have been researched and designed. They’re shaped a bit like scimitars. On the more pronounced versions, there are usually eight or more blades, and they have actually been used on some high performance turboprops. (Some piston propellers have a slight sweep-back, but they’re not all that eye-catching.) They are expensive to design and manufacture, and swept-back airfoils generate less lift and thrust than straight ones. However, they do generate less noise and drag, to a small extent. The correct answer is B.
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Based upon the FAA’s own data, which do you suppose have been the hardest FAA written tests?
Aircraft Dispatcher, Ground Instructor, ATP Helicopter, Flight Instructor Gyroplane, Inspection Authorization.
The FAA publishes average scores and pass rates for over 60 certificates and ratings, beginning with data for the year 2000. You’d be surprised at the results. Apparently, the basic Ground Instructor exam, the one for Flight Instructors adding a rating in gyroplanes, and the exam for Pilot Examiners in the glider category were really tough on applicants in 2003. The overall average pass rate for all 62 types of exams in 2003 was about 94% (and the average score was about 86%). And for these three exams mentioned above, the respective failure rates in 2003 were roughly one in six, one in five, and one in every three! (But the average scores were around 83% for all three.) One applicant in every four taking the exam for Inspection Authorization in 2002 flunked. In 2001, the failure rate for Inspection Authorization exams was one in four; the failure rate for Private Pilot Lighter-Than-Air (Airship) was one in three, and for Flight Instructor Gyroplane (as an added rating) it was one in two!
If you think that was bad, consider these numbers for average test scores:
Year Exam Average Score
2000 Pilot Examiner Gyroplane 71
2001 Private Pilot Gyroplane/Recreational Pilot – Transition 70
2001 Private Pilot Lighter-Than-Air (Airship) 68
2002 Private Pilot Lighter-Than-Air (Airship) 62
2001 Recreational Pilot Gyroplane 60
2003 Private Pilot Lighter-Than-Air (Airship) 58
2002 Recreational Pilot Helicopter 31
2002 Commercial Pilot Free Balloon – Gas 18
(Some of these numbers may not be statistically significant, especially for groups in which there may literally be only a very few dozen individuals. If you would like to see for yourself, have a look:
http://av-info.faa.gov/data/teststat/03passrate.html
http://av-info.faa.gov/data/teststat/03means.html
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Wherever the President visits, a large chunk of instant Special Use Airspace follows. In this case, ‘large’ means a cylinder with a radius of up to thirty miles (although at Camp David, it is ‘only’ 10), and going up to 17,999 feet MSL. What, if any, is the protective airspace around the Vice President?
A) They are exactly half the presidential dimensions: 15 miles in radius, going up to 8,999 feet MSL.
B) The Vice Presidential TFR has a radius of three nm and a height of 2,999 feet AGL.
C) The size is exactly the same.
D) There is none.
Answer: The Vice President’s protective bubble is much smaller, but there definitely is one: it’s choice B. <BR><BR>
