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Fan & Propeller shapes: Sickle and Curved

  1. Jul 13, 2007 #1
    Air or Water:
    I'm looking for information on fan and propeller shapes (ducted and non-ducted) which might lead to more quiet if not more efficient shapes.

    Noise is lost energy just as heat is lost energy in a mechanical system, right?

    I have a thread which contains many bits of information for the context of this request. It also has many pictures which if you cannot see tell me and I'll repost all the links here.

    I'm no engineer, pretty pictures tell me a lot. However if you have recommended text books, graphs, charts or other information you feel you can share, please add it to this thread.

    Reference Thread:
    http://www.hoverclubofamerica.org/forum/index.php?showtopic=1244&st=0 [Broken]

    Sample Picture and second link:
    http://www.hoverclubofamerica.org/forum/index.php?showtopic=1232&st=0 [Broken]

    Cheers, George/kach22i

    Attached Files:

    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jul 13, 2007 #2


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    And cavitation. Big problem in propellors. That's why they're curved.
  4. Jul 13, 2007 #3


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    Do realize that fan shape is an extremely active area of research. We are always looking for ways to tweak our fan blade designs to achieve exactly what you are asking about. In regards to noise, the tip geometry is pretty much king. That's where you are sure to go supersonic and that is where the noise is generated.

    For example, the forward swept tip is now the standard.

    You can also see the tips here on Honda's blatant copy of our engine:

    I'll see if I can dig up any more sources.
  5. Jul 13, 2007 #4


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  6. Jul 14, 2007 #5
    Turbofan Video-1:

    Turbofan Video-2:

    These details look very odd to me, taken at a local airshow last week at Willow Run (FedEx jet). Do these steps and bevels or cants help reduce noise?


    The Horten brothers and their WWII flying wings had some odd pusher propellers, I'll try to find my books and post those pictures too.

    Attached Files:

  7. Jul 14, 2007 #6


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    I didn't get a chance to get down to Willow Run this time. I was bummed out. Did the MING have one of their CH-47s there?

    Anyways, there are certain things that fan design need to accomplish:
    - High thrust
    - Low weight
    - Low Noise
    - Structural integrity

    It's difficult to say exactly what each fan designer had in mind with each specific piece of geometry. As I mentioned before. the tip area, especially on a large diameter fan, is going to have a lot of supersonic shock issues. That means noise. So pretty much any design is an effort to increase the efficiency and decrease the noise at the tips.
  8. Jul 18, 2007 #7
    Yes, the double rotor did a mock rescue of a downed flyer.

    My 34 picture album of the Willow Run airshow, mostly B-25 nose art.

    http://s184.photobucket.com/albums/x295/kach22i/B-25 Mitchell at Willow Run 2007/

    Back to the topic; does anyone know if NASA has any current or very recent projects dealing with curved blades? I know they did a super quiet prop. It was hollow and had air pumped through it. The escaping air controled the vortex's which make sound. The air did not escape out the tips, rather it vented on the backside of the prop surface via the hub.
  9. Jul 18, 2007 #8
    From page 12 of the Random Picture thread in the O.T. section of HCA.

    http://www2.nlr.nl/public/facilities/AVET-Info/Content/UK/PropBlades.html [Broken]
    http://www2.nlr.nl/public/facilities/AVET-Info/Content/Pics/Prop_apian_isolated.jpg [Broken]
    http://www2.nlr.nl/public/facilities/AVET-Info/Content/Pics/Prop_apian_kulites.jpg [Broken]

    Daniel T. Valentine, Ph.D.
    http://people.clarkson.edu/~space/VALPG1.html [Broken]
    http://people.clarkson.edu/~space/VALPG1_files/image003.jpg [Broken]
    Photo from the Naval Institute Proceedings web site;
    this propeller was the first highly-skewed propeller
    installed on a U. S. Merchant ship. The Project was
    supported by MARAD. It was designed by Valentine
    (when working at DTRDC under Dr. Wm. B. Morgan).

    From page 13.........................of HCA thread.
    http://www.hoverclubofamerica.org/forum/index.php?showtopic=779&st=180 [Broken]


    http://www.geae.com/aboutgeae/presscenter/ge90/ge90_20041116.html [Broken]
    http://www.geae.com/aboutgeae/presscenter/ge90/GE90_Blade.jpg [Broken]
    http://www.hoverclubofamerica.org/forum/index.php?showtopic=779&st=180 [Broken]

    Novel Engineering and Fabrication Techniques Tested in Low-Noise-Research Fan Blades
    Trailing Edge Blowing blade with top skin removed. Air enters at the retainer (bottom right) and exits through turning vanes at the trailing edge
    Last edited by a moderator: May 3, 2017
  10. Jul 18, 2007 #9


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    Wow! That NASA fan blade design is wild.
  11. Jul 18, 2007 #10
    It reminds me of a Aerial Tennis Birdie (badminton).

    http://www.everbe.com/Products/Sports/cwdata/badminton%20rackets.html [Broken]
    http://www.everbe.com/Products/Sports/badminton%20birdy.jpg [Broken]
    Last edited by a moderator: May 3, 2017
  12. Jul 19, 2007 #11
    Why do helicopter rotor blades have weights in them?

    Do other types of fans or propellers have end tip or leading edge weights?


    Helicopter blades have weights in them?
    http://www.cavalrypilot.com/fm1-514/Ch3.htm [Broken]
    http://www.cavalrypilot.com/fm1-514/IMG00039.GIF [Broken]
    Last edited by a moderator: May 3, 2017
  13. Jul 20, 2007 #12


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    For track and balance purposes. The blades need to be rotating in the same plane or as close to the same plane as possible. If you don't you get control and vibration issues. Since there is no real fine tune adjustment per blade on a rotor head, blade weights adjust the tracking.

    I can't be for sure, but I can not say that I have ever seen them anywhere else.
  14. Jul 21, 2007 #13
    I thought this paper may be of interest to some.

    Senior Thesis Project
    University of Virginia Department of Mechanical and Aerospace Engineering
    Raymond Scott Ciszek
    March 25, 2002

    Last edited by a moderator: Apr 22, 2017
  15. Jul 22, 2007 #14


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    If you mean the snubbers (the cantilevers sticking out of each blade at about 2/3 of the blade height) their function is to control vibration, not noise reduction.

    When the engine is running, the blades twist and the snubbers press against each other to act like a solid ring. That has a big effect on the lowest vibration frequency of the blades, and eliminates some aerodynamic stability problems (a.k.a. flutter). The disadvantage is they partly block the airflow through the fan.

    Modern fan designs have longer blade chords and "hollow" blades (actually, lightweight composite structures inside the blade) to control these vibration issues without using snubbers between the blades.
  16. Jul 23, 2007 #15
    Thank you, I was not familiar with that term.

    AlephZero are you calling the vane riblets "snubbers"?

    Will the turbofan's blades will be pulled foreward and well into the larger part of the duct opening under full speed? You see the level changes there (right?), one is sloping a little.

    This cannot be good for tip blade vortex generation, right?


    Last edited: Jul 23, 2007
  17. Apr 13, 2008 #16
    Here's a related question: Both (air) fan blades and ships' propellor blades often have one side that's curved (convex) and another side that's either straight or concave. But the orientation is OPPOSITE on fan blades and propellors! I.e., props seem to lead with their curved (convex) edge, as the leading edge, while fan blades seem to lead with the other side and use the curved (convex) edge as a trailing edge!

    How can the optimum blade shape for these two common fluids be OPPOSITE from each other? Seems weird to me.

    BTW, I'm old enough to remember when fan blades looked like prop blades. I don't think the shape of props has changes hugely in those decades, but the shape of fan blades has. Now some of them seem to have concave leading edges with swept-forward "points" at the outside! If that were a universally efficient way of slicing through the air at an angle to generate lift, I'd expect modern airplanes to have wings with concave LEs with swept-forward wing tips. But I haven't seen anything like that -- at least not yet!

    What gives?
  18. Apr 13, 2008 #17


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    Can you dig up an example of a propellor with a concave leading edge? I've never seen such a thing.
  19. Apr 13, 2008 #18
    No, Dave, I've never seen a propellor blade with a concave leading edge, they're all convex. But I've seen lots of FAN BLADES with concave leading edges, and some with straight ones, too. And their TRAILING edges are all convex, like the LEADING edges of the propellors. That's my point, the fact that they're totally opposite to each other.

    Got an answer to the puzzle?
  20. Apr 13, 2008 #19


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    Can you dig up an example of a fan blade with a concave leading edge? I've never seen such a thing.
  21. Apr 13, 2008 #20
    I've got one at home, but I'm not there now. Here, they've got ceiling fans that look exactly like ship propellors, but they go in only one direction, which would be reverse for a ship -- straight edge first, convex edge trailing. That's really what prompted the question. If you've got a relatively new table fan or pedestal fan around, check the blade shape, and I think you'll find the leading edges are either straight or concave.

    OK, I just did a quick check of "table fan" on eBay: [I can't post URLs here yet, so I'm leaving out the cgi and ebay and com parts at the beginning]
    Lasko-16-Performance-Oscillating-Table-Fan-Model-2506_W0QQitemZ160227774440QQihZ006QQcategoryZ20612QQssPageNameZWDVWQQrdZ1QQcmdZViewItem shows the kind of a fan blade with a concave leading edge that I'm talking about. Note that the fan blows air FORWARDS, so the back = farther-away edge of the blades is the leading edge -- that's the funny concave edge with the pointy bit near the circumference. If it turned BACKWARDS, it would look like a reasonable boat prop, with a "swept-back" trailing edge. Also, comparing it to the shape of an airplane wing yields paradoxical (= backwards) results, no?

    For comparison, here's the proof that fan blades USED to look exactly the other way around: VINTAGE-ANTIQUE-ELECTRIC-TABLE-FAN-ROTOR-ELECTRIC-CO_W0QQitemZ330227727933QQihZ014QQcategoryZ4037QQssPageNameZWDVWQQrdZ1QQcmdZViewItem . That's an old one, and the convex edge leads, and the concave edge with the point trails. That looks like conventional "streamlining" to me, and like a ship's prop -- but somebody's apparently discovered recently that the opposite shape works better for fans.

    (Judging by the pitch, both fans have to rotate clockwise, the way we're facing them. But the blade shapes are exactly opposite to each other!)
    eBay also has lots of fans with straight blade edges. Again, the "vintage" ones have the straight edge trailing, like a prop, and the new ones have the straight edge leading, backwards from a prop.

    What gives?
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