Hi Bob,
> A question for anyone .
>
> Does the Europa stall warner wing inlet have any use in Angle of
Attack instrumentation?
>
I don't think so. The pressure signal provided is designed to be very
alinear: a small positive pressure (reference static) at most angles of
attack, then a large negative pressure (reference static) near the stall
angle. The purpose is to operate a switch.
An AoA instrument using the signal would probably become just a stall
warner too.
I actually put a stall warning probe in the other wing with the
intention of using it for an angle of attack measurement some time in
the future. The orifice is at 4% of chord or so on the top surface.
I had forgotten what the plans were exactly but I think I have
documented now what my thoughts may have been. See the attachment.
Jan de Jong
AoA - calculated as percentage of maximum lift coefficient
determine also:
- current airspeed
- stall speed at current mass and acceleration
- reserve airspeed at current mass and acceleration
additionally (after extra calibration inputs):
- current lift as % of MTOW
and after averaging:
- mass as % of MTOW
- manoeuvring speed
uses a microcontroller
measure
q': differential pressure pitot and static
p: differential pressure AoA-orifice and static
correction
the dynamic pressure at stall is an important ingredient
it is probably necessary to correct q' for pitot position and angle to get a
good
value
correction depends on AoA more than q' - may involve iteration
assume succesfully done
q: dynamic pressure
airspeed
q = k1 x (v-squared)
k1 is a known constant
v is calculated and indicated as airspeed
percentage of maximum lift coefficient
lift = k2 x l x q
l is lift coefficient
k2 is an unknown constant that depends on airplane/wings configuration and size
calling L the maximum lift coefficient and Q the dynamic pressure at stall
speed:
lift = k2 x L x Q
it follows that for any particular lift:
100 x l / L = 100 x Q / q, percentage of maximum lift coefficient
(lift coefficient and dynamic pressure are inversely proportional)
a function of lift coefficient
with all things equal except airspeed and AoA we posit:
p = f(l) x q
where f(l) is a function of l only
the underlying assumption is:
- over the airspeed range of interest
- around the location of the AoA-orifice
the shape of the airflow (and the shape of the pressure distribution) does not
depend on airspeed
two-way function, locating an AoA-orifice
the location of the AoA-orifice is selected so that f(l) is a strictly monotonic
function of l over the whole range of positive AoA
there must be a 1:1 relationship between f(l) and l, that gives good resolution
in both directions over the whole range
---From NACA graphs it appears that an orifice in the wing top at a few % of
chord
should work
(there are other probes and locations that may provide a good, strictly
monotonic,
signal)
calibrating for a flight configuration (flaps position)
1. the airplane is repeatedly gently stalled
register 2 numbers: f(l) = p / Q and 100; a maximum (or minimum) for f(l) is
sought
2. select a higher airspeed, with the same lift (1 G)
register 2 numbers: f(l) = p / q and 100 x Q / q
3. repeat for a number of airspeeds up to vne
extra: register the current Q and put in and register:
the current % of MTOW,
the manoeuvring speed at MTOW according to the manual
calculating lift coefficient percentage for a flight configuration (flaps
position)
determine f(l) as p / q, find percentage P (after interpolation) from the
calibration
table
also:
determine current Q from: q / Q = 100 x P
determine current stall speed V from: Q = k1 x (V-squared)
determine current reserve airspeed R from: R = v - V
extra:
find registered Q, % MTOW and manoeuvring speed,
compare current Q to registered Q,
calculate current % MTOW and manoeuvring speed
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