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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)

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  • $\begingroup$ 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. $\endgroup$ Feb 3 at 14:28
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    $\begingroup$ @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. $\endgroup$
    – sdenham
    Feb 3 at 16:58
  • $\begingroup$ 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. $\endgroup$
    – Cort Ammon
    Feb 3 at 18:52
  • $\begingroup$ @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. $\endgroup$
    – sdenham
    Feb 3 at 22:28
  • $\begingroup$ @sdenham and that would be why it was a comment rather than an answer. $\endgroup$
    – Cort Ammon
    Feb 4 at 1:21

6 Answers 6

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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.

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    $\begingroup$ 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. $\endgroup$
    – sdenham
    Feb 3 at 17:38
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    $\begingroup$ "aerodynamic forces […] CANNOT change the […] aircraft structure." - well, they could, by breaking the aircraft apart :) $\endgroup$
    – Bergi
    Feb 3 at 22:43
  • $\begingroup$ Hah! yes, on that point, you are correct. $\endgroup$ Feb 4 at 3:26
  • $\begingroup$ @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. $\endgroup$ Feb 4 at 3:32
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    $\begingroup$ @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. $\endgroup$ Feb 6 at 3:01
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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.

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  • $\begingroup$ Re A380 -- see footnote in my answer. PS was typing reference Concorde url independently before seeing your not-yet-posted answer-- $\endgroup$ Feb 3 at 12:01
  • $\begingroup$ 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? $\endgroup$
    – John K
    Feb 3 at 14:06
  • $\begingroup$ @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. $\endgroup$
    – Koyovis
    Feb 3 at 16:18
  • $\begingroup$ 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?" $\endgroup$
    – sdenham
    Feb 3 at 16:39
  • $\begingroup$ @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, $\endgroup$
    – John K
    Feb 3 at 16:56
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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
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  • $\begingroup$ 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). ... $\endgroup$ Feb 5 at 15:20
  • $\begingroup$ (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.) $\endgroup$ Feb 5 at 15:25
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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.

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  • $\begingroup$ I have removed the thinking what the author possibly wanted to say. $\endgroup$
    – h22
    Feb 4 at 11:48
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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.

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  • $\begingroup$ 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. $\endgroup$ Feb 5 at 13:13
  • $\begingroup$ @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. $\endgroup$ Feb 5 at 15:43
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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.

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