Anhedral and winglets
 

Hi

After looking at pictures of planes with anhedral wing config it occurred to me that none seem to employ winglets as a means of reducing induced drag. As a number of medium to heavy Jet aircraft with wing anhedral use a swept wing config also, why do they not include winglets? What devices do they include?

Doesn't the C17 have winglets/sharklets?

yep fair enough it does. That puts that issue to bed. Maybe with regards to others the cost of retrofitting winglets costs too much?

Like most things with aeroplanes there is rarely only one consideration to balance up ... fuel is only one part of the equation for end plates ... there may be an adverse structural impact which outweighs the fuel gains .. all comes down to an overall cost/benefit analysis by the OEM/STC organisations.

I think that most other aeroplanes with anhedral wings were manufactured quite some time ago, and the retro-fit would be more than the value to be gained. There are also very few of these types knocking around, the cost of design, manufacture, testing and certifying would be astronomical and the number of potential sales would make it unviable. The C17 that I mentioned was a recent creation when winglets/sharklets had already been designed, so to create the wing with them already fitted was relatively simple.

To fit winglets requires beefing up the wing if there is no structural margin due to change in bending moments. The 737NG - installation adds 375-518lbs to the weight of the aircraft, depending upon whether they were installed at production or a retrofit. The blended winglet assemblies themselves weigh 300 pounds per pair. Beefing up the wing structure added 100 pounds. If the original wing had had no excess structural margin, it is estimated that 600 pounds of beefing up would have been required.

Edited to add: from (a Boeing paper) http://www.smartcockpit.com/docs/Wingtip_Devices.pdf

Ideal-induced-drag theory (the theory of minimum induced drag for a given total lift, based on Trefftz-plane theory) is useful conceptually for understanding the relative drag-reduction potentials of different device configurations, but it is a poor guide to the net level of benefit that can be achieved. The actual induced-drag reduction is always significantly less than ideal, substantially so in retrofit and derivative applications, and increases in viscous drag and weight generally offset some of the induced-drag reduction. The structural-weight impact is always a major player in the design trades. The magnitudes of all of the offsetting factors depend strongly on the design details of the baseline airplane and the device.

The last two sentences are significant.