Flytec 6020 * - Manual (Page 47)

Operating Manual Flytec 6020
The physics could be pictured as follows:
In order to create a certain lift a certain number of air particles need to hit the airfoil. Because
of the fact that at 6,500 m altitude there are now only half as many particles present per
meter, the wing surface has to fly faster, but not twice as fast, because each particle has a
higher striking energy and is then only 41% faster.
However, for the calculation of wind, arrival altitude or arrival time at goal, the true air speed
values are always required. The wind wheel sensor shows the true air speed (=TAS),
because it runs practically without friction.
The Flytec 6020 GPS indicates generally the true air speed - TAS.
9.2.2 Stall alarm
If a pilot slows down his glider gradually, he will cause a stall when falling below a certain
speed, which will have different consequences, depending on the type of aircraft.
If the air current is suddenly cut-off from the entire surface, then a completely unforeseeable
crash would be pre-assigned. For this reason, our airfoil manufacturers are building a
so-called cross-setting into the support parts of the wings; this means that during flight the
wing tips will always have a smaller stalling angle than the middle part of the glider. If the air
flow stalls in the middle part of the surface when falling below a minimum speed, then lift still
remains on the wing tips. The aircraft is now in the so-called descending
or stall. Due to the
fact that the wing tips are positioned behind the aircraft’s centre of gravity, the aircraft will
independently drop nose downwards and try to increase the air current by gaining speed.
It is certainly to misadvise to remain a longer time span in this excessive flight situation, as
the wing will react extremely sensitive to even minor air turbulences. A stall can be
dangerous, especially during the landing approach. The consequence could result in a stall
over one surface or a non intended 180-degree curve. The stall alarm is a loud, concise
acoustic signal that requests the pilot to fly faster. It assists first and foremost hang gliders,
however para gliders to a lesser extent. Anyone who has ever watched a hang glider come in
during an approach can observe the following situation:
always when a good headwind prevails, many pilots will pull out their steering bar too early;
the glider then climbs a few more meters to end up in a favourable case afterwards with its
keel stuck in the ground. With no wind or with a light tail wind most pilots wait too long
to pull out the bar. In this case, and with any luck the result will only be a belly landing;
but also a ground-loop with broken base side tubes may be the result, or the glider’s nose
absorbs the impact energy when touching ground and the pilot will swing pendulum wise,
hitting his helmet on the front keel tube.
There are different stall speeds to apply for each glider and varying weight of the pilots.
Several tests are necessary to determine for the own arrangement the setting of correct
stall alarm limit in Main Setup Menu
⇒
User Settings
⇒
Speed
⇒
Stallspeed. Furthermore
it is worth to note that close to the ground, and because of the air cushion under the wings,
the stall occurs at approx. 2 km/h lower speed than in the high air.
However, stall speed also depends on the specific weight of the air, meaning: the flight
altitude. On the 6020-GPS the stall alarm level is automatically raised with increasing flight
altitude, corresponding precisely to Indicated Airspeed. It is unimportant if the pilot has
selected True- or Indicated Airspeed for the speed display screen.
The border limit between stall alarm and the speed for minimal sink is indeed very small.
Hence several pilots have complained about the fact that while circling up in weak thermals
at the speed of minimal sink, the stall alarm is triggered sometimes. In this regard there is
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