The day had arrived. We were going to practice stalls in my flight training. As described in my first “Wings” blog post, airplane wings produce lift because of the proper application of airflow, wing shape and angle of attack. It’s the miracle of flight! However, pilots need to know how to react when wings stop producing lift.
I’m sitting in the left seat of a Cessna 150 two-seat trainer, literally inches from my instructor. The instructions are to clear the area, reduce power to idle, pull back on the yoke until the airplane stalls (i.e. quits flying!) and recover. From ground school instruction I was aware that terrible things can happen if a pilot does not properly recover from a stall. The main nugget of truth buried in my brain was to “push the yoke forward to reduce the angle of attack of the wing and reinstate lift.” Pretty simple, right?
So there I am, engine at idle, pulling back on the yoke, being pushed back into my seat as never before, seeing only blank sky out the window with the stall warning horn blaring its message into my left ear.
I failed to actually stall the aircraft the first two or three attempts (remember, terrible things can happen if an airplane stalls). Finally, my instructor demanded that I keep pulling back on the yoke until he told me to recover. So that’s what I did…until he finally told me we had stalled. I immediately shoved the yoke forward to recover! Both of us were now pointing straight at the ground hanging in our seats by our lap belts. I gradually pulled out of the dive and sheepishly looked over at my instructor, who graciously suggested that next time, ”just release pressure on the yoke”.
In spite of the initial stall practice, I soloed after only 4.7 hours of dual instruction. Unfortunately, I later was to learn from a different instructor that you can actually maintain control of a small aircraft at minimum controllable airspeed, i.e. in a stall. Where was he earlier?!? Who knew?
Airflow over transmission line conductors and structural support members is important to understand. Design principles for power lines obey the same laws of physics as an airplane wing.
To illustrate, I will expand on the galloping phenomena described in the first installment of this blog post. Galloping, if it occurs, typically exhibits single or double-loop amplitudes. In single-loop mode, the conductor looks just like a giant jump rope between two poles. In double-loop mode, the conductor has a node about half-way between the poles, and the conductor looks like two jump ropes. In double-loop, the amplitude of the conductor motion is much less than in single loop. In an electrical circuit, one does NOT want the conductors to touch each other, so the design must ensure this does not happen. Like a stalled airplane, terrible things can happen when the wires touch!
So what is an engineer to do in order to control the airflow and either eliminate or control the galloping? Some things to consider include:
- Determine if galloping is a risk where the line is located (galloping does not happen everywhere)
- If galloping is a risk, try to determine if single- or double-loop will control
- Select design criteria (tension, sag, weather cases) to help manage conductor movement risk
- Would specialized conductors (“T2”) eliminate or minimize the galloping?
- Does the line geometry allow sufficient space for galloping to occur without the galloping loops overlapping and allowing the conductors to touch?
In spite of an engineer’s best efforts, at times a transmission line will exhibit unforeseen movement and require remedial action. Occasionally a utility will install mid-span spacers to maintain a fixed separation between wires. These are essentially insulators that attach to both conductors. These, too, can be effective, but they have nothing to do with airflow.
An alternate remedy is to change the airflow characteristics by adding twist-on spiral dampers that effectively change the shape of the conductor ”wing”. It is an expensive fix, but it works. This works because the airflow over the conductor is disrupted.
To control Aeolian vibration, energy dissipation devices are often installed near the conductor supports at the structures to prevent fatigue failure of conductor strands. This methodology relies on energy dissipation and not airflow manipulation.
Years ago a fiber company installed hundreds of miles of conductor onto existing transmission lines in the Midwest. These wires almost immediately exhibited severe galloping, and often caused damage to the supporting transmission line. The owner initially added airflow spoiler wings to the spans, which very quickly failed, flew into the adjacent cultivated fields and were plowed under. The eventual fix involved unclamping the conductor at adjacent spans and manually twisting the conductors before reconnecting to the poles. This essentially crated a variable cross-section in the wire, thus minimizing the conductor movement. It stalled the conductor wing!
I close by imagining the creative solutions we might see had Bernoulli been a transmission line engineer in the Midwest. I suspect they would have been awesome. If airplanes had been invented, I also suspect he would have been a pilot.