Bud,
Not very often I disagree with you, but the stall warning switch does
not work as you describe. Under normal flight the switch senses +VE pressu
re (connecting an ASI up to the stall system) both the A/C normal ASI and s
tall warning see almost the same pressure and read the same value. As the
angle of attack increases the air separation transition point (-VE pressure
moves down the leading edge) and it is this suction that operates the swit
ch as the stall is approached. With the trial ASI still connected, at this
point it reads zero or below with the ASI pointer on the stop. Next test fl
ight will be conducted with a +VE and -VE 5" water gauge connected to the s
tall system to measure the actual +VE and -VE pressures. This work is being
done as part of the Permit work for the UK MG test flight programme as the
LAA are insisting on a stall warning system on the motor glider. On trying
to use the standard EUROPA XS system we are having trouble getting the swi
tch to work consistently. Note: the wing LE profile of the MG is different
to the XS and the stall speed is much lower at 42 KIAS at max 1370 lbs lowe
r with the airbrake just cracked open. On my machine, the stall is very ben
ign with very little airframe buffet.
Regards
Ian Cook
G-CBHI MG
From: owner-europa-list-server@matronics.com [mailto:owner-europa-list-serv
er@matronics.com] On Behalf Of Bud Yerly
Sent: 29 June 2017 03:23
Subject: RE: Europa-List: Re: Stall Warner - switch problems
Kelvin,
The switch is a pressure switch. With the relative wind nearly straight on
the air into the tube is under suction. The switch is assembled to the ho
se to the leading edge using its pressure side. The switch is normally ope
n so with suction on the tube, the light is off at low angles.
At about 14 degrees angle of attack the switch is now under pressure from t
he ram air. The switch is normally open and at about 3 inches of water ish
, the switch will close and allow current to flow to the light/horn.
Once you land, and the speed diminishes, the pressure drops and the switch
opens and the horn goes off.
Not rocket science.
See photo below:
Regards,
Bud Yerly[cid:image001.png@01D2F0C2.29A422B0]
Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Window
s 10
From: Kelvin Weston<mailto:kelv@kdweston.biz>
Sent: Tuesday, June 20, 2017 5:03 PM
Subject: Europa-List: Re: Stall Warner - switch problems
o:kelv@kdweston.biz>>
Hi Bud and all
Now I am really confused. I have fitted the Europa stall warning mod and sw
itch but not yet used in anger.
How is this switch designed to work?
Is it normally closed and held open by pressure above a certain speed.
Or is it normally open and is closed by low pressure (vacuum), like a conve
ntional Cessna stall warner, caused by the angle of the wing.
The mod leaflet suggests it is operated by vacuum, but your description of
it being on until above a certain speed suggests the first option.
The switch has a hose connection to both sides of what I assume is a diaphr
agm, so it is easy to connect incorrectly.
If it is normally closed then does that result in the buzzer operating cont
inually on the ground until above the stall speed?
I think that would eventually get to be a bit of a pain, especially during
long holds on the ground.
--------
Regards
Kelv Weston
Kit 497
kelv@kdweston.biz<mailto:kelv@kdweston.biz>
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