Tuesday, August 19, 2014

Allegro Lite Sailplane - Finished and Flying

At least ten years ago I heard of free plans for a 2-meter radio control sailplane that was about the ultimate in performance for a glider of this size at the time. Dr. Mark Drela famous for so many innovations in model aviation and Professor of Fluid Dynamics in the Department of Aeronautics and Astronautics at MIT designed this glider. It is interesting to note that the human powered airplane Daedalus that his team built set the human-powered flight record of 72.44 miles in just under 4 hours in 1988 has the same rather unconventional rudder and stabilizer layout as the Allegro Lite glider. Dr. Drela also set a record for the fastest human powered water vehicle, has shared record breaking free flight indoor glider model design, and developed subsonic airfoil program known as XFOIL.

Finished Allegro Lite RC Sailplane

Allegro in Flight

With the all the plans and instructions downloaded from the Charles Rivers website I started the construction of the Allegro Lite on a glass surface using bags of bird shot for weight as recommended. Building the wing was an overwhelming task for me, it is a combination of balsa and composite materials. Spar construction consists of end grain balsa sandwiched between carbon fiber strips. This is wrapped with Kevlar tow (loose fibers) and then painted with a slow curing epoxy. The design objective for this sailplane was for it to be able to withstand any flying load including full-pedal winch launches. A friend of mine did manage to break the tailboom on a particularly brutal launch with his Allegro Lite.

Wing Spar and Tip

Another View of Spar After Sheeting 

Obtaining the special components needed for this project was somewhat easier in that I obtained a partial kit that had those composite components and laser-cut ribs. Building the wing was such a task that at some point I felt like if I finished this plane and it crashed it I would be devastated. It sat for years at about 90% complete and then I started slowly working on it again last winter. Just within the last couple of weeks have I started flying the glider. It flies very well but somewhat faster than I had anticipated. Still nervous flying my pride and joy I have only made thermal flights of no more than 5 minutes by making big circles over the field. 

Hatch Held With Magnets

If more nose weight could be crammed in the tiny nose pod it would be good to try because the CG is slightly behind the recommended spot and it seems to be close to tip-stalling occasionally. I used powerful tiny magnets to hold on the nose hatch this has worked very well.

What a feeling of accomplishment I have from building this model airplane.

Bill Kuhl

Additional Resources

Charles River RC Article and Plans for Allegro Lite

Dr. Mark Drela MIT

XFOIL Airfoil Program

Daedalus Human-powered Record Setting Airplane

Download Plans for Rubber Power Free Flight Corsair Designed by Mark Drela

Human Powered Speed Record Set on Water - Decavitator

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