Propulsive Efficiency related to hub diameter

Discussion in 'Props' started by DogCavalry, Feb 11, 2021.

  1. DogCavalry
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    DogCavalry I aim to misbehave.

    In aerodynamics, prop efficiency, within limits, is improved by larger hubs, or spinners. What is the hydrodynamic position? How much work has been done on larger hubs? Or does the increased drag in incompressable flow offset blade performance gains?
     
  2. jehardiman
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    jehardiman Senior Member

    There are several things that go into selecting hub diameter. First there is the strength of the shaft-hub-blade coupling. Then there is the issues with blade bending and root geometry. Finally there is matter of reducing active disk area because cavitation is driven by diameter at any given n and Va. With the exception of specialized things, like exhaust hubs, hub diameters fit into a small window of ~0.15 to 0.2 D. There has been significant work on optimizing thrust gains to interference losses for hubs, see the Goldstein factors.
     
  3. DogCavalry
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    DogCavalry I aim to misbehave.

    Will do.
     
  4. DogCavalry
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    DogCavalry I aim to misbehave.

    Goldstein factors seem to primarily relate to the relationship between blade roots.

    Active disk area loss isn't that critical when even a .3 H/D ratio is only 9% of area. And blade inner sections contribute little thrust. Most of their contribution comes from viscous effects of outer blade area.
     
  5. philSweet
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    philSweet Senior Member

    Can you support that first assertion? They are mostly there to cover up the pitch adjuster. Under what conditions do they increase efficiency and what is the mechanism by which they do so?

    Aircraft puller props don't have a hub trailing vortex problem like pushers do. Spinners have a friction drag problem. Variable pitch propellers have serious efficiency issues operating at off-design pitch. Larger hubs can lower the total radial blade twist considerably (but root camber increases), helping dramatically at off-design pitch settings. So VPPs will often run hubs up to 30% of tip radius. Of course, their maximum efficiency is somewhat compromised in exchange for better all-round performance. The presence of a rudder has a big influence on overall optimization schemes. Hub-rudder interfaces to reduce hub vortex losses are out there.

    Overall, hubs aren't often a separable item. They can have a large effect on the pitch and camber of the blade root if you have some particular reason to need to manage that. Otherwise, they have little effect on efficiency on their own. They reduce thrust and torque about in equal proportions, so efficiency doesn't change much.

    Below is from Becker Marine Systems

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    Below is from CAT - https://www.pon-cat.com/application/files/9315/3908/2588/LEDM3457-23.pdf

    upload_2021-2-11_22-34-3.png
     
  6. DogCavalry
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    DogCavalry I aim to misbehave.

    Indeed I can.
    The inner part of the prop gives no thrust, only drag. Consider a prop. For ease of visualization, a square prop, 10 by 10, with a 10% H/D. It has, for this simple case, a blade section with an extremely good l/d ratio as well. Imagine flow around the blade. Towards the tip of each blade induced drag is a significant feature, so the chord line is not far from the plane of the disk so that blade loading approximates the ideal- an elipse. The force generated on the blade at that point is almost normal to the chord line, and therefore almost normal to the plane of the disk.
    Moving inward a little, induced drag isn't an issue anymore, so flow may be considered entirely chordwise. The angle of attack of the blade is increased somewhat but forces are still almost normal to the disk. Most lift happens here. Moving inward, because there is little spanwise flow, you can think of sections as being smaller D props. An 8 by 10 prop, then a 7,6,5,4 by 10 prop. The angle of attack keeps increasing, and the lift remains almost normal to the chord line.
    By the time we are close to the hub, its a 2" by 10" prop. The angle of attack of the blade has increased to the point that the hydrodynamic force on the blade segment is in the plane of the disc. There's no thrust anymore, only drag. But its worse than that. Now the blades are like the vanes on an impeller in a centrifical pump. They create spanwise flow. Induced drag reappears, but its not flow from the pressure side to the reduced pressure side of the blade. Its flow from the the pressure side of one blade to the reduced pressure side of the blade ahead of it.

    So, a big hub, or spinner, simply removes this area of the blade from the flow. Fluid is moved outward to a region where the net force on the blade is mostly forward, instead just in a circle.
     
  7. philSweet
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    philSweet Senior Member

    Okay, now does a fixed hub or spinner have lower drag than this? Friction is a huge part of a prop's problem, and hubs add a lot of surface area compared to just a blade faired to a shaft of maybe 7% prop dia. The inner portion of an ideal prop has very low loading and is basically shaped to produce the same amount of swirl at small r/R as at larger r/R. So if basic fairing of the shaft/prop mechanical coupling puts you at >10%, show me where a fairing bigger than that has proven advantages.

    And by the way, one thing we do know is that hubs reduce thrust, so you don't get any global thrust exploits from that displaced core and changed blade geometry.

    Induced drag is an integral function. You just have to fiddle with the local camber to take into account the presence of the other blades so that the result is the correct span-wise load distribution. It doesn't change the obtainable minimum induced drag, it just changes the shape that achieves it. That's the argument at constant disc loading excluding the hub area. If you include the hub area, then you have to have a higher load on the blade as the hub gets bigger, and that increases the lower limit of the induced drag. So a bigger hub means a bit larger diameter at constant disc loading.

    Now then, can we do that with the same amount of material? or with the same developed area? Or with the same friction losses? It's a tricky comparison to make because you have to design new blades for each hub size. Most comparisons use a standard blade generator line (basically fixing rake and skew) and vary pitch distribution, thickness, camber, chord distribution (maybe) and blade count to redesign the blades each time the hub changes. That's a lot of stuff to juggle. But the effect is so small that you have to at least try.

    So basically, I am still looking for a direct comparison of a minimal hub prop and a bigger hub prop, each with separately optimized blades, producing the same thrust at the same advance ratio, where the bigger hub is more efficient.
     
    Last edited: Feb 13, 2021
  8. DogCavalry
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    DogCavalry I aim to misbehave.

    Ah Phil, I'm sorry. You've been really eloquently chasing the blade in front of you in a circle. The correct answer to my question was 'That was worked out a long time ago. Hub diameters increased to as much as 30% of D, or more in some cases. Mature science now.' Still, thanks for your generous answer. And if I'd spent another minute on it, I would have seen that mature marine propellers already have relatively huge hubs. My own damn fault.
     
    Last edited: Feb 12, 2021
  9. jehardiman
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    jehardiman Senior Member

    Well yes it is about the blade root spacing, what do you think causes that loss of thrust?
    Anyway, reading on with your posts I believe that you have a technical misunderstanding about aircraft propellers. The actual size of a propeller hub, the part where and how the blades attach, hasn't changed that much since the mid-1940's, and they are much less than 0.1D. You seem to have confused the spinner, which is fitted for aerodynamic reasons, as being part of the propeller, which it is not. Aircraft propellers and spinners designs are driven by completely different requirements than marine propellers. FWIW, props on vessels are placed aft for efficiency reasons, which is the same reason they are placed forward on aircraft. The reason for this ratio between density and viscosity for the two different fluids. Water is much more dense for it's viscosity than air. It is more efficient to recover energy (i.e. the wake fraction) out of the water wake than it is to lose efficiency to wake variation (i.e. thrust deduction). See the thread below,
    Pusher vs Tractor/Puller Propeller https://www.boatdesign.net/threads/pusher-vs-tractor-puller-propeller.46309/#post-616026
     
  10. DogCavalry
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    DogCavalry I aim to misbehave.

    A distinction without a difference. Since I wrote at length about fluid flow around blade roots, and the structure in a boat prop that is relevant is the hub, while in an aircraft it is the spinner than guides the fluid flow, I'm not sure what your point is.

    It is a kindness to include the pusher/puller link, but that was intuitively obvious to me about 40 years ago.

    And I wrote about a hundred words about what causes the loss of thrust, so I'm good with that too. Anyway, we are being pedantic, just to be pedantic. The issue I raised was solved long ago. Let's get on to the next problem. I'm going to throw another rock into the CPP vs FPP pond.
     
  11. jehardiman
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    jehardiman Senior Member

    So basically what you are saying is that this was all to troll, that you knew that your original statement was false?
     
  12. DogCavalry
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    DogCavalry I aim to misbehave.

    No, not at all. I asked the question in the first place then realized that prop designers noticed the same thing long ago, and solved it. It is certainly true that a prop, in air or water, gets no thrust from the innermost part of the blades, if the hub is made as small as possible, but a hub at 20% is more than large enough to solve that issue. And since I already apologized for my carelessness, and agreed that it was my own damn fault, I don't think it's fair to say I was trolling. It would be fair to say I wasted your time asking a question the answer to which was obvious. And for that, I do apologize. It's not true that there's no such thing as a stupid question.
     

  13. DogCavalry
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    DogCavalry I aim to misbehave.

    Or leave out that part of the blade, altogether.

    I believe this proves my point nicely. The hub is a necessary evil in most props. Blades must be attached to it at their root. If there's no need for a hub, and the designer is free to end blades where they like, where do they find the best efficiency? In this case, the designer could have joined all these blades at their tips with no hub, or a tiny hub, or an enormous streamlined fairing. They chose not to connect them. To omit that portion of the blade I originally remarked on.
     

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    Last edited: Feb 12, 2021
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