30.06.2010, 04:35
Being a lapsed pilot is merely a matter of circumstance. Flying is expensive in Britain and like a great many other pilots flying on a budget I've had no choice but to give it up. Your attitude is a little disrespectful.
But - Regarding the matter of tailplane aerodynamics. I am quite amused by your assertion that my viewpoint is one of kindergarten level, since you clearly haven't looked closely at the issue. Let's look closer then.
What is "lift"? Basically it's the result of of an imbalance of air pressure on both sides of a surface moving through the air. That much is readily understood (the 'kindergarten' level). However, we need to realise that lift is a compound force. It arises from more than one effect.
The first effect is the result of air striking a surface. It stands to reason that air pressure will increase if a surface obstructs the airflow, even at a shallow angle, which we call Angle of Attack. Unfortunately, because the surface is obstructing the airflow, we also generate drag. According to conventional wisdom on aerodynamics, this effect provides around one third of the total lift generated, but notice that can only be true if the surface has an angle of attack. In a perfect minimum drag condition, there should ideally be zero lift from this source.
The second effect is one derived from the shape of the airflow. Because airflow across a surface reduces air pressure upon it, by making one side bulge slightly the air has further to travel and therefore must travel faster across it to meet at the trailing edge. We therefore get a region of reduced air pressure above the surface of a conventional wing. Approximately two thirds of lift from a wing can be generated from this source.
Lift is then a compound of higher pressure below, and reduced pressure above. The amount of lift varies according to various circumstances, such as forward velocity and angle of attack. The total is one thing. The value of lift from it's component sources varies according to conditions too.
That describes our main lifting surface, the wing. The tailplane is not there to lift the aeroplane (although I concede some designs, especially primitive edwardian aircraft, were effectively tandem winged aeroplanes) but to provide stability in the same way as the feathers of an arrow. In a perfect minimum drag condition, the tailplane should not need to generate any aerodynamic forces at all, though for practical reasons a very small amount is necessary to 'balance' the aeroplane in flight - the weight of the rear fuselage is always present.
If the angle of the fuselage diverges from that of foward travel, the tailplane presents a surface that the airflow will impact upon. There is no lift generated from the airfoil section - the taiklplane is ideally an aerodynamically neutral surface - but merely that caused by airflow impacting on the surface, and that condition is hopefully temporary because the effect is to return the fuselage to it's former attitude. It's a stability function, and the entire reason for the tailplane being there. In practice, there will likely be a very small component of this force generated all the time as the aeroplane is dragging its rear structure through the air against gravity. As already mentioned, this generates drag, and thus is an undesirable characteristic for performance reasons.
If we decide to impart a change of attitude from the controls, we move the elevator (or indeed, the entire tailplane on some aircraft) at an angle to the airflow. The air is deflected and the attitude of the aerplane changes, modified by forward velocity into a controllable manoever since the stabilising effect of the tailplane is still present. Slab tailplanes do not generate any appreciable lift from their airfoil section. How can they? The shape is neutral. But if fitted with elevators, does the modifying of the cross section by movement of the cobntrol surface actually form an airfoil that generates lift? Your proposition is based on that assumption. That is the cause of the misunderstanding.
The deflected elevator does not change the length of travel for airflow. By definition, it cannot generate reduced air pressure on the opposite surface. All we get is the first component of lift which is direct air pressure as the flow impacts against the obstructing control surface. The aeroplane responds by attempting to balance the forces generated by our control input, and so the manoever ensues.
What we have then is a force generated by increased pressure on a control surface. It is not 'lift' in either direction because it does not have all the component forces that define it.
That's my point of view. Take it or leave it.
Incidentially, there's no practical reason for requiring a wing to have its main spar on the center of gravity. Some aircraft have multiple wing spars. I'm pretty sure they don't have multiple centre of gravity's. The centre of gravity is a physical property - its where the total mass can be balanced - the structure does not need to conform to that.
If you wish to debate theoretical subjects on the forums then please show a little more courtesy, otherwise I'll have to consider your fourteen years as an active pilot in fixed and rotary wings as an extraordinary example of good fortune.
But - Regarding the matter of tailplane aerodynamics. I am quite amused by your assertion that my viewpoint is one of kindergarten level, since you clearly haven't looked closely at the issue. Let's look closer then.
What is "lift"? Basically it's the result of of an imbalance of air pressure on both sides of a surface moving through the air. That much is readily understood (the 'kindergarten' level). However, we need to realise that lift is a compound force. It arises from more than one effect.
The first effect is the result of air striking a surface. It stands to reason that air pressure will increase if a surface obstructs the airflow, even at a shallow angle, which we call Angle of Attack. Unfortunately, because the surface is obstructing the airflow, we also generate drag. According to conventional wisdom on aerodynamics, this effect provides around one third of the total lift generated, but notice that can only be true if the surface has an angle of attack. In a perfect minimum drag condition, there should ideally be zero lift from this source.
The second effect is one derived from the shape of the airflow. Because airflow across a surface reduces air pressure upon it, by making one side bulge slightly the air has further to travel and therefore must travel faster across it to meet at the trailing edge. We therefore get a region of reduced air pressure above the surface of a conventional wing. Approximately two thirds of lift from a wing can be generated from this source.
Lift is then a compound of higher pressure below, and reduced pressure above. The amount of lift varies according to various circumstances, such as forward velocity and angle of attack. The total is one thing. The value of lift from it's component sources varies according to conditions too.
That describes our main lifting surface, the wing. The tailplane is not there to lift the aeroplane (although I concede some designs, especially primitive edwardian aircraft, were effectively tandem winged aeroplanes) but to provide stability in the same way as the feathers of an arrow. In a perfect minimum drag condition, the tailplane should not need to generate any aerodynamic forces at all, though for practical reasons a very small amount is necessary to 'balance' the aeroplane in flight - the weight of the rear fuselage is always present.
If the angle of the fuselage diverges from that of foward travel, the tailplane presents a surface that the airflow will impact upon. There is no lift generated from the airfoil section - the taiklplane is ideally an aerodynamically neutral surface - but merely that caused by airflow impacting on the surface, and that condition is hopefully temporary because the effect is to return the fuselage to it's former attitude. It's a stability function, and the entire reason for the tailplane being there. In practice, there will likely be a very small component of this force generated all the time as the aeroplane is dragging its rear structure through the air against gravity. As already mentioned, this generates drag, and thus is an undesirable characteristic for performance reasons.
If we decide to impart a change of attitude from the controls, we move the elevator (or indeed, the entire tailplane on some aircraft) at an angle to the airflow. The air is deflected and the attitude of the aerplane changes, modified by forward velocity into a controllable manoever since the stabilising effect of the tailplane is still present. Slab tailplanes do not generate any appreciable lift from their airfoil section. How can they? The shape is neutral. But if fitted with elevators, does the modifying of the cross section by movement of the cobntrol surface actually form an airfoil that generates lift? Your proposition is based on that assumption. That is the cause of the misunderstanding.
The deflected elevator does not change the length of travel for airflow. By definition, it cannot generate reduced air pressure on the opposite surface. All we get is the first component of lift which is direct air pressure as the flow impacts against the obstructing control surface. The aeroplane responds by attempting to balance the forces generated by our control input, and so the manoever ensues.
What we have then is a force generated by increased pressure on a control surface. It is not 'lift' in either direction because it does not have all the component forces that define it.
That's my point of view. Take it or leave it.
Incidentially, there's no practical reason for requiring a wing to have its main spar on the center of gravity. Some aircraft have multiple wing spars. I'm pretty sure they don't have multiple centre of gravity's. The centre of gravity is a physical property - its where the total mass can be balanced - the structure does not need to conform to that.
If you wish to debate theoretical subjects on the forums then please show a little more courtesy, otherwise I'll have to consider your fourteen years as an active pilot in fixed and rotary wings as an extraordinary example of good fortune.