tick the answer "all the above" but do so with a smile
The question of winglets or no seems to be perplexing and is made doubly so if one assumes that every decision made is rigorous and thoroughly justifiable. My father was one of the “brain drain” and left his British employer in sunny Bedfordshire for the far sunnier climes of California. He was horrified to find that many designs of the sixties were as much influenced if not more influenced by their looks than pure science. In fact “streamlining” for the want of a better word could be anything but, which matters not when fuel is 5 cents a gallon. At that price it doesn’t even figure whose gallon we are talking about. In one of his writings Richard Shevell of Douglas pokes rather more than gentle fun at my father’s naivete and strict West Indian upbringing. “Last but by no means least there is always the fashion” and dad would splutter and reach for his pen. It is interesting to note that two of the great firms Dad worked for are no more and both were proud to be designers of “fashionable” aircraft. The last aircraft my father collaborated on is in production… just and enjoys a good but slow wing design that you might think would benefit from a bit of wingletting. Just supposing the weight and fixing pose no problems and there are no adverse resonances the looked for improvements must basically appear in significant reduction of induced drag because ideally zero-lift drag remains the same or increases only slightly whilst compressibility drag remains the same or might possibly reduce a little bit. At that decision moment pure science and economics should step in and do the speaking for you. But the wit of Richard Shevell can still be heard chuckling down the years. “A tarted up design is not so much mutton dressed as lamb as glutton dressed as glam”. The final decision might be exasperating but if the customer wants winglets for marketing reasons he’ll go for it and he well may be right if he fills the plane and his staider scientific rival does not. If winglets are to have a purpose they are best designed in from the start. If they are options then don’t bother if the distance is very short. I remember a particular 128 seater with an OWE of 37650kg and a wing area of 125sq m that cruises nicely at about 440ktas. It was being considered for a variety of routes, one of which was a real bread-winner although only 315nm. Realistically however it would take 90nm to get up and 90nm to get back down again leaving only 135nm for the cruise which meant an average of 30,500ft taking both there and back directions. As you might expect the lift coefficient at 55 tonnes was very low, about 0.378, the induced drag coefficient was also low, about 0.0061 and the compressibility drag coefficient similarly low at about 0.0010 for a total drag coefficient of about 0.0245. This gives a disappointing L/D less than 15.5 whereas a glib salesman would tell you that the wing capabilities were of the order of L/D = 17.1, without of course telling you the conditions. Try as I might with all the science I could muster to persuade my listeners that winglets could do nothing for such an application as this short route, “they” knew better. They went for the “looks” but it was to me that they came when they found the performance disappointing. The smaller aircraft such as I have described have a very difficult role to fill, being offered in many sizes for many routes. The economics of winglets or not is there to be considered but on the likes of certain large long-haul twins the wing design is much less of a compromise. Yet even here and now there will be someone whispering that it would look much nicer with winglets or that they look stupid, get rid of them. I hope this helps to keep us aware that one hat does not fit all. At the extremes of any industry there are those who know more and more about less and less so they are the experts. I think that at my age I know less and less about more and more so please spare me the ridicule and honour these grey hairs. Enjoy your ppruning.
Best Rgds
The “E”