Discussion
I understand the job of the flaps on an aeroplane and I also understand how they work (I think). But what I don't understand is when the flaps are fully extended (pointing down) why the aircraft doesn't descend towards the ground. Like when banking left or right the ailerons one each wing point up or down to the opposite which in turn makes the aircraft bank left or right and going up or down the ailerons both point in the same direction.
Perik Omo said:
Flaps (on the wings) create drag/moderate speed and ailerons (on the tail) are the things that point the aircraft up or down.
Ailerons are not on the tail. They are on the wings. Elevators are on the tail. My understanding is basic but.
Elevator change the pitch of the aircraft, I.e point the nose up or down
Ailerons are on the wings and work opposite to each otter, one goes up whilst the other goes down, this causes the plane to bank.
Flaps extend the wing area and shape of the wing creating more lift and reducing stall speed (drag is a side effect)
Note. This refers to aircraft with sings and a tail plane rather than delta wings or jets that just have wings and a vertical stabiliser.
Edited by blueg33 on Wednesday 20th July 08:03
Blue33 is basically correct. What he is explaining are referred to as the "Primary Effects" of control of the various devices on the wings and tail of an aircraft. All these controls also have Secondary Effects (and even Tertiary Effects as well).
From memory, I would list the control effects as follows -
Ailerons - wing mounted devices which move up and down in opposition. Their primary effect is to bank the aircraft i.e. raise one wing up and lower the other (referred to as "roll"). The secondary effect is to introduce an element of turn towards the lowered wing
Elevators - mounted on the tail planes and used to adjust the pitch of the aircraft i.e. nose up or nose down movement.
Rudder - mounted on the tail fin. The primary effect is to swing the nose from side to side (yaw). The secondary effect is to introduce a turning effect on the aircraft and an element of bank.
Flaps (and leading edge slats/droops etc - if fitted) These are designed to increase the wing area of the aircraft to allow the aircraft to fly more slowly. They are mainly used during take off and landing. When they are deployed, they usually cause the aircraft to change its attitude in the air i.e. introduce a pitch change. They can be used as air brakes as well.
In addition to the above devices, aircraft have other moving sections to the wings and tail for fine adjustment or changing attitude and direction in a more sedate manner. For instance, having established an aeroplane in level flight, or descent, or climb. or turn, trim tabs are set to maintain that attitude until it is time to change again. Trim tabs allow the control forces i.e. the amount of effort the pilot needs to put into pulling or pushing the control yoke or rudder pedals to be reduced and place less physical strain on the pilot.
Some airliners use smaller devices on the wing to perform low rate turns - such as wing spoilers or lift dumpers (which are also used as air brakes).
Some aeroplanes use different forms of control surfaces to perform the duties explained above. For example, supersonic aircraft tend not to have elevators but have all moving tail planes. Some high performance aircraft operate their entire all moving tail planes to execute a roll rather than wing mounted ailerons. The F-18 and Tornado would be examples.
Delta winged aircraft often don't have separate elevators but instead have dual purpose elevons i.e. moving surfaces which work in opposition (i.e. as ailerons) for banking and work together as elevators for pitch control.
From memory, I would list the control effects as follows -
Ailerons - wing mounted devices which move up and down in opposition. Their primary effect is to bank the aircraft i.e. raise one wing up and lower the other (referred to as "roll"). The secondary effect is to introduce an element of turn towards the lowered wing
Elevators - mounted on the tail planes and used to adjust the pitch of the aircraft i.e. nose up or nose down movement.
Rudder - mounted on the tail fin. The primary effect is to swing the nose from side to side (yaw). The secondary effect is to introduce a turning effect on the aircraft and an element of bank.
Flaps (and leading edge slats/droops etc - if fitted) These are designed to increase the wing area of the aircraft to allow the aircraft to fly more slowly. They are mainly used during take off and landing. When they are deployed, they usually cause the aircraft to change its attitude in the air i.e. introduce a pitch change. They can be used as air brakes as well.
In addition to the above devices, aircraft have other moving sections to the wings and tail for fine adjustment or changing attitude and direction in a more sedate manner. For instance, having established an aeroplane in level flight, or descent, or climb. or turn, trim tabs are set to maintain that attitude until it is time to change again. Trim tabs allow the control forces i.e. the amount of effort the pilot needs to put into pulling or pushing the control yoke or rudder pedals to be reduced and place less physical strain on the pilot.
Some airliners use smaller devices on the wing to perform low rate turns - such as wing spoilers or lift dumpers (which are also used as air brakes).
Some aeroplanes use different forms of control surfaces to perform the duties explained above. For example, supersonic aircraft tend not to have elevators but have all moving tail planes. Some high performance aircraft operate their entire all moving tail planes to execute a roll rather than wing mounted ailerons. The F-18 and Tornado would be examples.
Delta winged aircraft often don't have separate elevators but instead have dual purpose elevons i.e. moving surfaces which work in opposition (i.e. as ailerons) for banking and work together as elevators for pitch control.
Edited by Eric Mc on Wednesday 20th July 08:28
NordicCrankShaft said:
I understand the job of the flaps on an aeroplane and I also understand how they work (I think). But what I don't understand is when the flaps are fully extended (pointing down) why the aircraft doesn't descend towards the ground. Like when banking left or right the ailerons one each wing point up or down to the opposite which in turn makes the aircraft bank left or right and going up or down the ailerons both point in the same direction.
If you're talking about a conventional aircraft (with a tailplane), then deploying flaps does cause a degree of pitch up or down moment (depends on the aircraft), due to changing the point at which the total lift acts on the wing. On a conventional aircraft the centre of lift from the wing is behind the centre of mass of the aircraft, and the tailplane provides downforce to balance the aircraft in pitch. Flaps increase the lift (and drag) of the wing, which is behind the CoM already, and as the flaps are at the back of the wing it also moves the CoL further back. This will pitch the aircraft down, which is countered by increasing the downforce from the tail.
It's useful to consider what effects these controls directly have in order to understand the effect they have. It's the wing which takes the aircraft in which ever way it goes until such times as the wing stops flying.
Elevators, used to control pitch, work by changing angle of attack.
Flaps take various forms all achieving broadly the same effects in different proportions. Every wing or aerofoil section has a Coefficient of Lift, and a Coefficient of Drag.
As car people, we're all familiar with the Coefficient of Drag. It tells us that depending on the density of the fluid we're moving in (i.e. air) how great the force acting against our movement is as a proportion of our speed through the fluid.
The Coefficient of Lift is a similar thing which tells us, depending on air density, how great a lifting force is generated by a combination of both speed and angle of attack - the angle at which the wing meets the oncoming air flow.
For a given wing and at a given air density, the same total lift can be generated by flying faster at a lower angle of attack, or slower at a higher angle of attack. The upper speed limit is governed by structural strength and Mach number, whilst the lower speed limit is governed by the critical angle of attack for that particular aerofoil section. Below AoAcrit the airflow remains attached to the wing and delivers the desired lifting affect. Above AoAcrit the airflow begins to detach from the wing destroying lift and increasing drag. Some aerofoils offer progressive breakaway, whilst others have more sudden characteristics. One tends to find that lower performance aerofoils are more gentle in the stall whereas the higher performance sections are more often associated with sharper breakaways - not unlike car tyres.
Whatever the type of flap: slotted, Fowler, Fairey Youngman, plain / simple, split; what they all have in common is that they are used to alter both the Coefficient of Drag (Cd) and Coefficient of Lift (Cl). One goes with the other. To fly fast without being bounced around you want/need low Cd and accept low Cl (remember you can equal the aircraft's weight in lift by flying faster).
To get acceptable flying qualities therefore reducing take off and landing speeds, what you want in a fast aircraft is a different wing for flying slowly. This is achieved with flaps. Changing wing area etc is somewhat secondary coincidental really.
When flaps are deployed, one can often feel Cd increase from the passenger cabin (and you know it in the cockpit) because we feel a deceleration. As Cl increases though, the total lift generated increases too given that the air speed and angle of attack were suitable for a different Cl, so the pilots will either have to slow down, or more practically, lower the nose a couple of degrees to generate the same lift at the speed they're flying at.
Lowering the nose is handy in itself - it gives a better view of the ground over the nose.
Depending on the exact configuration of the particular aircraft type, trim change with flaps can be either nose up or down.
Elevators, used to control pitch, work by changing angle of attack.
Flaps take various forms all achieving broadly the same effects in different proportions. Every wing or aerofoil section has a Coefficient of Lift, and a Coefficient of Drag.
As car people, we're all familiar with the Coefficient of Drag. It tells us that depending on the density of the fluid we're moving in (i.e. air) how great the force acting against our movement is as a proportion of our speed through the fluid.
The Coefficient of Lift is a similar thing which tells us, depending on air density, how great a lifting force is generated by a combination of both speed and angle of attack - the angle at which the wing meets the oncoming air flow.
For a given wing and at a given air density, the same total lift can be generated by flying faster at a lower angle of attack, or slower at a higher angle of attack. The upper speed limit is governed by structural strength and Mach number, whilst the lower speed limit is governed by the critical angle of attack for that particular aerofoil section. Below AoAcrit the airflow remains attached to the wing and delivers the desired lifting affect. Above AoAcrit the airflow begins to detach from the wing destroying lift and increasing drag. Some aerofoils offer progressive breakaway, whilst others have more sudden characteristics. One tends to find that lower performance aerofoils are more gentle in the stall whereas the higher performance sections are more often associated with sharper breakaways - not unlike car tyres.
Whatever the type of flap: slotted, Fowler, Fairey Youngman, plain / simple, split; what they all have in common is that they are used to alter both the Coefficient of Drag (Cd) and Coefficient of Lift (Cl). One goes with the other. To fly fast without being bounced around you want/need low Cd and accept low Cl (remember you can equal the aircraft's weight in lift by flying faster).
To get acceptable flying qualities therefore reducing take off and landing speeds, what you want in a fast aircraft is a different wing for flying slowly. This is achieved with flaps. Changing wing area etc is somewhat secondary coincidental really.
When flaps are deployed, one can often feel Cd increase from the passenger cabin (and you know it in the cockpit) because we feel a deceleration. As Cl increases though, the total lift generated increases too given that the air speed and angle of attack were suitable for a different Cl, so the pilots will either have to slow down, or more practically, lower the nose a couple of degrees to generate the same lift at the speed they're flying at.
Lowering the nose is handy in itself - it gives a better view of the ground over the nose.
Depending on the exact configuration of the particular aircraft type, trim change with flaps can be either nose up or down.
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