**4. Flight envelope and operational limits of aircraft**

For any aircraft, performance and operational limits are defined. Performance limits (**Figure 16**) are mostly defined by the aerodynamic configuration of aircraft. On the other hand, operational limits are based on the type of aircraft, structural and engine limits, wind resistance parameters, and maximum Mach number. All these limits are presented in diagrams of altitude *h* versus airspeed *v* and the final diagram, which represents the intersection of all limits, is called flight envelope (**Figure 17**). The flight envelope can be represented in graphs of altitude versus true airspeed, altitude versus indicated airspeed, or altitude versus Mach number.

All the speeds presented on **Figure 16** were described in the previous subsection. Operational limits related to the type of aircraft are described from the point of view of aircraft application: if an aircraft is a passenger jet, it should apply to the requirements of comfort for passengers and not exceed load factors of comfortable flight; on the other hand, if an aircraft is a jet fighter its operational limits from the point of view of load factors should be derived from compromise between the prevention of health issues of pilot that may occur and maneuverability for the combat use.

Structural limits are mostly related to aircraft strength, while the engine limits can be the result of its design and performance at higher altitudes. Wind resistance limits can be derived from the requirements of operational use or comfortability of flight for passengers.

**Figure 16.** *Performance limits.*

Aerohydrodynamic Institute (TsAGI), and on his initiative was created Zhukovsky Air Force Engineering Academy—alma mater of three famous Soviet aircraft

Science lives in the research schools, and great scientists are those who both do great discoveries and develop the next generations of discoverers. Zhukovsky Air Force Engineering Academy is my alma mater as well, and with these few words I would like to express my appreciation to all my teachers who helped me on my way

designers Sergei Ilyushin, Artem Mikoyan, and Alexander Yakovlev.

of professional and personal development.

*DOI: http://dx.doi.org/10.5772/intechopen.92105*

*Flight Vehicle Performance*

**Author details**

Aram Baghiyan

**169**

Improvis Aerospace and Defense LLC, Yerevan, Armenia

provided the original work is properly cited.

\*Address all correspondence to: abaghiyan@improvismail.com

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Maximum Mach number can be defined from the conditions of aeroelastic effects and vibrations, effects of shifting aerodynamic center, causing severe pitching moments, which can lead to the crashes, or loosing effectiveness of aerodynamic surfaces. For example, for jet aircraft L-39 the critical Mach number is *M* ¼ 0*:*8 and exceeding this condition leads to the shift of aerodynamic center of L-39, which generates pitching moment, causing a descending flight with acceleration [3]. To prevent such unstable flight, on L-39 air brakes are used, which automatically act at the Mach numbers *M* ¼ 0*:*78. The critical Mach number is defined at airspeed flight that leads to generation of shock waves on the wing due to the acceleration of airflow on the upper surface of wing.

Finally, an example of flight envelope for analysis is presented in **Figure 17** where the intersection of all limiting conditions is described.
