# Helicopter aerodynamic dashed linear contrail

Discussion in 'Images and Videos: Contrails, Skies, and Aviation' started by Trailspotter, Sep 25, 2016.

1. ### TrailspotterSenior Member

While paying a visit to the place of my childhood in Russia ten days ago (September 15, 2016), I spotted this very unusual contrail:

A local Aero Club regularly hires a Mi-8 (Mi-17) "Hip" helicopter for skydiving and related activities. It takes the jumpers to the altitudes of 2500 metres (8000 ft) or higher. I watched them from the ground about one kilometre from the airfield. While zooming at the helicopter as it reached the jumping altitude, I noticed a short white trail coming from the tips of the rotor blades on one side of the helicopter. Apparently associated with the blade tip vortices, this linear trail looked nothing like the vortex spirals behind moving helicopter should look:

However, the linearity of the trail can be readily explained by the vapour condensation occurring not along the entire blade tip path, but only at a particular point, the position of which is fixed relative the helicopter. Indeed, this was the case, as the following video (slowed down ten times) shows:

Source: https://youtu.be/Y4Kw23px6vk?t=21s

As each retreating blade tip passes the point at about right angle to the forward direction, it adds a short curvy dash of new condensation at the beginning of the trail. Sometimes it also creates a longer but less persistent contrail arc after the dash.

I also have posted on youtube another video of the same helicopter with better close-ups but no contrails

Last edited: Sep 25, 2016
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2. ### TrailspotterSenior Member

The second time I witnessed the same contrail type happened yesterday, one year and a bit after the first one:

Like the first time, the rotor blade tip contrails were forming at one particular point, where retreating blade was at about right angle to the helicopter's forward direction. Unfortunately, these contrails were too short-lived with individual dashes dissipating before the formation of a new dash.

Source: https://youtu.be/ga3v8x33nZg

3. ### Clouds GivemethewilliesActive Member

In level flight both sides must produce the same lift moment, but the distribution of lift is different along the blade.

4. ### TrailspotterSenior Member

It is a bit more complicated, as in level flight the rotor is tilted, so aerodynamic forces acting on each blade, a.k.a. "lift", have both vertical and horizontal components. The moments of vertical components must be cancelled indeed, but those of horizontal components normally are counterbalanced by the tail rotor.

5. ### Clouds GivemethewilliesActive Member

No comment in the interest of on-topicness..