Bill Wightman
Well Known Member
Hi all,
In recent months, there have been a few discussions of how Vne (Never Exceed Speed) is defined and determined. Unfortunately, there have been several posts and one article published in the RVator that have muddied the waters in helping to correctly understand what factors lie behind determining Vne for an airplane.
To put it simply, the issue of defining Vne for an airplane IS NOT a one-variable problem. The RVator article accurately points out one facet: that of aerodynamic flutter, and how it's sensitive to True Airspeed, not Indicated Airspeed. This is well known by aero engineers. Unfortunately, we've now found that its possible to fly well into a possible flutter situation while still under what we think are published airspeed limits for our airplanes. This would NEVER work for a certified aircraft... so we have a special situation here.
What I want to highlight is that in conventional GA aircraft, Vne is almost exclusively driven by aerodymamic limits. But the Vn diagram in the RVator article clearly shows the region over Vne labeled as "structural failure". To date, I've been under the assumption that our airplanes were found to be flutter-free for operation within the Vn envelope. For higher performance aircraft, a Mach limit sometimes can come into effect to address compressible flow around the airframe. But its important to note, that for ALL certified aircraft with no special limitations, you can safely operate the airplane within the prescribed INDICATED AIRSPEED limits and AT ANY ALTITUDE within the altitude limits for the aircraft. Just to be clear, all operating speeds as described in the FAR's are "Equivalent" airspeeds. Equivalent speed is simply indicated airspeed corrected for compressibility, and installation/calibration errors.
FAR 23.335 defines this:
A certified airplane must be shown to be free of flutter within its entire flight envelope at any airspeed (less than redline) and any altitude (under absolute ceiling).
FAR 23.629 talks about this:
We've now found out this isn't true for our RV's, which I think was an unintended limitation on the design. Keep in mind, you don't necessarily need a turbocharged engine, or lots of installed HP to go too far into the Safe TAS window: all a pilot needs to do is point the nose down hill from high altitude, and we all know the airspeed needle will wind up pretty quickly. This was reported on in the same RVator article.
About airplane design, and Vne:
The airplane's structure must react all primary forces acting on it. The structure is designed to handle only so much force in any direction, with a safety margin applied over that. Loads applied to the structure create stress levels that must be controlled to remain under safe levels for the materials used.
The point is: structure in our aircraft, and all certified aircraft, is designed around STRESS LIMITS (sometimes deflection limits). Exceed the ultimate stress (loads) and you fail the structure.
Stresses in an airplane are generated by AERODYNAMIC FORCE acting on the aircraft. Aerodynamic force is directly tied to something we call "dynamic pressure" , or denoted by the letter "Q" in engineering equations. Dynamic pressure is a function of the density of the air you're flying in, and the speed of that airflow. Its essentially "how hard" (not "how fast") the air is pushing against the airplane. The equation looks like this:
Q = 1/2 x (rho) x (V squared) -or- (one half times air density times speed squared)
The airspeed indicator we read in the cockpit is actually a dynamic pressure indicator. It reacts to the speed and density of the air we fly in, and thus it's important to understand the FORCES acting on our airframes are coming from the Q (dynamic pressure) shown to us as INDICATED AIRSPEED.
So, airframe stress limits represent one area of the Vn diagram limiting what the airplane can do safely.
OK, so how do we distill all of this into something every RV'er can use to fly safe?
--> You must know and understand that we fly airplanes that have known limitations in two important areas: Structural load limits, AND flutter limits.
To obey both of these two limits, we must always stay UNDER "Vne", or the red-line on the airspeed indicator (IAS), AND we must obey the (now published) TAS limit that was accurately pointed out in the RVator.
BOTH APPLY to our airplanes!
Sorry for the rant, but this is a "hot topic" for me...
In recent months, there have been a few discussions of how Vne (Never Exceed Speed) is defined and determined. Unfortunately, there have been several posts and one article published in the RVator that have muddied the waters in helping to correctly understand what factors lie behind determining Vne for an airplane.
To put it simply, the issue of defining Vne for an airplane IS NOT a one-variable problem. The RVator article accurately points out one facet: that of aerodynamic flutter, and how it's sensitive to True Airspeed, not Indicated Airspeed. This is well known by aero engineers. Unfortunately, we've now found that its possible to fly well into a possible flutter situation while still under what we think are published airspeed limits for our airplanes. This would NEVER work for a certified aircraft... so we have a special situation here.
What I want to highlight is that in conventional GA aircraft, Vne is almost exclusively driven by aerodymamic limits. But the Vn diagram in the RVator article clearly shows the region over Vne labeled as "structural failure". To date, I've been under the assumption that our airplanes were found to be flutter-free for operation within the Vn envelope. For higher performance aircraft, a Mach limit sometimes can come into effect to address compressible flow around the airframe. But its important to note, that for ALL certified aircraft with no special limitations, you can safely operate the airplane within the prescribed INDICATED AIRSPEED limits and AT ANY ALTITUDE within the altitude limits for the aircraft. Just to be clear, all operating speeds as described in the FAR's are "Equivalent" airspeeds. Equivalent speed is simply indicated airspeed corrected for compressibility, and installation/calibration errors.
FAR 23.335 defines this:
23.335 Design airspeeds.
Except as provided in paragraph (a)(4) of this section, the selected design airspeeds are equivalent airspeeds
A certified airplane must be shown to be free of flutter within its entire flight envelope at any airspeed (less than redline) and any altitude (under absolute ceiling).
FAR 23.629 talks about this:
(a) It must be shown by the methods of paragraph (b) and either paragraph (c) or (d) of this section, that the airplane is free from flutter, control reversal, and divergence for any condition of operation within the limit V-n envelope and at all speeds up to the speed specified for the selected method. ... (snip, emphasis added)
We've now found out this isn't true for our RV's, which I think was an unintended limitation on the design. Keep in mind, you don't necessarily need a turbocharged engine, or lots of installed HP to go too far into the Safe TAS window: all a pilot needs to do is point the nose down hill from high altitude, and we all know the airspeed needle will wind up pretty quickly. This was reported on in the same RVator article.
About airplane design, and Vne:
The airplane's structure must react all primary forces acting on it. The structure is designed to handle only so much force in any direction, with a safety margin applied over that. Loads applied to the structure create stress levels that must be controlled to remain under safe levels for the materials used.
The point is: structure in our aircraft, and all certified aircraft, is designed around STRESS LIMITS (sometimes deflection limits). Exceed the ultimate stress (loads) and you fail the structure.
Stresses in an airplane are generated by AERODYNAMIC FORCE acting on the aircraft. Aerodynamic force is directly tied to something we call "dynamic pressure" , or denoted by the letter "Q" in engineering equations. Dynamic pressure is a function of the density of the air you're flying in, and the speed of that airflow. Its essentially "how hard" (not "how fast") the air is pushing against the airplane. The equation looks like this:
Q = 1/2 x (rho) x (V squared) -or- (one half times air density times speed squared)
The airspeed indicator we read in the cockpit is actually a dynamic pressure indicator. It reacts to the speed and density of the air we fly in, and thus it's important to understand the FORCES acting on our airframes are coming from the Q (dynamic pressure) shown to us as INDICATED AIRSPEED.
So, airframe stress limits represent one area of the Vn diagram limiting what the airplane can do safely.
OK, so how do we distill all of this into something every RV'er can use to fly safe?
--> You must know and understand that we fly airplanes that have known limitations in two important areas: Structural load limits, AND flutter limits.
To obey both of these two limits, we must always stay UNDER "Vne", or the red-line on the airspeed indicator (IAS), AND we must obey the (now published) TAS limit that was accurately pointed out in the RVator.
BOTH APPLY to our airplanes!
Sorry for the rant, but this is a "hot topic" for me...
Last edited: