The Flying Cherub

Discussion in 'Sailboats' started by Doug Lord, Oct 7, 2014.

  1. Doug Lord
    Joined: May 2009
    Posts: 16,679
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

    Clive Everest has put lifting foils on his Cherub-what he calls a Pi foil. The technical details posted here are highly unusual to see on DA but are perfect for some design analysis. According to his post on DA ,foils are legal in the class but his is only the second Flying Cherub.
    Excerpts from his post on DA( ) :

    Compared to the Moth our sail area to all up weight is similar upwind and potentially much better downwind.
    (15.5m^2 / 210kg Vs 8.25m^2 / 110kg upwind). (Cherub: 462lb./161sq.ft=2.87 sq.ft./lb UPWIND-much better downwind // Moth: 242lb/ /DL)
    With twin wires and 1.8m beam our sail carrying ability is better.
    However our hulls are only one foot longer, nearly 3x the water line width and carrying twice the weight so getting to take off speed is going to be a challenge. We do not have the aero dynamic efficiency of the Moths high aspect una rig and minimal hull. I have no expectations of getting close to their performance.

    Our solution is a twin centreboard Pi foil configuration with a large main foil.
    I was going to make the foil with a 1.6m span but have reduced it to 1.45m so that it still loads onto the trailer and so that we can roll it for rigging with the foil retracted against the hull. The foil area is 0.22m^2, giving 30% more area to weight than the more recent moth foils. This gives a 10:1 AR similar to the Blade Rider or first Mk2 foil but lower than the more recent ones.
    The Pi foil configuration allows the wand to control the pitch of the entire foil which should give better height control than just the flap.
    It also allows a centreline wand to be used.
    It is was much easier to build as there are no structural T joints or complex flap assemblies.
    We have used a DAE41 foil section that was developed for low Reynolds human powered flight.
    It has low Cd for Cl near zero to Cl ~1.5 which is higher than we would ever want to use because at that level the induced drag would be higher than the hull drag if we were low riding. The section has quite a high pitch moment. To counter this we have added a tail plane to the foil.


    Pictures by or posted by Clive Everest:

    Attached Files:

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