Ask Captain Lim

Hi Captain Lim,

What is the speed of sound at 10900 and 12000 meters? Is there any chance in a jet steam that the plane could get near to this speed or do the computers kick in and slow the engines down?

I take it that you need delta wings to break the speed barrier then after the speed barrier has been broken, the plane could revert back to normal shaped wings?

Regards

Tim J

Hi Tim,

The speed of sound at 10900 and 12000 meters is 582 knots (670 mph) and 573 knots (660 mph) respectively. A jet stream of 150 to 180 knots (from the tail) can easily blow a plane way above the speed of sound in relation to the ground. For instance, if a Boeing 777 cruising at 450 knots is being blown by a jet stream of 180 knots, the resultant ground speed is 630 knots (450 + 180). This is 48 knots (630 - 582) greater than the speed of sound at 12000 meters.

Many people are confused and say that the plane is now supersonic, fearing that it may cause structural damage to the aircraft as it is not designed to fly that fast! No, as far as the plane is concerned, it is still cruising at 450 knots in relation to the relative airflow. The computer will not kick in to slow down the plane. When you are walking on a walkalator, would you slow down? No, you would not because you are taking advantage of the faster walking speed!

The Concorde has delta wings for supersonic flight. I believe you are referring to "sound barrier" instead of "speed barrier" where supersonic flight is concerned. Yes, "breaking the sound barrier" is the process of accelerating through Mach 1 and going from subsonic to supersonic speeds.

As the plane crosses this barrier, there is a large increase in drag that would seem to indicate an infinite amount of thrust be required to fly at the speed of sound. In order to maintain the speed of sound, the delta shape wings must continue to be maintained. Look at the F-111. The wings are retracted to normal shaped wings only when their speeds are back to subsonic speed.