X-AC-5 (oz10999)


X-AC-5 (oz10999) by OFW Fisher 1967 - plan thumbnail

About this Plan

X-AC-5. Experimental free flight model with asymmetrical layout.

Quote: "Slightly psychic (sensitive to nonphysical or supernatural forces and influences - Webster), in shape, this fully asymmetrical free flight is a stable consistent flyer. X-AC-5, by OFW FISHER.

The experimental MK 5, X-AC-5 for short, is the culmination of several years design and development work in the field of asymmetric configuration free flight flying machines. The X-AC-5 is ultra stable and can be flown in strong winds, once it has been correctly trimmed. Stall recovery is faster than for orthodox layouts.

Design: The X-AC-5 was designed to combine the excellent lateral stability of a flying wing with the longitudinal stability of a medium moment-arm orthodox machine; and at the same time provide an extremely light and robust structure. Induced drag on the starboard tip and elevon is reduced by a large tip fin, also affects directional stability. The X-AC-5, due to its low wing loading, is fitted with a self-stabilizing 'flat plate' type airfoil section; this has proved very effective in practice and is also very easy to build.

If the wing loading was increased, by fitting RC for instance, it would then be necessary to incorporate an asymmetric reflex type airfoil section. The CG of the model is on the thrust line of the engine and located 4.5 in back from the leading edge of the wing. Considering the areas H1 (port tip), H3 (port center-X), H4 (starboard center-X) and H2 (starboard tip), the line joining the centers of pressures P1, P2 of H1 and H2 is behind the line joining the centers of pressures of H3 and H4, thus providing a longitudinal correcting moment about the latter.

If F1 to F4 are the forces acting through the centers of pressures P1 to P4 of the respective areas H1 to H4 and, if m1 to m4 are the horizontal distances between P1 to P4, respectively, and the thrust line; we can take moments about the thrust line as follows: (i) F1.m1=F2.m2; (ii) F3.m3=F4.m4.

It will be apparent that as F2 > F1; in order to ensure that the equation (i) holds true, it is essential that m1>m2. Similarly for areas H3 and H4 to produce equation (ii). The foregoing is not intended as anything like a complete design analysis, but merely to give the builder an idea of the basic concept. It can easily be seen from the equations (1) and (ii) that the center of pressure of the entire machine lies on the thrust line and behind the C of G..."

X-AC-5, American Aircraft Modeler, July 1967.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.


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X-AC-5 (oz10999) by OFW Fisher 1967 - model pic

  • (oz10999)
    by OFW Fisher
    from American Aircraft Modeler
    July 1967 
    32in span
    IC F/F
    clean :)
    all formers complete :)
    got article :)
  • Submitted: 10/03/2019
    Filesize: 235KB
    Format: • PDFbitmap
    Credit*: Circlip, RFJ
    Downloads: 578

X-AC-5 (oz10999) by OFW Fisher 1967 - pic 003.jpg

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User comments

Half of the pdf is missing.
BP - 15/08/2020
No. But to be honest, that's what I thought myself, the first time I saw this plan. Look again though - it is a complete drawing that shows a (very) asymmetrical design.
SteveWMD - 15/08/2020
I saw the redoubtable Mr Fisher fly this at the British Nationals at Hullavington in 1967.
He found himself an empty piece of airfield where, after some huddling over the model, the motor started and he launched it straight ahead, whereupon it proceded to bank into a stable tight turn and circulate around him like a freeflight control-line model!
A remarkable and talented designer and flyer with a distinctive line in sharp looking cockpits on all the Performance Kits designs.
Roger T, Harpenden - 07/11/2022
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