Cricket ball 'swing' and aerodynamic lift
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Cricket ball 'swing' and aerodynamic lift
Please forgive the probably basic question from a non-pilot who visits this site for entertainment as well as to learn.
If one accepts that a 'swing' bowler bowls a ball in such a way that the ball moves through the air with the ball rotating along the seam in the line of flight, ie so that each side of the ball maintains its original orientation throughout the flight, and the 'swing' is sideways movement of the ball in flight due to the rough side of the ball experiencing higher pressure than the smooth or polished side,
then, why do aircraft wings not have the underside made 'furry' to increase lift?
I'm aware of the complexities of lift beyond the simplistic explanation of the air above having farther to travel etc. Still, it seems that the aerofoil wing is in some ways in the same situation as the rough and smooth sided cricket ball.
Can anyone shed some light on this?
If one accepts that a 'swing' bowler bowls a ball in such a way that the ball moves through the air with the ball rotating along the seam in the line of flight, ie so that each side of the ball maintains its original orientation throughout the flight, and the 'swing' is sideways movement of the ball in flight due to the rough side of the ball experiencing higher pressure than the smooth or polished side,
then, why do aircraft wings not have the underside made 'furry' to increase lift?
I'm aware of the complexities of lift beyond the simplistic explanation of the air above having farther to travel etc. Still, it seems that the aerofoil wing is in some ways in the same situation as the rough and smooth sided cricket ball.
Can anyone shed some light on this?
and the 'swing' is sideways movement of the ball in flight due to the rough side of the ball experiencing higher pressure than the smooth or polished side,
You could achieve a similar effect by making the upper surface of the wing more draggy, but that would defeat the objective. The goal is not an upward force at any cost, but rather the optimisation of lift to drag. Merely increasing drag doesn't help.
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The movement of a cricket ball is connected to the way the flow seperates from the surface of the ball; any asymmetry in the flow seperation will induce a force upon the ball, causing it to deviate from the 'straight' line.
Roughness, and indeed also the seam, can influence the seperation characteristics, by changing the transition from laminar to turbulent boundary layer flow, which changes how the flow behaves. Turbulent boundary layers seperate less easily, so the flow on the rough side, or the side with the seam forward, will tend to stay attached longer. As a result the wake becomes larger on the other, shiny, side, and the asymmetric wake will tend to push the ball towards the rough/seam direction.
Incidentally, aircraft DO use this behaviour - but since it's concerned with controlling seperations, and the best solution is to have no seperation at all, we generally attempt to design airfoils etc to minimise them.
However, everything must seperate eventually, and so if the nature of the seperation is important various devices - like vortex generators, typically - are used to control the way the flow seperates - although usually we're striving for symmetric behaviour, not asymmetric.
Roughness, and indeed also the seam, can influence the seperation characteristics, by changing the transition from laminar to turbulent boundary layer flow, which changes how the flow behaves. Turbulent boundary layers seperate less easily, so the flow on the rough side, or the side with the seam forward, will tend to stay attached longer. As a result the wake becomes larger on the other, shiny, side, and the asymmetric wake will tend to push the ball towards the rough/seam direction.
Incidentally, aircraft DO use this behaviour - but since it's concerned with controlling seperations, and the best solution is to have no seperation at all, we generally attempt to design airfoils etc to minimise them.
However, everything must seperate eventually, and so if the nature of the seperation is important various devices - like vortex generators, typically - are used to control the way the flow seperates - although usually we're striving for symmetric behaviour, not asymmetric.
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Slightly off topic, but interesting 
Tests have shown that by shaping the leading edge of the wing like
this whale's flipper (Knolhval - dont know the english name),
we will get a lot more efficient wing
Note all the bumps on the leading edge!
The vortex created between the bumps makes the airflow stick to the wing much
better which has great effects.
- 8% more lift
- 32% less drag
More fuel efficient, better low speed performance, better range etc.
MM
Tests have shown that by shaping the leading edge of the wing like
this whale's flipper (Knolhval - dont know the english name),
we will get a lot more efficient wing
Note all the bumps on the leading edge!
The vortex created between the bumps makes the airflow stick to the wing much
better which has great effects.
- 8% more lift
- 32% less drag
More fuel efficient, better low speed performance, better range etc.
MM
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This is basically the job of vortex generators, however better not to need to use them at all as they cause drag.
They are often used on reducing the engine/pod drag on aircraft.
Reminds me of the old how can you tell an Olympic Airways aircraft jokes......hair under the wings.
They are often used on reducing the engine/pod drag on aircraft.
why do aircraft wings not have the underside made 'furry' to increase lift?
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I managed to get the direction of cricket ball swing wrong as well - sorry about that.
Thanks for the illuminating responses. It seems that an aircraft only flies by virtue of some interesting physics and subtle compromises.
Thanks for the illuminating responses. It seems that an aircraft only flies by virtue of some interesting physics and subtle compromises.
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There are two types of swing bowling in cricket: conventional and reverse swing. Conventional swing is where the airflow breaks away from the ball earlier on one (rougher) side of the ball than the other, resulting in more lift on the smoother side of the ball, thus pulling the ball towards the smooth side.
With reverse swing, one side of the ball is so rough that the drag caused produces a greater force than the lift on the smooth side, pulling the ball in the direction of the rough side, which is why reverse swing bowlers get the best results from an old ball.
With reverse swing, one side of the ball is so rough that the drag caused produces a greater force than the lift on the smooth side, pulling the ball in the direction of the rough side, which is why reverse swing bowlers get the best results from an old ball.
Last edited by CAT1; 3rd Oct 2005 at 10:18.
resulting in more lift on the smoother side of the ball
My initial reply assumed that it was about differences in drag (i.e. forces in the direction of flight), regardless of whether the cause is separation or skin friction.
More recent replies and the cited articles suggest lift, i.e. forces perpendicular to the direction of flight caused by differential pressure, which seems like a distinct mechanism.
Of course it could be a combination of the two, but what evidence is there for one rather than the other? Can we get some order of magnitude numbers?
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The ball is a projectile.
The force the original poster refers to is neither lift nor drag as it acts perpendicular to the plane of the balls trajectory which is the plane in which lift and drag are measured. The trajectory "bends" because of this force.
However, spinning objects in viscous fluid do develop "lift". Something that gave Mr Kutta & Joukowski a headache or two no doubt.
IMHO, if the ball is spinning as the original poster suggests, the difference is purely due to the fact that a turbulent boundary layer will remain attached longer than a laminar one, with the net effect being a resultant force in the direction of the swing.
Aircraft want as much of the wing as possible immersed in laminar flow in the high speed regime and the latest possible flow separation i.e., highest AOA for stall, in the low speed regime.
I seem to remember colleagues of mine trying to suck away the boundary layers on aerofoils in our wind tunnels to reduce Cd...
The force the original poster refers to is neither lift nor drag as it acts perpendicular to the plane of the balls trajectory which is the plane in which lift and drag are measured. The trajectory "bends" because of this force.
However, spinning objects in viscous fluid do develop "lift". Something that gave Mr Kutta & Joukowski a headache or two no doubt.
IMHO, if the ball is spinning as the original poster suggests, the difference is purely due to the fact that a turbulent boundary layer will remain attached longer than a laminar one, with the net effect being a resultant force in the direction of the swing.
Aircraft want as much of the wing as possible immersed in laminar flow in the high speed regime and the latest possible flow separation i.e., highest AOA for stall, in the low speed regime.
I seem to remember colleagues of mine trying to suck away the boundary layers on aerofoils in our wind tunnels to reduce Cd...
The force the original poster refers to is neither lift nor drag as it acts perpendicular to the plane of the balls trajectory which is the plane in which lift and drag are measured. The trajectory "bends" because of this force.
And thinking slightly more about the physics, the "differential drag" argument that I put seems to be flawed.
Intuitively, differential drag causes turning: if we deploy the airbrake on one side rather than the other... if we make one tyre of a motor vehicle softer than that on the other side... it will tend to turn.But fundamentally, Newton's Laws require a force perpendicular to the direction of motion to cause the turn. Differential drag simply causes yaw. In the intuitive cases, directional stability pairs that yaw up with a corresponding sideforce and causes the turn.
But for the ball, directional stability doesn't seem relevant. Thus the swing can only be caused by a difference in the mean pressure from one side to the other.
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I seem to remember the effect being called the 'Coander' effect, not sure of the spellung 
It is used on helicopters enabling the tail boom of some heavy helos to off load the tail rotor by giving an element of lift.
The Notar also uses this effect to provide a 'NO Tail Rotor' aircraft. This uses a variable slot along the length of the tail boom which is fed by low pressure air from a fan in the after avionics compartment. By varying the slot the lift produced by the circular section tail boom can be altered thus assisting the steering duct at the back.
However, as helos are, probably, the worlds worst aerodynamic beasties, coupled with the fact that the majority of drag effects felt by these uses are infact perpendicular to the flight path, drag is not so much of a factor.
Sticking curry house furry wallpaper to the underside of a wing will dramatically increase drag, requireing an increase in thrust for any given value of Cl.
It is used on helicopters enabling the tail boom of some heavy helos to off load the tail rotor by giving an element of lift.
The Notar also uses this effect to provide a 'NO Tail Rotor' aircraft. This uses a variable slot along the length of the tail boom which is fed by low pressure air from a fan in the after avionics compartment. By varying the slot the lift produced by the circular section tail boom can be altered thus assisting the steering duct at the back.
However, as helos are, probably, the worlds worst aerodynamic beasties, coupled with the fact that the majority of drag effects felt by these uses are infact perpendicular to the flight path, drag is not so much of a factor.
Sticking curry house furry wallpaper to the underside of a wing will dramatically increase drag, requireing an increase in thrust for any given value of Cl.
Last edited by john_tullamarine; 5th Oct 2005 at 01:49.
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The Coanda Effect (so named after Henri-Marie Coanda) relates to the phenomenon whereby a fluid stream tends to follow the profile of the bounding surface providing that the divergence doesn't get too silly .. not really related to this question.
Without getting into arguments of semantics, fluid forces basically relate to what happens if we can get a fluid to move .. in making it move, we need to exert a force (generally achieved by pressures acting over a surface) and we can get a reaction (sometimes referred to as a psuedo-force for reasons beyond me) which might be of use to us .
Aim is to maximise the useful force component (lift in aircraft-speak .. ie a force normally acting in a direction pendicular to the direction of flight) while giving away as little as we can to the undesirable bit (drag .. acting parallel to flight and requiring power to offset).
This exercise can be achieved by
(a) sticking anything into the wind .. bit like putting one's hand out of the car window .. depending on orientation, the perceived force can be varied from all drag to a useful amount of lift with an attendent drag penalty .. not too effective, though
(b) sticking a suitably shaped surface into the wind .. otherwise called a wing .. the aerodynamicist's black art is to maximise the desirable while minimising the undesirable
(b) spinning a body in the wind .. if we use a cylinder, for instance, very high lift forces can be generated .. this is a pretty stock standard undergraduate wind tunnel demo exercise in first year aerodynamics ...
We can also get the same sort of effect with a sphere (cricket ball etc) and this is the black art of our illustrious (if somewhat subject to media harassment for the odd peccadillo) Warney and his spin colleagues.
(c) manipulating laminar and turbulent flow separation to give a nett lateral force resulting from the different forces associated with the two sorts of flow. This is the alternative black art of the fast bowler who gets a good deal of swing in the delivery of the ball.
Ah .. fond memories of playing about in wind tunnels as a youngster ...
Without getting into arguments of semantics, fluid forces basically relate to what happens if we can get a fluid to move .. in making it move, we need to exert a force (generally achieved by pressures acting over a surface) and we can get a reaction (sometimes referred to as a psuedo-force for reasons beyond me) which might be of use to us .
Aim is to maximise the useful force component (lift in aircraft-speak .. ie a force normally acting in a direction pendicular to the direction of flight) while giving away as little as we can to the undesirable bit (drag .. acting parallel to flight and requiring power to offset).
This exercise can be achieved by
(a) sticking anything into the wind .. bit like putting one's hand out of the car window .. depending on orientation, the perceived force can be varied from all drag to a useful amount of lift with an attendent drag penalty .. not too effective, though
(b) sticking a suitably shaped surface into the wind .. otherwise called a wing .. the aerodynamicist's black art is to maximise the desirable while minimising the undesirable
(b) spinning a body in the wind .. if we use a cylinder, for instance, very high lift forces can be generated .. this is a pretty stock standard undergraduate wind tunnel demo exercise in first year aerodynamics ...
We can also get the same sort of effect with a sphere (cricket ball etc) and this is the black art of our illustrious (if somewhat subject to media harassment for the odd peccadillo) Warney and his spin colleagues.
(c) manipulating laminar and turbulent flow separation to give a nett lateral force resulting from the different forces associated with the two sorts of flow. This is the alternative black art of the fast bowler who gets a good deal of swing in the delivery of the ball.
Ah .. fond memories of playing about in wind tunnels as a youngster ...
Last edited by john_tullamarine; 5th Oct 2005 at 01:52.
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The circulation lift phenomenon developed by a rotating cylinder or sphere in an airstream often is referred to as the Magnus effect (after Heinrich Magnus). Actually, the effect's observation predated Magnus, having been described by Newton nearly 200 years earlier.
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We are a long way from a swinging cricket ball (!)... and as someone who was bowled for three consecutive ducks this summer...once by a twelve year old...I would be intrigued if anyone come up with a definitive theory.
My ATPL level aerodynamics is hopelessly inadequate; however I can follow the varying transition points and the disparity in drag, but surely the low px created by the smoother side would to some extent negate this and result in the ball remaining reasonably true ?
Thanks... paddling back to my depth now.
MoT
My ATPL level aerodynamics is hopelessly inadequate; however I can follow the varying transition points and the disparity in drag, but surely the low px created by the smoother side would to some extent negate this and result in the ball remaining reasonably true ?
Thanks... paddling back to my depth now.
MoT
I'm sure I read somewhere that the Soviets used the same principle to steer their early space capsules on re-entry. Apparently the landing point could be varied quite accurately over several hundred kilometers.
