What determines the maximum speeds for a particular aircraft?

Question

Aircraft have three different maximum airspeeds:

1. V_MO (maximum operating speed) - the maximum speed that the aircraft is allowed to reach during normal operations.
2. V_NE (never exceed speed) - the speed that should never be exceeded during operation, even in an emergency, as flying at speeds higher than this could damage the aircraft.
3. V_D (diving speed) - the fastest they got it to fly during testing.

What part(s) and/or characteristic(s) of the aircraft determine the values of these three maximum speeds? Is it that the plane's engines are incapable of pushing it through the air any faster? Would the wings snap off from excessive aerodynamic loads if it flew faster? Is it something else entirely?

Answer 1

A quick list:

• Maximum speed with flaps at maximum deflection (v$_{FE}$)
• Maximum speed with flaps at take-off setting. Fly faster and the flaps might be torn off.
• Maximum speed with gear down (v$_{LE}$). Here not the gear itself, but the open gear doors will fail if that speed is exceeded.
• Maximum speed for full control deflections (maneuvering speed, v$_A$). Yank the stick to full deflection at a higher speed and risk overstressing the structure.
• Maximum speed in gusty weather (v$_B$). Here it is assumed that the aircraft happens to fly into a vertical gust of 50 (FAR part 23) or even 65 (FAR part 25) ft/s, which will greatly increase the lift force on the wings. Above that speed such a gust will overstress the wings.
• Maximum operating speed (v$_{MO}$)
• Design cruising speed (v$_C$)
• Maximum dive speed (v$_D$). This is the speed reached when at v$_C$ the aircraft enters a shallow dive and the pilot needs 20 seconds to react.
• Maximum operating Mach number (M$_{MO}$)
• Design cruise Mach number (M$_C$)
• Maximum dive Mach number (M$_D$). At least 0.05 Mach above v$_C$.

And that is only what is listed in the certification regulations. You can also distinguish between a maximum speed at which the landing gear is operated (v$_{LO}$) and the aircraft is flown with the extended gear, since large doors open during operation but are closed again when the gear is extended. And yes, for aircraft with retractable landing lights, there is even a dedicated maximum speed at which the landing lights are operated (v$_{LLO}$). As usual, Wikipedia even lists a few more.

Aircraft have several limits, depending on altitude. At low level, the dynamic pressure limits how fast they can go, mostly because of structural loads. At high speed, a deflected aileron will twist the wing, reducing its effectiveness. Better limit the speed to a point where the aileron is still effective.

Higher up, it is the Mach number which might limit your speed. If the aircraft is not designed for transsonic flight, Mach tuck and buffeting will set a clear limit, and the maximum Mach speeds are defined to prevent the airplane from venturing into that territory.

Heat loads might pose another limit. The F-16 is limited by the heat loads on its polycarbonate canopy and must not go faster than 810 KIAS at low altitude.

And, of course, there is always flutter. However, for certification it must be demonstrated that no flutter will occur within the allowed speed range +20%.

Answer 2

There are various physical things that will limit the top speed of an aircraft. Which one you will hit first depends on the airframe in question but here are some things you may encounter (aircraft dependent):

• Drag will generally counter your thrust which will limit the speed you can achieve in level flight and is often the practical limiting factor on aircraft that actually fly. Level flight is important to note as you can gain a huge amount of speed if you simply enter a dive.
• For propellor driven aircraft there are some practical speed limits because of the propellors.
• At some point flutter will become a problem and you may literally shake the wings off the plane.
• Around the speed of sound (mach 1) you are going to have all sorts of interesting things going on if you are pushing a subsonic optimized airframe to such speeds.
• Ultimately, if you still have your engines and wings heat becomes the real problem. The Concorde pumped fuel through the nose to use a coolant in the mach flight regime and was in part speed limited by nose temperature. For manned flight this becomes the limiting factor at the extreme end of the envelope and aircraft like the SR-71 had a very physically demanding environment.

Answer 3

The Main Reason:

Structural integrity of the wing. The lift equation is listed below:

You can see that the airspeed is the most influential factor for lift since the term is squared. Also remember that lift is a force so too much lift can rip the wing off of the airplane. An aircraft wing is only rated to take so much force before the structural integrity fails.