Does an aircraft pivot on it's CG or CP in flight?

[solomon]

Hey guys, this is something that always get me thinking, I've seen articles that i read about weight and balance, and it said that plane pivots along their central gravity during flight... I thought the central pressure (CP) would act like a falcon on a leaver which is the pivoting point because that's where most of the lift is generated and so the aircraft pretty much get's lifted up from the CP. Anyone care to explain?

 

Cheers, Solomon.

 

 

 

[old man emu]

Good question, Solomon.

 

The Centre of Gravity of an airplane is a theoretical point through which the Force of Gravity is said to act. It is taken to be a point for the purposes of calculations. The C of G of an airplane can move fore and aft, side to side and up and down by altering the mass and location of loads.

 

The Centre of Pressure is a theoretical point through which the Force of Lift is said to act. It, too, is taken to be a point for the purposes of calculations. The C of P can move by the change of Angle of Attack and airspeed.

 

The confusion you have is due to the misrepresentation in the diagram you have. What is the horizontal stabiliser at the rear of the airplane doing? It also produces Lift. This Lift counteracts the imbalance between the Force of Gravity, acting through the C of G, and the Lift force acting through the C of G.

 

Ask your instructor what happens when an airplane is incorrectly loaded so that it has an aft C of G.

 

Old Man Emu

 

 

[Marty_d]

Not to mention that the CG seems to be in front of the leading edge! I think she's a bit nose heavy...

 

 

[Sapphire]

C of p is used to unstall an a/c. If you stall, the c of p moves back causing the nose to drop. Probably saved a lot of people. Where things turn to disaster is when the stalled wing drops, opposite aileron is applied, stick is pulled back as the ground comes rushing up. I said it before, you just signed your death warrent. Practice correct recovery at 4000ft

 

 

[facthunter]

It's a silly diagram. The centre of gravity would normally be around 1/4 of the chord from the leading edge The centre of lift does change with change of AoA and flap extension. The variable force of the tailplane ( horizontal stabiliser, controlled by the elevators) enables you to control pitch by overcoming the couple ( turning moment) that exists when the CofG and the centre of lift are not in the same place. eg if the centre of lift is behind ( towards the tail) the centre of gravity, the nose will tend to drop unless there is a downforce on the tail. This is the normal situation. If you have the plane requiring an upforce(LIFT) from the tailplane, it will tend to be unstable in pitch. Nev

 

 

[old man emu]

That's a bad diagram, for sure. If you look at it, it seems to have introduced prop wash into the story and labelled it "Thrust". The position that indicates C of G appears to be where the Datum Point for Weight and Balance calculations is likely to be for this make of airplane (at the firewall). Facthunter is correct in saying that in a balanced airplane the CofG is located about 1/4 of the chord from the leading edge, which is where the CofP is located when the airplane is flying straight and level.

 

The Lift Force generated by the tailplane is about zero at straight and level, but when you move the elevator down that flying surface generates Lift which upsets the straight and level equilibrium and pushes the tailplane up (causing the airplane's nose to go down), or if you move the elevator up, the opposite happens.

 

If the CofG is well ahead of the CofP, the airplane will pitch nose down and you will run out of back stick so you can't get the airplane to fly level or climb using elevators only. This means that you would be unable to level out or flare for landing (nose first into the ground).

 

If the CofG mis well behind the CofP, the airplane will pitch nose up, and you will run out of forward stick so you can't get the plane to fly level or descend using elevators only. This means that the airplane will stall. This is the usual cause of stall after take off crashes.

 

OME

 

 

[dodo]

Centre of gravity and centre of pressure have to be at the same point for the aircraft to be maintain stable flight. By introducing an imbalance where the two are not at the same point (you lean forwards, or increase lift at the tail), you pitch your aircraft. The position of the two will still be so close it makes no practical difference as to which it rotates around (but in theory, it rotates around the cg).

 

dodo

 

 

[Yenn]

Both centre of pressure and centre of gravity change in flight. With a taildragger if you lift the tail high enough on the ground it can move the C of G forward enough to tip it over on its nose. the same thing must apply in flight.

 

 

[facthunter]

It's better and the conventional thing is to treat it as doing that. Use the Cof G. When you do an analysis of what happens during the flare rotation, you will find that it is not that simple especially with long (stretched ) aeroplanes. A lot of pilots could probably go their whole lives without analysing all that, though. Nev

 

 

[pylon500]

Simple answer is 'All movement is around the Centre of Gravity'

 

But remember, the centre of gravity, or balance point, exists in 3 directions (axis).

 

 

 

[Kyle Communications]

From my limited aerodynamic knowledge CP really is only a major consideration when approaching and exceeding supersonic flight in our little birds the CP is always close to the CG anyway so it doesnt count....approaching mach speeds it is a huge consideration the aircraft can duck under the concord used to transfer fuel between forward and aft tanks to counter the effects of CP

 

 

[solomon]

Thanks guys! That clarified allot for me.

 

 

[Head in the clouds]

Thanks guys! That clarified a lot for me.

Pylon 500 gets the cigar.

 

The diagram you posted is a bit misleading but perfectly valid even though the CG and CP positions are shown in very extreme locations. Using the fairly 'conventional' type of aircraft shown in your example, to be positively stable (i.e. a condition where the aircraft would want to return to equilibrium after being upset by turbulence, for example) the CG should always be ahead of the CP at all Alpha (angle of attack) up to the stall angle. As alpha increases (on the round-out for landing, for example) the pressure field moves forward which is why we need a static margin (the distance between the CG and CP in level flight) that is great enough to assure that the CP does not move ahead of the CG at high alpha. If that were to happen then the airfoil would develop a negative pitching moment i.e the nose of the aircraft would want to rise even further as the stall is approached, a condition that would be described as unstable.

 

In level flight, and with 'conventional' airfoils (Clark Y, NACA 2412 etc) as generally used for aircraft that might fit your example the CP would be at about 35-40% chord position but it moves forward to about 25% chord at 15* alpha (approximate stall angle of attack), so the aircraft should be loaded so that the CG is forward of the 25% chord position in level flight. Theoretically it might be anywhere ahead of that position but the further forward it is, the greater the pitching moment (turning force in nose down direction caused by the difference between the CP and the CG (the static margin)), so the controlling factor in regard of how far forward the CG can be is the amount of downforce that can be generated by the tailplane to balance and exceed the pitching moment in level flight.

 

During the round-out the CP moves forward so it might be reasonable to expect that since the static margin has decreased, so also has the pitching moment and so the tailplane would need to generate less downforce at that time. That is true in theory but the problem is the reduced airspeed during round-out, which reduces the total force the tailplane can generate, so in effect it is during low speed, high alpha that the problem of having the CG too far forward will show up as an inability to hold the nose up sufficiently to reach the stall angle of the mainplane, resulting in an 'abrupt arrival'.

 

So, for practical purposes in a 'conventional' aircraft it should loaded so that the CG is between about 10% and 25% MAC (Mean Aerodynamic Chord).

 

Note though what Pylon said, the movement (rotations in all three axes) are about the three dimensional CG and since the CG is positioned somewhere along each of X, Y, and Z axes, and always acts downwards, then as the nose of the aircraft moves up the CG moves forward in a high wing aircraft and aftwards in a low wing, so the CG and CP move apart in a nose high, high wing, increasing it's pitching moment and also increasing its stability and the opposite is true for a low wing. Before the howling starts, this is a very small effect due to the CP always acting perpendicular to the wing whereas the CG always acts vertically, but nonetheless it is the reason that midwing and biplane aircraft became popular for aerobatics since in those configurations there is no pitching moment change with change of attitude.

 

So Solomon, the simple answer to your original question is 'CG'.

 

 

[J170 Owner]

The aircraft will pivot or rotate about the CG.

 

 

[cscotthendry]

I read somewhere that the CG is always designed to be ahead of the CL for stability. In effect. the tail plane always has to supply a little down force when flying straight and level. This constant down force acts as a damper on pitch.

 

 

[facthunter]

That is true Scott . You will get better economy ( less drag with the tail providing some lift) but this situation doesn't normally exist with our type of aircraft. The LAST thing you need is for the horizontal tail feathers to stall. Nev

 

 

[facthunter]

I think the term "normal " means at right angles ( 90 degrees) to. Nev

 

 

[Head in the clouds]

The LAST thing you need is for the horizontal tail feathers to stall. Nev

Tailfeathers generally won't stall before the mainplane in a properly designed aeroplane because the critical angle (angle at which an airfoil begins to stall) is dependent on the aspect ratio (AR - span:chord ratio). Lower aspect ratios stall at higher alpha and so all conventional tailed aircraft have a tailplane which is of lower aspect ratio than the mainplane.

 

Running out of elevator authority is often confused with the tailplane having stalled before the mainplane, whereas what is usually happening is that the elevator is not able to produce enough downforce to keep raising the nose further while speed decreases in the round-out, and even though it is not stalled.

 

Increasing the horizontal stabiliser's (HS's) efficiency will provide considerable improvement and the first means of doing so is to close the gap at the hingeline, usually with a fabric strip, which can increase the co-efficient of lift (CL) of the HS quite significantly. These days it is quite common to see VGs added to the HS ahead of the hingeline but since they only serve to keep the boundary layer attached at greater alpha (i.e. delay the stall/increase the critical angle), and since the HS should not be stalled in any case, then in theory the VGs cannot provide any improvement and would only increase drag.

 

If closing the hingeline gap doesn't do the trick of allowing the wing to be brought to the critical angle in ground effect, then, assuming the plane is rigged correctly, the static margin (distance between CG and CP) is probably too high meaning that the aircraft is too stable. The CP cannot be changed without changing the airfoil so a wise operator might then check his/her CG position which might be too far forward...

 

the 'vertical'axis is usually called the 'normal'axis these days, that is at right angles to both the longitudinal and transverse axis....its rarely vertical!!

 

I think the term "normal " means at right angles ( 90 degrees) to. Nev

Quite correct. In the earlier post we were discussing a plane in straight and level flight, at which time the force through the CG (just the weight of the aircraft if in straight and level flight) would be vertically downward, at any other time the 'CG' is usually mis-described, especially in the classroom, but for the well-intentioned purposes of simplicity.

 

Of the forces acting on a manoeuvring aircraft, the total forces acting through the CG produce a vector which is neither downward nor normal to (or - at right angles to) the longitudinal and lateral axes, since the weight (1G) is always acting downward toward the ground regardless of the attitude of the plane, and centrifugal forces due to turning are acting at right angles to the long and lat axes. So the vector of those combined forces acts in a direction somewhere between the 'normal' axis and the ground.

 

Isn't thread drift wonderful? Apologies that this has nothing to do with the OP's question

 

 

[facthunter]

Any reference I have made to the tailplane stalling is in the "tail heavy" case where loading is the cause. Lockheed hudson/ loadstar's went in because of it. If the elevator effectiveness is not sufficient to stall the plane onto the ground because of nose heaviness, this is not a particularly dangerous situation as a bit of extra speed fixes it, or a burst of power will energise the HStab which would be a reasonable reaction if you had near full backstick and the plane kept descending. Down force on the tail adds to the lift requirement the wings must provide, although it is of a low order it is still there.. Nev

 

 

[solomon]

"as nose of the aircraft moves up the CG moves forward in a high wing aircraft and aftwards in a low wing"

 

Why do high wings and low wings move in opposite direction? and wouldn't this mean that low wing need to be more nose heavy than high wing so the nose drops at the stall angle?

 

 

[solomon]

Thanks guys! i dont really mind if we move off topic i'm learning a few more things in the process.

 

 

[Head in the clouds]

Any reference I have made to the tailplane stalling is in the "tail heavy" case where loading is the cause. Lockheed hudson/ loadstar's went in because of it. If the elevator effectiveness is not sufficient to stall the plane onto the ground because of nose heaviness, this is not a particularly dangerous situation as a bit of extra speed fixes it, or a burst of power will energise the HStab which would be a reasonable reaction if you had near full backstick and the plane kept descending. Down force on the tail adds to the lift requirement the wings must provide, although it is of a low order it is still there.. Nev

A forward CG can be just as dangerous as an aft one, if it is too far forward. The addition of power to accelerate the flow over the HS so as to have enough elevator authority to hold the nose up at low speed will solve the landing situation if you have a slightly too far forward CG and it will mean that you take a longer distance to land. A medium too far forward CG means you will take a very long distance to land. Take it to the obvious conclusion and a very far forward CG will mean you cannot hold the nose up without using considerable power and that means you can't land at all. The scary part of it is that you probably won't notice the problem until you reduce power for the approach because at high power settings for the take-off, climb and cruise, with lots of airspeed as well as propwash over the HS, the craft is perfectly controllable. Most pilots should notice that the elevator trim needed to be full back to maintain level flight but by that time it's a bit late.

 

And the aircraft incorrectly loaded with the CG too far aft, is also a dangerous beast.

 

Say the Cp was at 40% and the Cg at 35% in cruise, it would still have positive stability at that time and fly quite normally, actually more efficiently than usual because the HS wouldn't be producing as much downforce for the mainplane to compensate for. But as the aircraft slows and increases alpha for the landing the pressure field moves forward until the CP might be at 25% while the CG is still at 35%.

 

As the CP moves in front of the CG the plane changes from positive stability to negative stability (or becomes 'unstable') so the nose of the plane wants to continue to rise without any change in the force being produced by the HS. So now the pilot has to start making the HS produce positive (upward) lift to compensate for the negative pitching moment, so he has to start pushing the stick forward... If he catches it in time a successful landing could be made, albeit a sweaty one.

 

If he doesn't catch it early enough the mainplane will continue increasing alpha and the CP therefore keeps moving forward until a situation is reached where the HS might not be able to produce enough lift to correct the situation and the mainplane (not the HS) will stall. If the stall happens just above the ground you got away with it, if it's at 50ft like the Lockheed examples, you wouldn't.

 

This aft CG example is still a case of 'loss of elevator authority' rather than HS stall. It's even more difficult for the HS to stall earlier than the mainplane in this condition because the aircraft is rigged with the HS incidence about 3* lower than that of the mainplane, which in combination with the HS's lower AR would provide an even larger margin between the critical angles of each of them. However the elevator will have less authority in the aft CG case than the forward CG case because of that incidence difference affecting the HS's alpha so that when it is producing positive lift at max elevator deflection its co-efficient of lift (CL) is less than its CL would be when producing downforce at max elevator deflection. That statement pre-supposes that the elevator deflections were not incorrectly rigged, allowing deflections that were too high and thus producing a stall (EDIT - flow separation, not stall) at the hingeline which is another matter entirely.

 

"as nose of the aircraft moves up the CG moves forward in a high wing aircraft and aftwards in a low wing"

 

Why do high wings and low wings move in opposite direction? and wouldn't this mean that low wing need to be more nose heavy than high wing so the nose drops at the stall angle?

If you look at the image you posted at the beginning of the thread and remembering that the CG is in a 3D position, and note that the CG is both ahead and below the CP. As the nose rises, rotating around the CG, the CP moves down and backward, so when the nose is high the CG and CP are spaced further apart horizontally. Then consider a low wing aircraft, the CP would be below and behind the CG, so as the nose rises, rotating around the CG, the CP moves down and forward so when the nose is high the CG and CP are spaced closer together horizontally.

 

It is a very small difference but yes the weight and balance charts for low and high wing aircraft are slightly different, assuming airfoil and and all other considerations are the same. The allowance/difference (for landing) is even further reduced because of differences between the ground effect of the two types, the closer proximity of the ground to the low wing suppresses the wing downwash/fluid circulation more than is the case for the high wing so to all intents and purposes it can be ignored below the critical angle.

 

 

[David Isaac]

Good stuff guys, thanks

 

 

[solomon]

I've been wanting to use this formula to calculate CG:

 

CG= MAC/6+(3 x Tail Area x Tail Moment Arm/ 8 x Wing Area)

 

But i'm not sure what value im suppose to use for the MAC. Could the MAC value be the distance from the root chord to the MAC?

 

Also does any one have any other diffrent methods of working out the CG, my technique i've been using so far was to hoist my aircrft from the spar and see which direction the nose was leaning towards and how much it's leaning. I would then be ok with the weight and balance if it's leaning only slightly forwards about (5 degrees). This also worked well with the scaled model i've made of my aircraft.

 

 

[facthunter]

You weigh it in a level attitude. and have the values for the weight on each wheel.. If a T/W you need the scales to be on a trellis of the right height to keep it level You know where the wheel locations are on the fore to aft axis. If the mainwheels differ you could take an average of the two, but the variation is usually not much. Do all measurements from a datum. ( reference). ie the back of the propeller or it can be forward of the plane in space. As long as you use the same one all the time This way when you remove or add weight you can calculate the new Cof G without a reweigh. (Radio or battery etc). Nev