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But birds do it …

8 February 2010

At times the obvious eludes me.

Recently I met pilot Fred Schlafly, a director of the Wings Over Miami Air Museum, and he showed me his airplane— a Yakovlev Yak-52. He spoke about how his was imported in crates, I think two, and he assembled the airplane with the help of friends. I’ve spoken before about the dynamic nature of the people associated with this museum in the post Wings Over Miami Air Museum — a gem hidden within the Kendall-Tamiami Executive Airport. Their expertise is simply a wonder to see.

He also mentioned that he flies with two other pilots who fly similar aircraft, the Nanchang CJ-6. I quipped that the CJ-6 was a Chinese version of his Yak. Politely and expertly he told me that I was mistaken and why.

They differ by a subtle but significant difference and it gives the Yak-52 has more aerobatic ability than the CJ-6!

Why?

Basic physics, and birds have known how to use this advantage for eons.

Recall that aircraft wings have a dihedral angle, they angle upward from the fuselage, and this angle produces a dihedral effect when the airplane rolls. Dihedral effects are incorporated in aircraft designs to provide more stability since the effect resists the rolling of the wings. Simply, as an airplane rolls the lower wing generates more lift and the upper wing loses lift — this tends to return the wings to level. The lower wing generates more lift since the rolling of the wings causes the aircraft to fly a bit to that side, also called sideslipping. Now that the lower wing has more lift, it tries to reverse the rolling direction. Voilá — a restoring force — stability. The greater the angle, the greater the initial roll rate but also the greater the restoring force with a bit of time — but a lower dihedral angle has the wing perform more efficiently in level flight. So a balance is struck in each aircraft design.

Looking at the Yak-52 wings one can see a classic dihedral angle.

Fred Schlafly's Yakovlev Yak-52 (note the dihedral angle) — photo by Joe May

Not the CJ-6! It has a polyhedral angle and it generates stability using the same effect. These wings are level with respect to fuselage at first, but then have an upwards angle once away from the fuselage a certain distance. The angle must be greater to generate the same amount of force from a dihedral  angle, though. I’m not sure of the engineering design choice but it may be in order to simplify the manufacturing process or to increase the robustness. In the case of the Nanchang CJ-6, the wing is horizontal until just outside of the main gear stations. The inner portion of the wing, fuselage and structure to support the main gear are in the same plane.

Looking at the CJ-6 wings one can see an archetypical polyhedral with wings beginning to angle upward about 6 feet (2m) from the fuselage.

Nanchang CJ-6 (note the polyhedral angle) — photo by Joe May

So, the CJ-6 has a simpler manufacturing characteristic but the Yak-52 is quicker in a roll?

How about the birds? Vultures spend their time soaring in search of food, but don’t need to react to opportunities quickly, so they minimize soaring efforts with a low dihedral angle. The vulture gets the most lift from its wings during level flight this way but at the expense of roll rate. Vultures do not need to turn quickly so this is not a disadvantage. The hawks do need to turn quickly. Using keen eye sight from on high they most often need to turn rapidly and dive to catch unsuspecting prey. Prey that will not stay in one spot for a predictable amount of time. Hawks have a higher dihedral angle than vultures, trading a loss of soaring efficiency for higher roll rates (quicker turning). This aids in the identification of birds when they are soaring high over head. Vultures soar calmly with wings straight out. Hawks soar as if a bit too caffeinated — constantly tipping to one side then the other side — with the wings forming a noticeable “V”.

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