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Pitch controls airspeed?

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  • MCM
    replied
    I think you nailed it when you said that you only learned this stuff AFTER you learned to fly. For the majority of people at the initial level, they have enough trouble with the "cows get bigger, cows get smaller" concept. It is only after you have flown a fair bit that you understand the control inputs required to achieve an outcome, afterwhich you can understand the in depth answer.

    Why it does it is important to know, but more importantly (especially at training level) is what the pilot needs to do to achieve what he is trying to achieve, be it a constant speed descent, or a glidepath.

    By the way, have you recently heard of the case where a 737 in final approach had it's A/T uncommandedly go to idle and disengage, and that the A/P kept adding ANU trim to keep the plane on the glide slope, and that by when they realized they initiated a go-around setting TOGA but couldn't prevent the plane from keeping nosing up and speeding down despite the application of full AND elevator? The plane stalled and during the fall it accelerated enough to give the elevator enough authority to prevent a secondary stall. Luckily they had enough altitude.
    A great example of speed on thrust . The aircraft path was linked tightly to the elevators, and they followed that path religously. The airspeed decayed due to the thrust being low (yes, due to the many aerodynamic reasons you've given, but that isn't important to the pilot at this point). The recovery was due to underslung engines and being "behind the drag curve"

    It is all about frame of reference... and yours is an aerodynamic one whereas mine is a pilot training and "ultimate effect" one.

    I'm impressed you wanted to learn about it at age 11, and would have been capable of understanding. I, and I fancy the vast majority of airline pilots, would not have been either interested or capable of learning it... especially when they are at the standard of not knowing why the aircraft stalled

    Leave a comment:


  • Gabriel
    replied
    Originally posted by MCM View Post
    Gabriel,

    Of course they are all inter dependant. That is not in question.

    The question was specifically to do with a textbook on how to fly an aircraft, and the term pitch controlls airspeed, throttle controls altitude is specifically to do with approaches.

    It is the two major ways of flying the aircraft from a practical point of view... thrust for speed, or pitch for speed, and I was explaining the differences.
    I say again: In my (strong) opinion, for any given configuration there is only one way to take the plane from state A (a given combination of steady airspeed and steady climb/descent rate) to a new state B, which is by using the only combination of change in elevator/trim and thrust that will make it work. There is, however, more than one way to explain how this happens.

    And the plane and the Physics which said plane flies on care very little whether it is an approach or other phase of flight.

    You are taking this to an entirely different place, which my answer was not intended to go remotely near.
    My answer was. Remember I said I'd explain my view for you to comment, not that I was specifically arguing against your answer. In fact, I said I agreed with your answer 100%.

    A lot of what you say is technically correct,
    Meaning that some of what I've said is not technically correct? Ok, never minds...

    but has nothing to do with the day to day flying of the aircraft of trying to achieve a goal, and the best manner to do so.
    That's just because our responses get automated by repeating the same responses to the same signals we receive. But in fact, yes, that's exactly what we do when we fly everyday.

    It happened to me that I've learned to fly before understanding this. And only AFTER I learnt this I realized that when I was adjusting my descent during the final approach, but keeping the same speed, I was making TWO corrections with the elevator, not one: I'm low, pull up to the new desired descent, then push down again to the previous elevator position (of course, add power in the middle). If you don't do the second motion, the plane just keeps nosing up and, no matter what you do with the power, the plane WILL slow down. And that after the correction, the trim was EXACTLY as before. Of course, it was trimmed for the right speed and I'm still flying at the same speed, even if the GS is of a different angle, that doesn't need a change in trim. I didn't changed what I was doing after learning this. But, suddenly, I realized that I was doing things I didn't know I was and I understood what and why it worked.

    I'll say it again. It's simply impossible to sustain a change in speed without changing the elevator/trim position, and it's simply impossible to sustain a change the elevator/trim position without having your speed change. The truth is that elevator = AoA, and for each AoA there is one and only one speed at which the plane will fly at 1G. That, for me, sounds very much as "elevator controls speed". (I like better "elevator controls thrust/power required, throttle controls thrust/power available", but I figured that THAT WOULD have been too much)

    There is of course a complex interrelationship between pitch, thrust etc... doesn't teach a guy how to fly flight sim though.
    That depends on how serious is the guy.

    Does he wants just to "play the game" or he really wants to "learn to fly a plane" (even if it's a virtual one)?

    By the way, have you recently heard of the case where a 737 in final approach had it's A/T uncommandedly go to idle and disengage, and that the A/P kept adding ANU trim to keep the plane on the glide slope, and that by when they realized they initiated a go-around setting TOGA but couldn't prevent the plane from keeping nosing up and speeding down despite the application of full AND elevator? The plane stalled and during the fall it accelerated enough to give the elevator enough authority to prevent a secondary stall. Luckily they had enough altitude.

    The "not everyday" situation is what counts really. Is this too much for our friend? Maybe. Let him/her judge that.

    I wish I had someone explain me this in my early days of flight sim when I was 11. Hell, I wish an instructor had ever explained me this. I had to go to the 4th year of the university and have Aerodynamics to learn what for me should be basic pilot knowledge at Sport Pilot student level.

    I think in this kind of stuff lives the seed of what makes a person a "pilot" and not an "airplane driver", as 3we has said. Now we only need to know what our friend wants to become, even if only at a virtual level.

    EDIT TO ADD: For the rest of the guys: MCM is a proffessional airliner pilot (I am not, not even remotelly close), for what I've seen in these forums he deserves all my respect and we are not really disagreeing except in what is the right ammount of knowledge our friend is looking for.

    Leave a comment:


  • MCM
    replied
    Gabriel,

    Of course they are all inter dependant. That is not in question.

    The question was specifically to do with a textbook on how to fly an aircraft, and the term pitch controlls airspeed, throttle controls altitude is specifically to do with approaches.

    It is the two major ways of flying the aircraft from a practical point of view... thrust for speed, or pitch for speed, and I was explaining the differences.

    You are taking this to an entirely different place, which my answer was not intended to go remotely near.

    A lot of what you say is techincally correct, but has nothing to do with the day to day flying of the aircraft of trying to achieve a goal, and the best manner to do so. There is of course a complex interrelationship between pitch, thrust etc... doesn't teach a guy how to fly flight sim though.

    Leave a comment:


  • 3WE
    replied
    Oooo Gabe you meany....telling poor Praehapt that he belongs nowhere near a flight simulator!!!!!!!

    I'm afraid I have probably made myself known as a fan of ITS who dishes out some crap- but in the vein of fun.

    Come on MCM- don't you see the ironing- "you belong nowhere near a flight simulator".

    And to Praehapt- A couple of things. 1) I like to say that pitch-airspeed, power-altitude is a rule of thumb that works very well to make someone a pilot and not an airplane driver. No doubt there are exceptions and the true fact is that you really can't do much without changing both pitch and airspeed- but the rule really does work.

    2) I cannot belive that Gabriel did not provide a 2-page answer on what a stall is- If you point the nose too high or go to slow, a plane will stall. It kind of sounds like that's what you did.

    I don't know what book you are reading, but I would suggest Wolfgang Langewishche's "Stick and Rudder". Another thing I would recommend is to make a paper airplane or to buy a balsa wood or styrofoam glider.

    The other thing you should learn about is Phugoid behavior (and that's what the paper airplane is for.

    In the meantime, I would cast a vote that we ban Gabriel for being a jerk.

    Leave a comment:


  • Gabriel
    replied
    Hi MCM.

    In fact, I agree with everything you've said. I was not trying to say that "elevator controls speed, throttle controls altitude" is a universal truth regardless the type of plane. Instead, I was trying to say that flight mecanics is the same regardless the type of plane.

    I will now give you my answers for you to comment.

    There are not several ways to do the maneuvers I described (note that I was very specific in their definition). There are several ways to explain the same thing.

    If you are in an MD-80 at 33,000ft and want to start the descent at the same IAS, what do you do with the controls (step by step)? What will be the position of the controls (comparing before starting the maneuver, during the transition and with the steady descent established)?
    First of all, note that I've said "same IAS" (or better make it CAS), and I didn't mention any specific vertical speed, so it will be that of the pilot's choice.

    One can say "push down to descent at the desired vertical speed and thottle back as needed to prevent the speed from increasing".

    One can also say "reduce trhust for the desired descent rate, use the elevator just to damp the phugoid"

    But it's the same thing. No matter what, at the end once established in the descent at the same IAS, you'll find yourself flying with a reduced thrust and with the elevator in the same position compared to the initial one (no stick force and no trim needed).

    And if you are at 1000ft and want to slow down from 300KIAS to 250?
    Version1: Cut trhust, slowly apply ANU elevator to prevent the plane from descending, when at 250kts add thrust as needed.

    Version2: Cut thrust, slowly apply ANU elevator to slow down (if you don't, the plane will nose down and tend to mintain it's original trimmed speed), when at 250kts add thrust as needed.

    At the end, you'll find yourself flying at a reduced thrust because the plane makes less drag at 250 than 300kts, and you'll be pulling pretty hard on the yoke (until you apply ANU trim).

    And if you are level at 2000ft, plane fully configured for landing and established at Vref, and want to transition to a 700fpm descent to track the GS you are just intercepting but keeping Vref?
    The same as in quesion 1.

    The interesting thing here, again, is that at the end you'll be flying with a reduced trhust and THE SAME elvator, with no need to adjust trim.

    Of course you'd use small elevator inputs to correct short term small vertical speed deviations, and small thrust adjustments to correct fro gusts and other short term deviations.

    But if you find that, for whatever reason, 700fmp is not working and you have to change it to 600fpm, you cannot simply pull up because you'll lose speed (and you want to keep Vref). Explain it however you want, but what you'll do is increse thrust, pull up on the elevator, and return the elevator to its original position.

    What is important to understand is that there is a thing called trim speed, on which either the thrust has no influence or has a small influence for not too big changes in thrust. The trim speed, for a given airplane configuration (weight, GoG, flaps/slats) is function ONLY of the position of the elevator (ok, and of the trimmable stabilizer if there is one). And that's the ONLY speed the plane WILL fly at 1G.

    If you are flying at Vref and -700fpm and want to change it to -600fpm keeping Vref, you pull up (say to 1.1Gs), but if you keep the stabilizer at that new position the plane will slow down (phugoid oscilations in the midle) until the reduced speed compensates for the increased AoA as to make Lift = Weight again and return to 1G, so you'll have departed Vref, NO MATTER WHAT YOU DO WITH THE THROTTLES.

    So what you do is pull up (say to 1.1Gs) until you get -600fpm, then releave the back pressure to return the elevator to it's original position (and hence keep trim speed = Vref), and then (or better simultaneously) add trust as needed to keep the -600fpm (you have to compensate for the lower energy that is coming from loosing altitude). You could also just add the same thrust and do nothing with the elevator and the plane will eventually reach at the same final state, but not without several long phugoid oscilations in the middle.

    But in the end, you just cannot change the speed IN THE LONG TERM without changing the elevator (or trim), and you cannot change the elvator or trim and keep the same speed IN THE LONG TERM.

    That's my view.

    How rude! That is just a rediculous statement, and it is no-wonder people don't like to post new questions. Flight Sim is a great way of learning the basics... and it isn't just for "experts" who recreate reality. There is no need to be rude to people who are just trying to learn.
    I agree. It was intended to be a joke, but it was too much. I want to publicly apoplogize for that.

    I know Praehapt Bubtah is not offended because I've kept PM's with him/her, and he/she knows that I'm a FlightSimmer since the Sublogic Flight Simulator II in the Commodore64, when I was just a kid.

    Leave a comment:


  • MCM
    replied
    Gabriel,

    My answer was specifically in the context I set - that of final approach. I was trying to keep it simple considering the question asked... I am more than happy to go into detail.

    Whether you use pitch or thrust for airspeed is different depending what you are trying to acheive, and if you are using an FMC driven mode to acheive it.

    I'll answer your questions, but I think you might not like a couple of the answers.

    If you are in an MD-80 at 33,000ft and want to start the descent at the same IAS, what do you do with the controls (step by step)? What will be the position of the controls (comparing before starting the maneuver, during the transition and with the steady descent established)?
    It depends what sort of descent you are trying to acheive. If your descent is PURELY based on speed (which is the *basic* concept) then yes, you reduce the thrust to idle and use "Speed on Pitch". This works when you are flying with no flight director or FMC guidance and have manually *guessed* a top of descent point.

    There is a catch... the modern flight director system does NOT predicate a descent based purely on speed. Noting your desire to remove the underslung pitch issue, we'll move to the Boeing 737 through 777 family (I do not know the FMC on the MD80). Instead, it works out its constraint, and taking into account the forecast winds works out a descent point, and a fixed path through space.

    The aircraft attempts to fly this path (called VNAV PATH) and will use, yes, pitch to maintain it. Speed is managed through the use of thrust and drag. Speed on pitch would take you away from this path, which is not the desired result (although it is used to some extent when the Path is not appropriate, and on older aircraft without FMC's).

    And if you are at 1000ft and want to slow down from 300KIAS to 250
    You can look at this either way, but in reality is a combination of controls.

    And if you are level at 2000ft, plane fully configured for landing and established at Vref, and want to transition to a 700fpm descent to track the GS you are just intercepting but keeping Vref?
    This is simple, and the original question I was answering... you use PITCH to maintain the path, and THRUST to manage speed. Thats just how it is done! You don't have to believe me... there are others here who will probably explain it more eloquently. AJ should be able to explain... he's the one with the training background .

    In theory, yes, you could reduce the thrust and pitch for airspeed, adjusting thrust to maintain the RoD, but in reality it is not effective for a large aircaft, and the desired vertical path is far more sensitive and so pitch is used to maintain it, NOT thrust. It does work on small aircraft too... try it sometime.

    And finally
    If you don't understand what I'm talking about, you belong nowhere near Flight Simulator.
    How rude! That is just a rediculous statement, and it is no-wonder people don't like to post new questions. Flight Sim is a great way of learning the basics... and it isn't just for "experts" who recreate reality. There is no need to be rude to people who are just trying to learn.

    Leave a comment:


  • Gabriel
    replied
    Originally posted by Praehapt Bubtah View Post
    Thanks to everyone for their replies, although I think I am more confused since there seems to be some disagreement.

    Today, I noticed the strangest behavior. After I missed the runway, I went to full power and pulled up, but the plane only climbed briefly, In fact, it seemed to try and flip over and I crashed. The plane was also making strange beeping and buzzing sounds.
    Congratulations!

    Now you are ready to take-off an MD-80 with the flaps forgoten up: you already know how to stall a plane!

    (as an alternative, you can try to fourtyone a CRJ, or to land a Colgan)

    On a more serious note, the elvator controls the angle of attack (one angle of attack for each elevator position), and that in turn controls how much speed you need to make lift = weight, what is called the trim speed.

    If you don't understand what I'm talking about, you belong nowhere near Flight Simulator.

    Leave a comment:


  • Praehapt Bubtah
    replied
    Thanks to everyone for their replies, although I think I am more confused since there seems to be some disagreement.

    Today, I noticed the strangest behavior. After I missed the runway, I went to full power and pulled up, but the plane only climbed briefly, In fact, it seemed to try and flip over and I crashed. The plane was also making strange beeping and buzzing sounds.

    Leave a comment:


  • Gabriel
    replied
    Originally posted by MCM View Post
    It depends what type of aircraft you are flying.

    When people talk about this, it is usually specifically in relation to the final approach.

    In light aircraft, such as a C172, it is common to be taught to maintain your airspeed using pitch, which isn't hard to do... if you pitch up, you lose speed, and if you pitch down, you increase speed.

    If you pitch for speed, then adding power naturally makes you climb (or decrease your rate of descent), or reducing power makes you descend (or increase your rate of descent)... so in this respect the textbook is correct... it is certainly how I (along with 90% of students in Australia) were taught to fly.

    This works fine in a light aircraft, however isn't particulally effective in a large one. A jet is usually flown so that pitch is rate of descent and thrust is speed, which as you point out is a little more intuitive.

    Note you can use the second method on a light aircraft usually quite successfully.
    [sarcasm]

    I thought that the Fiziks that governed flight was the same regardless the size of the plane.

    [/sarcasm]

    I don't agree with your view.

    If you are in an MD-80 at 33,000ft and want to start the descent at the same IAS, what do you do with the controls (step by step)? What will be the position of the controls (comparing before starting the maneuver, during the transition and with the steady descent established)?

    And if you are at 1000ft and want to slow down from 300KIAS to 250?

    And if you are level at 2000ft, plane fully configured for landing and established at Vref, and want to transition to a 700fpm descent to track the GS you are just intercepting but keeping Vref?

    (Ok, we'll establish some rules. Full manual flight. No AT no AP. The plane is trimmed for the steady state before the maneuver, but no additional trim will be used to acomplish the maneuver. I said MD-80 just to filter-out the effect of the change of power on trim associated with under-wing engines)

    Leave a comment:


  • ThreeOneBravo
    replied
    Another example of pitch controlling airspeed is when the CF-18 demo does the high alpha pass. It's at full throttle, 35 degrees nose up yet going only 130-140 knots.

    Leave a comment:


  • MCM
    replied
    It depends what type of aircraft you are flying.

    When people talk about this, it is usually specifically in relation to the final approach.

    In light aircraft, such as a C172, it is common to be taught to maintain your airspeed using pitch, which isn't hard to do... if you pitch up, you lose speed, and if you pitch down, you increase speed.

    If you pitch for speed, then adding power naturally makes you climb (or decrease your rate of descent), or reducing power makes you descend (or increase your rate of descent)... so in this respect the textbook is correct... it is certainly how I (along with 90% of students in Australia) were taught to fly.

    This works fine in a light aircraft, however isn't particulally effective in a large one. A jet is usually flown so that pitch is rate of descent and thrust is speed, which as you point out is a little more intuitive.

    Note you can use the second method on a light aircraft usually quite successfully.

    Leave a comment:


  • Praehapt Bubtah
    started a topic Pitch controls airspeed?

    Pitch controls airspeed?

    While trying to learn to fly flight simulator I read a textbook that said pitch controls airspeed and power controls altitude.

    I don't see how that can be right. To descend, you must point the plane downward. And to speed up you would obviosly need more power.

    This must be an error.
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