Pipistrel Virus 912, Flight And Maintenance Manual

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Appendix
At high speeds the air circulating the airframe should cause tremendous noise. Wrong!
In fact the noise is caused by drag.
Modern aircrafts like Sinus and Virus
, manufactured of com-
posite materials, have so little drag, 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 familiarise 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 pressuse 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.
TAS
: stands for TrueAirspeed. 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 alti-
tude, 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 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 airspeed which is being in-
dicated. The indicated airspeed value may actually be pretty much lower than speed of air to which
the aircraft is exposed, the TAS.
So is VNE regarded as IAS or TAS? It is in fact regarded as TAS!!! You should be aware of that so
that you will not exceed VNE like the two pilots mentioned above have.