Vortex Generator question

[RFCC]

A recent article in Flight International states the Boeing 777-300ER has had seven outboard vortex generators added to the mainplane. These are part of a mod kit fitted to reduce drag and improve performance. The VGs were added to improve handling with the raked wing tip, but "had a surprising fuel burn benefit of around eight-tenths of a percent".

Question - why do the VGs improve fuel burn?

[ifleeplanes]

Vortex generators can be used to create a 'virtual' wing fence and reduce span wise flow...as such they can reduce induced drag and thus increase fuel economy....I think

[wonderboysteve]

They can be used to delay shock-induced separation, hence improving cruise drag characteristics. I wouldn`t expect them to do very much to change the spanwise flow; flow visualisation of vortex tracks will show them heading outboard.

[ifleeplanes]

They do head outboard but due to the increased airflow energy not as rapidly as free air hence they can create a virtual fence and DO help to reduce induced drag.

[mehdi72]

vortex generators

VG's are small parts about few square inch/each fixed on the extrados of a wing.The meaning of VG's is to create vortices which will transfer energy from the general flow of the air towards the layers that sustain the most "braking effect" close to the surface of the wing.In other words the time the flow will stall will be delayed thus the friction drag is lowered so will be the fuel flow.The general idea is to transform laminar flow into turbulent (more energy).The ratio between inertia/viscosity of a flow is called Reynolds number(Re).In this relation you can see that if you reduce the viscosity, Re will increase:the higher the number the more turbulent the air flow will be thus the more energized and the less friction agaisnt the wing.This is exactly the purpose of VG's.
Hope I could help

[Ozgrade3]

I want to know how do they measure eight tenths of 1 percent. Is it calculated or shows up in testing.

[RFCC]

Thanks for the replies guys/gals

[barit1]

If it's "a surprising fuel burn benefit", I read that as "not predicted by analysis calculations".

In other words, the measured fuel burn was below predictions.

But I stand to be corrected...

[Old Smokey]

I want to know how do they measure eight tenths of 1 percent. Is it calculated or shows up in testing.

All performance data is first calculated, and then proven by testing.

Don't confuse this with APD (Aircraft Performance Deterioration) factor, which is a negative performance adjustment arising from monitoring of 'in-service' aircraft over a lengthy period of time.

Regards,

Old Smokey