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Mark George wrote:
>
>Marv -
>
>I read with great interest your editor's comments on Scott Krueger's
>recent post about the importance of AOA instrumentation. I am a new pilot
>and a Lancair/experimental dreamer/hoper/believer, and perhaps might speak
>for a few others in asking for more info on your position on the benefits
>of AOA indication, i.e. is this primarily stall avoidance? Any
>links/references
>for reading?
I decided to copy my reply to The List at large, as I thought there might
be a few others out there who are not familiar with Jim Frantz's AOA
instrument and the benefits its use provides. My apologies to those of you
who are already flying with one or have at least committed to its
installation in your partially completed project.
First of all, as you are probably aware, there is no such thing as a "stall
speed" for any aircraft, as it is possible to stall the wing at any given
airspeed. The wing will stall, however, at only one critical angle of
attack, and it is angle of attack that is indicated by the AOA
instrument. With one person and partial fuel aboard, a given aircraft
might stall at 60 knots in a particular configuration (let's say in the
landing configuration with gear and flaps down). Load that same aircraft
up with a couple more people and full fuel and the stall speed will
increase to 85 knots (or some other higher airspeed). Do a high speed
fly-by and pull back on the stick, and you will find that it is possible to
stall the wing even at an airspeed which is much higher than the speeds you
will experience in the typical landing configuration. The beauty of the
AOA instrument is that it doesn't focus on speed at all, rather it gives
the same indication for the critical AOA irrespective of the wing loading
and airspeed. This is the reason that the AOA instrument can be considered
the primary stall warning device.
Jim's AOA does much more than give you a stall warning, however, as it is
designed in such a way that it gives you a graphical indication of where
you are operating on the power curve... indications that are 'in the green'
are on the front side of the curve, the operational regime called normal
command. Indications that are 'in the yellow' or 'in the red' are showing
operations on the back side of the power curve, the area known as reverse
command. Herein lies the beauty of the AOA instrument. The lift/drag
curve is sometimes referred to as "the drag bucket" and it is at the bottom
of this "bucket" where you find the best lift/drag ratio. Like the
critical AOA, this is a fixed angle of attack independent of airspeed and
loading. Consequently it gives you more than a simple stall warning, as
flying the airplane with the indicator showing only green and flickering
the bottom yellow indicator means that you're flying at the best L/D, the
angle which corresponds with your best engine out glide angle, your best
angle of climb, and your maximum endurance angle of attack. Pretty slick,
eh?
While flying the pattern the AOA instrument will remind you to lower the
nose when trying to tighten up that base-to-final turn, as when you
increase the bank and load up the wing the AOA barks at you to advise you
that you're approaching the critical angle of attack. Besides displaying
the critical stall angle and the best L/D angle, the instrument is also
used to setup the best angles for a standard approach or one that will be
used for a short field landing.
While working the booth at Oshkosh I heard Jim mention that high density
altitude operations become much safer as well, and so I got the short
course in using the AOA for those types of operations. If, after you've
rotated and setup for the best angle of climb (best L/D) you find that the
trees are coming up faster than you're comfortable with, you will
positively know to abort the takeoff and get the airplane back on the
runway ASAP. Since you're already flying at the best L/D, any additional
back pressure on the stick will only result in your "hitting the trees
where they're fatter" (thanks, Jim, great line). You simply can't
outperform the best L/D, and the AOA provides you with that information
regardless of whether you're flying alone or with full passengers and fuel.
That's about it in a nutshell... if you'd like more info you can contact
Jim at 952-474-4154, me at 941-694-3757, or check out Jim's AOA website at
http://www.angleofattack.com/. I hope this has shed a little light on what
this little instrument is all about.
<Marv>
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LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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