Why does an airplane turn? (Long)

[UnAnswerd]

It is still not evident to me exactly why the airplane turns. The "horizontal component of lift" just doesn't explain anything. I would like to analyze the events leading up to a turn, and explain why I do not understand what actually causes the airplane to change heading. For this example, we'll use a left turn...

The pilot turns the control yoke to the left. This causes the left aileron to move up, and the right aileron to move down. Due to the reconfiguration of the ailerons, the left wing now has less lift, and the right wing now has more lift. In other words, each wing is now producing a different amount of lift.

The unequal lift on each wing causes the airplane to roll about it's longitudinal axis, which in this case is to the left. Okay, so the airplane is banked. I know why the airplane can bank, but what makes it turn????

Anyway, the airplane continues to roll about it's longitudinal axis as long as the ailerons are deflected. The pilot establishes the angle of bank, and then the yoke is neutralized. The airplane is now maintaining the bank, even though each wing is now producing the same amount of lift. I can understand this, but still don't see how the bank has anything to do with enabling the aircraft to turn...

So now the airplane is banked. Unfortunately, this means less lift from the wings is now acting in direct opposition to total weight. The result? The airplane starts sinking. The pilot pulls back on the yoke, causing the elevator to move up. Relative wind strikes the elevator and pushes the tail down. This in turn causes the airplane to rotate about its lateral axis, and brings the wings to a steeper angle of attack. The wings now generate more lift so the airplane maintains altitude regardless of it's banked attitude. Unfortunately, induced-drag also goes up, due to more lift. The airspeed drops. No way around this one unless there's more power available. Okay, so I understand what causes the airplane to bank, lose altitude, and then maintain altitude again with airspeed decreasing. I understand all this, but still don't understand why the airplane should be turning!!!! All I can visualize right now is the airplane traveling in a straight line, but banked......

Okay, so earlier we said that total lift was no longer acting in direct opposition to total weight. Where is some of it acting then??? Yes, it must be acting in a more horizontal state. But hold on. Wouldn't that mean that the aircraft is simply being PULLED to the left. Meaning that it tracks a diagonal line, but the nose is still pointed in the same direction??? Maybe the airplane is being pulled to the left, but I STILL don't understand why it TURNS or CHANGES HEADING.

This is all I can think of: It's the elevator, isn't it? Because the elevator is also banked with the rest of the airplane, it too now acts more horizontally. In short, the elevator is almost like a rudder when the airplane is banked. Relative wind hits the "up" elevator and pushed the tail the outside of the turn. The engine keeps pulling one way, the tail is constantly being pushed another way, and the airplane turns!!!! Finally, maybe this is what causes the nose to turn, and make the airplane fly in a circle????

So I will ask one more very interesting question: Although altitude may start to drop, can the airplane still turn if the elevator is neutral????

 

[mmmdonut]

Just stop while you're ahead....

 

[VNugget]

It helps if you break the situation down into the two parts of what's going on in the turn -- 1) the lateral acceleration, or, in simpler words, the change in the plane's path. You already figured this one out correctly -- the horizontal component of lift. So far so good. 2) The change in the plane's heading. This part may or may not be a little tricky, depending if you've already learned about stability trim settings, and the plane's tendency to return to it's trimmed angle of attack. This page explains it not extremely well, but hopefully it does the trick. Anyway, if you don't already know about stability, then I'm not sure how to explain it. Bit if you have, remember how the plane pitches in reaction to changes in the relative wind (gusts), in order to return to its original AOA?

Well, in your left turn, once the airplane has initially moved a little to the left, the AOA on the wing has now decreased, so the plane will pitch "up" to compensate for that. The only difference is that this isn't a momentary gust, but a repeating "cycle". Except that it's not a "cycle," but a continuous motion.

If you're having trouble picturing the the change in AOA, just imagine the plane flying straight and level instead of in a turn. Well, if you grab it and push it UP as it's moving forward (this is the analogy of the lateral acceleration), can you now see how the air is coming from a little bit up-and-from-the-front instead of just from the front? That's the decrease in AOA that the plane will then naturally pitch up to compensate for. The only difference is that all of this is going on sideways.

 

[philo beddoe]

Fear.

It has long been established that aircraft are terrified of flying in straight lines, and are only capable of doing so when there is no oncoming traffic.

The advantage of this natural desire to turn that most aircraft possess was put to great use in the early days of airline flying. By maintaining an almost imperceptible bank, the aircraft was kept happy even as the load factor increased slightly. This load factor, as we know now, is the key to airline profitability.

So, while pilots were merrily arcing their way through the skies, everyone (including the pilots) was making a lot of money.

This, incidentally, was the origin of great circle navigation.

To the ignorant masses banished to their dreary lives afoot on the soil, the curved paths caused them to speculate that the flight crews, whom they both loathed and envied, were partaking a little from the high-caliber liquors then served to passengers.

Hence, the term "rum line" was originally used as a derogatory term by jealous ground-pounders.

The "h" and the "b" were added later, creating "rhumb line”, which was nothing more than a cheap PR trick to play down the pilot's suspected (and often very real) propensity for aerial libations.

Things were going along well enough until the sky got more crowded, and radar control became necessary. Now remember, airplanes get scared if they fly straight for too long, so a radar vector would make them jittery. However the soothing baritones of air traffic controllers managed to keep most of the airplanes relatively calm.

But with increasing inability to maintain the critical, but nearly imperceptible bank angle, load factors began dropping. Deregulation was hastily introduced so that it could later ne used as an excuse, but those of us flying the tri-motors knew better.

Today, like an electronic straightjacket, FMS’s and autpilots keep airplanes going straight enough, but since it is under duress, they certainly are nowhere near as contented. Thus, they punish their owners and pilots by being as recalcitrant as an old mule at times, pretending to have failed components, hot starting engines, anything they can do in their bid to shake off their percieved enslavement to ‘the man’. How often do you see 'could not duplicate' on the squawk sheet? You're catching on.

This has escalated, and the current form of aeronautical rebellion being shown by these machines is in the form of the laughable ‘clear air turbulence’.

Everyone knows clear air is smooth. But preying on the fears of the flying public, and as a way to spite those in charge, aircraft will often spontaneously begin to shake and bounce around, as if to remind those aboard that they live or die based on the whims of the airplane.

Check it out next time you are on board an airliner and you experience “turbulence”.

Look out at the wings. They’re shaking, aren’t they?

Don’t be fooled, these aircraft know exactly what they’re doing, and only the threat of being flown off to the boneyard is preventing an all-out revolt.

So watch it on those repo flights.

 

[VNugget]

So I will ask one more very interesting question: Although altitude may start to drop, can the airplane still turn if the elevator is neutral????

Yes. And not only "it can," but "it must." If you remove elevator input, you still haven't removed the horizontal component of lift, nor have you removed the plane's tendency to naturally pitch for the original (trimmed) AOA.

 

[cessna_driver2]

Ok, I suggest the Pilot's handbook of aeronautical knowledge. My old copy (that I had to dig out ) is AC61-23B. Im sure it's been updated since. But it is a start.

here you go.

1 the rudder does not turn the a/c.

"The airplane must be banked because the same force (lift) that sustains the airplane in flight is used to make the a/p turn. The a/p is banked & back elevator pressure is applied. This changes the direction of lift & increases the angle of attack on the wings, which inturns increases lift. The increased lift pulls the a/p around the turn. The amount of back elevator pressure applied, & therefore the amount of lift, varies directly w/ the angle of bank used. As the angle of bank is steppened the amount of back presure must be increase to hold altitude..

In level flight the force of lift acts opposite to & exactly equal in manitude to the force of gravity (G). G tends to pull all bodies to the cneter of the earth, therefore this force always acts in a vertical plane w/ respect to the relative wind, which for the purpose of this discussion is considered to be the same as acting perpendicular to the lateral axis of the wind.

W/ the wings level, lift acts directly oppsite to gravity. However, as the a/p is banked, gravity still acts in a vertical plane, but lift will now act in an inclined plane.

The force of lift can be resolved into two compnents - vertical & horizontal. During the turn entry the vertical component of lift still opposes gravity, & the horizontal componemnt must overcome centrifugal force; consequently, the total lift must be sufficient to counteract both of these forces. The a/p is then pulle daround the turn, not sideways, because the tail section acts as a weathervane which continually keeps the a/p stramlined with the curved flightpath.

Also note that as the turn develops, centrifugal force will act opposite to the horizontal component of lift & the vertical component of lift will act opposite to gravity. The total resultant lift acts opposite to the total resultant load. So long as these opposing forces are equal to each other in magnitude the a/p will maintain a constant rate of turn. If the pilot moves the controls in such a manner as to change the manitude of any of the forces the a/p will accelerate or decelerate in the direcitn fo the applied force. This will result in changed the rate at which the a/p turns."


Get it.

In short, the combination of vertical and horizontal componets of lift is acting to pull the a/c through the turn. It is the combination of both of those components of lift. Then the tail trying to straighten the a/c like a weathervane.

 

[Mr. Cole]

It's the horizontal lift and the elevator. The horizontal component of lift alone is not enough to cause a turn. Left to its own devices it would simply cause a translation in the direction of the horizontal component. In order to obtain a perfect circle, or anything that resembles a circle, the force must be central. One of the conditions of this is that the horizontal component of lift has to point towards the center of the circle. This is what the horizontal stabilizer and elevator do. Even with the elevator neutral there is still a force on the horizontal stab. Remember the horizontal stab in straight and level flight generates a downward force that creates a upward moment to balance out the downward moment created by the wings. A plane can also fly at knife-edge, i.e. a 90 degree bank, and not turn. At that point all of the lift generated by the wings is horizontal, with vertical lift generated primarily by the fuselage. But even with the wings generating only horizontal lift the plane doesn't turn. Why? Because the elevator is used to keep it flying straight.


Consider a plane in straight and level flight. It has a vertical lift component but doesn't fly in a circle. Granted that lift is usually balanced by weight in steady state. But even when there is an imbalance the plane doesn't turn vertically. How do you get the plane to fly a vertical circle, ie a loop, you pull back on the elevator? As someone else mentioned, there is a specific elevator deflection at each coefficient of lift that results in a moment coefficient about the lateral axis of zero. This is also what is called the trimmed condition to which a stable plane likes to return because the moments about that axis are balanced.

Dave

 

[BEECH-SLAPPED]

Then the tail trying to straighten the a/c like a weathervane.

This is a key statement as to what keeps changing the aircraft's heading in a coordinated turn. I say "coordinated" because if you were to apply enough rudder to keep the heading constant in a bank, you will just be slipping sideways.

Also note that as the turn develops, centrifugal force will act opposite to the horizontal component of lift & the vertical component of lift will act opposite to gravity.

You are correct about the vertical component of lift acting opposite the aircraft weight. However, please remember that CENTRIFUGAL force does not exist! Never has! In order for a body to accelerate, it must be acted upon by a unbalanced, or net, force. Remember that in a turn, the aircraft is accelerating. It may be flying at a constant airspeed, but it's DIRECTION is changing. That is an acceleration. If the horizontal component of lift exactly balanced "centrifugal" force, there would be no net force and the aircraft would not turn. The force that is so often mistaken for "centrifugal" force is the equal and opposite reaction force felt on the body being accelerated. That force you feel on your arm when you turn your car is the reaction of the door pushing on YOU, keeping you in the turn. Without it, you would fly out of the car in a straight line tangent to your velocity at that point in the turn. The force keeping a body in a turn is known in the physics world as CENTRIPETAL force. The horizontal component of lift is the centripetal force keeping the aircraft in the turn.

 

[LarryinTN]

It is still not evident to me exactly why the airplane turns.

It turns for the same reason that a ball on a string turns when you spin it above your head. In the case of the airplane, the horizontal componant of lift takes the place of the string.

 

[UnAnswerd]

I thank everyone for the information. Unfortunately, I've yet to visualize what enables the plane to perform this elementary maneuver. Some say the angle of attack decreases as the plane rolls in and that the nose pitches into the turn. I'm sure this guy is correct, but nevertheless I don't understand how that happens. I'm going to have to do some research. It's really aggravating me that I don't fully understand the mechanics of something seemingly so simple.....

 

[avbug]

Rather than coming here for the explaination (which most are willing to offer), spend the thirty bucks for an hour of a ground or flight instructor's time, and get it first hand. The concept is insanely simple, and easy to understand. Quite obviously you don't understand it from the written word; get someone to hold a model airplane, bank it, push it with their finger and you'll have it down cold, and crystal-clear.

The elevator does not turn the airplane. The rudder does not turn the airplane. The ailerons do not turn the airplane. A horizontal force vector that can come from several sources, severally or singally, works to effect acceleration; a change in direction or velocity.

In the case of a slight bank, the oft-touted horizontal component of lift, or the amount of lift vectored in the direction of the turn rather than straight "up," is serving to push (or pull, depending on your wants and tastes) the airplane around a turn. This division of the vertical lift vector means that less vertical lift is available; the airplane will descend. Angle of attack is increased to compensate. Induced drag increases, the aircraft slows unless additional thrust is provided.

 

[VNugget]

According to his original post, he already understands how the horizontal component of lift causes the plane to change path. What he doesn't get is what causes it to change heading instead of the plane just slipping in the new path, with its original heading.

 

[Mr. Cole]

Avbug,

While there is little that I disagree with you on when it comes to aviation as you've probably forgotten more about aviation since yesterday than I will ever know, I will respectfully disagree that the elevator/horizontal stabilizer has nothing to do with the turn.

Actually I apologize in that I should have said the tail, along with the horizontal component of lift (HCL), causes the plane to turn, in my original post. The character of a turn is determined by the elevator. If the vector known as the horizontal component of lift doesn't continue to towards the center of the circle, the circle will be something less than ideal. In the limiting case I use the elevator to negate the impact of the tail and its moment and the plane flies in a bank but doesn't turn.

Even in the case of the the ball on a string it doesn't have to turn in a circle. In fact lets build a analogy to the ball on the string, say a kite. This is a slightly easier point to illustrate because forward motion by the kite will create lift. I can run forward with the kite into the wind and the kite will fly forward in the direction I pull it. But I can also start pulling it in laterally while running forward, which mimics a horizontal component of lift. The kite simply moves forward and to the side. But if I stand in a circle and turn, the kite will turn in a circle as well. By standing at the center of the circle and spinning with the string in my hand I'm constraining the HCL to remain pointed in the center of the circle. Even with the ball on a string it is the action of your fingers and/or arm that keep the circular motion going. Try spinning a ball on a string and watch what your arm is doing. If you stop actively doing something the ball will either stop spinning or fly away on a tangent if you release it.

Turning flight is no different that straight and level flight with regards to circular motion. In straight and level flight you have weight and lift equal. However, weight acts through the center of gravity and doesn't cause a moment about the CG. However, if the lift vector is behind the CG it will cause a downward pitching moment. This is desirable in a stable plane. What counters that is the upward pitching moment of the tail. The plane is trimmed when the moment caused by the wing, at some value of lift coefficient, is equal to the moment caused by the tail. The overall moment of the tail is caused by the deflection of the elevator. When the elevator is deflected it will change the plane's AOA by changing the pitch attitude. There will be one value of AOA, and therefore wing lift coefficient, that will produce a moment sufficient to counter the moment caused by the tail in that condition. Any other value of AOA for that elevator position will not create equilibrium and the plane will oscillate until it finds the value that does. Of course that AOA and resulting lift coefficient will correspond to some velocty such that the lift equal weight.

So to make a long story longer the elevator controls the pitching moment in normal flight and hence the ability to make "vertical turns" or loops. In a turn in the horizontal plane you have the same forces acting as in the vertical plane. It doesn't matter whether weight and lift equal or not, since the weight always acts through the CG and does not contribute to the moments. The elevator is what determines the character of the turn once the HCL is present.

Dave

Rather than coming here for the explaination (which most are willing to offer), spend the thirty bucks for an hour of a ground or flight instructor's time, and get it first hand. The concept is insanely simple, and easy to understand. Quite obviously you don't understand it from the written word; get someone to hold a model airplane, bank it, push it with their finger and you'll have it down cold, and crystal-clear.

The elevator does not turn the airplane. The rudder does not turn the airplane. The ailerons do not turn the airplane. A horizontal force vector that can come from several sources, severally or singally, works to effect acceleration; a change in direction or velocity.

In the case of a slight bank, the oft-touted horizontal component of lift, or the amount of lift vectored in the direction of the turn rather than straight "up," is serving to push (or pull, depending on your wants and tastes) the airplane around a turn. This division of the vertical lift vector means that less vertical lift is available; the airplane will descend. Angle of attack is increased to compensate. Induced drag increases, the aircraft slows unless additional thrust is provided.

 

[VNugget]

Some say the angle of attack decreases as the plane rolls in and that the nose pitches into the turn.

You took that concept a little too early into the picture. First the airplane banks (which of course involves some changes in AOA separately on the individual wings, which is NOT what I was talking about), and then it starts moving sideays due to the HCL as you already understand. It is this sideways movement that decreases the AOA. Once again, the word "sideways" however is misleading, and you can picture it easier with straight-and-level flight. If you move the plane up without rotating it you should be able to se how the AOA decreases. The only difference in a turn in real life is that the wing(s) is banked instead of level.

I think that some of the confusion came from my use of the word "wing," which you might have taken to mean just one of the wings. What I really meant was both wings... a lot of times people will use the singular form of the word, just to express the idea that the topic is focused on how the "wing," ignoring the fuselage, is behaving in the air.

 

[Metro752]

Why does your gay ass have a picture of the cessna that a kid comitted suicide in as your avatar. You ass munch.

 

[VNugget]

7500.....

 

[TonyC]

Considering the history of threads started by this memeber, I hesitate to invest much effort in answering. I fear the boy is crying wolf again.

If he indeed has 2.2 hours in the Cherokee 140, he must have possession of or access to some sort of ground training manuals complete with diagrams and pictures explaining this fundamental concept. I seem to hear the whiz of the reel as the proverbial hook is being set securely in our collective cheeks.

 

[KigAir]

90 degree bank

What force opposes gravity when a plane is banked 90 degrees?

 

[Kingairrick]

Why does your gay ass have a picture of the cessna that a kid comitted suicide in as your avatar. You ass munch.

I agree. I worked in that building when I used to have a real job. It's not remotely funny. What an a$$hole.

 

[Mr. Cole]

The fuselage is the primary lifting surface with the rudder acting as the elevator.

What force opposes gravity when a plane is banked 90 degrees?