So when an airfoil creates lift. The air above the foil moves faster. This creates lower pressure in tip of the wing and relatively speaking higher pressure under the wing this create lift l. Along with meetings 3rd law of motion. You know it deflects aid downward the reaction is forcing the wing up. But how did this work with an elevator to my understanding. When the elevator goes up so the airflow across the top speeds up, so this creates low pressure. And the air below the elevator is relatively speaking higher pressure. But this would cause the elevator and tail of the plane to go down along with the pitch of the plane. But in realty when the elevator goes up, the tail of the plane goes down and the pitch goes up. Same with when it goes down the the airflow below the elevator speeds up and become low pressure and the air above it is higher causes the tail of the plane to go down and the pitch up. But in reality when the elevator goes down the pitch also goes down. Really confusing, someone please help.
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$\begingroup$ No the elevator creates lift to the other direction, it pushes the tail down not up. It's a wing rotated upside down. You can think the h stabilizer being a wing and the elevator as flap. $\endgroup$– user3528438Nov 3, 2017 at 18:42
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$\begingroup$ "When the elevator goes up so the airflow across the top speeds up" - it does? Where did you read that? I would have expected the opposite. $\endgroup$– Tanner SwettNov 3, 2017 at 19:29
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$\begingroup$ @TannerSwett well when the angle of the elevator goes up, the air above it can't move out of the way in time so it become constricted and squeezed so it moves faster which creates lower pressure above the wing so higher pressure below the wing so the net force is up lifting the tail of the plane and dipping the nose of the plane $\endgroup$– ItzyoboiNov 3, 2017 at 21:33
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$\begingroup$ If the last comment were true, you could create positive lift from the wings of an aircraft by attaching them to the fuselage with the trailing edge much higher than the leading edge. Wings do not work that way. $\endgroup$– David KNov 3, 2017 at 21:45
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$\begingroup$ @Itzyoboi Well, that's an interesting chain of reasoning, and I don't understand aerodynamics well enough to say where it goes wrong. But anything which pushes air away from itself (as the top of an up elevator does) is going to experience higher pressure, and anything which pulls air towards itself (as the bottom of an up elevator does) is going to experience lower pressure. Newton's laws tell you that much. $\endgroup$– Tanner SwettNov 3, 2017 at 21:59
4 Answers
When an elevator moves (trailing edge) up, it partly moves in the way of the airflow. That means pressure goes up.The elevator is an airfoil itself, in the shade of another airfoil. If it moves up, the same thing happens when a wing moves trailing edge up: lift works downwards.
Picture above shows an airfoil with trailing edge down: lift is upwards.
- Upper aft side of the airfoil: air only moves faster if the surface moves out of the way of the stream, if the shape moves away gently enough the air still tries to follow. Note that the nose of the shape had pushed air out of the way, so there is less air along the upper aft side of the picture: lower pressure.
- Lower side of the airfoil: air tries to stream like it did before, but now there is a shape in the way of the airstream. The air needs to stream through a narrower stream tube: higher pressure. When the stream bumps into the airfoil at a shallow angle, it gets deflected downwards: impulse lift.
Stick your hand out the window of a moving car. Think of your hand as the elevator. Now twist the trailing edge of you hand up and down. Observe what happens in each case.
Think of the horizontal stabilizer as just a piece of metal to mount the elevator. The elevator is the 'working' surface.
On aircraft where the whole stabilizer moves, it is acting as a wing.
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$\begingroup$ How do you twist the "trailing edge of your hand"? : ) $\endgroup$– user18035Nov 4, 2017 at 8:04
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1$\begingroup$ My bad. Hold your hand out, fingers closed, parallel to the ground. Twist the whole hand palm facing the airflow and then back of the hand into the airflow. $\endgroup$ Nov 4, 2017 at 9:40
If you say that when you are trying to go up and the elevator goes down as the same time the nose try to goes down ( nose down pitching tendency) it came from the thrust line. The thrust line is an imaginary line that criteria moment around lateral axis if an airplane. You can find it in privet pilot manual book in chapter 1 section D.
It seems very difficult to me to guess what the speeds of the air flow will be at various places on the top and bottom of an airfoil just by looking at the shape of the airfoil, as you appear to be trying to do.
There are ways to calculate approximate speeds of the air flow using advanced mathematics, but of course that is not especially easy either.
A much easier way to think about what kind of lift you will get (positive or negative) is to look at the trailing edge of the airfoil. Unless the airfoil is stalled, the air flowing off the top and bottom of the airfoil tend to continue flowing in about the same direction, at least while they are still near the airfoil. On the other hand, the air flow in front of the airfoil comes from the direction the aircraft is flying. Look at the difference in directions coming in and going out, and this will give you an idea of which direction the reaction force is in.
This will not tell you the magnitude of the reaction force; that depends on how much the air is deflected at points farther above and below the airfoil. For the magnitude of the force, you still need to do the advanced mathematics or observe the actual force on an actual airfoil as air flows past it.
