Rudder blade design - problem

Discussion in 'Hydrodynamics and Aerodynamics' started by WildCard, Apr 16, 2020.

  1. Kim Klaka
    Joined: Feb 2017
    Posts: 14
    Likes: 3, Points: 3, Legacy Rep: 10
    Location: Fremantle, Western Australia

    Kim Klaka Junior Member

    Hi. yes, I am a naval architect familiar with this subject, so happy to talk technical if necessary. Is your background nav arch or aero eng? Terminology differs slightly between disciplines.
     
  2. markdrela
    Joined: Jun 2004
    Posts: 307
    Likes: 28, Points: 28, Legacy Rep: 324
    Location: MIT Aero & Astro

    markdrela Senior Member

    I think Jehardiman identified the problem: The pivot axis is too far forward on the rudder.

    Any symmetric airfoil has the center of pressure very near its 25% chord point, at any angle of attack within the stall limits. Your average chord is 400mm, so a rough estimate of the center of pressure is 100mm ahead of the area centroid, or 38 mm behind your present axis. This 38mm is the rudder-force moment arm which is what's resulting in the large tiller forces.

    I suggest moving the rudder forward on the axis by 25mm. This will reduce the moment arm from 38mm to 13mm, and thus reduce the hydro forces on the tiller by almost a factor of 3.





     
  3. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Good, with that sorted we can settle this up pretty fast. Do a FBD for the proposed blade and geometry using the rudderhead as the point of reference for zero, 5 degrees, 10 degrees, etc tiller movement. Since this thread is about tiller load and balance area, the movement of the CP on the surface of the foil is small compared to its physical movement of the CP due to rudder shaft rotation, especially considering the spanwise AoA effect of the rudder stock rake and span sweep.

    So for reference, I retired after 35 years as a NA&ME PE out of Webb and UofM.
     
  4. Kim Klaka
    Joined: Feb 2017
    Posts: 14
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    Location: Fremantle, Western Australia

    Kim Klaka Junior Member

    Nope, don't understand that, sorry. Tiller torque is generated by the hydrodynamic moment about the axis of the stock, regardless of rake angle (ignoring buoyancy/weight forces, which have not been part of this thread so far, and ignoring bearing friction). Therefore, surely the change in torque on the tiller with AoA is only due to:
    - chordwise CP movement and
    - the change in magnitude of the normal force.
    Maybe I am just blind to some other geometric effect. There is a subtlety that the flow angle onto the rudder is probably not exactly at right angles to the stock, but that is just one or two degrees uncertainty, and not likely to alter CP much. All experimental results I have seen show that the CP moves aft with increasing AoA, the amount of movement depending on aspect ratio AR ( less movement for high AR).
    Also puzzled by your last clause "the spanwise AoA effect of the rudder stock rake and span sweep". I guess it needs a diagram, words can be confusing for some things. I am worried that I have been getting all this wrong over the years.

    PS: I might not outrank you on wisdom or knowledge, but I probably do on age - did my first tank tests on a rudder 47 years ago :)

    PPS: For anyone reading this thread who is using the formulae in Principles of Naval Architecture (publ. SNAME) to calculate the chordwise centre of pressure, there is a mistake in the book. The first term in the formula for quarter-chord moment coefficient (eqn 125 in the 1989 edition, eqn 25 in the 1967 edition), should be multiplied by AoA; it is missing in the formula. There is also a mistake in the accompanying fig 139 in the 1989 version; the scale for CP needs to be moved up by 20% (it is correct in the 1967 edition).
     
  5. Kim Klaka
    Joined: Feb 2017
    Posts: 14
    Likes: 3, Points: 3, Legacy Rep: 10
    Location: Fremantle, Western Australia

    Kim Klaka Junior Member

    I agree. This is a very high aspect ratio rudder so the CP will stay close to 25% chord. Puzzling that Jefa put the stock at 18% chord. Moving the stock back into the blade could be an expensive exercise; maybe worth talking to Jefa first?
     

  6. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Sorry, I was unable to enlighten you with my poor comments to get your focus off the foil and onto the rudder shaft, but as markdrela commented, everyone agrees that the balance is poor. I suspect Jefa will build a rudder with a shaft anywhere a customer will pay for, but why that one was selected as a "racing rudder" for WildCard's boat <shrug>.

    No one "outranks" anyone on wisdom or knowledge. Over my years in engineering dealing with others of different experiences, I have found that most "disagreements" are when two people have very different methods and understanding of how to approach the same problem. I will state that after all the tank and operational experiments I have done/had done for me over the years, tank water appears to have very little to do with real water; I really cannot express how messed up real water is. I decided to get my PE after a contracted PhD tried to explain to me and the head of a US Navy Lab, how my real time data did not agree with his model because I only had a "Bachelor's understanding". Fortunately for me, the PhD head of the Lab and I had been working together on this problem for several decades, and though we had differences of opinion, I had been brought in as the Navy's technical expert to evaluate the contracted PhD's system because I had the most experience with all the different models and real operations. It was enlightening to see how meaningless letters changed people when money was on the table. Thus my signature.
     
    Last edited: Oct 17, 2020
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