Let's suppose that in Cruise flight I exceed the VNE in TAS or in ground speed, why isn't the structure of the aircraft affected by the high speeds?
I am sorry for my english, I am not a native speaker.
3
-
2$\begingroup$ When the speed in the air is within limits, how do you imagine that the speed over the ground could cause problems with the structure of the aircraft? $\endgroup$– Ralph J ♦Mar 3 at 1:17
-
3$\begingroup$ Suggestion: pick one or the other to ask about, TAS or ground speed. They are two totally different things. Or post two different questions. $\endgroup$– quiet flyerMar 3 at 1:27
-
1$\begingroup$ See "dynamic pressure". They talk about that with rockets too. Ground speed does not matter because the plane speed is relative to the airmass, but True Airspeed, relative to Mach number, can have stress effects. Read on. $\endgroup$– Robert DiGiovanniMar 3 at 15:16
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
2
All aerodynamic forces scale with the dynamic pressure ($q=0.5\,\rho\,V^2$). This is true for lift and drag -- hence the use of lift coefficient ($C_L=L/(q\,S)$. This is also true for pressure coefficient ($C_p=(P-P\infty)/q$).
Here, $V$ is the true airspeed.
While we're at it, the equivalent airspeed is essentially a measure of dynamic pressure.
When you are at altitude, the density ($\rho$) reduces, so to get to the same dynamic pressure, you go much faster. For the same airplane - at the same angle of attack - to get the same forces, you have to go faster.
So, structural loads for an airplane are all in terms of equivalent or indicated airspeed. A V-n diagram used to express the load envelope of an aircraft is generated in terms of equivalent airspeed.
A fighter jet - or a rocket launch, they will talk about $q_{max}$ -- the maximum dynamic pressure the structure can take.
As far as groundspeed -- the only time the aircraft structure cares about ground speed is when you are in contact with it. That happens during takeoff, landing, and crashes.
-
-
$\begingroup$ @Frog perhaps so, but the role of ground speed in them is related, but different. $\endgroup$ Mar 3 at 5:44
$\begingroup$
$\endgroup$
0
why isn't the structure of the aircraft affected by the high speeds?
As also explained in this answer, Velocity Never Exceed (Vne) is defined as 90% of the Velocity Dive (Vd) which is the highest speed after which structural damages do occur. The 90% is given as a safety margin and it is the speed at which the "red range" begins. So theoretically at speeds between Vne and Vd no structural damages should happen.
Let's suppose that in Cruise flight I exceed the VNE in TAS or in ground speed
Please note that all those speeds are given as Indicated Air Speed (IAS) and not in True Air Speed (TAS) nor in ground speed. This answer gives a good overview about TAS and IAS.
