Questioning airfoil design of WIG craft

Discussion in 'Hydrodynamics and Aerodynamics' started by MantaRay, Mar 10, 2013.

  1. daiquiri
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    daiquiri Engineering and Design

    IMO, the canard in the two WISES videos has too small chord and flies too high above the water surface to feel a sensible ground effect. Looks like it's purpose is to level up the nose of the craft when it comes too close to the water surface, but as soon as the nose starts pointing up (and too much, by the way) and the flight level increases (also too much) the pitch-down restoring moment appears to come mainly from the negative Cm of the wing.
    Frankly, it doesn't look very pitch-stable in that video. The amplitude of the flight-level oscillations appear to be pretty high.

    Cheers
     
  2. Petros
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    Petros Senior Member

    it is a little more complicated than that. On an aircraft with a convetional configuration (horizontal tail in the back), the tail makes it stable by producing a down force. On a canard configuration the fore plane makes it stable but both the wing and the canoard produce up force, this is why may feel the canard is more efficient.

    When in the ground effects (generally taken as the wing span distance, with diminishing beneficial effects from zero up to the height of the wing span), the pitch trim is affected and you have to trim out the change with the tail, or the canard. this means with a short coupled horizontal tail you have to produce more downward lift (that the wing has to overcome) to remain stable, reducing the efficiency of the combination. But the canard configuration efficiency is not much affected by the trim change since both surfaces create lift.

    You can also make a tailless (all wing) configuration work, but the trim changes can be more difficult to deal with since the moment arm to the trim surface (on the trailing edge of the wing) is much smaller than the canard or conventional tail in the back.

    One way to improve the efficiency of the tail in the back configuration is to make a longer tail boom, so not as much now force is needed to trim out the pitch change. Of course a long tail boom means you can not fly as close to the surface of the water or it would strike it when at high angles of attack (at lower speeds). Hence the reason for the T-tail, to get it higher. Which also adds structural weight.
     
  3. daiquiri
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    daiquiri Engineering and Design

  4. MantaRay
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    MantaRay Junior Member

    Thanks for the links, I saw that video some time ago (EDIT: I watched it again just now and I learned a lot more this time around now that I know a bit more about aerodynamics - thanks for posting it). It made me realize how easy it could be to test v small scale models. I may try giving a few designs a go within the next few weeks (after I have done some more study on aerodynamics). It will help my learning in a practical sense.

    While skimming through the thread you sent me I saw this concept design.

    [​IMG]

    Looks fantastic, but then again would it be practical? 'Inverse' v-tail with standard delta rather than reverse-delta (EDIT: though not reverse delta it still does have the ram wing to induce lift) - wondering if that thing would be able to achieve ground effect. Here is the site page for this concept design.

    EDIT: on closer inspection, it looks as if it does not have wings per se, but rather the body acts as the wings. Kind of like the 'flying wing' that Submarine Tom mentioned on the previous page, or a 'blended wing body' as they call it in aviation. Again, looking at the 'wings' it has a nice broad span like the WISES model:

    wingspan.jpg
     
  5. daiquiri
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    daiquiri Engineering and Design

  6. MantaRay
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    MantaRay Junior Member

  7. Michael Y
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    Michael Y Junior Member

    The chord doesn't impact ground effect, the span does, so the canard's span is what is important.

    I agree that it does not look very pitch stable. As before, coming out of ground effect is in general destabilizing. Along with the movement of the center of pressure, the drag decreases as you get close to the ground and the lift increases. But the drag decrease is the major impact, the lift increase is not as much. So depending on changes in the lift of the canard to provide stability is not going to be terribly effective.
     
  8. daiquiri
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    daiquiri Engineering and Design

    No, the chord is the most important factor. It is the longitudinal extent of the wing (the chord), set at an angle and close to the ground, which acts as an obstruction which augments the RAM effect.

    The biggest effect of ground proximity is felt within around 20-25% of the chord. It is a region of flight where viscous effects carry an important part of flow choking below the wing. The second significant interval of flight levels is comprised between 1 chord and several span lengths (some authors say up to 10 span lengths). In this region inviscid flow phenomena dominate and the ground effect is significantly lower. Above this region the craft is considered to be flying OGE.
    The region in between the first two (i.e. between 0.2 and 1 chord lengths) is a zone where both mixed viscous and inviscid phenomena act and the analysis is more complex.

    Hence, the chord is the main parameter in ground effect analysis.

    The above consideration is also tightly related to safety issues during bank turns (there's always a slight bank during turns of a WIG craft). Take two WIG crafts, one (WIG "A") with AR = 6 (say, a 6 m span wing, 1 m chord), and the other one (WIG "B") with AR = 1 (say, 3 m span, 2 m chord. They both have the same wing area of 6 sq.m .
    Suppose we want to have the maximum benefit, in terms of L/D ratio. of the ground effect. So we decide to make them fly at a height equal to 25% of the chord. So first one will fly at 0.25 m height, the other one at 0.5 meters from the water surface. Then, due to the wingspan, the first one will ideally be able to bank only 5° before touching the water surface, while the other one can get to 19°.

    Hence, there are some good reasons why nearly all WIG vehicles have a low aspect-ratio wing and a very prominent chord, besides the problems relative to maneuvering in ports or restricted areas with long wings protruding from your fuselage. ;)

    Cheers
     
  9. MantaRay
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    MantaRay Junior Member

    I have been playing around with cardboard models all day. I will draw the models up in 3D as my cardboard models are looking and beaten up.

    Firstly I made a ram wing (in image). Right off the bat it worked OK - a nice glide. I added a rear vent to release some of the pressure form under the wing. The problem I encountered is when I pushed it enough it gained too much upward pitch and flipped over. That was to be expected.

    deltaram.jpg
    (note - the bend at the nose of the bottom model is simply there to hold the nose up. It is not a part of the design per se.)

    I then made a ram reverse-delta which you see on most modern WIGs. I used this model as a guide but without the tail to start with. Of course, without the tail it gained too much up pitch and flipped. However, it was much easier for it to flip compared to the simple ram wing. It was actually hard to get it to glide as all it wanted to do is flip. Again, it was expected so I added the horizontal wing but I did not get it correct after a few tries as it kept flipping. I believe I got the COP and COG completely out of balance. But I then decided to try something different - something from this design.

    It is basically a delta ram wing (bottom model in image). It was very stable. I could not get it to flip over. When I applied a lot of thrust to it, it would pitch up, stall and pitch down. It would pitch down too low to resume stability though. I was wondering why this happens. I think I can understand why the delta wing is more stable though - because the pressure is more evenly distributed throughout. At least it seemed that way when I watched it glide.

    ADD: I think the pitching down maybe to do with that silly nose bend I used to prop the nose up. I will make a better model and try adding a canard for further lift and possible stability.

    ADD: I removed the nose bend and replaced it with a support near the nose. I also tried the canard which caused down pitch which resulted in very little lift of the model. Now I see why the WISES model used a canard (if configured correctly balances the up pitch of the ram wings). However, I wonder what the results maybe if the canard used an aerofoil wing rather than a flat one. Without the canard but with the replace support the model glided/hovered fine.

    deltaram2.jpg

    Time now to see how it performs with a body on top of the wing. Using this design as a guide for the body I am thinking that it may produce some good lift due to the negative pressure the body will create.

    ADD: I will have to work on the body another day as it will take more time than I thought and I will need to go somewhere with a larger floor as I will need to push it further and harder as it weighs more. I tried a a badly constructed one though and there seemed to be some unwanted down pitching. If this issue resumes I am sure it can be remedied by an additional wing/tail configuration to supplement the delta ram wing.

    Hhmm, I wonder how a delta and reverse delta ram combo would work out.
     
  10. Michael Y
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    Michael Y Junior Member

    So what is a good reference for reading up on ram and ground effect?
     
  11. daiquiri
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    daiquiri Engineering and Design

  12. MantaRay
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    MantaRay Junior Member

    Wow, nice one. I have been needing a document like this.
     
  13. daiquiri
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    daiquiri Engineering and Design

    Looks like NASA server with technical reports is being censored right as I am writing it. See what happened: http://www.boatdesign.net/forums/al...e-unhappy-times-we-live-46638.html#post621742

    Hence, I am attaching a document from my private collection. It plots aerodynamic characteristics of wings having AR = 1 to 6.

    What you will notice is that for all wings the IGE starts to become noticeable at around h/c = 0.4-0.6 . However, the corresponding h/b values are very different, ranging between 0.5 and 2.5. So, again, the h/c value is the one which commands.
    The various symbols mean:
    h = flight level
    b = wing span
    c = chord length
    A (or AR) = Aspect Ratio, c/b for rectangular wings

    Cheers
     

    Attached Files:

  14. kalymnos
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    kalymnos New Member

    Wig

    hi, i have been reading your posts. any chance we can chat about a WIG project iam interested to do? my email is: ekoumbaras@hotmail.com.

    thanks
     

  15. kach22i
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    kach22i Architect

    Both on the same craft, similar to the Dragonfly aircraft?

    Lippisch was a big fan of the reverse delta because of the stability it gave his craft. Might be a good stating point if you have decided to forego the simplicity of a wide aspect straight wing.
     
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