VISITORS IN 2018
Newsletter February 2018.
Next meeting Thursday February 15.
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
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 Dan Oldridge
From Jan Nademlejnsky