Dimples

[buzzc152]

I was watching a programme last night about the designing of the bouncing bomb (what a awesome tuesday night !!).

ANyhow, whilst designing the ideal bomb they experimented with golf balls of varying design. They found that golf balls with dimples (as all golf balls have now) travelled further because of the greatly reduced drag when compared to a smooth golfball (no dimples). It has something to do with delaying the separation point of the airflow over the surface I think.

SO, my question is, why don't they make aeroplanes out of golfballs ? Or better still, why not have a dimpled aerofoil/fusalage surfaces like a golf ball has if it reduces drag ?

[Hippy]

Flippin good question that! :D
It got me thinking and I came up with the following totally non-technical answer: I recon the dimples would cause not just a loss of drag but also a severe loss of lift. Golf balls and bouncing bombs are supposed to drop out of the sky, aeroplanes aren't! ;)

Hippy.

[Capt. Amazing]

Just to recap on the golf ball airflow stuff and then answer your question.

When a golf ball is flying through the air, it has three forces on it. Two of them are gravity and drag from the air. In addition, if the golf ball is spinning, there will be a Magnus force on it (which may point up or down or either side, depending on how it is spinning -- the Magnus force cannot point along the flight path of the ball however.

The main goal of making a golf ball go farther however is to reduce the force of drag as it flies throught the air. In general, turbulence increases drag, because the energy needed to stir the air up and make it swirl around is energy that the ball has lost. One might think that a rough ball will induce more turbulence in the air than a smooth one, but it depends on how fast the ball is going. For balls going slowly through a viscous fluid, then the fluid just moves a bit to the side as the ball passes, and then it returns more or less to where it was. If the fluid motion is smooth, we call the motion "laminar", otherwise it is "turbulent." A ball moving quickly through a fluid like the air will have air flowing in a laminar fashion in some places and in a turbulent fashion in others. Directly behind the ball there will be a turbulent "wake", and surrounding that will be smoothly flowing air. The whole idea behind reducing the drag is to make the turbulent wake small.

The air that slides past the ball very close to it is called the "boundary layer". At the place where the turbulent wake starts is called "separation of the boundary layer" where the smoothly flowing air departs from the ball and does not close up behind the ball nicely but rather swirls around in small vortices. If the boundary layer can be encouraged to stick to the ball a little longer, then the turbulent part of the wake can be reduced. It turns out that adding a little extra turbulence in the boundary layer itself all over the ball allows the main smoothly-flowing air currents to stay closer to the ball and delays the separation of the boundary layer. There are some nice pictures of balls in wind tunnel test's showing this effect, and they can be found around the net. Sorry can't remember off hand where.

There is also an increase in the Magnus force, giving the ball some lift when it is spinning in the correct direction. This force helps keep the ball in the air longer, allowing it to travel farther.

People have thought of putting dimples on everything from swimsuits to cars to airplanes. You only get an advantage from these dimples if the boundary layer can be made to stick longer to the object. Some cars just have vertical flat ends to them where the trunk comes down and there is no way to reduce the turbulent wake of these no matter how dimpled the paint is. And the boundary layer stays with airplane wings, except maybe a bit at the ends (which can be helped by putting winglets out on the tips, or small vortex generators, or else vortilons on the wings). So overall you do not need them, as they would not be doing anything that is'nt already being taken care of.

At least, I think that's what may be happening, but I am sure someone will correct me if they think I am wrong.
:D ;)

[lomapaseo]

>When a golf ball is flying through the air, it has three forces on it. Two of them are gravity and drag from the air.<

That's strange, I can only think of two forces and only one due to gravity, the other being due to drag.

How does it develop a second gravity force while flying through the air.

I like the rest of the explanation though:)

[Flying Clog]

Hasn't Cathay or Singapore Airlines fitted one of their 74s with a dimpled surface to test the effect it would have on drag/fuel efficiency. Thought I saw it on a telly programme a few months ago.

[Capt. Amazing]

Three forces:

1) Gravity
2) Drag
3) Magnus force (if ball is spinning)

Magnus Force is the force acting on a rotating or spinning object.


Magnus force (http://www.marist.edu/physics/applets/magnusg.html)

Did I miss write it, or do you think I am missing something? If I am then it is ether. Magic ether.:D And the rest is just a cover story.

C.A.

[411A]

Hmm, 'tis called "area ruling" and suspect 747FOCAL could explain it better than most...:)

[Capt. Amazing]

The concept of area ruling was conceived by R.T. Whitcomb and was first employed on the Convair XF-102. The original XF-102 experienced problems achieving its design speed. However, redesigning the aircraft using the area rule yielded improved transonic performance. As a result, area ruling was used on future aircraft.

Definition of the area rule: The area rule provides that the cross-sectional area of an aircraft stays relatively constant along the length of the fuselage.

Use of this rule can minimize air flow disturbance and decrease drag.

[Mad (Flt) Scientist]

Just to clarify:

Addition of dimples to golf balls is in order to control the transition from laminar to turbulent flow in the boundary layer. This is, as noted, in order to delay separation of the boundary layer from the surface.

This is generally not required on an aircraft due to the much larger scale of an aircraft. The boundary layer will generally turn turbulent of its own accord before the possibility of separation occurs. Therefore there is no need to artificially promote trubulent flow.

With smaller bodies the need arises. And indeed with wind tunnel models it is commonly necessary to add "transition fixing" to ensure that the transition occurs at the right position, otherwise the flow will not be representative of the fulls cale aircraft.

The experiments being conducted with small scale devices on modern aircraft (such as the various films and coatings trialled recently) are generally an attempt to maintain laminar flow (which has less drag IF it does not separate than turbulent flow) - precisely the opposite of the golf ball dimpling.

And area ruling (which incidentally is concerned primarily with maintaining a smooth rate of change of cross sectional area, rather than a constant cross sectional area) has absolutely nothing to do with either delaying or promoting flow transition.

[buzzc152]

Thanks for all your answers chaps. I'm quite chuffed that I came up with a thought of my own that holds some engineering merit.

If one of you does decides to make a plane with dimpled wings, please make sure it is registered as G-BALL. :)