ANGLE of ATTACK INDICATOR
(similar to a Lift Reserve Indicator or
LRI)
2011/03/25 Peter Cowan
Installation in aircraft, where they should not be a primary
control device, is at your own risk.
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This simple, inexpensive
instrument quickly becomes your instrument of choice when maneuvering at slow
speeds. I’ve heard from several other pilots who confirm this (see some
comments from users at the bottom of the page and a comment from my first
user).
Why use this kind
of an instrument?
The reason that the angle of
attack indicator is more useful than airspeed is because a stall can occur at different
indicated airspeeds depending on the angle of bank and weight so that if you
want to maneuver at the edge, the airspeed indicator is not the best
indication. For example, at a bank angle of 75 degrees, the stall speed
has doubled. On the other hand, "they" say that no matter what the
conditions, stall always occurs at the same angle of attack of the wing to the
air stream. This device gives an indication of the angle with a needle moving
into the red bar when at the stall angle. Overall, the green, yellow, red
segments on the gauge make for an easy to read instrument.
The gauge measures the
difference in pressure at two points on a probe mounted outboard of the prop
slipstream (or on a pusher, anywhere in smooth air at the nose). My S7 shown below is approaching the stall
angle.
Above, the gauge is mounted
on the surface of the panel over a 2 1/8” hole (not inserted). This location
gets the gauge mounted nice and high and easy to latch onto when you look at
the panel.
If there is room, a good place
to mount the gauge is on top of the glare shield on top of the panel. This area
is close to your line of sight:
Below is a similar bracket
made for a Rans S6 (and the probe bracket is for an S6 jury strut):
The gauge body, at 2 5/8”, is an odd size. This one will mount in a 3 ¼
instrument hole using the optional adapter plate:
This probe is mounted at the end of the lift strut on
this S7
I used a length of 3/16 steel
rod pushed out to the inspection hole from the cabin to tape the tubes to for
pulling through the wing into the cockpit. The top hole is for the high
pressure line.
The tubes follow the
pitot/static lines out to the strut where the probe is mounted. An alternate routing is down the inside of
the strut and into the cabin near the bottom of the strut.
On the production version,
the shaft is a slightly different shape which puts the probe a little further
ahead and the spacer is not used.
If you fly a pusher, the
probe could be placed at the front of the fuselage. One guy put his at the
front of a wheel pant. It is not necessary to go right out to the end of the
lift strut either. On a SuperCub, a logical place is at the top of a jury
strut. In this case the shaft need not be bent as it is above.
Here is a jury strut mount on
the Rans S7. It is the same as the
previous bracket but
with a 2nd
plate made out of thinner stock so that it just clamps around the jury
strut. Further down in the “Feedback”
section is a picture of another jury strut mount using a plastic block made by
a customer.
The probe must be in unobstructed free air away from
the prop blast and at least 6” away from the wing.
Here are a couple of instruments showing the two faces
available.
Setup: also see:
Installation
Instructions
Start with the centre line of
the probe tilted about 35 degrees nose down to the center line of the aircraft.
Take a flight and stall the plane. As the nose goes up and speed drops the
needle should come off the right hand stop and move towards the red area. Note
the position of the needle when the stall occurs. You want the needle to be
near its zero speed, at rest position in the red arc at the stall (on most gauges
this is not right at the left stop). If the needle gets to the left side before
the stall then the nose of the probe is too high so rotate it down before the
next flight.
Conversely, if the needle is
still in the yellow at stall, then the nose of the probe is too low so raise it
some before the next flight. When you
have it set where you want it, tighten the mounting bolt and put a mark on the
mounting bracket so you can see that it hasn’t been moved later. Now, when you set up on final approach, you can
experiment with where in the yellow arc you feel comfortable.
The
Ready-To-Install kit includes the following (see below for unfinished parts):
Instrument with either black or white face (and white needle or
black needle),
Mounting
hardware for flush or surface mount but not diagonal brace mount or adapter
plate for 3 ¼ hole,
Adapter tubes to go from hose barbs on instrument to 3/16
lines,
Up
to 28 feet of 3/16 tubing
(the length required for through the wing routing on an S7
where
strut attaches 8’ out from the fuselage, 22’ for jury strut mount,
Probe (aluminum) with all drilling complete,
Probe support shaft in 6061 aluminum drilled and optionally
bent for Rans S6,7 strut
Mount,
Hardware to attach probe to shaft.
Price: $ 175 Cndn. Or US Plus $15 shipping (in
Ask
about price reduction on orders for more than one unit.
Tandem option:
If you have a tandem seating aircraft and fly from the back
seat, it is very useful to put a second instrument
up high
where you can see it without the front seat passenger shifting sideways so you
can see the panel.
Since
the AOA is pretty much all you need on approach it makes things a lot easier
and safer.
Here
is mine mounted in an S7. Even with a
passenger or student in the front seat you have an unobstructed view and it is
in your line of sight.
One
probe can drive two instruments so this option gives you a second instrument
with modified barbs to enable you to easily splice this gauge into the existing
tubes.
It
also includes the mounting plate shown in the picture. Specify flat mounting surface or bent to
conform to tubular spar carry thru as in the Rans S7.
Price is $ 110. plus $15 shipping if
not ordered with the primary gauge.
Price for primary instrument plus
Tandem Seating option: $260 + $15
shipping
Unfinished parts
kit option:
Unmodified gauge (you remove bezel and face plate)
Black or white face (uncut, unpunched, existing face
plate untrimmed),
Probe (not drilled or finished),
Straight bracket only
Up to 28’ of tubing
2 adapter tubes
Price $100.
Mounting plate to put gauge in a 3 ¼ instrument hole $ 20
Mounting bracket for top of glare shield/panel $ 30
Jury strut mounting bracket $ 20
Click for instructions on
finishing required
For the unfinished kit
If you want one, I’ll need to know:
Length of tubing if not going on a Rans S7 or S6
White or black face
Bent, straight, or Jury strut bracket
Options
Your address
How you want to pay: (
Reply to: Peter Cowan PayPal account id is peterc@pipcom.com
)
Telephone:
705 8778404 Eastern time
Mailing address is 996/ RR 2 Lakefield
K0L2H0
If you are not going to order
one, I’m curious to know why not because many folks feel this is very good
value for a helpful instrument. Please send me a note: Peter
Cowan
Back to Rans
topics page.
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First feedback (2009/03/31) from a purchaser (Danny Hayman,
Challenger):
“Very pleased but thanks for the heads up. I am now to a point that airspeed is no longer my primary gauge, instead it's the AOA.............”
Feedback from Mike Arnold C-150:
Well I got it hooked up and after several trips up to set the stall point I took this into the shortest strip I have ever gone into. It is a 1000ft long paved private strip at about 7000ft AGL. At 90 deg OAT,. it worked just as advertised. So I would give you good feedback.
This one is on an X-Air:
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Peter,
Some more feedback for
you at last.
Photos attached of installation on an Xair "H" (Hawk over here), bit temporary at the
time but tidied up now.
Mounted the head on the jury strut with a nylon block,
gauge is not really line of sight but there were two handy bolts to mount it
without cutting the panel
I intend to move it down onto the top of the dash
panel soon. Did have a problem with a sticky needle but sorted that.
Works just like it says on the tin, quite
interesting to see just how much excess speed you can carry on approach if
you use conventional methods.
Takes a little practice to have confidence in the
AoA, but then it makes for much easier low speed handling.
Many Thanks,
Nick
Here is an opinion from one user of
a similar instrument:
I finished an
RV-8 two & a half yrs ago and have flown it over 425 hrs all over our great
country. During flight testing (after very little adjustment for calibration) the
LRI gauge indicated the imminent and actual stalls at the same points on the
gauge, regardless of loading, acceleration/deceleration or attitude. In
fairness, I will admit I have never purposely stalled the RV-8 while inverted,
but, due to the orientation of the probe, would not expect it to indicate
properly (all my stall testing was done while in positive-g flight).
Since completing
the stall testing, I can sincerely say that I've flown every landing pattern,
particularly at unfamiliar airports, referring almost exclusively to the LRI
gauge when my eyes are in the cockpit. My only complaint with the instrument
has nothing to do with its value or effectiveness: my complaint is that the
small gauge was not available when I installed mine, and mine is larger than I
need. It certainly is easy to install/calibrate and you incur very little
weight penalty for having it on board.
Having flown 2
late-model Couriers, I find the wing to be exceptionally forgiving or docile or
whatever adjective you prefer, and the handling is most impressive at the
low-speed range. However, when I'm close to gross, and it's high and hot (I
live in Albuquerque), I'll appreciate being able to make that base-to-final
turn with more evidence than my fingertips and butt cheeks telling me that I
have plenty of "reserve lift" to keep me, my plane and passenger from becoming a statistic.
One man's humble
opinion and personal experience.
And another who flys a Rans S7:
I have one, I use it a lot, and I really like having it.
Here is how I look at it:
Maybe some of the benefit
or the reason I am comfortable with it is the simple, colour coded read out
which, to some extent, would be achievable with an airspeed also. But knowing
that the information it gives is the same no matter the circumstances of the
plane is a big plus. I find that while I occasionally cross check to the
airspeed, for the most part, I set up my approach by glancing at the AOA
indicator. While I rarely go into short strips and have lots of room ahead, my
"thing" is to get down in the shortest distance in a variety of
inlets around the lakes so safely coming in at minimum speed is important.
And one more
thoughtful opinion from a non user:
I have to say that I've been watching this thread with
interest.
Whilst I don't have any great feeling for or against the LRI I think
that some stuff needs to be clarified. The LRI is not a true AoA
system because it does not measure the actual relative airflow and
compare it against the aerofoil chord datum. What it does is
approximate the angle of attack by measuring differential air
pressures acting on a probe in the free airstream. As does the
system described by another poster, where the sense ports are on the
wing upper and lower surfaces. That particular system cannot be
anything more than a sophisticated stall warning setup because it
senses air pressures at the aerofoil surface – incidentally similar
system is in common use in high performance gliders to set optimum
angles and speeds for reflex flap use.
Because these systems are using air pressure as a reference they are
subject to errors just as the ASI, ALT etc etc are. As far as the LRI
goes its main limitation is that it is therefore necessarily
inaccurate outside the confines of straight and level un-accelerated
flight [not so with a true AoA system] – which incidentally is
exactly where it is calibrated?? It could however still be a
valuable tool in a light recreational aircraft for use in takeoff and
landing where it gives a visual indication of how close you are
getting to a stall – providing it has been calibrated correctly. But
then the same applies to the ASI it does pretty much the same in the
S7 – again provided it is correctly calibrated – and bugs don't take
up residence in the small orifices.
Where the difference is in presentation of the information. I have
not used the LRI so cannot comment on its practical operation but
see it as an aid to safe slow flight particularly in the S7S which as
I have said before only has two flaws 1; no step to get in it, and 2;
exceptionally good low speed handling. I had the opportunity to fly
my first S7S flight on its very first flight since build with an ASI
that failed on climb out so the whole flight was conducted without
meaningful airspeed information What this revealed was that the S7S
controls are effective right up to and into the stall without as much
reduction of `feel' as say a C152 or Cub etc as the airspeed decays,
so I flew the first approach faster [by way of feel] than gut feeling
was saying I could be. I have many hundreds of landings now into
short strips with the S7S and even yet I still like to keep one eye
on airspeed as I pass over the hedge just to make sure I am not
getting dangerously close to the stall flying by feel.
I look forward to testing the LRI in my S7S to see how its
theoretical limitations affect its practical operation, but I do
suspect that it will turn out to be a valuable aid. My day job is in
aircraft accident investigation particularly light recreational stuff
and it remains a fact even today that a high percentage of fatalities
occur from stall/spin related accidents and these often being in
aircraft that have stall warning devices. So flight using the ASI
alone clearly wasn't enough for some unfortunate folk. Using
something like the LRI, providing it is used with due regards to its
capabilities and limitations [as with everything else], ought to
provide the pilot with additional valuable safety information.
At the price being given by Peter, his was hardly a commercial
venture, and you were not being forced to buy it. Lets face it you
can always give Spruce £1000 for a device which will likely give no
more reliable information if you feel the need.
The following note is from Jim
Covington on the Titan group:
So many follow-up posts - this is a good discussion! I'll
try to answer
a lot of questions in one post.
The fundamental point I have here is that I don't think the instrument
Peter described is a true AoA instrument, but I *do* think it's very
valuable and I encourage anyone who doesn't have one to look into it. If
you gave me the choice between an airspeed indicator and Peter's
instrument and said I could only have one (FAR's notwithstanding) I'd
choose Peter's.
Also, for all of the assumptions below, I'm assuming *no flaps.* Also,
for the discussions of pressure, all discussions are about differences -
so I'm ignoring static pressure (since it will be the same at both
ports; only the dynamic pressure changes.)
Lucien's statement: "Stall is determined only by angle of attack and can
happen at any airspeed or loading"
Absolutely right!
Hugh's instrument sounds like a true AoA instrument - a vane in the
wind. From Hugh:
"I have heard the assertion that an AOA indicator will always tell you
when your wing is about to stall."
Yes, that's true.
"But when you're in a steep bank, meaning higher G forces, how does that
happen? I guess it's because the wing has to generate higher lift in the
turn, so it must be at a higher angle of attack."
Exactly! You do need to carry a higher AoA at any given airspeed to
generate the extra lift required to hold the plane at a constant
altitude. If that given airspeed is just below stall in level flight,
you will stall in a steep bank if you try to maintain altitude with the
same airspeed - because your AoA has increased.
Now onto Peter's fundamental question -
you got it right in the last
email I received. "Jim was saying that there is a reason why this is not
a true angle of attack instrument and I'm curious about that reasoning"
Yes, that's what I'm saying. Here goes:
This instrument measures the difference in dynamic pressure at two
ports. The pressure at each port is proportional to two things - the
dynamic pressure due to airspeed, as would be measured by a perfect
pitot tube, and the cosine of the angle of the axis of port opening
relative to the angle of the oncoming air - the angle of attack of that
particular port. As the angle of attack of the instrument changes, the
difference between the dynamic pressures at the two ports also changes.
As you increase the angle of attack, the angle of attack of the top port
increases and the bottom one decreases, so the pressure at the top one
decreases and the bottom one increases, so our instrument shows a change
in the difference between the two. So far, so good - it's pretty close to
an angle of attack instrument.
Here's where the problem is - changing the angle of attack is not the
only way to change the difference in pressure between the two ports. You
can also change this by changing the airspeed without changing the angle
of attack. Imagine the top port is directly into the wind, and the
bottom port is angled downward about 45 degrees. The pressure at the top
port will be 100% of the dynamic pressure, and the pressure at the
bottom port will be about 70% of the dynamic pressure. Our instrument
shows us some reading (I don't know what reading - it depends on how
it's calibrated.)
Now imagine you double the airspeed in the wind tunnel. The dynamic
pressure with go up by a factor of 4. Here's the important point: *The
difference in pressure between the two points will also go up by a
factor of 4.* So the differential pressure gauge changes! This is why
one of the major manufacturers of a similar system calls it a "Lift
Reserve Indicator." It shows the *lift* your wing is producing, not the
angle of attack. Now if you're not turning hard tight turns, this really
doesn't matter all that much. You do want to know how much lift you're
making, especially during landing and when trying to maintain Vx, Vy or
best glide.
How does the $1800 system avoid this? They *also* take input from the
pitot tube, and do some math on the differences in pressure at all
*three* ports to come up with a real AoA.
So why is the cheaper instrument useful? Because for the majority of
flight, and for almost all one G flight, your airspeed doesn't generally
double at a given angle of attack. You typically only get high angles of
attack at very low airspeeds and low angles at high airspeeds, and the
instrument can be calibrated to show you valuable information in both
these flight regimes.
So to sum up - it's a great instrument. If you don't have one, consider
getting it. It's well worth the investment. Your landings will be
smoother and shorter, guaranteed. But don't count on it to tell you when
you're about to stall in a 5-G pull-up from a split S. For that you need
a real AoA instrument with a display readout near the center of your
field of vision.
This is from another RV-8 guy:
Flying with AOA - it's about Maximum Performance
----------------------------------------------------------
I must admit that, as an Aeronautical Engineer by training, I have always
wanted to have an Angle of Attack indicator in my airplane because, well, we
all know that is how wings stall, and geeks like instrumentation. However, I
also leaned to fly many decades ago in J-3 Cubs (and the like), and have spent
many, many more flight hours without an AOA than with one. In fact, most of my
AOA flying is in simulators at hypersonic velocities - very important there,
but bearing no resemblance to using it at low speeds. I know many Naval
Aviators who have never flown without an AOA, and learning to use one from
scratch probably gives on a unique perspective on the device. But picking it up
"later in life" is interesting, because, well, I tend to feel the
wing first, and then look at the AOA later. (To be fair, I don't generally have
to look at my ASI in a light plane that I am used to on final either - you get
to know what it feel like - unless my loading is way different from normal).
But flying with the new AOA software on my GRT EFIS for the past couple of
months has given me some interesting insights into how it can be useful - even
for those of us that "don't need it". The bottom line is that if you
operate away from the edges of the low-speed envelope, it is not going to
dramatically change your scan or habit patterns. But if the goal is to get the
maximum performance out of your bird at the low end , then yes, AOA can really
help!
The best example that I can give is that at my normal approach speeds in the
-8, I can land and stop in about 1,000' - 1,200' of runway. Because I almost
always have 3,000' or so available, I rarely worry about doing a short field
landing (and I get at least two years on a set of brake pads), and concentrate
on being safely over the threshold, and working on a nice touchdown. This is
easily accomplished with an approach speed of about 70 knots, and the C/S Prop
brakes you pretty quickly when it's time to dump speed. At my normal approach
speed, the GRT AOA indication doesn't even appear on the screen until I am down
below 70 knots (calibrated to a 1.3Vso approach speed), popping on with a pitch
up indication as I come over the fence. You can say that I am carrying plenty
of margin over the fence - or that I am flying way faster than I need to.
Either way, the AOA works fine - I am just choosing to have a little better
"penetration" speed down final.
Now, if I go out for some short field landing practice, the AOA is fantastic!
Pitch up until I get the green dot, and the plane just seems to hover its way
down final - in fact, there almost isn't enough energy to flare - a little bump
of the throttle makes the touchdown much softer. And, of course, it
re-emphasizes just how short an RV can land - I spend more of my time enjoying
the high speed aspects of the design than the low speed. Playing around with
slow flight at altitude, the AOA is a great tool for seeing when you are
getting the most out of the wing - but it just reinforces what my
seat-of-the-pants already knows.
So I guess I will never be accused of being an "AOA Cripple", unable
to fly comfortably without one, but that is just because I have flown without
one too much in my life. But, for precise speed control on an approach where it
makes a difference, the AOA is a very handy tool to have. I guess that sort of
sums up many of the features I have built in to my aircraft - Synthetic
Approaches, HiTS, ILS, multiple GPS's..much of the time, they are superfluous.
But when you really need that one tool to make the flight safer and more
successful, it sure is nice to have! (Let's face it, that's how we justify
buying shop tools as well.right?)
Paul
__________________
Paul F. Dye
RV-8 - N188PD - "Valkyrie" - 890+ Hours!