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The US military emits more CO2 annually than several countries. Winglets have been proven to work well in the commercial sector to reduce fuel burn, therefore reducing both CO2 emissions and fuel cost. The US military doesn't really lack research or production/modification capability, so why have winglets not been installed on non-fighter/attack aircraft? (Think C-130, E-3, KC-135...) They would reduce CO2 emissions and fuel costs, so why not implement them?

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    $\begingroup$ I don't think they care that much about cost or carbon footprint, but any improvement in fuel efficiency improves range, which they do care about, so good question... $\endgroup$ – Bianfable May 3 at 12:38
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    $\begingroup$ Winglets are definitely not proven to reduce fuel burn. If you are designing a new aircraft, just make the wing appropriate span, that's most efficient. Winglets only improve things when there is a desire to use shorter wingspan—usually when scaling up existing design to keep the same wingspan as previous generation either to avoid larger redesign of the wing, or to make sure the new aircraft fits in the slots designed for the older one. When the plane isn't enlarged, there is rarely reason to add winglets. $\endgroup$ – Jan Hudec May 3 at 17:20
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    $\begingroup$ Winglets reduce wingtip vortices (induced drag). If you have less drag, you need less power to go the same speed. If you use less power, you burn less fuel. Therefore, winglets reduce fuel burn. $\endgroup$ – MD88Fan May 3 at 19:49
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    $\begingroup$ medium.com/war-is-boring/… Lockheed Martin already have OEM approved aero modifications for C-130 and C5 including winglets, fuselage microvanes, and automated deflection of control surfaces to adjust wing loading. The issue is bit about not wanting to reduce CO2 or make savings, it’s about capital costs of upgrading vs any future savings, loss of capability while these aircraft are in being upgraded, and the effect these mods will have on tactical activity such as airdrop which will require costly T&E. $\endgroup$ – Arkhem May 4 at 7:22
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    $\begingroup$ If you really want this to happen, contact your congress-person, who is hopefully on the armed services committee, and convince them to formulate a bill to help save the environment to support this. Good luck with that. $\endgroup$ – CGCampbell May 5 at 16:23
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I think the main reason is that many of these planes were produced before winglets were common

Look at a list of USAF planes and see what years each one were produced (I am not including every plane, but most of the common ones)

  • C-5: 1968–1973, 1985–1989
  • C-17: 1991-2015 (has winglets)
  • C-130: 1954-present
  • E-3: 1977-1992
  • KC-10: 1979–1987
  • KC-46A: 2013-present (does not appear to have winglets, even though recent)
  • KC-135: 1955–1965

But when you look at this story about winglets, there is the sentence

The first 747-400 was delivered to Northwest Airlines in 1989, the first commercial aircraft with proper winglets.

So for the most part you are talking about adding winglets to an existing airframe, rather than aircraft with winglets as part of the design. This increases the expense of adding winglets, which reduces their cost-effectiveness. Look at this article on modifiying the B-52 to use four modern engines and the problems with that (not all of these would apply to winglets).

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  • $\begingroup$ Right. I understand that most of the US fleet was built prior to winglets being a big thing, but the question is why they haven't been implemented with aftermarket modifications. $\endgroup$ – MD88Fan May 3 at 17:12
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    $\begingroup$ Aftermarket modifications that affect aerodynamic stability would likely have to go through a certain amount of testing & certification, which isn't free, and is likely just not cost-effective when spread over a small number of planes. Consider: KC-46A (related to 767 but not identical, so you can't just transplant parts from one to the other) planned production 179 vs. 767 over 1,200 produced and more on order. $\endgroup$ – manassehkatz-Moving 2 Codidact May 4 at 1:04
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All the discussions of engineering difficulties and cost-effectiveness are valid arguments but based on my USAF career in program offices and in operating commands, including 4 years in TAC, it boils down to money and priorities.

Once the a/c is operational the program office is concerned with sustainment. They need to provide the engineering, tools, and materials needed to keep the planes combat ready. In addition to providing spares to the operating units, they are also responsible for depot level maintenance. They have a constrained budget and the priorities are set by the operators.

Spare parts are driven by in-field failures. Safety related issues take top priority. Service life extension becomes a concern on older a/c. And the operators have a wishlist of upgrades they would like.

The operator’s top priority is being mission capable. The wishlist will be things that keep the planes operational or improve its ability to perform its mission. There’s never enough money or time to get everything on the wishlist. So the list is a rough plan that projects out 10 years or so. It gets updated every year when they can lock down what they will do next year.

So the marginal increase in fuel economy from adding winglets never comes close to the top of that priority list. In addition to the direct costs of a large mod such as winglets, there is the operational cost of having the a/c spend an extra week in the depot. If a squadron has 24 a/c, that week in the depot equates to almost 6 months the squadron will be short an a/c. That’s a big hit to operational readiness.

When we were retrofitting GPS to the entire AF fleet in the 90’s it was a constant battle to get the operating commands to give us the time in depot to do the upgrades. They wanted the a/c back ASAP. It was about a 10 year effort and it took Congress threatening to pull upgrade funding for any aircraft that wasn’t GPS equipped after a cutoff date.

Any significant upgrades are usually addressed in the development of the replacement a/c. But even that will go through numerous trade offs to (try to) meet schedule and budget.

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  • $\begingroup$ The wonderful world of weapons systems supply chain and logistics... it's my world right now, and this is spot on. $\endgroup$ – CGCampbell May 5 at 16:16
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They have.

For example, look at the wingtips of the P-8. They don’t look like the commercial variant (because the commercial variant will snap off if you hang out in icing conditions for hours), but the wingtips it has essentially work the same way.

In general though, the answer to your specific question is that there is no “aftermarket”. Air Force and Army each rely almost entirely on the OEM for engineering design work the Navy does to a lesser extent, and no service has a good, recent record of in-house metal bending.

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  • $\begingroup$ True! I hadn’t considered the P-8. However, with the KC-135 fleet, I think it could be done to have Boeing or LM or someone similar build a winglet kit for guard/reserve/active units to install. $\endgroup$ – MD88Fan May 5 at 0:16
  • $\begingroup$ @MD88Fan why do you think that, and what what advantage do you think it would provide? C-130 is under a PBL with LM, and is run out of a USAF program office. How do you envision that contract being structured? $\endgroup$ – fectin May 5 at 0:21
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Winglets work by using the span-wise air flow around the wing tip to generate forward 'lift'. This airflow is highly dependant on the angle of attack, so the winglet needs to be designed for a specific condition or it just adds weight and drag.

Airliners spend almost all their time at cruise height and speed while loaded, so it's possible to design a winglet that works for that case. Military aircraft have to operate in a wider range of conditions, and the improvement in cruise performance might not be worth the reduction in loiter time.

It might also help to consider using the weight of a winglet to just extend the wing. That would reduce wing-tip losses at all speeds, allow the plane to fly higher and reduce take-off and landing speeds. Why isn't it done? Because the designer has already worked out the optimum aspect ratio, and improving performance in one area would reduce it in another.

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    $\begingroup$ True, but the question was about support aircraft (transport, tanker, AWACS, etc), so they spend a lot of time in a similar envelope to airliners. $\endgroup$ – MD88Fan May 4 at 11:39

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