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.