Hey guys and gals, welcome back to Helicopter Lessons in 10 Minutes or Less. This is Jacob and thanks for tuning in. What I'll be going over today is how rotor systems compensate for the symmetry of lift. Now if you haven't seen my video explaining the symmetry of lift, I'd recommend watching that video first.
I'll put the link to that video in the description of this one. So for a quick summary, the symmetry of lift is the difference in lift that exists between the advancing side and the retreating side of the rotor disc. If the symmetry of lift were allowed to continue without compensation, there'd be no way to maintain forward flight. flight. The advancing side would continue to produce more lift than the retreating side and cause a constant roll to the left.
Luckily helicopters are designed to compensate for this using blade flapping and cyclic feathering. These allow the pilot to reduce some of the lift of the advancing side while simultaneously increasing the lift of the retreating side. So what does that look like?
Well we have the same setup as we have for all of our other diagrams. The counter clockwise turning rotor system and the helicopter transitioning into forward flight. have cross sections of the blade in their respective field of travel. On the advancing side of the blade, as the blade advances into the relative wind, the amount of relative wind that's hitting the blade is increasing, it's speeding up.
So it has a faster air velocity than on the retreating side. So this is kind of like when you put your hand out the window, you know, pitching it up and as you draw faster, your hand's wanting to climb. So the same thing's happening with the advancing side. It's flashing.
flapping up into the relative wind. Now this is obviously going to change the lift characteristics of the blade. So here we have the shape of the airfoil.
The line is the cord line intersecting the trailing edge and leading edge of the airfoil. Now in its axis of rotation... This is the blade's rotational relative wind.
This is the tip path plane. This is the path that the blade travels or wants to travel as it rotates around the mast. Chord line is how much angle you have in the blade. And now what we're looking for is the resultant.
The resultant amount of airflow compared between the relative wind and any kind of up flap or down flap. Now as we said, this is going to up flap, so that's going to be a downwards flow of air through the rotor system. going to look like is this downwards flow of air right here and so now our resultant relative wind is going to be right here. So what does this mean? This means this area right here is our lifting area of the blade.
That's how much lift this blade is creating on this side. Now let's look at it on the other side. So this blade has flapped up advancing or into the the relative wind as it advances on the advancing side and then as it slows down begins to flap downwards. So now we have a down flap. Now that's manifested on this side where we have the same diagram with the blade pitch.
We have our rotational relative wind, the path that it takes as it travels around the mast, and now we have the down flap or an up flow of air. So that is looking or depicted like this. It's down flap, the up flow of air, giving us a resultant down here. That resultant relative wind. The resultant is the difference in between the the relative wind and the induced flow or the amount of upflow or downflow of air in the rotor system.
Now this is changing our angle of attack just like it did in over here and so now our lifting area is this region. So that downflow has increased the angle of attack. It's increased our amount of lift generated on this side of the blade. So if you look at the nose and the tail this is going to be relatively the same on both the forward and aft portions of the blade. But now Now we have a clear difference in lift.
We're compensating for that dissymmetry of lift by just having the blades flapping up and flapping down. That wraps up blade flapping as a means to compensate for dissymmetry of lift. Now after blade flapping, we have cyclic feathering, slightly different diagram. This one's a little bit easier, less in-depth.
The same diagram, rotating counterclockwise forward. motion and what we have is cyclic feathering. This is saying I want to go forward so I am going to move my cyclic forward. Now what happens with gyroscopic precession is this input has to be made 90 degrees in advance so it's actually increased ...using the pitch here so that we have the increase and lift. on the aft portion of the blade.
So that 90 degree delay covered in my gyroscopic precession video, this is causing the rotor disc to tilt forward and produce more lift on the aft portion than on the forward, producing that forward momentum. Now when that input is actually made it's actually increasing the pitch on the nine o'clock position of the rotor disc. So what does that look like?
Now we have our pitch over here it's actually pitching this up to say, for sake of reference we'll say it's pitching pitching the 9 o'clock position up 15 degrees and simultaneously reducing the pitch on the advancing side to say, five degrees. So now we have another compensation for this dissymmetry of lift by having an increase in pitch on the retreating side and a decrease in pitch on the advancing side. So increase in angle of attack, increase lift, decrease angle of attack, decrease lift. And that is cyclic. feathering.
Now this concludes my discussion on the compensation for dissymmetry of lift. We're using blade flapping and cyclic feathering to increase the lift of the retreating blade and decrease the lift of the advancing blade. So thanks for watching.
If you have any questions write them in the comment section and I hope you guys stay tuned for more videos. Thanks for watching.