Flying the Plane
Tuesday, 30 December 2014 17:03
Hi Captain Lim
I have just finished reading a book about flying that I bought on a trip to Taiwan. From the book I know that an aircraft is flying at around 3 degrees when cruising in the air. I also know that the table on the passenger's seat is also designed to be at 3 degrees.
I have a question. Is the aircraft flying upwards at 3 degrees or downwards at 3 degrees? And if it flies upwards at 3 degrees, will it climb to a higher level? Flying from an opposite direction, would it descend at 3 degrees towards the ground?
Thank you for your answer.
What you have read is partly correct.
In aerodynamics, the 3 degrees you mentioned in known as the angle of attack (or alpha angle). It is in fact the angle of the wing chord to the relative air flow. I don’t want to be too technical. Take relative air flow as the flow of air parallel to the earth. In other words, when a plane is cruising at a constant height, say at 35,000 feet, it is flying at that constant height but its angle of attack is still at 3 degrees. It does not climb. The 3 degrees provides the lift to maintain that constant height.
As a rule of thumb, passenger planes are designed to cruise with a two or three degree nose-up angle at cruise flight. Engineers have a good idea of the angle of attack required to produce the required lift at cruise. As such, they may design table on passengers’ seat to cater for the inclination.
I believe the Airbus A330 is designed with a slight nose-down angle when parked on the tarmac so as to give it a more level cabin attitude in flight.
In some planes, the angle of attack or pitch angle may change slightly in flight depending on the load. For example, a Boeing 777 may take off with 100 ton of fuel during the takeoff but burns off 90 tons during landing at the destination. As such, the weight change is quite big.
A 2 degrees of pitch up would cause the front end of a Boeing 777 (about 209 feet or 60m) long to be about 7 feet (2m) higher than the back during the cruise.
A plane does not fly 3 degrees down during the cruise. But is flies a 3 degrees gradient during an approach in an auto land. In aerodynamic, a 3 degrees gradient and a 3 degrees angle of attack is different. Please watch the video above and below on what angle of attack is all about.
To answer your question again, a plane flying at 3 degrees (angle of attack) will not climb but maintain a constant altitude. It does not fly 3 degrees (gradient) down unless coming in for a landing.
How Airplanes Fly