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Air Plane, Ground Beef

Science brain teasers require understanding of the physical or biological world and the laws that govern it.

 

Puzzle ID:#16848
Fun:*** (2.45)
Difficulty:*** (2.75)
Category:Science
Submitted By:airhead3***
Corrected By:cnmne

 

 

 



Pete the Pilot was trading flying stories with Aaron the Arrogant, when Aaron started in on a particularly juicy tale: "Well, I got that story beat. There I was, rolling down the runway for takeoff 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 don't know how I didn't graze that cow's back! He he, 'graze,' no pun intended", Aaron deadpanned, elbowing Pete jokingly. Sensing he was losing his audience, though, Aaron quickly continued, "But that's not all! To top it off, not 5 seconds after takeoff, my left engine quit on me!" To this, Pete raised an eyebrow in interest. Aaron 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 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", Aaron finished with great gusto. Pete, with an amused look on his face, said, "Well, that's a great story, Aaron, but because you are sitting here telling it, I know it didn't actually happen." How does Pete know?



Answer

Pete knows that a cow on the runway isn't out of the question for a small airport, but it's the second half that he has a beef with (sorry, I couldn't resist the pun). Pete 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, Aaron'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, Aaron's left turn into the bad engine would have made the circumstances even worse. Had Aaron 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 Pete the Pilot is glad his boastful friend is sitting there to tell him this story, Pete knows Aaron is full of hot air.
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