Miniflyer

Hy,

do you still need that file? If yes drop me a line...judging from the graphic it's the file i created a while back and which was then graphically made attractive by Spunki. Nice to see that people are using it :-)

Best regards
Hank

HarryC

My Feedback: (1)

Mike, if the stab/canard has a symmetrical section and is pivoted at the AC (25% of MAC) then there is no aerodynamic loading.

BaldEagel

Harry

Of course you are correct, I was trying to keep the explanation simple, by not going into the requirement to pivot at the center of pressure which is not neccesaraly 25% of MAC, it would start to get complicated explaining the difference between the C of G and the C of P at a given AOA, but the point is that in flight the loading would be equallised on the top and bottom of the airfoil and therefore diminished. Can't think of a canard or full flying stab which would not have a symmetrical section though.

Mike

HarryC

My Feedback: (1)

Mike, are you confusing the Centre of Pressure with the Aerodynamic Centre? The CofP moves around wildly, at times it can be an infinite distance behind the wing. The AC on the other hand is stationary, it's exact location depends on the section being used but it is at or very close to 25% MAC. The CofP is a very outmoded concept due to its movement and infinite values it is of little help in explanations or calculations. Wings are explained in terms of lift force acting through the AC and a rotational force, the Moment Coefficient of Pitching, around the AC.

The relationship between CG and CP is irrelevant to the aerodynamic rotational force. For a symmetrical section there is no rotational force around the AC so for a symmetrical stab or canard pivoted at the AC there is no airflow generated force for it to be blown back to the neutral position by the airflow and hence no compensation there for it not being mass balanced. It looks like since 3/4 of the area (roughly) is behind the AC it stands to reason that it gets blown level, but it doesn't!!! Rotational pitching force comes from having camber, with zero camber i.e. symmetrical there is no pitching force, as camber increases the pitching force increases. The pitching force also depends on speed, so the faster you go the greater the pitching force. CofP is an amalgam of lift and pitching so at the zero lift angle when pitching still exists there is zero CofP force therefore in order to have it still cause pitching, the lift has to be an infinite distance behind the wing. The conceptualisation and maths becomes silly, which is why CofP is largely ignored now.

There are plenty of planes with non-symmetrical stabs, Tornado and Phantom to name 2. They have inverted sections to help generate the enormous downforce at the tail, the Phantom having an amazing undercamberd, inverted section with inverted leading edge slats on some variants. These generate rotational pitching forces around the AC, and because it is inverted it pushes the leading edge of the stab upwards and its t/e downwards.

H

BaldEagel

Harry

I can't for the life of me see how that works, the C of P is the highest point of pressure on the section due to the airflow, I am afraid at my age I am a very outmoded kind of guy and the Aerodynamic Centre remaining stationary at different AOA is a new concept on me, to sugest that the C of P could be behind the section to me is a nonsense concept, you can only get positive and negative presure so long as the air is being seperated by the section, now addmitedly the presure buble can move a small amount in front or behind the wing according to the AOA, to suggest that this can still happen an infinite distance behind the section is beyond my conprehension.

Thanks for the info on the non-symmetrical section all flying stabs, if I had given it some thought I would have quickly realised that some airframes (Blackhawk especially) need lift or downforce at the tail end to have any chance of flying.

Mike

HarryC

My Feedback: (1)

No it's not Mike, the CofP is as I explained above.
H

HarryC

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Mike, this from wikipedia

On a cambered airfoil the center of pressure does not occupy a fixed location. For a conventionally cambered airfoil, the center of pressure lies a little behind the quarter-chord point at maximum lift coefficient (large angle of attack), but as lift coefficient reduces (angle of attack reduces) the center of pressure moves toward the rear. When the lift coefficient is zero an airfoil is generating no lift but a conventionally cambered airfoil generates a nose-down pitching moment, so the location of the center of pressure is an infinite distance behind the airfoil.

The way the center of pressure moves as lift coefficient changes makes it difficult to use the center of pressure in the mathematical analysis of longitudinal static stability of an aircraft. For this reason, it is much simpler to use the aerodynamic center when carrying out a mathematical analysis. The aerodynamic center is a slightly more difficult concept to comprehend, but the aerodynamic center occupies a fixed location on an airfoil, typically close to the quarter-chord point.

BaldEagel

Harry

Wikipedia is not an authoritive sourse of information anyone can go on there and put forward anything they like, however from the NASA web site:

As an object moves through a fluid, the velocity of the fluid varies around the surface of the object. The variation of velocity produces a variation of pressure on the surface of the object. Integrating the pressure times the surface area around the body determines the aerodynamic force on the object. We can consider this force to act through the average location of the pressure on the surface of the object. We call the average location of the pressure variation the center of pressure in the same way that we call the average location of the weight of an object the center of gravity. In general, the pressure distribution around the object also imparts a torque, or moment, on the object. If a flying airfoil is not controlled in some way it will tumble as it moves through the air.

If we consider an airfoil at angle of attack, we can (theoretically) determine the pressure variation around the airfoil, and calculate the aerodynamic force and the center of pressure. But if we change the angle of attack, the pressure distribution changes and therefore the aerodynamic force and the location of the center of pressure and the moment all change. So determining the aerodynamic behavior of an airfoil is very complicated if we use the center of pressure to analyze the forces. We can compute the moment about any point on the airfoil if we know the pressure distribution. The aerodynamic force will be the same, but the value of the moment depends on the point where that force is applied. It has been found both experimentally and theoretically that, if the aerodynamic force is applied at a location 1/4 chord back from the leading edge on most low speed airfoils, the magnitude of the aerodynamic moment remains nearly constant with angle of attack. Engineers call the location where the aerodynamic moment remains constant the aerodynamic center (ac) of the airfoil. Using the aerodynamic center as the location where the aerodynamic force is applied eliminates the problem of the movement of the center of pressure with angle of attack in aerodynamic analysis. (For supersonic airfoils, the aerodynamic center is nearer the 1/2 chord location.)

Therefore the Aerodynamic center is a theoreticall center and far from being an outmoded concept the Center of Pressure is the more accurate means of establishing what happens to an airfoil section, just more complicated, the suggestion that the C of P can be infinatly behind the section is not one I subscribe to and can find no authority explanation for it happening, once an airfoil gets to AOA that no longer supports differential pressure the pressure bubble moves off the TE and collapses.

If you can guide me to an authoritive explanasion I would be keen to see it.

Thanks
Mike

RAPPTOR

My Feedback: (41)

pivot goes on mac. balance goes 25% of mac. used this many times, high leading edge taper angles make this a pain!!!worst one i did was an a-7 corsair..f-22

HarryC

My Feedback: (1)

Mike, what you have quoted from NASA says exactly what I have told you, and what is on wiki. It contradicts what you then assert!

Try your local library for proper aerodynamic texts e.g A C Kermode, etc.

It is basic aerodynamics! For a start you could read "model aircraft aerodynamics" by martin simons in which he says "the CofP was an abstract theoretical point.... the calculations produce the extraordinary conclusion that the CofP at zero lift must lie an infinite distance behind the wing". At low angles the CofP is already behind the wing, no longer on it.

H

HarryC

My Feedback: (1)

It can't, the MAC is a line from l/e to t/e. Did you mean to say the pivot goes on 25% of MAC, i.e the AC?

BaldEagel

At low agles of attack the C of P is near if not on the C of G not behind the wing, we will have to agree to dissagree on this one, the theory put forward for the so called aerodynamic centre is just that its in theory not practice, still an interesting disscusion, I will get out all my old notes and books and re swat the subject of fluid dynamics to see if I have missed anything, if I have I will come back to you, perhaps in a PM to prevent us getting to embroiled on this thread on what is obviously a favorite subject to both of us.

Mike

HarryC

My Feedback: (1)

What? I'm sorry Mike but you are just so completely wrong! You really do need to read some proper aerodynamics text books.

BaldEagel

Harry

For you to think I am that wrong in my assersions must mean we are talking about totally different things, see Birch and Bransom Flight Briefing for Pilots Page 29 or Welch and Irving Page 353, maximum lift happens at the C of P which is approximatly at the highest point of an airfoil section where maximum differential occur between the negative and positve pressures above and below the wing, this is approximatly around the C of G and cannot occur behind the wing section, there being no pressure differential.

Mike

HarryC

My Feedback: (1)

Indeed, we certainly are talking about a different thing. You are describing what is essentially a fixed, unmoving point, "approximatly at the highest point of an airfoil section where maximum differential occur between the negative and positve pressures". Something has gone awry in your definition of the CofP! The CofP is not what you have described, it is not at the point of max difference between top and bottom surface pressures, it is not defined as being at at the highest point of an airfoil and it is not around the CG.

The definition of the CofP -
From NASA - Integrating the pressure times the surface area around the body determines the aerodynamic force on the object. We can consider this force to act through the average location of the pressure on the surface of the object. We call the average location of the pressure variation the center of pressure in the same way that we call the average location of the weight of an object the center of gravity. ( http://www.grc.nasa.gov/WWW/K-12/airplane/ac.html which also describes the Aerodynamic Centre and why it is used and the some of the problems with CofP, but see quote at the end and link for the main description and problems with CofP)

From the RAF manual for its pilots - The overall effect of these pressure changes on the surface of the airfoil can be represented by one single aerodynamic force activity from the point at which all forces balance. This point is called the Centre of Pressure.

From The Air Pilot's Manual Vol 4 (for the UK PPL - ... the overall effects of these changes in static pressure using the single aerodynamic force Total Reaction acting at a single point on the chord line - the Centre of Pressure..... At normal crusing speeds the CofP is back towards the centre of the wing. As the angle of attack is increased..the CofP moves forward. The furthest forward that it moves is to about 25% aft of the l/e..... The lift force acts through the CofP, at 4 degrees AoA the location of the CofP is about 40%of the chord back from the l/e. These numbers apply to typical flattish bottom sections found on light planes for PPL students, the CofP for a symmetrical section is fixed at 25% MAC which is not the highest point of the wing, not the point of max difference between top and bottom pressures, and definitely not where the CG is!

From Simons - The CofP was always an abstract theoretical point for there was no way the measuring equipment could be moved back and fore in the wind tunnel to track its supposed movement. At moderatley low AoA, calculation and plotting of the CofP showed it had moved far to the rear, so far it was no longer within the wing but must be considered as lying somewhere beyond the trailing edge... The calculations produce the extraordinary conclusion that the centre of pressure at zero lift (e.g. vertical climb or dive) must lie an infinite distance behind the wing... Providing it is remembered that it is an abstraction, this rather old method of describing the wing forces..... Nonetheless it can cause confusion because it is often quite wrongly assumed thatthe centre of pressure cannot move beyond the trailing edge or that it stops somewhere before the t/e. This impression is reinforced by the older textbooks of aircraft engineering..... That symmetrical sections would have a fixed centre of pressure at the quarter chord point was predicted by theory long before wind tunnel engineers discovered it to be so in practice

No mention of the highest point, maximum difference point or the CG in any of those descriptions of the CofP.

This from NASA - There are several important problems to consider when determining the center of pressure for an airfoil. As we change angle of attack, the pressure at every point on the airfoil changes. And, therefore, the location of the center of pressure changes as well. The movement of the center of pressure caused a major problem for early airfoil designers because the amount (and sometimes the direction) of the movement was different for different designs. In general, the pressure variation around the airfoil also imparts a torque, or "twisting force", to the airfoil.
To resolve some of these design problems, aeronautical engineers prefer to characterize the forces on an airfoil by the aerodynamic force, describedabove, coupled with an aerodynamic moment to account for the torque. It was found both experimentally and analytically that, if the aerodynamic force is applied at a location 1/4 chord back from the leading edge on most low speed airfoils, the magnitude of the aerodynamic moment remains nearly constant with angle of attack. Engineers call the location where the aerodynamic moment remains constant the aerodynamic center of the airfoil. Using the aerodynamic center as the location where the aerodynamic force is applied eliminates the problem of the movement of the center of pressure with angle of attack in aerodynamic analysis. From http://www.grc.nasa.gov/WWW/K-12/airplane/cp.html

See also http://www.grc.nasa.gov/WWW/K-12/airplane/ac.html for the Aerodynamic Centre

BaldEagel

Harry

I am not ignoring you, just very busy at the moment digging a base for a new workshop, it has to be sunk 1.3M into the ground and its taking up all my time, I will reply soon, looking at your latest post it will probable be a humble pie answer.

Mike

HarryC

My Feedback: (1)

I've no doubt that some of you are thinking that it is preposterous that the lift can be behind the wing and as far as an infinity behind the wing. Well, it is because the centre of pressure is not the lift! Do not confuse them as being the same thing. The CofP is a combination of two effects - the lift and the rotational force of a cambered wing. That is why aerodynamics is far better using the Aerodynamic Centre, where the two effects can be dealt with entirely separately.
The CofP theory says that the lift force causes the rotation of a cambered wing. The rotation force, or pitching moment as it is usually called, is dependent on the coefficient of pitching for that section, and airspeed. If measured from the AC (the 25% of MAC point) the pitching force is constant for all unstalled angles of attack. So the faster you go, the stronger the nose down pitching regardless of (unstalled) angle of attack. The nose down pitching is still there at the zero lift angle and that's what causes the infinity problem.

The rotational force is a torque. Torque = force x distance. In the case of a wing using Centre of Pressure theory, torque = lift x distance from AC. Since torque is constant for all unstalled AoA, then any change in lift force will alter the distance at which the lift force is applied. For level flight at 1G, changing speed will change the torque, but since force is constant then the distance must change, hence the movement of the CofP. For flight involving a change of lift force at a constant speed, since torque must then be constant (since speed is constant) any change in lift force must be matched by a change in distance. For example, torque = lift x distance from AC, if speed is constant then torque is constant, if you reduce lift by pushing the nose down then to balance the equation the distance at which the lift is applied must increase. At very low lift levels the distance has to be huge since a very low force has to be applied at a great distance in order to generate the torque. Or, at very high speeds in level flight the torque is very high but since lift is constant at 1G then the distance at which the lift force is applied has to be very high to generate the torque. Now take the case where the lift force is made very small, approaching zero as might happen in a push-over, a maximum acceleration ballistic dive, vertical climb or dive etc. The large torque is still there but the force producing it is approaching zero, so the lever arm required is enormous, and eventually as the lift force creating the torque reaches zero it will require an infinitely long lever arm to create the torque.

Clearly this is bonkers. The only things that can affect the wing must be touching it, they can't be an inch behind it let alone at infinity. Yet the maths of the centre of pressure is inescapable!! The problem is not the maths, it is the concept of the CofP. Imagine a wing that has some lift force on its upper surface towards the t/e and some down force on the lower surface towards the l/e. The up and down force cancel out so that there is no net up or down lift force, i.e it is at zero lift. But due to the fore and aft displacement of the forces, a rotational force exists. Now we can see that all the causes of the lift (zero in this case) and of the rotation are on the wing and that the problem is trying to explain the rotational force by using the non-existent lift force!!! But that is exactly what CofP tries to do and that is why it is such a problematic idea.

BaldEagel

Harry

From Simons - This impression is reinforced by the older textbooks of aircraft engineering..... That symmetrical sections would have a fixed centre of pressure at the quarter chord point was predicted by theory long before wind tunnel engineers discovered it to be so in practice.

My Welch and Irvin is dated 1977 and my Pilots Briefing Books are even older, that's my excuse and I am sticking to it.

Harry thanks for the update on moden thinking about aerodynamics its nice to know the yongsters are still telling us oldies how to do things. [:@][&:][sm=red_smile.gif][sm=cry_smile.gif]

Regards
Mike

BaldEagel

Harry

Forgot to thank you, its was an iteresting disscusion and I learnt something, shades of Old dog new Tricks come to mind.

Thanks again
Mike

Bob.R

My Feedback: (2)

Harry, is the above a typo? I thought the pitching coefficient was constant, not the pitching force. Perhaps it would be correct to say that the pitching force at a constant speed is constant for all unstalled angles of attack? I think you say that in your prior sentence.

HarryC

My Feedback: (1)

Yes Bob, thanks for picking that up, the pitching coefficient is constant, so the pitching force that is created is constant for any one speed, regardless of AoA.

H