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Martin XB-51 for twin Rapier L2 power



At long last I am pleased to finally present the plan for my XB-51, should any of you fancy having a go at it. As a design it has a lot going for it, as the two motors are close to the centreline, and more or less on the centre of gravity. It is also a relatively simple build, with the fuselage being a basic box with formers added top and bottom.



I have made one significant change to the design of the prototype, which is that the wings are now drawn attached to the 3/32" fuselage sheeting. On the original I added a centre section piece to the wings which passed through the fuselage. This left an ugly gap round the wing, and in a crash the outer wing panel just broke away from the centre section anyway, so it might as well have just been attached to the fuselage side. Instead of a centre section I have added cross braces inside the fuselage to help transmit shock loads through the fuselage.

The original weighed 64 grams with 2 L2 motors on board and came out slightly nose heavy. It was trimmed for the required slightly steep glide by bending the hinged elevators up a touch.



The ideal power has proved to be a pair of L2LT motors rated at 80mN. I have flown it on more powerful motors (100 - 105 mN) but it gets a bit exciting, and you can end up with a spiral descent at the end of the flight. Or even the start of the flight. You really have to get this model flying as straight as possible for as long as possible, so I advise fitting a small wing tip trim tab for fine tuning.

It is possible to hand launch the model by holding the engine nacelles, but I strongly recommend using a catapult, which gives more repeatable launches. A simple hook is shown on the plan, and all you need for a catapult is a loop of 1/8" or 3/16" rubber tied to the top of a stick.

The model will also fly on 80 or 85 mN rated L1 motors - in which case you need to put a temporary spacer in each motor tube to get the back of the motor in the same place as the L2 would have been.

Although this is a big model, I think it needs enlarging a bit to fly reliably with standard L2 motors in the 110 mN power range - maybe 20 inch span? Whatever you do, make every effort to keep the airframe light.

Click here to see "proof of flight" video taken at Old Warden Scale weekend July 2005 (3.8 MB)

Click here to see a video of another good flight made during the SAM 35 Gala at Old Warden a couple of weeks later (4.5 MB)

Thanks to Graham Potter for taking the video footage.

If you have any specific questions about building or flying the model, just drop me an e-mail .

You can read more about the model here



To read and print the pdf file you need Adobe Acrobat Reader, which is a free download from www.adobe.com

The plan should print off full size if you set your printer settings to A4 paper, even if it has got different sized paper in the printer. If in doubt, check against the small T shapes on the plan sheets which are spaced 50 mm apart.

Click here to download the plan as a pdf file (301 KB)

Click here to download the plan as zipped bitmap files (273 KB)



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