20
$\begingroup$

When helicopters take off and land is it preferable to point the nose into the wind? Since a helicopter is providing its own lift does it matter which direction wind is coming from?

Improve this question
$\endgroup$
2
  • $\begingroup$ Tom, the question about wind and hovering is complex and should be asked separately. It's paragraphs of answer on it's own. What the wind is doing when hovering is a critical factor and has caused many accidents. $\endgroup$
    – Simon
    Nov 20, 2015 at 8:42
  • $\begingroup$ @Simon The first part has been answered pretty well. I'll separate out the hovering part as its own question if I can word it so it isn't too broad. You say it has paragraphs of answer. $\endgroup$
    – TomMcW
    Nov 21, 2015 at 22:22

4 Answers 4

Reset to default
11
$\begingroup$

In general, helicopters require less power while flying forward (or backwards) compared to hover as translational lift (of Effective Translational Lift, ETL) is produced. Even though the blades are providing lift by rotation, the airspeed experienced by the blades is different if wind is present. For example, if there is headwind $v$, the advancing blades will have an airspeed of $r \omega + v$, while the retreating blade will have an airspeed of $r \omega - v$. As lift is proportional to square of airspeed, the implications are obvious.

For this reason, it is preferable to take off into the wind, rather than take off vertically.

Power required

Note that the power required is same for headwind or tailwind. However, tailwind landing (or takeoff) has some disadvantages from stability and power management point of view that is it generally discouraged, like:

  • In case of landing in tail wind, it is very difficult to abort landing and re-gain forward airspeed. Moving from negative speed to forward speed initially decreases the air speed the main rotor experiences, and the Power Curve shows an initial increase in Power Required. As the helicopter accelerates from negative (ground) speed, the main rotor system loses translational lift before it goes through zero airspeed. As a result, the power required increases, which requires increased pedal application and tail rotor power, precisely when the power requirement is high.
  • If there is a power failure, the chances of safe landing is reduced.
  • There is more risk of running out of directional control. In case there is a tailwind, the helicopter vertical tail (and fuselage) may align try to align with the wind, resulting in uncommanded yaw. If not corrected with proper pedal input, this may lead to loss of control.
  • In case of landing, the possibility of brownout or whiteout conditions is more in case of tailwind.
Improve this answer
$\endgroup$
2
  • $\begingroup$ As I said elsewhere, ETL can be produced by forward speed, but it only requires airflow over the rotor disc from any direction. Also the end, "Note..." with the four bullet points are completely wrong. $\endgroup$
    – rbp
    Nov 21, 2015 at 19:20
  • 1
    $\begingroup$ How did this get 5 upvotes? $\endgroup$
    – Simon
    Nov 22, 2015 at 10:14
9
$\begingroup$

Moving forward through the air helps to make the helicopter more efficient, and it does not matter if that movement comes from hovering in a headwind or from flying forward. For the same reason, a helicopter will climb faster when flying in a spiral than when going straight up.

Technically, flying backwards or sideways will also improve the efficiency of the rotor, but in reverse the helicopter will become directionally unstable. Moving the fuselage sideways will create much more drag, so flying forward is the better choice.

Improve this answer
$\endgroup$
4
  • $\begingroup$ I've been referenced! (Sort of...) $\endgroup$
    – FreeMan
    Nov 19, 2015 at 21:11
  • 1
    $\begingroup$ Translational lift comes from airflow over the rotor, not just from forward speed, although that is one way to accomplish it. But translational lift can happen in a hover with just 15knots of wind, and it can come from any compass direction. Although helis are subject to weathervaning, their ability to yaw far outstrips any airplane's, and so reverse directional control is merely a minor nuisance that is easily accomplished. $\endgroup$
    – rbp
    Nov 21, 2015 at 19:23
  • $\begingroup$ @rbp: With "forward speed" I meant air speed. The next sentence should have made that clear. For flying in reverse, the instability goes up with speed and the time to double goes down. Slow backwards flight is manageable, but going faster is challenging. $\endgroup$
    – Peter Kämpf
    Nov 21, 2015 at 20:54
  • $\begingroup$ well i know you know the difference, and i think you could write a clearer answer. regarding instability, its unclear where the instability comes from. $\endgroup$
    – rbp
    Nov 21, 2015 at 21:44
5
$\begingroup$

Here is something off an FAA document about helicopter performance. https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch07.pdf

Winds

Wind direction and velocity also affect hovering, take off, and climb performance. Translational lift occurs any time there is relative airflow over the rotor disk. This occurs whether the relative airflow is caused by helicopter movement or by the wind. As wind speed increases, translational lift increases, resulting in less power required to hover. The wind direction is also an important consideration. Headwinds are the most desirable as they contribute to the greatest increase in performance. Strong crosswinds and tailwinds may require the use of more tail rotor thrust to maintain directional control. This increased tail rotor thrust absorbs power from the engine, which means there is less power available to the main rotor for the production of lift. Some helicopters even have a critical wind azimuth or maximum safe relative wind chart. Operating the helicopter beyond these limits could cause loss of tail rotor effectiveness. Takeoff and climb performance is greatly affected by wind. When taking off into a headwind, effective trans lational lift is achieved earlier, resulting in more lift and a steeper climb angle. When taking off with a tailwind, more distance is required to accelerate through transla tion lift.

Improve this answer
$\endgroup$
3
  • $\begingroup$ although this is a citation rather than an answer in the author's own words, it is the only correct answer. $\endgroup$
    – rbp
    Nov 21, 2015 at 19:24
  • $\begingroup$ @rbp I agree. It's the only one that has any real detail specific to why a headwind is preferable. I just hate to click accept on a cut-and-paste answer $\endgroup$
    – TomMcW
    Nov 21, 2015 at 22:14
  • $\begingroup$ that's why i added my own answer (hint, hint) $\endgroup$
    – rbp
    Nov 21, 2015 at 22:16
5
$\begingroup$

Effective Translational Lift (ETL) is lift generated by airflow over the rotor, and increases the effectiveness of the airfoil by about 15%. ETL is generated beginning about 15knots of airflow over the rotor.

Assuming the helicopter is landing with the nose pointed into the wind, a headwind decreases the ground speed at which ETL is generated. So a helicopter landing into a 15 knot wind can maintain ETL all the way into a hover (when groundspeed is zero).

Maintaining ETL as long as possible into the landing is important for 2 different reasons:

  1. it requires less power to fly the approach, increasing the safety factor in the case of a go-around, or in high density altitude or high gross weight landings, the requirement to make a run-on landing

  2. by maintaining ETL, there is no chance of entering ring vortex state, which is nearly unrecoverable close to the ground, especially in steep approaches

Improve this answer
$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .