# What is this flange on the engine?

Flying home from the Denver ComiCon, on a 737, I saw an angled flange on the engine. Could you describe its purpose?

• Jun 20 '14 at 4:16
• Good thing the photos are copyright. You wouldn't want someone steeling those. Nov 21 '14 at 23:56
• @kjmccarx I posted that because I improperly failed to accredit photos in another stack exchange and was flagged for copyright violation. I have cleaned up all of my questions and answers so that any photos I used are either my own, or copyright as creative commons and attributed correctly. Nov 22 '14 at 0:03
• @CGCampbell Fair enough. I was just giving you a hard time. Always better to be safe than sorry. Nov 22 '14 at 0:10
• You know what? This question is 5+ years old, they're my photos, they're in the PD, I attributed them... I'm good with it. Dec 7 '19 at 2:03

This is called a "strake" or "chine" (Aerodynamic fan cowl strake/chine, Nacelle strake/chine).
They allow the aircraft to generate more lift at lower speeds, which entails such positive consequences as lower stall speeds, lower landing speeds, lower take-off speeds and shorter runways.

Strakes should be used if the nacelles are mounted closely under the wing of the aircraft, see also Why don't all aircraft use/need nacelle chines?. Nacelles are usually mounted like that in order to

With an optimal arrangement and at high angles of attack, such vortex generators, which are known as nacelle strakes or chines, generate a powerful vortex that flows over the wing, where on a slat in front of said wing it delays airflow separation until the aircraft flies at greater angles of attack.

The idea dates back to 1971 (US Patent 3744745: Liftvanes):

in operational conditions wherein high angles of attack are encountered, such as in landing or takeoff, the vanes oppose the strong upwash around the nacelle, reducing the flow separation on its upper areas, and providing a strong downwash marked by marginal trailing vortices

DC-10 wind tunnel tests showed a significant loss in maximum lift coefficient in the flap deflected configurations, with landing slat extension, compared to predictions. This resulted in a stall speed increase of about 5 knots in the approach configuration. The initial wing stall occured behind the nacelles and forward of the inboard ailerons. The problem was traced by flow visualization techniques to the effects of the nacelle wake at high angles of attack and the absence of the slat in the vicinity of the nacelle pylons. The solution was developed in the NASA Ames Research Center 12 ft. pressurized tunnel and turned out to be a pair of strakes mounted forward on each side of the nacelles in planes about 45 degrees above the horizontal. The final strake shape was optimized in flight tests. The strakes are simply large vortex generators. The vortices mix the nacelle boundary layer air with the free stream and reduce the momentum loss in the wake. The vortices then pass just over the upper surface of the wing, continuing this mixing process. The counterrotating vortices also create a downwash over the wing region unprotected by the slat, further reducing the premature stall. The effect of the strakes is to reduce the required takeoff and landing field lengths by about 6%, a very large effect.

• Very nice, thorough answer with great references. Interesting stuff too! Jun 20 '14 at 12:08
• So what happens to airflow during cruise? Do these strakes have very little effect or drag for most of the flight? If they did, I would think they would be made in-flight adjustable. Jun 20 '14 at 14:04
• @PhilPerry It's claimed that the chines provide these benefits "without having a negative influence on the drag at cruising", and that "The longitudinal inclination of the vortex generator is selected such that the drag is as low as possible during cruising."
– user2168
Jun 20 '14 at 16:11
• @PhilPerry But, there are retractable chines (The Boeing Company: Retractable nacelle chine. US Patent 8087617). They say that there is some unwanted drag associated with fixed chines.
– user2168
Jun 20 '14 at 16:16
• @PhilPerry When cruising, the angle of attack is lower, the chine is more-or-less aligned with the airflow, and it does not generate large vortices. This is all made visible by condensation when flying on a humid day. In a sense, the reduction of angle of the attack in cruise is the adjustment you are thinking of. May 5 '15 at 15:20

Adding to user2168's answer, here are some pictures visualizing the principle of nacelle chines, also called nacell strakes or aerodynamic fan cowl strakes.
Please note that the vortices are always there, but they can only be seen in special conditions, see the end of this answer for an explanation.

As was stated above, some conditions have to be true if vortices are to be seen:

• To be more precise, the humidity, pressure and temperature of the ambient air has to "match" the pressure drop induced by the nacelle chines/strakes (this pressure drop is due to the flow acceleration, i.e. a higher flow velocity, caused by the nacelle chines/strakes). For passenger aircraft, this is usually the case on humid days not too far above the ground.

• Let's assume this is true. In addition, we assume that the air is incompressible, which is usually true for velocities $\mathit{Ma} \leq 0.3$ - which is the case for take-off and landing.
Now, the increase in flow velocity $c$ caused by the nacelle chines/strakes leads to the decrease of static pressure $p$, which leads to the decrease of static temperature $T$ (the total/stagnation pressure $p_0 \approx p_t$ and total/stagnation temperature $T_0 \approx T_t$ approximately stay constant, $p_0 \approx p_t \approx \mathit{const}$, $T_0 \approx T_t \approx \mathit{const}$).
If the temperature drops below the local dew point, condensation occurs: the gaseous water vapor transforms into liquid water droplet clouds - this is what can be seen in pictures.
As a side note: During condensation, the "enthalpy / latent heat of condensation" is released.

• Similar effects and explanations can be found e.g. in How does this vortex form inside a jet engine?, Why does condensation form on the wing especially during take-off and landing? and What caused a fluctuating cloud to form in a jet engine intake on a humid day?.

Image sources:

I do not believe the teaching in the patent is correct.

Instead of the downwash from the strake suppressing the wing upwash (and remember, the other side of the vortex adds to the wing upwash!) we believe that the vortex is well organized and can withstand the adverse pressure gradient of the wing better than the 'junk' wake of the nacelle without the strake.

The size and location of the strake determine when the vortex will burst, and hence govern when the inboard wings stalls. Ideally you would like this to be at the same time as the outboard wing. Too late and you get unacceptable pitch up. Too early and you lose low speed performance (higher stall speeds equated to higher speed schedules and longer takeoff field lengths).

A side note, the outboard strake is of little to no value at all, which is why Boeing airplanes only have strakes on the inboard side.

• hi and welcome. this is a comment on another answer (that includes a patent), on its own it's not clear how your answer answers the question itself.
– ymb1
Jan 14 '20 at 17:06
• @Dougball The answer contains valuable information. Please edit the answer so that it forms a comprehensive answer to the question. Jan 15 '20 at 3:13