Extra 3.25 (oz7306)
About this Plan
Extra 3.25. Radio control sport scale model. For .19 to .28 power. Wing area 340 sq in.
Quote: "It seems that, from every category of airplane, there emerge one or two that stand out and endure the passing of time. In the warbird world, it's likely to be the P-51 Mustang; classic 'rag' fans would probably choose Piper's ubiquitous J-3 Cub; and the F-16 would get the nod from the under-25 crowd.
But what about in the upside-down world of high-performance aerobatic airplanes? No, I'm not talking about the Decathlon, although it is a great starting point and confidence builder. What I mean is the neck-twisting, gut-wrenching, eyeball-popping, aerial hot-rods with vertical speeds of 50 feet per second and roll rates 60 times faster than the second hand on your watch! Proponents of the biplane school stand strongly behind the Pitts and the Ultimate, while the monoplane mavens cheer the Lasers, Sukhois and, the subject of this article, the Extra 300.
To most R/C modelers, the Extra Series of aerobatic airplanes should need no introduction. It has been around for a number of years in a variety of forms from full kits to ARFs, giant scale to .40 size. With the design presented here, I set out to provide performance similar to that available in the larger models, but in a .25-powered package - hence, the name Extra 3.25.
Because some sport acrobatic airplanes tend to get a little 'squirrelly' as their dimensions shrink, I opted to use a partially flat-bottom airfoil section rather than one that was fully sym-metrical. This tames the flying qualities down to the ability level of the average sport flier, but it doesn't detract from the plane's appearance with regard to the full-size Extra.
The absence of dihedral beyond that created by the taper of the wing itself makes the model neutrally stable with little or no trainer-like, self-righting characteristics. This allows it to be quite acrobatic even with its lifting airfoil. All that means is that I don't recommend the Extra 3.25 to beginners. The tendency to want to build it because it's "cute" or just the right size to fit in your little hatchback should be tempered with the con-sideration that it's not a trainer, and you should feel comfortable flying a 4-channel, aileron-equipped model before tackling the Extra.
Building this model is not difficult and can be easily accomplished by anyone with a few kits and a scratch-building project or two under his belt. (This is a great time to plug my 'Scratch-build-ing R/C Airplanes' book, which is available from Air Age Publishing; see the ad elsewhere in this issue.) Building materials are all conven-tional and available through your local hobby shop or favorite mail-order supplier: there's not a lot of material required, so it's not going to bust your budget!
Rather than forcing you to make your way through a narrative from which you'd need to pick out the construction methods, I decided to present the sequence in the way most successful kits are presented: step by step. These instructions, used with the notes on the plan and the construction photos, should answer most of your questions and make building your version of the little Extra an enjoyable and rewarding experience.
To simplify some of the project and to cut down on your carving and sanding time, I have available a set of vacuum-formed plastic parts for the Extra. The package includes a cowl, wheel pants and a transparent canopy..."
Direct submission to Outerzone.
Supplementary file notes
Article pages, text and pics, thanks to MarkWinstanley
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ScaleType: This (oz7306) is a scale plan. Where possible we link scale plans to Wikipedia, using a text string called ScaleType.
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User commentsHi, it's Uwe again. I found a website full of pictures from the Extra 3.25: cobrajet.rchomepage.com/Extra_3_25
Uwe - 22/12/2015
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