Mixing and the Jet Flap

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Abstract
Summary: One of the main problems associated with the “ Jet Flap ” concerns the discrepancy in thrust between idealised theory and the experimental results. This discrepancy is attributable to the mixing with the surrounding flow of the thin two-dimensional jet while still in close proximity to the aerofoil. The effect of the mixing may be calculated to a first approximation from a formula derivable from first principles, while certain second order effects, which can be significant, may be considered qualitatively.It is concluded that. (i) the full thrust should be experienced by a jet flapped aircraft at cruise, (ii) it should be possible to attain a low form drag at cruise in comparison with a conventional aircraft, (iii) at take-off, an aircraft using a shrouded jet flap would have better thrust recovery than one using a pure jet flap (which shows appreciable losses), (iv) the use of by-pass engines would further improve the thrust characteristics, (v) the practical gains from thrust augmentation, as obtained by controlling the mixing, seem likely to be small.

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