Extra 300s (oz7722)


Extra 300s (oz7722) by David Boddington 1995 - plan thumbnail

About this Plan

Extra 300s. Radio control scale model. For .40 sized engine.

Direct submission to Outerzone.

Update 26/2/2024: Added article, thanks to RFJ.

Quote: "The EXTRA 300S. Full-size FREE pullout plan for 40 size, 42 in wingspan version (enlargeable to 70 size, 56 in span) and a separate Plans Service introduction for a 64 in wingspan model for 108 to 25cc engines.

Modifications to the two seat Extra 300 to the single seat version brought about improvements in modelling terms. The canopy is smaller and the wing is lower set on the fuselage, allowing a one piece wing to be fitted from the underside (with the 300 two seat version plug-in wing panels are a more obvious solution).

Competition aerobatic aircraft can be somewhat 'aggressive' in appearance but the Extra 300 designs have an elegance and proportions which improve the look of the aerobatic manoeuvres they perform. I have taken a little artistic licence in reducing the width of the front end of the fuselage and cowling a little. On the full size aeroplane it is quite wide, to encompass the Lycoming engine, and any modeller wishing to have a more scale representation should take a look at the scale drawings and modify the plans accordingly.

Most of the aerobatic prototypes of the modern generation feature a wing aerofoil with a generously rounded leading edge and then, from about 30% chord rearwards, the upper and lower surfaces are virtually flat, terminating in quite a thick trailing edge. No doubt this gives the pilot the qualities he needs for the punishing schedule of aerobatics he must perform in competitions. The change from the convex curvature of the leading edge to the straight rear section probably allows a rapid breakaway of the airflow, giving positive control. Fine for the pilot sitting in the aeroplane, but would it be acceptable when the scale model was remotely controlled? Stories abounded of sudden stall phenomena when a scale aerofoil section was used and a wing-drop on landing approach without the slightest warning. I had experimented with near-scale wing sections before, on the SU26, without any adverse effect and decided to go ahead with drawing the model with the original aerofoil - or as near to it as I could get with the information at hand.

Two of the larger models were to be built, one by David Toyer who was fitting a Super-Tigre 2500, and Alan Blount, going for an ASP 108 - plus smoke system. Alan decided to try a standard (NACA 0015) aerofoil on his model and this gave the opportunity of comparing the two models for flying characteristics. David flew both of the models and, although it is early days yet, it would appear that there is little difference between the two models in terms of aerobatic and general performance. Importantly, the scale aerofoil version shows no signs of sudden wing drops, it stalls sedately and can be slowed up reasonably on the landing approach.

Both wing profiles are shown on the feature plan (Plans Service) drawing, Alan also included about 3 degrees washout on his foam wing but this is optional and only necessary if you want to tame the performance and increase stability. No NACA wing profile is shown on the FREE plan version but this could be incorporated if desired, the use of non-scale strip ailerons is also an option on the smallest design.

Perhaps the wing construction method is not to your liking (I promise you that it works OK). If you prefer to go for a standard veneered foam wing, or a sheeted, full chord rib style wing that's fine - but don't come running if you have a wing failure. Seriously, if you wish to use the alternative conventional methods of wing construction, using either wing section, please do so. At least the system shown on the drawings and an obechi veneered wing has been proven on the large size models.

Prototype Extra 300S aeroplanes are fitted with four-blade propellers, perhaps we will see this trend coming into modelling before too long, the three bladed propeller is already commonplace. Louisa Knapp should have been flying D-ETXO in the British Aerobatic Championships at Wickenby last year but she unfortunately was unwell and unable to participate. The aircraft was parked in the hangar, but with the canopy cover in place, hence the photographs of the aircraft in that state. The Close Up photos show the interesting twin exhaust/silencer arrangements, something emulated on David's model. Other Extra 300S' have the more standard twin exhaust pipes discharging at the lower rear of the engine cowling. Louisa's mount was also different from the Pat Lloyd drawings in having a more rectangular air inlet at the cowl front for oil cooling.

Looking decidedly slippery on the ground the models really come to life in the air where they positively scythe through the sky. There is no doubt that the smoke system adds to the interest of the model, you will find details of smoke generators elsewhere in this issue.

Although not designed specifically for scale aerobatic competitions I feel sure that, in the right hands, they would acquit themselves well in all but the large scale contests. Essentially these models are for fun aerobatics, designed to sharpen your flying skills with a model that will give you a good build.

Jeff Barringer has written the instructions for building and flying the smaller Extras, the larger version is not that different although it uses a little more in the way of veneered foam. Experienced modellers - and this is not a training or number one design - will have no problems with the large model, it is conventional in all respects except for the wing. You can follow the other instructions for this structure, so over to Jeff.

EXTRA 300S by Jeff Barringer. Introduction: Walter Extra has done it again! Just when the world was taking up his Extra 300 two-seater aerobatic masterpiece to the extent that it has become seemingly, standard equipment in Third World Air Forces, he has introduced the ultimate (with a small 'u') - the Extra 300S. Three hundred horsepower of Lycoming in a ridiculously light fuselage gives rise to the Scale Modellers Dream - a device you can stuff full of engine and still claim, rightly, to be scale.

David Boddington is no sluggard when it comes to new designs either, especially ones with the potential of the 300S, and so I duly asked to build the prototype. Once again, as with his successful 'Ultimate Biplane' design, David has drawn up the plans for two sizes, '40' and '70' size versions.

Now some of you may remember my previous Extra, the Goldberg 300 wrecked in a mid-air at Cranfield a few years ago. I always said that, of all the aircraft I have built, this is the one I would willingly build again - it was just so good! So, to me fell the honour of building the 70 size prototype for David - into which I intended to put my OS.70 Surpass four stroke motor.

Overall design: The Extra 300S is of built up construction, apart from the wing leading edges, which are foam. This comes about partly as an attempt to try to emulate that characteristic wing shape of the full size and partly because David wanted to revisit this form of construction - long used for Combat aircraft. The fuselage is sheeted around conventional formers with balsa sheet turtle decks to front and rear. The empennage is made up from balsa sheet and the mainplane is also fully sheeted.

Preparation: I begin to write this part way through the build, and I seem to have been cutting formers, ribs and foam for my entire life! When I begin assembling I like to have all the pre-cut parts to hand so that I can get into a rhythm hence I cut every conceivable part from the plan beforehand - and woe betide Boddington if he gets it wrong! Having cut all formers and ribs, I set to on the foam leading edges. My thanks here to David Toyer for his assistance. After some effort we ended up with a very nice pair of leading edges which I subsequently skinned with 0.4mm ply using Copydex. And now into the building proper..."

Update 27/2/2024: Added article part 2 (Prototype Parade) thanks to Graham.

Supplementary file notes

Article part 2 (prototype parade).


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Extra 300s (oz7722) by David Boddington 1995 - model pic

  • (oz7722)
    Extra 300s
    by David Boddington
    from Radio Control Scale Aircraft
    June 1995 
    42in span
    Scale IC R/C Civil
    clean :)
    all formers complete :)
    got article :)
  • Submitted: 13/05/2016
    Filesize: 739KB
    Format: • PDFbitmap
    Credit*: JoeFergusson, Poacher
    Downloads: 8465

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User comments

Anyone have the RCSA Jun 1995 magazine article for this beauty. If so can you submit to OZ to add.
George Cooke - 25/02/2024
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