This blog was formatted from comments on last week's Energy Management blog (>35, pitch/power discussion). Rich Stowell checked in and his comments, reprinted here, are especially valuable for all pilots and educators. His FREE "Learn to Turn" course offers even more extensive guidance.
While a good start, the new “Energy Management” chapter in the 2021 FAA Airplane Flying Handbook devolves into the usual trap of a designer-oriented approach to training. As pointed out by Langewiesche in Stick and Rudder, “What is wrong with ‘Theory of Flight,’ from the pilot’s point of view, is not that it is theory. What’s wrong is that it is the theory of the wrong thing—it usually becomes a theory of building the airplane rather than of flying it.”
The first principle of flying light airplanes is “Pitch + Power = Performance.” This succinct statement points to energy management. But it is so much more than that. Substitute angle of attack (AOA) for Pitch; thus, “AOA + Power = Performance.” Airspeed (V) and G-load (G) are proxies for AOA. Hence, “V + G + P = Performance.”
This simple statement takes us to the two types of airplane performance we care about: Climb Performance and Maneuvering Performance. The V-P diagram illustrates climb performance. In this context, “climb” can be positive (climbing), zero (level), or negative (descending). We can talk about energy; which axis is controlled with throttle; which axis, with elevator; what happens on the back side, the front side, and way on the front side. We can include excess power (Pxs = Pav – Pre), which translates into rate of climb (ROC).
We can overlay and discuss critical V-speeds such as Vm, Vs, Vh, and Vne (vertical lines drawn on the diagram); Ve and Vy (horizontal lines); and Vx and Vbg/Vbr (tangent lines). We can discuss the effects on the performance of things such as airplane configuration, density altitude, and bank angle.
We can map out a go-around. Say, for example, we add go-around power while at 60 knots. Achieving max ROC is important in this example, which is a touch over 70 knots. Pushing the elevator control forward allows us to climb at a faster rate. This is counterintuitive (just like stall recovery) unless we understand the V-P relationship. Similarly, there are speeds on either side of Vy that, with go-around power applied, result in the same ROC albeit slower than max ROC. In one instance, we would have to push to speed up and thereby climb at a faster rate; in the other, pull to slow down and thereby also climb at a faster rate. Again counterintuitive, but normal in terms of the V-P relationship. We can correlate key speeds like Vy with their corresponding pitch attitudes, too, should we ever lose our airspeed indicator.
We can even map out and discuss a complete engine failure (P = 0), in which case the V-P diagram flips and becomes a Rate of Descent vs. Speed diagram (ROD-V). Correlate the glide attitude with your best glide speed here, too.
The V-G diagram, on the other hand, illustrates maneuvering performance within the confines of the airplane’s aerodynamic and structural limits. The same Vne line from V-P transfers to V-G. The same Vs line transfers as well, but the V-G diagram shows us all the stall speeds from zero G to the design limit G.
Whatever we say about the V-P diagram must be consistent with, and transferrable to, the V-G diagram and vice versa. Does anyone think we can substitute power settings along the V-axis on the V-G diagram, making it a P-G diagram?
When they happen to be discussed, V-P and V-G are presented as totally unrelated pictures. The reality is that three parameters tell us what the airplane is doing at any point in flight: V, G, & P.
Since V-P and V-G diagrams share the same speed axis, imagine connecting the two diagrams at right angles to each other along the V-axis. Imagine an airplane in flight in the space between the two diagrams. Projecting the airplane’s shadow onto the V-P diagram informs us about its climb performance at that moment. Likewise, projecting the airplane’s shadow onto the V-G diagram tells us how it is maneuvering at that moment. Alternatively, we might think of the V-G diagram as a toaster with an infinite number of thin slots to accept an infinite number of thin slices of V-P bread. The graphic shows this concept for level flight at 1G. This slice of the envelope is bounded by the 1G stall speed, Vne, and the available power.
The interplay between pitch and power on performance should be clear, as should the primary use of elevator and throttle controls. Also note that pitch is assigned as AOA/speed control and power as altitude control during the most critical of flight operations:
- The mountain, canyon, and backcountry flight environments (Mountain, Canyon, and Backcountry Flying, Amy L. Hoover & R.K. Williams; Mountain and Canyon Flying Training Manual, Lori MacNichol)
- The technical logic of airplane performance (Performance of Light Aircraft, John T. Lowry)
- AOPA articles (Fatal Instinct and Power and Pitch, Barry Schiff)
- The flippers and the throttle (Stick and Rudder, Wolfgang Langewiesche)
- Fundamentals of flying technique (FAA)
- Fundamentals of stall recovery (FAA)
- Go-around/Rejected Landing (FAA)
Likewise, airplane manufacturers assign pitch for AOA/speed control and power for altitude control during critical flight operations and emergencies in their airplanes. For example, see the amplified procedures in the Cessna 172P:
- Short field landing
- Landing without elevator control
- Glassy water landing (floatplane version)
- Engine failure
- Emergency descent through clouds
- Recovery from a spiral dive (in clouds)
Teaching Pitch and Power
Pitch. The elevator control moves fore and aft. The airplane’s response is seen as head-to-feet movement by the pilot. The primary effect is on angle of attack, which presents as changes in at least a couple of these: V, G, attitude, flight path. Possible secondary effects might include changes in rigging and engine effects, gyroscopic precession, angle of bank, and altitude.
Power. The throttle moves fore and aft. The primary effect is to move the airplane here-to-there (taxi from one location to another on the airport; fly from airport A to airport B; climb or descend from one altitude to another). Possible secondary effects might include changes in torque, p-factor, slipstream, and airspeed.
Do we inadvertently, negatively reinforce “use the secondary effect of pitch for altitude” as the norm, which by default makes all the above scenarios the exceptions? Or is “using the secondary effect of pitch for altitude” really the exception, especially since we must be in just the right slice of the envelope, close to the correct altitude profile already, and with speed that we either don’t need or don’t really care about to achieve the desired effect?
Does our approach to teaching the V-P relationship along with the trainee’s practical experience spent mostly in one part of the envelope contribute to the potentially dangerous pull response in other parts of the envelope? How do we push learning to the correlation level for the entire performance envelope?
Stripping away the apparent complexity in the FAA’s energy management chapter—especially with regard to the most critical energy state scenarios—returns us to the primary roles of pitch and power as illustrated on V-P and V-G diagrams. Hence my motto: “Pitch for the speed you need.”
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This is an excellent post. I highly recommend anything written by Rich Stowell.
A typical exchange between a student and myself during an early session of dual;
“Now Mrs. Jones, there are several ways you can put this airplane in a climb. There’s the FAA method where Ps= dh/dt v/g dv/dt. OR you can put the nose at this angle to the horizon, add climb power, retrim the airplane at that attitude and walla. You are now in a climb !
Just remember……THIS is your climb attitude. No matter what you are doing with the airplane, if you want to enter and maintain a climb, just put the nose at that attitude, apply climb power, trim the airplane and you will climb safely”.
Mrs Jones just learned how to establish a climb in about a minute of dual. And she’s a happy camper too because she did it herself and she is now beginning to feel like she can actually learn to fly an airplane !
Dudley, yours is a wonderful example of the pilot-centric approach to flight training. Simple. Effective. Rooted in V-P principles. As your trainee progresses, and assuming the trainee wishes to dig deeper into this, it is an easy step to show how what was taught/learned is derived from and can be illustrated on the V-P diagram.
What, no Greek letters and formulas to make it “complicated and confusing?” 😉
I agree entirely, especially for the initial exposure. As experience (and comfort) progress, we can add more nuance and deeper understanding. You demonstrated two important guiding principles: “what performance do I, as a pilot, want to achieve with this aircraft? and how do we achieve that?” Second, the importance of conveying to the learner a sense of control and confidence ASAP for motivation and eventual mastery.
Have you see new AOPA videos on UPRT Dudley? (This is in your wheelhouse: https://youtu.be/el6rQw-yTYY ) I thought this modeled good CFI pace (in addition to the other points they intended to present)
Love it. But that is the FAA way. “A straight climb is entered by gently increasing back pressure on the elevator flight control to the pitch attitude referencing the airplane’s nose to the natural horizon while simultaneously increasing engine power to the climb power setting”. pg 3-19 Airplane Flying Handbook. I have no idea why it has gotten so complicated.
Dudley, as a follow up to your post in “Teach (and fly) Energy Management!” about how to get the FAA to be more accurate with info presented in its handbooks (e.g., getting rid of the erroneous centrifugal force in turns thing), consider this:
Since 1987, I’ve been teaching the first principle that “The motion of the airplane is described relative to the pilot.” Derived from this is developing the ability to see roll as head-to-hip movements; yaw as ear-to-ear movements; and pitch as head-to-feet movements.
In an interesting coincidence, nearly 30 years later virtually the same language suddenly appeared in the FAA Airplane Flying Handbook! The only change was from my “ear-to-ear” yaw motion to “shoulder-to-shoulder” in the handbook (maybe to avoid the appearance of blatant plagiarism?). Anyway, it took a long time but an important change happened organically.
One way I think of understanding pitch and power is to imagine that there is a string attached to the middle of the wing and another string attached to the propeller hub. When power is applied for takeoff, just imagine you are pulling horizontally on the string attached to the propeller. When you’ve reached flying speed (rotation speed), start gradually pulling vertically up on the string attached to the wing – there’s your takeoff roll and initial climb. Level-off and cruise – reduce the upward pull on the vertical string just enough so the airplane doesn’t move up or down (i.e. reduce pitch to maintain an altitude) – adjust the pull on the horizontal string (rpm) for desired rpm/speed. Descent and landing – reduce the upward pull on the vertical string so the airplane follows the glideslope to the runway threshold (pitch down as necessary). Reduce the pull on the horizontal string to maintain proper approach speed (adjust throttle for recommended approach speed, NOT any particular rpm or vertical speed but APPROACH AIRSPEED).
I was lucky to have an instructor who really knew how to fly airplanes and signed me off with the minimum time required, 40 hours (I passed first attempt). He could fly everything on the airport (SE, ME, taildragger) with phenomenal smoothness and accuracy. The essay about the strings was not his but is one of the ways I try to make this subject easier to understand, and it was the way he flew and taught me to fly and is the way the controls are designed to be used. For me I’ve also never had any problem with putting the airplane in the right place at the right speed even in some very strong weather following the basic techniques above.
It is astonishing to me that there is so much confusion and debate about pitch and power. But quite a while back, I realized the main reason. There is an inexplicable misunderstanding about the concept of The Region of Reversed Command or flying on the Back Side of the Power Curve. It is widely interpreted to mean that below L/D Max (best glide speed), the pitch and power controls ‘reverse’. How many times have you heard that someone uses power to control altitude on approaches. What does that then mean that that pilot is attempting to do? It means that that pilot is trying to use the horizontal string attached to the propeller to control altitude. That should make for an interesting discussion in a physics class. And also that pilot must think he can use the vertical string to provide thrust. So when an airplane gets low on approach, one of those pilots will add power. That means the horizontal string is being pulled with more force and since it is pointed downward, it will try to make the airplane go down faster rather than move up. Usually those pilots are very lucky to be flying a light single-engine airplane that is reasonably well trimmed on approach. That means the increased power will usually energize the propeller slipstream and force the tail downward increasing angle of attack and move the airplane upward. So they escape a total misuse of the controls. But it’s a sloppy way to fly an airplane and when the weather is bad, forget it. Your controlling pitch with a cushion of air on the tail. If you have plans for flying twins and jets (which don’t have a propeller slipstream down the fuselage to push the tail down), you will only fly downward at a faster speed. The correct technique for correction of a low approach – add power to hold airspeed (horizontal string) as you increase pitch to regain altitude (vertical string) (pg 9-30 Airplane Flying Handbook).
So what is the Region of Reversed Command – it is that range of speed from best glide to stall where drag gets higher with lower speed, reaching the highest amount of drag at the last knot above stall speed. The lower the speed, the higher the drag – that’s what reverses (pg 11-11 PHAK). So be judicious with your use of power as you fly in this range and don’t get too slow. Fly the ‘strings’ correctly for maximum control and accuracy.
If the string theory is an accurate representation of the V-P relationship, you should be able to tie it directly to, and illustrate the idea on, a V-P diagram. If not, the theory is flawed.
I don’t think of it as a theory or where it fits on a diagram. It makes it easier to visualize the source and direction of each force and what each can and cannot do.
A deep thinking exercise:
CFI: What does the green curve on the V-P diagram represent?
STU: Power required.
CFI: Required for what?
STU: Required for level flight.
CFI: In other words, POWER required FOR constant ALTITUDE?
STU: Well…um…I suppose so.
CFI: That is what the curve represents, correct?
STU: Correct.
CFI: What insight does that give you into the relationship between pitch and power, as well as the primary control functions of elevator and throttle?
Driving, Flying, & Gliding:
In driving mode, the V-P curve represents the “power required for constant speed.” You can only ever be somewhere on this curve when driving your car (or driving your flying car to the airport).
The ROD-V curve in gliding mode represents the “altitude burn rate required for constant speed.” Analogous to driving mode (and ignoring atmospheric effects that could influence the glide), you can only ever be somewhere on this curve when in a glide.
The beauty of flying mode, however, is that we are not relegated solely to flight on the green “power required for constant altitude” curve. We can be off the curve. Move above it and we are climbing; stay on it and we are in level flight; move below it and we are descending.
Flying is not like driving, or even gliding for that matter.
Regions of Command
The region of reversed command can be defined in two ways. Traditionally, Ve is the crossover point for “reversed command.” Moving left or right away from Ve demands an increase in power to remain on the green curve (power required for level flight). That is, relative to Ve, increasing speeds require increasing power for level flight; yet decreasing speeds also require increasing power for level flight.
A case can be made that Vy is a crossover point for “reversed command,” too. Considering rate of climb instead of power required for constant altitude, and with max available power applied, moving left or right away from Vy results in a decrease in rate of climb. That is, relative to Vy, increasing speeds yield slower rates of climb; yet decreasing speeds also yield slower rates of climb.
“Reversed command” does not mean “reversed controls” any more than “inverted flight” means the elevator works “backwards” compared to upright flight. Habits developed in the region of normal command and in upright flight can lead to erroneous impressions about how the controls function in other regions/attitudes.
“Habits developed in the region of normal command and in upright flight can lead to erroneous impressions about how the controls function in other regions/attitudes.”
Could you give an example which can happen in normal flight, not aerobatic flight?
Every time I think of reverse command I think of Barty Brooks and his unfortunate failed landing in the F100 at Edwards.
Popularly known as the “Sabre Dance” what happened to Brooks was simply that he slipped past the line and into reverse command. He got in so deep that power became useless in any effort to recover. He needed to reduce his angle of attack to stop the sink rate and he didn’t have the air under him to allow that.
This situation could happen to anyone landing any airplane. In fact much of the “thinking” behind pitch for airspeed and power for altitude is based on an attempt to keep pilots from entering this situation.
I would note that there were additional factors involved with Brooks fatal landing. The Hun was noted for its ability to couple between roll and adverse yaw at high angles of attack and low airspeed. These were indeed players in the crash as were the stall characteristics involved with the swept wing on the 100.
But there is no mistake. Had Brooks avoided reverse command and remained on the front side of the power curve he might have avoided what took place.
I have always referred to the deepest corner of reverse command as a “coffin corner” not to be confused to the classic coffin corner at altitude we all know so well. It’s that horrible situation you can enter on the last phase of a final approach were you get yourself into reverse command and develop a sink rate that is unrecoverable with power and where the wing MUST be unloaded to save the day. If this happens you either have the remaining altitude to do or you don’t.
This is a subject instructors are well advised to cover with every student before solo. It doesn’t require a highly technical explanation. It simply is important that every pilot who flies be aware of the dragon who lives behind the curve and who has such big and sharp teeth.
Here is a similar predicament with a better outcome: “Accidental test flight F-16.” Similarly, high AOA and even all that thrust would not climb without lowering the nose: https://twitter.com/i/status/1219027486035582977
Short final/landing — low and slow mush accidents.
Short final/landing — low and slow stall accidents.
Late go-arounds where max angle of climb or max rate of climb is necessary, but not attained.
Short field takeoff at high density altitude with obstacles on the departure end.
A common theme is the pilot’s attempt to climb higher or faster by pulling the elevator control farther aft.
Your turn. Can you give a quick example of teaching practical flying by referencing the V-P diagram?
Rich; I like your analogy using the parts of the body as references. It shows creativity and fits well within the KISS principle theory on flight instruction. The more we as instructors allow the student to identify with everyday things the easier it becomes for the student to comprehend the complexities involved with flying an airplane.
There is plenty of time on the ground for the more “serious” side of flying. Students will usually do much of that learning between dual sessions while they are relaxed and not under the stress of handling the aircraft.
Rich your comment July 7, 2022 7:42pm no reply button. “Your turn. Can you give a quick example of teaching practical flying by referencing the V-P diagram?”
Thanks. Agree completely. I got on a tangent misinterpreting your previous comment. I also do what you listed working carefully also with what happens in ballooned landings and how much power is needed to avoid a stall if the decision is made to re-attempt the flare. Also after a flying lesson with a DPE at Leadville, CO years ago, I’ve always included some climbouts at reduced power to simulate the extreme high density altitude (also simulates partial engine failure) and assure two things, that the student keeps the nose much lower to maintain the right speed, and to have patience and keep the nerves as calm as possible with the much flatter angle and lower rate of climb. This is also covered during climbs in slow flight. And also we do a couple of go-arounds leaving the flaps down simulating a flap failure – I’ve had flaps fail more than once in Cessnas.
Dudley July 7, 2022 at 5:49pm comment – no REPLY button again.
Reverse command is normal on final for short field approaches. The recommended speeds are lower than glide speed. The aircraft is still under perfectly safe control as long as the pilot coordinates the controls. If not flown in that manner, it could be very difficult to accomplish landings near the distances in the POH. There is an important point in the Airplane Flying Handbook that would have applied to the Brooks approach. He was undershooting – the correct control input is to add power first, then pitch up. The extra power first is to hold the airspeed when drag increases with the extra pitch. Brooks pitched up first and was late to add power, and it just kept getting worse from there. The F-100 was a new model and they were to discover it had vicious stalling characteristics. The stall on that aircraft starts at the wing tips and as it progresses inboard, it shifts the center of lift ahead of the center of gravity resulting in a tendency to pitch up. When the afterburner was lit, it combined to raise the nose even more. Brooks was not experienced in the airplane and in general at that time, the characteristics of the airplane were not well understood yet. So anyway, if a short field approach is properly executed, there shouldn’t be any issues with control of the airplane.
Oh my……….where to go with this answer? LOL
Generally speaking here. Slight variances considering a 50′ obstacle being present or not present.
Well, first of all, I have to respectfully disagree with your analysis of a short field landing. Short field landings are NOT flown on the back side of the power curve. It is possible of course to allow the aircraft to slide into reverse command during the very last phase of the approach but the correct way to do a short field landing is to set up properly for a stabilized approach, then fly the approach at 1.3xVso (unless otherwise stated in the POH) which is NOT in reverse command. I would imagine that if an applicant for a PPL set up for a back side short field approach with an examiner in the airplane that student would be on the way back for a bit more dual. :-))))))))))
1.3Vso is an area where you want to be careful of course but it’s still on the front side of the curve.
The area where you CAN go into reverse command and where the real danger lies is as you get close in on final. The “coffin corner” I often speak of can occur if you allow the airspeed to get so low and angle of attack so high that you don’t have the power available to solve the sink rate. NOW you are in reverse command and in as we say……..”deep trouble” (for lack of the better word)
As for Barty and the Hun. His problems were many that day and much of what you said about the Hun is true. But in the end analysis he simply allowed the aircraft deep into that “coffin corner” area I often speak about. He applied all the power he had trying to save it including full burner. It wasn’t enough to fly out of the backside. The rest was simply a combination of all both you and I have commented on about the problems involved with the 100 and how those problems interfaced with the reverse command scenario Brooks had allowed his aircraft to enter.
It was indeed a sad event. Brooks was doing a friend a favor that day and ordinarily wouldn’t have been the pilot on that flight.
Every “soft field” takeoff puts the aircraft into the induced drag side of the curve (reversed command). Done correctly, full power is already in use (flying in ground effect below stall speed) and the nose must be lowered to accelerate and (eventually) climb. (no afterburners on a piston Cessna though🤣)
I’m basing it, for example, on a Skyhawk which has a best glide of 65 and short field approach of 61 and an Archer I fly with best glide of 76 and short-field speed of 66. On very short final, I go below the 61 or 66 to get ‘little to no float’, the objective right out of the Handbook.
David is aware of the reply button issue and is working on it.
These V-P diagrams are based on real data for a Cessna 172 for two conditions: sea level and 10,000 feet density altitude. The available range of speeds between Vs and Vne is unchanged. What changes are the amount of power available and the power required for level flight (i.e., power required for constant altitude). Consequently, the amount of excess power (and with it, rate of climb) that can be used to gain altitude has shrunk as well.
Density altitude affects power, which in turn affects the range of possibilities where the pilot is able to maintain a constant altitude or engage in climbing flight. Again, and notwithstanding the attendant performance consequences, the range of speeds between Vs and Vne is unaffected and remains available to the pilot.
Edit: I tried to add the diagram to the reply, but it didn’t work. See it here: https://www.richstowell.com/v-p-g-diagram/