Winglet Question from a university exam...
Yes, at degree level I think I'd have given that zero as well.
The tips increase the effective aspect ratio through modification of the Oswald span efficiency factor (e) in the term Cd = Cdo+(1/e.pi.AR)Cl^2.
This effective increase in AR reduces the induced drag term. (NOT the profile drag term, very occasionally called "pressure drag").
Physically, it reduces the energy loss through creation of tip vortices, as admirably demonstrated by some of the pretty pictures above.
So drag is reduced, Cl/Cd is increased, wake vortices reduced thus reducing wake separation requirements, fuel burn is reduced, particularly in low speed flight where the Cl^2 term predominates, although there will still be some effect at high speed (where the Cdo term predominates).
At anything above foundation level, I'd have thought that your lecturers would be looking for an explanation along the lines I've given above, with a sketch diagram of the tip vortices and how they reduce in magnitude.
G
The tips increase the effective aspect ratio through modification of the Oswald span efficiency factor (e) in the term Cd = Cdo+(1/e.pi.AR)Cl^2.
This effective increase in AR reduces the induced drag term. (NOT the profile drag term, very occasionally called "pressure drag").
Physically, it reduces the energy loss through creation of tip vortices, as admirably demonstrated by some of the pretty pictures above.
So drag is reduced, Cl/Cd is increased, wake vortices reduced thus reducing wake separation requirements, fuel burn is reduced, particularly in low speed flight where the Cl^2 term predominates, although there will still be some effect at high speed (where the Cdo term predominates).
At anything above foundation level, I'd have thought that your lecturers would be looking for an explanation along the lines I've given above, with a sketch diagram of the tip vortices and how they reduce in magnitude.
G
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Sorry, but wake vortices are not reduced. They are different. With the winglets there are the two components, the small core and the large core, virtually 2 separate vortex, and they act much differently.
The small core are very high velocity, and a smaller core, which may break up earlier. The large core are slower moving, and tend to last longer.
Under the right conditions, a small core will transform into a large core. This tend to happen when the air is drier, and calmer. Under this scenario, the vortex is much longer lasting, and therefore a much higher chance of WVE.
The flaps on the ac have the largest effect on wake vortex. A 737-800 on final with flaps 20, generates a much different vortex than the same aircraft with flaps 30 and/or 40. The same aircraft.
IF you want to see some real interesting vortices, look at the A380 on final....
The small core are very high velocity, and a smaller core, which may break up earlier. The large core are slower moving, and tend to last longer.
Under the right conditions, a small core will transform into a large core. This tend to happen when the air is drier, and calmer. Under this scenario, the vortex is much longer lasting, and therefore a much higher chance of WVE.
The flaps on the ac have the largest effect on wake vortex. A 737-800 on final with flaps 20, generates a much different vortex than the same aircraft with flaps 30 and/or 40. The same aircraft.
IF you want to see some real interesting vortices, look at the A380 on final....
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For the question: "Describe performance benefits of aircraft winglets"
The prformance benefits are specifically, increased take off and landing weights and lower cruise fuel burn.
Degree level education should be testing understanding, not tick-box glib answers as might be preferred for pilot TK exams.
Without seeing the module descriptor and whole exam paper of-course, it's hard to be at-all sure of the level that was expected of this student.
G
Without seeing the module descriptor and whole exam paper of-course, it's hard to be at-all sure of the level that was expected of this student.
G
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Genghis,
I assume that you are asking about the A380 wake? I have not published anything about about this aircraft yet, as the data collection is still ongoing.
There are a few significant factors that influence the creation of the wake turbulence on final. Flaps are the most significant factor in vortex creation. (as noted with the 737-400 with outboard flaps vs 737-800 without, or flap setting 25 vs 30/40). Think about this, on final in a crosswind, the AC is crabbing, exposing one wing more than another, so you have a completely different vortex off of each wing...(in all the illustrations about crosswind blowing the vortex out of the way, they are incorrect, and actually, the vortex rollup can be increased on one side and decreased on another...I have even seen them rollup and bounce along the wind like a stone skipping on water)
The GPA is another, and a shallower GPA tends to reduce the strength and velocity of the vortices.
What I will also note on the A380, is a slow FAS, and that once the wheels are down, the airflow is so scrambled, that the turbulence doesnt rollup...
I assume that you are asking about the A380 wake? I have not published anything about about this aircraft yet, as the data collection is still ongoing.
There are a few significant factors that influence the creation of the wake turbulence on final. Flaps are the most significant factor in vortex creation. (as noted with the 737-400 with outboard flaps vs 737-800 without, or flap setting 25 vs 30/40). Think about this, on final in a crosswind, the AC is crabbing, exposing one wing more than another, so you have a completely different vortex off of each wing...(in all the illustrations about crosswind blowing the vortex out of the way, they are incorrect, and actually, the vortex rollup can be increased on one side and decreased on another...I have even seen them rollup and bounce along the wind like a stone skipping on water)
The GPA is another, and a shallower GPA tends to reduce the strength and velocity of the vortices.
What I will also note on the A380, is a slow FAS, and that once the wheels are down, the airflow is so scrambled, that the turbulence doesnt rollup...
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My understanding of the physics of the wake is this:
The aircraft's lift force is matched by a transfer of momentum downwards to the airflow in the wake (Mr Newton), which is what drives the vortex. In level flight, the rate at which the momentum is transferred matches the aircraft's weight, and is inevitably associated with some energy imparted to the air: mostly kinetic energy in bulk flow, with some thermodynamic effect from changes in pressure, both smoothly and in shocks.
By minimizing the kinetic energy given to the wake, for a certain amount of momentum, you reduce the rate at which energy is lost to the bulk airflow (induced drag) for a certain amount of lift.
The amount of kinetic energy imparted to the air in the wake depends on the product of its density and the speed-change-squared - while the momentum transferred depends on the product of the density and the speed-change. The key goal is then to minimize the average change in speed in the airflow in the wake, especially of the fastest-moving parts of the wake - the core of the vortex, where the speed squared term is most significant.
If you can reduce the average speed of the same mass of air in the vortex, by making the extent of the fast parts of the vortex less, as suggested in the Boeing cartoon above, or otherwise, using a fence, increased span, raked tip, winglet etc., then you're making the owner happier.
It's not dissimilar to the efficiency of a jet/propellor - you want to move as large a mass of air as possible, by as small a change in speed as possible, to get the maximum force from the minimum amount of energy expended.
The aircraft's lift force is matched by a transfer of momentum downwards to the airflow in the wake (Mr Newton), which is what drives the vortex. In level flight, the rate at which the momentum is transferred matches the aircraft's weight, and is inevitably associated with some energy imparted to the air: mostly kinetic energy in bulk flow, with some thermodynamic effect from changes in pressure, both smoothly and in shocks.
By minimizing the kinetic energy given to the wake, for a certain amount of momentum, you reduce the rate at which energy is lost to the bulk airflow (induced drag) for a certain amount of lift.
The amount of kinetic energy imparted to the air in the wake depends on the product of its density and the speed-change-squared - while the momentum transferred depends on the product of the density and the speed-change. The key goal is then to minimize the average change in speed in the airflow in the wake, especially of the fastest-moving parts of the wake - the core of the vortex, where the speed squared term is most significant.
If you can reduce the average speed of the same mass of air in the vortex, by making the extent of the fast parts of the vortex less, as suggested in the Boeing cartoon above, or otherwise, using a fence, increased span, raked tip, winglet etc., then you're making the owner happier.
It's not dissimilar to the efficiency of a jet/propellor - you want to move as large a mass of air as possible, by as small a change in speed as possible, to get the maximum force from the minimum amount of energy expended.
