Pitch & Power, Altitude & Airspeed Question

[Alamanach]

Undaunted Flyer, if ALTITUDE CHANGE MODE is characterized by large changes in altitude, then is it fair to say that altitude changes in FLIGHT PATH MODE are always relatively small in magnitude? (I can see how descent in final approach, as important as it is, is perhaps not all that large a descent.)

 

[UndauntedFlyer]

Undaunted Flyer, if ALTITUDE CHANGE MODE is characterized by large changes in altitude, then is it fair to say that altitude changes in FLIGHT PATH MODE are always relatively small in magnitude? (I can see how descent in final approach, as important as it is, is perhaps not all that large a descent.)

In FLIGHT PATH MODE pitch changes for altitude correction represent small immediate changes in altitude which are really just corrections. While on final approach this may represent a 10,000 foot loss in altitude if it's a 30 mile final but it is still a FLIGHT PATH MODE if it is on a path such as a VASI or an electronic glide slope.

 

[3BCat]

What UndauntedFlyer meant to say was...

All conventional airplanes are subject to the same principles of aerodynamics.

I agree with what you said. Your answer, however, (which by the way is not the only correct one) seems a little complicated. It's beyond me why you would answer a basic question about the relationship between pitch, power, airspeed and altitude using terminology that only has meaning in advanced aircraft.

When you learned to fly, is that how your instructor answered the same question from you? Probably not.

 

[UndauntedFlyer]

3BCat: Thanks for your reply. I really don't think there is anything that any student pilot with an 8th grade education can not understand. Whether it's a Technically Advanced Aircraft (TAA) like a Cessna 172 with a Garmin 1000 and an A/P, a B777 or a Cessna 172 with just steam gauges it's all the same.

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

And in answer to your last question, I really do wish someone could have explained it this way. It has always been one way or the other as some still do on this board. But I never did hear an explaination of two modes of power and pitch relationships, FLIGHT PATH MODE and ALTITUDE CHANGE MODE, because no one ever heard of such concepts then.

 

[Alamanach]

Two pertinent comments from Stick and Rudder, by Wolfgang Langewiesche. First, on page 150:

"(T)he elevator determines the Angle of Attack at which the airplane will fly, and this is really what the elevator is for: it is the airplane's Angle of Attack control."

On page 158, commenting on the idea that the elevator elevates the airplane:

"This notion is reinforced by the name of the control, the fake erudition that prefers the Latin and misleading word, elevator, to the honest old pilot's word, flippers."

Reading his chapter titled "The Flippers and the Throttle", he seems to want to come down on the side of elevator:airspeed throttle:altitude, but he is forced to concede that there are exceptions. He doesn't have a solution with the simplicity of Undaunted Flyer's FLIGHT PATH MODE/ ALTITUDE CHANGE MODE (FPM/ ACM).

I'm not entirely clear on the defining difference between FCM and ACM. Either one can include substantial altitude changes. Does ACM consist of high-performance changes, while FCM is, for lack of a better word, easy? If ACM is high-performance flying, and if as KeroseneSnorter says, GA planes are underpowered, that might explain why ACM elevator/throttle flying is taught to us beginners.

 

[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.