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Foxtrot_Sim

Center of pressure?

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Hello! 

I recently got myself the complete private pilot kit with all the textbooks and stuff (ill list everything in the package if you wanted to know). But the main textbook is called "The Complete Private Pilot". And in the first chapter it discusses aerodynamics and forces on a plane. In the section with CG, it explains something called "center of lift." It didn't do a good job explaiing it, and the pictures didn't help. I asked my physics teacher and he said he never heard of it before. 

 

So what it "center of life''? From what I got from the text was that it was the point on the airfoil BEHIND the CG that is hit hardest with the relative wind. Is that correct? If not, what if it? And are there any examples or experiments I can do to test and observe it? 

 

THanks-

Dmitri. 

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Centre of Lift or Centre of Pressure is a concept similar to Centre of Gravity. The pressure distribution over the aerofoil varies along its length. The CoP is the single point through which the Total Reaction Force (and Lift itself) is considered to act - the concept being similar the CoG where that is the singular point through which the weight of the aircraft is considered to act - it makes conceptualising it much easier. Typically, CoP would be designed to lie behind the CoG, so that in the event of a loss of thrust, the couple between the two will tend to make the aircraft pitch nose down, thereby assisting in maintaining gliding speed.

 

Hope that helps.

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I think I got it. So to recap, it's the point on the wing (airfoil) where there is most lift. Like the opposite concept of the CG? i.e. CG is about weight and a force down, and CoP is the opposite where there is most lift? 

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I think I got it. So to recap, it's the point on the wing (airfoil) where there is most lift. Like the opposite concept of the CG? i.e. CG is about weight and a force down, and CoP is the opposite where there is most lift? 

Almost there - not the most lift, simply the single point where Lift is considered to act - the pressure pattern is too complicated to look at it any other way. It doesn't matter how much lift is being produced, the CoP is the SINGLE point through which it is considered to act. CoP also moves depending on Angle of Attack, but that's for another day!

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Okay, so lift only happens in one spot on the wing? And that is the CoP? I thought the whole wing produced wing. 

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Okay, so lift only happens in one spot on the wing? And that is the CoP? I thought the whole wing produced wing.

No, lift is produced over the whole wing. CoP represents a single point where it is considered to act. Try looking at it on Wikipedia, some diagrams will help you see the picture.

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I'll go ahead and take a look at a few other sources. Thanks so much for explaining! I'll report back when I finally understand it. 

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Any expression which starts Centre of... in relation to aviation is a resultant force, location or concept. It is not an actual point, but a virtual one. As your posting in a forum about virtual aircraft flying around a virtual atmosphere in a virtual airspace, you probably already are well on the way to understanding !

 

If one were to take ALL the points on the wing, measure the lift at all those points and then display them as they actually are, we wouldn't be anything other than confused, and there's no way to conceptualise such physics in a way that is readily understandable. Like the concept of `average` which is a calculated mean, mode or median, (`the average family has 2.2 children...') Centre of Lift is simply a way of representing the huge number and changing nature of force, velocity and gravitational vectors as a more easily assimilated single point.

 

As you grow your aerodynamic understanding you will find a very large number of these concepts, which are simplifications of hugely sophisticated mathematical models.

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The centre of lift is the point on an aircraft where all the various lifting forces act.

 

If the aircraft is trimmed then the centre of lift is in the same point as the centre of gravity.

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The centre of lift is the point on an aircraft where all the various lifting forces act.

 

If the aircraft is trimmed then the centre of lift is in the same point as the centre of gravity.

 

Thats where the OP was confused, Gerry. It's NOT a physical point at all.You describe it as if all the forces act toward that point. They don't, it's a virtual point which is a summation of the forces involved for the convenience of calculation only.

 

And as a technical point the CG need not be at the same point as the Centre of Lift for balanced flight. The other forces at play influence the relationship actively - thrust and drag, weight and lift. CG does need to be in a range accommodated by the CoL. The Centre of Lift is not a fixed point, it's constantly moving as a result of the forces acting, so while the CG may need to move, it is not always the case and may actually need to NOT be in the same point as the Lift vector for a balanced flight.

 

As a simple analogy consider the difference created by an offset or vectored thrust line. Or control surface movement. For balanced flight the CG should now NOT  be at the point of centre of lift.

 

It's the basis of the work of the great aerodynamicists like Bernoulli and Rutan that they can consider the totality without being hamstrung by the conventions of single line thinking.

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If the aircraft is trimmed then the centre of lift is in the same point as the centre of gravity.

Absolutely incorrect. Trimming corrects the imbalance between the Thrust/Drag and Lift/Weight couples by using the lever of the trim tabs and the CG. It in no way moves the CoP

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And as a technical point the CG need not be at the same point as the Centre of Lift for balanced flight.

 

There are Centres of Lift  for the wing, the fuselage, the tailplane. All of with vary the angle of attack and controst goil settings. What is your definition of Centre of Lift?

 

The Centre of Lift of the aircraft is the combined lift force from all those components. For a trimmed aircraft the lift, drag, and thrust forces most all go through the the Centre of Gravity if the aircraft is trimmed. if not there must be a moment which makes the arcraft untrimmed. The purpose of the elevator in trimming is to move the position of the Centre of Lift and eliminate any moments on the aircraft.

 

 

 


Trimming corrects the imbalance between the Thrust/Drag and Lift/Weight couples by using the lever of the trim tabs and the CG. It in no way moves the CoP

 

I am talking about the Centre of Lift, not the Centre of Pressure which are not the same.

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Gerry

As a rule of thumb, a conventional aircraft (main wing forward, horizontal stabiliser aft) the COL should be aft of the COG. While it is true to a point that the less distance between them the better, as drag lessens, but can invoke potential stability issues due to lack of control authority. Too much distance and you get increase in drag (Trim Drag) whcih creates difficulty in holding the nose up as speed drops. The reason for this is that the tailplane acts as an inverted airfoil (negative lift) to force the tail down and for balancing (trim) purposes the force it applies needs be compensated by a COG that is beyond the COL in the opposite direction to the fulcrum activity of the tail:

imageck7.jpg

 

 

Conversely, some Fly By Wire aircraft often use the instability of a close relationship between COG and COL to maximise maneuverability as a consequence of relaxed stability. If we take your COL=COG argument you don't make the aircraft trimmed, you actually make the aircraft inherently unstable, and less controllable, so while it might work for an F16 with FBW and all-flying tail feathers, it will not work for a conventional, stable aircraft !

 

There's a lot more detail to this explanation (and better pictures) in the AOPA technical document 

 

http://flighttraining.aopa.org/pdfs/8083-25_chap3.pdf

 

Pages 3-11 onward discuss stability and there's a good set of diagrams on there that illustrate the need for COL to be aft of COG for normal flight.

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In your diagram, the total lift on the aircraft is the sum of the lift of the wing  and of the lift on the tailplane. The total lift is equal to the aircraft's mass if it is trimmed.

 

So the Centre of Lift acts through the Centre of Gravity of the aircraft if it is trimmed. By definition, a trimmed aircraft has no pitching moment about the Centre of Gravity - if it did have pitching moment then it would pitch and, therefore, not be trimmed.

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You're only seeing the COG in a single plane. There is no requirement for the COL to act through the COG as long as the resulting forces create (induce), or reduce stability as desired. In your idea, there can be no place for high wing or low wing aircraft, because the COL does not act through the COG.

 

All aircraft would therefore have to be mid-wing. But they aren't.

 

COG can be above or below the COL. Indeed it can even be to one side or the other as balanced flight results from the confluence of forces acting, not about a single plane of motion but about a confluence of a multiplicity - even COG is a movable feast as the vertical component changes according to the AOA of the wing. Yet you can create a stable climb or descent, even when the vertical COG component has been swung aft (climbing) or forward (descending).

 

Are you confusing Centre of Lift with centre of rotation

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Thanks guys for the discussion and explanations. I just read my initial post and, my god is my grammar horrible XD. I am trying to find out what the center of pressure is, as well as the center of lift. I am so sorry if that caused confusion. I have done more research and look at all of the links you guys supplied and I'm still piecing it together. 

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Are you confusing Centre of Lift with centre of rotation ?

 

No - you misunderstand your own diagram (17-24), which does not show the Centre of Lift.  It shows only the lift force from the wing (L) and the lift force on the tailplane (T)

 

The total lift on the aircraft (La) is given by La = L + T

 

The moments about the Centre of Gravity consist of a a nose-down moment from L and and a nose-up down from T. The resulting moment has to be zero for a trimmed aircraft. The Centre of Lift of the aircraft is therefore at the Centre of Gravity.

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Not true. If it were true then any change in COG would NOT cause an increase or decrease in stability. And you could not have high wing v. low wing arguments... there would be one place only to place a wing.

 

pic4-6.gif

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Your latest figure 4-6 reads:
 

CENTER OF GRAVITY (CG) IS THE POINT AT WHICH THE WEIGHT FORCE IS CONSIDERED TO BE CONCENTRATED
CENTER OF LIFT (CL) IS THE POINT AT WHICH THE LIFT FORCE IS CONSIDERED TO BE CONCENTRATED

 
Unless the point at which CG and CL are the same then there must be pitching moment. In that case the aircraft is not trimmed. That is a fundmental statement about mechanics.

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Aircraft in trim...

 

As it says

For trimmed flight, no rotation about c.g...

 

 

So the Centre of Lift and Centre of Graviity are at the same point; otherwise there would be a rotation about  the Centre of Gravity.

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Unless the point at which CG and CL are the same then there must be pitching moment. In that case the aircraft is not trimmed. That is a fundmental statement about mechanics.

 

But the force produced by the tailplane is not, for the purposes of understanding the physics of flight on a course of flying instruction, considered to be part of the total lifting force.

 

I can understand the basis of your argument. However, I cannot find any relevant literature that explains the forces of an aircraft in flight in the way that you do. If you can find an aviation reference to substantiate your position, I would be genuinely interested to read it.

 

It may well be quite proper to describe the force produced by the tailplane as part of the total lifting force, but that is not how any pilot training (or examination) material considers it. Instead, the force generated by the tailplane is considered to be a separate, downwards force which balances the moment between the CofP and the CofG.

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