32
$\begingroup$

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?

$\endgroup$
  • 1
    $\begingroup$ I don't know but there are a couple of ways props are unlike wings: 1. considerable longitudinal tension. 2. moving through wake of prior blade, not clean air. $\endgroup$ – RedGrittyBrick Apr 11 '15 at 8:47
  • 1
    $\begingroup$ Related if not duplicate $\endgroup$ – DeltaLima Apr 11 '15 at 9:09
  • $\begingroup$ Id say related as I'm not talking about a ring but actual winglets ;) $\endgroup$ – Maverick283 Apr 11 '15 at 9:12
  • $\begingroup$ Propellers often have a very high aspect ratio to start with. $\endgroup$ – Thunderstrike Apr 11 '15 at 11:13
  • 8
    $\begingroup$ There are a few pictures on the Internet. Generally, the owners are not happy about their proplets. $\endgroup$ – Simon Richter Apr 11 '15 at 14:32
24
$\begingroup$

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.

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

$\endgroup$
  • 3
    $\begingroup$ @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. $\endgroup$ – Peter Kämpf Apr 11 '15 at 16:35
  • 2
    $\begingroup$ 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" $\endgroup$ – user2357112 supports Monica Apr 11 '15 at 16:42
  • 2
    $\begingroup$ @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? $\endgroup$ – Peter Kämpf Apr 11 '15 at 16:47
  • 2
    $\begingroup$ @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. $\endgroup$ – Peter Kämpf Apr 11 '15 at 16:52
  • 3
    $\begingroup$ @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. $\endgroup$ – Peter Kämpf Apr 11 '15 at 20:31
6
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ 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. $\endgroup$ – Meower68 Sep 13 '16 at 13:57
2
$\begingroup$

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.

$\endgroup$
2
$\begingroup$

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).

$\endgroup$
  • 1
    $\begingroup$ your first paragraph does not really sound like a good reason, the FAA would know which models use those props. $\endgroup$ – Federico Oct 6 '16 at 17:26
  • $\begingroup$ 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. $\endgroup$ – FreeMan Oct 6 '16 at 19:00
2
$\begingroup$

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.

$\endgroup$
-4
$\begingroup$

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.

$\endgroup$
  • 3
    $\begingroup$ almost all competition gliders have winglets $\endgroup$ – Federico Apr 15 '15 at 5:15
  • 4
    $\begingroup$ 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. $\endgroup$ – Peter Kämpf Apr 15 '15 at 18:14
  • $\begingroup$ 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. $\endgroup$ – KorvinStarmast Oct 6 '16 at 17:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.