Aerodynamics

Part 1 – Propellers

Aim

To gain a basic understanding of the principles of propeller aerodynamics, and how they are used to create thrust for unmanned aircraft.

Objectives

At the end of this briefing you will be able to:

  • Describe the naming convention for propellers
  • Discuss the meaning of the pitch of a propeller
  • Discuss propeller pitch and performance

Propeller Design

Propellers in Simple Terms

The aircraft propeller consists of two or more blades and a central hub to which the blades are attached.

Each blade of an aircraft propeller is essentially a rotating wing.

As a result of their construction, the propeller blades are like aerofoils and produce forces that create the thrust to pull, or push, the aircraft through the air.

The engine furnishes the power needed to rotate the propeller blades through the air at high speeds, and the propeller transforms the rotary power of the engine into forward thrust.

Propellers for Unmanned Aircraft

Examples of different sizes of propellers for unmanned aircraft.

Pitch

Pitch is the displacement a propeller makes in a complete spin of 360° degrees.

Example of how propeller pitch effects the translational motion.

This means that if we have a propeller of 40” pitch it will advance 40 inches for every complete spin as long as this is made in a solid surface; in a liquid environment, the propeller will obviously slide with less displacement.

The pitch concept is not exclusive to propellers, other mechanical devices like screws also use it. For instance, a screw with 10 mm of pitch will advance 10 mm for every complete turn of the screwdriver.

Propeller Blade Angle

Relative wind speed affecting the angle of attack of a propeller blade.

Fixed Pitch

Fixed-pitch and ground-adjustable propellers are designed for best efficiency at one rotation and forward speed.

They are designed for a given aircraft and engine combination. Since the efficiency of any machine is the ratio of the useful power output to the actual power input, propeller efficiency is the ratio of thrust horsepower to brake horsepower. Propeller efficiency varies from 50 to 87 percent, depending on how much the propeller “slips.”

Propeller slip is the difference between the geometric pitch of the propeller and its effective pitch.

Geometric pitch is the theoretical distance a propeller should advance in one revolution; effective pitch is the distance it actually advances.

The Twist on a Propeller

The reason a propeller is “twisted” is that the outer parts of the propeller blades, like all things that turn about a central point, travel faster than the portions near the hub.

Propeller blade twist.

If the blades had the same geometric pitch throughout their lengths, portions near the hub could have negative AOAs while the propeller tips would be stalled at cruise speed.

Propeller Pitch and Efficiency

The pitch of the propeller is generally chosen to provide the speed characteristic of the aircraft for the purpose required.

Increasing the blade pitch increases the blade drag, and decreasing the blade pitch decreases the blade drag.

A larger (coarser) blade angle, for a given RPM, will adsorb more power and require more torque to turn it at the requested RPM.

Usually 1° to 4° provides the most efficient lift/drag ratio, but in flight the propeller AOA of a fixed-pitch propeller varies — normally from 0° to 15°. This variation is caused by changes in the relative airstream, which in turn results from changes in aircraft speed. Thus, propeller AOA is the product of two motions: propeller rotation about its axis and its forward motion.

Propeller Designation

Propellers are designated by two numbers:

  • Diameter
  • Pitch

A propeller designated as a 12-6 propeller is therefore:

  • 12″ in diameter
  • 6″ of pitch.

…where pitch is the distance a propeller will move forward in one revolution in a perfect fluid (which air is not).

Theoretically a 6″ pitch will move forward 6″ with each complete (360°) revolution of the propeller.

How Pitch Affects Propulsion

The properties of a propeller with high pitch:

  • High speed flight
  • Poor Acceleration
  • Poor Climb
  • Can be difficult to slow down for landing

The properties of a propeller with low pitch:

  • Low speed flight
  • Good Acceleration
  • Good Climb
  • Finer speed control throughout throttle range – particularly at low throttle settings

Pitch in Simple Terms

An easy way grasp the concept of propeller pitch is to draw a parallel to the gearing in a car.

Low pitch propellers = low gear in your car.

It will get you up hills well but will not take you any where fast.

High pitch propellers = Beginning your drive in fifth gear.

It will take forever to accelerate to speed but the plane is cruising when it gets there.


Propeller Performance

Propeller Balance

An out of balance propeller can be the cause of a lot of problems.  Some of these problems manifest as:

  • Prevents the engine from developing full power.
  • Causes excessive vibration through the airframe.
  • Causes excessive vibration through on-board electronics, leading to premature failure.
  • Causes fuel foaming which can cause the engine to run ‘lean’.
    • The result is the engine loosing performance and power, to stall, or just not run smoothly.

This is all amplified in a smaller aircraft.

Trimming Balance

Before you attempt to balance a propeller, be sure to clean it.

Most propellers are close to being in balance when purchased, so they should only need a small amount of work to bring them into perfect balance. 

If the propeller is severely out of balance – return it because too much material would have to be removed which would significantly change the shape of the blade.

Propeller Balancing Device

If one blade is heavier than the other, then the usual method to bring the propeller into balance is to remove material from the heavy blade using sandpaper.

Trimming Heavy Propellers

Do not trim the tip of the heavy blade!

Although the blade may balance statically, it will not be balanced when it starts to spin, because of unequal mass distribution.  Material is generally removed from the face (front) of the propeller or from the back of the propeller.

Generally all that is required is to sand the face a little.

Propeller Tracking

Occasionally you may encounter a propeller that does not track properly.  Either the mounting hole is off-centre or the hub is not square to the plane of rotation. 

In either case, if the propeller is noticeably out of track you should not use it.

It is easy to see if the propeller is tracking correctly.  Stand back for safety and look at the propeller from the side and from the rear.

From the side, both tips should be clearly visible in the same line.  If you see two lines, then the hub is not square to the plane of rotation.

The recommended procedure is to return the propeller – it is defective, and may require too much modification to ‘repair’.


Selecting Motors & Propellers

Propeller Selection

Beware the “hobby mentality

The propeller should be chosen to match the aircraft — not the engine. An appropriate engine should then be chosen! 

Consider this. Mounting a racing propeller to a scale WWI aircraft will severely limit the model as an early warbird has so much airframe drag that the propeller will never be able to deliver it’s full potential, with the result that the aircraft will be a sluggish flyer at best.

Additionally, using too ‘slow’ a propeller – one with low pitch – on a model intended to go fast may prevent the aircraft from gaining enough speed to fly at all. A basic mistake is finding a propeller that works great – with a certain engine in a certain aircraft – and then imposing that propeller on that engine regardless of the aircraft!

Number of Blades

With ‘model’ sized aircraft the most efficient propellers are two bladed.  Because the diameter of our propellers tend to be small, multiple blade propellers disturb the air that the trailing blade is entering tending to make them less efficient.

Generally, with smaller aircraft, for best overall performance, it is recommended to utilise 2-blade propellers.

Matching Propeller and Motor

Propeller-Motor Thrust Output

The Reality of Motor and Propeller Selection

Test & Verification Procedure (Actual procedure – V-TOL Aerospace):

  1. The propeller was fitted to the motor, which was mounted on the scales to provide down thrust, pushing against the scales.
  2. The current meter was setup in line with the battery and Electronic Speed Controller to measure current drawn by the motor.
  3. The servo checker was setup to control the motor through the ESC.
  4. A camera was mounted such that it could see the current meter and the display on the scales.
  5. For safety the video display and servo checker were operated from behind a safety barrier.

Test Results

ThrottleVoltsAmpsThrust (g)
μs%9×4.59×69×4.59×69×4.59×6
1000012.5712.570000
11001012.5512.550.30.36560
12002012.5212.491.61.7165150
13003012.4712.423.13.7285290
14004012.2712.216.27.6480500
15005012.1512.0620.312.5680705
16006011.9711.9116.119.6900910
17007011.7811.7322.825.811401085
18008011.5211.4429.434.513301240
19009011.3911.0638.139.515251345
200010011.1311.0138.944.315501390
Collected thrust data for differently sized propellers
ThrustVoltsAmpsWatts
(g)8×69×69×4.58×69×69×4.58×69×69×4.5
10012.412.512.51.61.10.919.8413.7511.25
20012.412.412.53.42.22.142.1627.2826.25
30012.312.412.45.53.83.567.6547.1243.4
40012.212.412.38.75.54.9106.1468.260.27
50012.112.312.312.77.86.7153.6795.9482.41
6001212.212.216.89.98.7201.6120.78106.14
70011.812.112.1221310.7259.6157.3129.47
80011.7121226.215.713.4306.54188.4160.8
90011.711.911.934.118.415.5398.97218.96184.45
100011.611.711.739.223.418.1454.72273.78211.77
110011.611.627.622.6320.16262.16
120011.411.633.324.5379.62284.2
130011.311.437.828.9427.14329.46
140011.211.444.232.4495.04369.36
150011.238.1426.72
Collected power draw data for different thrust values

Given the test data, and noting that cruise speed for the condor aircraft is attained at 8A/100W with a 9 x 4.5 propeller, it can be seen that the most efficient propeller to produce the required amount of thrust is the 9 x 4.5 propeller.


Safety

Hazards of Propellers

Safety must always be first and foremost in your thoughts when handling unmanned aircraft, or components from unmanned aircraft.

  • Propellers spin at relatively high speed.
  • Propellers are made from hard material.

This is why should we pay special attention to the propeller when completing an inspection of an aircraft!

What happens when things go wrong?

Example of a propeller that was broken while spinning.
A broken propeller that was found after an incident during an indoor test.

All that stopped the blade segment from passing all the way through the partition was the fabric on the other side!


Summary

You should now be able to:

  • Describe the naming convention for propellers.
  • Discuss the meaning of the pitch of a propeller.
  • Discuss propeller pitch and performance.

Part 3 – Flight (Multirotors)

Aim

To gain a basic understanding of the principles of aerodynamics, as they apply to unmanned aircraft.

Objectives

At the end of this briefing you will be able to:

  • Describe the aerodynamic principles of multirotor flight and how multirotor aircraft attitude is controlled
  • Outline considerations when Hovering In Ground Effect and operating in confined spaces
  • Outline the normal and non-normal flight modes available for Academy multirotor flight operations

Movement

Aircraft Axes

MovementAxesLimits
PitchLateral35o (180o/sec)
RollLongitudinal35o (180o/sec)
YawNormal90o/sec

Motor Configuration

Note: Refer Specifications for YOUR airframe

To facilitate flight path control:

  • The motor numbering sequence and direction of rotation are linked to the orientation of the autopilot
  • The firmware interprets any control inputs and commands the correct motor(s) in the correct sequence to change motor speed (RPM)

Aerodynamics – Rotary Wing Flight

  • Aerofoil (rotor) is rotated by the aircraft power plant
  • Air is drawn down from above and accelerated though rotor disc
  • There will be lower pressure above the rotor, and higher pressure below the rotor
  • There will be a large blade tip vortex
  • Maximum downwash velocity will occur approximately two rotor diameters below the disc

The Multirotor as a System

Flying a Multirotor

Question: Is it possible to take the theory we have learned for fixed wing aircraft and make it apply to multirotor aircraft?

Vertical Flight

Each rotor provides ¼ of TRTR

Horizontal Flight – Theory

Control inputs required ⟹ Pitch & Roll ⟹ Tilting the TRT

From previous theory, for horizontal flight:

  • TRT needs to increase
  • RTVC supports weight
  • RTHC moves airframe along flight path

Horizontal Flight – Example

Pitch Forward1234
HoverKKKK
Entry
MaintainKKKK
Exit
HoverKKKK
Roll Right1234
HoverKKKK
Entry
MaintainKKKK
Exit
HoverKKKK

Yaw

Newton’s Third Law: For every action there is an equal and opposite reaction.

MotorsRotationEffect of Torque
1 & 2Counter-clockwiseClockwise
3 & 4ClockwiseCounter-clockwise
Right Yaw1 & 2 3 & 4
Left Yaw1 & 2 3 & 4

Operating in Confined Spaces

  • Low pressure (i.e. suction) in region above disc and region of the tip vortex
  • Objects such as poles, walls or ceilings will interfere with the flow similar to the strong winds felt in alleyways around bases of tall buildings close together – venturi effect)
  • Increased air velocity will lower the pressure
  • Airframe likely to be sucked into collision with object

Operations

Multi-Crew Operations – Crew Stations

Person stationed at GCSPerson stationed with hand controller
1st Pilot / Captain2nd Pilot / First Officer
Responsible for:
– Start, continuation, diversion and end of a flight by the aircraft
– Operation and safety of the aircraft (including payload) during flight time
– Conduct and safety of members of the crew
Actively support the Captain in:
– Safe flight outcomes
– Appropriate operation of the aircraft and payload
– Achievement of mission objectives (as planned and subsequently amended)
Crew stations.

Multi-Crew Operations – Responsibilities

Pilot Flying (PF)Pilot Monitoring (PM)
Person controlling the aircraft:
– Direct manipulation of the flight controls and flight path; or
– Entering commands into a navigation system (e.g. GNSS) that is coupled to an autopilot
Person monitoring the aircraft:
– Monitoring flight path and other aspects of operational performance
– Authorised to TAKE CONTROL if undesired aircraft state develops and remains uncorrected
Crew responsibilities.

Undesired Aircraft State: aircraft position or speed deviations, misapplication of flight controls, or incorrect system configurations associated with a reduction in margins of safety

Flight Modes

ModeAnnunciationSelectedFunction
StablisedSTABLabelled switch on right side hand controller– Co-pilot is Pilot Flying with autopilot engaged (or stability board)
– Continuous control inputs required to maintain station (i.e. position & height)
Height HoldHHLabelled switch on right side of hand controller– Co-pilot is Pilot Flying with autopilot engaged (Barometric sensor on autopilot)
– Power lever in neutral position = aircraft will maintain height with no pilot input
– Co-pilot makes roll / heading inputs to maintain station / manoeuvre laterally
NavigationNAVON and OFF labelled position switch on left side of hand controller– Only selected ON when authorised by Captain
– When selected ON, Captain becomes Pilot Flying
– When selected ON, autopilot will command the aircraft to navigate vertically and laterally to the waypoint from the Active Flight Plan that is annunciated on the Captain’s Primary Flight Display. Waypoint sequencing will automatically continue
– When selected OFF, Co-Pilot becomes Pilot Flying in either HOV or STAB mode depending on that switch position
Return to LaunchRTLON and OFF labelled position switch on left side of hand controller– GNSS coupled to autopilot
– Airframe will track direct from present position to starting point
– If > 50 ft AGL, present height will be maintained
– If < 50 ft AGL, airframe will climb to 50 ft AGL then track to launch site
– At launch site, hover for 5 secs, autoland, then motor shutdown and lock
– Mode cancelled by cycling RTL switch to OFF
Normal operations.

Emergency Operations

PriorityStateConditionsAction
1Critical BatterySTAB, HH or NAV; GNSS; and
> 2m from launch position
– RTL then autoland
– GCS annunciations (Low Battery; Critical Battery – RTL)
STAB, HH or NAV; and / or
< 2m from launch position
– Autoland
– GCS annunciations (Low Battery; Critical Battery, Landing)
2Loss of C2 link + 10 seconds
(Set to risk)
HOV or HH; and GNSS; and
> 2m from launch position
– RTL then autoland
– GCS annunciations if C2 link re-established
< 2m from launch position– Autoland
3Nil GNSSSTAB or HH– GCS annunciation indicating NAV not available
NAV– Autoselects HH mode
– GCS annunciations
– Co-pilot recovers aircraft via hand controller to land in STAB
4GeofenceBreach– RTL then autoland
– GCS annunciations
Breach + 100 meters– Autoland
– GCS annunciations
Non-normal operations.

Example Flight Profile

Take-OffMissionLanding
Pilot Flying (PF)Co-Pilot:
– Gentle climb in STAB to visually establish aircraft outside ground effect to check control operation, then HH mode
– Position to vicinity of Take Off Rally and wait for authorisation from Captain to select NAV mode
Captain:
– NAV mode commands autopilot to navigate vehicle both laterally and vertically in accordance with Active Flight Plan, or as directed by specific command buttons on GCS
Captain:
– When Landing Rally has been sequenced as the active waypoint, the autopilot will navigate a pre-determined descent and landing profile
Pilot Monitoring
(PM)
Captain:
– Validate PFD/MFD information during climb (max 1 minute)
– Once (a) satisfied PFD/MFD checks out OK and (b) Captain is ready to take control, Co-Pilot can be advised that NAV mode may be selected  
Co-pilot:
– Watches vehicle throughout flight
– If undesired aircraft state observed, Co-pilot (as Pilot Monitoring) is authorised to immediately deselect NAV and take over to assure safe flight outcome
– Maintains situational awareness over operating environment (e.g. intruding aircraft, spectators, obstructions on runway, wind, weather etc)
Co-pilot:
– Watches vehicle throughout flight
– If undesired aircraft state observed, Co-pilot (as Pilot Monitoring) is authorised to immediately deselect NAV and take over to assure safe flight outcome
– Maintains situational awareness over operating environment (e.g. intruding aircraft, spectators, obstructions on runway, wind, weather etc)
Example of changes in pilot roles.

Hand-over/Take-over Procedure

What: The process of a pilot in command positively giving control of   the aircraft to another pilot or positively assuming   control from another pilot and the acknowledgement of this action by the pilot or co-pilot.

How:  Verbally

  • Handing Over – response: Taking Over
  • Taking Over – response: Handing Over

Captains:

  • If you authorise your Co-Pilot to select NAV mode, what you are implying is I’m happy and ready to take over.
  • Be watching out for your mode annunciation to change to NAV; your Co-pilot may not be able to fly and talk at the same time!
  • Acknowledge the NAV annunciation with Taking Over (and Handing Over if NAV changes to FBWA or UNAS)

Note:

  • A remote crew member can take control of the aircraft at any time should they believe an abnormal situation has arisen (legal implications may apply)!
  • If you are not ready or able to take over, withhold or rescind the NAV mode authorisation!

Summary

Covered Information:

  • Described the aerodynamic principles of multirotor flight and how multirotor aircraft attitude is controlled
  • Outlined considerations when Hovering In Ground Effect and operating in confined spaces
  • Outlined the normal and non-normal flight modes available for Academy multirotor flight operations

Part 2 – Flight (Aeroplanes)

Aim

To gain a basic understanding of the principles of aerodynamics, as they apply to unmanned aircraft.

Objectives

At the end of this briefing you will be able to:

  • Describe the basic principles of Lift
  • Illustrate an aerodynamic cross-section and show how lift is generated

Aeroplanes

The Aeroplane Structure

Axes, Planes of Movement & Flight Controls

The three cardinal axes

The “Normal” Axes

The “Longitudinal” Axes

The “Lateral” Axes

Aerodynamic Forces

Airfoil Cross Section


Aeroplane Design

Basic Aerofoil Shape

Terminology:

Cambered Aerofoil Section

Cambered Aerofoil Section

Mean Chord: Average Chord Between the Wing Tip and Wing Root

Aspect Ratio

Other Basic Terms – Wing Loading

Wing Loading: (WEIGHT) / (WING AREA)

Other Basic Terms – Angle of Incidence

The Angle of Incidence is the fixed angle between the chord line and the longitudinal axis.


Dihedral

What is Dihedral?

Recall what happens when we roll a fixed wing aircraft about its longitudinal axis….

ROLL ⟹ SLIP ⟹ YAW

Dihedral on a Boeing 777

Why have Dihedral?

Dynamic Inherent Stability!

An aircraft without dihedral will not naturally return to wings level.

The horizontal component of lift causes the aircraft to slip right.

How Dihedral Works in a Practical Sense

Adding dihedral improves lateral stability when the aircraft rolls left or right.

The aircraft is banked to the right:

  • Firstly, the airplane will slip to the right.
  • This means the relative wind is no longer approaching directly head-on to the aircraft, and instead is approaching slightly from the right.
  • This means that there is a component of the relative wind that is acting inboard against the right wing.

An Example of Dihedral Working

Aircraft banks right, aircraft slips right – there is a change in relative wind.

As the relative wind has an inboard component, and the wings are tilted up slightly, a portion of the relative wind strikes the underside of the low wing, pushing it back up toward wings level.

How Dihedral Fixes the Main Problem

As the aircraft slips right, relative wind contacts the underside of the ‘low wing’ (Because of dihedral) rolling the aircraft to the left.

The greater the dihedral an aircraft has, the greater the effect becomes. Most aircraft have only have a few degrees of dihedral – just enough to keep the wings level during small disturbances, like turbulence.

The Problems Dihedral Creates

Dihedral results in reduced lift. There’s a vertical component and a horizontal component to the total force.

Dihedral will work ‘against you’ when commanding a roll right or left. When the aircraft is commanded to bank, the dihedral effect constantly tries to return wings level. As dihedral is increased, roll rate is decreased.

Dihedral reduces roll rate as the lower wing produces more lift (more than with wings straight and level).

Dihedral in Multirotors

Dihedral in an aircraft is intended to stabilise the aircraft in ‘roll’. The natural position of the aircraft is roll-centred.

Dihedral on a multirotor has exactly the same effect!

The multirotor should have a natural tendency to correct slight shifts in roll. Remember that in a multirotor ‘roll’ is taken to mean any horizontal axis – as in this context multirotor aircraft don’t have a ‘nose’ or ‘tail’.

In the diagram above, the craft is completely horizontal. The vertical component of thrust is what counters weight. In this arrangement the horizontal components of thrust are opposing, and so no horizontal motion results.

Multirotor Dihedral Example

A small dihedral (4° – 6°) will result in a small reduction in efficiency, and will result in an aricraft that is less inclined to randomly yaw when moving forward, and that is more stable during hover and descent.


Aerofoil Properties

Flaps

Flaps lower the stall speed, change the effective camber of the wing and increase drag allowing flight at lower airspeeds.

Essentially flaps redesign the properties of the airfoil.

  • When the flaps are lowered:
    • Aircraft initially ‘balloons’ due to an increase of lift – unless corrected
    • Allows a lower ‘nose attitude’ for same airspeed due to the increase in drag
  • When the flaps are raised:
    • Aircraft initially ‘sinks’ due to a decrease in lift – unless corrected
    • Nose attitude needs to be raised for same airspeed due decrease in lift

Basic Types of Flaps

Effects of Different Types of Flaps – Simple Flap

Effects:

  • Increased Camber
  • No change to Wing Area

Therefore:

  • Increased Lift
  • Increased Drag

Effects of Different Types of Flaps – Split Flap

Effects:

  • No Change to upper surface Camber
  • Increase to Lower Surface Camber
  • No change to Wing Area

Therefore:

  • Increased Lift
  • Large Increased Drag

Effects of Different Types of Flaps – Slotted Flap

Effects:

  • Increased Camber
  • No change to Wing Area

Therefore:

  • Better increase in Lift due slot
  • Increased Drag

Effects of Different Types of Flaps – Fowler Flap

Effects:

  • Increased Camber
  • Increased Wing Area (Chord Increased)

Therefore:

  • Better increase in Lift due slot
  • Increase in Lift due increased wing area
  • Increased Drag

Leading Edge Devices

Allows Lift to be enhanced while producing minimal extra Drag.

Primary goal: increase leading edge radius.

Trim Tabs

Trim Tabs provide long term relief from aerodynamic control forces.

Tab introduces a force which holds the control surface in the desired position.

This removes the pressure from the control inputs that are required to be held. It is a labor saving device which reduces the fatigue on the Pilot.

The Pilot’s Lift Equation

What do we have influence over – what don’t we have influence over?

The Coefficient of Lift (CL) is a dimensionless coefficient that relates the lift generated by a ’lifting body’ (read: wing) to the fluid density (read: air) around the body, the fluid velocity (read: relative air speed) and an associated reference area (read: wing area).

CL is a function of the angle of the body to the flow, its Reynold number and its Mach number. The lift coefficient CL  refers to the dynamic lift characteristics of a two-dimensional foil section, with the reference area replaced by the foil chord.

Flaps, and the CL and Angle of Attack Curve

Slats, and the CL and Angle of Attack Curve

Total Reaction Force

The centre of pressure is the point where the total sum of a pressure field acts on a body. In aerospace, this is the point on the aerofoil (or wing) where the resultant vector (of lift and drag) acts.

Aerodynamics (Revision)

Main components:

  • Lift is Directly Proportional to CL (α)
    • Increase Angle of Attack (α) results in increase in Lift.
  • Lift is Directly Proportional to Velocity (V²)
    • An increase in speed results in an exponential increase in Lift.
  • Lift is Directly Proportional to Surface Area
    • An increase in Surface Area results in an increase in Lift.

Aerodynamic Effects in Flight

Stall or Flow Strips – What are they?

Stall strips are small wedge-shaped strips on the front of the wing.

Stall strips typically consist of a small piece of material, usually aluminium, triangular in cross section and often 15–30 cm in length that are riveted or bonded on the point of the wing’s leading edge, usually at the wing root.

Stall strips are used to initiate flow separation at chosen locations on the wing during high-angle of attack flight, so as to improve the controllability of the aircraft when it enters stall.

They are typically employed in pairs, symmetrically on both wings. On aircraft where wing airflow is affected by asymmetrical propeller wash, a strip may be used on a single wing to reduce risk of entering a spin.

Stall or Flow Strips – What do the look like?

Small wedge-shaped strips on the front of the wing.

Stall or Flow Strips – Summary

A stall strip attempts to ensure that the wing root stalls before the wing tips.

As the wing increases its angle of attack, airflow is eventually disturbed by the stall strip.  This causes this part of the wing to stall at a lower angle of attack than it would otherwise stall at.

Causing the wing root to stall before the wingtips is desirable as should the wingtips stall there will be a loss of aileron control making it difficult if not impossible to recover from the stall (Ailerons are typically at the wing tips).

By using a stall strip, the overall angle of attack at which your airplane stalls is lowered, however as the wing root will stall before the wingtip, stalls are more recoverable.

Washout

Washout is a design characteristic built into the wing, where the angle of attack is reduced span-wise from root to tip, typically 1° to 2°.

Washout creates a situation where the root of the wing stalls before the tip, softening the stall and allowing the ailerons to be functional deep in the stall.

Washout is a twist in a wing that causes the wingtip to meet the airflow at a lower angle than the root in normal upright flight. 

Washout – Summary

Washout is a twist in a wing that causes the wingtip to meet the airflow at a lower angle than the root in normal upright flight. 

Washout can be added after construction by slightly raising both ailerons. This is recommended for the maiden flights of a new model.

Turning

Stability

What: Natural ability for the vehicle to return to original condition following a disturbance with no pilot input

Why: So we can determine the nature of the human to vehicle interface (Do I have to fight it or caress it?)

Longitudinal Stability

  • Aircraft in normal flight suffers a pitch down disturbance
  • Increased angle of attack on tail plane compared to main plane:
    • Causes greater % increase in lift on tail plane compared to main plane
    • Creates downwards force restoring moment about the C.o.G.
  • Positive dynamic stability
  • Loading with C.o.G. beyond aft limits could make vehicle uncontrollable in pitch

Lateral Stability – Dihedral

  • Aircraft with dihedral in normal flight suffers roll disturbance:
    • Causes sideslip toward downgoing wing
    • Dihedral causes an angle of attack on the downgoing wing to generate Lift
    • This creates a restoring roll moment
  • Positive dynamic stability at small angles of bank
  • Negative dynamic stability at high angles of bank (spiral tendency)

Lateral Stability – Pedulosity

  • High wing aircraft in normal flight suffers roll disturbance:
    • Causes sideslip toward downgoing wing
    • This creates a small sideways drag force
  • The resulting aerodynamic force is displaced sideways i.e. no longer in direct alignment with C.o.G.:
    • This creates a restoring roll moment

Directional Stability

  • Aircraft experiences direction disturbance:
    • Resulting air flow acting between tail and C.o.G. creates restoring yaw moment
  • Positive dynamic stability

Part 1 – Lift

Aim

To gain a basic understanding of the principles of aerodynamics, as they apply to unmanned aircraft.

Objectives

At the end of this briefing you will be able to:

  • Describe the basic principles of Lift
  • Illustrate an aerodynamic cross-section and show how lift is generated
  • Examine what is a stall

Forces

A basic question – What Exactly is a Force?

What is a force? ⟹ A force is a push, a pull or a twist.
How is a force illustrated? ⟹ A vector  (direction, magnitude)
How is a force measured? ⟹ Force = Mass × Acceleration
What are the unit of force? ⟹ kg.m/s2 (also called Newtons)

Description of a Force

force is a push or pull upon an object resulting from the object’s interaction with another object.

A force is a vector quantity – it has both magnitude and direction.

It is common to represent a force by an arrow.

Because forces are vectors, the effect of an individual force upon an object is often cancelled by the effect of another force.

A force is represented diagrammatically as an ‘arrow’, the length of the arrow representing magnitude, and the direction the arrow points represents the direction of the force.

Resultant Force

A resultant force is the single force which represents the vector sum of two or more forces.

The ‘start’ of the second force needs to be moved to the ‘end’ of the first force, with the resultant going from the start of the first force directly to the end of the second force (as shown in the diagram).

Components of a Resultant Force

A force acting on a point can be broken up into it’s horizontal (X) and vertical (Y) components:

Rotating a Force

Note: For Straight and  Level flight ⟹ Lift = Weight

Principle of Moments

The moment of a force is a measure of its tendency to cause a body to rotate about a specific point or axis.

The magnitude of the moment of a force acting about a point or axis is directly proportional to the distance of the force from the point or axis.It is defined as the product of the force and the moment arm. The moment arm or lever arm is the perpendicular distance between the line of action of the force and the centre of moments.

Principle of Moments

When a system is in equilibrium, the TOTAL anticlockwise moment = TOTAL clockwise moment.

Consider the following system:

Horizontal Stabiliser and its Turning Moment

Most airplanes are designed so that the wing’s centre of lift (CL) is to the rear of the centre of gravity.

Applying simple physics principles, it can be seen that if a bar was suspended at point L with a heavy weight hanging on it at the CG, it would take some downward pressure at point T to keep the ‘lever’ in balance. 


Creating Lift

What is Lift?

Lets consider and develop a concept!
(Lets not make reality fit our theory, but rather simply consider and apply known fact!)

Consider this!

What if I want to squirt a friend with the garden hose, but the pressure is not quite enough for the water to reach them.
Is it possible for me to do this?

A Question?

How does covering the end of the garden hose make the water squirt further?

Answer: When you put your finger over the tip of the hose you partially block the end, or in other words: decrease the amount of space the water has to flow through!

…but what if we just PINCH the hose?

When you squeeze the hose, you decrease the amount of space the water has to flow through, and in the pinched area of the hose the water flows faster!

Making the hose squirt!

Since the same amount of water has to flow out of the hose as flows in to the hose, the water must shoot out faster – to keep the amount of water flowing out a constant.

The hose can’t expand to accommodate more water, so the water has to shoot through the ‘reduced’ opening faster. Pressure has to do with how an object will feel as a result of a force exerted on it. Because pressure causes the water to shoot out of the hose faster, it will feel harder, and it will travel farther.

The Principle of Mass Continuity

In fluid dynamics, the continuity equation states that, in any steady state process, the rate at which mass enters a system is equal to the rate at which mass leaves the system.

The Venturi Effect

Area is inversely proportional to Velocity. Reduced cross sectional area therefore Increased Velocity

Basically: Halve the Area, Double the Velocity

What about a ‘one finger’ pinch?

This still causes a reduced area in the hose for the water to flow through, and in the reduced area of the hose, the water will still flow faster!

Measuring Venturi Effect

Velocity of airflow increases through the restriction.
Velocity after restriction is equal to velocity before the restriction.

Airflow “Velocity” through Venturi can also be referred to as the “Dynamic Pressure” of the airflow through the Venturi.

Why Venturi Effect Happens

In fluid dynamics, a fluid’s velocity must increase as it passes through a constriction in accord with the principle of mass continuity, while its static pressure must decrease in accord with the principle of conservation of mechanical energy.

Static Pressure decreases more through the restriction.
Static Pressure after restriction is equal to Static Pressure before the restriction.

Bernoulli’s Theorem

Relationship between Velocity & Pressure
Total Pressure (H) = Dynamic Pressure (q) + Static Pressure (p)
(𝐻 =𝑞 + 𝑝)

Bernoulli’s principle tells us that fast-moving air is at a lower pressure than slower-moving air (i.e. Faster Air = Lower Pressure)

So how could squirting a garden hose possibly have anything to do with how an aircraft flies?

A similar mechanism to that which causes water to squirt further when a finger is placed over the end of a hose causes one of the “types” of lift experienced by an aircraft wing in straight and level flight!


Aerofoils

The Basic Aerofoil Shape

…its all about the shape!

Cambered Aerofoil Section

The Bending of Air

The Flow of Air

The fluid on top of the wing is accelerated and the fluid on the bottom of the wind is slowed down compared to velocity of the aircraft itself because the wing geometry and angle narrows the flow area above the wing and widens the flow area below the wing. (Venturi Effect!)

How Aircraft use this to Fly

Note: For Straight and  Level flight ⟹ Lift = Weight

What is Lift with Regards to Flying?

Lift is:

  • An aerodynamic force
  • Lift opposes weight

Aerodynamic Forces

Pressure Differential

The Forces

Vector Diagram

Aerofoil Diagram

Ways of Generating Lift

Straight & Levelat altitude, straight and level flight
Ground EffectOne wingspan from the ground
Angle of AttackLanding – slower speed

Aerodynamic Effects

Spanwise Lift Distribution

Wing-Tip Vortices

http://www.ryanwaters.com/wingtip-vortices/ accessed 21 Sep 2013
http://en.wikipedia.org/wiki/File:Airplane_vortex_edit.jpg accessed 21 Sep 2013

Stalling

A stall occurs when the Angle of Attack exceeds the Critical Angle.

https://sites.google.com/site/flightsafetysystems/anti-stall accessed: 20/07/2020

There is a significant loss of Lift and increase in Drag. This is caused by the majority of airflow over the upper surface of the wing separating.

Symptoms of Approaching Stall

Level Flight – 1 G Stall:

  • Low and reducing Airspeed
  • High nose attitude
  • Buffeting
  • Stall warning device activation
  • Significant aft control column
  • Sloppy Controls: be very careful with aileron input at low speeds!

Characteristics of Stall

Level Flight – 1 G Stall

  • At Stall Speed
  • Pronounced nose drop
  • Possible wing drop:
    • Maybe caused by Aileron / Rudder use
    • Wind Gusts / Turbulence
    • Flow Strips
  • Increased Buffeting
  • Increasing Rate of Descent

Stall Recovery

Simultaneously:

  • Lower the nose
  • Full Power
  • Rudder to stop further wing drop (if any ?)
    • Do Not use Aileron (Why ?)
  • When recovered:
    • Level the wings with Aileron
    • Regain normal flight

Stall Speed

REMEMBER!

  • An Aircraft will stall when the CRITICAL ANGLE OF ATTACK is exceeded.
  • This can occur at any speed within the speed range of the aircraft.
  • G. Stall?
    • Dive recovery – Be careful