Caveats of high lift airfoil

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by durabol, Jan 9, 2011.

Help Support HomeBuiltAirplanes Forum by donating:

  1. Jan 9, 2011 #1

    durabol

    durabol

    durabol

    Well-Known Member

    Joined:
    Mar 2, 2010
    Messages:
    169
    Likes Received:
    3
    Location:
    Lethbridge, Alberta / Canada
    I'm going to attempt to build a low powered (50hp) WIG flying boat type thing using a biplane design to hopefully get off the water. I have been considering using a high lift airfoil such as an S1223 since drag isn't an issue and I don't plan on using flaps. I believe the airfoil was designed for low Reynolds numbers (model airplanes) but am unsure if this is a problem? I also wondered if the extreme under camber on the airfoil would be a problem to cover?

    Brock
     
  2. Jan 9, 2011 #2

    Dana

    Dana

    Dana

    Super Moderator Staff Member

    Joined:
    Apr 4, 2007
    Messages:
    8,831
    Likes Received:
    3,178
    Location:
    CT, USA
    The undercamber will create some issues with covering, but not insurmountable... you'll need to attach the fabric to the ribs at close intervals to maintain the profile shape. It's hard to say how it would behave at higher Reynolds numbers, The pitching moment is rather high, and with the aft loading that means, getting adequate structural strength in the aft part of the wing could be an issue. I'm also guessing (intuitively, with no facts to back it up) that this airfoil might not be a good choice for a biplane, I suspect the efficiency hit from making a biplane might be more than for a standard airfoil.

    Why do you think drag isn't an issue?

    -Dana

    If at first you don't succeed, skydiving is not for you.
     
  3. Jan 9, 2011 #3

    WBNH

    WBNH

    WBNH

    Well-Known Member

    Joined:
    Oct 5, 2006
    Messages:
    307
    Likes Received:
    82
    Location:
    Portsmouth, NH
    An ultralight WIG I was familiar with as a kid in the 80's used an inverted delta wing (flat end forward) with an inverted airfoil...the idea wasn't to provide lift in the conventional sense...but to float on a cushion of air, a la hovercraft with no skirt, by using the higher flow side of the airfoil facing down and angle of attack to compress the air against the water...

    ...I don't see how this would work with a biplane planform.
     
  4. Jan 9, 2011 #4

    orion

    orion

    orion

    Well-Known Member

    Joined:
    Mar 3, 2003
    Messages:
    5,800
    Likes Received:
    135
    Location:
    Western Washington
    Actually, it probably wasn't an inverted airfoil but a reflexed one. In ground effect you need an airfoil with very little pitching moment since close proximity to the ground results in a very high nose-down tendency.
     
  5. Jan 9, 2011 #5

    GESchwarz

    GESchwarz

    GESchwarz

    Well-Known Member

    Joined:
    Oct 23, 2007
    Messages:
    1,179
    Likes Received:
    112
    Location:
    Ventura County, California, USofA.


    Thanks Orion, although it makes perfect since, I never realized that.

    Let me ask you this before I commit to my choice of airfoil (the time to decide is coming soon). For my 1800# gross, tandem seat, pilot-in-front machine. Having near zero pitching moment is critical for me, do you think that I could do better than the GA37A-218, and still have the benefits of this airfoil?
     
  6. Jan 9, 2011 #6

    orion

    orion

    orion

    Well-Known Member

    Joined:
    Mar 3, 2003
    Messages:
    5,800
    Likes Received:
    135
    Location:
    Western Washington
    For your application it's not really a consideration since your proximity to the ground is nowhere near as close as it would be for a classic wing-in-ground-effect craft. As such, the section you selected for your project seems reasonable.

    WIGs on the other hand operate within about a tenth of the MAC off of the surface (give or take a bit, sometimes closer) and as the trailing edge nears the surface the cp shifts further aft: From 25% chord to about 50% chord, or even a bit further back than that. That results in a severe trim change, which can be further exacerbated by an improper section. The magnitude of the moment change is strongly a function of the section selected and conventional airfoils tend to fare the worst. For this reason WIG designs tend to do better with a reflexed section, similar to what you'd have in a flying wing.
     
  7. Jan 10, 2011 #7

    durabol

    durabol

    durabol

    Well-Known Member

    Joined:
    Mar 2, 2010
    Messages:
    169
    Likes Received:
    3
    Location:
    Lethbridge, Alberta / Canada
    -Thanks for the helpful and knowledgeable responses.

    -I didn't think drag mattered too much as this is going to be a slow flying aircraft.

    -As far as covering is concerned I was planning to glue the fabric on ala Hipec or similar and after some thought it probably wouldn't be a problem.

    -I realize the top wing of a biplane won't benefit from ground effect but liked the idea because biplanes have a lot of wing area which I though was important to getting off the water. The top wing will probably add some stability in flight as well.

    -As far as the pitching moment (due to the airfoil characteristics and from flying in ground effect) and the fact that the TE of the airfoil is quite thin I think I will make the TE out of solid foam covered with fiberglass. Perhaps I'll need to add a few more ribs as well to help transfer forces. Perhaps I could just use a high lift airfoil on the top wing and a reflex airfoil on the bottom wing that will be in ground effect?

    Brock
     
  8. Jan 10, 2011 #8

    orion

    orion

    orion

    Well-Known Member

    Joined:
    Mar 3, 2003
    Messages:
    5,800
    Likes Received:
    135
    Location:
    Western Washington
    Due to the characteristics of ground effect flight, that top wing could dramatically destabilize your craft. WIGs are difficult to design since they have two stability criteria and getting both to work simultaneously is difficult at best.
     
  9. Jan 10, 2011 #9

    CNCRouterman

    CNCRouterman

    CNCRouterman

    Well-Known Member

    Joined:
    Nov 26, 2003
    Messages:
    52
    Likes Received:
    0
    Location:
    Minnesota, USA
    The sparse reading on WIGs I recall tended to place the Horizontal Stab rather high to keep it OUT of the ground effect, supposedly to improve its effectiveness, the operating theory (Yeah, mechanics illustrated I think) was that if the craft rose to high, and left ground effect, the main wing lost much of its lift, so the craft sank back down into ground effect. I think it was something I read in one of Bill's (Orion) white papers that mentioned WIG airfoils are not friendly OUT of ground effect in terms of stability (fuzzy memory there). Perhaps it is more a case of designing for an operating envelope, and don't expect the same benign behavior outside the design envelope?

    I think I may be recalling the same or similar designs for ground effect watercraft you mentioned. I believe the wing was similar to a revered delta, and had a significant negative dihedral as well, almost Klingon Bird of Prey in landing configuration.
     
  10. Jul 13, 2011 #10

    kj9p

    kj9p

    kj9p

    New Member

    Joined:
    Jul 13, 2011
    Messages:
    2
    Likes Received:
    0
    Location:
    Normal, Illinois
    durabol
    You might start looking (browsing) for the paper: High-Lift Low Reynolds Number Airfoil Design by Michael S. Selig and James Guglielmo. The paper covers the subject nicely. It also gives the S 1223 airfoil coordinates. I am also experimenting with this airfoil.
    kj9p
     
  11. Jul 13, 2011 #11

    RCMusall

    RCMusall

    RCMusall

    Member

    Joined:
    Sep 18, 2003
    Messages:
    9
    Likes Received:
    0
    Location:
    Ft Lauderdale, FL
    CAUTION! To anyone considering a flying a surface close to or on a restricting surface (WIG).
    The surface providing an increasing and major portion of lift is bottom side as it compresses the air and causes a higher pressure as the air is squeezed under the surface. However, the air velocity is increased as section area is reduced by the angle of the plane and the pressure is decreased at the back as in a venturi This can cause a catastrophic result as in a racing hydroplane taking off into a backward flip as the back edge pressure is reduced and pressure on the forward portion is rapidly increased with a rapid increase in the angle of attack.

    Dick_SF@msn,com
     
  12. Jul 14, 2011 #12

    orion

    orion

    orion

    Well-Known Member

    Joined:
    Mar 3, 2003
    Messages:
    5,800
    Likes Received:
    135
    Location:
    Western Washington
    Racing hydroplane flip mechanism is only slightly affected by this accelerating flow and certainly is not at all a contributor at the beginning of the nose-up motion when the hull's aft end is still in the water. The primary mechanism of the flip is due to vortex flow created by the geometry of the pickle-forks. This creates an extreme low pressure field on the forward part of the boat, lifting the nose out of the water and over backward. The mechanism is almost identical to that of the strake used on many modern fighters - for instance, in a high G turn, the F-16's strake carries almost 70% of the lift force. One of the most ideal strake (and VG) shapes is the gothic arch, which is almost precisely the planform shape of the unlimited hydroplane forked bows. It takes only a few degrees of aoa to create this flow.

    Some years back I was working on this issue with Ron Jones Marine - as a partial fix we created a larger edge radius on the bow forks. The result was substantially increased stability since the larger radius kept the flow attached longer, thus delaying the shed vortex.
     

Share This Page

Group Builder
arrow_white