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| Air Plane Episode Ascertainment (Posted on 2022-10-06) |
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Pierre was trading flying stories with Alistair when Alistair started in on a particularly juicy tale: "Well, there I was rolling down the runway for take off in my Piper Twin Comanche. Suddenly, as I was approaching takeoff speed, a cow just ambled onto the runway right in front of me! I pulled back hard on the controls and got into the air, and I swear I didn't know how I didn't graze that cow's back! heh heh, "graze", no pun intended," Alistair deadpanned, elbowing Pierre jokingly.
Sensing he was losing his audience, though, Alistair quickly continued, "But that's not all! To top it off, not 5 seconds after takeoff, my left engine quit on me!" To this Pierre raised an eyebrow in interest. Alistair gestured self-assuredly as he went on, "Thankfully, I was able to use my skill and lightning reflexes to save the situation. I immediately went to full power on my right engine and pulled up the nose up to keep the plane in the air and all I had to do was make a quick turn to land on the perpendicular runway to my left," Alistair finished with great gusto.
Pierre, with an amused look on his face said, "Well, that's a great story, Alistair, but because you are sitting here, telling it, I know it didn't actually happen."
How did Pierre know?
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Submitted by K Sengupta
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Rating: 5.0000 (1 votes)
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Solution:
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(Hide)
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Pierre knows about an aerodynamic phenomenon called Vmc Roll in multi-engine airplanes, where the airplane can, at the loss of an engine in certain conditions, lose control and roll over. An engine failure on a twin-engine propeller-driven airplane (such as the Twin Comanche) creates a tendency to turn into the dead engine due to both asymmetrical thrust and the drag from the windmilling propeller on the dead engine. (Think rowing a boat only from the right side, and in addition dragging the left oar through the water.) Further, the propellers normally create what is called induced acceleration over the wing, creating additional lift. (The propellers act as fans, creating faster moving air over the wings, and consequently, more lift.) When only one engine is working, that wing is then generating more lift than the wing that is no longer receiving induced acceleration, so the plane will want to roll into the dead engine. Vmc, or "Velocity minimum controllable" is an airspeed below which the rudder no longer has enough effectiveness to counter these turning/rolling tendencies caused by an engine failure, and the plane can then enter a Vmc Roll. Because of his already slow speed caused by cow-evasion maneuver, Alistair's situation was ripe for a Vmc Roll. His reaction of immediately increasing the power on the good engine would have only exacerbated the situation, due to the increased induced acceleration over the right wing creating even greater differential lift (stronger rolling force). Also, Alistair's left turn into the bad engine would have made the circumstances even worse. Had Alistair reacted the way he stated, he would have flown the airplane right into the ground (and most likely inverted). Although it may be counterintuitive, the correct action would have been to *reduce* power in the right engine and point the nose slightly downward to increase the airspeed above Vmc to give the rudder enough effectiveness to counter the aerodynamic tendencies. While Pierre the Pilot is glad his boastful friend is sitting there to tell him this story, Pete knows Alistair is full of hot air.
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