Forward C of G limits
Forward C of G limits
Guest
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as,
Erm.......have to question your response there! The aircraft cofg must be infront of the aircraft neutral point in pitch for the aircraft to be stable - as we all know! As the cofg is moved rearwards, it approaches the neutral point of the aircraft, and stability is reduced. Thus, the rearward limit of the cofg is a stability limit (as you stated).
During most flight phases, the tail load for a commercial aircraft is a download (look at the tail incidence on a jet after landing!). This would indicate that the effective centre of the pressure of the main wing is behind the centre of gravity, necessitating the tail download to trim. Thus, as the cofg moves forwards, a greater moment is required from the tailplane to trim the a/c. Therefore, the forward cofg limit is imposed as a result of a minimum required control power in critical flight phases i.e. typically at landing, where all high lift devices are deployed and airspeed is low. To extend, the aerodynamic centre of the wing does not HAVE to be infront of the cofg!
Regards,
Cuban_8
Erm.......have to question your response there! The aircraft cofg must be infront of the aircraft neutral point in pitch for the aircraft to be stable - as we all know! As the cofg is moved rearwards, it approaches the neutral point of the aircraft, and stability is reduced. Thus, the rearward limit of the cofg is a stability limit (as you stated).
During most flight phases, the tail load for a commercial aircraft is a download (look at the tail incidence on a jet after landing!). This would indicate that the effective centre of the pressure of the main wing is behind the centre of gravity, necessitating the tail download to trim. Thus, as the cofg moves forwards, a greater moment is required from the tailplane to trim the a/c. Therefore, the forward cofg limit is imposed as a result of a minimum required control power in critical flight phases i.e. typically at landing, where all high lift devices are deployed and airspeed is low. To extend, the aerodynamic centre of the wing does not HAVE to be infront of the cofg!
Regards,
Cuban_8
Guest
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As Checkboard says:
Fwd C of G limit: Manoeuvrability considerations
Aft C of G limit: Stability considerations
The C of G limits applied to the aircraft, during loading on the ground, also have to cater for all likely movements of the C of G and also the C of P in flight. For example:
Mechanics of Flight by A C Kermode (ISBN 0-273-31623-0) is worth reading should you wish to know more.
Fwd C of G limit: Manoeuvrability considerations
Aft C of G limit: Stability considerations
The C of G limits applied to the aircraft, during loading on the ground, also have to cater for all likely movements of the C of G and also the C of P in flight. For example:
- The C of G can move in flight as a result of fuel usage, fuel transfer, fuel jettison, mid-air refuelling, passenger movement, weapon release, water dropping or parachute dropping.
- The C of P can move in flight as a result of a change in aircraft speed, flap selection or altered wing sweep.
Mechanics of Flight by A C Kermode (ISBN 0-273-31623-0) is worth reading should you wish to know more.
Guest
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For the "light metal" I fly, the forward limit for CoG is determined as much as anything, by the effectiveness of the elevator when in ground effect. Ground effect by reducing the authority of the elevator, necessitates up to 15 deg of extra elevator to allow the aircraft to flare for landing.
The flare is simply bringing the wing to its critical AoA and allowing it to stall. As this mathematically links to Lift (by that long equation with Coefficient of Lift, density and velocity) it must also relate to Weight.
For a better description see Kerschner's Advanced Pilots Manual.
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[email protected]
The flare is simply bringing the wing to its critical AoA and allowing it to stall. As this mathematically links to Lift (by that long equation with Coefficient of Lift, density and velocity) it must also relate to Weight.
For a better description see Kerschner's Advanced Pilots Manual.
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[email protected]
Guest
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Mr moto, I am suggesting that a tail can produce lift becasue it does, but lift doesn't have to be in the positive sense. As has been pointed out, often the incidence of the tailplane on a large aircraft is negative. As it produces some kind of force, it therefore produces a moment about the c of g.
Cuban - I humbly go into reverse over the aerodynamic centre issue, I was just considering a simple case where the tailplane has a positive incidence.
Apologies for any confusion!
Cuban - I humbly go into reverse over the aerodynamic centre issue, I was just considering a simple case where the tailplane has a positive incidence.
Apologies for any confusion!
Guest
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aerostude
Sorry, I don’t agree - may we discuss what you posted?
Why? On most aircraft the C of G will not lie behind the C of P, although on some types, frequently for instance on seaplanes, it could. You may have assumed that the tailplane only provides lift, but it can, and does, often provide a downforce. There are also other forces/couples acting on the aircraft to consider, principally the Thrust/Drag couple. It is only when these four main forces cannot be balanced satisfactorily by themselves, that the tailplane will be called upon to provide a balancing force, either upward or downward.
There is in fact an advantage in having the C of G behind the C of P; in the event of thrust loss, there is a nose down (stabilising) pitch.
Why must it be lift? Ideally, at normal cruise speed, the tailplane will carry no load, but at high speed, it will normally be a download.
As the distance between the C of G and the C of P increases, more elevator deflection is required, not less, thus the elevator is, by definition, less effective. Thus the forward C of G limit is determined by the minimum acceptable level of manoeuvrability.
As the C of G moves back, stability is reduced, and the aircraft does become more difficult to control, but this is because it is becoming unstable, not too statically stable! Thus the aft C of G limit is determined by the minimum acceptable level of stability.
Regards
Bellerophon
Sorry, I don’t agree - may we discuss what you posted?
<font face="Verdana, Arial, Helvetica" size="2">The centre of gravity HAS to lie BEHIND this point or you are in trouble.</font>
There is in fact an advantage in having the C of G behind the C of P; in the event of thrust loss, there is a nose down (stabilising) pitch.
<font face="Verdana, Arial, Helvetica" size="2">If you than consider the input from the tail lift</font>
<font face="Verdana, Arial, Helvetica" size="2">if the c of g moves forward, the elevator effectiveness is increased</font>
<font face="Verdana, Arial, Helvetica" size="2"> if the c of g moves back….the aircraft is difficult to control and ultimately uncontrollable-effectively the aircraft becomes too statically stable.</font>
Regards
Bellerophon
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Aerostude
Have to say I agree with Bellerophon
Although it is nothing like so important to appreciate as the stuff you are both discussing, there can be a structural problem with a forward CG that actually sets the forward limit.
It goes like this: Forward CG increases stability therefore aircraft needs more down force at the tail to manoeuvre. When pulling a very forward CG aeroplane to its g limit, this force may be so great as to shear the tail off downwards. In this case the rear fuselage structural strength limit sets the forward CG limit of the aircraft (The Harrier is an example). Why not make the rear fuselage stronger? Because nobody wants to increase structure weight if you can make do with a less forward CG. Another example of compromise in aircraft design.
JF
Have to say I agree with Bellerophon
Although it is nothing like so important to appreciate as the stuff you are both discussing, there can be a structural problem with a forward CG that actually sets the forward limit.
It goes like this: Forward CG increases stability therefore aircraft needs more down force at the tail to manoeuvre. When pulling a very forward CG aeroplane to its g limit, this force may be so great as to shear the tail off downwards. In this case the rear fuselage structural strength limit sets the forward CG limit of the aircraft (The Harrier is an example). Why not make the rear fuselage stronger? Because nobody wants to increase structure weight if you can make do with a less forward CG. Another example of compromise in aircraft design.
JF
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The thread thus far attempts to put the question into far too simplistic a box.
What must be kept in mind is that the envelope is the critical loci of a series of lines each of which prescribes a limiting (forward/aft CG, and/or min/max gross weight) condition for one design criterion (which is influenced by cg/weight) as required by the design standard. There may be quite a number of such individual lines, none of which necessarily is obvious from a simple inspection of the published TC envelope.
Some of the posts are relevant, while others are somewhat innovatively or mistakenly wide of the mark.
The following is not suggested to be exhaustive, but some of the considerations which feed into establishing the forward limit include
(a) nose assembly components - structural limits
(b) aft body structure for gust loading cases - structural limits
(c) pitch trim capability for various flight stage cases
(d) wing/empennage - structural limits
Bookworm's specific interest as originally posted appears to be the sloping forward cg line at higher gross weights typically seen in GA aeroplanes other than the smallest types. Some of the considerations which may feed into this prescribed limit include
(a) trim/flare capability
(b) nose/wheel/tyre - structural limits
(c) wing/empennage - structural limits
Keep in mind that the aircraft loading configuration, or even the fuel use cg movement, may result in pragmatic truncation of limits - not much point in working up a more extensive envelope than reasonably required. In general, the stall speed is critical for forward cg/higher weight test points - it may be a pragmatic solution to restrict the higher weight forward envelope to avoid undesired performance penalties associated with a higher than desired or necessary stall speed.
What must be kept in mind is that the envelope is the critical loci of a series of lines each of which prescribes a limiting (forward/aft CG, and/or min/max gross weight) condition for one design criterion (which is influenced by cg/weight) as required by the design standard. There may be quite a number of such individual lines, none of which necessarily is obvious from a simple inspection of the published TC envelope.
Some of the posts are relevant, while others are somewhat innovatively or mistakenly wide of the mark.
The following is not suggested to be exhaustive, but some of the considerations which feed into establishing the forward limit include
(a) nose assembly components - structural limits
(b) aft body structure for gust loading cases - structural limits
(c) pitch trim capability for various flight stage cases
(d) wing/empennage - structural limits
Bookworm's specific interest as originally posted appears to be the sloping forward cg line at higher gross weights typically seen in GA aeroplanes other than the smallest types. Some of the considerations which may feed into this prescribed limit include
(a) trim/flare capability
(b) nose/wheel/tyre - structural limits
(c) wing/empennage - structural limits
Keep in mind that the aircraft loading configuration, or even the fuel use cg movement, may result in pragmatic truncation of limits - not much point in working up a more extensive envelope than reasonably required. In general, the stall speed is critical for forward cg/higher weight test points - it may be a pragmatic solution to restrict the higher weight forward envelope to avoid undesired performance penalties associated with a higher than desired or necessary stall speed.
Guest
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On the L1011 we always have nose up pitch trim. This shows that th CoG is fwd of the CoP. It is good form to load the aircraft with as far back a CoG as possible thus ofloading the "STAB" (elevator for lesser mortals). This way you burn less fuel, make more money, are likely to remain employed for longer.
The only real restictions come at light weights because the stab doesnt go very far nose down and without the weight turning moment it could run out of authority if for eg. going around with the center engine out. all the pitching moment from the wing engines is nose up !!!!!!!!!
We do have weight related go around thrust settings to compensate for this.
The only real restictions come at light weights because the stab doesnt go very far nose down and without the weight turning moment it could run out of authority if for eg. going around with the center engine out. all the pitching moment from the wing engines is nose up !!!!!!!!!
We do have weight related go around thrust settings to compensate for this.
Guest
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Sorry Stude, I think I misread the original post.
I'm used to people arguing that a conventional aircraft could not produce positive lift from the tail.
The Pilatus Porter, for example, has a very high lift airfoil placed upside down at the tail.
I'm used to people arguing that a conventional aircraft could not produce positive lift from the tail.
The Pilatus Porter, for example, has a very high lift airfoil placed upside down at the tail.
Guest
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I'm afraid that virtually all of my FJ experience is on the P&HQ or operational side, so I shall have to bow to JF's far greater experience concerning the Fwd CG limit of aircraft like the Harrier.
However, on the little aeroplane side, I've been directly responsible for finding the CG limits on quite a few, and without fail the limiting factor for the aircraft that I've dealt with has been the ability to adequately control the aircraft in the roundout and flare. (Actually not strictly true, on one type it was defined by the point when we ran out of ballast).
The aft CG limit incidentally is *usually* defined by an acceptably low limit of LSS, but I have come across one type (with a rather small fin) where we defined it by directional stability running out.
Bear in mind however that many aircraft are designed right to the limit of permitted stalling speed, which is usually pitch authority limited in a light aircraft. So, a fwd CG limit defined by Vso is certainly a possibility, and I suspect that this is probably the case with some of the higher performance FAR-23 aircraft.
G
However, on the little aeroplane side, I've been directly responsible for finding the CG limits on quite a few, and without fail the limiting factor for the aircraft that I've dealt with has been the ability to adequately control the aircraft in the roundout and flare. (Actually not strictly true, on one type it was defined by the point when we ran out of ballast).
The aft CG limit incidentally is *usually* defined by an acceptably low limit of LSS, but I have come across one type (with a rather small fin) where we defined it by directional stability running out.
Bear in mind however that many aircraft are designed right to the limit of permitted stalling speed, which is usually pitch authority limited in a light aircraft. So, a fwd CG limit defined by Vso is certainly a possibility, and I suspect that this is probably the case with some of the higher performance FAR-23 aircraft.
G
Guest
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Thanks for all the responses, particularly those from John Farley and John Tullamarine which uncovered a number of aspects that I had not considered.
My interest stems from recent experience with a light twin. A reweighing revealed not just a substantial weight gain (almost at the level where I would suspect the previous documentation of being fraudulent) but also a substantial forward movement of the C of G, despite a load of new equipment in the tailcone!
We're now operating on the forward limit of the C of G, and I'm concerned about the consequences of leaving my kneeboard on the glareshield during flight, and issues like the need for longer legs.
What sort of tolerances are built in to the certification process limits?
My interest stems from recent experience with a light twin. A reweighing revealed not just a substantial weight gain (almost at the level where I would suspect the previous documentation of being fraudulent) but also a substantial forward movement of the C of G, despite a load of new equipment in the tailcone!
We're now operating on the forward limit of the C of G, and I'm concerned about the consequences of leaving my kneeboard on the glareshield during flight, and issues like the need for longer legs.
What sort of tolerances are built in to the certification process limits?Guest
Posts: n/a
Bookworm,
There are a few traps for young players when it comes to weight control .... may I offer just a few thoughts which might be pertinent to your situation ?
(a) there are many errors to be found in inappropriate use of scales. Indeed, one occasionally sees scales which are, themselves, quite inappropriate for the intended use. I recall a case, many years ago, where a rebuilt classic taildragger was weighed on quite inappropriate jackpad cells. To cut a long story short, the test flight just avoided disaster and when the machine was reweighed with appropriate platform scales, the empty cg jumped aft a great distance.
(b) likewise if the weigh is made with lots of fuel and uses inappropriate "standard" sg values to account for the fuel weight. Also, one ought to check that the tanks called up in the AFM or TCDS actually are in the example aircraft ... I have seen just this sort of mistake cause acute embarrassment.
(c) check that the loading system is both reasonably pertinent to the aircraft and that the entry data, if it is a trimsheet style of system, are appropriate to the system's design. It is generally common for the weight control chap (or chap-ess) to design such systems using a datum position which differs from that suggested in the manufacturer's data. While we try to make such systems difficult to misuse, sometimes it doesn't always work out as well as we might hope. I recall a case where an operator was using one of my trimsheets (with a typical non-standard datum) and one of that particular fleet was modded and reweighed. The weigher, not being quite up to speed on trimsheets, thought to do the right thing by rescheduling the entry data - but used the manufacturer's standard datum rather than the non-standard datum for which the trimsheet was designed. Fortunately, the chief pilot, who was a friend of mine, couldn't quite make his load fit the system and thought to give me a call to check. Quite sobering.
(d) have you made a research check through the aircraft's weight control history ? That often gives a good guide as to where the error(s) is(are) ?
(e) is the aircraft behaving strangely in flight at your calculated forward cg ? If so, how so ?
I would be interested in your observations. The story, as you have given it so far, is a little inconsistent with what I have seen over the years with lighties.
There are a few traps for young players when it comes to weight control .... may I offer just a few thoughts which might be pertinent to your situation ?
(a) there are many errors to be found in inappropriate use of scales. Indeed, one occasionally sees scales which are, themselves, quite inappropriate for the intended use. I recall a case, many years ago, where a rebuilt classic taildragger was weighed on quite inappropriate jackpad cells. To cut a long story short, the test flight just avoided disaster and when the machine was reweighed with appropriate platform scales, the empty cg jumped aft a great distance.
(b) likewise if the weigh is made with lots of fuel and uses inappropriate "standard" sg values to account for the fuel weight. Also, one ought to check that the tanks called up in the AFM or TCDS actually are in the example aircraft ... I have seen just this sort of mistake cause acute embarrassment.
(c) check that the loading system is both reasonably pertinent to the aircraft and that the entry data, if it is a trimsheet style of system, are appropriate to the system's design. It is generally common for the weight control chap (or chap-ess) to design such systems using a datum position which differs from that suggested in the manufacturer's data. While we try to make such systems difficult to misuse, sometimes it doesn't always work out as well as we might hope. I recall a case where an operator was using one of my trimsheets (with a typical non-standard datum) and one of that particular fleet was modded and reweighed. The weigher, not being quite up to speed on trimsheets, thought to do the right thing by rescheduling the entry data - but used the manufacturer's standard datum rather than the non-standard datum for which the trimsheet was designed. Fortunately, the chief pilot, who was a friend of mine, couldn't quite make his load fit the system and thought to give me a call to check. Quite sobering.
(d) have you made a research check through the aircraft's weight control history ? That often gives a good guide as to where the error(s) is(are) ?
(e) is the aircraft behaving strangely in flight at your calculated forward cg ? If so, how so ?
I would be interested in your observations. The story, as you have given it so far, is a little inconsistent with what I have seen over the years with lighties.