Asymmetric Partially Aerodynamic Turboprop Contrails at 25,000 and 27,000 feet

Mick West

Administrator
Staff member


This photo is interesting for a number of reasons. It's a propellor plane flying at 27,000 feet and leaving a contrail. One side of the contrail is significantly larger than the other. The photo was taken at about 18:20 UTC (11:20 PDT) on Friday Oct 23, 2015.

It appears at first as if it is only leaving one contrail, but as it spreads out, it becomes more apparent that there are two:


The plane itself is N575RD, at twin engine turboprop:


Earlier I'd been taking photos of what I though were 737's or similar climbing out of SFO, and leaving aerodynamic contrails. I was surprised when I zoomed in on one and found that was a turboprop too.


This was Alaska flight 2586, a Bombardier Dash 8 Q400 (N404QX), at an even lower 25,000 feet, leaving a very large contrail. It was flying from San Jose.

It also had a curiously asymmetric contrail, with the trail on the pilot's left (bottom in the photo), seeming to diverge a lot more than the other. I wonder if this is to do with the rotation of the engine wake interacting with the vortex sheet from the wing. If both engines rotate in the same direction (which it seems they do, looking at other photos of the plane), then that will produce two different wake interactions.

Both contrails were persistent, and at one point they crossed. The large contrail here is from the Alaskan flight.


The Alaskan contrail was also very long, going from horizon to horizon (my horizon is a bit limited by hills and trees though).


The propellor had a very interesting effect on the trails, which made me wonder if the primary formation mechanism here was actually aerodynamic from the props?


Remember this is at 25,000 feet. Exhaust contrail formation temperature is -40°F, and US Air Net gives this forecast for Sacramento (very close to here).


Meaning you would not expect any exhaust contrails until well above 30,000 feet, and neither of these planes were even close.

Looking at nearby soundings for this morning (12Z = 5AM)
25,000 feet is 7620m, the temperature is only -24° to -26°C (and that's at 5AM).

So are these purely aerodynamic contrails?
 
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Trailblazer

Moderator
Staff member
Looking at that last photo I would say they do look purely aerodynamic. And I think you are right about the interaction with the wing vortices being responsible for the asymmetry. The wing vortices would have opposite "handedness" on either side of the plane, as the wings are mirror images, whereas the propellers are rotating in the same direction.
 

skephu

Senior Member.
Wouldn't aerodynamic contrails start directly at the propellers like in this photo?

I wouldn't exclude the possibility of exhaust contrails. Turboprop engines have higher efficiency than high-bypass turbofans, so the critical temperature of contrail formation is higher as well.
 

Hama Neggs

Senior Member.
Wouldn't aerodynamic contrails start directly at the propellers like in this photo?

Maybe they are started by the props, but then added to by the engine exhaust to where they become visible JUST where the prop portion already exists?
 

Mick West

Administrator
Staff member
Wouldn't aerodynamic contrails start directly at the propellers like in this photo?
Yeah, but I think the ice crystals are just initially too small to see. The pressure drop below the critical temperature is shorter when it's warmer. You see this with full-wing aerodynamic contrails too.

This is the other plane I photographed about a minute earlier (made the "bottom" trail on the curved panorama)


A320-211, Around there it was climbing through 25,000 feet. I think this is an inner wing aerodynamic contrail.

I wouldn't exclude the possibility of exhaust contrails. Turboprop engines have higher efficiency than high-bypass turbofans, so the critical temperature of contrail formation is higher as well.

Not -25°C though surely? The pressure at 25,000 feet would be around 375mb.



Hmm, need to get some figures going here. What's the contrail factor for a turboprop?
 
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Trailspotter

Senior Member.
Earlier I'd been taking photos of what I though were 737's or similar climbing out of SFO, and leaving aerodynamic contrails. I was surprised when I zoomed in on one and found that was a turboprop too.


This was Alaska flight 2586, a Bombardier Dash 8 Q400 (N404QX), at an even lower 25,000 feet, leaving a very large contrail.

The upper ceiling of Q400 is only 25,000 ft, yet there are more records of its contrails than those of other passenger turboprops of similar types. I presented quite a few in my earlier thread:
https://www.metabunk.org/contrails-from-turboprop-planes-at-lower-than-typical-altitudes.t6082/
Unfortunately, some of old youtube videos and flickr photos listed there are not available anymore, so I've updated my post with new examples of Q400 contrails. Interestingly, all these trails display similar asymmetry. Some of them could be aerodynamic ones:


others look like ordinary exhaust contrails:


My initial explanation of the preponderance of Q400 in contrail formation over other passenger turboprops was its having the more efficient engine hence the cooler exhaust. Notion of persistent aerodynamic contrails usually occurring at lower altitudes than exhaust contrails suggests a plausible alternative explanation - the Q400 preponderance is due to its lower ceiling, that is, flying at lower altitudes than its competitors.
 
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Hama Neggs

Senior Member.
The "puffs" might indicate an interaction between the prop and the pressure differential from the top to the bottom of the wing.
 

Mick West

Administrator
Staff member


This photo is interesting for a number of reasons. It's a propellor plane flying at 27,000 feet and leaving a contrail. One side of the contrail is significantly larger than the other. The photo was taken at about 18:20 UTC (11:20 PDT) on Friday Oct 23, 2015.

There was an interesting similar thing posted on Twitter today:


There's a video, but it's hard to see the trail. It is there though.
Source: https://www.youtube.com/watch?v=vxt_uRktyPk

 
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