Pitch & Power, Altitude & Airspeed Question

[Caveman]

'Stick and Rudder' is considered by many to be the definitive book on the subject of flying. A lot of what he says about the way an airplane moves in a mass of air is helpful but the rest of it is a crock IMO. There. I said it. I know, blasphemy, right? A soon as I figured out that his idea of the relationhip between pitch and power was wrong (IMO), I lost all faith in the rest of his book. That relationship is the base knowledge from which all other discussions must evolve. If we don't agree on that that, then further conversation moot.

This topic is like the debate over abortion. It's unlikely that anybody will ever change their mind baed on rational argument. I'll fall on my sword CONVINCED that pitch = altitude and no amount of logic will persuade me otherwise. My colleagues in the pitch = airspeed camp will probably do the same.

 

[Alamanach]

This topic is like the debate over abortion. It's unlikely that anybody will ever change their mind baed on rational argument.

I'd have to disagree with you there. My background is in mechanical engineering, and as such I'm inclined to believe that there must be a mathematically rigorous relationship between pitch, power, airspeed and altitude that can be objectively determined. Once we work out what that relationship is, anybody who disagrees will be demonstrably wrong.

 

[3BCat]

I really do believe that this is the only explanation that can be understood because it is totally correct.

Again, I agree with your statements, except that "this is the only explanation that can be understood because it is totally correct." Everyone learns differently, and at their own pace. Some students are visual learners...some are tactile...you know all this. To say that there is only one way of explaining this is absurd.

The terms FLIGHT PATH MODE, and ALTITUDE CHANGE MODE are highly descriptive, but not intuitively obvious. Terms such as SPEED ON THRUST, SPEED ON ELEVATOR, THRUST CLIMB, OPEN CLIMB could also be used and confused. Hence, the question from Alamanach; trying to totaly understand.

There is more than one way to take the skin off a cat.

 

[UndauntedFlyer]

I'd have to disagree with you there. My background is in mechanical engineering, and as such I'm inclined to believe that there must be a mathematically rigorous relationship between pitch, power, airspeed and altitude that can be objectively determined.

This relationship has already been worked out and it's demonstrated on every airplane with an FMC. My prior post on this confirms the mathematical - mechanical - engineering relationships do work. And as I have said so many times in my post, "This is the same in any airplane from a Cessna 172 being flown by hand to a B777 being flown by the auto-flight system."

The answer is known and proven in every B757, 767, 777 as well as many other FMC equipped airplanes. If Stick and Rudder, by Wolfgang Langewiesche says something different it is because he didn't really understand what we know now. Wolfgang just didn't understand the key which is two-fold, so instead he tried to answer this question in a singular sense. But as I have said, the answer is two-fold:

• FLIGHT PATH MODE vs.,
• ALTITUDE CHANGE MODE.

FLIGHT PATH MODE applies when the airplane is operating on a fixed FLIGHT PATH, such as in cruise or descending on a glide path like an ILS glide slope, VASI or even a VNAV path. In the FLIGHT PATH mode the FMS computer holds the flight path (altitude) with pitch and holds the airspeed with (automatic) throttles. This is the same in any airplane from a Cessna 172 being flown by hand to a B777 being flown by the auto-flight system.

ALTITUDE CHANGE MODE applies when the airplane must make LARGE CHANGES IN ALTITUDE either in a climb or in a descent without concern to a particular path. A typical climb or a descent to a newly assigned altitude while holding a constant airspeed, is an example of ALTITUDE CHANGE MODE. And in this mode, the FMS computer controls altitude with power by setting (max) climb power for climbs, or by setting idle power for descent. And in either a climb or a descent, pitch controls airspeed. So in automatic airplanes large altitude changes are made by using power for altitude control and pitch for airspeed changes. This is the same in any airplane from a C-172 being flown by hand to a B777 being flown by an auto-flight system.

Your Questions or Comments are Welcome............

 

[VNugget]

Both control both. It's that depending on the mode of flight, it's more of one or the other.

According to the hard mathematical relationships, it's pitch controls airpseed and power controls altitude

Those hard mathematical equations being:

angle of climb = arcsin((thrust-drag)/weight)

rate of climb = (power available - power required) / weight

Airspeed = sqrt(Wing loading/(Coefficient of lift*density ratio))

No arguing with that. Things like high/low thrustline, downwash on tail etc. notwithstanding, there is an absolute relationship between AOA and speed. Whether you're climbing, level, or diving, A AOA is gonna get you X airspeed and B AOA is gonna get you Y airspeed, always, period.

The thing is that those hard mathematical relationships are correlations, and correlations aren't always the most useful in determining the best control inputs, which are a more fluid, sensual thing. A speed increase from one low speed to another on the back side of the power curve is gonna involve a huge trim change, and the fact that AOA = airspeed will be a lot more apparent than a speed change from a cruise speed to a faster cruise speed, in which case the trim change will be tiny, and the pilot will more readily think of the primary control as the throttle.

 

[UndauntedFlyer]

Both control both. It's that depending on the mode of flight, it's more of one or the other.

According to the hard mathematical relationships, it's pitch controls airpseed and power controls altitude

Those hard mathematical equations being:

angle of climb = arcsin((thrust-drag)/weight)

rate of climb = (power available - power required) / weight

Airspeed = sqrt(Wing loading/(Coefficient of lift*density ratio))

No arguing with that. Things like high/low thrustline, downwash on tail etc. notwithstanding, there is an absolute relationship between AOA and speed. Whether you're climbing, level, or diving, A AOA is gonna get you X airspeed and B AOA is gonna get you Y airspeed, always, period.

The thing is that those hard mathematical relationships are correlations, and correlations aren't always the most useful in determining the best control inputs, which are a more fluid, sensual thing. A speed increase from one low speed to another on the back side of the power curve is gonna involve a huge trim change, and the fact that AOA = airspeed will be a lot more apparent than a speed change from a cruise speed to a faster cruise speed, in which case the trim change will be tiny, and the pilot will more readily think of the primary control as the throttle.

This all sounds impressive but are you really going to tell a student this when he's on final approach and getting a little low and slow? I don't think so. I am certain that it is much better to say,

"Pitch the nose up a bit to hold the GP and add some power for the airspeed, remember we're operating in FLIGHT PATH MODE now."

Doesn't that seem easier?

Your questions or comments are welcome...

 

[VNugget]

No, but I'd tell him to add power to get back up to glidepath.

If a student flares too high and is about to drop it in, would you tell him to pitch up to arrest his sink rate?

 

[MauleSkinner]

Again, I agree with your statements, except that "this is the only explanation that can be understood because it is totally correct." Everyone learns differently, and at their own pace. Some students are visual learners...some are tactile...you know all this. To say that there is only one way of explaining this is absurd.

The terms FLIGHT PATH MODE, and ALTITUDE CHANGE MODE are highly descriptive, but not intuitively obvious. Terms such as SPEED ON THRUST, SPEED ON ELEVATOR, THRUST CLIMB, OPEN CLIMB could also be used and confused. Hence, the question from Alamanach; trying to totaly understand.

There is more than one way to take the skin off a cat.

I agree with 3BCat...Over the years I've found that I tend to learn things differently than others, and can say from personal experience that the explanation you understand isn't necessarily the explanation that I'm going to understand.

As flight instructors, it's our responsibility to be able to find the explanation that clears it up for the student...NOT the one that makes the instructor feel good.

Fly safe!

David

 

[Caveman]

I'd have to disagree with you there. My background is in mechanical engineering, and as such I'm inclined to believe that there must be a mathematically rigorous relationship between pitch, power, airspeed and altitude that can be objectively determined. Once we work out what that relationship is, anybody who disagrees will be demonstrably wrong.

I already think they are demonstrably wrong, but this is one of those debates where people are so firmly entrenched that no amount of logic or reason is going to change anybody's mind. Mine or theirs.

 

[UndauntedFlyer]

The FLIGHT PATH MODE & ALTITUDE CHANGE MODE are approximately the terms that will be used in all future aircraft with autopilots. So if your student pilot has any aspirations of ever flying such airplanes it would be very nice for them to learn it this way now. Otherwise that student will have to learn this later and un-learn what was taught to them earlier. Besides, this is totally correct now, will be correct in the future and was correct in Wolfgang's "Stick & Rudder" time too.

If a pilot wants to understand Technically Advanced Aircraft (TAA) they might as well start thinking in terms of FLIGHT PATH MODE & ALTITUDE CHANGE MODE now.

Questions or Comments are welcome.....

 

[VNugget]

I repeat my question.

 

[Holding Short]

With everything else staying the same(no extra drag added) and the same performance wanted(same A/S; and same vertical speed), you will still end up with the same combination of AOA(elevator) and power setting. If you could some how measure it. Whether your from one school of thought or the other.

Just pick one.

HS

 

[3BCat]

UndauntedFlyer - Care to explain?

If a pilot wants to understand Technically Advanced Aircraft (TAA) they might as well start thinking in terms of FLIGHT PATH MODE & ALTITUDE CHANGE MODE now.

Questions or Comments are welcome.....

I have a question.

The airspeed is set to maintain 250 kts. with the autothrust on. A high angle of descent (-5 degrees) is set in the FMS. The airplane reaches idle thrust, and starts to accelerate beyond the selected speed. The elevator is now controlling BOTH altitude and airspeed, because thrust is fixed at idle.

Which mode am I in, Flight Path Mode, or Altitude Change Mode?

 

[UndauntedFlyer]

No, but I'd tell him to add power to get back up to glidepath.

If a student flares too high and is about to drop it in, would you tell him to pitch up to arrest his sink rate?

Personally I would tell him to fly the airplane. And in the case of over flaring it would, of course, require the additon of power to assure a safe landing. In this case the airplane has actually exceeded the limits of the flight path mode and is now moving over the the altitude change mode. Thus power is needed to slow the descent rate.

 

[UndauntedFlyer]

I have a question.

The airspeed is set to maintain 250 kts. with the autothrust on. A high angle of descent (-5 degrees) is set in the FMS. The airplane reaches idle thrust, and starts to accelerate beyond the selected speed. The elevator is now controlling BOTH altitude and airspeed, because thrust is fixed at idle.

Which mode am I in, Flight Path Mode, or Altitude Change Mode?

No man can serve two masters and that is the situation here. The airplane is neither in a Flight Path Mode or an Altitude Change Mode; it is in an airspeed hold and an angle of descent hold or on Boeings a rate of descent mode. This is, as I have said, requiring it to serve two masters which it can not always do unless the airplane is well within a small envelope.

In any case, what we are talking about here is teaching a concept to new students such as "Flybiewire" who started this thread. He is a 15-year old high school student. So I think what you are addressing is light-years beyond what he needs to know. His objectives are to make normal climbs, fly straight and level, do slow flight, descents and landings. I am certain that the best approach here, and for any new pilot, is to teach FPM and ACM that I have presented in this thread.

Do others on this Board like the Flight Path Mode and Altitude Change Mode method of teaching what controls altitude and airspeed?

Your comments and questions are welcome...