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Thread: Strutted flexwing vs negative G?

  1. #1
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    Strutted flexwing vs negative G?

    Just something that kept me awake last night.....

    I know that a strut will take both tensile and compression loads but in compression the strut is able to/likely to 'buckle'!

    Once this has happened (even slightly) then the ability of a slender strut to withstand continued compression loads is dramatically reduced.

    Small sub struts on majority of 3 axis strutted wings are designed to reduce/prevent this process but of course on a flex wing there is no 'anchorage' for a sub strut!

    So question is:- on a strutted flex wing how does the design cater for negative G?

    As usual any and all answers greatly received and appreciated.

  2. #2
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    Strutted flexwing vs negative G?

    Paul Wilkinson wrote:
    So question is:- on a strutted flex wing how does the design cater for negative G?
    Maybe we should start from the topless hang glider that has no struts. In this case all the bending loads in negative G are resisted by the cross tubes and leading edges. Conventional cross tubes aren't strong enough for this so they get beefed up.

    In the case of the strutted wing the negative G loads are resisted by the cross tubes and leading edges in bend and the struts and base bar in compression, so the struts aren't alone in load resistance. By their very nature it's very hard to get flex wings into negative G, and on the load test rig they very rarely get to -3G because aeroelasticity results in load shedding as the structure bends under the heavier negative loads. Strutted wings tend to use the more conventional alloy cross tube layouts of their king posted brethren rather than the very large and carbon fibre cross tubes fitted to topless hang gliders.

    Pitch stability from tips, washout and sweepback tends to make the wing fly at the tips first in negative and to raise the nose.

    In practice with most HG when they are negatively loaded (failed loop?) the point of failure is the junction of the outer leading edge with the cross tube and inner. This is where the bending moment is greatest on the unsupported outer.

  3. #3

    Strutted flexwing vs negative G?

    The codes call for -3G ultimate regardless. However trike struts are shorter than their fixedwing cousins and at a less steep angle, and due to heavy planform taper the loads are focussed far more in board so compression loads are reduced. But the struts are still generally thicker than most microlight fixedwings to help resist buckling.

    One might want to consider the A frame uprights in comparison - although a bit shorter they are subject to 6g worth of ultimate compression loads from positive G.. No dents or bends please! - every student pilot should have this very heavily emphasised during training..

    Paul

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    Strutted flexwing vs negative G?

    Paul Dewhurst wrote: The codes call for -3G ultimate regardless.
    It would be interesting to test the differences between the codes and real life, as I have a theory that aeroelasticity means the mathematical modelling is some way off. For a topless HG, like the early La Mouette Topless, they got nowhere near -3G on the test rig even when run at the maximum negative angle of attack and Vne. The flexibility of the carbon cross bar and the lack of lower struts meant load got seriously shed.

    Gerard proofed all the early Topless gliders by sand bagging (pics in XC Mag from way back), so he was sure that they would survive -3G loads anyway, but there was no indication that aerodynamically they would ever create such a load, and in fact the rig showed that it wasn't possible to get near -3G. Sand bagging != aerodynamic loads.

    However, this would be very different for the strutted version, as the struts would limit downward bending of the leading edges in a way that doesn't happen on the HG versions with side wires. There would be less load shedding, but I'd expect that it would still be debatable about getting to -3G at Va, and maybe needing to get close to Vne to achieve it. It would be interesting to see what the difference between real life and theory is on the BHPA test rig, or if any of the BHPA/DHV/USHGMA has ever load tested a strutted wing. It would have to be a wing for a single seat trike, as it's unlikely the rig would be able to load a two seater wing to -3G.

    Some HGs have survived up to +7G on the test rigs. As Paul says, the effect of the smallest of bends or dinks on the uprights resisting this cannot be overemphasised.

  5. #5

    Strutted flexwing vs negative G?

    -3 is of course ultimate and is allowed to deform and break after 3 seconds, based on a no damage limit load of 2G. That's where rig testing can be a problem where effectively you might only be able to apply limit loads due to speed and truck power limitations. Hopefully we will never expereince any sustained negative loads duri g our time as Flexwing pilots and the aerodynamics don't work well in that regime. The big negative loads you can get are when tumbling, but you are basically f*^#^d whatever if that happens so maybe academic!

    Paul

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    Strutted flexwing vs negative G?

    So,
    They take negative G because they are 'beefed up' (so as to not buckle up go 2. / 3 G) as assisted by cross tube and leading edges?

    Seems to be asking alot of leading edge and cross tube hinge points but I'll take that (for now!).

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    Strutted flexwing vs negative G?

    Paul Wilkinson wrote: So,
    They take negative G because they are 'beefed up' (so as to not buckle up go 2. / 3 G) as assisted by cross tube and leading edges?

    Seems to be asking a lot of leading edge and cross tube hinge points but I'll take that (for now!).
    Don't forget that the cross tube junction is held onto the keel by a strap, which transfers the load. This limits upward bending of the cross tube junction.

    Microlight wings I assume are only sandbagged, but having seem how much bending the La Mouette Topless survived on the BHPA test rig with real aerodynamic loads I'd not be worried.

    Don't forget that manufacturers like Ellipse made their first strutted wings in the mid to late 1980s.

    Worth having a look at the cross tube/leading edge plates on some wings... they won't be exactly the same as they are on their king posted brethren....

  8. #8

    Strutted flexwing vs negative G?

    Paul Wilkinson wrote: So,
    They take negative G because they are 'beefed up' (so as to not buckle up go 2. / 3 G) as assisted by cross tube and leading edges?

    Seems to be asking alot of leading edge and cross tube hinge points but I'll take that (for now!).
    Loads in those is not so different to using wires - struts or wires it just creates a fixed point for the leading edges to react the loads.

    Paul

  9. #9
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    Strutted flexwing vs negative G?

    Agree that Paul - providing the strut doesn't buckle!

  10. #10
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    Strutted flexwing vs negative G?

    Paul Wilkinson wrote: Agree that Paul - providing the strut doesn't buckle!
    think the maths for that is covered by Euler bending of slender struts. Maybe the struts in question aren't thin enough (length v depth) to be covered by this...

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