PERFORMANCE Flashcards
Cruise ceiling
Altitude at which the max rate of climb capability at max continuous power and best CLB speed is 300ft per min
Service ceiling
Altitude at which max rate of climb capability at max continuous power and best climb speed is 100ft per min
VT/VF
Specific range. Is TAS/Total FF, OR nm per pound of fuel
VT/VF MAX
The max Value of specific range for a given weight & altitude
99% VT/VF MAX
The value of specific range resulting from increasing TAS from the speed at VT/VF MAX, to give a value of specific range which is 1% less than VT/VF max
Long range cruise
Term that identifies flight at the speed of 99% VT/VF MAX
VH
Structurally limited max level flight speed (recommended limit speed)
Normal bleed
When compressor bleed air is used to operate the aircon & pressurisation system
(800”lbs extra TQ)
All bleed
When compressor bleed air is used to operate the engine & wing/tail surface anti ice systems as well as the normal bleed functions
(extra 1200”lbs)
Drag index
that is proportional to drag change asssociated with a change in aircraft configuration
What advantage do props have over jet engines
advantage in power/weight ratio over jet engines at speeds up to about 350 k
What is the general power utilization of a turboprop
Over 90% of the energy is extracted by the turbines to drive the compressor(s) and the propeller; the remaining energy appears in the jet exhaust as thrust
What are the four factors affecting power output AND specific fuel consumption
- Forward speed
- Temperature
- RPM
- altitude
How does temperature affect power output?
An increase in intake temperature = an expansion of air particles. This will reduce power output because the mass flow through the engine will be reduced due to reduced air density.
How does altitude affect power output
At a constant TAS, power output falls with increase in altitude due to the decrease in air density. This is mitigated however by a decrease in air temp = increased air density
How does forward speed affect power output?
As forward speed increases, ram effect increases air pressure at the intake, increasing power output (up to about 400kts)
Define specific fuel consumption
The measurement of fuel used/hr/kW (power)
How does RPM affect SFC
Turboprop engines are designed to operate most efficiently at Max continuous RPM. For the minimum (best) SFC, engines should be operated at this at all times
How does temperature affect SFC
SFC improves in cold air because of the increased thermal efficiency of the engine.
How does altitude affect SFC
No direct effect as fuel control unit will reduce fuel consumption as air density decreases.
How does forward speed affect SFC
Shaft power increases with speed because of the improved mass flow through the engine. The GFC will be higher overall.
Summarise the best configuration for minimum SFC (max efficiency)
a. At MCrpm.
b. At high altitude (better thermal efficiency in colder air).
c. At a TAS between 300 kt and 400 kt, so that advantage may be taken of ram effect while the
propeller efficiency is still high.
define specific air range
defined as air distance travelled per unit quantity of fuel
Power required is a product of?
TAS and drag
Where does the maximum TAS/power ratio occur?
At the minimum drag speed (Vimd)
What is power required a product of
Drag & TAS
How does altitude affect TAS/power ratio
As altitude increases, TAS increases, however the power required also increases. Thus the ratio is unaffected
How does wind affect TAS/power ratio
Should be a ground speed/power ratio
Tail wind: a speed less than Vimd can be flown
Head wind: a speed more than Vimd should be flown
What are the two reasons aircraft should be flown at a speed slightly more than Vimd
- The variation of TAS/power at speeds near VIMD is negligible and a slightly higher speed will allow a faster flight without undue loss of range.
- When flying at VIMD, any turbulence or manoeuvres will cause a loss of lift and height. which can only be regained by the application of more power and consequent fuel wastage.
Describe the conflict when flying for endurance between flying for airframe and flying for engine efficiency
- Airframe demands least power at minimum power speed,
plus about 10 kt for control, at as low an altitude as possible = minimum fuel flow. - Flying for engine efficiency: engines are most efficient at MCrpm,at a high altitude (cold temperature) and a high TAS (around 300 kt).
How to fly the aircraft for both max endurance AND engine efficiency
At a medium altitude where an airframe speed in the region of VIMD is maintained by throttling the engines
How to achieve maximum range in a turboprop
- cruise climb using MCrpm and an IAS slightly above VIMD.
During the cruise climb, SAR improves as both drag and SFC fall.
What do temps colder than ISA cause (3)
(1) The aircraft to fly higher.
(2) A slight increase in TAS.
(3) An increase in SAR.
How to achieve Maximum endurance in a turboprop
achieved at a medium altitude where the engines are throttled to produce an IAS of about VIMD.
STEP-CLIMB cruise profile (process)
Recommended step is 2000ft
1) determine AUW
2) determine cruise ceiling
3) Select desired alt below cruise ceiling
4) Use range summary charts to get cruise speed, FF & TQ
5) Set recommended power & adjust as AUW decreases.
what to do if HWC is >70kts
increase TAS by 4kts for every 10kts above 70.
What is driftdown
due to failure of 1 or 2 engines, it may be necessary due to loss of power, to descend to a lower alt.
What is the driftdown procedure
1) maintain the recommended driftdown airspeed until ROD decreases to 100FPM.
2) maintain 100FPM ROD until reaching service ceiling altitude (= max height for terrain clearance with some sacrifice in range)
OR until reaching cruise ceiling.
What are the 5 climb speeds & when would you use them
1) Normal speeds (not international & fuel not critical, good to reduce floor angle)
2) Four engine climb speeds: when a/c is climbing to cruise alt/for oceanic legs
3) High speed climb (medium heights/is light - 210kts)
4) Max angle
5) Best RoC
When is a climb possible?
- Once the engines have been throttled back to 930 deg
- Or if employing 970deg cruise, once TAS has increased to 304kts
Cruise climb procedure?
- Once at cruise ceiling, request a block from ATC
- Set 970deg or 1010 & maintain LRC speed
- Accept a constant climb
Long range cruise - when would you use it?
- when fuel becomes a consideration
When using LRC speesds, when would a setting of 1010 be justified
If large headwinds dictate a large correction to the LRC speed, 1010 will be more efficient.
When would a reduction in LRC speeds be required
in Tail winds in excess of 100kts
LRC procedure
1) determine initial cruise ceiling with current AUW
2) Use specific range graph with current AUW & correct altitude
3) Determine TAS to fly using LRC on graph
What are the 6 TOLD GECs
- Full TQ?
- Vmcg dry?
- Vmcg wet
- Vrot limited by?
-Vmcg IGE/OGE?
- Dump time rqd?
- full TQ is available up to & incl 21deg C
- vmcg dry should always be in the 80s
- vmcg wet is +/- 18kts faster
-Vrot is limited by Vmca IGE at weights <120k, limited by Vto >120k - Vmcg IGE is always. 6-7kts less than Vmca OGE
-Dump time required when >120k lbs & more than 10,000lbs of fuel
Max endurance definition
Speed that results in min FF for a given weight, altitude and temp.
How would you prove LRC formula using the specific range chart
LRC SAR/MAX SAR = 99%
Relationship between weight & Vimd
Vimd is proportional to AUW, as weight decreases, power must be reduced to fly best range speed
Why do we set 300KTAS as a general rule of thumb?
- As this is a ballpark number that is the closest to LRC for most given weights and altitudes.
- Usually works out a bit faster than our actual LRC, with little to no effect on fuel consumption.
If our TAS is 290, and we are struggling to get to 300KTAS, are we underperforming?
- No, LRC is generally around 285-295KTAS, so we are actually more efficient at this speed than we would be at 300KTAS. Use book to find “top of curve” if rqd
- Only if time is an issue would you necessarily need to look at speeding up.
Considerations when climbing above F300:
- RVSM asx: can CLB into, however ATC must be told “negative RVSM” and will then provide 2000ft vertical sep rather than standard 1000ft for RVSM equipped a/c
- OXY: above FL300, all crew must have access to quick don oxy masks (Must have <10 POB as only 10 regulators are available)