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Аэродинамика вертолета/Helicopter Aviation17.htm

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<BODY><A href="http://www.cybercom.net/~copters/helicopter.html">[Top]</A> <A 
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<H1>Flapping</H1>Flapping can be pretty hard to understand at first, although it 
really is a fairly simple concept. A couple things you need to understand is the 
relationship between angle of attack and relative wind. Relative wind is simply 
the direction the air seems to be coming at you because of your motion. When you 
stick your hand out of a moving car window, the wind hitting it seems to be 
coming from directly in front of you. The wind will always seem to be moving in 
the opposite direction of your motion. For instance, in the car example, the car 
is moving forward, and the wind seems to be moving backward. 
<P>Now lets change the example a little bit. Take a hardcover book in your hand, 
and drop it. Think about a little person glued to the bottom of the book. As the 
book falls, he will feel the wind seem to come directly up at him from the 
ground. Again, this is relative wind due to the motion of the book: 
<P><IMG height=256 src="Helicopter Aviation17.files/relative1.gif" width=256 
align=bottom> 
<P>Instead of dropping the book, let's slide it along a table sideways. The 
table is there to prevent it from falling, so the only motion will be sideways 
motion. For this exercise, we let the little guy stand on top so he won't be 
squished: 
<P><IMG height=256 src="Helicopter Aviation17.files/relative2.gif" width=256 
align=bottom> 
<P>What if we remove the table, throw the book sideways, allowing it to also 
fall toward the ground. It will have two components of motion, one horizontal 
and one vertical, like this: 
<P><IMG height=256 src="Helicopter Aviation17.files/relative3.gif" width=256 
align=bottom> 
<P>Note that the relative wind is the combination of the horizontal and vertical 
motions. To the poor little guy about to be squished again, it seems to be 
coming at him from an angle, somewhere between the vertical and horizontal. Now 
let's use a tiny bit of math. If we draw the blue arrows to represent the two 
components of motion, horizontal and vertical, and then we extend red lines down 
from the top arrowhead and to the left from the bottom arrowhead until they 
meet, we can then draw the green line back to the origin of the two vectors. The 
green line is a vector which will show us the direction the relative wind is 
coming at the little guy on the book: 
<P><IMG height=256 src="Helicopter Aviation17.files/relative4.gif" width=256 
align=bottom> 
<P>Notice that the two blue lines are about the same length, and that the angle 
ends up being about 45 degrees. That's not by chance. Let's take a look at what 
happens if we make the horizontal component of motion be twice as fast as the 
vertical component: 
<P><IMG height=256 src="Helicopter Aviation17.files/relative5.gif" width=256 
align=bottom> 
<P>Notice how the angle has changed! Because the horizontal component is larger, 
the angle seems to be more horizontal than vertical. We can calculate the angle 
this way, as long as we know the horizontal component and vertical component of 
motion. 
<P>Okay, what's all this got to do with flapping? If it's not already obvious, 
the book represents the rotor blade. Let's see a picture of a rotor blade that 
is spinning, but not flapping: 
<P><IMG height=256 src="Helicopter Aviation17.files/relative6.gif" width=256 
align=bottom> 
<P>This is sort of a complex diagram, but lets take it part by part. The blue 
line shows the motion of the rotor blade, moving forward due to rotation. The 
green line shows the relative wind. Since there is only a horizontal component 
to motion right now, the green line is horizontal. The pink line labeled tip 
plane path shows the orientation of the rotor disk. It's just another line 
showing the horizontal motion of the rotor blade. The yellow line which is 
labeled chord line can just be thought of the direction the airfoil is facing 
(but not necessarilly moving). In the case of this diagram, this would all be 
consistent with a helicopter at hover: the chord line is tilted because the 
pilot has raised the <I>collective</I> control, thereby tilting the blade up 
from the horizontal. Angle of attack is defined as the angle between the 
relative wind and the chord line. That is shown by the purple line. 
<P>In the next figure, we've added some vertical motion to the rotor blade: 
<P><IMG height=256 src="Helicopter Aviation17.files/relative7.gif" width=256 
align=bottom> 
<P>The way I've drawn this is on top to show the two components of motion of the 
rotor blade. Note that the blade is flapping <I>up</I>. I've copied the blue 
lines of motion down by the airfoil, and used the red lines to show where we 
would extend the blue lines to. Then I drew the green relative wind line from 
this point back to the origin. There are a few interesting things to notice now. 

<P>First of all, remember that the relative wind is equal and opposite to the 
direction the object is going, so the green relative wind line also gives us an 
idea of the path of the blade. Clearly it is moving forward and <B>up</B>. The 
important thing to notice, however, is that the angle between the relative wind 
and the chord line is now cut about in half. In this case, the lift would also 
be cut in half. If this is not clear to you, lets see the same diagram, but 
rotated so that the relative wind is horizontal to us: 
<P><IMG height=256 src="Helicopter Aviation17.files/relative8.gif" width=256 
align=bottom> 
<P>It should be clear that the angle of attack in this picture is much smaller 
than the diagram without flapping. 
<P>One final key to all this is, what makes the blade flap up? The answer is 
very simple: it's the excess lift. Remember that at a hover, the blade angle is 
where the forces of lift and centrifugal force balance out. If we increase the 
lift (because of the excess airspeed on the advancing blade) and the centrifugal 
force stays the same, the extra lift will cause the blade to flap up to a higher 
position until lift and centrifugal forces are once again in balance. 
<HR>

<ADDRESS>Paul Cantrell<BR><A href="mailto:paul%20at%20copters.com">paul at 
copters.com</A> (replace " at " with "@" to email me - this avoids SPAMMERS I 
hope) </ADDRESS>
<P><A href="http://www.cybercom.net/~copters/helicopter.html">[Top]</A> <A 
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avsim_ru.diz	20.07.2007 17:09	362 B
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Helicopter Aerodynamics

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Helicopter Aerodynamics

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