Before we start the design of aircraft there are couple of things that needs to be clear. For any design to take place there should be a clear defination of requirement. This is called High level requirements. The range of the aircraft, payload, the operation condition, the cruise speed.
Important questions:
A. What are the operational Requirements?
1. Payload- size, type and weight
2. Range- long range or short range
3. Mach Number- design Mach Nr. ie cruise speed
4. Runway requirements- length and texture of runway
5. Mission requirements- longer distance flight or more maneuverability more important
6. Operation cost
B. What are the safety and legal requirements?
1. Start and landing distance
2. Noise restriction
3. Staggering distance during takeoff and landing
4. Performance requirement for engine failure.
There two requirements are with different perspective. the second is only concentrated on the safety of the operation.
Here i would like to specifically focus on the cruise speed. For the configuration design of an aircraft, the most important high level requirement is Speed. Not the speed at which the aircraft takes off or lands but the speed at its cruising altitude. Crusing altitude of long range aircraft is normally above 36 thousand feet to 40 thousand feets.
Before going deeper in the catagories. We need to understand Mach Number.
Mach number is the ration of flow speed of aircraft to the speed of sound at the same temperature, pressure and altitude ( called local speed of sound).
For context lets take some of the fastes moving object built by human.
1. Koenigsegg Agera RS: top speed of 447km/hr.->0,36 Mach Nr
2. LO series trains in Japan: 603km/ hr.->0,5 Mach Nr
Mach 1= 1234.8km/hr.
If we hear a Mach number 0.3 then it could be interpreted as the 30% of the speed of sound at that particular temperature pressure and altitude. Thus speeds can be divided into different categories like:
1. Subsonic flights: the flights which are at a speed below 0.8 Mach Nr are called subsonic flights.
Most of the commercial turbofan aircrafts lie in this category.
Characteristics:
1. High aspect ratio. 6 to 10.
2. rounded nose and leading edge of wing.
2. Transonic Flights. the flights which are in the transition from subsonic to Supersonic so called transsonic speed of flight. The are speeds between 0.8 to 1.2 Mach Nr.
Characteristics:
1. Swept wing design to delay drag divergence
2. Supercritical Airfoil to delay wave drag.
3. adhering to the principle of Whitcomb Area rule
3. Supersonic Flight. the flight which is more than 1.2 and less than 5 Mach Nr is called supersonic flight
Characterisctics
1. Sharp edges
2. thin airfoil-sections
3. all-moving tailplane/canards
4. Hypersonic flight. the flight which has speed of more than 5 and less than 10 Mach number.
Characteristics:
1. Cooled nickel or titanium skin;
2. the design is highly integrated, instead of assembled from separate independently-designed components
5. High hypersonic flight flights between 10 and 25 Mach Nr.
Characteristics:
1. Hypersonic designs are often forced into blunt configurations because of the aerodynamic heating rising with a reduced radius of curvature.
2. Thermal control becomes a dominant design consideration.
3. Structure must either be designed to operate hot, or be protected by special silicate tiles or similar.
6. Re-entry speeds. flight speed above 25 Mach Nr.
characteristics
1. Ablative heat shield; small or no wings; blunt shape.
Comments
Post a Comment