21
$\begingroup$

The Republic Day celebrations at New Delhi, India had a series of flypasts by the Indian Air Force.

What struck me the most was that in a formation with a C-17 in middle and two Su-30MKI flanking it, there were contrails visible from the Su-30MKI but not from the C-17:

C-17, Su-30MKI Formation on Republic Day 2018 C-17, Su-30MKI Formation on Republic Day 2018

Both the C-17 and Su-30MKI have turbofan engines. For C-17, Pratt & Whitney F117-PW-100 and for Su-30MKI Lyulka AL-31.

As per 'How does contrail formation differ from turbofan to turbojet?', contrails depend on the bypass ratio of the turbofan engine:

(...) higher bypass engines available today increase the contrail occurrence.

As per available sources, the F117 has a bypass ratio of 5.9:1 and the AL-31 has a bypass ratio of 0.59:1. So, the F117 has higher bypass compared to the AL-31 (or maybe I'm wrong).

What would be the reason that in the first two photographs, the C-17 engines do not form contrail whereas the Su-30MKI seem to?

One possibility is that, this difference is due to possible different power settings on the two aircraft engines for the same speed (considering the different intended role).

Another thing was that such contrails were not visible on any other fighter aircraft (e.g., Tejas/LCA). Or even another pair of Su-30MKI along with a AEW&CS aircraft:

Tejas/LCA Formation on Republic Day 2018 Su-30MKI along with a AEW&CS aircraft

Notes:

  1. Sorry for the poor quality of photos due to the poor visibility in Delhi (winter fog/smog).
  2. Some photos are here:

$\endgroup$
  • 23
    $\begingroup$ Are you sure that they weren't using some sort of smoke generation system? Those don't look like contrails to me. $\endgroup$ – Ron Beyer Jan 26 '18 at 18:41
  • $\begingroup$ @RonBeyer not sure/aware about smoke generation... But now that you mentioned it, I also doubt these are contrails... $\endgroup$ – Kanchu Jan 26 '18 at 18:46
  • 4
    $\begingroup$ Absolutely not contrails. Smoke generators for sure. Actual contrails, condensation from the motors, won't form near sa level in warm air. $\endgroup$ – Ralph J Jan 26 '18 at 18:50
  • 4
    $\begingroup$ And amazingly, in that second pic the Sukhoi's can apparently produce a giant bird in their trails! :D $\endgroup$ – TomMcW Jan 26 '18 at 19:10
  • 2
    $\begingroup$ They must be chemtrails $\endgroup$ – MCMastery Jan 26 '18 at 23:34
38
$\begingroup$

This 'party trick' is covered nicely on metabunk.org.

[It's the Su-30's] ability to inject unburnt fuel into the exhaust plume via the afterburner system. The super hot air of the exhaust instantly evaporates the volatile fuel, and then when it mixes with the cold air behind the plane the fuel reforms as a fine mist, essentially a fuel smoke cloud.

enter image description here(YouTube)

$\endgroup$
  • 2
    $\begingroup$ Ha, we actually came up with the same source moments apart. $\endgroup$ – Jamiec Jan 26 '18 at 18:53
  • 1
    $\begingroup$ Seconds apart! :D $\endgroup$ – ymb1 Jan 26 '18 at 18:54
  • 1
    $\begingroup$ 'party trick' :) Interesting read! +1 $\endgroup$ – Kanchu Jan 26 '18 at 18:56
  • 1
    $\begingroup$ And a +1 for the both of ya's (@Jamiec) for the "great minds think alike" effort. $\endgroup$ – FreeMan Jan 26 '18 at 21:18
  • 1
    $\begingroup$ Is there any practical reason to do this? Or is it just for cool points? $\endgroup$ – BruceWayne Jan 27 '18 at 3:15
31
$\begingroup$

From: Forum post - "Trails from the back of Fighter Jets", Metabunk.org

Jets practicing for air-shows (or actually in air-shows) sometimes use smoke to create trails. This is made by injecting something like paraffin into the hot exhaust where it vaporizes and condenses as smoke.

And the accompanying image

Looks a lot like the image in your question.

$\endgroup$

protected by ymb1 Jan 28 '18 at 2:21

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.