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Tuesday, September 17, 2013

Turning the Corner - Part 3 of "First to What?"

Although initially dispirited after the 1901 gliding season was ended, Wilbur soon recovered his natural optimism.  After only 2 weeks, his sister Katharine commented "We don't hear anything but flying machine and engine from morning till night....."  In September of the next year (1902), Wilbur made a presentation to the Western Society of Engineers, in which he recapped the Wright's experiments to date, and his comments were published in their transcripts, which were highly sought after by fellow experimenters.

Wilbur and Orville had become suspicious of Lilienthal's list tables, which they now thought may be in error, and also wondered if Smeaton's cofficient, used to calculate lift and drag for an airfoil, was also wrong.  At the time, Smeaton's coefficent (k) was thought to be 0.005, but it had also been experimentally measured as being anywhere between 0.0027 and 0.0054, albeit with poor instruments.  The US Weather Bureau had started to use k=0.004 instead.

The first test rig

To test their ideas the Wright's placed a small wheel horizontally on the front of one of their bicycles that balanced the drag from a plate of known size against the lift of an airfoil, but they found speed control difficult.  However, their early results seems to confirm their doubts in both Lilienthal and Smeaton.  So Orville, the most practical of the two brothers, made a small 18" long wind tunnel with a fan to generate airflow and vanes to ensure the flow was turbulence free.  They also created a very innovative way to measure lift and drag, again balancing the unknown force against known drag from flat plates also exposed to the same airflow.  In this way, they could make precise measurements of force, without having to measure force directly with springs and complicated mechanism.

The Wright's larger wind tunnel

In October 1902, Wilbur wrote to Octave Chanute, saying that "Lilienthal's table is very seriously in error".  Armed with this data, and using the results from their 1901 glider, the Wrights had calculated Smeaton's coefficient to be 0.0033, which is within a few percentage points of the value accepted today.  Using the new value for k and a new larger wind tunnel they built in October 1902, the Wright's tested a variety of airfoil shapes.  Although the wind tunnels are lost, the balance mechanism and drag plates are on display at the Smithsonian museum.  With the correct value for k in hand, Wilbur recalculated the proper coefficient of lift (CL) for their airfoil designs, and determined which wing shape was optimal for the speeds they flew.

Now the Wrights could design an airplane and know the critical functions of lift, drag, and glide angle (or power needed) in advance.  They had become the first true aeronautical engineers in the sense that their future designs were engineered for a specific level of performance, not just built in hope they would get off the ground if they were lucky.  They were also fully controllable designs with moveable airfoils, not just weight shifting.

Now the Wrights built a new glider, the 1902 glider - the first engineered aircraft.  They shipped it South to Kill Devil Hills in the September of that year, where Octave Chanute visited and brought, really against the Wright's desires, a few of his designs for "comparative testing".  The Wrights by this time were so far advanced that they already knew those designs were flawed and why they wouldn't fly, but not wishing to hurt the old man they accepted his gliders, but spent as little time as they could with them.

The 1902 glider was the largest aircraft built to that time.  The wings were longer and narrower (high aspect ratio), which they now knew reduced parasitic drag.  The airfoil had a camber of 22:1, and wing area of 305 feet.  It flew marvelously.  The Wrights started flying it a kite, and immediately noticed that it's lift was enough to make the wires almost vertical, rather than at the substantial angle of the 1901 glider.  The wing flew at a substantially lower angle of attack, and generated much less drag, with a glide ratio about 10:1.

The 1901 and 1902 gliders flying as kites (see the difference?)

Wilbur make the first flights, making glides of over 500 ft, and finally Orville took the controls of an airplane for the first time.  In the first 3 days they had flown further than their previous two trips combined.  One control problem remained, the strange "Well Digging" behavior when in a banked turn, named because the low wing would leave a crater in the sand as it dropped and pivoted.

Today we know this as "adverse yaw", a phenomenon caused by the increasing lift on the rising wing, which since drag is proportional to lift, also increases the induced drag for that wing.  As a result, the nose is pulled in the opposite direction to the turn, something we overcome with applying rudder in the direct of a "coordinated turn", one of the first things a student pilot learns (especially in a glider).  At the low speeds the Wrights flew, just above the stall, pulling the rising wing backwards would yaw the glider into a side slip, dropping the low wingtip into the ground.

Wilbur flying the first true airplane - with dual fixed vertical rudders (later replaced with a single moveable one)

Orville invented the next innovation, a movable vertical stabilizer/rudder.  Wilbur, the more experienced pilot, recognized that with their limited controls of the time, a third control would be too much, and proposed tying the moveable rudder to the wing warping controls, inventing the coupled turn used on many modern aircraft, such as the Beechcraft Bonanza.  Around the same time, the Wrights re-rigged the glider to have drooping wingtips, inventing dihedral wings (although most wings use positive dihedral today, some still use negative dihedral or "anhedral", as did the Wrights).

With control over all three axis, the Wrights never again experienced well digging, and the airplane was substantially invented in 1902.  That year, the Wrights made nearly 1,000 flights, with the longest being 622 ft and lasting 22 seconds.  Enthusiastically, and in great contrast to the previous year, Orville wrote home "We now hold all records!"