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