Dimpled Skin, Lower Drag??

[Hot High Heavy]

I was watching Mythbusters tonight and they were testing the 'Golf Ball' effect, ie the idea that the reason why a golf ball is dimpled is to make it create less drag (in the form of turbulence) and therefore make it fly further.

They tested it on a car by altering the skin with clay to dimple it and after putting some controls in place, discovered that there was an 11% fuel burn decrease. They were pretty amazed as was i!

My question is, if this is the case, why has it not been attempted on aircraft?

I always thought that smooth surfaces were better for airflow and too reduce parasite drag etc as indeed this is what is in text books. Is this dimple effect a new area of research? :confused:

(I'm not an engineer by the way, only a pilot!)

HHH

[SeldomFixit]

Shark skin film has been trialled successfully - to dimple Alclad would be to court disaster I would imagine.

[GS John]

Thought Airbus had trialled a boundary layer control film a few years back. Certainly gliding knew a lot about it, boundary layer control / drag reduction and was doing things about it in the late 80's.

[PPRuNe Towers]

Proof of concept stuff has been around for many years.

A bit of digging with 3M in the search string should show you info on the limited production runs of low drag film they've done.

Rob

[Gas Bags]

Come on guys,

What is a vortex generator for????????


This is basic aerodynamics!

[glhcarl]

Come on guys,

What is a vortex generator for????????




To create a vortex!!!!!

However, if the design engineer does his job correctly vortex generators are not required.

[aveng]

And we thought the bag chuckers and caterers were idiots for all the dents over the years. Who would have thought they were on to something!

[doppleganger]

It is all to do with tripping the boundary layer to make it turbulent sooner (i.e. moving the transition point forward). Turbulent air has more energy, so will stick to the surface further around the back-side of the golf ball. This decreases the profile drag a large amount, but for a small increase in skin friction drag created by the dimples.

Aircraft wings do not need dimples because they are not golf ball shaped. However, at strategic places, like in-front of flaps or ailerons, where there is a danger of flow separation, this movement of the transition point forward is done with vortex generators (as gas bags stated). It has exactly the same effect.

(Sure, dimples would do the same thing, but the fact that an aircraft wing is far more predictable re its orientation through the air than a golf ball is, the designers can be far more discerning when it comes to placing the flow-trip devices)

Great description of the golf-ball effect and how it is applied to aircraft is here:

Aerospaceweb.org | Ask Us - Golf Ball Dimples & Drag (http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml)

Hope that helps

Doppleganger
Aerodynamics Lecturer

[CV880]

GS John,
Yes, Airbus delivered an A340-300 in the late 90's to Cathay Pacific that was almost entirely covered in 3M's Riblet film. I don't know the official verdict of the trial but think the initial minor improvement disappeared with time. The film was removed after about 5 years. It gave the aircraft an odd satin finish.
I think the aircraft was B-HXC msn 142 (or B-HXD msn 147)

[Hot High Heavy]

Thanks for the detailed info Doppelganger, that does make a lot of sense. The link is very interesting!

One question though, surely the airflow around a car is quite predictable to designers? I am thinking that maybe they (the car designers) have probably decided to go with aesthetics on their vehicles. I couldn't imagine a nice shiny Jag that was dimpled all over like a golf ball!


Cheers - HHH

[doppleganger]

Airflow over/around a car is even more predictable than an aeroplane. There is a much smaller speed range, and no change in angle of attack to worry about.

However the back end of a car is so blunt that there is not a hope of the airflow sticking to the surface beyond the boot lid . The Austin Allegro was famously proven to be more aerodynamic backwards than it was forwards - a fact often touted by the presenters of Top Gear. Actually, the same applies to most, if not all motor cars, especially estates.

For best aerodynamics, the depression behind a car that causes most of its drag, should be filled in thus:

http://www.coolthings.com/wp-content/uploads/2010/01/boattail1.jpg

Not sure why this hasn't caught on! But in its absence, the best that can be done for that depression behind the car is to minimise it. This is done by deflecting the air sharply upwards as it leaves the boot lid, thus tightening the air-curl and dissipating its energy much closer behind the car, rather than leaving a long trail of eddies way behind the vehicle. That's why you often see the bootlid spoilers like these:

http://www.themotorreport.com.au/wp-content/uploads/2009/03/alpina-d3-bi-turbo-3.jpg

It is a flow-trip device of sorts, but is to dissipate the energy in the airflow more quickly, and over a shorter distance, rather than try to make it stick to any surface.

Don't expect to see dimples on car or aeroplane skins any time soon. They are a blunderbus approach to flow tripping, and most inefficient at that. They are a drastic compromise given that most other flow-trip devices wouldn't tolerate the repeated impact from a 5-iron.

[fly_antonov]

I' d like to add a few potential reasons:

A. The big difference between jet aircraft compared to cars and golf balls is the speed they travel at.
I believe that a dimpled airplane would help formation of shockwaves and sensibly reduce critical mach, increase shockwave-induced drag at high Machs.

B. The manufacturing could become a true cost nightmare.

C. From a maintenance point of view, it would be a nightmare to discern surface damage on a dimpled surface.

D. The dimples would require a sensible addition of material, hence more weight.

E. The dimples could become a hiding place for dirt, hence increase drag.

My 2 cents.

[Gas Bags]

On the mythbusters note, they did an episode regarding a utility vehicle comparing the fuel efficiency with the tailgate installed and without the tailgate installed.

Interestingly the utility vehicle was more fuel efficient with the tailgate installed and in the closed position.

GB

[Lyulka]

Some 737's have some golf-ball pattern on the fuselage, starting right behind the wing trailing edge. See this (http://www.airliners.net/photo/Royal-Air-Maroc/Boeing-737-4B6/1651611/L/)picture for an example

[SeldomFixit]

Incoming - some Muvvers do 'ave em :ok:

[doppleganger]

Not dissimilar from a golf ball is Amy Williams' skeleton bobsleigh helmet.

However, since the helmet enters the air the same orientation all the time, and does not have to withstand repeated beatings from a golf club, the designers have fitted much more efficient flow-trip devices in the form of vortex-generators.

Controversial as it may be, the results speak for themselves.

WINTER OLYMPICS 2010: Green light for Amy Williams as protest is given cold shoulder | Mail Online (http://www.dailymail.co.uk/sport/othersports/article-1252441/WINTER-OLYMPICS-2010-Green-light-Amy-Williams-protest-given-cold-shoulder.html)

[Rogercrier]

Doppleganger :- I read your article with interest. From somewhere I am under the impression that the dimples are also on a golf ball to help with lift. The club induces top backwards spin which gives faster airflow above the ball and slower under hence lift and the dimples grasp more air and drag it into this lift inducing spin. I am not an engineer, so may be talking out of a lower orifice?

Please advise.

[doppleganger]

I haven't heard of that as being another reason for dimples on a golf ball but the theory is good, so it is a reasonable idea.

Any object which is spinning as it sails through the air will experience a disymmetry of lift - sideways (eg the swerve of a cricket ball) or vertical, depending on the relative orientation and direction of spin. Anything which thickens the boundary layer (eg dimples) will aid the process.