High Speed Flight
As World War II progressed aircraft flew faster and faster. By mid-war P-51s, Spitfires and other types were reaching speeds close to that of sound, especially in dives.
Pilots began to report control difficulties and unexpected problems, which experts determined were due to flying too close to the speed of sound.
In 1940 NACA commissioned the Bell aircraft company to build a special research aircraft for the purpose of exploring the speed range near and beyond the speed of sound. It was considered better to do the research using an actual aircraft because the USA had no wind tunnels capable of operating at supersonic speed.
The Germans mean time, were building the first supersonic wind tunnel and doing research in that way.
The X-1
The research aircraft Bell built for NACA was designated X1. Two operational aircraft were finally built, although they did not fly until the war was over. By then the German research data had been captured and several of the questions the X1 was intended to answer were already known. Nevertheless the X1 became the first aircraft to fly faster than the speed of sound in October, 1947 when Chuck Yeager flew it to mach 1.1
Subsonic, Transonic and Supersonic
We will be using the terms is Subsonic, Transonic and Supersonic to describe different speed flight ranges.
These terms are used frequently and depending on the context can mean slightly different things. For example the average person on the street simply thinks of supersonic as flight at more than the speed of sound. Therefore, subsonic would be less than the speed of sound.
The above will be an acceptable starting definition but we will learn that some of the airflow around the aircraft reaches the speed of sound before the actual TAS of the aircraft equals the speed of sound. Thus, we define the three regions of flight as follows:
- Subsonic - All flow everywhere on the aircraft is less than the speed of sound.
- Transonic - Some flow is subsonic and some is supersonic.
- Supersonic - All flow everywhere on the aircraft is supersonic.
In addition to these speed ranges future aircraft will need to recognize a Hypersonic speed range which begins at approximately Mach 5. In this speed range the air is heated so much by the friction of the aircraft passing through it that the air molecules ionize. This disrupts radio communications and is the main reason space agencies loose contact with space craft for several minutes when they first enter the atmosphere on return from space. We will not be considering hypersonic flight in this course.