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 references. 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 as referenced to a point on the windshield. Sighting as targeting with a gun.
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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