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The following quote is from G. Bristol’s book, Ace the Technical Piloting Interview:

[The] movement of the centre of gravity is due to a change in weight. The distribution of the aircraft’s weight can change for three reasons and thus cause the centre of gravity to move. The three reasons for a change in aircraft’s weight are:

  1. Fuel Burn
  2. Passenger Movement
  3. High Speeds*

*Note. This is so because the greater the speed, the greater the lift created. To maintain straight and level, the aircraft adopts a more nose-down attitude, accomplished by creating lift at the tailplane. This lift effectively reduces the weight of the tailplane section of the aircraft.

I was surprised to read that the centre of gravity (CG) changes with “High Speeds.” As I understand it, the horizontal stabiliser produces a downwards force to counteract the moment about the CG produced by lift acting through the centre of pressure.

Though the adoption of a nose down attitude at higher speeds to maintain straight and level does make sense due to the increased speed (assuming constant $ \rho $).

But I am still struggling with the sentence, “This lift effectively reduces the weight of the tailplane section.” I just can’t see how speed could change the actual CG.

Does the actual CG change at “High Speeds”?

(Though I do understand that aerodynamics is universal to all aircraft, if there are differences in discussion between light aircraft compared to turboprops and passenger jets, I would prefer the discussion to be more towards the latter, thank you)

tedioustortoise
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  • Perhaps more grammer than physics " reason for a change " implies moving CG to re-balance it with aerodynamic forces. Moving crew was commonly done in airships for trimming. – Robert DiGiovanni Feb 03 '22 at 14:28
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    @RobertDiGiovanni If nothing else, the sentence "this lift effectively reduces the weight of the tailplane section of the aircraft" is a non-sequitur even though trimming is sometimes done by mass redistribution. It is a non-sequitur because the center of gravity is determined by actual weight, which does not change as a result of aerodynamic forces on the tailplane. – sdenham Feb 03 '22 at 16:58
  • As a technicality, anything which applies a force to the aircraft can cause bending, which would move the CG. It would surprise me if drag forces from high speed move it significantly, but technically it does have an effect. – Cort Ammon Feb 03 '22 at 18:52
  • @CortAmmon This would be yet another way to answer the question posed by the title without addressing the issues tedioustortoise is asking about in the body of the question. No doubt someone will soon point out that if the airplane ls moving along at high speeds, then so is its CG. – sdenham Feb 03 '22 at 22:28
  • @sdenham and that would be why it was a comment rather than an answer. – Cort Ammon Feb 04 '22 at 01:21
  • @CortAmmon Indeed, which is why I wrote "would be"... I may have become oversensitive to anything that might be read as suggesting that the argument quoted in the question is in any way reasonable. – sdenham Feb 04 '22 at 14:06

6 Answers6

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Changing the aerodynamic forces on any part of the aircraft CANNOT change the distribution of mass within the aircraft structure. The center of mass, (AKA center of gravity) can ONLY be changed by moving mass around within the structure, (or adding/removing mass), and remains constant unless you do that. Aerodynamics does not (CANNOT) have anything to do with it. It doesn't even change if you move the airplane to another planet, or out into space, in free fall, where there is "no" gravity.

Now although this may only confuse the basic point here, it is true that if you do something that requires redistribution of mass (be it fuel, cargo, passengers, whatever) then that mass redistribution, (if/when it occurs), will change the CG, nevertheless, it is mass redistribution, and only mass distribution, that can directly cause a change in the CG.

Any action you take that requires or necessitates mass redistribution cannot by itself change the CG.

Charles Bretana
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    This gets my vote for best answer, but may I propose a rephrasing of the last paragraph, which I had to re-read? I think what you are saying is something like "True, if you do something that actually redistributes mass, then you may change the CG, but even if an action you take creates a need to redistribute mass, it does not mean that the action has itself changed the CG." My apologies if I am not being clear or have misunderstood your intent here. – sdenham Feb 03 '22 at 17:38
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    "aerodynamic forces […] CANNOT change the […] aircraft structure." - well, they could, by breaking the aircraft apart :) – Bergi Feb 03 '22 at 22:43
  • Hah! yes, on that point, you are correct. – Charles Bretana Feb 04 '22 at 03:26
  • @sdenham, No actually, I meant exactly what I wrote. If you word it the way you suggest, you would be conflating the two things. How can you "Do something that actually* redistributes mass* ..." without redistributing mass ? Examples abound and are mentioned in some of the other answers, but in all cases, it is the actual mass distribution (if/when it occurs), that changes the CG, not the action that necessitates the mass redistribution. – Charles Bretana Feb 04 '22 at 03:32
  • @CharlesBretana Clearly my attempt at rephrasing failed in my goal to be unambiguous, but I feel there is a concern here. Maybe it's just me, but when I first read that last clause, I thought someone might disagree thus: "but if you did something that necessitated mass redistribution (in a way that changes the CG), then that first action caused the GC change". This would be a valid use of 'cause', and the chain of causality is often important (e.g. in accident analysis), but it is clear from the passage quoted in the question that this is not what the book's author is talking about... – sdenham Feb 04 '22 at 13:56
  • @CharlesBretana ...WRT "if you do something that requires redistribution of mass ... then that mass redistribution will change the CG": mass redistribution does not necessarily change the CG, even though it may well do so. I thought you were pointing out that only mass redistribution can change the CG (putting aside asymmetrical changes to the gravitational field!), and so, to change the CG, you actually (as in 'nothing else will suffice') have to redistribute mass. Clearly, my proposed change utterly failed to make anything clear! – sdenham Feb 04 '22 at 14:00
  • @sdenham, I'm not sure if I am reading your comment accurately, but if you are speaking from the perspective that the chain of causality must include connections that only exist due to a human's intentions, then semantically, you might be correct, but, the point of analysis is to isolate the direct cause of effects, not the indirect causes that are five, or three, or even only one connection away... This, I believe, is true especially when the intermediate connections are not absolutely required, but must require some other independent event to occur to make the connection. – Charles Bretana Feb 04 '22 at 15:45
  • As to your comment "mass redistribution does not necessarily change the CG", well, technically that is true, you could, by sheer chance, move the mass around in such a way so that the resultant CG ends up exactly where it was, but that would a fluke (and dependent on the stated tolerances to your definition of "exact"). I did not see the need to break out this exception... (as I did not see the need to state clearly, every time I use the phrase "mass distribution", that this also includes the addition and/or removal of mass.) – Charles Bretana Feb 04 '22 at 15:49
  • @CharlesBretana Human agency is not an issue here; the hypothetical response "but if you did something that necessitated mass redistribution (in a way that changes the CG), then that first action caused the GC change" would be a valid chain of causality even if both actions were performed by automatic machinery. What I am trying to say here is that if someone responded to your final phrase with this thought, they might go on to think that the passage quoted in the question is correct after all. As far as I can tell, that would not be a conclusion you support (quite the opposite, in fact!) – sdenham Feb 04 '22 at 16:49
  • You may not think there is any real risk of anyone thinking that way (but see Koyovis's reply...), and it is up to you whether you think I am making an issue out of nothing. WRT mass transfer without CG movement, one can seek to do it quite deliberately, as in drawing fuel from both left and right tanks. As it is clear what you mean here, this is not an issue that I would have mentioned, except that it is the first part of a sentence that I felt could be misunderstood... To be clear, I strongly support your answer as being the correct one. – sdenham Feb 04 '22 at 16:52
  • I may be misunderstanding the point you are trying to make (it is getting a bit convoluted), but if I understand you, I am quibbling because to my mind, there cannot be any such action that "necessitates" a mass distribution. And unless the actual mass distribution is an absolutely unavoidable, physical consequence of the first action (and I cannot think of any first action that could be described such), then the first action cannot be argued to be a direct cause of the change in CG. – Charles Bretana Feb 05 '22 at 02:45
  • As to Koyovis answer, yes, it is true that when an aircraft goes supersonic, the center of pressure changes (moves aft), and this shift changes the trim forces and, everything working as designed, a fix is to transfer fuel and case a CG change. But using this type of logic, you could equivalently argue that a pilot's decision to fly to a destination further away (which would permit supersonic flight) causes a change in the CG. Or, you could argue that a patient having a medical issue while supersonic that necessitates an early landing for medical treatment causes a CG change. – Charles Bretana Feb 05 '22 at 02:52
  • I mean you are probably aware of the theory (argument) that the flapping of a butterfly's wings can completely change the weather on the other side of the globe at some point in the future. Yes, everything is connected in a chain of indirect causality. That is why it is so important to emphasize clarity in direct causes of effects. Otherwise, we are forced to make a decision as to where (when) in the infinite chain of past events, we are going to stop. I remember a story told about a fly, and a bird, and a cat, and a dog, and a horse and a general on the horse, and a battle, and a war.... – Charles Bretana Feb 05 '22 at 02:58
  • I'd suggest that while this answer is correct it discusses mass, while the text quoted by the OP discusses weight- in an uncomfortably vague fashion. At the very least I'd expect somebody responding to such a question in a technical interview to consider the possibility that "weight" in this context included aerodynamic contributions. – Mark Morgan Lloyd Feb 05 '22 at 11:26
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    To @sdenham's point, that last paragraph is a mouthful... Yes, I completely understand the point you are making, but had to read it a few times, and ultimately I don't think it adds any value or clarity to the first paragraph. CG is only affected if you redistribute mass. Period. – Michael Hall Feb 05 '22 at 18:18
  • I took the OPs reference to weight instead of mass as nothing more than than the ubiquitous misstatement or misuse of terms or mis-identification of weight for what is actually mass. In no way would I ever assume that a reference to weight is intended to include aerodynamic forces. Weight is simply the force of gravity on an object in a gravitational field equal to the mass times the strength of the field is local units of acceleration. – Charles Bretana Feb 06 '22 at 02:59
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    @Michael Hall, Yes, in hindsight, I agree with your reaction to my last paragraph. As I believe removing it completely would make all these comments confusing, I will edit it to make your point clear. – Charles Bretana Feb 06 '22 at 03:01
  • @CharlesBretana What work are you quoting in your answer? – CGCampbell Jun 02 '23 at 12:47
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The author of the book makes a correct introduction: "..movement of the centre of gravity is due to a change in weight. The distribution of the aircraft’s weight can change for three reasons". The footnote is incorrect: a change in tailplane lift never causes a shift in Centre of Gravity. Aerodynamics have no business with gravity forces acting on mass - CoG does not change according to tail plane trim, the Centre of Lift does.

enter image description hereImage source, earlier mentioned in this answer.

But there is a mitigating factor, in case of fuel transfer. Modern passenger planes do change CoG by re-distributing fuel when cruising, exactly for purpose of reduced drag from positive tail plane lift. For instance in the A380, by means of the trim fuel tanks in the horizontal tail.

enter image description hereImage source

The other case where shifting the CoG helps efficiency at high speeds, is of course for supersonic aircraft. Wing CoL shifts from about quarter wing chord at subsonic speeds, to half wing chord at supersonic speeds. Fuel transfer around transsonic speed then shifts the CoG aft as well, to prevent large trim drag.

Koyovis
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  • Re A380 -- see footnote in my answer. PS was typing reference Concorde url independently before seeing your not-yet-posted answer-- – quiet flyer Feb 03 '22 at 12:01
  • I think it's better to say the tail is lifting for trim, not so much longitudinal stability, which would be more of a transient case. If the tail is lifting for trim, it must mean the net pitching moment of the aircraft, absent the tail, has reversed from nose down to nose up, even with the wing operating at some positive angle of attack, correct? – John K Feb 03 '22 at 14:06
  • @JohnK The tail is lifting for trim indeed. Longitudinal stability must always be maintained though, which in cruise is no problem with a lifting tail. – Koyovis Feb 03 '22 at 16:18
  • While this answers the question posed by the title, it does not help with the issue actually being raised here, which is whether the quoted passage is correct - and it is not, a fact that is clarified by its author's note, which leaves no doubt that he is fundamentally confused. @tedioustortoise presumably wants the latter to be addressed, and was unaware that the title allows for an answer which avoids that issue. In retrospect, perhaps a better phrasing would be "Do the aerodynamic forces on the tailplane contribute to determining the location of the center of gravity?" – sdenham Feb 03 '22 at 16:39
  • @sdenham Perhaps the CG is considered an "effective" CG, and relates to trim drag, not longitudinal stability per se. As AOA goes down with increasing speed, pitching moment goes down, and trimming forces required to regulate that target AOA go down. For the purpose of trim drag, it's as if you moved the CG aft, offloading the tail, and maybe that's what they are really talking about. Ideally you'd want the tail to be completely unloaded for trim at cruising AOA, with its lifting forces only contributing to longitudinal static stability, – John K Feb 03 '22 at 16:56
  • @JohnK I won't go into details of why the author's way of looking at the issue is not just useless but confusing - and probably incoherent - even if you call it an "effective CG", because the more germane point here is that the question tedioustortoise wants answered is not answered by noting that trim tanks are used in some aircraft. – sdenham Feb 03 '22 at 17:14
  • @Koyovis Indeed; the answer could be tightened up considerably by deleting everything else. – sdenham Feb 03 '22 at 22:07
  • Aerodynamic forces on the tailplane contribute to determining the [CoL (+1) and therefor its relationship with the] location of the center of gravity, which you need to account for "To maintain straight and level". – Mazura Feb 04 '22 at 03:41
  • @sdenham as per suggestion. – Koyovis Feb 04 '22 at 07:35
  • Perhaps also change "The other case where CoG shifts at high speeds ..." to "The other case where shifting the CoG helps efficiency at high speeds ...". It's something real jets do in practice, but it is optional. The whole question is about whether speed directly affects CoG, rather than just motivating you to change it by other means, so unambiguous wording is good whenever possible. – Peter Cordes Feb 05 '22 at 12:37
  • (I made a suggested edit with that change, but I didn't find great wording for the new end of that paragraph; could use improvement.) – Peter Cordes Feb 05 '22 at 13:00
8

NO.

The "center of gravity" is where the total weight of the body, or alternatively it is total mass, may be thought to be concentrated.

It is often seen as the internal physical feature of the solid body and (as commonly understood) is not altered by velocity, acceleration or density of the surrounding environment. It may be altered only by altering the internal structure of the physical body, like relocating some mass differently inside it. Hence, for the aircraft, only moving any mass in, out or relocating inside would change it. Fuel burning and passenger movement are cases for that but change of velocity is not.

h22
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8

There are two separate issues here.

Firstly, as long as we are not physically redistributing the mass of the aircraft, then a change in the aircraft's pitching tendencies, as speed is varied, absolutely should not be described as a "movement of the centre of gravity... due to a change in weight." So the quoted text is unambiguously in error.

The second issue concerns whether or not the horizontal tail actually develops more upward lift, or less downward lift, as cruise speed is increased. The quoted passage suggests that this is necessary to keep the aircraft's nose down to keep the wing meeting the air at the increasingly small angle-of-attack that is associated with increasingly higher airspeeds. And there's no doubt that the wing's angle-of-attack must indeed be decreased as the airspeed is increased.

But the truth is that due to the nose-down pitching moment from the cambered wing, which may be expressed as a pitching coefficient, the tail typically must produce more downforce or less upforce in higher-speed cruise than in lower-speed cruise. Despite the fact that the elevator is positioned lower at higher airspeeds (in the simple case where the whole stab itself is not moveable for trimming the aircraft.) This seeming discrepancy can be accounted for by the fact that the aircraft as a whole flies at a lower pitch attitude and angle-of-attack in high-speed cruise than in low-speed cruise, and this affects the angle-of-attack of the horizontal tail. So it is a mistake to imagine that the tail effectively seems to "get lighter" as airspeed is increased. The opposite is actually true, if we are intending to describe the combined effects of the actual weight of the tail, and whatever upforce or downforce it must generate to trim the aircraft.

It is the case that during supersonic cruise, an aircraft's overall center of lift can move quite far aft, which in the absence of a change of C.G., would have to be compensated for by a strong nose-up change in pitch control trim. In the Concorde, this is handled by pumping fuel aft during acceleration to high-speed cruise. But similar procedures are not normally used on aircraft that cruise firmly in the subsonic range.1

So in summary, the quoted passage is riddled with errors.

Footnotes:

  1. Quote from a PDF on Airbus A380 fuel system: "Center of Gravity (CG) Control Transfers: CG control transfers maintain the aircraft's CG forward of the aft certified limit by transferring fuel from the trim tank to the appropriate wing tanks. There are no aft CG transfers, because the CG only has a minor impact on the cruise performance. " Source: "FUEL SYSTEM OF AIRBUS A380 AIRCRAFT SYSTEMS MOHAMAD HAMDI NAJJAR Faculty Number: 363215016", https://pdfcoffee.com/fuel-system-of-airbus-a380-aircraft-systems-pdf-free.html
quiet flyer
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  • The logic of "*Note. This is so because the greater the speed, the greater the lift created. To maintain straight and level, the aircraft adopts a more nose-down attitude, accomplished by creating lift at the tailplane. This lift effectively reduces the weight of the tailplane section of the aircraft." is so circular and utterly flawed. The author is claiming that A) the wing angle-of-attack, and therefore aircraft pitch attitude, needs to be lower/ more nose-down at high speed (true). ... – quiet flyer Feb 05 '22 at 15:20
  • (Ctd) B) the tail need to produce more upward lift to accomplish this (my answer argues that this is false, but for the sake of argument, let's pretend that it is true.) C) Since the tail is supposedly doing what it needs to do to keep everything balanced, we've now created a problem, similar to a shift in CG-- and the plane is no longer in balance. (Makes no sense whatsoever.) – quiet flyer Feb 05 '22 at 15:25
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You should not rely on Mr. Bristol's book to ace the Technical Piloting Interview. He makes three errors in the quoted section:

  1. Mass is not weight. The mass of something can only be changed by adding or taking away stuff. Weight is a force, the product of mass and acceleration. For the nitpickers: Yes, mass does increase once you reach a substantial fraction of the speed of light, but we can comfortably neglect this relativistic effect in aviation.
  2. The center of gravity does not change when weight changes from, say, a change in acceleration. The turning airplane is not only subject to gravitational acceleration, but also to inertial acceleration resulting from its movement on a curved path. Change the curvature of the path and weight changes, but the mass does not. Now it can be argued whether gravitational and inertial mass are two distinct things, but so far nobody could demonstrate any difference between them, apart from semantics.
  3. The gravest error is the third one: Tail load becomes more negative as speed increases when the wing has positive camber. Since the center of pressure moves back on a cambered wing as angle of attack decreases, the lift distribution between wing and tail (or canard) must change in order to keep the center of pressure of the full airplane at the center of gravity.

When a symmetric airfoil is used on the wing, the tail load will stay constant over angle of attack if we only look at the aerodynamic forces. But powered airplanes also produce thrust, and an offset thrust line will also affect the pitching moment. When engines are mounted below the wings and drag rises with airspeed above minimum drag speed, more thrust is needed to balance drag and will add to the engines' pitch-up contribution. This requires more lift on the tail, too! Only very high engine placements (like on the Lake amphibians or an Ekranoplan) will cause tail lift to decrease (or downforce to increase) when speed rises near the airplane's top speed.

Peter Kämpf
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  • Is it normal to talk about "weight" in terms of forces other than gravity? (In the non-inertial reference frame of a turning airplane, you're considering centrifugal force part of the weight?) With my non-aviation physics background, I'd have said weight = m g, and the atmosphere is thin enough that g is close enough to the same 9.81m/s^2 everywhere you can fly a plane on Earth. (And certainly not varying with time enough to matter.) Other forces acting on the mass of the plane are obviously significant and need to be accounted for somehow, of course. – Peter Cordes Feb 05 '22 at 13:13
  • @PeterCordes If you think how the effects of a load factor >1 on a pilot are described: Yes. If you make a distinction between gravitational and inertial weight: No, but that is approaching scholastics. – Peter Kämpf Feb 05 '22 at 15:43
1

Reasons for a change in aircraft’s weight are no more than what the word says, reasons. The only difference between fuel burn and passenger movement on the one hand and high speeds on the other, is that the first two themselves actually cause a change in CG, whereas the latter only changes the lift. Both fuel burn and passenger movement may be corrected by changing Lift, just as altered lift due to high speeds may be corrected by changing the CG, thus making it a plausible reason to do so.

There is however no such thing as a Bernoulli effect on the influence of gravity. You can't outrun it.The text is somewhat misleading. "...reasons and thus cause.. " suggests both are and/or mean the same, which is not correct as causes do not reason.