The usual story given is flutter and this seems proven in many case.
But is that always the case?
Putting it another way, if there is no G loading (straight and level flight in smooth air) and there is no flutter, what will be the first to break and why?
It seems to me that this will depend on the design of the airframe, and perhaps even on the state of the individual airframe.
And would all types be able to exceed their Vne in straight and level flight? Mine can, being overpowered for its slender weight, and with a relatively low Vne of 100kts, but I can imagine not all can.
I know of an Isaacs Fury that was built with oversize wires. Apparently it wouldn’t reach Vne in a vertical dive with full power.
I agree with Jan: too general a question to be meaningful
Fair point in that not much can reach Vne in level flight. But almost anything in the SE/ME business can reach Vne in a stable descent.
There won’t be a single answer for all types but the discussion could be interesting. @Pilot_DAR might know quite a bit about it.
In principle you might have trouble with torsional divergence, buzzing of control surfaces, static breaking of structures due to too high loads after fatigue, structural damage, corrosion, gust loads or too high control input, thermal weakening of the structure, miscalculation of loads or structures, faulty and too stiff repairs…
mh – for those among us who have little engineering knowledge perhaps a wider explanation of the terms you use would be helpful. For example who or what is going to be buzzing my control surfaces? If an insect then it surely is of little consequence!!!!
There are many failures which might occur – this is all driven by the aircraft structure. And don´t be thinking the only critical condition is Vne + high G load. When you have full fuel in your tanks the high G loading might be even less critical than the 0 G.. But this is not generally applicable, this is really case by case. Sometimes there is a wing structure limit, sometimes the limit might be at control system, sometimes somewhere else.
bottom line for pilots – fly inside the envelope, Vd (dive speed) is to be demonstrated only during flight test by flight test crew.
Michal wrote:
There are many failures which might occur
Exactly. But I guess the majority of breakups occur due to flutter/aeroelasticity and overstressing during pullout. Fast aircraft (especially jets) experience loss of control or control reversals due to transonic flow over the control surfaces (as countless aircraft have “demonstrated” catastrophically during attempts to break the sound barrier). Some aircraft, especially very light planes, might fold their wings backward even in low-g situations as their wing structure is not really strong in the fore-aft-direction. I think there is such a breakup on YouTube somewhere.
On some aircraft, maybe a glider with washout, the tips will have a negative angle of incidence at a high enough speed. The tips will deflect downwards. Not what the designer intended.
There is also the question of tailplane load at high speed.
Fenland_Flyer wrote:
mh – for those among us who have little engineering knowledge perhaps a wider explanation of the terms you use would be helpful.
I’ll try to outline some basic concepts.
When you have a wing creating lift, the integrated lift vector creates a torsional moment around the elastic axis of the wing. Normally in an aircraft, this axis is located aft of the lift vector, so this moment increases the angle of attack by twisting the wing. This is counteracted with the torsional stiffness of the structure. However, given a certain speed, the forces produced by the air exceeds the load carrying capacity of the wing and it breaks. If you calculate this, the “additional” angle of attack strives for infinity, thus this phenomenon is called torsional divergence (the AOA diverges). Unlike Flutter, Buffeting or Buzz, this is a static aeroelastic problem.
Classic flutter is a coupling of a torsional oscillation and a bending oscillation that occurs out of phase and thus is not dampened. You don’t need any control surfaces for that, just one eddy can set the flutter in motion (read: in real flight always), once the airspeed is given to couple two oscillations. With increasing airspeed the natural bending frequencies increase, while the torsion frequencies decrease If two modes lock at one frequency, flutter can occur. Very stiff structures have high eigenfrequencies to begin with and thus don’t tend to flutter much, or at very high airspeeds (think Van’s aircraft). Long elastic structures are more prone to flutter (think sailplanes).
There is another kind of flutter, if you add one degree of freedom, i.e. a control surface. Usually the centre of gravity of the control surface is not in it’s hinge, so every bending oscillations leads to a change of control surface position, depending of the stiffness of the control system. Cables and pulleys are more elastic (less stiff) than rods. This impacts the lift created by the surface and this can lead to torsional oscillations that might couple with the bending motion of the wing and start control surface flutter. This is the reason, why many aircraft have balance masses at their control surfaces, to minimise the lever arm of the control surfaces centre of gravity and reduce the induced oscillations.
As no hinge is perfect and all have some “play”, this play can be enough to start moving of the control surface. Although this might not lead to a coupled oscillation, it can lead to fatigue or resonance of the structure or part of it. But usually they have a too high frequency for the aircraft structure to resonate.
As the aircraft velocity increases, so does the frequency of the bending oscillation of the wing. Often, when the wing moves up or down, it transits the angle of attack for zero lift. This leads to a series of starting vortices that run off the trailing edge of the wing. These vortices could induce resonance when they hit other aircraft structures, or, as everything that vibrates, lead to fatigue.
The rest should be clear. If you run over V_NE, the wing is able to produce way more load than the structure could carry. Every weakening of the structure by fatigue, manufacturing failures, unknown damage, thermal impact on material properties or previous overloading of the structure, does increase the risk of a classic static break, for instance by exceeding maximum tolerable bending moments.
And, of course, the engineers could have made an error and the structure is not that strong as they think to begin with (one of the main reasons we have and want certification.)
