Take the 2-minute tour ×
Aviation Stack Exchange is a question and answer site for aircraft pilots, mechanics, and enthusiasts. It's 100% free, no registration required.

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

Flange1 Flange2 Flange3

share|improve this question
1  

1 Answer 1

up vote 28 down vote accepted

This is called a "strake" or a "chine". They allow the aircraft to generate more lift at lower speeds. (Lower stall speed, lower landing speeds, lower take-off speeds, shorter runways.)

From US Patent 20100176249: Engine Nacelle Of An Aircraft Comprising A Vortex Generator Arrangement:

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.

enter image description here

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

enter image description here

From R.S. Shevell. Aerodynamic Bugs: Can CFD spray them away? 1985

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.

share|improve this answer
2  
Very nice, thorough answer with great references. Interesting stuff too! –  dvnrrs Jun 20 at 12:08
1  
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. –  Phil Perry Jun 20 at 14:04
1  
@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 at 16:11
3  
@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 at 16:16

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

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