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Tail Wind


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Well if you don't know what Effective Translational Lift is then that's going to require some studying. To make it brief, a helicopter's rotor disk is more efficient at creating lift when it's moving horizontally through the air. It doesn't matter what direction the aircraft is moving, although it's generally moving forward when this effect occurs because pilots like to fly in the direction that they can see.

 

The reason that 4-6 m/s is the worst tailwind is because that's generally the approach speed when you're just about to land a helicopter. You're not yet in ground effect and you are still getting some benefit from ETL. But if you have that tailwind, you are effectively in an out-of-ground effect hover as far as the air mass is concerned. And an OGE hover takes much more power than an IGE (in ground effect) hover.

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If tail wind is more than 6 m/s at that time also i think helicopter will be like hovering OGE. Can you please clarify?

 

Once the tailwind exceeds the forward ground speed of the helicopter, it is flying backwards relative to the airmass, regardless of the helicopter's forward speed over the ground. The rotor doesn't care what direction the horizontal movement comes from.

 

A: If you are on final approach to a landing, and let's say that at this moment there is no wind, and you have a forward airspeed of 5 m/s. Since there is no wind, you are moving across the ground at 5 m/s. You are also moving within the airmass with a forward airspeed of 5 m/s. The rotor disk of the helicopter is able to generate more lift due to the 5 m/s headwind, thus reducing the power requirement during the final landing approach.

 

B: If in that same example you are landing with a 5 m/s tailwind, then in that case you are still moving across the ground at 5 m/s, however the helicopter is not moving at all relative to the airmass, since both the airmass and the helicopter are moving in the same direction at the same speed. In this case, much more power would be required to maintain this approach since it is effectively hovering. And if the aircraft is more than maybe 30-40 feet off the ground, it would be an out of ground effect over.

 

C: Same example, but with a 10 m/s tailwind. The helicopter is still moving across the ground at 5 m/s, but since the airmass is moving faster than the helicopter's groundspeed, the helicopter is moving backwards relative to the airmass. Effectively, the helicopter is flying forward over the ground at 5 m/s, but BACKWARDS through the airmass at 5 m/s. This horizontal airflow over the rotor provides the same assistance with lift that was realized in example A, just from behind instead of from the front.

 

For the sake of these examples, I have left out how the tailwind affects tail rotor effectiveness (it helps a little) and how the tailwind will make it a real pain to keep the nose straight due to the weathervaning effect.

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Once the tailwind exceeds the forward ground speed of the helicopter, it is flying backwards relative to the airmass, regardless of the helicopter's forward speed over the ground. The rotor doesn't care what direction the horizontal movement comes from.

 

A: If you are on final approach to a landing, and let's say that at this moment there is no wind, and you have a forward airspeed of 5 m/s. Since there is no wind, you are moving across the ground at 5 m/s. You are also moving within the airmass with a forward airspeed of 5 m/s. The rotor disk of the helicopter is able to generate more lift due to the 5 m/s headwind, thus reducing the power requirement during the final landing approach.

 

B: If in that same example you are landing with a 5 m/s tailwind, then in that case you are still moving across the ground at 5 m/s, however the helicopter is not moving at all relative to the airmass, since both the airmass and the helicopter are moving in the same direction at the same speed. In this case, much more power would be required to maintain this approach since it is effectively hovering. And if the aircraft is more than maybe 30-40 feet off the ground, it would be an out of ground effect over.

 

C: Same example, but with a 10 m/s tailwind. The helicopter is still moving across the ground at 5 m/s, but since the airmass is moving faster than the helicopter's groundspeed, the helicopter is moving backwards relative to the airmass. Effectively, the helicopter is flying forward over the ground at 5 m/s, but BACKWARDS through the airmass at 5 m/s. This horizontal airflow over the rotor provides the same assistance with lift that was realized in example A, just from behind instead of from the front.

 

For the sake of these examples, I have left out how the tailwind affects tail rotor effectiveness (it helps a little) and how the tailwind will make it a real pain to keep the nose straight due to the weathervaning effect.

 

 

Hlw sir

What if our wind speed is 7 m/s? Then at 25 kph we are suppose to loose our relative wind and also we have come below ETL. At that time will the helicopter feels like hovering OGE??

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Hlw sir

What if our wind speed is 7 m/s? Then at 25 kph we are suppose to loose our relative wind and also we have come below ETL. At that time will the helicopter feels like hovering OGE??

 

If the helicopter is moving over the ground at 25 kph and the helicopter is experiencing a tail wind of 7 m/s, then the aircraft is in a hover as far as the air mass is concerned. If the aircraft is far enough off the ground to be out of ground effect, then yes, the helicopter would be in an OGE hover with regard to power requirements and airflows through the rotor system.

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Well there is LTE - Loss of tail rotor effectiveness.

 

 

The other thing to consider when decelerating out of translation in a tail wind you can misjudge your power requirements as your airspeed drops to zero while you still have positive ground speed. Add in a descent and you can find yourself experiencing a rapid change in power requirements to arrest descent. it can be very deceptive.

 

 

It's much safer to decelerate into the wind, where you still have positive airspeed during all phases of flight.

 

 

HTH


Edited by ben1101

i5 4690K, GTX1070, 24GB 1800mhz, HP WMR, Custom FFB helicopter controls.

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