FAQ-Questions
Does the Elevator Control Altitude in Descent
What is Indicated Airspeed (pressure-speed)
What Controls Indicated Airspeed (pressure-speed)
What is Sustaining Thrust and Excess Thrust
What Causes an Aircraft to Stall
What is Aircraft Performance Level
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The history of aviation is replete with specific kinds
of incidents and accidents that continue in spite of discussion, teaching, and
regulation. Attempting to find some reason for these things, I have found
there is a common contributing factor that seems to always exist...aircraft
control!
Resulting study and discussion has revealed a basic
misunderstanding of how aircraft are controlled and a lack of sufficient
proficiency in aircraft control when involved in marginal situations.
This book is an attempt to clarify some of these basic
shortcomings of aircraft control.
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Flight is “falling” through
the air. Control is balance like a teeter-totter adjusting the forces
around an effective center of gravity.
An aircraft has a positive
angle above its direction of motion called “angle of attack”. Aircraft
design is to be loaded with the “static center of gravity” slightly forward of
the center of lift of the airfoils.
When moving with enough
velocity within the displacing airmass to cause enough vertical pressure, the
aircraft lifts from the surface. The forward mass loading causes the
aircraft to want to fall forward.
The pressure of the
oncoming airmass displacement offsets that loading to allow continued flight by
offsetting the gravitational force of the forward center of mass moment arm around
an effective center of gravity. The aerodynamic tail loading and any
engine vectored-thrust lifting balance these forces.
The specific pressure
required for a given velocity depends on the angle of attack the aircraft is
moving into the airmass. The adjustment of that angle is by change in the
balancing aerodynamic lift caused by position of the elevator at the tail and
the vertical component of thrust lifting at the attachment of the engine.
Successful flight is
coordination of elevator position for an angle of attack and engine power for
thrust.
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Does the Elevator Control Altitude in Descent
Elevator pitch can cause
climb and descent. What then controls indicated-airspeed? The
perceived change is climb or descent with input of either power or pitch
change.
A change of elevator
position causes a change of angle of attack pitch. That is all it ever
does. The response to an elevator pitch input will cause a change of
altitude and attitude by angle of attack change while calling for an
indicated-airspeed change. The inertia and momentum at the time
determines the extent of change. Most flight is with higher powered aircraft at
an indicated-airspeed sufficiently above any minimum to always get this as a
kind of energy exchange zooming response.
A change of power in your
tractor-engine airplane causes a change of climb-pitch and a related changed
direction of motion. That is all it ever does in level or climbing
flight. If maintaining level flight or climb, power change can cause
acceleration or deceleration but there is always required coordination of
elevator pitch to allow acceleration or deceleration.
Normal flight with the
usual atmospheric anomalies within any airmass requires need of continuous
monitoring of attitude and a control input to counter any undesired change of
motion, either, a change of elevator-pitch or engine powered climb-pitch.
Coordination of the two is normally required.
The question always asked,
does power control indicated-airspeed or does elevator-pitch control
indicated-airspeed?
Consideration of the basic
physics makes it obvious that elevator-pitch sets an angle of attack for a
specific indicated-airspeed the aircraft wants to fly.
The lifting of the outward
component of the engine thrust-vector determines the direction of aircraft
motion as a climb, level, or descent angle.
Descent requires reducing
the level flight sustaining thrust which in turn reduces a portion of engine
thrust-lift vector that contributes to the angle of attack as set with
elevator-pitch.
Now, in descent, without
coordinated elevator-pitch input to maintain constant indicated-airspeed,
increased thrust will cause some increased angle of attack and
deceleration. Decreased thrust will cause some decreased angle of attack
and acceleration.
In descent the technique
used by an individual Pilot will determine what it takes to maneuver into a
desired attitude or maintain a desired indicated-airspeed. In all cases,
it will be continuous coordination of both engine-pitch and elevator-pitch.
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What is
Indicated-Airspeed (pressure-speed)
Indicated airspeed is an indication on the Indicated
Air Speed (IAS) instrument. The reading is calibrated in speed, miles per hour
(mph) or nautical miles per hour (kts.). The instrument sensing is from a pitot
tube, an open-ended tube facing the oncoming airmass flow encountered by the
aircraft.
The real measurement is from the rammed air into the
instrument’s pitot tube. It is an air-pressure measurement.
The reading of indicated-airspeed is actually
indicated-air-pressure-speed. Though aircraft are flown relative to a reading
of mph or kph., flight operation, the operational limitations, are based on the
encountered air-pressures.
The maximum indicated-airspeed, never exceed speed,
(Vne) is a structural limitation at which the aircraft is approaching its
maximum designed strength.
Ascertaining minimum indicated-airspeed from the Pilot
Operating Handbook (POH) is dependent on the gross weight at any given time.
In all cases, indicated-airspeed has little to do with
velocity across the ground. That is the realm of “true-airspeed” and
“ground-speed”, both used for navigation.
The aircraft and all its operational limitations are
relative to Indicated-airspeed and the related pressures being applied to the
structure.
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What Controls Indicated Airspeed (pressure-speed)
Pitch adjustment of the elevator and horizontal
stabilizer with control wheel fore and aft input (pushing or pulling), or the
elevator-trim control (nose-up, nose-down), controls the balance of the
aircraft in-flight by setting a slight nose-up pitched attitude
(angle-of-attack) of the aircraft into the direction of motion.
The aircraft’s thrust-sustained forward movement
encounters the free-stream mass of the air resulting in an air-pressure-speed.
The size of the frontal area of the aircraft’s encountering determines the
displacement volume of airmass, and the air-pressure resulting from a
particular velocity determines the indicated-airspeed the aircraft will try to
fly.
The larger the frontal area, the more airmass
displaced, the less velocity required to maintain the aircraft weight, and the
smaller the frontal area, the less airmass being displaced, the greater the
velocity required to maintain the same lift.
From this it follows, a specific encountering
angle-of-attack will require a specific velocity through the airmass to cause
the lift required to sustain the flight. It can be said, the elevator setting allows
an indicated-airspeed, the thrust from the engine or gravity cause and
sustain that indicated-airspeed.
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What is Sustaining Thrust and Excess Thrust
In theoretical perfect level, constant altitude,
constant indicated-airspeed flight conditions with the aircraft trimmed for
hands-off perfect flight there is an engine thrust setting that sustains this
condition. For this trimmed indicated-airspeed condition, no matter the
attitude or maneuver there will be this constant sustaining thrust required.
Any change increasing thrust, for any reason, will
become excess thrust for this condition and result in an attitude change,
maneuvering away from the initial conditions. For a climb, excess engine
thrust will cause a climb angle with increasing altitude.
For a turn, to maintain level, it requires adding
excess thrust to maintain the constant vertical component of lift. For descent,
it requires negative engine excess, reduced engine thrust allowing a horizontal
component of gravity as thrust to add to maintain the total sustaining thrust.
For all flight attitudes, at a constant
indicated-airspeed, there is always a constant sustaining thrust required, plus
for maneuvering, some excess thrust, either engine or gravity .
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What
Causes an Aircraft to Stall
All aircraft have a maximum wing angle-of-attack at
which the airflow over the wing continues to conform to cause required lift.
Controlling with aft input to the elevator (pulling the control wheel)
increasing into an attitude exceeding the wing critical angle-of-attack will
cause sudden loss of lift and the aircraft will start to fall. This is stalling
an aircraft.
The conditions that can cause an aircraft to stall are
only attained with pilot input, pulling the control wheel, excessive nose up
trim, failure to monitor autopilot, etc. “The Pilot Stalls the Aircraft”!
The pilot pulling the control wheel is the usual cause
of stall!
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There are different kinds of pitch. A pilot must be
careful when referring to a kind of pitch. Pitch is steering the
direction of thrust.
1. Elevator-pitch controls the angle-of-attack, the
angle of the aircraft encountering the free-stream air is a nose-up pitch
angle. For all flight, there is always a positive wing angle of encounter to
the free-stream air in order to generate lift.
This elevator-pitch is caused by the position of the
elevator or horizontal stabilizer and their related aerodynamically generated
lift on the tail from motion through the air.
The small outward angle-of-attack has also created a
small engine thrust-vector of lift from current thrust. Any small engine lifting
contributes to part of the constant level flight, angle-of-attack along with
the elevator or horizontal stabilizer trimmed position.
2. Climb-pitch angle is the angle at which the
aircraft motion is no longer horizontal, but with increasing or descending
altitude with the forward motion. Excess thrust causes climb angle without
change of indicated-airspeed. Excess thrust causes an outward lifting thrust
component at the engine from the small upward angle of travel into the relative
wind of motion. With excess thrust being the only change, there has been an
increased nose-up attitude at the current constant indicated-airspeed, which
caused the climb angle. This is vertical steering of the aircraft.
3. Engine-pitch or engine vectored-thrust lifting is
part of both the level and climbing sustained pitch angles.
4. Additionally, the rudder yaw is a side-pitching
also directing or steering thrust.
5. Pitch Angle is merely a measurement of the attitude
of the aircraft profile as an angle of the horizontal axis to the surface. The
pitch angle is the sum of the climb angle and the body angle. The climb angle
is from the surface to the direction of motion (the relative wind) and the body
angle-of-attack is from the direction of motion to the longitudinal axis.
There is often reference to “angle of inclination” of
the wing, which is the attachment of the wing to the fuselage, an angle above
the longitudinal axis. This angle has no measurement and allows wing angle of
attack when cruising with zero body angle of attack. It is of no concern
to a pilot, just a nice to know thing.
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What
is Aircraft Performance Level
An aircraft can only climb to an altitude that its
power/thrust available can cause. The limitation of altitude is a power
limitation.
As altitude increases, the indicated-airspeed pressure required for flight is
constant. There is a change of true-airspeed due to the increased velocity,
relative to the airmass being penetrated, to maintain the constant encountering
air pressure required for the flight.
The aircraft aerodynamics does not know. It only responds to air pressure.
However, dramatically affecting the engine is the reduced mass-of-the-air of
higher altitudes. The airflow into the engine induction system is a constant
volume with full open throttle. The reduced elemental mass of less dense air
means there is reduced oxygen for burning in each volume of air intake.
Continued climbing to higher altitudes reduces available oxygen in the same
manner as slowly closing the throttle at lower altitudes. As this occurs, the
aircraft reaches an altitude at which the engine is only able to produce the
sustaining thrust. At that point, there will be no further climb.
Maneuvering is not possible without excess engine power so requires some
descent using a horizontal component of gravity for thrust to an altitude at
which sufficient oxygen is available to develop the excess power.
Awareness of the reduced thrust capabilities of engines when operating from
high altitude airports is very critical. It can easily become possible that the
engine will not allow a takeoff from a high altitude airport.
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There are five ways to input control to an aircraft.
1. The Rudder, actuated by pushing foot pedals, causes
the aircraft to yaw side-ways. The rudder steers with side-pitching
changing the direction of thrust.
2. The Ailerons, actuated by turning the control
wheel, create differential aerodynamic lifting on the wings causing the
aircraft attitude to roll or bank. The banked attitude redirects the
aerodynamic and engine lifts to an angle, creating a horizontal component of
lift for turning the aircraft, but simultaneously reducing the current vertical
components.
3. The Elevator rotates from the tail area to create
outward aerodynamic lifting either, from the top or bottom of the tail and acts
on a moment arm from its center of lift to the current center of gravity. This
causes pitch of the attitude to or from the pilot while changing the
angle-of-attack and associated frontal area. Use of the elevator is to
maintain aircraft balance around that current center of gravity for a desired
indicated-airspeed.
4. Elevator Trim, actuated by a small wheel in the
cockpit adjusts the elevator or horizontal stabilizer to an elevator control neutral
position for ease of pilot input. A fixed trim setting allows constant
indicated-airspeed control without elevator control wheel input. A set elevator
trim allows manual elevator control input and when released the aircraft
resumes the set indicated-airspeed…a kind of cruise control.
5. Engine vectored-thrust lift results in a pitch
input from its location similar to the elevator and acting over a moment arm
from its attachment. This small angle has a resulting outward component
of lift acting at the engine. The angle-of-attack always requires an
angle of encounter to the relative wind (direction of motion).
In level sustained thrust flight, level turning
flight, or climbing flight the portion of lift from the sustaining thrust
continues as part of the elevator and horizontal stabilizer indicated-airspeed
setting while added lift from the outward component of excess (increased)
thrust will cause increased lift for turn or climb angle.
Reduced thrust for descent will reduce the engine
thrust-vectored lift portion of elevator trimmed indicated-airspeed and allow
some acceleration. This will require added nose-up elevator trim if desiring to
maintain the same constant indicated-airspeed.
Thereafter, throughout all descending flight, thrust changes below the
sustaining thrust will affect angle of attack and the related
indicated-airspeed flown. Continuous coordination of elevator-pitch and
thrust-pitch are required during all descent maneuvering.
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Directed Course flight is controlling the aircraft by
directing with the controls toward visual objects. Maintaining a wings level
constant altitude directed course is by visually sighting the horizon level and
fixed across the windshield. A directed course heading is fixing a point on the
horizon or surface and maneuvering the aircraft to maintain that point unmoving
on the windshield.
A directed course descent is maneuvering the aircraft
to cause a desired destination to be sighted fixed and unmoving on the
windshield. A directed course approach to landing is maneuvering the chosen
landing spot to be unmoving low on the windshield.
All these situations require adjustment of heading and
power to cause the sight picture desired. The elevator has little play since it
is trimmed to maintain the desired indicated-airspeed.
All these maneuvers, utilizing the concept of
“directed course”, are actually maneuvering into a collision course with the
destination, point, or horizon. The term directed course is a technique for
controlling the aircraft visually. It is not desirable to call flight by
collision course, but in reality, that is all it is.
Maneuvering from the visual sightings allows
continuous and positive control. When established on a collision course,
it is quickly obvious if you are too high or too low.
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Level flight maneuvering is turning, which with its
rolled attitude, reduces the aerodynamic and engine thrust-component vertical
components of lift so requires coordinated thrust increase to sustain constant
vertical lift for level flight.
A level turn requires maintaining constant vertical component
lift.
Coordinated increase of thrust, adding excess thrust,
throughout a turn will maintain the lift balance with associated increased
engine thrust-component lifting while traveling level along an angled plane.
The angle of attack set indicated-airspeed will remain constant.
The level, constant indicated-airspeed turning
maneuver, increases the effective structural gross weight of the aircraft.
Level or climb turning flight requires added power to carry this load. Most
small aircraft do not have enough power to sustain more than a 30-40 degree
banked level constant indicated-airspeed turn.
Use of gradual increased aft elevator-pitch when
turning will add some momentary lift to allow level flight. However, this
increases angle of attack, slowing the aircraft, while rapidly increasing the
loading on the wings with load factor (“g” force) and an associated increased
stall indicated-airspeed. Therefore, this turning requires cautious
consideration that there is sufficient indicated-airspeed above Vy or Vso to
allow increasing lift in this manner.
To make constant indicated-airspeed level turns to any
bank angle, just coordinate with added power for lift and rudder as necessary
to coordinate steering. There will be a banking limit depending on the power
available, beyond that limit descent will begin, after all, this is all the
power so all it can do at this indicated-airspeed. That is a turning
limit for your aircraft.
Sure, if you pull on the elevator it will stay level
slightly longer as the indicate-airspeed decreases with increased angle of
attack toward the critical angle and a stall. See, to stall, you have to
do it to yourself.
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