CG and stability
Re the original subject, a gentleman by name John S. Denker has written a fascinating lot about almost every aspect of how aircraft fly here: Welcome to Av8n.com. There is a long section on Angle of Attack Stability, Trim, and Spiral Dives here: 6 Angle of Attack Stability, Trim, and Spiral Dives (av8n.com). He discusses something called decalage which I think is about ensuring that the tailplane normally has a smaller angle of attack than the wings. Would be interesting to hear some of you experts comment on this, and how relates to earlier explanations here!
Anyone know why forward CG improves longitudinal stability? Or why aft CG worsens it?
I can visualize that an arrow shot from a bow would be unstable if the CG were at the tail end (if that's an appropriate analogy), I just can't figure out why that would be...
I can visualize that an arrow shot from a bow would be unstable if the CG were at the tail end (if that's an appropriate analogy), I just can't figure out why that would be...
A Newton = 1 Kg over 1 metre.
So a body in motion can only be maintained resisting deviation (wobble) by an external body that is equal but that isn't always enough: Unstable air adds complex additional forces. It was then discovered, from no doubt the observation of nature, air resistance could be used as an additional and variable force to neutralise the erratic wobbling forces. Three feathers provided the required force effect in pitch, roll and yaw.
The aeroplane trimmed stability has to be varied to change vectors using pitch, roll and yaw. This is achieved by adding a flap to the three planes (feathers) in order to vary the force of each by the pilot at will. If the CG is in the middle then to raise the nose weighing 1 kilo 1 metre then the force from the horizontal tailplane downward must also be effective to an additional 1 kilo for 1 metre. Move the CG forward to 25% then to raise the nose 1 metre the tail plane must be lowered 2 metres. If the tailplane force available cannot do that then it will be impossible to raise the nose sufficiently. This will also be the case if restricted by the runway on landing and the nose lands first. So by moving the CG aft the balance will be eventually be found and this will be the most forward limit but on the edge. Keep moving the CG further aft and reach a safe forward limit published by the manufacturer. Moving further aft again to the point the aircraft becomes unstable and now an advanced level of skill is required not to overcontrol. Then bring the CG forwards again to a point where only the average pilot skill is required; this is the published aft CG.
Last edited by Fl1ingfrog; 8th Mar 2024 at 13:10. Reason: to expand and clarify
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A Newton is the force required to accelerate a mass of 1kg at 1m/s/s. Or at 1g, the force to hold up 98.1 grammes.
Traditional scales are designed to change the leverage as the pans go from even. Also, it's a 1 kg mass, not a weight; weight is a force proportional to acceleration.
In any case holding the item against the fall from gravity is accelerating the item.
There are several demos on YouTube of accelerometers being wired to speakers to make a noise proportional to the acceleration. They all go silent when dropped, indicating they are not accelerating in their own frame of reference when in free fall.
In any case holding the item against the fall from gravity is accelerating the item.
There are several demos on YouTube of accelerometers being wired to speakers to make a noise proportional to the acceleration. They all go silent when dropped, indicating they are not accelerating in their own frame of reference when in free fall.
Avoid imitations
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Sdwings really answered his own question in his first post by mentioning the arrow shot backwards.
Theoretically, if a normal arrow was fired backwards very accurately it would continue in a straight line. But if there was the tiniest displacement at the tail, the aerodynamic force at the tail feathers (now at the front) would become greater with a greater displacement until the arrow “flipped” over the C of G. From then onwards, the aerodynamic forces would reduce until the arrow flew straight as it was really designed to do.
An aircraft needs to be stable in all axes to some extent but unlike an arrow it needs to fly at various all up masses, in other than a straight line, ie have manoeuvrability. The centre of lift and the C of G are therefore placed relatively close together to find a safe compromise between outright stability and reasonable manoeuvrability. To prevent it “flipping” like the arrow fired backwards (a theoretical extreme only mentioned to illustrate the point) the designer works out the safe C of G envelope, which must be adhered to if the aircraft is to stay in full control.
Theoretically, if a normal arrow was fired backwards very accurately it would continue in a straight line. But if there was the tiniest displacement at the tail, the aerodynamic force at the tail feathers (now at the front) would become greater with a greater displacement until the arrow “flipped” over the C of G. From then onwards, the aerodynamic forces would reduce until the arrow flew straight as it was really designed to do.
An aircraft needs to be stable in all axes to some extent but unlike an arrow it needs to fly at various all up masses, in other than a straight line, ie have manoeuvrability. The centre of lift and the C of G are therefore placed relatively close together to find a safe compromise between outright stability and reasonable manoeuvrability. To prevent it “flipping” like the arrow fired backwards (a theoretical extreme only mentioned to illustrate the point) the designer works out the safe C of G envelope, which must be adhered to if the aircraft is to stay in full control.
Talking about "mass" is a little esoteric, every flight manual I've seen talks about "weight", Concorde and SR-71 both, the speeds of those two aircraft and the direction in which they flew influenced the "weight" to an extent that was measurable by the boffins.
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Conversely, the manufacturers’ flight manuals for aircraft I’ve flown for a living since 1985 have used the term “Mass” rather than “Weight”.
I remember there being an amendment to the RAF Gazelle Flight Reference Cards at that time which caused some discussion.
But argument over that point has little to do with this question.
I remember there being an amendment to the RAF Gazelle Flight Reference Cards at that time which caused some discussion.
But argument over that point has little to do with this question.
For a whole bunch of reasons, "weight" and "mass" can be treated as interchangeable when you do everything at 1g. That includes doing your shopping, or calculating an aeroplane's weight/mass and balance from a weighing on the ground. This is how the incorrect market metric unit of kilogrammes-force comes into such widespread use, and in imperial units so many people get confused between the pound-mass (lb) and pound-weight (lbf).
In flight however, we are regularly NOT at 1g, thus it's important for e.g. doing aircraft stability or performance calculations, we differentiate between the two. It's also where SI becomes suddenly massively superior to Imperial, since SI only has one unit for weight/force, the Newton, and one for mass, the kilogramme, whilst Imperial has two mass units (the slug and the pound) and two force units (the pound and the poundal).
G
In flight however, we are regularly NOT at 1g, thus it's important for e.g. doing aircraft stability or performance calculations, we differentiate between the two. It's also where SI becomes suddenly massively superior to Imperial, since SI only has one unit for weight/force, the Newton, and one for mass, the kilogramme, whilst Imperial has two mass units (the slug and the pound) and two force units (the pound and the poundal).
G
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Yes, groundlooping arises due to instability because the centre of mass is behind the lateral forces when the plane veers every so slightly off line.
It's a bit like balancing a pencil on it's tip, or a cricket bat on it's handle on your outstretched palm. (Do Americans do that with a baseball bat?)
As long as everything is perfectly in line, it's stable, but nothing in the real world is ever perfect, and as soon as it deviates, even slightly, it is unstable and gets worse.
It's also the reason trailers need their centre of mass forward of the axle(s) for stability, resulting in downforce at the coupling.
Nobody seems to have answered you.
March 6th, 23:33, second paragraph.