Newsletter February 2018.
Next meeting Thursday, February 8.
7:30pm, Blair Field Clubhouse.
By Dan Thomas
Angle of Attack (part 2)
Last month I wrote a little about Angle of Attack, or AoA
as we’ll call it. As I said, it’s an often misunderstood or completely ignored
aspect of flying, and that lack of knowledge causes innumerable loss-of-control
accidents. This month we’ll expand on the subject somewhat, using pictures.
After all, most of us see a lot better than we hear or read.
Around ten years ago, while a Director of Aircraft
Maintenance at a college-based flight school, I also did some flight instructing
and taught a course on Aircraft Systems. I found that visualizing AoA was
difficult for many students, so I came up with this thing, building it out of
steel rod and flat bar and some plywood, and using one of the models we made for
the instructors while they were briefing the students.
The rods represent the relative wind. The nearest set
represents a steep turn. The middle set represents level, climbing or descending
flight, and the farthest combines the two for climbing or descending turns. Some
interesting geometry appears once we place the model on those rods.
We’ll start with the level cruise flight. The airspeed is
high, so the angle of attack is relatively low. Safe enough. Remember, lift is a
function of airspeed and angle of attack: if speed is low, you need more AoA.
Speed high, less AoA.
Note the low AOA, signifying a good speed.
All the angles on this AoA table are exaggerated to make
understanding easier, and the turn radii are much smaller so that AoA
differentials show up clearly. We don’t really want to descend at the angle
Low speed descent. The AoA is large, to make up for the low
airspeed and still generate 1G of lift for the airplane. The sink rate will tend
to be high due to the large AOA and the induced drag that goes with it. This can
be used instead of slipping to descend more quickly in flapless airplanes, but
speed control is critical since it will be low and can fall further quickly. My
own Jodel will sink at truly alarming rates if I slow it up when high on
approach and can leave insufficient airspeed for flaring.
And this points out two common mistakes pilots make: (1)
Diving at the runway when too high increases airspeed, decreases AoA and
therefore drag, and the airplane arrives in ground effect with far too much
speed and several types of accidents can happen. (2) Pulling the nose up to
stretch the glide doesn’t work. It increases AoA, reduces speed, and you
actually fall short of your intended spot.
Several lessons can be drawn from this one. With the nose
high and the airplane travelling in a level flight path, we have here either
slow flight (where the low speed demands lots of AoA), or we’re on the edge of a
stall (which is what slow flight is, anyway), or the pilot has pulled up real
hard in an attempt to get that spectacular zoom. Let’s address the last
scenario: the zoom. Remember Newton’s First Law of motion? “An object in motion
stays in motion with the same speed and in the same direction unless acted upon
by an unbalanced force.” This airplane, because it has mass, wants to keep going
straight ahead, and if we want it to climb, the wing has to provide extra lift.
If the change in direction takes some time, the wing has more time to do it, and
therefore the extra load is less. If we pull up suddenly, we need a lot of extra
lift and we feel significant G force as we do it. If we pull just a bit too
hard, however, we can get what’s called an “accelerated stall,” and stall that
wing and crash the airplane if we did it at low level. Aerobatic pilots do snap
rolls by pulling back hard and stomping the rudder; in effect, they’re getting a
horizontal spin. They spend many hours at high altitude, often wearing a
parachute, getting training and practicing this sort of thing. They’re not the
ordinary weekend warrior like you or me fooling around at low level trying to
impress someone. They’re not that foolish
This shows the airplane climbing. See that the relative
wind is at such an angle that the AoA is completely manageable? The airplane’s
path has changed. The relative wind reflects that. The AOA is higher than in
cruise, because the airspeed in a climb is lower than in cruise. If we keep
pulling back, though, the speed falls, AoA increases and adds drag and speed
falls further, and we’ll stall the airplane
In a level turn, the AoA is essentially the same on both
wings. But wait a minute. Most airplanes exhibit an overbanking tendency caused
by the speed differential between the wings, since the inside wing is travelling
a shorter path than the outside wing, so it’s going slower and wants to keep
dropping. SO we usually end up adding opposite aileron to stop the roll, and now
we have a higher AoA on the inside wing and it might stall sooner if we get too
slow or too steep. Some pilots tend to slip the airplane a bit so it won’t keep
rolling. Safe but a little sloppy.
The relative wind’s Angle of Attack is between it and the
wing’s chord line. Dropping an aileron or flap increases AoA. It also increases
camber, which increases lift, which is the real function of a flap or aileron.
It’s not just deflecting air up or down; it’s changing the camber of the whole
airfoil, and therefore the speed of airflow over the top relative to the bottom.
More camber = more speed differential = more lift.
If you’re still with me, things are about to get
interesting. See the difference in AoA? It’s visible as a gap under the leading
edge of the inside wing. It’s a result of the small radius of the flight path of
that wing compared to the outside wing, and in a smaller radius the path has to
be steeper to descend at the same rate as the outside wing. It helps to counter
the overbanking tendency, but can cause this sort of trouble:
Skidding the turn increases the difference
in AoA between the wings. And, as we noted last month, a skid requires opposite
aileron, which increases further the AoA on that inside wing, bringing it closer
to stall angle if we get slow. Remember that chord line changes with aileron
input? Don’t do this. It’s a common cause of base-to-final turn accidents. If
you’re skidding in this scenario, you’ve messed up the approach. Go around and
try again. A good approach starts way back on downwind.
A slipping turn actually decreases the AoA differential and
makes the turn much safer. In fact, this is commonly taught as an
altitude-losing maneuver if you’re a bit high on base. The inside aileron is up,
decreasing the AoA further on that wing.
The angles apparent in the pictures are considerably larger
than they are in real life. In a typical base-to-final turn, 30° bank, 70 MPH,
the radius is such that there’s only a small fraction of a degree difference in
AoA between the two wingtips, but it’s enough to cause grief. The accident
history bears this out.
Aha! The classic departure stall scenario. A climbing turn
puts the outside wing at a higher AoA, and it will stall first if we pull back
too hard, and will try to roll into a spin out of the turn. This sort of thing
is the cause of accidents when a pilot takes off from a too-short strip and
tries to clear some obstacles at the end. Read the POH! It has charts regarding
takeoff distances over obstacles, and you have to pad those distances a lot if
the strip has long grass or is soft or there’s no wind or it’s a hot day.
Wishful thinking won’t lift the airplane clear of the trees.
As I said last month, the typical trainer like a 172 will
probably just make lots of noise and drop its nose a bit and go on flying if you
do this at altitude. They’re very forgiving (but will still hit the trees on
that short strip). It’s like learning to drive in Grandpa’s 1971 Pontiac station
wagon. However, go buy an old Champ or Citabria or Super Cub and see what it
thinks of this sort of thing. It will bite. There is no better way to spend your
money than on thorough and competent training. It’s the best life insurance.
When buying an airplane, you need to leave money aside for the inevitable
maintenance surprises as well as that really good checkout.
From Paul Parsons
Who killed the Red Baron?
From Jan Nademlejnsky
182. Short Flight over North Ridge Downtown
and Sandbars, Kamloops, Jan 13, 2018
The first flight in the new year. Kamloops
received a bit of sunshine on snow covered ground and mild temperature, just
below freezing. It was short time, which was limited by my level of freezing
fingers. I counted 23 eagles on the ground and in the trees, and one in flight.
It was good flight.