RE: Hobbyzone Challenger
Here is an excerpt from that article I mentioned that is relevent to this discusion:
Air Speed vs. Ground Speed
The plane rises from the ground based on the lift generated by the wings. The amount of lift generated is directly related to the speed of the air moving over the wing and not the plane’s speed over the ground. It is air speed that is important and the two are rarely the same. A simple illustration should help.
You are flying the plane at 20 MPH ground speed. What is the plane’s air speed? That depends on what the air around the plane is doing.
If you stand still and hold your plane facing into the wind, its ground speed will be zero, but its air speed will be equal to the wind’s speed. If the wind is coming at you at 7 mph then the plane’s air speed is 7 mph even though its ground speed is zero.
Even though the plane is not moving, the wing is developing lift equal to the speed of the air traveling over the wing. In fact this is how a kite flies. Being held by a string, the kite’s ground speed is zero, but its air speed is equal to the movement of the speed of the wind, so it develops lift and rises. If the wind stops blowing, its air speed drops to zero and it falls to the ground.
Let’s put some simple numbers to it.
In dead calm air, you learn that your plane’s top speed is 20 mph at full throttle. In dead calm air your air speed and your ground speed are the same. Remember that 20 mph because we are going to use it in the rest of the discussion.
WHY DOES MY PLANE DO THAT?
If you are flying into a 10 MPH head wind, and your plane is at full throttle moving at 20 mph (remember the 20 from above?) your ground speed is 10 MPH. Think of it this way, you are trying to go forward at 20 mph and the wind is pushing you back at 10 mph. So your ground speed is a net of 10 mph. OK? With me?
Now let’s look at the air speed of the plane. The wind is blowing at you at 10 mph and you are flying into it at 20 mph. While your ground speed is 10 MPH, your air speed is still 20 MPH. Your plane’s wing will develop lift based upon its 20 mph air speed, not its 10 mph ground speed.
Now, this same plane makes a 180 degree turn so that it has a 10 mph tail wind. The plane is still moving through the air at 20 mph, however, in this case our ground speed is 30 MPH since the tail wind pushes us forward, adding its speed to our own. However the wing is still developing lift based on the 20 mph speed of the plane through the air. It is developing the same lift even though the plane is moving across the ground more quickly.
The difference between air speed and ground speed becomes very apparent when our plane is flying into a headwind that is equal to our own speed. The net effect is zero ground speed. The plane appears to hover in the air. This is actually a lot of fun. On windy days I get a kick out of balancing my plane’s speed against the wind so that it hovers over a point. Since the wind speed is not consistent, it is a throttle balancing game, or with my gliders, it is a matter of balancing the glide path to the wind, but you can appear to hover a glider this way.
If the head wind is greater than our plane’s speed, the plane will appear to be flying backwards, which can appear very odd, but once you understand air speed vs. ground speed, it begins to make sense.
If you find this interesting, take a look at the rest:
[link=http://www.rcezine.com/cms/article.php?cat=&id=17]Beginners Bootcamp - March[/link]