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FAQnewred.gif (906 bytes)          

Frequently Asked Questions (FAQ's) on principle of flights

Go back to the first set of FAQs on Principles of Flight.

 

FAQ on instruments of airplanes

Please go to http://www.allstar.fiu.edu/aero/princ1.htm. Halfway down the page are materials on instruments.

 

FAQ about different explanations for lift

First you must understand that it took quite a long time to show that Bernoulli didn't explain everything about lift and that finally with the use of sophisticated equipment Bernoulli was shown not to give all the answers. Bernoulli was simple but if you understand the Eberhardt paper (which can be found at http://www.allstar.fiu.edu/aero/airflylvl3.htm), you see there are some situations in which bernoulli doesn't work.

 

FAQ on how to calculate the coefficient of lift

The coefficient of lift is normally obtained by experimental work using wind tunnel tests. Many tests were run by the NACA, the predecessor to the NASA, and a booklet NACA Report 824 (1945) was created to give the relation of coefficient of lift, coefficient of drag, pressure and moment coefficient for many different types of airfoil. you can get that from your library. you can find examples of this information in our level 3 materials...use the search engine and type in "coefficient" to get information about coefficients of lift, drag, moment and pressure. From Dommasch, Sherby and Connolly "airplane aerodynamics", Pitman Press, fourth edition 1967, p. 106-110, we find that the coefficients of lift, drag and moment depend upon the angle of attack, the mach number and the Reynolds number. For subsonic speeds, normal airfoils have a linear relationship between angle of attack and coefficient of lift until just before stall occurs (the airfoil or wing experiences a loss of lift).   For higher speeds, the mach number is higher than 0.3 (mach number is the velocity of the aircraft divided by the velocity of the sound), then the coefficient of lift = the coefficient of lift at low speed divided by the square root of (1- mach number squared). This correction for the mach number effects is based on Glauert who proposed it in 1928. Von Karman proposed a more complicated. Note that the coefficient of lift at low speed is the value that is normally obtained experimentally.

The lift coefficient is also dependent on Reynolds number. Actually the Reynolds number determines the type of flow(whether laminar or turbulent), which in turn determines where the flow separates from the wing, which in term affects the lift, drag and momentcoefficients. We note that as Reynolds number increases, the maximum lift coefficient increases. But this does not occur indefinitely; when flows become very turbulent, the maximum lift coefficient begins to drop.

 

FAQ on Gliders

Gliders are unpowered airplanes that use the thermal properties of air to create lift. You can learn about lift and airplanes on our site by going to http://www.allstar.fiu.edu and clicking at the top the principles level 2 button. You can also find more information about gliders on this site http://travel.howstuffworks.com/glider.htm.

 

FAQ on minimum speed necessary for take off

It varies but it can go from 120-150 mph. There is a formula that is used to determine the speed. Normally, you try to get lift to equal weight and that has to be equal to the airdensity at sea level x surface area of the wings x a lift coefficient based upon the wing x the square of the velocity divided by 2. Since wings are different, the surface area will be different and so will the lift coefficient which will vary based upon the angle of the wing to the wind direction. Check out http://www.allstar.fiu.edu/aero/forcesactairflght.htm. You can also look at our pages for lift and drag.

 

FAQ on speed of an airplane

Airplanes can fly at any speed from 100 miles an hour to about 1500 miles an hour depending on the type.

Some crop dusters fly very slow. Some jet planes fly at 500 miles an hour, and some attack aircraft can fly as fast as 1500 miles an hour. The shuttle when it comes back from space is flying even at faster speeds.

 

FAQ on the effect of earth rotation and flight direction on time of flightnewred.gif (906 bytes)

The rotation of the earth has no measurable bearing on time of flight.  Inertially, we start out already moving with the earth. There is not force that wants to slow us down relative to the earth's rotation. 
For example, the atmosphere basically moves with the earth, it does not sit stationary while the earth rotates within it (creating a several hundred mile per hour wind).  For example, if you're riding in a bus, and throw a ball from the front to the back of the bus it takes the same time to throw it forward as it does backwards.  From the point of view of a person on the street, it appears that in one instance the ball has traveled farther and faster than in the other. But since the air in the bus is also moving with the bus, there is no time difference between the two throws.

What DOES take longer is flying west (northern hemisphere) into the wind versus flying east with the wind.  But that is not directly related to the earth's rotation (evidenced by the fact that at some latitudes, the wind prevails from the east).  If we were approaching the planet from outer space - with no rotational bias, and then decided to make a turn to proceed to a destination on earth, then we would have to adjust our speed and direction relative to the earth and its rotation to get there - essentially if the destination were moving toward or away from us, for example, when the Apollo missions returned from moon missions.

The great circle routes which many airplanes travel are portions of paths of a circle. The plane that contains the circle passes through the earth's center.  Imagine a piece of paper slicing the earth and passing through its center.  Where the earth's surface and the paper intersect would be the path of the great circle. 
Why are they important?  To follow lines of latitude above the equator is a waste of time and money. The great circle route is shorter, assuming the winds are the same on either route.  The great circle routes are most pronounced at the high latitudes and not at the equator.

We thank Capt. Bill Palmer for this explanation.

 

FAQ on "gliding" a commercial aircraft into a landing without engine power

Think of a shuttle as a minor example. It is a perfect glider when it comes in to land, unpowered. Commercial aircrafts do glide very well. The big fans on the engines do increase drag when they are not running, but commercial transports still have a very good glide ration, without power. The primary difference between a commercial transport and a glider is that a glider has a very light wing loading, so its airspeed is low. A commercial transport has a high wing loading and so must glide at a very high airspeed (~120+ knts). Commercial aircraft can stall, but due to the dangers, it is not usually practiced or recommended.

 

FAQ on making a flight model

If you want to make a radio controlled flight model, you will need to go to a hobby store and buy a kit to build a radio controlled plane. Otherwise, get a kit of balsa wood and attach two strings about two meters long at the end of one of the wings so that they are parallel to each other. Then attach the strings to a stick you can hold. When you rotate (spin around yourself) the strings will move the airplane and when you move the stick up down or left or right, you can get the plane to move as well.

 

FAQ on wings for models

Read the following section on flight theory and click on wing design. At the end of the document is information that can help you in the name of a book or books you can look at.

 

FAQ on the causes of climb of an airplane

If lift is greater than weight, there is a net excess force upwards, and, since that excess force equals mass times acceleration, the plane accelerates in the direction of the excess force, in this case, upwards. And this is without thrust. If the thrust has an upward component, during climb, for example, this adds to the overall excess force upwards that leads to an upward acceleration. Examples of lift being greater than or less than weight without thrust are birds that soar or dip without the need to flap or generate thrust through rearward wing motion.

 

FAQ on the conversion from pounds-thrust to horsepower

Please look at the following page for the example on HP. HP is basically power and thus requires a velocity of some kind. http://www.allstar.fiu.edu/aero/Hydr03.htm
1 HP is 550 lb-ft/sec. Also remember to take into consideration the number of engines used.

 

FAQ on Breguet XIX biplane

If you want schematics of the biplane, try contacting the National Air and Space Museum in Washington, DC, USA. They may have that information available.

 

FAQ on the altitude reached by an airplane in flight

The altitude of flight depends on the type of aircraft and the weather. Some run at 33,000 ft and some run higher. Some military jets have flown at altitudes over 100 thousand feet. Most commercial jets fly between 30-50 thousand feet. The SR-71 has been reported, off the record of course, to have a ceiling altitude of about 100,000 feet. I have searched other stealth aircraft pages and their ceilings are lower. It may be possible to fly that high but as one webmaster says, you would have to get your astronaut wings. Perhaps some of the future aircraft will be able to do it, take off as aircraft fly into space and land as aircraft.

 

FAQ on the design of rotor blades

Visit the Glenn Research Center (NASA-Lewis) of the NASA website http://www.nasa.gov and go to the NASA-US army portion of the website. There is work that is being performed there on helicopter engines and I am sure on rotor blades. That might be a beginning.

 

FAQ on the capabilities of a magnetic compass

A magnetic compass definitely detects motion in the horizontal plane. In the vertical plane it might near the poles, but I do not belive there will be much of a change near the equator.

 

FAQ on having planes flip over

It is the change in airpressure from one side to the other that will induce a moment and cause it to roll around the lengthwise axis. Also if the front is heavier than the back there will be a tendency for it to rotate (pitch forward) and it will flip over (see flight). That is why airplanes have ailerons and trim tabs.

 

FAQ on lift and Bernoulli

You can read about lift and Bernoulli at http://www.allstar.fiu.edu/aero/princ2.htm. You will find a lot of information there.

 

FAQ on left turns for paper airplanes

Make your airplane as required and put the left aileron tab up at about 60-90 degrees to the horizontal and the right aileron tab down at about 60-90 degrees to the horizontal. Less than that will make the plane turn the airplane slowly to the left. Also, most important, don't forget to turn the vertical part (below the wing) about 30 degrees to the left. That means you will have to make two cuts in the vertical section so that you can bend the vertical section.

For more information on paper airplanes, go to our homepage and click on principles level 3 at the top. Then go to the theory of flight link and click on it. Go to the physical description of flight and click on it. You will find a very interesting article that describes flight and its application to paper airplanes or balsa airplanes whose wings are flat.

 

FAQ on DME

This answer is provided by Capt. Bill Palmer, a commercial airlines pilot:

Each aircraft sends pulsed interrogation signals to the DME station in basically a random pattern (over a very short period of time), but remembers what the random manner was. It then looks for a reply from the station matching the pattern it used. The randomness ensures that no two aircraft are sending the same interrogation signal. DME interrogations are sent first at about 150 times per second, until communication is made with the station, and then about 30 times per second. The DME station can only answer to interrogations from about 100 or so aircraft at a time (that's a lot).

 

FAQ on upper airfoil camber

The upper camber airfoil formulas depend on the type of airfoil. Normally each airfoil is given a number (see for example explanations for theory of flight in Level 3 principles of our website). The numbers describe different characteristics of that particular airfoil.

 

FAQ about  laminar flow around a golf ball !

The golf balls without dimples would have a higher drag coefficient as seen in many pictures found in Schlichting's book on Boundary Layer Theory. However, dimples are placed to trip the flow from laminar to turbulent and keeping it attached for a longer distance along the golf balls' surface, thus reducing the overall drag and allowing it to go farther. Here is what I wrote to another email sender, which is more in depth. "With golf balls, the dimples are there to affect the boundary layer of air closest to the golf ball surface. A smooth golf ball while in flight, unlike your car, is moving much faster causing the flow around it to separate from the golf ball surface very close to the front of the ball. The air flow behind the ball (in the wake of the ball) is very turbulent causing an increase in drag. If we could cause the flow around the ball NOT to separate so close to the front but remain attached for a longer distance along the ball's surface, we could decrease the drag. The dimples do this by causing the flow near the golf ball surface to become turbulent AND also keeping the flow near the surface attached for a longer distance, thus decreasing the drag, WHEN compared to a smooth golf ball case. However, in comparison to the case of laminar flow over a smooth ball where no separation occurs (the least drag case), the dimpled ball case has higher drag. Dimples are used on golf balls for a specific reason. The golf balls without dimples would have a higher drag coefficient as seen in many pictures found in Schlichting's book on Boundary Layer theory. However, dimples are placed to trip the flow from laminar to turbulent and keeping it attached for a longer distance along the golf balls' surface, thus reducing the overall drag and allowing it to go farther.

"With golf balls, the dimples are there to affect the boundary layer of air closest to the golf ball surface. A smooth golf ball while in flight, unlike your car, is moving much faster causing the flow around it to separate from the golf ball surface very close to the front of the ball. The air flow behind the ball (in the wake of the ball) is very turbulent causing an increase in drag. If we could cause the flow around the ball NOT to separate so close to the front but remain attached for a longer distance along the ball's surface, we could decrease the drag. The dimples do this by causing the flow near the golf ball surface to become turbulent AND also keeping the flow near the surface attached for a longer distance, thus decreasing the drag, WHEN compared to a smooth golf ball case. However, in comparison to the case of laminar flow over a smooth ball where no separation occurs (the least drag case), the dimpled ball case has higher drag.

Some airplanes do have "fences", little metal spikes jutting out from the top of the wing surface, just for that purpose of tripping the airflow from laminar to turbulent in order to keep the flow attached longer to the wing surface.

 

Go back to the first set of FAQs on Principles of Flight.

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Updated: June 14, 2004