AIRCRAFT OPERATING INSTRUCTIONS – VIRUS 912 S-LSA GLIDER
Page
36
You might say: “Why did they not keep their speed within safe limits? How could they be so thoughtless
to afford themselves exceeding the V
NE
?” Speaking with the two pilots they both confessed they went
over the line inadvertently. “Everything just happened so suddenly!” was what they both said. Therefore
it is of vital importance to be familiar to all factors that might influence your flying to the point of
accidentally exceeding the V
NE
.
Here is the relationship between the human factor and performance:
The human body is not intended
to be travelling at 250 km/h (135 kts), nor is it built to fly. Therefore, in flight, the human body and its
signals should not be trusted. To determine the speed at which you are travelling, one normally relies
upon two senses – the hearing and sight. The faster the objects around are passing by, the faster one is
travelling. True enough. The louder the noise caused by air rushing past the airframe, the faster one
must be cruising. True again. But let us confine ourselves to the scenarios associated with both of these
events. At higher altitudes, human eye loses its ability to determine the speed of movement precisely.
Because of that pilots, who are flying high up feel like they are flying very slowly. Additionally, it seems
that at high speeds the air rushing past the airframe ought to cause a tremendous rushing noise. But this
is wrong! In fact, rushing air noise is caused by drag. Modern aircraft like Sinus and Virus, manufactured
of composite materials, have so little drag in cruise attitude, that they actually sound quieter than you
would expect. Especially if you are used to wearing a headset when flying you must not rely on your ear
as the instrument for determining speed. REMEMBER! When flying high, the only reliable tool to
determine airspeed is the cockpit instrument - the airspeed indicator!
How to read and understand what the airspeed indicator tells you?
Let us first familiarize with the terms used below:
IAS: stands for Indicated Airspeed. This is the speed the airspeed indicator reads.
CAS: stands for Calibrated Airspeed. This is IAS corrected by the factor of aircraft’s attitude. No pitot tube
(device to measure pressure used to indicate airspeed) is positioned exactly parallel to the air flow;
therefore the input speed – IAS – must be corrected to obtain proper airspeed readings. With Sinus and
Virus, IAS to CAS correction factors range from 1.00 to 1.04 (not a big deal). Now for the critical variable-
TAS: stands for True Airspeed. TAS is often regarded as the speed of air to which the aircraft’s air-frame
is exposed. To obtain TAS you must have CAS as the input value and correct it by pressure altitude,
temperature and air density variations. The maximum structural speed is linked to IAS. But light planes,
manufactured of carbon reinforced plastics, with long, slick wings are more prone to flutter at high
speeds than to structural failure. So flutter, a function of TAS, is the main factor of determining VNE for
us, and most other carbon-reinforced-plastic aircraft producers. Flutter speed is linked to TAS, as it is
directly caused by small differences in speed of air circulating the airframe. Hence air density is not a
factor. For all who still doubt this, here are two quotes from distinguished sources on flutter being
related to TAS: “Suffice to say that flutter relates to true airspeed (TAS) rather than equivalent air-speed
(EAS), so aircraft that are operated at or beyond their VNE at altitude - where TAS increases for a given
EAS – are more susceptible to flutter...” New Zealand CAA’ Vector Magazine (full passage at page 5 of
http://www.caa.govt.nz/fulltext/vector/vec01-4.pdf
). “The critical flutter speed depends on TAS, air
density, and critical Mach number. The air density factor is almost canceled out by the TAS factor; and
most of us won’t fly fast enough for Mach number to be a factor. So TAS is what a pilot must be aware
of!” Bob Cook, Flight Safety International. The airspeed indicator shows you the IAS, but this is sadly
NOT the speed of air to which the aircraft’s airframe is exposed. IAS and TAS are almost the same at sea
level but can greatly differ as the altitude increases. So flying at high altitudes, where the air is thinner,
results in misinterpreting indicated airspeed. The indicated airspeed value may actually be much lower
than speed of air to which the aircraft is exposed, the TAS. So is VNE related to IAS or TAS? Although the
redline on our altimeter may imply that it is associated with IAS, in reality, for all gliders which are
