Jet Aircraft Applied Performance
Variations in the thrust required curve
- The importance, Nea the necessity, of analyzing thrust required curves under certain conditions are the basis of all phases of performance.
- These curves define the requirements of the airplane and must be considered when deciding which power plant to hang on the plane.
Weight pg 99 navweps
- Changing the weight influences the parasite drag only a minimal amount because of changes in surface area of engine nacelles fuel pods ect. Graph
- Induced drag changes significantly because of the higher AOA needed to generate the extra lift required.
- This causes the thrust required curve to move greatest in the lower region.
Effect of a Weight Change on the Tr Curve
–More AOA for a given airspeed
–More lift required
–More DI produced
–More thrust required
- Note that the curves’ greatest difference is present in the lower velocity region of the curve
- This represents the increase in DI
Configuration Changes pg 101 navweps
- The change in configuration causes a change in the L/Dmax curve as well as forcing the thrust required curve up and to the left.
- This could be thought of as being the opposite to the effect of weight because the increase in parasite drag will effect the curve most in the higher region.
Configuration Change (Overall)
Altitude Changes pg101 navweps
- This is useful for understanding how range and endurance is effected by changes in altitude.
- Lets say we have an airplane with a min drag of 1250lbs at 160kts
- If operated at the same q or 160kts at 22,000 ft, you would have a true airspeed of 227kts.
- This would then cause the thrust required curve to flatten out and move to the direction of higher velocity.
- Why? Because the curves are plotted using TAS
- What conclusion can you draw about Min Drag?
Tr and Ta for Altitude
- The airplane at 22,000 feet has the same drag as the one at sea level if operated at the same q (160kts)
- However, the TAS at 22,000 is 227 kts
- This extra speed then flattens the curve and moves it in the direction of higher velocity
- This also increases the range of the aircraft because of higher speed (note the tangent lines on the curve 0 wind)
- Minimum drag does not change with altitude
- But the TAS for Min drag does change
- L/D max does not change with altitude, remember the curves are in True so if the curves were plotted in IAS you would see the same L/D max
Altitude Effect on the Thrust Curve
Go Over page 102-103 Dole
Read over these items
- Remember SR is nm/lbs of fuel
- As fuel is burned the weight becomes less
- To maintain the correct amount of thrust as weight is reduced, reduce throttle settings to lower thrust and a lower airspeed.
- Speed and thrust decrease as weight decreases to get max range
- The drag curve is the thrust required curve because it take 1lb of thrust for 1lb of drag
- Thrust available from a jet engine is less at altitude because of density
- Thrust required to overcome induced drag varies inversely with V2
- For a jet 4 items occur at L/Dmax:
- Max range and endurance occur at higher altitudes because:
–Fuel flow is less
–Lower temp increases efficiency
–Reduced ct from ops at higher rpm
–Higher tangent point on the TR Curve
- In a climb thrust must overcome both drag and the aft component of weight
- To achieve max range as fuel is burned, reduce power
- Best angle of climb Vx is a function of excess thrust
- Best rate of climb Vy is a function of excess power
- If the weight of the jet is increased through in-flight refueling, the TR curve moves up and to the right
- If you lower the landing gear on a jet, parasite drag becomes more than induced drag in most cases