The Doppler Effect By James Goss      If you have read my article on tuning your twin engine you will see that I love the sound of our model engines. If you haven't you may want to go to my web site and read it. If you are on my site reading this article then read the other article while you are here. I try to add a new article at least every two months that has something to do with radio control modeling. This article came to mind while I was writing the twin engine article after talking about the unique sounds our engines produce. It is rather short, but I think you will enjoy it if you are not already familiar with the Doppler effect.      Like I have said in other articles about the sound our engines generate, there is many factors that contribute to this sound. One of the most unusual, and one that seems to be really strange, is known as the Doppler effect. It is named for the Austrian physicist Christian Doppler (1803-1853), who discovered the effect. While flying our models, I am sure that you have noticed this effect at one time or another. It does make one stop and think about why your engine sounds like it is changing its rpm when it is not really changing at all. In general, a Doppler effect is experienced whenever there is relative motion between the source of a sound and the observer. As long as the sound generating object is stationary and you, the observer, is also stationary, then you will here the exact same frequency as is being generated by the sound source. When the object is moving toward you, the frequency that you hear will be higher than the source frequency. When the object is moving away from you, the frequency will be lower than the source.      This effect is not easy to understand, but if you think about it like this you can comprehend it better. Lets say that the source and the observer are stationary, in reference to each other, and the source is producing a sound that is 500 Hz in frequency. At this time the sound that reaches the observer's ears will be exactly 500 Hz. Now lets say that the source of the sound (your airplane) is moving toward the observer at a rate of 50 miles per hour, this produces the relative motion that is needed to obtain the Doppler effect. Relative motion simply means that one object is moving in respect to the other object. While stationary, you were receiving 500 Hz per second. That is to say that your ears would receive 500 pulses of sound waves each second. With the sound source moving toward you, your ears will receive a higher frequency than the 500 Hz. At fifty miles an hour, your plane will be moving at 4400 feet per minute, or 73.3 feet per second. Since sound travels at about 1120 feet per second, you can see that in one second your ears will receive the 500 pulses per second that the source is producing, plus about 6% more, or about 530 sound pulses of air per second. So even though the source is generating 500 Hz per second, you hear 530 Hz per second.     Another way to look at this phenomenon is to think of it like this. While both objects are stationary the source has enough time to push 500 pulses of air past your ear in one second. If you now start moving toward the object, or if the object starts moving toward you, you will go past 500 wavefronts of air in one second, plus about 30 more because you are moving toward the source. If it is still not clear, here is another explanation of the Doppler effect. This time lets say that you are standing at the end of a conveyor belt with boxes moving toward you at the rate of 10 boxes per minute. The boxes are spaced exactly three feet apart.  So in one minute 10 boxes will pass you. During that same minute if you walk toward the oncoming boxes and take three steps (9 feet), you will now have 13 boxes to pass you during the same one minute of time, you will gain three boxes. On the other hand, if you turn around and take three steps in the other direction, you will loose three boxes and only 7 will pass you in one minute of time. It works the same way with sound waves. This concept is amazing to me and I thought you might enjoy my definition of the Doppler effect. The next time you are at the field, make a low pass down the center of the field and notice the pitch produced by your engine as it comes toward you, and then its pitch as it goes away from you. There should be a noticeable change in its frequency. When you dive your plane from a high altitude you will also get the Doppler effect. So the Doppler effect is just another aspect of our engines that helps create excitement in our sport. Another effect that I would like to mention at this time is related to the fact that I stated above, sound travels at about 1120 feet per second. Sometimes you will have your airplane, especially a glider, a great distance from you when you decide to chop the throttle. The engine seems to keep on running for a while before the throttle responds and the engine reduces its rpm. For a split second you think your radio is not working and you are in trouble again.  Before you read on do you know what is happening? Yes, the engine is responding just as fast as it always has, it's just that it takes time for you to hear the change because the plane is so far away. If you are flying 2,200 feet away, it will take the sound waves about two seconds to get to your ears. Two seconds is a long time to wait for a change to occur after you move a stick when flying radio control planes. So the next time you are doing some wild aerobatics and you move sticks but nothing happens, that's ok don't worry about it, just remember you are waiting for the waves to reach you! The End

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