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On the first look this question might sound ridiculous and maybe it is. But as propeller blades act by the same physical laws as wings, and winglets reduce the induced drag by quite a bit, then why are they not put on propeller blades?

I could imagine that it might be a material problem. But are there any more reasons? And why exactly might it be a material problem?

Maverick283
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Winglets on wings help because they increase the volume of air on which the wing can act. Extending the wing span would be much more efficient, but when span is restricted or the maximum wing bending moment is limited, winglets bring a small improvement in efficiency at high lift coefficients.

On propellers, however, the winglets would run through air which is already affected by the tips of the propeller. No additional air will be involved, so no efficiency increase will be possible. Please note that propulsive efficiency is increased by accelerating more air by a smaller amount. The formula for the propulsive efficiency $\eta_p$ of an air breathing engine is $$\eta_p = \frac{v_{\infty}}{v_{\infty} + \frac{\Delta v}{2}}$$ where $v_{\infty}$ is the speed of the inflowing air and $\Delta v$ the speed increase of the air affected by the propeller disc. A smaller $\Delta v$ acting on a higher mass flow makes the engine more efficient. This effect is most pronounced when $v_{\infty}$ is low.

The prop tip winglets would operate in a region of high dynamic pressure and generate more friction drag without contributing to the prop's efficiency.

Things become different when the propeller blades have a very low aspect ratio and there is a hard restriction on propeller diameter: This is true for ships, where draught puts a hard limit on propeller size. Here a sort of winglet does indeed help: The Kappel propeller has its tip bent forward and increases efficiency by 3 - 6 percent. Given that marine propellers even for large ships have efficiencies between 50% and 60%, this is a noteworthy increase in efficiency.

By the way: Whoever tells you that winglets reduce induced drag quite a bit has something to sell to you, but I digress.

Peter Kämpf
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    Pretty much everything that comes up when I google "winglets" says they're there to reduce energy losses from wingtip vortices. Do you have any links that analyze them from the perspective you're stating, and that weren't written by you? – user2357112 Apr 11 '15 at 16:03
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    @user2357112: Ask yourself: Can the vortex forming behind the wing still cause drag? Is there any mechanism which allows it to hold the aircraft back? Next: Don't read Wikipedia, the Web's collection of politically correct opinion. Don't read blogs and run away from short news articles: Their authors have no idea what they write about. Instead, look at scientific studies. PDFs of 30 or 50 pages. Light reading: Maughmer's article. – Peter Kämpf Apr 11 '15 at 16:35
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    Can you link such studies? – user2357112 Apr 11 '15 at 16:37
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    The link you provided doesn't seem to support your claim: "In essence, the improvement in aircraft performance due to winglets results from their ability to reduce induced drag traded off against their added wetted area increasing the profile drag" – user2357112 Apr 11 '15 at 16:42
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    @user2357112: Take your time, read the whole article. BTW, why would that not support my claim? What do you actually understand my claim to be? – Peter Kämpf Apr 11 '15 at 16:47
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    The last sentence of your answer strongly implies that the purpose of winglets is not to reduce induced drag. The article seems to contradict that. Your claim has other aspects that may or may not be contradicted by the article; I'm still reading. – user2357112 Apr 11 '15 at 16:50
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    @user2357112: My objection is to the "quite a bit" part. Read also the first paragraph, not just the last sentence. Winglets help at high lift coefficients and improve L/D by single-digit percentages. Read Maughmer: At high speed they are adding drag, at low speed they help. A bit, not "quite" a bit. – Peter Kämpf Apr 11 '15 at 16:52
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    Still reading. The article is now talking about how winglets reduce the energy wasted in producing spanwise flow that ends up going into wingtip vortices, and how induced drag can be calculated by determining the energy contained in the trailing vortex system. "To minimize this drag, the amount of energy used in producing the required downwash must be minimized, that is, the energy that is “wasted” in creating unnecessary spanwise flow and in the rolling up of the tip vortices must be minimized." To a rough approximation, it's not that wingtip vortices cause induced drag... – user2357112 Apr 11 '15 at 17:10
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    ...it's that the mechanism of induced drag dumps the wasted energy into wingtip vortices. – user2357112 Apr 11 '15 at 17:11
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    While the improvement in overall L/D ratio is only a few percentage points, and not at all speeds, the reduction specifically in induced drag appears to be quite substantial. The difference appears to be due to a combination of the increase in profile drag and Amdahl's law - reducing a part of the drag substantially can only reduce the overall drag by however big that part was. – user2357112 Apr 11 '15 at 17:39
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    @user2357112: At optimum L/D, induced drag is half of total drag. And again: If you spend the added profile drag on a span extension, your gain in induced drag reduction is 2.5 times bigger. Whatever you think is substantial, this is substantialler. See here for a meta study on winglet performance. – Peter Kämpf Apr 11 '15 at 17:54
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    @sanchises: You are wrong. If the prop-winglet would increase lift locally, it would distort the lift distribution and increase induced drag for the same lift. If you need more thrust, increase RPM! I hate it when people down vote an answer simply because they did not understand it. – Peter Kämpf Apr 11 '15 at 20:31
  • This answer is wrong! Winglets on a prop would limit spanwise flow, leaving more air to work on. Furthermore, winglets can be designed in such a way so as to suck air inboard towards the prop hub, further increasing the air being worked on, they would also reduce induced drag from tip vortices. Because of the strong outboard flow along the prop, there is also air circulation from the bottom to the top surface which significantly reduces lift on the prop tips because the high pressure air from the bottom ends up in the low pressure region on top, proplets would reduce this. – MishaP Oct 03 '18 at 17:40
  • @MishaP … that's why we see winglets being added to props all around, don't we? If you think you know better, don't just put unproven claims in a comment but write a better answer. – Peter Kämpf Oct 10 '18 at 03:19
  • @PeterKämpf Many things that improve airplane efficiency are not in productions for a variety of reasons. For instance, why don't we see blended wing or prandtl wing airplanes? Wing tunnel and homebuilt experiments have confirmed the theory, but companies such as Boeing, Airbus, etc, do not want to risk millions and/or billions on such radical changes. A sausage with wings flies well enough, why take risks? On top of that - sometimes manufacturing costs are too high, or technology does not exist to reliably implement theory. So the claim "they don't exist, so it's inefficient" is false. – MishaP May 10 '20 at 11:53
  • @MishaP: Blended wings are pure marketing and Prandtl wings fly all around you, only not by that name. All and more have been studied intensively and only the designs that work have been built. Do you really think the engineers are that stupid? – Peter Kämpf May 10 '20 at 15:00
  • Engineers - no. Marketing, execs etc, that are happy to keep selling old stuff as long as it sells? yes. – MishaP May 10 '20 at 23:44
  • @MishaP: I agree, but what they sell is not old stuff. It might seem that way because we have arrived at an optimum and can improve this only very gradually. All radical departures lead downhill. – Peter Kämpf May 11 '20 at 06:48
  • @PeterKämpf most notable "radical departures" that lead downhill were attempted when technology was significantly less developed than it is now - no composite materials, no CFD or FEA analysis, and many many more advances that have been made since then.

    A great example - canards were deemed statically unstable, until Burt Rutan made then long ez which is statically stable. Fighters solved the issue with electronic stability augmentation. However, still no canards on commercial airplanes. Just one example.

     – MishaP May 11 '20 at 12:08
  • @MishaP: Where did you get that nonsense from? Of course were canards considered as stable as conventional aircraft. See Focke-Wulf 19 Ente or MiG 8 Utka. Just because the Wrights didn't get stability and Rutan is a blowhard? But nobody sane wants to fly with those spoilers up in front. – Peter Kämpf May 11 '20 at 13:44
  • @PeterKämpf both the FW19 and Mig8 showed unstable behavior. I've lost interest in this discussion. goodbye. – MishaP May 11 '20 at 22:16
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Hartzell make propellers that appear to have winglets (tips bent aft) called Q-tips. The name seems to refer to them being quieter. I have read that they do the same job as a slightly larger diameter prop. They seem to have no life issues. Since noise costs energy, and a smaller prop does the same job.... it is possible that they are more efficient. The bent tip is small - on the order of a couple of inches.

Thomas McKelvey
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  • It's my understanding that the bent tip helps reduce the air pressure immediately outboard of the propeller, not unlike a winglet. This does two things: keeps the pressure inboard, where it can add to the propulsive thrust, and reduce the ability for sound to propagate outwards. So, yes, it would make for a quieter prop. But that's just a side-effect; the main effect is improved efficiency of the prop. And no, it doesn't need to be big; the ratio of winglet to wing is pretty small so the ratio of bent tip to propeller radius would be pretty small, too. – Meower68 Sep 13 '16 at 13:57
  • Strange that increased sweep at the tip of a wing/prop is now called a winglet with much more frequency. Along these lines - there are scimitar and other form factor props that have similar benefits. – MishaP May 10 '20 at 11:39
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Pros:

  1. Winglets on a prop would limit span-wise flow, leaving more air to work on, because it does not "fly away".
  2. Furthermore, winglets can be designed in such a way so as to suck air inboard towards the prop hub, further increasing the amount of air being worked on.
  3. They would also reduce induced drag from tip vortices. Anyone who says winglets are trash can explain that to all competitive gliders, Boeing, Airbus, Cessna, and any other modern aircraft manufacturer, most of whom use some sort of wingtip device.
  4. Because of the strong outboard flow along the prop, the low pressure air from the top of the blade, and the high pressure air from the bottom of the blade both end up outboard of the propeller, where the low pressure region sucks high pressure air from the bottom of the blade onto the top, thereby reducing the high pressure on the bottom, and increasing the low pressure on top, resulting in a completely useless portion of the blade, which produces very little thrust but still creates drag, and requires more power to turn the propeller.

For all of the above reasons, a propeller of the same diameter that is placed into a shroud can potentially create up to 85% more thrust using the same engine, as a propeller that is not shrouded.

Cons:

  1. A wingtip located on the tip of a propeller would be constantly accelerated outboard, if a regular propeller blade deals with this acceleration rather easily, because it is an axial load on the blade, the winglet would be a cantilever beam, and the forces would rapidly add up creating a rather formidable force on any significantly large winglets on the tip of a propeller.
  2. Besides the magnitude of the force, and the fact that it is a cantilever type load, it is also a variable load which is less on low RPM, and higher on high RPM, which quickly leads to material fatigue and structural failure.
  3. The cost to manufacture such a propeller would be higher.
  4. The weight of the propeller would be higher to account for the structural loads listed above.

Boils down to cost, weight, reliability.

MishaP
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Winglets on propellers look like propellers that have gone through "ground strike" and the FAA grounds prop-driven aircraft with propellers damaged due to ground strike.

It's simply more expensive to manufacture, certify, and maintain; even though it does provide improved thrust in take-off position; but potentially more drag during cruise and feather conditions (based off propeller pitch, will be different with different propellers).

BERP tips most likely more effective overall considering take-off, cruise, and feather conditions (feather is the desired pitch during engine failure to prevent wind-mill and drag from the prop free rotating).

user17361
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    your first paragraph does not really sound like a good reason, the FAA would know which models use those props. – Federico Oct 06 '16 at 17:26
  • Simon Richter mentioned in his comment that pictures of ground strikes are readily available online. I'm going to go out on a limb and say that even FAA investigators would be familiar with what a ground strike looks like and with the design of proplets and be able to tell the difference. – FreeMan Oct 06 '16 at 19:00
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The concept of 'winglets' was applied in early days of aviation by the inventions of Henri Coanda, patents: 1937GB191112740, 1910; CA370885, 1937; and to boat propellers; you can have a look, also in ESPACENET, at patents: ES-0444150, Inventor: 'Gonzalo Perez Gomez', and ES-8300608, 1987, same; and ES-0293837_U by 'Ramon Ruiz Fornella', all from: 'Astilleros espanoles'. 'NASA Technical Memorandum 87771', by Milton A. Beheim: 'NASA Research in Aircraft Propulsion', shows a propeller with 'winglets'.

I'd say that one of the reasons for winglets in wing tips is to reduce the marginal vortex, due to the compensation of the difference in pressures between the upper and lower surface of wing, this 'vortex' or 'swirl' increases drag, and thus impairs overall airplane efficiency.

Urquiola
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  • There has been much research into winglets. early on they were not considered effective, however, I challenge you to find a modern commercial plane without winglets, and that's because they improve overall airplane efficiency. To say otherwise is misleading. – MishaP May 10 '20 at 11:41
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The same reason why there are no winglets on gliders. The majority of propellers tips are small & pointy enough that there is a negligible spillover to cause wingtip vortices that makes the wing inefficient. Prop blades with wide tips like the ones in C130's or Huey helicopter blades may be improved with winglets but such design also adds complexity(cost), weight and forces that may impede the reliability & safety the blades.

Wallen
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    almost all competition gliders have winglets – Federico Apr 15 '15 at 05:15
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    Be careful not to upset the members of the winglet cult, for they are easily irritated and punish you with downvotes if you don't share their beliefs. – Peter Kämpf Apr 15 '15 at 18:14
  • A Huey helicopter blade is not a propeller: it is a rotating wing (airfoil). Helicopter blades on a great many models improve performance (and reduce noise) around the tip by sweeping the tip back. The constantly changing air flow over a rotor blade in flight is significantly different from the airflow over a swept wing airliner. This answer was down voted due to misinformation. – KorvinStarmast Oct 06 '16 at 17:19