About Reversed Command Area

4 Answers

1. 
   

   
   

   Bryan on May 09, 2022

   Pitch for airspeed, power for altitude is a flying strategy, not a mathematical formula. The strategy is taught to overcome the counterintuitive nature of the region of reversed command where a slower speed requires more power to maintain steady flight.

   Nevertheless, the math is interesting and informative. As you’ve noted, level flight is maintained by a combination of factors. The pilot can control three of those factors: the shape of the wing (flaps), angle of attack, and speed. The rest, including air density, can only be influenced indirectly, if at all (by climbing for less dense air or descending for more dense air).

   Notice that thrust is not part of the equation for lift. Thus, engine power alone is not going to alter your lift. The lift will only change if the speed, angle of attack, or shape of the wing changes, right? Why then, does the flying strategy suggest powering for altitude? Because the addition of power will increase the speed, cause a pitch-up moment that will increase the angle of attack, or both.

   So if you’re going along in slow flight right above stall speed and you add full power without adjusting the elevator at all, the airplane is going to feel like a balloon. Two things are causing that. First, your airspeed in the lift formula is low so that lift is very sensitive to even the slightest increase in speed (remember, velocity is squared in the formula for lift so it has a large relative effect). Second, the act of adding power causes a pitching moment because the power is applied forward of the center of gravity. This causes the airplane to rotate (countered by the downward pressure exerted by the tail) the nose rises, increasing the angle of attack without any elevator input from the pilot. The act of adding power also puts more air over the wings even if the airspeed doesn’t increase and that increases lift, too. Unchecked, this will lead to a stall because the airspeed will decrease in the climb until the critical angle of attack is exceeded. Ask your instructor to demonstrate an elevator trim stall to see this in full action.

   So back to your question–adding power doesn’t change the air density, but it will change the angle of attack and airspeed. The pilot then uses the elevator to control the airspeed so that the airplane remains in control.

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2. 
   

   
   

   Warren Webb Jr on May 09, 2022

   “I studied Reversed Command Area with PHAK and I have a question for that.
   As we know, Region of Reversed Command is about “Pitch for speed, Power for altitude” to maintain altitude.”
   I believe you heard someone’s mis-interpretation of the Region of Reversed Command. Pitch and power don’t ‘reverse’. The wing and engine are in fixed positions (as opposed to the engines on a V22 Osprey which move from horizontal to vertical). I think you will find these maneuvers interesting to do with your CFI. From cruise speed at safe altitude, gradually reduce the power to idle (don’t shock cool the engine), and hold altitude with the elevator gradually increasing angle of attack until the wing stalls and recover from the stall normally with pitch and power. You will notice nothing ‘reverses’ – the lift vector will oppose gravity easily and maintain altitude to the stall. Now stabilize the airplane in level flight at several speeds – for a C172 make that 85, 75, 65, 55, and 45, all flaps up. Jot down the power needed to hold each airspeed. You will find that 75 requires less power than 85, and 65 requires less power than 75. Then the interesting part – 55 will require MORE power than 65, and 45 MORE power than 55. You have just demonstrated the regions of Normal (85-65) and Reversed (65-45) Command. Less speed in the region of normal command – less power. Less speed in the region of reversed command – MORE power. Exactly what is said in the PHAK. Throughout the exercise (85-45), the lift vector opposes gravity – the thrust vector opposed drag. Why is this important. The FAA wants you to know that as you get very slow, drag increases, not decreases (like in a car) and so you can’t be bashful about using the power to overcome a lot of drag which gets worse as you get slower, especially at full flaps, to avoid a stall.

   I had an interesting experience with someone who was applying for a job. We were in a Cherokee Six and approaching a runway with a PAPI. On final, he got low (flying toward a point past the numbers but below the glideslope) – red-over-red. He increased power. The only thing that happened was that we accelerated 10 knots from 80 to 90. But we stayed on exactly the same line of flight and we were still red-over-red (there were no obstacles). Later he was scratching his head and said he didn’t understand why we didn’t get back up to glideslope when he increased the power. Think about the forces. He increased the thrust vector which was pointed downward at the beginning of the threshold. We therefore accelerated straight toward that point. To go up to the glideslope, he needed to also increase the lift vector which was pointed at the glideslope – i.e. as is explained in the Airplane Flying Handbook chapter 9, increase power to hold speed while simultaneously increasing pitch to correct altitude.

   There are two main factors, I think, that cause confusion with this subject. First is simply that many people misinterpret the term ‘region of reversed command’. As the PHAK explains, it is how drag reverses, not how controls reverse because they don’t. And second, in light single-engine aircraft on approach and properly trimmed out, the descent can in fact be controlled with the power, only because the propeller creates that corkscrewing slipstream around the fuselage and downward pressure on the tail, so indirectly you can control the pitch with the power. But what you are really doing is controlling pitch with a cushion of air which as you can imagine can get very hazardous if conditions aren’t near perfect. So if you increase power and the propeller slipstream lowers the tail, it gives the illusion that power controls altitude. Unfortunately for the pilot I told you about in the Cherokee Six, he didn’t understand how that all fit together, the pitch didn’t change, and we stayed on the same low path. The proper technique was increase power and simultaneously apply back pressure, each as needed.

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3. 
   

   
   

   Warren Webb Jr on May 09, 2022

   This video includes an excellent explanation and illustration of the Regions of Command starting at 2:34.

   https://www.youtube.com/watch?v=ht8Mh6ELicw&lc=UgxRtLeAEGrzY7rMAJZ4AaABAg

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4. 
   

   
   

   Warren Webb Jr on Jul 21, 2022

   This of course does not mean the pilot can add power and raise the pitch in an unlimited way to control altitude. Every airplane has it’s limitations which should by then be well understood – for a Skyhawk flaps up at full power, the most the pitch can be raised without a loss of airspeed is to about positive 10 degrees – with flaps down, much less than that. The maximum force in any airplane against a downdraft would be to apply full power and pitch for Vx.

   With that said, pitching to airspeed would be obviously correct for go-arounds (follow the POH), climbs, engine failures, or partial engine failures when the engine cannot maintain the desired speed.

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