Sticky solution
Question: I have a friend who uses molasses to clean rust off old iron items
collected from the bush. The rusty iron article is placed in a jar of molasses
solution (nine parts water, one part molasses) and left for two weeks. After
this time, the article comes out clean and almost shiny.
What is happening here?
Answer: Molasses contains chelating agents. These are made of molecules that
are shaped a bit like the claws of a crab鈥攖he word chelation comes
directly from the Latin word chele, meaning claw. They can envelop
metal atoms on the surface of an object, trapping them and removing them.
Molasses owes its properties to cyclic hydroxamic acids which are powerful
chelators of iron.
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More of these compounds are found if the molasses is derived from sugar beet
rather than cane sugar. The plants from which molasses is made presumably use
these chelating agents to help them extract minerals from the soil.
Interestingly, there are aerobic microorganisms that use similar cyclic
hydroxamic acids to scavenge iron. So plants and microbes appear to use the same
chelation strategy to obtain their daily ration of iron.
The same process is at work when you clean old coins with Vegemite or cola.
The power of chelating agents also explains why the insides of tomato tins need
to be lacquered. The citric acid in the tomatoes would dissolve the metal of the
container if the lacquer were not present. Household cleaning agents, especially
detergents and shampoos, also rely on chelation. These soften water to make it
more effective during the cleaning process.
Chelation has its uses in medicine, too. EDTA or ethylenediamine tetraacetic
acid is used as a chelating agent to control levels of calcium in the body and
can reduce the effects of mercury or lead poisoning.
Ben Selinger
Department of Chemistry
Australian National University
ACT
Ben Selinger is the author ofChemistry in the Marketplace
andWhy the Watermelon will not Ripen in your Armpit(Allen &
Unwin)
Travel tips
Question: Why are the wing tips on modern passenger jets upturned?
Answer: The small upturned fins on the end of the wing are to stop the
formation of wing-tip vortices. Forming these vortices takes quite a lot of
energy, and so leads to increased drag. On planes that do not have the wing-tip
fins, you can sometimes see the vortices during take-off and landing; on humid
days they appear as condensation trails.
Nature got here first, however. Many large birds have special separated
flight feathers at their wing tips that achieve the same effect.
R. L. Harris
Malvern, Worcestershire
Answer: The history of aircraft wing-tip appendages started in 1897 when
Frederick Lanchester patented the idea of using end plates to cut drag. The
first flight experiments with them were not conducted until about 1924, and in
1955 James Clements tried an improved version.
In 1976 NASA tested these winglets in wind tunnels and showed their potential
to reduce drag and fuel consumption. In 1979 John Spillman, H. V. Ratcliffe and
A. McVitie of the Cranfield Institute of Technology in Bedfordshire conducted
flight tests using multiple wing-tip sails, mounted on a Paris MS760 aircraft.
They found that three narrow sails, arranged in a cascade, with the leading one
about 30掳 from the horizontal and the trailing one horizontal, gave better
drag reductions than single winglets.
However, the NASA designs were adopted for use in production aircraft. The
theoretically superior Cranfield designs languished, possibly because their
complexity outweighed performance advantages or perhaps because the British
engineers didn鈥檛 have contracts with Boeing.
I have attached winglets to aerobatic kites and found lift-to-drag ratios
increased by about 25 per cent while the radii of tight loops decreased in a
similar way.
David Blackburn
Adelaide, South Australia
Answer: Perhaps not surprisingly, the arguments about whether winglets are 鈥渁
good thing鈥� are not clear-cut. When they were first introduced, it appeared that
improvements in efficiency of about 10 per cent could be produced. For newer
wing designs the improvements were smaller. In 1980, Boeing engineers found that
winglets were unlikely to prove commercially attractive for their new 747 jumbo
jet derivative.
In a new design, increasing the wingspan generally proves to be a better
alternative to winglets if the vortex drag is too high. However, there are
limitations on wingspan imposed by airport operations. If these apply, it can be
worthwhile to design the wing with winglets. The benefits are not usually large,
perhaps 2 or 3 per cent in efficiency, but even these small benefits can prove
important in airline economics.
However, aircraft are not bought solely on the basis of performance.
Appearance also counts. This is particularly true of business jets, so if an
aircraft has attractive, efficient-looking winglets it can be hyped. It is then
only necessary for the winglets to be harmless, rather than beneficial.
Gliders are the aircraft with the highest aerodynamic efficiency. They have
large wingspans, but I don鈥檛 know of any with winglets.
Michael Freestone
Engineering Sciences Data Unit
London
This week鈥檚 questions
Beer buoys: I recently held a barbecue, and to help the party along I filled
a plastic dustbin with ice and cans of beer. When the ice melted, the remaining
cans were left floating at all angles in the water. By the following morning,
however, all were floating vertically鈥攅xcept for those containing widgets,
which remained either disoriented or had dived to the bottom of the dustbin.
What is going on here?
Simon Hallowell
Manchester
Fight strateg: When I have been ill with a high temperature, my wife has
tried to persuade me to take paracetamol to reduce my temperature. However, I
prefer to take nothing because I have heard that the body increases its
temperature to fight the infection. Which of us is right?
Paul Stevens
London