(L/D)max vs. minimum total drag
01-06-2021 | 01:47 PM
#11
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From: 747 Captain (Ret,)
Harry Hurt did not make many mistakes when writing Aerodynamics for Naval Aviators, but he sure blew this one.
His chart clearly confuses min drag for L/D max.
I have also seen other publications which make the same mistake, possibly after reading Harry's book.
Note: While the min drag speed is theoretically the optimum holding speed, no human pilot, and very few autothrottle systems can actually control speed that well. For this reason, many aircraft flight manuals chart a slightly higher speed for optimum holding speed. For some aircraft, this slightly higher speed might actually be L/D max.
Joe
His chart clearly confuses min drag for L/D max.
I have also seen other publications which make the same mistake, possibly after reading Harry's book.
Note: While the min drag speed is theoretically the optimum holding speed, no human pilot, and very few autothrottle systems can actually control speed that well. For this reason, many aircraft flight manuals chart a slightly higher speed for optimum holding speed. For some aircraft, this slightly higher speed might actually be L/D max.
Joe
01-06-2021 | 01:57 PM
#12
Gets Weekends Off
Joined: May 2010
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From: Baja Vermont
Actually, he didn’t. Look at page 98, there’s a fundamental difference between propeller and jet aircraft on their drag polars and where min drag occurs and where L/D Max occurs due to the form of propulsion—propellers do WORK, jet apply FORCE. Read thru it for the details.
01-06-2021 | 09:16 PM
#13
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Galaxy is correct. When considering a Thrust Required curve, the lowest point is L/D max, whether piston or jet airplane. However, when determining Max Range and Max endurance, a piston airplane uses the Power Required curve, not Thrust required.
For a reciprocating engine, specific fuel consumption (commonly abbreviated SFC) is defined as the weight of fuel consumed per unit power per unit time. Anderson, John. Introduction to Flight (p. 501). McGraw-Hill Higher Education. Kindle Edition.
Thus Breguet Endurance and Range equations are determined from using Power Available versus Power Required curve. See attached. For a propeller airplane, maximum range is CL / CD max. Max endurance occurs at a CL^3/2 / CD max.
To maximize range for a reciprocating-engine, propeller-driven airplane, we want the following:
1. The largest possible propeller efficiency η.
2. The lowest possible specific fuel consumption c .
3. The highest ratio of W0 / W1 , which is obtained with the largest fuel weight WF .
4. Most importantly, flight at maximum L / D .
Anderson, John. Introduction to Flight (p. 505). McGraw-Hill Higher Education. Kindle Edition.
For a jet airplane, the Breguet Equations are derived from Thrust Required. As such, for jet airplanes CL / CD max corresponds to Max Endurance. Max range occurs at CL^1/2 / CD max.
to obtain maximum range for a jet airplane, we want the following:
1. Minimum thrust-specific fuel consumption ct .
2. Maximum fuel weight Wf .
3. Flight at maximum CL^1/2 / CD
4. Flight at high altitudes
Anderson, John. Introduction to Flight (p. 512). McGraw-Hill Higher Education. Kindle Edition.
For a reciprocating engine, specific fuel consumption (commonly abbreviated SFC) is defined as the weight of fuel consumed per unit power per unit time. Anderson, John. Introduction to Flight (p. 501). McGraw-Hill Higher Education. Kindle Edition.
Thus Breguet Endurance and Range equations are determined from using Power Available versus Power Required curve. See attached. For a propeller airplane, maximum range is CL / CD max. Max endurance occurs at a CL^3/2 / CD max.
To maximize range for a reciprocating-engine, propeller-driven airplane, we want the following:
1. The largest possible propeller efficiency η.
2. The lowest possible specific fuel consumption c .
3. The highest ratio of W0 / W1 , which is obtained with the largest fuel weight WF .
4. Most importantly, flight at maximum L / D .
Anderson, John. Introduction to Flight (p. 505). McGraw-Hill Higher Education. Kindle Edition.
For a jet airplane, the Breguet Equations are derived from Thrust Required. As such, for jet airplanes CL / CD max corresponds to Max Endurance. Max range occurs at CL^1/2 / CD max.
to obtain maximum range for a jet airplane, we want the following:
1. Minimum thrust-specific fuel consumption ct .
2. Maximum fuel weight Wf .
3. Flight at maximum CL^1/2 / CD
4. Flight at high altitudes
Anderson, John. Introduction to Flight (p. 512). McGraw-Hill Higher Education. Kindle Edition.
01-07-2021 | 12:26 PM
#14
Gets Weekends Off
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From: Volleyball Player
01-07-2021 | 02:02 PM
#15
Living the SloBus life
Joined: Jan 2018
Posts: 2,178
Likes: 92
Galaxy is correct. When considering a Thrust Required curve, the lowest point is L/D max, whether piston or jet airplane. However, when determining Max Range and Max endurance, a piston airplane uses the Power Required curve, not Thrust required.
For a reciprocating engine, specific fuel consumption (commonly abbreviated SFC) is defined as the weight of fuel consumed per unit power per unit time. Anderson, John. Introduction to Flight (p. 501). McGraw-Hill Higher Education. Kindle Edition.
Thus Breguet Endurance and Range equations are determined from using Power Available versus Power Required curve. See attached. For a propeller airplane, maximum range is CL / CD max. Max endurance occurs at a CL^3/2 / CD max.
To maximize range for a reciprocating-engine, propeller-driven airplane, we want the following:
1. The largest possible propeller efficiency η.
2. The lowest possible specific fuel consumption c .
3. The highest ratio of W0 / W1 , which is obtained with the largest fuel weight WF .
4. Most importantly, flight at maximum L / D .
Anderson, John. Introduction to Flight (p. 505). McGraw-Hill Higher Education. Kindle Edition.
For a jet airplane, the Breguet Equations are derived from Thrust Required. As such, for jet airplanes CL / CD max corresponds to Max Endurance. Max range occurs at CL^1/2 / CD max.
to obtain maximum range for a jet airplane, we want the following:
1. Minimum thrust-specific fuel consumption ct .
2. Maximum fuel weight Wf .
3. Flight at maximum CL^1/2 / CD
4. Flight at high altitudes
Anderson, John. Introduction to Flight (p. 512). McGraw-Hill Higher Education. Kindle Edition.
For a reciprocating engine, specific fuel consumption (commonly abbreviated SFC) is defined as the weight of fuel consumed per unit power per unit time. Anderson, John. Introduction to Flight (p. 501). McGraw-Hill Higher Education. Kindle Edition.
Thus Breguet Endurance and Range equations are determined from using Power Available versus Power Required curve. See attached. For a propeller airplane, maximum range is CL / CD max. Max endurance occurs at a CL^3/2 / CD max.
To maximize range for a reciprocating-engine, propeller-driven airplane, we want the following:
1. The largest possible propeller efficiency η.
2. The lowest possible specific fuel consumption c .
3. The highest ratio of W0 / W1 , which is obtained with the largest fuel weight WF .
4. Most importantly, flight at maximum L / D .
Anderson, John. Introduction to Flight (p. 505). McGraw-Hill Higher Education. Kindle Edition.
For a jet airplane, the Breguet Equations are derived from Thrust Required. As such, for jet airplanes CL / CD max corresponds to Max Endurance. Max range occurs at CL^1/2 / CD max.
to obtain maximum range for a jet airplane, we want the following:
1. Minimum thrust-specific fuel consumption ct .
2. Maximum fuel weight Wf .
3. Flight at maximum CL^1/2 / CD
4. Flight at high altitudes
Anderson, John. Introduction to Flight (p. 512). McGraw-Hill Higher Education. Kindle Edition.
I guess you went back for an aero degree after almost orphaning your kid
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