Why do I need a stall warner
What is the principal of operation?
What is so unique about the
ACI stall sensor?
What is the correct sensor
position on the leading edge?
What is the correct sensor
position along the wing?
What is the sensor mounting
position for the RV-6?
What is the sensor mounting
position for the Europa?
Can the sensor be installed on a tube and
fabric wing?
Can the sensor be installed inside the wing
with vane projecting through the leading edge?
How loud is the alarm?
Can the interconnect cable be cut and joined?
How detailed are the installation
instructions?
If I use a 9 volt battery
as power source how long will it last?
1.
Why do I need a stall warner?
Stall/spin accidents are one of the most common causes of light aircraft
fatal accidents. A stall warning can provide a timely reminder if speed
control on approach is lacking, and contributes towards a safe controlled
arrival.
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2. What is the
principal of operation?
The sensor unit is mounted on the leading edge of the wing, and the alarm
is actuated by upward movement of the sensor vane. The vane is deflected
upwards when the angle of attack (aoa) reaches a point where the airflow
division at the leading edge changes dramatically in favour of the upper
wing surface. This begins to occur well before the stall. As the unit
senses angle of attack, it gives proper warning in both level flight and
steep turns. The point at which the alarm operates is established experimentally
for each individual aircraft - by adjusting the sensor's vertical position
on the leading edge aerofoil section. The operating concept was developed
in the 1940's in the USA, and has been used on most production light aircraft
ever since.
Note: Alarm operating airspeed rather than
angle of attack (aoa) is referred to in these notes for convenience, and
refers to level flight stall speed only.
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3. What is so unique
about the ACI stall sensor?
Firstly, it is surface mounted, and the sensor is thus easily adjusted
to provide the desired operating speed on any Experimental or Permit aircraft.
No wing leading edge cut out is required, just a 3.5 mm diameter entry
hole for the cable behind and underneath the sensor. Secondly, the switching
system is hermetically sealed and magnetically operated - there is no
micro switch. Thus the required aerodynamic operating force is low. .
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4. What is the
correct sensor position on the leading edge?
The vertical position on the leading edge decides the alarm operating
speed. The advantage of the ACI sensor is that surface mounting allows
simple repositioning between flight tests - duct tape is used to temporarily
secure it. The aim is to get the sensor to activate the alarm several
knots above the stall, and it is repositioned up or down to achieve this.
Two or three flights may be required after which the mounting plate is
mechanically fastened. Normal alarm setting is about 7 knots above the
level flight stall, possibly more for low inertia, high drag aircraft
or less experienced pilots. The sensor is very small and light, weighing
20 grams (2/3 oz.) and is normally fitted with no. 6 self tapping screws,
but can also be bonded on with silicone rubber caulk.
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5. What is the
correct sensor position along the wing?
The lateral position must be at least I metre (3 ft) outboard of the propeller
arc to keep it clear of propeller wash, and at least 600mm (2 ft) from
the wing tip to avoid tip vortex effects.
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6. What is the
sensor mounting position for the RV-6?
The nominal position for the RV-6 is for the top edge of the sensor mounting
flange to be 520mm (20.5 ins.) forward of the leading edge wing skin line
(ie above the main spar). Must be measured with flexible tape/rule. Most
builders fit well outboard, at the inboard end of the last wing panel,
allowing internal access to wiring by wing tip removal if required.
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7. What is the
sensor mounting position for the Europa?
We can provide a sensor position drawing showing the nominal position.
Initial trials are carried out with the sensor duck taped on as described
in 4. above. The lateral position should NOT be further inboard than the
outboard end of the flap. The 3.5 mm dia cable can be taped directly aft
under the wing (with minimum 25mm wide high performance tape) to the aileron/flap
channel. It can then run inboard to the cockpit alarm. On the XS wing
the wire is easily fed into the wing behind the main spar. As an alternative
to self tap screws, the sensor unit can be bonded to the leading edge
using silicone rubber caulk. The bond area on the mounting plate is large
and the mounting plate is easily bent to conform with the L.E. profile.
It is still possible to remove the sensor unit for service/repair etc.
With the stall warning unit fitted, PFA Engineering may not require the
inboard stall strips, thus significantly improving take off performance.
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8. Can the sensor
be installed on a tube and fabric wing?
On some Kitfox, Avid, and Rans aircraft, the leading edge tube is highly
stressed at some locations and the presence of screw fixing holes is a
concern. The manufacturer's engineering dept will normally approve a sensor
installation well outboard on the wing - say 3 feet inboard of the last
rib where bending moments are low. The sensor has operated very adequately
at this position on several Rans S6 aircraft. Alternatively, and bearing
in mind the sensor weighs only 20 grams (2/3ounce), the sensor can be
bonded on with silicone rubber caulk which is flexible and kind to the
fabric. Initial testing to establish the correct vertical position must
be carried out with the sensor duck taped on as explained above.
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9. Can the sensor
be installed inside the wing with vane projecting through the leading
edge?
The difficulty is to establish where to cut the hole in the wing.
A solution is to test fly it with the sensor unit duck taped outside to
establish correct position (see 4. above), then mount inside, allowing
for some further minor adjustment.
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10. How loud is
the alarm?
It can be heard through standard headsets in any normal enclosed cockpit
aircraft. If the sound output is considered too loud, the sounder diaphragm
can be taped over to reduce output. However the sound output is not adequate
for an open cockpit environment.
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11. Can the interconnect
cable be cut and joined?
There is no problem in cutting the interconnect cable. However keep the
connector within the aircraft structure, strapped up or fixed clear of
any moisture trap, and protected against oxidation. A simple miniature
2 position screw terminal block (preferably with wire protectors under
the screw) well treated with silicone grease or vaseline is the most reliable
method.
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12. How detailed
are the installation instructions?
The installation instructions are comprehensive, for both sensor and alarm
box installation, and wiring.
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13. If I use
a 9 volt battery as power source how long will it last?
Current is only taken while the alarm is sounding, and life for an alkaline
9V battery of Duracell type will normally be at least 2 years. However
your preflight check must include lifting the vane to ensure correct alarm
function.
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