J
Jimko
Guest
This topic got rolling in the thread about the Fokker crash, so I thought that I would start a new thread as this subject threatened to hijack the original thread.
This is a LONG post!
A question came up in the original thread about the ability of the WW1 airplanes to glide without power. I was wondering about that very same issue at the time. Since I have a great amount of curiosity about the physics of flight in those circumstances…not that I care about the mathematics and formulas to derive such information, I wanted to find out enough about results that others have calculated so that I can make some informed comparisons and have some consequent better information about gliding in a ‘no engine power’ situation, something that was a fairly common occurrence in WW1. And, I don’t mean to apply this as a criticism of the flight models in OFF or to question their validity when compared to real-life flight. It’s just a desire to understand how the actual WW1 aircraft performed in a no-power situation.
Most material that discusses glide characteristics uses comparative analysis of various modern flying objects and I find that these figures are useful when trying to appreciate the gliding ability of the older aircraft. It’s easier for me to understand and to relate to the figures for the early aircraft that way.
I have gathered information from various sources which I will try to identify should you want to look them up. In some cases, I neglected to note the source and I just saved part of the article, but they can often be tracked down. I’ve also underlined bits of pertinent information to draw them to your attention.
This is not absolute proof of anything, it’s just for your information and your consideration should you be interested in this subject. I think that the information which follows does provide a reasonably good idea of the glide characteristics of the early aircraft. So for any others who, like me, may wonder about the gliding ability of WW1 aircraft, here is some information to ponder and to use for a better understanding…
First, a couple of definitions:
Glide ratio, also called, Lift-to-drag ratio, (L/D), glide number, or finesse, is an aviation term that refers to the distance an aircraft will move forward for any given amount of lost altitude (the cotangent of the downward angle). Alternatively it is also the forward speed divided by sink speed (unpowered aircraft):
The terms glide ratio and lift-to-drag ratio are interchangeable. This is true because the force vectors also determine the direction of travel with the engine off. Glide ratio is the preferred term for unpowered aircraft, and lift-to-drag ratio the preferred term for aerodynamics literature and powered flight. This parameter effectively describes the efficiency of the airframe.
From another source:
L/Dmax is sometimes termed the glide ratio because for light aircraft it is just about the same ratio as distance covered/height lost in an engine-off glide at the optimum still air gliding speed. For example if L/Dmax = 8 then the glide ratio is 8 :1 meaning the aircraft might glide a horizontal distance of 8000 feet for each 1000 feet of height lost, in still air with the wings held level.
Now here are some interesting figures in the next chart for the glide ratio of various flying objects.
Note the “Gimli Glider”, an Air Canada Boeing 767 which ran out of fuel over Gimli Manitoba and glided down to a safe landing.
Flight Object.....................Scenario............L/D or Glide Ratio
Modern Sailplane.................gliding....................~60
Virgin Atlantic G. Flyer..........Cruise......................37
Lockheed U-2......................Cruise...................~28
Rutan Voyager.....................Cruise.....................27
Albatross .........................................................20
Boeing 747..........................Cruise.....................17
Hang glider.......................................................15
Gimli glider.....Boeing 767-200 with fuel exhausted.~12
Common tern....................................................12
Paraglider....................high performance..............11
Herring gull.......................................................10
Concorde M2...................Cruise........................7.14
Cessna 150.....................Cruise..........................7
Space Shuttle................Approach......................4.5
Concorde ......................Approach.....................4.35
House sparrow...................................................4
Space Shuttle...............Hypersonic......................1
Apollo CM.......................Reentry.....................0.368
From the NASA research “Quest for Performance: The Evolution of Modern Aircraft” we have this interesting chart with glide (L/D)max figures for WW1 aircraft:
Aircraft...........................Engine............................(L/D)max
Fokker E-III.....................Oberursel U.I...........................6.4
DeHavilland DH-2.........Gnome Monosoupape......................7.0
Nieuport 17....................Le Rhone 9J..............................7.9
Albatross D-III................Mercedes DII............................7.5
Fokker triplane, Dr.-1.......Oberursel Ur II...........................8.0
Sopwith F.1 Camel.............Clerget 9B..............................7.7
SPAD XIII C.1...............Hispano-Suiza 8Ba........................7.4
Fokker D-VII.....................BMW IIIA................................8.1
Sopwith 5F.1 Dolphin.......Hispano-Suiza............................9.2
Fokker D-VIII.................Oberursel Ur II............................8.1
Junkers D-I.......................BMW IIIA................................7.0
Handley Page 0/400..........Liberty 12.N..............................9.7
Gotha G.V......................Mercedes DIVa...........................7.7
Caproni CA.42.....................Liberty.................................8.2
B.E. 2c..............................R.A.F.la................................8.2
Junkers J-I........................Benz Bz.IV............................10.3
DeHavilland DH-4..................Liberty................................8.1
And, for comparison, here is glide data for some later vintage aircraft also form NASA’s “Quest for Performance”...
Aircraft.................................Engine..............................(L/D)max
Douglas DC-3....................Pratt & Whitney R-1380....................14.7
Boeing B-17G....................Wright R-1820 Cyclone......................12.7
Seversky P-35...................Pratt & Whitney R-1830.....................11.8
Piper J-3 Cub.......................Continental A-65.............................9.6
Stinson SR-8B......................Lycoming R-680.............................10.8
Beechcraft D17S...........Pratt & Whitney R-985 Wasp Jr................11.7
So, for example, a Sopwith Camel, calculated using the above L/Dmax or glide ratio of 7.7 can glide 7700 feet of horizontal distance for every 1000 feet of altitude loss in still wind conditions (7.7 to 1 ration). Thus, at 5000 feet of altitude with power loss, a Camel could conceivably glide for (7700 X 5 )= 38,500 feet or about 7.3 miles (11.75 Km). Headwinds, tailwinds, other factors can enter into the equation and change the actual glide distance. Most of the WW1 aircraft in the chart are theoretically able to glide this distance and even a bit farther.
Interesting to note that the figures in the NASA chart put most WW1 aircraft gliding figures in the “Cessna 150 at cruise” to the “Herring Gull” gliding range.
For one more comparison of a gliding scenario in a plane that almost everyone is familiar with, Jim Van Namee who has done tests with his Cessna 172 mentions these glide figures in his “Maximize Glide Performance”:
“You’re heading from Taos, New Mexico to Los Alamos, just on the other side of the mountains, for a $100 taco at Dos Colores in your Cessna 172. While enroute, you’ve climbed to 13,500 feet for some sightseeing. Ten miles out over Cerro Peak, drilling along, fat, dumb and happy, the iron wind maker suddenly coughs and sputters. What to do, what to do?
Knowing the maximum glide speed of your trusty steed, you immediately go through your emergency actions and slow to 65 KIAS. This gives you a glide ratio of nine miles for each 6,080 feet of altitude….… By “zooming” your aircraft to gain altitude until you slow down to maximum glide speed and then begin your descent you might be able to get a little more glide distance.”
Jim actually goes on to explain why he can achieve a glide ratio that is higher; as much as 12:1, but it’s not necessary to copy the lengthy article here. Suffice to say that even with a ratio of 9:1, at 6000 feet you should be able to glide for about nine miles in a 172 unless other factors interfere.
And finally, we have this very interesting and informative set of facts presented by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation:
Glide Considerations
To glider pilots the word “glide” is the life-blood of flight. To a powered aircraft pilot the word “glide” sometimes implies an emergency last-ditch-effort maneuver to get to the ground safely after an engine failure.
Regardless of how you define it, the glide is an important maneuver and understanding what constitutes a glide and what variables affect performance in a glide is critical to safe flight.
Referred to by some as “dead-sticking it in,” gliding flight may be better described as a process by which we use the momentum created by gravity to maintain sufficient forward speed in order to maximize lift.
In fact, the idea of “dead-sticking it” may not be quite accurate since it is said to refer to the stationary wooden propeller of early aircraft experiencing engine seizure.
In any case, true gliding flight, although simple to perform, is surrounded by much confusion and misperception. In this discussion I will attempt to dispel some of the confusion!
In the initial study of the glide maneuver it should become apparent that the forces at work are essentially the same as for a climb. One noteworthy difference is the lack of power in the glide.
The aerodynamicists will tell us that the goal of the glide is to minimize the gliding angle in order to maximize the range (or distance covered) during the glide. The glide angle is the angle between the flight path of the aircraft and the gliding range.
In order achieve the max glide angle; the airspeed that maximizes the ratio of the coefficient of lift to the coefficient of drag must be flown.
Note that nowhere in this discussion has weight been mentioned. In fact, weight will not affect the glide angle or glide ratio of an aircraft unless the wrong airspeed is flown. So the only thing affected by changes in weight is the TAS at which the maximum coefficient of lift to drag ratio is achieved.
If you think that your gliding ability improves as the aircraft gets lighter, you’re kidding yourself for sure! The best one can do is to realize that the lower weight will require a lower gliding speed in order to produce the same gliding range as for the gross weight condition. It’s all about flying the max L/D ratio. Flying the “gross weight” gliding speed when the aircraft is below that weight will result in reduced performance as seen by an increased glide angle as well as increased rate of descent.
Another way to think about this is to compare two different weight aircraft with the same aerodynamic design. Both aircraft will glide exactly the same distance if each is flown at its optimum airspeed for the optimum lift to drag ratio for that weight. The heavier aircraft must fly faster to achieve the same lift to drag ratio but will glide no further nor less than the lighter aircraft.Pilots rarely fly at max gross weight, and when they do, it’s not for very long. So if we assume that most pilots fly around at something less than max gross, then the best glide speed advertised in the pilots operating handbook is not going to produce optimum glide performance.
The reason for this is clear. Manufacturers of most light aircraft list a single glide speed predicated on the aircraft being at max gross weight at sea level. Holding off a minute on the sea level part, if the weight is not a max gross then published glide speed is not quite correct.
Luckily, the normal range of weight an aircraft is flown at usually will only have a few knots of effect on the proper best glide speed.
One other common misunderstanding related to weight is that a heavy aircraft (e.g. Boeing 747) will have poorer gliding performance than a smaller aircraft. That’s not necessarily true!
The truth is that the 747 may actually have better gliding performance than a smaller piston single-engine aircraft due to the larger aircraft having a more aerodynamically clean design.
On the other hand, most sailplanes likely have better glide capabilities than a 747. It’s all about aerodynamic design and not at all about weight.
But then there is the consideration of altitude and its affect on glide performance. The general thought is that glide performance should decrease in the thinner air of higher altitudes.
In actuality the only thing that changes with a change in density (altitude) is the true airspeed at which the optimum glide performance will be achieved. Indicated airspeed used for glide remains the same with an increase in altitude. The best true glide airspeed varies inversely with the square root of air density and, as such, an increase in air density decreases the true airspeed for best glide increases.
A pilot does not need to compensate for the change in air density since the aircraft actually moves through the air at the true airspeed necessary to achieve a given indicated airspeed. If the air is less dense, in order to show any particular indicated airspeed, the aircraft will actually have to move through the air faster than what is indicated.
Aircraft configuration is also a consideration for best glide performance. Generally, the clean configuration will produce the highest lift to drag ratio. As a result of increased parasitic drag of the landing configuration and increase in coefficient of lift due to flap application the best glide airspeed in the landing configuration will be less than for the clean configuration.
How much less is anyone’s guess but its generally recommended that once the landing configuration is set, the landing indicated airspeed appropriate to that configuration should be used.
It is difficult to say with any certainty what intermediate flap settings (with gear retracted) do to the lift to drag ratio. One clear result of application of flaps is an increase in lift which offsets the drag increase at low flap settings, with increasing disparity between lift and drag production as flap settings increase.
Many pilots attempt to “stretch” the glide either by increasing the angle of attack with pitch or by the addition of flaps. The danger of doing either should be clear. Any momentary gain in performance is just that…momentary!
Once the aircraft achieves equilibrium at the new angle of attack for a pitch and/or flap setting, the glide performance will be worse than previous and confirmed by an increase in rate of descent.
The last area of concern is the effect of wind on glide performance. In simplistic terms, it should be clear that a headwind will reduce performance by increasing the glide angle and a tailwind will improve glide performance by decreasing the glide angle.
The more complex aspect of gliding with a wind is that the optimum glide performance may not be accomplished at the published best glide speed. In a strong headwind, the optimum glide performance will be achieved at an airspeed above that which the max lift to drag ratio occurs.
Likewise, with a tailwind, a slightly lower airspeed will maximize glide performance. Luckily, the wind conditions need to be pretty strong to have a significant impact on the glide performance. As a general rule, if the direct wind velocity component is greater than 25% of the gliding speed, a change in the best glide speed will be necessary to maximize gliding range.
Again, it is hard to generalize from the theory to the specific requirements for each aircraft so a caveat about gliding in winds is warranted. If the winds exceed 25% of the best gliding speed, pick your landing spot very carefully as flying at best glide will undoubtedly produce performance significantly different from the normal light wind conditions in which most of us fly.
Hopefully by this point you have had a good review of the glide maneuver and refreshed your working knowledge what variables affect the glide performance. Some of the above may also be new to you as well!
Rest assured that all of it is pertinent to understanding the glide characteristics of a powered light aircraft. There’s more to talk about when flying a sailplane, but we’ll reserve that discussion for another time.
This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 160 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.
Note his comment about “lighter aircraft” which pretty much destroys my notion and the comment I made in the original post about the possibility of these lighter WW1 aircraft having some gliding advantage. It doesn’t work that way!
That’s about it. Take from it what you will. I hope this didn’t put you to sleep, but if it does, you might want to print it and keep it on the night table next to the bed, so it can be of some use on those sleepless nights!
Cheers!
This is a LONG post!
A question came up in the original thread about the ability of the WW1 airplanes to glide without power. I was wondering about that very same issue at the time. Since I have a great amount of curiosity about the physics of flight in those circumstances…not that I care about the mathematics and formulas to derive such information, I wanted to find out enough about results that others have calculated so that I can make some informed comparisons and have some consequent better information about gliding in a ‘no engine power’ situation, something that was a fairly common occurrence in WW1. And, I don’t mean to apply this as a criticism of the flight models in OFF or to question their validity when compared to real-life flight. It’s just a desire to understand how the actual WW1 aircraft performed in a no-power situation.
Most material that discusses glide characteristics uses comparative analysis of various modern flying objects and I find that these figures are useful when trying to appreciate the gliding ability of the older aircraft. It’s easier for me to understand and to relate to the figures for the early aircraft that way.
I have gathered information from various sources which I will try to identify should you want to look them up. In some cases, I neglected to note the source and I just saved part of the article, but they can often be tracked down. I’ve also underlined bits of pertinent information to draw them to your attention.
This is not absolute proof of anything, it’s just for your information and your consideration should you be interested in this subject. I think that the information which follows does provide a reasonably good idea of the glide characteristics of the early aircraft. So for any others who, like me, may wonder about the gliding ability of WW1 aircraft, here is some information to ponder and to use for a better understanding…
First, a couple of definitions:
Glide ratio, also called, Lift-to-drag ratio, (L/D), glide number, or finesse, is an aviation term that refers to the distance an aircraft will move forward for any given amount of lost altitude (the cotangent of the downward angle). Alternatively it is also the forward speed divided by sink speed (unpowered aircraft):
The terms glide ratio and lift-to-drag ratio are interchangeable. This is true because the force vectors also determine the direction of travel with the engine off. Glide ratio is the preferred term for unpowered aircraft, and lift-to-drag ratio the preferred term for aerodynamics literature and powered flight. This parameter effectively describes the efficiency of the airframe.
From another source:
L/Dmax is sometimes termed the glide ratio because for light aircraft it is just about the same ratio as distance covered/height lost in an engine-off glide at the optimum still air gliding speed. For example if L/Dmax = 8 then the glide ratio is 8 :1 meaning the aircraft might glide a horizontal distance of 8000 feet for each 1000 feet of height lost, in still air with the wings held level.
Now here are some interesting figures in the next chart for the glide ratio of various flying objects.
Note the “Gimli Glider”, an Air Canada Boeing 767 which ran out of fuel over Gimli Manitoba and glided down to a safe landing.
Flight Object.....................Scenario............L/D or Glide Ratio
Modern Sailplane.................gliding....................~60
Virgin Atlantic G. Flyer..........Cruise......................37
Lockheed U-2......................Cruise...................~28
Rutan Voyager.....................Cruise.....................27
Albatross .........................................................20
Boeing 747..........................Cruise.....................17
Hang glider.......................................................15
Gimli glider.....Boeing 767-200 with fuel exhausted.~12
Common tern....................................................12
Paraglider....................high performance..............11
Herring gull.......................................................10
Concorde M2...................Cruise........................7.14
Cessna 150.....................Cruise..........................7
Space Shuttle................Approach......................4.5
Concorde ......................Approach.....................4.35
House sparrow...................................................4
Space Shuttle...............Hypersonic......................1
Apollo CM.......................Reentry.....................0.368
From the NASA research “Quest for Performance: The Evolution of Modern Aircraft” we have this interesting chart with glide (L/D)max figures for WW1 aircraft:
Aircraft...........................Engine............................(L/D)max
Fokker E-III.....................Oberursel U.I...........................6.4
DeHavilland DH-2.........Gnome Monosoupape......................7.0
Nieuport 17....................Le Rhone 9J..............................7.9
Albatross D-III................Mercedes DII............................7.5
Fokker triplane, Dr.-1.......Oberursel Ur II...........................8.0
Sopwith F.1 Camel.............Clerget 9B..............................7.7
SPAD XIII C.1...............Hispano-Suiza 8Ba........................7.4
Fokker D-VII.....................BMW IIIA................................8.1
Sopwith 5F.1 Dolphin.......Hispano-Suiza............................9.2
Fokker D-VIII.................Oberursel Ur II............................8.1
Junkers D-I.......................BMW IIIA................................7.0
Handley Page 0/400..........Liberty 12.N..............................9.7
Gotha G.V......................Mercedes DIVa...........................7.7
Caproni CA.42.....................Liberty.................................8.2
B.E. 2c..............................R.A.F.la................................8.2
Junkers J-I........................Benz Bz.IV............................10.3
DeHavilland DH-4..................Liberty................................8.1
And, for comparison, here is glide data for some later vintage aircraft also form NASA’s “Quest for Performance”...
Aircraft.................................Engine..............................(L/D)max
Douglas DC-3....................Pratt & Whitney R-1380....................14.7
Boeing B-17G....................Wright R-1820 Cyclone......................12.7
Seversky P-35...................Pratt & Whitney R-1830.....................11.8
Piper J-3 Cub.......................Continental A-65.............................9.6
Stinson SR-8B......................Lycoming R-680.............................10.8
Beechcraft D17S...........Pratt & Whitney R-985 Wasp Jr................11.7
So, for example, a Sopwith Camel, calculated using the above L/Dmax or glide ratio of 7.7 can glide 7700 feet of horizontal distance for every 1000 feet of altitude loss in still wind conditions (7.7 to 1 ration). Thus, at 5000 feet of altitude with power loss, a Camel could conceivably glide for (7700 X 5 )= 38,500 feet or about 7.3 miles (11.75 Km). Headwinds, tailwinds, other factors can enter into the equation and change the actual glide distance. Most of the WW1 aircraft in the chart are theoretically able to glide this distance and even a bit farther.
Interesting to note that the figures in the NASA chart put most WW1 aircraft gliding figures in the “Cessna 150 at cruise” to the “Herring Gull” gliding range.
For one more comparison of a gliding scenario in a plane that almost everyone is familiar with, Jim Van Namee who has done tests with his Cessna 172 mentions these glide figures in his “Maximize Glide Performance”:
“You’re heading from Taos, New Mexico to Los Alamos, just on the other side of the mountains, for a $100 taco at Dos Colores in your Cessna 172. While enroute, you’ve climbed to 13,500 feet for some sightseeing. Ten miles out over Cerro Peak, drilling along, fat, dumb and happy, the iron wind maker suddenly coughs and sputters. What to do, what to do?
Knowing the maximum glide speed of your trusty steed, you immediately go through your emergency actions and slow to 65 KIAS. This gives you a glide ratio of nine miles for each 6,080 feet of altitude….… By “zooming” your aircraft to gain altitude until you slow down to maximum glide speed and then begin your descent you might be able to get a little more glide distance.”
Jim actually goes on to explain why he can achieve a glide ratio that is higher; as much as 12:1, but it’s not necessary to copy the lengthy article here. Suffice to say that even with a ratio of 9:1, at 6000 feet you should be able to glide for about nine miles in a 172 unless other factors interfere.
And finally, we have this very interesting and informative set of facts presented by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation:
Glide Considerations
To glider pilots the word “glide” is the life-blood of flight. To a powered aircraft pilot the word “glide” sometimes implies an emergency last-ditch-effort maneuver to get to the ground safely after an engine failure.
Regardless of how you define it, the glide is an important maneuver and understanding what constitutes a glide and what variables affect performance in a glide is critical to safe flight.
Referred to by some as “dead-sticking it in,” gliding flight may be better described as a process by which we use the momentum created by gravity to maintain sufficient forward speed in order to maximize lift.
In fact, the idea of “dead-sticking it” may not be quite accurate since it is said to refer to the stationary wooden propeller of early aircraft experiencing engine seizure.
In any case, true gliding flight, although simple to perform, is surrounded by much confusion and misperception. In this discussion I will attempt to dispel some of the confusion!
In the initial study of the glide maneuver it should become apparent that the forces at work are essentially the same as for a climb. One noteworthy difference is the lack of power in the glide.
The aerodynamicists will tell us that the goal of the glide is to minimize the gliding angle in order to maximize the range (or distance covered) during the glide. The glide angle is the angle between the flight path of the aircraft and the gliding range.
In order achieve the max glide angle; the airspeed that maximizes the ratio of the coefficient of lift to the coefficient of drag must be flown.
Note that nowhere in this discussion has weight been mentioned. In fact, weight will not affect the glide angle or glide ratio of an aircraft unless the wrong airspeed is flown. So the only thing affected by changes in weight is the TAS at which the maximum coefficient of lift to drag ratio is achieved.
If you think that your gliding ability improves as the aircraft gets lighter, you’re kidding yourself for sure! The best one can do is to realize that the lower weight will require a lower gliding speed in order to produce the same gliding range as for the gross weight condition. It’s all about flying the max L/D ratio. Flying the “gross weight” gliding speed when the aircraft is below that weight will result in reduced performance as seen by an increased glide angle as well as increased rate of descent.
Another way to think about this is to compare two different weight aircraft with the same aerodynamic design. Both aircraft will glide exactly the same distance if each is flown at its optimum airspeed for the optimum lift to drag ratio for that weight. The heavier aircraft must fly faster to achieve the same lift to drag ratio but will glide no further nor less than the lighter aircraft.Pilots rarely fly at max gross weight, and when they do, it’s not for very long. So if we assume that most pilots fly around at something less than max gross, then the best glide speed advertised in the pilots operating handbook is not going to produce optimum glide performance.
The reason for this is clear. Manufacturers of most light aircraft list a single glide speed predicated on the aircraft being at max gross weight at sea level. Holding off a minute on the sea level part, if the weight is not a max gross then published glide speed is not quite correct.
Luckily, the normal range of weight an aircraft is flown at usually will only have a few knots of effect on the proper best glide speed.
One other common misunderstanding related to weight is that a heavy aircraft (e.g. Boeing 747) will have poorer gliding performance than a smaller aircraft. That’s not necessarily true!
The truth is that the 747 may actually have better gliding performance than a smaller piston single-engine aircraft due to the larger aircraft having a more aerodynamically clean design.
On the other hand, most sailplanes likely have better glide capabilities than a 747. It’s all about aerodynamic design and not at all about weight.
But then there is the consideration of altitude and its affect on glide performance. The general thought is that glide performance should decrease in the thinner air of higher altitudes.
In actuality the only thing that changes with a change in density (altitude) is the true airspeed at which the optimum glide performance will be achieved. Indicated airspeed used for glide remains the same with an increase in altitude. The best true glide airspeed varies inversely with the square root of air density and, as such, an increase in air density decreases the true airspeed for best glide increases.
A pilot does not need to compensate for the change in air density since the aircraft actually moves through the air at the true airspeed necessary to achieve a given indicated airspeed. If the air is less dense, in order to show any particular indicated airspeed, the aircraft will actually have to move through the air faster than what is indicated.
Aircraft configuration is also a consideration for best glide performance. Generally, the clean configuration will produce the highest lift to drag ratio. As a result of increased parasitic drag of the landing configuration and increase in coefficient of lift due to flap application the best glide airspeed in the landing configuration will be less than for the clean configuration.
How much less is anyone’s guess but its generally recommended that once the landing configuration is set, the landing indicated airspeed appropriate to that configuration should be used.
It is difficult to say with any certainty what intermediate flap settings (with gear retracted) do to the lift to drag ratio. One clear result of application of flaps is an increase in lift which offsets the drag increase at low flap settings, with increasing disparity between lift and drag production as flap settings increase.
Many pilots attempt to “stretch” the glide either by increasing the angle of attack with pitch or by the addition of flaps. The danger of doing either should be clear. Any momentary gain in performance is just that…momentary!
Once the aircraft achieves equilibrium at the new angle of attack for a pitch and/or flap setting, the glide performance will be worse than previous and confirmed by an increase in rate of descent.
The last area of concern is the effect of wind on glide performance. In simplistic terms, it should be clear that a headwind will reduce performance by increasing the glide angle and a tailwind will improve glide performance by decreasing the glide angle.
The more complex aspect of gliding with a wind is that the optimum glide performance may not be accomplished at the published best glide speed. In a strong headwind, the optimum glide performance will be achieved at an airspeed above that which the max lift to drag ratio occurs.
Likewise, with a tailwind, a slightly lower airspeed will maximize glide performance. Luckily, the wind conditions need to be pretty strong to have a significant impact on the glide performance. As a general rule, if the direct wind velocity component is greater than 25% of the gliding speed, a change in the best glide speed will be necessary to maximize gliding range.
Again, it is hard to generalize from the theory to the specific requirements for each aircraft so a caveat about gliding in winds is warranted. If the winds exceed 25% of the best gliding speed, pick your landing spot very carefully as flying at best glide will undoubtedly produce performance significantly different from the normal light wind conditions in which most of us fly.
Hopefully by this point you have had a good review of the glide maneuver and refreshed your working knowledge what variables affect the glide performance. Some of the above may also be new to you as well!
Rest assured that all of it is pertinent to understanding the glide characteristics of a powered light aircraft. There’s more to talk about when flying a sailplane, but we’ll reserve that discussion for another time.
This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 160 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.
Note his comment about “lighter aircraft” which pretty much destroys my notion and the comment I made in the original post about the possibility of these lighter WW1 aircraft having some gliding advantage. It doesn’t work that way!
That’s about it. Take from it what you will. I hope this didn’t put you to sleep, but if it does, you might want to print it and keep it on the night table next to the bed, so it can be of some use on those sleepless nights!
Cheers!
