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Thread: Pitch Mode

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  1. #1
    Captain Thor's Avatar
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    Pitch Mode

    One of my aviation mentors, who also happened to be delivering ground school to me, challenged me with a question.

    He asserted (just to get me going and thinking, I now understand - I'm pretty easy to bait into debate on these kinds of things) that if Pitch Mode is used in descent along with a thrust reduction, the result will be a descent and an increase in airspeed.

    We never use pitch mode except during takeoff and other times by accident so I didn't have experience to back up my argument. Nonetheless, I asserted that this was simply not possible, that it would result in a descent with a reduction in airspeed.

    He countered with "Attitude plus power equals performance. Everybody knows that!" or some such, just to throw me off the trail.

    I asserted my understanding of physics and aerodynamics told me that a change from level flight to descent at a constant pitch could only result in an increase in angle of attack, and "everybody knows" that is almost synonymous with a reduction in airspeed.

    We continued to debate and agreed to test my postulate during my proficiency check in the airplane the following day. It turned out we never got a chance to test it due to the examiner's busy schedule, traffic, and other factors but I still couldn't let it go.

    I called a friend, retired heavy jet Captain and current sim instructor but he couldn't seem to grasp what I was asking from my description over the phone. Another Airbus pilot could agree in principle with my theory but wasn't sure it would work in real life. It turns out that this concept is not that easy to discuss, even with highly experienced aviators.

    Wanna give it a try?

    (the discussion, I mean)
    Last edited by Thor; April 7th, 2013 at 01:10. Reason: correct typo
    Thor Odinson... you have betrayed the express command of your king. Through your arrogance and stupidity, you've opened these peaceful realms and innocent lives to the horror and desolation of war! You are unworthy of these realms, you're unworthy of your title, you're unworthy... of the loved ones you have betrayed! I now take from you your power! In the name of my father and his father before, I, Odin Allfather, cast you out!

  2. #2
    Captain Thor's Avatar
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    I finally got a chance during a repositioning flight to give this theory a try. It was a week ago so the numbers or from my not so great memory but should be pretty close.

    Basic result is that maintaining pitch and reducing thrust results in decreasing altitude and reducing speed.

    Started at FL290, 92% N1, 2.5 degrees nose up, 251 Knots indicated airspeed (IAS) in level flight.

    We got the clearance to descend so after briefing the FO on what I was going to do, I selected pitch mode on the autopilot and reduced thrust to about 80% N1.

    Result was 2.5 degrees nose up, 236 Knots IAS, and I don't recall exactly but think the rate of descent was 800-1000 FPM. At any rate, we were going down fast enough not to get "reminded" by ATC...

    I'm going to try to make some rough sketches on my iPad and then upload them here to illustrate why I think this is...

    Pics will not be pretty. I generally cannot draw.

    Stand by....
    Thor Odinson... you have betrayed the express command of your king. Through your arrogance and stupidity, you've opened these peaceful realms and innocent lives to the horror and desolation of war! You are unworthy of these realms, you're unworthy of your title, you're unworthy... of the loved ones you have betrayed! I now take from you your power! In the name of my father and his father before, I, Odin Allfather, cast you out!

  3. #3
    Captain Thor's Avatar
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    I drew some rough sketches to illustrate the concept we're discussing. (I'm really hoping someone will chime in so I'm not just talking to myself!)

    Just a reminder that the "relative wind" is in exact opposition to the path of the aircraft and the "angle of attack" is the angle between the relative wind and the chord line of the wing. Thus in level flight the the aircraft pitched up 2.5 degrees, it looks roughly like this:

    Name:  pitchlevel.jpg
Views: 2624
Size:  51.9 KB

    You can see the relative wind and the path of the aircraft or opposite but more or less parallel (small 2.5 degree difference).

    Then, after engaging Pitch mode (i.e. commanding the aircraft to maintain the 2.5 degrees nose up attitude), and reducing thrust, the situation looks more or less like this:

    Name:  pitchdescending.jpg
Views: 2609
Size:  75.7 KB

    As the aircraft descends, the relative wind is still exactly opposite the path of the aircraft but the pitch remains the same. The chord line remains at the same angle relative to the horizon but the path of the aircraft is different, hence the direction of the relative wind has changed. The angle between the relative wind and the chord line has increased significantly and in this poorly drawn cartoon, it's very steep for illustrative purposes.

    When pitch remains the same and the aircraft descends, the angle of attack is increased.

    And as we learned as private pilots, increasing angle of attack in otherwise normal flight results in a decrease in airspeed.

    What am I missing?
    Last edited by Thor; April 7th, 2013 at 02:10.
    Thor Odinson... you have betrayed the express command of your king. Through your arrogance and stupidity, you've opened these peaceful realms and innocent lives to the horror and desolation of war! You are unworthy of these realms, you're unworthy of your title, you're unworthy... of the loved ones you have betrayed! I now take from you your power! In the name of my father and his father before, I, Odin Allfather, cast you out!

  4. #4
    I'm surprised there hasn't been more discussion on this brain teaser...

    ;-)
    I intend to fly until my beard gets caught in the propeller. And since I don't plan on growing a beard, that may be a while. Ernest K. Gann, age 69


    Daily Diatribe - (Carl's Blog)

  5. #5
    I think there hasn't been much discussion because someone's afraid to formulate a big theory and then get it smashed by someone who knows what they're talking about. (Just speaking for myself, of course). That said, we generally use a combination of speed mode and vertical speed mode in the CRJ. Several of the gray-hairs use pitch mode, a holdover from one of our older fleet types I suppose. From time to time, I use it too. But, I've never used it for a descent - climbs only. In the CRJ, speed mode leaves the jet wallowing through a range of pitches to hold a constant airspeed or mach, and I've seen it go from 50FPM to 1500 fpm trying to hold that speed in turbulence. Vertical speed mode is smoother, but requires a constant eye on the speed - Jets being normally-aspirated firebreathers, they are losing power from the moment climb power is set, so 2300 fpm might hold 290 knots at FL180, but you're doing 700-1000 FPM at top of climb or when you transition to MACH. Pitch mode seems to hold nearly constant speed with zero hunting in the climb and a lot less wheel-spinning to keep things happy.

    Anyway, that bunch of fluff serving as the foundation for the theory, here goes. In the lower speed ranges, the correlation between speed, power and pitch are clear in level flight, up to a certain point. Between 250K and 310K, there isn't a whole lot of change in pitch to hold level flight, but as you get below 230K the changes are marked. I'd imagine that at lower speeds, you might see the numbers act more like you anticipate. I'd imagine that the characteristics to counter critical mach and such are to blame for airplanes not acting like airplanes when they're flown at spaceship altitudes and speeds.

    my $0.02.

  6. #6
    Captain Thor's Avatar
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    PP,

    First, I think one of the cool things about this site is that gentlemen and ladies can disagree like ladies and gentlemen.

    I have it on good authority that TPTB (the powers that be) on this site won't tolerate the kind of flaming and baiting that goes on in other online forums.

    Anyway, the original question wasn't really about "technique" for using the autopilot but rather understanding geometry and aerodynamics.

    Most autopilots have trouble with descent in airspeed mode but the CRJ autopilot is particularly crappy. The Avro did a reasonable job but it actually had "adult" avionics (and auto throttles!). <grin>

    Anyway, the point was to understand why the plane would decrease speed if pitch remained the same.

    So please, if my amateurish drawings and explanations have that wrong, tell me how!



    Quote Originally Posted by Pasturepilot View Post
    I think there hasn't been much discussion because someone's afraid to formulate a big theory and then get it smashed by someone who knows what they're talking about. (Just speaking for myself, of course). That said, we generally use a combination of speed mode and vertical speed mode in the CRJ. Several of the gray-hairs use pitch mode, a holdover from one of our older fleet types I suppose. From time to time, I use it too. But, I've never used it for a descent - climbs only. In the CRJ, speed mode leaves the jet wallowing through a range of pitches to hold a constant airspeed or mach, and I've seen it go from 50FPM to 1500 fpm trying to hold that speed in turbulence. Vertical speed mode is smoother, but requires a constant eye on the speed - Jets being normally-aspirated firebreathers, they are losing power from the moment climb power is set, so 2300 fpm might hold 290 knots at FL180, but you're doing 700-1000 FPM at top of climb or when you transition to MACH. Pitch mode seems to hold nearly constant speed with zero hunting in the climb and a lot less wheel-spinning to keep things happy.

    Anyway, that bunch of fluff serving as the foundation for the theory, here goes. In the lower speed ranges, the correlation between speed, power and pitch are clear in level flight, up to a certain point. Between 250K and 310K, there isn't a whole lot of change in pitch to hold level flight, but as you get below 230K the changes are marked. I'd imagine that at lower speeds, you might see the numbers act more like you anticipate. I'd imagine that the characteristics to counter critical mach and such are to blame for airplanes not acting like airplanes when they're flown at spaceship altitudes and speeds.

    my $0.02.
    Thor Odinson... you have betrayed the express command of your king. Through your arrogance and stupidity, you've opened these peaceful realms and innocent lives to the horror and desolation of war! You are unworthy of these realms, you're unworthy of your title, you're unworthy... of the loved ones you have betrayed! I now take from you your power! In the name of my father and his father before, I, Odin Allfather, cast you out!

  7. #7
    Well, I think you got it right, Thor.

    I'm still surprised we couldn't coax more folks into the discussion.

    It's a brain teaser...
    I intend to fly until my beard gets caught in the propeller. And since I don't plan on growing a beard, that may be a while. Ernest K. Gann, age 69


    Daily Diatribe - (Carl's Blog)

  8. #8
    I think the Thunder God is right.

    I also think that we could do a lot better teaching new students fundamentals like angle of attack.
    Hagar likes boats. I like airplanes!

  9. #9
    Captain Thor's Avatar
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    I got to have a brief meal with my heavy jet friend and asked the question again.

    When I outlined the parameters for him in person, he immediately said, "Airspeed will drop."

    That was a relief!
    Thor Odinson... you have betrayed the express command of your king. Through your arrogance and stupidity, you've opened these peaceful realms and innocent lives to the horror and desolation of war! You are unworthy of these realms, you're unworthy of your title, you're unworthy... of the loved ones you have betrayed! I now take from you your power! In the name of my father and his father before, I, Odin Allfather, cast you out!

  10. #10
    I asserted that this was simply not possible, that it would result in a descent with a reduction in airspeed.
    Last edited by Carl; April 24th, 2016 at 06:22.

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