# LESSON 7 Chapter 6 Basic Jet Performance ANA Chapter 2

Chapter 6

Jet Aircraft Basic Performance

X-29

• 1984 to 1992
• Designed to prove and test the forward swept wing concept
• Swept back wings stall at the tips first, but with a forward swept wing, the wing root stalls first
• Gave control up to 45° α
• Composites allowed this wing to be built
• Supercritical wing
• Unstable design, a computer using 40 adjustments a sec is needed to fly it
• Max speed Mach 1.6

The Curves

• When studying the curves in this chapter look for some key points:
• –Max level flight velocity
• –Max climb angle
• –Velocity for max climb angle
• –Max rate of climb
• –Velocity for max rate of climb
• –Velocity for max endurance
• –Velocity for max range

Thrust vs Power

• There is a difference between aircraft that produce thrust and ones that produce power.
• The turbojet, fanjet, ramjet, scramjet and rocket are examples of thrust producing power plants.
• Thrust is measured in pounds.
• Fuel burn is proportional to thrust
• Which in turn affects range and endurance

Thrust vs Power

• The piston engine and turbo prop are examples of power producing aircraft.
• Power is measured in horsepower.
• Performance considerations are then based on the amount of thrust or the amount of power respectively.

Thrust vs Power

• For example, fuel flow for a turbofan engine would be related to thrust whereas the fuel flow for a piston would be related to power
• Each pound of drag requires a pound of thrust to offset
• Thrust Required
• Since 1lb of thrust is required to offset 1lb of drag, we may use the Total drag curve as a thrust required (TR) curve for jets

Propulsion

• Newton’s 2nd law an unbalanced force acting on a mass will accelerate the mass in the direction of the force (F=ma)
• Newton’s 3rd law for every action there is a opposite and equal reaction
• These apply to the jet engine

Propulsion

• Air goes in the engine, is accelerated by expansion and comes out the other end faster than it went in.
• The result is thrust
• The equation explains thrust available, mass airflow, inlet velocity, exit velocity:
• Where:
• TA= Thrust Available
• Q=Mass Airflow
• V1=Inlet V in fps
• V2=Outlet V in fps

Propulsion

• Thrust can be increased by 2 ways:
• 1. increasing the airflow, which means a bigger hole, which causes more drag in the early jet engines
• 2. increasing the exit velocity with respect to flight velocity.
• –This method reduces efficiency because of all the kinetic energy wasted in the exhaust stream.

Propulsion

• Propulsive efficiency is ηp
• The equation is therefor:
• Where η=eta

Propulsion

• Turbojet and turbofan engines were then developed to increase efficiency
• This allowed the engine to develop more ability to handle higher values of Q. (mass airflow)
• The high bypass turbofan also has the advantage of being much quieter.

Specific Fuel Consumption pg 117 navweps

• SFC or ct is the fuel flow in lbs/hr per pound of thrust.
• The equation is:
• This is a measure of the efficiency of an engine.
• Lower values of ct are most desirable suggesting less fuel per lb of thrust
• So said another way, ct is an expression of how much fuel it takes to get a pound of thrust

Specific Fuel Consumption

• The minimum specific fuel consumption is obtained at relatively high power settings and high altitudes.
• The lowest values of ct are achieved between 95% to 100% rpm
• The lowest inlet air temperature reduces the ct.
• This is because when the inlet air temperature is lowered a given heat addition can provide relatively greater changes in pressure or volume.

Specific Fuel Consumption

• Generally for the jet engine, ct decreases steadily with altitude until the tropopause is reached where the value is approximately 80% of the sea level value.
• The lowest values of ct are obtained near altitudes of the tropopause where the temp levels out
• Above the tropopause, ct values can actually increase because of the density decrease with no corresponding temp decrease

Total Fuel Flow

• Total fuel flow is:
• Where T is thrust
• ct is specific fuel consumption
• Thrust varies with rpm and altitude and ct also varies with rpm and altitude

Thrust Required Curve

• At stall, drag is about 2000lbs
• At 485kts drag is also about 2000lbs
• Dmin occurs at L/Dmax about 240kts
• Note the sharp increase about 600kts
• Mach 1 is 661.5kts sea level standard day

Thrust Required Curve

• Jet engines are most efficient at high rpm
• Note the difference in TA from 95% to 100%
• This results in a speed increase of more than 100kts
• Vmax occurs at the intersection of TA and TR or about 605kts TAS

Thrust available and Thrust required

• If thrust available is equal to the thrust required, the plane can fly straight and level but cannot accelerate or climb.
• This is because drag and thrust are balanced

Thrust available and Thrust required

• This happens where the thrust available line intersects the thrust required curve.
• This can happen at two places in the curve
• –one at high speed or Vmax and
• –one at the low side near stall (back side of the power curve).
• If the thrust were reduced below 830lbs the plane will not be able to hold altitude
• This occurs at the plane’s absolute ceiling which happens at about 47,500 feet for the T-38

Other Facts About Thrust

• Thrust is reduced with altitude
• Mass flow through the engine decreases with an increase in altitude
• This occurs because density is less at higher altitudes
• Other Facts About Thrust
• Temperature also effects thrust
• Lower temps at altitude improve efficiency
• This helps offset the density decrease
• It also helps keep the temps down inside the engine allowing higher fuel flows

Climbs

• Two types of climbs:
• 1.  zoom climb is where the pilot levels off to build airspeed.
• Used for setting climb records, fighter interceptor and really looking cool.
• F-16 weighs 30,000 lbs
• Thrust in after burner is 29,100

Climbs

• 2.  Steady velocity climb vx, vy and cruise climb.

Forces in a Climb

• Lift is less than weight in a climb.
• The thrust can be broken into two vectors one of which is a vertical component.
• This is what offsets the value of lift that is less than weight.

Forces in a Climb

• The steeper the climb angle the less the lift vector supports and the more the thrust vector must support.
• A vertical climb then would require no lift but thrust would have to equal the weight of the airplane.

Vx

• The maximum climb angle or Vx will occur at the point at which there is the biggest difference in thrust available and thrust required.
• For a jet this occurs at L/Dmax

Vx

• Dole points out that in real life, headwind or tailwind will effect climb angle.
• Also a slower airspeed may have to be used because of the time required to reach the L/Dmax velocity.

Vx and Vy

• The difference in Vx and Vy is illustrated in the figure
• The formula for rate of climb is:
• It is not so easy to get best rate.
• The best rate of climb depends upon velocity and excess thrust.

Vx and Vy

• For Vy, figure your rate of climb using the RC formula for different airspeeds and plot a curve.
• The top of the curve will give the best rate of climb.
• By the way this style of graph is called a hodograph

Endurance

• The amount of time an aircraft can remain airborne.
• This is independent of wind however, turbulence will tend to decrease endurance.
• So where does max endurance occur for a jet?

Endurance

• Max endurance occurs for a jet at Max L/D
• Specific endurance is time per lb. of fuel
• This is because this is where the minimum thrust required is located
• Min thrust required means the lowest fuel flow.

Endurance

• Find the bottom of the Curve, drop straight down.

Specific Range

• Specific range is distance in nautical miles per lb. of fuel
• Max range is the max distance the plane can fly on a given amount of fuel.
• The formula for specific range is:
• Max specific range will not occur at max L/D but at a point where the ratio of the square root of CL to CD is at a max value

Specific Range

• So where the hell is that?
• Assuming no wind, draw a Tangent to the curve, drop down

Wind

• Wind effects the Spec Range.
• A head wind will decrease it and a tailwind will increase it.
• To minimize these effects, a faster speed should be flown into the wind and a slower speed with the wind.

Wind

• Your book shows how to do it by plotting a tangent to the thrust required curve.
• As weight decreases because of fuel burn specific range increases
• Less AOA to maintain the same altitude thus less drag thus less thrust required.
• Account for wind by adjusting your starting tangent baseline
• Headwinds are positive
• Tailwinds are negative
• Effects of wind on SR

Fly 290kts for Tailwind, 320kts for Headwind

Total Range

• This depends on fuel available and specific range.
• Because specific range is a variable and fuel is not, these two together make up total range.
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