Back side of the Power Curve vs Drag Curve
Power curves vs Drag curves — More similar concepts talked about interchangeably creating confusion and conflict in understanding while gumming up communications and discussion about them.
Summary up front:
1 — There is a region of speed instability that runs from the maximum endurance Angle of Attack (AOA) and slower till stall. We’ll typically look at this as maximum endurance airspeed in lieu of max endurance AOA.
2 — This area is typically labeled the “region of reverse command” though should more properly be considered the ‘region of reverse demand.’
3 — True reverse command relates to how we use pitch if we need to use pitch to control altitude or path. This area has the front side or normal side faster than or at lower AOAs than Lift over Drag maximum (L/Dmax). At AOAs higher than L/Dmax AOA or slower than the associated speed for given weight, if pitch must be used to control path, one must pitch opposite ie reverse command so as to attain favorable second order effect.
4 — Power techniques work both to the front side and back side of power and thrust curves. Power techniques work the same way regardless of where on the curves you are. If you have means to do so, power techniques are consistent, more precise, and are easier to use. Pitch for AOA or airspeed and power for altitude or path. It is the best way to fly.
In a previous piece(1), I wrote about how we misapply airspeed often using airspeed in lieu of AOA forgetting we mean in lieu of AOA. Similarly, I made mention we often misuse pitch in lieu of AOA forgetting we mean AOA. These are not fully interchangeable concepts and such misuse hides dangers. Trying to compensate for the differences creates confusion and pushes us closer to the dangers.
Similarly, thrust and power get interchanged despite differences. Such is much less egregious than airspeed not recognized as in lieu of AOA yet it still creates friction in communication and misunderstandings. I am guilty of wrongly interchanging these concepts in discussion creating such confusions.
These confusions impact not only what we control and the significance thereof, they also impact how we look at curves and our understandings thereof. More specifically, we have a Drag curve aka Thrust Required curve and a Power curve meaning Power Required curve. Thrust and power are closely related but they’re not exactly interchangeable. Yet we regularly use the two terms as synonymous.
Confusion resulting from using the two terms and thus inadvertently the two curves interchangeably includes what is and where is ‘the back side.’ “The back side of the power curve” is a common expression yet depending on what you’re looking at, you may be back side of the power curve or back side of the drag curve. In my previous piece, I referred to back side of the curve meaning back side of the drag curve.
This particular confusion gets worse as we have notions of “reverse command.” What is considered “reverse command” by the FAA is typically in regards to the power curve. Yet what the FAA sees as “reverse command” is really ‘reverse demand’ and ‘alternate command.’ With this, their use of the power curve is only appropriate with propeller airplanes while the same sort of thinking for jets should be done on the thrust required curve. Yet the FAA itself sometimes uses power curve and sometimes uses drag curve while discussing where “reverse command” lies.
In the Pilot Handbook of Aeronautical Knowledge chapter eleven(2), the FAA gives us:
“When an aircraft is in steady, level flight, a condition of equilibrium must prevail. An unaccelerated condition of flight is achieved when lift equals weight, and the powerplant is set for thrust equal to drag. The power required to achieve equilibrium in constant-altitude flight at various airspeeds is depicted on a power required curve. The power required curve illustrates
the fact that at low airspeeds near the stall or minimum controllable airspeed, the power setting required for steady, level flight is quite high.”
“Flight in the region of normal command means that while holding a constant altitude, a higher airspeed requires a higher power setting and a lower airspeed requires a lower power setting. The majority of aircraft flying (climb, cruise, and maneuvers) is conducted in the region of normal command.”
“Flight in the region of reversed command means flight in which a higher airspeed requires a lower power setting and a lower airspeed requires a higher power setting to hold altitude. It does not imply that a decrease in power produces lower airspeed. The region of reversed command is encountered in the low speed phases of flight. Flight speeds below the speed for maximum endurance (lowest point on the power curve) require higher power settings with a decrease in airspeed. Since the need to increase the required power setting with decreased speed is contrary to the normal command of flight, the regime of flight speeds between the speed for minimum required power setting and the stall speed (or minimum control speed) is termed the region of reversed command. In the region of reversed command, a decrease in airspeed must be accompanied by an increased power setting in order to maintain steady flight.”
This is where my problem lies, ‘the regime of flight speeds between the speed for minimum required power setting and the stall speed (or minimum control speed) is termed the region of reversed command.” ‘… Is termed…’ means this is a social construct. We have chosen to define “reverse command” as this phenomenon. Yet reverse command is misleading. The reality is this describes ‘Reverse Demand’ not “reverse command.” We’re not commanding anything, the phenomenon is demanding it of us. Either we accept descending in these speeds or we add more power. If we want to go slower in this region, we accept a greater energy loss rate hence need to feed this with chemical energy, more throttle giving either power or thrust, or with potential energy, descending. It is not, however, us commanding in a reverse fashion. Speed is commanded through AOA which is commanded via pitch control. There is no reverse command here due to being slower than minimum power. Power works the same way regardless of where you are on the curve. Add power and you reduce descent, perhaps as much as to level off or even climb. If you have power available and you add power, you climb relative to your previous trend or line. Power always works this way, there is no reverse command with power. There is, however, reverse demand as more gets required as you move up the curve moving left. Perhaps chapter eleven means the FAA’s knowledge is bankrupt.
The FAA stated position, while correct in its understanding of the phenomenon, is misleading in how it suggests action on the part of pilots. And it is confused within itself stated in different ways in different places. See chapter five of the Airplane Flying Handbook(3),
“When flying above the minimum drag speed (L/DMAX), more power is required to fly even faster. When flying at speeds below L/DMAX, more power is required to fly even slower. Since slow flight will be performed well below L/DMAX, the pilot should be aware that large power inputs or a reduction in AOA will be required to prevent the aircraft from decelerating. It is important to note that when flying below L/DMAX or on the backside of the power curve, as the AOA increases toward the critical AOA and the airplane’s speed continues to decrease, small changes in the pitch control result in disproportionally large changes in induced drag and therefore changes in airspeed. As a result, pitch becomes a more effective control of airspeed when flying below L/DMAX and power is an effective control of the path.”
They’ve got most the facts correct though some are context specific while the application suggested is misleading. Your power demand may change one side or the other, but you really shouldn’t be changing how you fly. Pitch for speed power for altitude applies across the whole curve.
The FAA’s discussion in the above cited paragraph is contextually only applicable to jet airplanes as jets produce thrust not power. Propeller airplanes produce power, therefore all description of such logic moving about a curve must be done on the power curve not the thrust curve. Minimum power is slower than L/Dmax hence a propeller’s “reverse command” region begins at less than L/Dmax. (To play it safe, one can say the “reverse command” region really, ‘reverse demand,’ is left of maximum endurance. A jet’s maximum endurance is L/Dmax while a propeller’s maximum endurance is minimum power required. In both cases, maximum endurance is the proper dividing line for reverse demand. Associate reverse demand with magnitude of power (propeller) or thrust (jet) required.) In this we see the FAA is just as guilty as the rest of us with mixing up our curves.
Worse, they give an impression that forward of (east of) L/Dmax, you should fly “front side techniques” while aft (west), you should fly “back side” techniques. This is pure crap. It perpetuates a misunderstanding of what are front and back side techniques. It is true that “power is an effective control of the path” and “pitch becomes a more effective control of airspeed when flying below L/Dmax,” but power is an effective control for path front and back while ‘more effective control’ with pitch to airspeed merely means it gets more sensitive to pitch the slower you are. Pitch is more effective a speed control on the back side relative to itself as the speed control on the front side. Pitch is not only effective back and front, it is the control for AOA hence for airspeed back and front. This means you should be using ‘power techniques’ regardless of which side you’re on. You should only use true front and back techniques should power be unavailable while also lacking means to control drag, or if power response times make power techniques difficult to unusable. The FAA’s statement fails to consider that aft of L/Dmax but forward of minimum power required, a propeller airplane is still front side of the power curve despite being back side of the drag curve.
In slow flight, you will likely be in or near the ‘reverse demand’ and regardless will be near stall. Hence the notions to reduce AOA and/or needing large power to prevent decelerating are valid. This does not necessarily mean you are on the back side of the power curve, however. You could be though you may not be. You are definitely on the back side of the thrust curve. Reducing pitch to reduce AOA to reduce drag to gain speed to be able to climb is correct. Adding power to climb if needing to maintain AOA works though sluggishly presuming you have power to give.
The applicability of back side flying really only applies to fighter pilots in slow or one-circle fights as these are the “I’ll hit the brakes and he’ll fly right by” situations yet these typically use power techniques not back side ones. Even pilots in competitive short takeoff and landing (STOL) events rely on power techniques not true back side.
For us normal pilots, back side is not useful. It is interesting, but doesn’t do squat for us. When might you like to know about this? Nose high unusual attitudes, stall avoidance, stall recovery… yet in all of these you don’t actually need to know you need more power if slower as your first priority is to break the AOA, by so doing, you’re accelerating rightward on the curve and you don’t care about power excess or how power responds at the left end. You only care about when can you safely add power in order to add it. Light twins should go idle initially as they bunt as a precaution against asymmetry, low slung engines should too so as to avoid pitch up thrust moments; you the likely reader likely in a single engine light plane can add simultaneously though remember right rudder for propeller effects. Concerns for need to increase power in turns and/or to climb? That’s true regardless of front side or back side. Slow flight may amplify awareness of this as it requires such to a greater degree, but it isn’t a unique aspect of slow flight. It is not part of the reverse demand phenomenon.
While not using ‘speed stability’ in this paragraph, the FAA does allude to it. And they write to it subsequently.
“It is also important to note that an airplane flying below L/DMAX, exhibits a characteristic known as “speed instability” and the airspeed will continue to decay without appropriate pilot action. For example, if the airplane is disturbed by turbulence and the airspeed decreases, the airspeed may continue to decrease without the appropriate pilot action of reducing the AOA or adding power.”
Speed stability relates to maximum endurance. You need more power to sustain lower speeds when going slower than maximum endurance while you need more power to sustain faster speeds when going faster than maximum endurance. Assuming pilot in the loop, an excursion faster than max endurance will tend to correct to initial condition while an excursion slower than max endurance will amplify itself either accelerating to a new stable condition forward of max endurance or slowing into stall. With the pilot out of the loop, you still have a basic stability with trim setting. If you get slower than max E without stalling, the nose is going to drop and you will trade altitude regaining speed back toward your trimmed equilibrium. More often than not, the instability arises with getting slower than maximum endurance while the pilot tries to maintian altitude or glide path with pitch. Very few aircraft are truly pilot out of the loop speed unstable even behind (left of) the curve. The Lancair 360 with a relatively flat trim setting for speed curve as well as the fly-by-wire F/A-18 with trimming to constant g force are the only two I can recall off the top of my head. I’ve observed it in the Hornet though in the Hornet it is true both sides of the curve. I’ve only read about it for the Lance, so take that for what it is worth.
This means this cited paragraph is only correct for jets; for propellers, the same applies about minimum power required. Jets are on the drag curve aka thrust required curve while props are on the power curve. And it is typically only correct regarding demand not airframe instability.
Note the precision of word ‘airplane’ not ‘aircraft.’ Airframe speed instability in helicopters is real. Pilot in the loop not required to see such in helicopters. But all our conversation here really is for fixed wing.
The FAA has it correct in chapter eleven of the Pilot Handbook of Aeronautical Knowledge,
“An airplane performing a low airspeed, high pitch attitude power approach for a short-field landing is an example of operating in the region of reversed command. If an unacceptably high sink rate should develop, it may be possible for the pilot to reduce or stop the descent by applying power. But without further use of power, the airplane would probably stall or be incapable of flaring for the landing. Merely lowering the nose of the airplane to regain flying speed in this situation, without the use of power, would result in a rapid sink rate and corresponding loss of altitude.”
Note the sink rate is a short term occurrence and if the sink occurs high enough, on the back side, you “push to float.” Consider the second order effect. Pushing initially loses lift while accelerating but then reduces energy loss via less drag.
“If during a soft-field takeoff and climb, for example, the pilot attempts to climb out of ground effect without first attaining normal climb pitch attitude and airspeed, the airplane may inadvertently enter the region of reversed command at a dangerously low altitude. Even with full power, the airplane may be incapable of climbing or even maintaining altitude. The pilot’s only recourse in this situation is to lower the pitch attitude in order to increase airspeed, which inevitably results in a loss of altitude.”
This is done accepting the cost of the first order effect to buy the second order effect, less drag more energy addition.
Per the Airplane Flying Handbook chapter five immediately preceding the errors in this same chapter five mentioned above,
“Slow flight should be introduced with the target airspeed sufficiently above the stall to permit safe maneuvering, but close enough to the stall warning for the pilot to experience the characteristics of flight at a low airspeed. One way to determine the target airspeed is to slow the aircraft to the stall warning when in the desired slow flight configuration, pitch the nose down slightly to eliminate the stall warning, and add power to maintain altitude and note the airspeed.”
and per the Airplane Flying Handbook chapter nine(4),
“A pilot executing a go-around needs to accept the fact that an airplane cannot fly below stall speed, and it cannot climb below minimum power required speed. The pilot should resist any impulse to pitch-up for a climb if airspeed is insufficient. In some circumstances, it may be desirable to lower the nose briefly to gain airspeed and not be on the backside of the power curve.”
Pitch for AOA, pitch for speed! Power for climb. Maybe not bankrupt after all, just conflicted.
“When the final approach is too high, the pilot may lower the flaps as required. Further reduction in power may be necessary, while lowering the nose simultaneously to maintain approach airspeed and steepen the approach path. Alternatively, the pilot could use a forward slip to increase the descent angle and rate of descent while maintaining proper approach speed.”
If you reduce power, you don’t need to lower the nose as your trim setting will do it for you. Though I am a fan of the forward slip!
“When the base leg is too low, insufficient power is used, landing flaps are extended prematurely, or the velocity of the wind is misjudged, the airplane may be well below the proper final approach path. In such a situation, the pilot would have to apply considerable power to fly the airplane (at an excessively low altitude) up to the runway threshold. When it is realized the runway cannot be reached unless appropriate action is taken, power should be applied immediately to maintain the airspeed while the pitch attitude is raised to increase lift and stop the descent.”
Rubbish! Power to climb, maintain on-speed AOA with pitch. Pulling up is what endangers you. It’s almost like these books are written by committee. And they mislead you with that passive voice, “… while the pitch attitude is raised…” Does this mean such is an action for you to do? Or does it happen naturally?
If you add power, you might actually have to push just a little due to the prop wash enhancing the horizontal stab and elevator’s downward lift causing a pitch up moment. Consider the trim stall with tractor style propeller airplanes. But if we disregard this, adding power means the trim set AOA will cause the pitch to appropriately rise for us. Add power to flatten out, use pitch to maintain proper “speed.” This is how airplanes work. They’re built to work this way. You don’t raise the pitch for the low, the power addition does this for you. You add power to flatten and maintain AOA with pitch.
Icon, Sea Rey, Lake pilots, you can raise the pitch; you’ve got negative pitch moments with power due to high mounted engines. You’re the exception. But for an entirely different reason.
An older FAA(5) understood:
“Since the conditions of steady flight predominate during most flying, the fundamentals of flying technique are the principles of steady flight:
- Angle of attack is the primary control of airspeed.
- Power setting is the primary control of altitude.”
“NOTE: The Instrument Flying Handbook, AC 61–27B, contains basic information needed to acquire an FAA Instrument rating, and a valuable section on the attitude instrument flying. In that section, it is brought out that power control must be looked at in relation to its effect on altitude and AIRSPEED, since any change in power setting can result in a change in airspeed or altitude, or a combination of both. ‘At any GIVEN (CONSTANT) AIRSPEED, the power setting determines whether the aircraft is in level flight, in a climb, or in a descent. On the other hand, IF YOU HOLD ALTITUDE CONSTANT, THE AMOUNT OF POWER APPLIED WILL DETERMINE AIRSPEED.’ This is not meant to contradict the earlier stated principles of steady flight, but to present a refined and proven means of control coordination for the attainment of precision performance during attitude instrument flying.”
Note one must hold altitude or path constant for power to affect speed. This is why throttle impacts speed with most autopilot modes excluding FLC or IAS. ALT, BARO, VS, VNV, GS, GP all force one to “front side techniques” but as soon as you disconnect, you’re back to power techniques. If you discount prop wash and/or engines vertically displaced from cg, power in the absence of stick forces determines climb, level, descent and rates thereof while AOA will remain constant at the trimmed value and as weight won’t have changed in the moment, airspeed will likewise remain constant.
“THE REGION OF REVERSED COMMAND.
Airplane configuration and altitude define a specific variation of POWER SETTING required versus airspeed (jet thrust or propeller power). At low airspeeds near the stall, the power setting required for steady, level flight is quite high. From that point, an increase in airspeed reduces the required power setting until some minimum power is reached at (or near) the conditions for maximum endurance. Increased speed beyond the condition for maximum endurance then increases the power setting required for steady, level flight.”
Understood the distinction between prop and jet with respective need to power or thrust curves while they appreciated in both the dividing line for reverse demand in maximum endurance and saw that speed stability east of instability west of.
Thanks to David St George(6) for pointing this older FAA out.
With that, look to this early inadvertent liftoff:
https://www.youtube.com/watch?v=qtx3De3iX2o
Since we’re mentioning L/Dmax, here’s a reminder from Aerodynamics for Naval Aviators regarding everything true of this specific AOA:
“Many important items of airplane performance are obtained in flight at (L/D)max. Typical performance conditions which occur at (L/D)max, are:
- maximum endurance of jet powered airplanes
- maximum range of propeller driven airplanes [in still air]
- maximum climb angle for jet powered airplanes
- maximum power-off glide range, jet or prop [in still air]
The most immediately interesting of these items is the power-off glide range of an airplane. By examining the forces acting on an airplane during a glide, it can be shown that the glide ratio is numerically equal to the lift-drag ratio. For example, if the airplane in a glide has an L/D of 15, each mile of altitude is traded for 15 miles of horizontal distance. Such a fact implies that the airplane should be flown at L/Dmax to obtain the greatest glide distance.”
“An unbelievable feature of gliding performance is the effect of airplane gross weight. Since the maximum lift-drag ratio of a given airplane is an intrinsic property of the aerodynamic configuration, gross weight will not affect the gliding performance. If a typical jet trainer has an L/Dmax of 15, the aircraft 1 can obtain a maximum of 15 miles horizontal distance for each mile of altitude. This would be true of this particular airplane at any gross weight if the airplane is flown at the angle of attack for L/Dmax. Of course, the gross weight would affect the glide airspeed necessary for this particular angle of attack but the glide ratio would be unaffected.”
With this, another neat thing to consider regarding AOA from Andreas Horn via James Albright of Code 7700(7) has us considering a normalized AOA. With a normalized AOA, the maximum AOA aka stall is 1.0. This let’s us find AOAref based upon knowing Vref standards. In the case of the transport, we have Vref = 1.23 Vstall and Code7700 shows:
For the rest of us, Vref = 1.3 Vstall and normalized AOAref is 0.6. Though I prefer to use clock codes for those displays wise enough to put such in an analog format. 0.6 normalized AOAref at three o’clock. You can go to 4 o’clock, faster and lower AOA if it be gusty. From the Garmin Perspective Plus manual:
While we don’t have a published AOA for best glide in many aircraft, James Albright(8) has a ROT that will be close enough in a pinch:
“Commit the Angle of Attack for L/DMAX (usually 0.3) to memory, it is your best glide and endurance speed and can save your life”
Just note he’s talking jets so that ‘best endurance’ doesn’t apply for props; instead such is a good guess for AOA best range in no wind. It is also best glide in no wind. Gotta go faster into headwinds. The glider books do a good job showing this. Shift your origin for tangent lines on drag polars with headwind and tailwind right and left, shift your rising and sinking air down and up.
This brings us to consideration of what are back side techniques. As you may have gathered from the above, I disagree as to what is commonly accepted “understanding.” Power techniques are valid on the back side but they are not back side techniques. Power techniques work front side and back side. True front side versus true back side techniques relate strictly to the drag curve. And they are truly reverse command. The command in one situation gives you the opposite effect in the other situation when you allow the second order effect to propagate. If front side and unable to use power, or forced to use front side techniques due to slow power response, pitch may be used for altitude or path in a normal sense while if available, power can be used to catch up on speed with its delayed response. Pull up to flatten or move upward though realize you are zooming as opposed to climbing till you catch up with power for that speed. Push down to dive till power reduction adequately makes for descent. Back side of the thrust curve, you’d still be using pitch for altitude and power for speed but you’ll pull up to go down and push down to go up. Anticipate the counter corrections. Pitch for altitude, power for speed are the tools of both front side and back side techniques though the required pitch input gets reversed for the same desired output. This is reverse command not reverse demand. With that back side pulling up to fall if high and trying to anticipate to dump the nose to catch, you can see why power techniques are preferred. Having to push if low while not necessarily having the room for the immediate effect to work, or lacking perception of such, getting you to the second effect also really deters from true back side flying. Power techniques work the same way regardless of side, power for altitude, pitch for speed. No reversals required. Power techniques keep you safer and more comfortable on the back side while enabling better precision on both sides. Power techniques help prevent you from over-controlling in either side too.
Note gliders use “power techniques” despite having no power. How, you may ask. They use spoilers. Normal approaches are with spoilers at midrange thus one can pull back on the spoiler as if pulling back on power if high thus getting more drag and destroying more lift thus sinking. If low, push the spoilers in thus reducing drag and restoring lift.
Another thought stemming from these discussion: Consider for a moment what constitutes least energy lost. Typically we will default to thinking least energy loss over time. Least energy loss over time is minimum sink and is maximum endurance. Yet we can also consider least energy lost over distance which is maximum range and best glide. Yet both of these, max range and best glide, need to be considered over the ground not through the air mass yet numbers are derived through the air mass. So, really you have three different conditions to which you can describe least energy loss: over time, distance through air mass, distance traversed over ground. Reverse Command happens at L/Dmax adjusted to air mass effects (aka least energy loss per distance traversed over the ground). If you’re west of adjusted L/Dmax and you pull up (get slower), you fall shorter whereas if you’re east of adjusted L/Dmax you push down (get faster) to fall shorter. Reverse Demand meaning going slower demands more power (and thrust) to sustain potential energy condition including trend happens west of least energy loss over time being also west of maximum endurance. Reverse Command and Reverse Demand are not synonymous.
If you have a choice in the matter, power technique is the only way to fly. If you don’t have the option, try to stay on the front side. If you must fly true back side with no power option, you’re flying like a U-2. There’s reason it is named the Dragon Lady.
Though lengthy, I highly recommend reading both my recent pieces regarding Improve your Landings with AOA and Power techniques(1) and its associated case study with the crash of the F-35C in the SCS(9). Anyone interested in learning more about spins can see the same Medium story set with Going Beyond Procedure(10).
(1) https://medium.com/@jamesmcclaranallen/improve-your-landings-with-aoa-power-techniques-04601584fb3a
(2) https://www.faa.gov/sites/faa.gov/files/13_phak_ch11.pdf
(5) https://faaflighttest.us/AC6150a.pdf
(6) https://faaflighttest.us/pitchandpower.html
(7) https://code7700.com/angle_of_attack.htm
(8) https://code7700.com/1979_angle_of_attack.htm
(10) https://medium.com/@jamesmcclaranallen/going-beyond-procedure-an-open-letter-b181ac14a22f