Wren (oz8217)

About this Plan

Wren. Radio control sport biplane. For .15-.19 power.

Quote: "Sometimes streamlining is hard to love. An old bipe stirs the memories. This one builds easily, for horsing around. Wren, by Norman Rosenstock .

I remember riding my bicycle for two hours as a youngster to get to Floyd Bennett Field to watch and be near the airplanes. This was around 1938, before the site became a military field. The impressions which I carried awry with me still remain, so when I decided to build a biplane, its design was influenced by my early memories.

The Wren is a mental composite of all the old biplanes I saw as a boy. I could have made a dandy biplane in the contem-porary style with symmetrical airfoils and the like; such a ship would be capable of performing the whole AMA Pattern with ease, but this was not what I wanted. Like that ol' time religion, I wanted a biplane that not only looked ol' time, but flew like it.

The aerobatic capabilities of the old bi-planes during the 30's were limited. The 'modern' airfoil of the time was the Clark Y. It performed much better than the earlier airfoils left over from World War I, however, even the Clark Y had its limitations. Inverted flight required a high angle of attack to generate lift, for instance. Another disadvantage was its great center of pressure travel which created very abrupt stalls. The resulting spin was the terror of the times. The advantages of the Clark Y airfoil were that it gave a higher top speed than the older under-cambered airfoils, but still gave a low landing speed. The Wren uses a modified Clark Y airfoil which functions well for the model as long as certain conditions are met.

The rigging of all biplanes is very important; incidence and decalage should be carefully established before trying to fly the model. The upper and lower wing must be at 0° incidence, using the flat bottom of the airfoil as a reference. The stabilizer. The third as a reference. The stabilizer needs 2° positive incidence; specifically the hinge line spar is 1/8 in lower than the leading edge of the stabilizer. The third requirement is that the center of gravity should be between 25 to 30% of the mean aerodynamic chord. (The former is preferable.) Using measurements, this means that the CG should be between 2-1/2 in and 2-5/8 in from the leading edge of the top wing.

When these three conditions have been met, the plane is a dream to fly. It is quick and nimble. The ship becomes airborne after a takeoff run of about 15 to 20 feet. Rolls and inside loops are excellent; however, outside loops are not in its bag of tricks. Inverted flight is a joy. You simply do a half roll and shove the stick forward, and it flies with the tail hanging low, just like in the old days. I have found that pushing the stick all the way forward gives me very little climb and a higher angle of attack. Perhaps with a larger engine and more down, outside loops could be achieved, but such changes spoil the performance of the airplane and turn it into a bomb.

The Wren is a fun plane to fly. I don't suggest a larger engine as it would spoil the characteristics of the airplane. It would probably fly even more realistically with a .15 engine. The building of this model is not suggested for beginners and a certain amount of experience is assumed.

Construction. Start with the wing. As the top wing has a span of only 38 in, a large board is not necessary. I used an old drawing board which is very flat. After cutting out the required ribs and four wing tips start assembling the top wing first. It is easy to construct, as there is no sheeting and no dihedral. After the wing is completely assembled and dry it is ready to be formed and sanded to shape at its leading edge and wing tips.

Next, the lower wing is assembled without the ailerons. Each half is constructed separately and the space between the over and under 3/16 spars in the last bay is filled with some scrap 3/16 balsa. After this the trailing edge block is glued in place. When both halves are dry, remove them from the plan, then sand each panel at the joint with a large sanding block so that when joined with the other half, it will give the desired angle of dihedral. With one panel flat on the board the other wing tip is raised..."

Attached is Norman Rosenstock's Wren from Flying Models magazine issue 12-76. Regards, 'theshadow'.

Direct submission to Outerzone.

Supplementary file notes

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