Hello and welcome back to Helicopter Lessons in 10 Minutes or Less. Jacob again. Just want to thank you guys and gals for watching the videos. If you like the videos so far, let me know. Like the video, leave a comment, leave some feedback.
Any kind of feedback and suggestions help me make these videos better and better. Alright, so let's get started. Today I'll be explaining the transverse flow effect. This is the right rolling motion and slight vibrations that occur in a helicopter as it begins to accelerate into forward flight.
So let's take a look at this. To preface everything, take a helicopter at a stationary hover. We can all agree for the most part that airflow is coming vertically through the rotor system, stationary. And if we're at a high rate of forward speed, we can agree that airflow for the most part is coming at the helicopter horizontally. So now we're going to talk about the transverse flow.
This is that changing of the flow of airflow from a stationary hover to forward flight. It's the changing of the flow from vertical to horizontal. So what does that look like?
Well, this usually occurs between 10 to 20 knots of forward airspeed, or airspeed in any direction for that matter. What happens is that airflow is starting to hit the leading, or the front half of the rotor system in a horizontal direction, and then transitioning more and more to a vertical direction as it gets to the aft portion of the rotor disc. So, more horizontal on the front half, more horizontal on the back half. Now we can kind of see this explained in these two diagrams of the airfoil.
So, this is the airfoil for the aft half of the rotor system. We have the cord line running... through the leading and trailing edge of the blade, a rotational relative wind, which is the path that the blade travels around.
And now we're going to have the induced flow, or the downwards flow of air, this downwards flow of air through the airfoil. which is going to reduce the amount of angle of attack we have in the blade. So we have that downwards flow of air.
It's affecting the air flow like that, giving us our resultant. Keep in mind our resultant is the rotational relative wind adjusted for the up flap or down flap, the up flow, down flow of air flow in the blade. This, compared with our chord line, is going to give us our angle of attack. So this angle of attack is our lifting region of the blade.
for the Apt-Aff. Now let's look at the front half. We've got the chord line, the rotational relative wind, but in this case it's more horizontal, less induced flow, less vertical flow of air through the rotor system.
So now it's not going to be as drastic of a change. So let's look at this angle right here. It's adjusting.
for the induced flow. Now we have our resultant relative wind. Now we have this area for our angle of attack, this area of lift.
It's a larger angle than this angle. We're getting more lift on the front. half of the blade than on the aft half of the blade.
So more vertical air, less lift, more horizontal air, more lift. So what does that look like? Well it's more lift in the front half than the retreating, or on the front half than the aft half. So let's see what that looks like.
If we were to do a top down look at our rotor system and divide this into quadrants, this is more lift on the front half, that's the direction of travel, less lift on the aft half. Now we have to compensate for this gyroscopic recession. that are covered in the other video that all of this stuff manifests 90 degrees later.
So what does it actually look like? We're gonna have that 90 degree delay, 90 degree difference in the manifestation of this, where this increased lift is actually gonna rotate to the left side of the disc, and this will rotate to the right side of the disc. So what we have here is an increase in lift on the left side, a decrease in lift on the right side.
So more lift on the left than the left. the right, it's going to cause that right roll. So the only thing that we can do to compensate for this is that left cyclic in the direction of the roll to compensate.
So what you may see in young pilots is, say this is your runway, you know, got your lanes down the middle. What you're going to see is you're going to have your helicopter, say starting right here, correction, say he's going to start right here, begin his takeoff, he's going to get that right roll, it's going to push him to the right of the lane. he's going to see it, correct it back to the center lane, and continue its takeoff.
Or, when they're in on approach and they get between that 10 to 20 knots, they're going to notice that right roll, it's going to push them to the side, they're going to correct back to it, or they might just continue forward and land right out of the lane. So, it's anytime you're in between that 10 to 20 knot airspeed, you're going to have that pushing to the right, that right roll. The aircraft is going to try to drift to the right.
You're going to have a little bit of... vibrations as well because the difference in drag in between different parts of the rotor system. So in a nutshell that's transverse flow.
It's that right rolling motion. It's a simple concept. It's pretty easy to compensate for.
You just have to know it's there and be ready with that left cyclic once you get in between the air speeds 10 to 20 knots. Well that's it. Thanks for watching.
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