Aircraft aerodynamics question
Discussion
I know we have a few pilots, aircraft engineers and enthusiasts on PH. So, this is something I was musing over whilst on a flight yesterday sitting in a window seat overlooking the wing. How come the aircraft’s control surfaces appear to be in the same position on take off and landing? That is with the flaps fully down and the leading edge slats fully extended (or at least that’s how it looked from the window).
I would have thought the control surfaces would at least be in different positions during take off and landing if not opposite. So in simple terms how do the same control surface positions allow an aircraft to gain height during takeoff and loose it during landing?
I understand the principals of how a wing section generates lift (air over the top has to travel a greater distance than air under the bottom, hence has to travel faster, hence has a lower pressure, hence lift is generated) but this one’s got me perplexed.
I was on an MD80 BTW
I would have thought the control surfaces would at least be in different positions during take off and landing if not opposite. So in simple terms how do the same control surface positions allow an aircraft to gain height during takeoff and loose it during landing?
I understand the principals of how a wing section generates lift (air over the top has to travel a greater distance than air under the bottom, hence has to travel faster, hence has a lower pressure, hence lift is generated) but this one’s got me perplexed.
I was on an MD80 BTW
t1grm said:
I understand the principals of how a wing section generates lift (air over the top has to travel a greater distance than air under the bottom, hence has to travel faster, hence has a lower pressure, hence lift is generated) but this one’s got me perplexed.
You've answered your own question in a way!!!
By extendeing flaps/slats, the surface area of the wing is increased, giving greater lift, which allows for a lower airspeed at landing/takeoff.
On an aircraft that size the control surfaces are likely to be pretty big. As the aircraft should be in equilibrium when in straight and level flight a small deflection of the control surfaces will have a big effect.
Also the surfaces you can see on the wing control the roll of the aircraft along the axis of the fuselage so are unlikely to move all that much during take-off and landing. The surfaces that point the nose up and down are on the tailplane. There will be flaps on the wings however that give the aircraft greater lift at lower speed at the expense of fuel economy etc, you should have seen these move as you come to land.
The above may or may not be bollox!
Ben
Also the surfaces you can see on the wing control the roll of the aircraft along the axis of the fuselage so are unlikely to move all that much during take-off and landing. The surfaces that point the nose up and down are on the tailplane. There will be flaps on the wings however that give the aircraft greater lift at lower speed at the expense of fuel economy etc, you should have seen these move as you come to land.
The above may or may not be bollox!
Ben
t1grm said:
I understand the principals of how a wing section generates lift (air over the top has to travel a greater distance than air under the bottom, hence has to travel faster, hence has a lower pressure, hence lift is generated
Yeh, I understand this as well. What puzzles me is that some of them can fly upside down as well. How is this possible?
sparkythecat said:
I understand the principals of how a wing section generates lift (air over the top has to travel a greater distance than air under the bottom, hence has to travel faster, hence has a lower pressure, hence lift is generated
Yeh, I understand this as well. What puzzles me is that some of them can fly upside down as well. How is this possible?
Sounds like you need Combat Flight Sim.
You'll find you can make planes do all sorts of things when some git is trying to
you down! I often wish CFS had a 'record' or 'replay' facility so I can watch some of my manoeuvres again...
crazycats said:
I understand the principals of how a wing section generates lift (air over the top has to travel a greater distance than air under the bottom, hence has to travel faster, hence has a lower pressure, hence lift is generated) but this one’s got me perplexed.
You've answered your own question in a way!!!
By extendeing flaps/slats, the surface area of the wing is increased, giving greater lift, which allows for a lower airspeed at landing/takeoff.
Not really. My question is how come this has the effect of making the aircraft gain height when taking off and loose height during landing?
ywouldi said:
On an aircraft that size the control surfaces are likely to be pretty big. As the aircraft should be in equilibrium when in straight and level flight a small deflection of the control surfaces will have a big effect.
Also the surfaces you can see on the wing control the roll of the aircraft along the axis of the fuselage so are unlikely to move all that much during take-off and landing. The surfaces that point the nose up and down are on the tailplane. There will be flaps on the wings however that give the aircraft greater lift at lower speed at the expense of fuel economy etc, you should have seen these move as you come to land.
The above may or may not be bollox!
Ben
I think you’re talking about Ailerons. They control the roll of the aircraft and yes I know these move very slightly and cause quite large movements of the aircraft in comparison.
I’m talking about the flaps on the trailing edge (in board of the Ailerons on an MD80) and the slats at the leading edge. These are huge surfaces and move a huge amount (about 1-2 ft) from the wing and actually sort of detach themselves from the wing in that you can see clear daylight between the flap/slat and the main structure of the wing. They are lowered only during takeoff and landing and are retracted during cruising.
t1grm said:
Not really. My question is how come this has the effect of making the aircraft gain height when taking off and loose height during landing?
Because the flaps dont make the aircraft lose or gain height on their own.
The flaps provide greater lift at a given speed.
So when taking off they are down, this combined with the rotational force from the elevators on the tailplane pointing the nose upward and making the aircraft lift off (or rotate) at a lower speed.
When landing the flaps are lowered as the aircraft can then maintain a lower speed for landing whilst the wings still produce enough lift to make the plane stay in the air. Of course the lift produced is just under the amount needed to maintain straight and level flight so the plane loses height gradually. Other control inputs simply fine tune the loss of height.
Ben
>> Edited by ywouldi on Saturday 26th March 15:59
t1grm said:
crazycats said:
t1grm said:
I understand the principals of how a wing section generates lift (air over the top has to travel a greater distance than air under the bottom, hence has to travel faster, hence has a lower pressure, hence lift is generated) but this one’s got me perplexed.
You've answered your own question in a way!!!
By extendeing flaps/slats, the surface area of the wing is increased, giving greater lift, which allows for a lower airspeed at landing/takeoff.
Not really. My question is how come this has the effect of making the aircraft gain height when taking off and loose height during landing?
The flaps don't make the aircraft gain or lose height. The power of the engines combined with the elevators on the tail make the plane go up or down. The flaps create more lift under the wing so the stall speed is lower, that way the the plane can take off and land at lower speeds.
As for making the plane go up and down, if a plane is flying at say 1000 feet and you open the throttles on the engines but did not alter any surface controls, the plane will go higher, you do not need to alter the elevators for this.
>> Edited by falcemob on Saturday 26th March 16:07
Having written all that Im still not sure exactly why the answers given havent answered your question? I think my last reply may have done so though. The flaps allow the aircraft to fly slower and maintain a decent attitude and allows the pilot full control and also keeps the aircraft from stalling.
I think I am getting a little deep for my basic airmanship stuff I have studied!
Ben
I think I am getting a little deep for my basic airmanship stuff I have studied!
Ben
ywouldi said:
Because the flaps dont make the aircraft lose or gain height on their own.
The flaps provide greater lift at a given speed.
So when taking off they are down, this combined with the rotational force from the elevators on the tailplane pointing the nose upward and making the aircraft lift off (or rotate) at a lower speed.
When landing the flaps are lowered as the aircraft can then maintain a lower speed for landing whilst the wings still produce enough lift to make the plane stay in the air. Of course the lift produced is just under the amount needed to maintain straight and level flight so the plane loses height gradually. Other control inputs simply fine tune the loss of height.
Ben
>> Edited by ywouldi on Saturday 26th March 15:59
Exactly what I wanted to say, but just can't type quick enough
t1grm said:It doesn't do that, the pilot makes the aircraft gain or lose height and change its attitude using the controls. The use of flaps etc gives more lift at low airspeed so, as has been said earlier, take-off and landing can be achieved at lower airspeeds. You still need lift just before you've landed as well as just after you've taken off!
Not really. My question is how come this has the effect of making the aircraft gain height when taking off and loose height during landing?
Flying upside down ... if you look at an aircraft flying like this, its attitude is rarely perfectly level, the nose is up and the tail down even though the whole thing is moving horizontally, near enough. In this attitude a component of the thrust is acting upwards and opposing both weight and any downward aerodynamic forces. In such an attitude, the wing and cockpit surfaces also help with lift from the 'angle of attack' idea, where the airflow hits the wings and body when they are sloping upwards, and this also exerts an upward force on the wings and body.
ywouldi said:
Having written all that Im still not sure exactly why the answers given havent answered your question? I think my last reply may have done so though. The flaps allow the aircraft to fly slower and maintain a decent attitude and allows the pilot full control and also keeps the aircraft from stalling.
I think I am getting a little deep for my basic airmanship stuff I have studied!
Ben
Don’t worry mate. You probably answered the question perfectly correctly it’s just it wasn’t obvious to me when reading your post
So, I think I understand it now. I wasn’t factoring in the elevators. So, the flaps and slats have nothing to do with the attitude of the aircraft, the elevators do that? The flaps and slats just allow the wing to generate enough lift at low speeds?
Ailerons (end of the wings) control roll, elevators (at the back) pitch, rudder yaw. On most aircraft
The flaps/leading edge slats change the shape of the wing to increase the overall lift at a given speed. This allows the aircraft to land and take off at slower speeds. However proportional to lift is drag, so the wings go back to a smaller profile at higher speeds.
google for Bernoulli effect for why
>> Edited by john_p on Saturday 26th March 17:23
The flaps/leading edge slats change the shape of the wing to increase the overall lift at a given speed. This allows the aircraft to land and take off at slower speeds. However proportional to lift is drag, so the wings go back to a smaller profile at higher speeds.
google for Bernoulli effect for why
>> Edited by john_p on Saturday 26th March 17:23
turbobloke said:
Flying upside down ... if you look at an aircraft flying like this, its attitude is rarely perfectly level, the nose is up and the tail down even though the whole thing is moving horizontally, near enough. In this attitude a component of the thrust is acting upwards and opposing both weight and any downward aerodynamic forces. In such an attitude, the wing and cockpit surfaces also help with lift from the 'angle of attack' idea, where the airflow hits the wings and body when they are sloping upwards, and this also exerts an upward force on the wings and body.
So, if I've understood this right, it's only the propulsion unit that's keeping it up and if that fails it'll plummet to earth rather than glide, unless you turn it the right way up. Won't it?
andygo said:
I thought the way some control surfaces extended themselves from the wings on landing with large angles of attack made them into air brakes.
Do you mean on the upper surface of the wing..?
If so, then yes they are speed brakes, only normally used just after landing, when both side are deployed together to increase drag.
However their primary role is as spoilers, which extend in conjunction with the opposing aileron, to break up the airflow across the upper surface of the wing, reducing lift, and causing that wing to drop...and dramatically increasing roll rate...blah, blah
sparkythecat said:
So, if I've understood this right, it's only the propulsion unit that's keeping it up and if that fails it'll plummet to earth rather than glide, unless you turn it the right way up. Won't it?
Pretty much..
If you look at the cross section of a wing from a fighter, or aerobatic aircraft, then the classic aerofoil shape is kept to a minimum.....so the distance from leading to trailing edge is very similar, across upper and lower wing surfaces
This enables inverted flight with far less control input, and at lower airspeeds, the trade off however is that far less lift is created in conventional flight attitudes.
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