Pitch & Power, Altitude & Airspeed Question

[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...

 

[VNugget]

Controlling airspeed in a power off glide is light years beyond what a student needs to know?

 

[FlyBieWire]

I just got home from school and have now read all the replies to my question. Thank you all for your help.

I can see the answer to my question is not cut and dry. I do like UndauntedFlyer's Flight Path Mode (FPM) and Altitude Change Mode (ACM) explanation. I don't think I have read it anywhere else but it seems to make the most sense to me at this point in my training.

I’m sure there are going to be exceptions to any rule but I like the concept of the role reversal of power and pitch as the Modes change.

 

[UndauntedFlyer]

Controlling airspeed in a power off glide is light years beyond what a student needs to know?

Controlling airspeed and angle/rate of descent when beyond the limits of the demand is light years beyond the objectives of this thread.

As for the above question only, then you are in a Altitude Change Mode until the assigned altitude is captured.

 

[UndauntedFlyer]

I’m sure there are going to be exceptions to any rule but I like the concept of the role reversal of power and pitch as the Modes change.

The role reversal concept is the most important difference in this explanation vs. Wolfgang's "Stick & Rudder" explanation. No automatic airplane could possibly fly until the two modes, FLIGHT PATH MODE & ALTIDUDE CHANGE MODE were separated. And I will say that no pilot can fly an airplane until he/she understands this either, consciously or unconsciously.

Your questions or comments are welcome.....

 

[pilotyip]

I only know what I learned landing solo on an aircraft carrier in the T-28C with a total of 125 hours of flt time. If the meatball goes one index high reduce power 1", if the meatball goes low one index add 1" of power. Power effects your descent rate. If airspeed in above 82 Kts, raise the nose, if airspeed is below 82 Kts lower the nose. Nose attitude effects your airspeed. Seemed to work for me.

 

[Alamanach]

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

Can you explain what "power available" and "power required" are?


I agree with FlyBieWire, I like the FPM/ ACM explanation too. And I think the existence FMS is a strong point in favor of Undaunted Flyer's main argument. But...

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... 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.

from an engineering standpoint, what makes the "fixed" flight path of FPM different from the unspecified flight path of ACM? There are similar forces and similar dynamics at play, and it is not obvious what distinguishes one flight mode from the other (in a mathematical/ physical/ engineering sense). Unless the distinction is simply that FPM is a more precise mode of flying. If that is the case... we may be back to square one.

 

[UndauntedFlyer]

Can you explain what "power available" and "power required" are?


I agree with FlyBieWire, I like the FPM/ ACM explanation too. And I think the existence FMS is a strong point in favor of Undaunted Flyer's main argument. But...



from an engineering standpoint, what makes the "fixed" flight path of FPM different from the unspecified flight path of ACM? There are similar forces and similar dynamics at play, and it is not obvious what distinguishes one flight mode from the other (in a mathematical/ physical/ engineering sense). Unless the distinction is simply that FPM is a more precise mode of flying. If that is the case... we may be back to square one.

I would not really want to explain the Power available vs. Power required idea. That is something altogether different from the concept in this thread.

Now regarding the FPM differences from the ACM. Besides what has already been mentioned the answer is that ACM mode always has the throttle in a fixed position such as idle or maybe 1800 RPM for descents. And then the throttle will be at takeoff or climb power for climbs. In the FPM the throttle would normally be somewhere in the middle moving as necesasry for airspeed control.

 

[Alamanach]

...the answer is that ACM mode always has the throttle in a fixed position such as idle or maybe 1800 RPM for descents. And then the throttle will be at takeoff or climb power for climbs. In the FPM the throttle would normally be somewhere in the middle moving as necesasry for airspeed control.

Someone must have paid my electric bill... the lights just went on! Thanks, I'll go think this over.

 

[3BCat]

FlyBieWire

Pitch , Power and Trim. I'm glad you found an explanation that you like. Just remember, its best not to run out of airspeed, altitude and ideas all at the same time!

Keep 'em flyin'!

 

[VNugget]

Can you explain what "power available" and "power required" are?

Power required is the power required to maintain level flight at any given speed. Power available is the actual power produced by the engine/prop at the moment. If the Pavailable = Prequired you're gonna be in level flight. If Pavailable exceeds Prequired, then you have excess power and you're gonna climb. If Pavailable is less than Prequired, then there's a deficiency of power and you're gonna descend.

If you have an engineer's aproach to things, I highly recommend this book: Aerodynamics for Naval Aviators. It may not be easy to digest; I'm not an engineer (I failed out of first year college calculus twice) but each section makes sense after a couple of read-throughs. The more I understood from it, the more I connected the basic underlying flight mechanics principles to how an airplane behaves in real life, thus increasing the connection between man and machine.