Performance and Limitations Flashcards
As air becomes less dense, aircraft performance is reduced among what 3 factors?
Reduced power since engine takes in less air
Reduced thrust since propeller is less efficient in thin air
Reduced lift since thin air has less force on airfoil
Standard day at sea level (C, F, Hg, Mbs)
15C / 59F
29.92Hg
1013.12 Millibars
What is the standard temperature lapse rate?
What altitude is it accurate to?
2C / 1000 feet
36,000 feet
What is the standard pressure lapse rate?
It is accurate to what altitude?
1 in Hg / 1000ft
to 10,000 feet
Why is it important to understand pressure altitude?
To determine aircraft performance
How can you determine pressure altitude?
- Setting Baro to 29.92 and reading indicated altitude
- Applying correction factor to indicated altitude according to reported altimeter
- Using a flight computer
Define density altitude
Pressure altitude corrected for non-standard temperature
What type of altitude determines how the plane performs?
Density altitude
What factors affect changes in air density?
- altitude
- temperature
- humidity
How is density of air and temperature related?
Inversely - meaning as temperature increases, air density decreases.
Increase in humidity causes a ______ in air density
decrease
What is Relative Humidity?
A measurement of how much water vapor is in the air compared to how much the air can hold at a given temperature. It’s expressed as a percentage.
Can warm air or cold air hold more water vapor?
Warm air can hold more water vapor
Is water vapor heavier or lighter than air vapor?
Lighter.
A water molecule is lighter than a nitrogen or oxygen molecule.
Humid air is lighter: Because of this, humid air (air with water vapor) is lighter than dry air.
When landing at high elevation airports the IAS for approach and stall is the same. What is different?
TAS is faster
Ground Speed is faster
Longer approach, longer rollout
Greater distance to clear obstacles
Longer runway needed
What are the main aircraft performance characteristics?
Takeoff and Landing Distance
Rate of Climb
Ceiling
Payload
Range
Speeds
Maneuverability
Fuel Economy
What is Induced Drag?
How does Induced Drag vary?
Induced drag is the total drag created by the production of lift.
As airspeed increases, induced drag decreases.
More lift = more drag.
At a lower airspeed, a higher AOA is required. This equates to greater induced drag.
What is Parasite Drag?
How does Parasite drag vary?
Parasite drag is drag from the friction of air as it moves over the aircraft structure.
Parasite drag is broken into three types:
- Skin Friction Drag: Due to exposed rivets and how the flaps, rudders, and ailerons are reinforced.
- Form Drag: Caused by the aerodynamic shape of the airplane disrupting the air it moves through.
- Interference Drag: Caused by structures attached to the fuselage and wing, like the juncture of the wing and fuselage.
As airspeed increases, parasite drag increases.
If you double the airspeed, what happens to parasite and induced drag?
Parasite drag becomes 4x greater (8x power needed)
Induced drag becomes 1/4 of original value (1/2 power needed)
Define Service Ceiling.
Can service ceiling always be reached?
The altitude where an aircraft can no longer climb greater than 100 FPM.
No. Service ceiling may not be reached due to actual density altitude.
Climb performance is a result of what two energy types?
Excess power above that required for level flight.
The aircraft’s kinetic energy (trading airspeed for climb)
Define Absolute Ceiling.
The altitude where climb is no longer possible
What is the Power Loading formula?
What units is it expressed in?
Gross weight of the plane divided by HP of the engine.
lbs/hp
What is the Wing Loading formula
What units is it expressed in?
Total mass or weight of plane, divided by the wing area (includes ailerons).
lbs/sq. feet
Why is power loading important?
Why is wing loading important?
Power loading is important for takeoff and climb capability.
Wing loading is important for determining the landing speed.
Define maximum range.
At what point is maximum range obtained?
The maximum distance flyable with a given fuel supply.
L/D max (maximum Lift to Drag ratio)
Define maximum endurance.
At what point is maximum endurance obtained?
The maximum time flyable with a given fuel supply.
Minimum power (lowest fuel flow)
Ground effect is caused by what change?
Where does ground affect occur?
Interference of airflow around the wing, caused by the ground.
Ground effect occurs one wingspan above the surface.
A wing in ground effect has reduced upwash, downwash and wingtip vortices. What does reduced wingtip vortices cause?
What happens at 1/4 and 1/10 of the wingspan above the ground?
Reduced wingtip vortices reduces induced drag
1/4 span of wing above ground induced drag is reduced by 25%
1/10 span of wing above ground induced drag is reduced by 50%
Ground effect causes what problem in landing and what problems in takeoff?
Landing problem - excess speed may result in excess float due to ground effect.
Takeoff problem - aircraft may takeoff below recommended speed, but out of ground effect induced drag may mean the aircraft isn’t capable of flight and could descend back to ground.
What is “flight in the region of reversed command?”
(also known as the back end of the power curve)
Higher airspeed requires lower power setting to hold altitude
Lower airspeed requires higher power setting to hold altitude
A decrease in airspeed must be accompanied by an increased power setting to maintain steady flight.
Where may an airplane encounter the region of reverse command? (2)
- A low airspeed high-pitch attitude powered approach for a short field landing
- Soft-field takeoff and climb while attempting to climb out of ground effect with too high of a pitch
What runway surface factors affect performance?
Non-hard surface and unsmooth surfaces increase ground roll.
Muddy + Wet surfaces reduce friction and act as obstructions to reduce landing distance
Runway gradient or slope
Braking effectiveness is defined as:
The amount of power that is applied to the brakes without skidding the tires
Takeoff and Landing performance is affected by:
- air density
- surface wind
- runway surface
- runway gradient (slope)
- weight
- powerplant and thrust
What does increased density altitude cause on takeoff and landings?
- increased takeoff distance (greater TAS required)
- reduced rate of climb
- increased TAS on approach and landing
- increased landing distance roll
What calculation is made before a flight prior to every takeoff in regards to a rejected takeoff?
The point along the runway at which the airplane should be airborne or discontinue the takeoff (need remaining runway using normal measures to safely stop aircraft)
Why are there various manifold pressure and propeller settings for a given power output?
So pilot can choose between best efficiency and best power for given conditions.
A higher RPM causes what issue regarding oil and fuel?
More friction and more fuel
What happens to the manifold pressure of a non-turbo aircraft operating at higher altitudes?
Constant power output will decline due a drop in manifold pressure
What is brake horsepower (BHP)?
The power delivered to the propeller shaft of an aircraft engine. It’s the percentage of normally rated power available at sea level on standard day.
When do you lean a normally-aspirated direct-drive engine?
- lean anytime power is 75% or less
- at high altitudes
- at high-altitude airports lean for taxi/takeoff/pattern/landing
List 3 methods for leaning aircraft engines
Tachometer Method
Fuel Flowmeter Method
Exhaust Gas Temperature Method
Describe the Tachometer Method of leaning an engine
For best economy - lean from full rich to maximum power (peak RPM).
Then lean until engine starts to run rough and enrich mixture slightly until engine runs smoothly.
Describe the Fuel Flowmeter Method of leaning an engine
Use the fuel flowmeter to lean the mixture to the POH marked fuel flow
Describe the Exhaust Gas Temperature Method of leaning an engine
Lean slowly to peak EGT then enrich it 50 degrees rich (cooler) of peak EGT
What is the CG equation?
CG = total moments / total weight
Weight x arm = moment
What does an Aft CG cause (5 things)?
- Lower stall speed (due to less wing loading)
- Higher cruise speed (due to less drag)
- Less stability (spin and stall recovery is difficult)
- Less fuel burn (from less drag)
- Increased range
What is the CG change equation?
(Weight shifted X distance moved) / aircraft gross weight = CG change
Assumed weight (if unknown) for
Crew + Pax
Gas
Oil
Water
Crew + Pax = 190lbs
Gas = 6lbs/gal
Oil = 7.5 lbs/gal
Water = 8.35 lbs/gal
3 ways to control lift during flight
- AoA
- airspeed
- flaps
Performance chart data assumes what important items (which may not exist)?
Paved, level smooth, dry runway surface.
Age of aircraft, pilot skill and proper technique.
Where does Maximum ROC (Vy) and Maximum AOC (Vx) converge?
At the aircraft’s maximum ceiling.
Does horizontal stabilizer produce lift?
Yes, a horizontal stabilizer produces lift downwards.
Thrust hits top of horizontal stabilizer. With less thrust there is less lift on the wings and less downforce on horizontal stabilizer.
What could cause the CG to move aft during flight?
- fuel burn
- passenger movement
What happens to elevator effectiveness while in ground effect?
The elevator is much less effective in ground effect
Define payload
The total weight of everything an aircraft carries that’s not part of its structure or fuel.
Includes passengers, cargo, baggage, and any other items necessary for the flight’s purpose.
Define useful load
The maximum amount of weight the aircraft can carry in Payload (people/crew, baggage, cargo) along with useable fuel and oil.
Define basic operating weight
The weight of an aircraft ready for flight, excluding usable fuel and payload (cargo, passengers, etc). It includes the aircraft’s empty weight plus items like crew, unusable fuel, and fluids necessary for operation.
Define empty weight
The weight of an aircraft with all its fixed equipment, including unusable fuel, undrainable oil, and all operating fluids, but without any crew, passengers, cargo, or usable fuel.
Does warm or cold air hold more water vapor?
Warm
What is the definition of the term “relative humidity”?
The percentage of the maximum amount of water that a sample of air can hold.
What effect does humidity have on air density?
Water vapor is lighter than air, so moist air is lighter than dry air.
As the water content of the air increases, the air becomes less dense, increasing density altitude (lowering density) and decreasing performance.
Since temperature and pressure decrease with altitude, how will air density be affected overall?
The decrease in temperature and pressure have conflicting effects on density as you go up in altitude, but the fairly rapid drop in pressure with increasing altitude is usually the dominating force. Hence, the density is likely to decrease with altitude gain.
What effect does temperature have on air density?
Air density and temperature are inversely proportional
(i.e. higher temperatures lead to lower air density)
What effect does atmosphere pressure have on air density?
Air density is directly proportional to pressure. If air pressure gets lower, density gets lower which will yield a higher density altitude.
What factors affect air density?
Changes in altitude, temperature, and humidity. High density altitude refers to thin air while low density altitude refers to dense air.
How is density altitude determined?
Find pressure altitude, and then correct it for nonstandard temperature variations.
