LESSON 7 Chapter 6 Basic Jet Performance ANA Chapter 2

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Flight Theory

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.
  • About 250kts

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
  • About 330kts

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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