Straight and Level
Aims
- to fly the aircraft directionally straight, laterally level, in balance, at a constant attitude and speed.
- To understand the design/stability of our particular aircraft.
Objectives
- Normal cruise, slow cruise (with and without flap) and fast cruise.
- The forces and stability in S&L flight.
- Describe how the aerofoil produces lift.
- State all the factors in the production of lift and drag.
- What happens to induced drag with increasing AoA.
Revision of Effects of Controls
- What are the three primary flight controls and their primary effects?
- How do you counteract propellor slipstream when you increase power?
Straight & Level Flight
The Four Forces
- Lift, Weight, Thrust, Drag
Forces During Straight & Level
- During straight and level flight, the aircraft is said to be in “equilibrium.”
- Therefore, lift equals weight and thrust equals drag.
- If any of these weren’t true, we wouldn’t be in straight and level flight. (We’d either be climbing/descending or accelerating/decelerating).
- Lift = Cₗ * ½ * ρ * v² * s
- Cₗ ~ wing shape and angle of attack
- ½ * ρ ~ air density
- v ~ speed
- s ~ wing surface area
- As pilots, we generally only control Angle of Attack and Airspeed.
Aircraft Stability
- Aircraft stability is classified as static and dynamic.
- Positive static stability;
- Neutral static stability;
- Negative static stability;
- Positive dynamic stability;
- Neutral dynamic stability;
- Negative dynamic stability
Longitudinal Stability
Distance from CP to tailplane is greater than from CP to mainplane – hence, greater reaction.
As the mainplane and tailplane are attached to the fuselage at different angles of incidence i.e. due to a vertical wind gust, that increases AoA on the mainplane by 2 degrees, the AoA on the mainplane will increase by 50% and the tailplane by 100%.
Directional Stability
Lateral Stability
Upper keel surfaces tend to roll the aircraft level, as well as lower CG i.e. pendulum
Dihedral (low AoB) tends to roll wings level
Drag
Induced Drag
- Induced drag is a function of angle of attack.
- As the AoA decreases and airspeed increases, induced drag decreases.
- As airspeed decreases and AoA increases, induced drag increases.
Parasite Drag
- Parasite drag comprises of
- Form drag
- Skin friction
- Interference drag
- It increases with the square of velocity i.e. 50 knots equals 100kg of drag, while 100 knots equals 400kg of drag!
- Perhaps why the biplanes of yesteryear only cruised at 50 knots.
Total Drag
- Combination of:
- Induced drag;
- Parasite drag;
- Total drag (combination of Induced & Parasite).
Establishing Straight & Level
- Power + Attitude = Performance!
- Power: As required for flight phase.
- Attitude: To maintain altitude.
- Speed: Allow the aircraft to stabilise.
- Trim: Trim the aircraft to relieve control pressure.
- P.A.S.T is the Method to attain P + A = P.
Maintaining Straight & Level
- Lookout
- Attitude
- Ensure the selected attitude is maintained with reference to the horizon.
- Instruments
- Adjust the nose attitude by external reference. Do not set the attitude by the instruments.
- Instrument scan: heading/landmark, balance indicator, altimeter, VSI.
Airmanship/TEM/HF
- Threat: Traffic
- Error: Failure to identify and take effective action to avoid conflicts
- Management: Effective situational awareness, lookout, communication and evasive actions
- Mitigation: SA! Scanning outside for traffic
IMSAFE
- Pilot Fitness Checklist
- Illness
- Medication
- Stress
- Alcohol
- Fatigue
- Emotion
Quiz to Objectives
- The forces in S&L flight
- Slow cruise, normal cruise, and fast cruise
- Types of stability
- Power + Attitude = Performance
- What happens to induced drag with increasing AoA