Effective Translational Lift ETL Lesson

Aug 26, 2018

Effective Translational Lift ETL Lesson

Dave Redmond CFII gives an excellent presentation in this video on Effective Translational Lift, also known as LTE. This video is from the aerodynamics section inside Helicopter Online Ground School.

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 Now we're going to do a video on a helicopter going from stable hover flight all the way through ETL and what happens aerodynamically and what happens to the ship. For these scenarios, we're going to pretend like there's no wind, so dead calm, and we're going to start off in a hover.

 

Effective Translational Lift ETL Lesson

Step one, we are just in a hover. Everything is pretty uniform, air is coming down like this, got our vortices, we're in ground effect. There you go. That's our starting point. Now the term you want to say is with any horizontal movement, which means if you get two knots of wind, that will cause things to change, or if you just start going at a walking pace, things will start changing.

Step two is any horizontal movement. I should say horizontal airflow. Now we're starting, again, this can be just a walking pace. We're starting to move forward in the back half of the helicopter, really nothing has changed. Still coming down, going out. Vortices, front half, mostly the same, probably right here is the same, but then the air isn't quite coming straight down.

Now it's starting to shift and come in a little bit more like this, and your vortices is being reduced a little bit and being blown away just a little bit. Now when you change your airflow from coming straight down to a little more horizontal, you have reduced your induced flow. You can watch one of my other videos where I explain why reducing induced flow increases angled attack, but when you reduce induced flow, you get more lift, so the helicopter wants to pick up a little bit.

Also, with a smaller vortices, it will produce a little bit more lift, as well, and the helicopter and you'll feel this if you do a nice, slow takeoff, you feel the helicopter at the beginning just kind of want to pop up a little bit, and that's getting any beneficial lift form any horizontal movement.

Now we're going to start accelerating and going a little bit faster and a little bit faster, and we're going to get into a transverse flow effect. This happens between 10 and 20 knots. What happens is you can almost think of it as the front half of the blade is in clean air, the back half is still in dirty recirculated air.

We're starting to go a little bit faster. Front half is more horizontal like this and coming through nice and pretty clean, vortices been blown away. The back half is still coming down like this because it hasn't fully been blown away, and you have a vortices. You can think of it as front half clean, back half is still kind of dirty.

Effective Translational Lift ETL Lesson

This is and the way I picture it is you have a blade and it's going back and forth between oh, clear air, my vortices blown away. I got good horizontal flow, not as much induced flow. Oh, bad air in the back, I got a lot of induced flow, I got a vortices not producing as much lift, so clean, dirty, clean, dirty, clean, dirty, it's just the vibration of the rotor system going through that.

The second thing is a right roll. Now we talked about gyroscopic procession in another video, so you may want to review that.

If I look top down at the helicopter, so here's the body of the helicopter and the tail, and I have the blade, you're getting more lift at the front of the blade. Because of gyroscopic procession, that has a maximum effect 90 degrees later over here. You have that increase in lift affecting you on the left side, which will cause the helicopter to roll to the right a little bit.

Effective Translational Lift ETL Lesson

Third thing, we have a left yaw. This tail rotor back here, remember, it's doing the same thing as the main rotor as far as pushing air, creating lift, all that kind of stuff. It has vortices, it has induced flow, and just like the main rotor. As you start moving forward in a clean air, those vortices will get blown away, the airflow will be more horizontal, not going straight in like this, so your tail rotor will become more effective, which means it's going to push harder, which means all that left pedal you have in for a hover, now that tail rotor's going to become more efficient, so it's going to yaw you to the left.

As you're pushing through this, you need to be backing off that tail rotor and pushing some right pedal to keep your nose straight ahead. Now this is a really a funky transition that takes a while for students to get through because they're not anticipating it. The nose will kind of dip up and they weren't anticipating so they'll nose over, then it will roll to the right, so they'll push left and then the tail rotor will swing you to the left, so then you're too late pushing right pedal, and you get this kind of fishtailing weird thing until you've blown through it.

Next, we get to ETL, which is just effective translational left. This stage should actually be called translational left, as well. Translational lift is any horizontal airflow. This is effective translational lift. This now happens between 16 and 24.

Effective Translational Lift ETL Lesson

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