K.Haroon Site Admin
Joined: 11 Dec 2005 Posts: 1311

Posted: Tue Apr 17, 2012 3:45 pm Post subject: Questions on Thrust, Power and Drag 


Graph of "Drag against Speed"
Range speed VMR for the jet is found at the tangent to the drag curve.
The tangent from the origin makes contact at a point ahead of VMD.
Graph of "Power against Speed"
Range speed for the prop is found at the tangent to the power curve.
The bottom of the power curve is Vmp.
The tangent from the origin makes contact at the point of VMD.
For Props: Thats the point for best range.
For Jets: Thats the point for best endurance.
Consider the graphic representation of the power required versus true air speed (TAS), for a piston engined aeroplane with a given mass. When drawing the tangent from the origin, the point of contact (A) determines the speed of "Maximum Specific Range"
The tangent from the origin to the power required against true airspeed curve, for a jet aeroplane, determines the speed for: "maximum endurance"
The tangent from the origin to the power required curve for a jet aircraft gives:
a) VMD < Correct
b) the minimum power speed
The airspeed for jet aeroplanes at which "power required" is minimum:
a) is always lower than the minimum drag speed < Correct
b) is lower than the minimum drag speed in the climb and higher than the minimum drag speed in the descent
c) is always higher than the minimum drag speed
d) is the same as the minimum drag speed
The point at the bottom of the curve is Vmp. The velocity for minimum power. It is the minimum speed for straight and level flight. The tangent to the curve is Vmd. So Vmp is less than Vmd. Since it is proportional to Vmd so it is always less.
Source: (http://www.atpforum.eu/showthread.php?t=7702)
The point at which a tangent out of the origin touches the power required curve:
a) is the point where the Lift to Drag ratio is a maximum < Correct
b) is the maximum drag speed
c) is the point where drag coefficient is a minimum
d) is the point where the Lift to Drag ratio is a minimum
The bottom of the thrust required curve, which is the drag curve, is Vmd and where CL:CD is at a maximum.
The tangent to the thrust required curve,drag curve, is 1.32 Vmd which is max range speed for a jet.
The bottom of the power curve is Vmp.
The tangent to the power curve is Vmd and where CL:CD is at a maximum and the point where the prop aircraft flies for max range.
Source: (http://www.atpforum.eu/showthread.php?t=5674)
The lowest point of the drag or thrust required curve of a jet aeroplane, respectively, is the point for:
a) minimum drag < Correct
b) maximum endurance
c) maximum specific range
d) minimum specific range
Practically thats the speed for max endurance too though theoretically one comes before the other. Unfair choice for such a small difference.
How does the thrust of fixed propeller vary during takeoff run? "The thrust decreases slightly while the aeroplane speed builds up"
When does THRUST = DRAG?
a) Flying level at a constant IAS < Correct
b) Climbing at a constant IAS
c) Decreasing at a constant IAS
d) All of the above
On a reciprocating engine aeroplane, with increasing altitude at constant gross mass, angle of attack and configuration the power required:
"Increases and the TAS increases by the same percentage"
With increase in altitude Thrust decreases and TAS increases. Drag remains the same.
Power Required = Drag x TAS
Power Avaialble = Thrust x TAS
Which of the following diagrams correctly shows the movement of the power required curve with increasing altitude (H1<H2)?
The curve moves up and to the right.
(Refer to figure 032_81)Assuming constant L/D ratio, which of the diagrams provided correctly shows the movement of the THRUST REQUIRED CURVE? Mass M1 is higher than mass M2.
As speed is reduced from Vmd to Vmp: "power required decreases, drag increases"
In which of the flight conditions listed below is the thrust required (Tr) equal to the drag (D)?
a) In level flight with constant IAS < Correct
b) In a descent with constant TAS
c) In accelerated level flight
d) In a climb with constant IAS
The induced drag of an aeroplane at constant gross weight and altitude is highest at:
a) VSO (stalling speed in landing configuration) < Correct
b) VS1 (stalling speed in clean configuration)
The induced drag of an aeroplane at constant mass in unaccelerated level flight is greatest at "THE LOWEST ACHIEVABLE SPEED IN A GIVEN CONFIGURATION"
When flying an aircraft on the back of the drag curve, maintaining a slower speed (but still faster than VS) would require:
a) more thrust < Correct
b) more flap
c) less thrust due to less parasite drag
d) no change
Maximum Horizontal Speed:
Q.1. The maximum horizontal speed occurs when:
a) The maximum thrust is equal to the total drag < Marked Correct
b) The thrust is equal to the maximum drag
c) The thrust is equal to minimum drag
d) The thrust does not increase further with increasing speed
Q.2. With which conditions would the aircraft need to be flown, in order to achieve maximum speed in horizontal flight?
a) Thrust set for minimum drag
b) Best lift  drag ratio
c) Maximum thrust and maximum drag < Marked Correct
d) Maximum thrust and minimum drag
Go for the option that mentions "Total Drag". If "Total Drag" is not mentioned then select the option that says "Maximum Drag".
See the discussion about this ambiguity here
When flying the Backside of Thrust curve means:
a) a lower airspeed requires more thrust < Correct
b) the thrust required is independent of the airspeed
c) a thrust reduction results in an acceleration of the aeroplane
d) a lower airspeed requires less thrust because drag is decreased
The induced drag of an aeroplane at constant mass in unaccelerated level flight is greatest at:
a) the lowest achievable speed in a given configuration < Correct
b) VS1
VS1 is the stall speed or the minimum steady flight speed with the aeroplane in a configuration appropriate to the case under consideration.
What condition is found at the intersection of the thrust available and the drag curve?
a) Unaccelerated level flight < Correct
b) Unaccelerated flight in a climb
For a pistonengined aeroplane at a constant altitude, angle of attack and configuration, an increased weight will require: "more power and more speed"
More weight = more lift = more induced drag = drag curve shifts to the right = more speed.
Power Required = Drag x TAS. More drag = More power.
Profile drag is:
a) directly proportional to the square of the EAS < Correct
b) directly proportional to the square root of the EAS
c) inversely proportional to the square root of the EAS
d) inversely proportional to the square of the EAS
Be careful its "Square" not "Square Root"
All other factors being equal minimum drag speed is
a) proportional to weight < Correct
b) a function of pressure altitude
c) a function of density altitude
d) proportional to temperature
The thrust of a jet engine at constant RPM:
a) increases in proportion to the airspeed < Correct
b) is inversely proportional to the airspeed
c) does not change with changing altitude
d) is independent of the airspeed
Gas Turbine Engines and Jet Thrust
The intersections of the thrust available and the drag curve are the operating points of the aeroplane: "in unaccelerated level flight"
Region of REVERSED COMMAND means: "a lower airspeed requires more thrust"
Which of the following variables will not affect the shape or position of the drag vs. IAS curve, for speeds below Mcrit:
a) altitude < Correct
b) configuration
c) weight
d) aspect ratio 
