AVIA 222 Advanced Flight Operations
Today’s Topics • Mach Numbers, Speed of Sound • Weight and Balance • V Speeds
High Speed Flight • Speed of Sound… (S) • The speed of sound is the speed at which the pressure waves (sound) move through the air. • Temperature is the controlling factor, not altitude, or density • Or as the know-it-all people at the NASA Glenn Research Center like to explain it… • Air is a gas, and a very important property of any gas is the speed of sound through the gas. Why are we interested in the speed of sound? The speed of "sound" is actually the speed of transmission of a small disturbance through a medium. (Sound itself is a sensation created in the human brain in response to sensory inputs from the inner ear. The transmission of a small disturbance through a gas is an isentropic process. The conditions in the gas are the same before and after the disturbance passes through.
Cold Temperatures • The speed of sound at sea level on a standard atmosphere day (+15C) is 660kts. • S @ -40C = 594 kts • S @ -60C = 575 kts
The speed of sound for any temperature can be calculated with the following formula: S = 39 X Temp K • S = speed of sound in kts • K = temp in Kelvin (C + 273) Example: What is the speed of sound when the outside air temperature is 32 C? S = 39 X 305 S = 39 X 17.46 S = 681 kts.
Note: Most flight computers will also calculate for speed of sound by turning the Mach index to the outside temperature in the TAS window. The speed of sound for that temperature is read on the outer scale opposite the 10.
Mach # • The mach number is the ratio of TAS to the speed of sound at that temperature. Review…TAS is Calibrated Airspeed corrected for Altitude and Temperature CAS is corrected for instrument and position error, but at cruise speed and higher altitudes is basically the same as IAS Mach # = TAS S
NASA - GRC • As an object moves through the air, the air is disturbed. The disturbances are isentropically transmitted through the air at the speed of sound. You can study how these disturbances are transmitted with an interactive sound wave simulator. If the object moves much slower than the speed of sound, conditions are said to be subsonic, and compressibility effects are small and can be neglected. If the object moves near the speed of sound, conditions are said to be transonic, and compressibility effects like flow choking become very important. For speeds greater than the speed of sound, the conditions are said to be supersonic, and compressibility effects are important. If the object moves more than five times the speed of sound, conditions are said to be hypersonic, and the high energy involved under these conditions has significant effects on the air itself. Depending on the specific shape and speed of the object, shock waves may be produced in the supersonic flow of a gas. Shock waves are very narrow regions in the flow where the flow conditions are irreversible. The isentropic flow relations can not be used across a shock wave, although they can be used on either side of the shock wave. The important parameter in each of these flow regimes is the ratio of the speed of the object to the speed of sound. Aeronautical engineers call this ratio the Mach number. A Mach number less than one indicates subsonic flow, a Mach number near one is transonic, and a Mach number greater than one is supersonic or hypersonic.
Subsonic airflow: no speeds above the speed of sound are produced by any part of the airframe • Transonic: The dividing range between subsonic and supersonic (mach 1) • Supersonic: Speed above mach 1
When the pressure waves ahead start to be compressed by the supersonic a/c, a sonic boom is triggered.
Link to Mach…and S of S. Check out this web site if you do not understand the speed of sound or mach number stuff. Or check it out if you want to know more: http://www.grc.nasa.gov/WWW/K-12/airplane/mach.html
What does this mean? • This means that aircraft reporting there speed in Mach is always referencing a valid (corrected) index to the speed of the aircraft……still with me? • This eliminates the need to correct the IAS with temperature and pressure (pressure altitude) to find the TAS.
Machmeter • An old style mach gauge used for an aircraft that does not exceed the speed of sound.
Critical Mach Number (Mcrit) • This is the highest speed at which a specific aircraft can fly at without creating supersonic flow over any surface airframe parts. • Above this speed a shock wave starts to form over parts of the airframe. As the speed increases the shock wave grows from covering parts of the upper airframe to covering the entire upper and lower surfaces.
Drag Rise or Drag Diversion: At about 5% above Mcrit the shock wave begins to form over the upper surface of the wings and starts to cause separation of the boundary layer of air thus resulting in a sudden increase of DRAG. This condition can shift the center of pressure (lift) suddenly forward, and result in loss of control or even airframe breakup. • The exact speeds that this occurs depends on airfoil design.
Maximum Mach Operations (Mmo) • As a result of Mcrit, Mmo is set at a speed to avoid undesirable flight characteristics and avoid speeds that produce excess drag (increased fuel consumption).
Mach Buffet • Buffeting occurs when the shock wave on the top of the wings begins to create turbulence that can disturb the airflow over the tail surfaces. At higher speeds the elevator can become ineffective. This has be the cause of more than a few crashes from learjets to p-51s when the a/c have been pushed too close to the sound barrier.
Sweepback • To help rise the Mmo to higher speed the wings are designed with sweepback to help minimize the portion of the wings that is starting to produce drag rise/drag diversion.
Various aircraft with different degrees of wing sweepback. In general the more sweepback the less lift that is generated. Most cargo, well subsonic a/c, will have next to no sweepback.
Note the sharp sweepback of the only passenger plane that cruised above the sound barrier…Mach 2.02 was the typical cruise at about 50,000ft.
Dutch Roll Anyone? • An undesirable effect of sweepback on a wing is the tendency to produce a “dutch roll”. This is a yawing and rolling movement produced when an advancing wing produces more lift than the retreating wing, but then the lift is offset by the vertical stabilizer thus resulting in a oscillation. • To correct a dutch roll use aileron input only or yaw dampener
MAC • Mean Aerodynamic Chord: the average width of the wing from the Leading Edge to the Training Edge • LEMAC represents 0% of the MAC • TEMAC represents 100% of the MAC
Weight and Balance • For larger aircraft with many varieties of different loading configurations the weight and balance is considerably more complex than what we do with our training aircraft. • C of G can be expressed as inches aft of the reference datum, or as a % of the MAC. • C of G limits would be something such as between 12% and 47% of the MAC.
To determine % MAC from C of G ARM: % MAC = ARM” – LEMAC” X 100 MAC”
Aircraft Weights • Empty Operating Weight (EOW): basic aircraft weight with unusable fluids • Basic Operation Weight (BOW): The EOW plus crew and their baggage, oil, fluids, equipment. • Zero Fuel Weight (ZFW): BOW plus PAYLOAD (passengers, baggage, cargo) • Landing Gross Weight (LGW): ZFW plus reserve and alternate fuel. • Take Off Gross Weight (TOGW): ZFW plus enroute fuel. • Gross Weight for Taxi (GWFT): TOGW plus run-up and taxi fuel.
Payload Payload = ZFW – BOW (zero fuel weight less the basic empty weight) • The payload is what the carrier gets paid for…passengers, baggage or cargo.
Payload Example • Determine payload where: • total fuel = 2,000 lbs, • crew and baggage = 600 lbs, • ZFW = 10,000lbs • EOW = 6,000lbs • EOW + Crew = BOW (6,600lbs) • ZFW – 6,600lbs = 3,400lbs.
REPOSITIONING OF THE C of G • To shift weight from one location to another to get within the C of G limits use this formula: wt = d WT D • wt is the weight to be moved in lbs • WT is the gross weight of the a/c in lbs • d is the distance the C of G moves in inches • D is the distance the item moves in inches
Moving the C of G Example: • An aircraft has a gross weight of 15,000 lbs with a CG 2” aft of the max. aft limit allowed. • What is the minimum amount of weight that can be moved from its current location of 320” to the fwd compartment at 170” ? wt = 2 (distance the CG has to move) 15,00 150 (station 320 – station 170) Answer: 200lbs is the minimum amount that must be moved fwd to the front compartment to get into limits.
Mid Term Exam Next Week!! • Use the presentations slides for subject reference. • Also use your Turbine Pilots Flight Manual