In Van's RvAtor mag there is an eye-opening
article on Vne (never exceed airspeed). We all (most?) have it marked on
our Indicated Airspeed Gauges with the Red Line along with our stall speeds,
flap extension limits, turbulent air limits, etc.
Well, it appears that all of the other
airspeed limits mentioned are a function of the "Q" factor (aerodynamic
pressure) as is our Indicated Airspeed. As the relative Q changes
with changing airspeed/altitude those limits stay in sync with the
indicator. So most of us (I know I did) assumed the Vne on the
indicated airspeed is also a function of the "Q" factor. Apparently Vne
is NOT, it is a factor of True Airspeed!!
Normally this does not mean much, BUT, if you are
turbocharging your aircraft and intend to fly at altitudes greater than 10,000
ft (especially with a turbocharged engine which can give you sea level power)
it can be very important. The reason is that Indicated airspeed
continues to decrease (for a constant true airspeed) as altitude
increases(air density decreases). However, in most cases our True
airspeed increases somewhat with altitude. Van
apparently figures that with a normally aspired engine your power
decrease would limit your true airspeed potential to below Vne (at those
higher altitudes) - but, if you have the power (turbocharged) to give
you a higher airspeed then there could be a potential problem.
The bottom line is that at the higher altitudes
your indicated airspeed gauge can be showing you safely below the Red Vne
mark but, your True Airspeed may be exceeding Vne by a considerably margin
putting your airframe in a potentially danger zone of flutter.
Just though I would pass it on. The full
article is in the latest Rvator and was a response to folks asking about
turbocharging the engine for the RV-10 to make it a high speed long range
cruiser.
The rest of you may have been aware of this
difference, but I know I was not.
FWIW
Ed