## Chapter 6

Jet Aircraft Basic Performance

## Thrust vs Power

There is then a difference between aircraft that produce thrust and ones that produce power.

The turbojet, fanjet, ramjet, and rocket are examples of thrust producing power plants.

Thrust is measured in pounds.

## 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

## The Power, Thrust, Velocity Thing

The power required for flight depends on the thrust required and flight velocity.

The formula that relates power required to thrust required to velocity is:

Remember, the drag curve is really a thrust required curve

## The Power, Thrust, Velocity Thing

So by this equation it takes 1 hp to offset 1 lb of drag at 325kts

at 650kts it takes 2 hp to offset 1 lb of drag

at 162.5 kts it takes 1/2 hp to offset 1 lb of drag

## 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 ηp equation is therefor:

Where η=eta

## Propulsion

Turbojet and turbofan engines were then developed.

This allowed the engine to develop more ability to handle higher values of Q.

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 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

## Specific Fuel Consumption

The minimum specific fuel consumption is obtained at relatively high power setting an high altitudes.

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 ct are obtained near altitudes of 25,000 to 35,000 feet for the turboprop.

## 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 curve 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).

## 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 pg87 Dole

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 on page 89

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

You basically have to 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.

See page 90 Dole

## 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 is that?

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

Dole 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.

## 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.