What causes heading to change in an airplane turn?

5 Answers

1. 
   

   
   

   Matthew Waugh on Jul 27, 2010

   I’m not aerodynamics expert – but – once an aircraft is banked, regardless of how it got there or how the other controls are held, there is a horizontal component of lift, and thus a tendency to turn. Now you could hold the controls in such a way that the effect of the horizontal component of lift is counter-acted by the controls (applying rudder for example).

   Your examples all relate to elevator. So using different elevator will change the AOA which will change the total lift which will change both the horizontal and vertical components of lift. Since most of our bank angles aren’t very extreme, there is more of an effect on the vertical component.

   Any bank will cause a turn, the more bank, the more horizontal component, the higher the rate of turn.

   +3 Votes 4 Votes  1 Votes

2. 
   

   
   

   Brian Konsko on Aug 11, 2010

   To make this easier to follow here is an outline of the sections to follow: General Question | Rudder in Turn | Question & Answer

   General Question: “What causes heading to change in an airplane turn?”

   The horizontal component of lift causes the resultant heading change.

   Horizontal component of lift (HCL) = Lift * Cosine (Bank Angle)

   To make it easier to understand component interaction we can remove the cosine:

   HCL = Lift * Bank Angle

   Grade school level of math teaches us that zero multiplied by any number results in zero. Applying this knowledge to the above formula tells us that if either lift or bank angle is zero; HCL will be zero. Thus, a turn cannot be present if either lift or bank angle isn’t present.

   (Disclaimer: A loop is a vertical turn that doesn’t have a bank angle. There may be a select few other exceptions, but in general the above is true.)

   From this formula we also know that HCL is directly proportional to bank angle and lift. Let us make a quick topic detour, as this knowledge can help to see relationships between components in formulae. It allows us to quickly determine how certain factors affect our aircraft. Anything on the top of the formula is directly proportional to the resultant and anything on the bottom of a division formula is inversely proportional to the resultant. Example:

   X = D / I

   X would be directly proportional to D and inversely proportional to I. That means, if D goes up X goes up (directly), where as if I goes up X goes down (inversely).

   Back on topic, we now know that a zero bank angle or zero lift gives us zero HCL and thus no heading change. We also know that the speed at which our heading changes (amount of HCL) can be increased or decreased by increasing or decreasing bank angle or lift respectively. Now that we know how all of this works on paper, though, what does it tell us in an airplane?

   Early on in private training we are told lift equals weight in straight and level flight. This is an important distinction to make before moving on, one that Matt touched on. Since we start any turn from a level flight position, we should always have lift equal to weight to begin with. So, as soon as bank is other than zero, we have an HCL because lift was already greater than zero (it’s equal to weight). The result, as Matt said, is that we will have a turn and a heading change from just adding bank, without adding any backpressure.

   In an airplane this merely confirms what we already know. We know that the steeper our bank the faster we turn. We also know that the harder we pull back, when banked, the faster we will turn.

   Rudder in a Turn: “Aerodynamically is the vertical fin (not the rudder) generating horizontal lift creating the heading change?”

   First, the rudder is not used in creating HCL.

   A rudder experiences what the aerodynamic world calls weather cocking. What this means is that whenever the rudder is not in line with the aircrafts relative wind (wind flowing parallel and opposite the path of flight) it will weather cock into the wind. This is similar to a weather vane.

   Whenever an aircraft is turned a sideslip is induced. This sideslip continues throughout the entire turn, even with perfect use of rudder. What is happening is a very small, unnoticeable to the pilot, sideslip that is quickly corrected by the rudder. This weather cocking continues throughout the entire turn. The result is, after a 360 degree turn, the airplane has effectively pivoted 360 degrees around its vertical axis.

   Questions & Answers: (1) “When an aerobatic airplane puts in a hard bank with no elevator, does it’s heading begin to change?”

   If the pilot successfully eliminates lift while performing this maneuver, then yes, it is possible. Google knife edge to see an example of an aerobatic maneuver in which this occurs.

   Questions & Answers: (2) “I have a lot of flight hours and would like to understand this better.”

   You should pick up an 8th grade level physics book and open to the chapters on vectors. You might also try google, but finding something easy to comprehend in this fashion might prove difficult.

   You might also try aerodynamic related books, listed in order of ease to understand:

   Emergency Maneuver Training (NO MATH!!!)
   Illustrated Guide to Aerodynamics
   Aerodynamics for Naval Aviators

   -6 Votes 1 Votes  7 Votes

3. 
   

   
   

   Ryan on Aug 12, 2010

   Another way to think about this is the consider the tail-down force that the horizontal stabilizer provides during straight and level flight. Google tail down force if you’re not familiar. To summarize, the horoz stab is actually providing downward(negative) lift and effetively keeps the nose of the airplane UP (counteracting gravity, you could say, but actually keeping the centers of lift and gravity in equilibrium.) When you bank the airplane, this tail-down force still exists even (especially) if you’re keeping the elevator neutral (i.e. trimmed as it was for level flight.) In the bank, the nose will naturally drop due to the change in vertical component of lift, and the tail-down force now also has a horizontal component, effectively pushing the tail to the outside of the turn, and the nose into the turn.

   Hopefully that wasn’t too confusing, but it’s a different look at the forces involved. The end result is that your TOTAL lift never changed… in the bank you have less vertical lift and more horizontal lift even though you don’t change elevator position; and that horizontal component will still turn the plane.

   +4 Votes 6 Votes  2 Votes

4. 
   

   
   

   Wesley Beard on Nov 09, 2010

   This question has nothing to do with aerodynamics at all but how the heading indicator works.  Here is a thought for you, flying along in straight and level flight you decided to kick in full rudder.  What is going to happen to the heading indicator?  Undoubtedly it will move with the nose of the airplane moving with respect to the horizon.
   A heading indicator uses the principle of “rigidity in space” meaning the airplane is moving around the heading indicator.  Read up on how this works and you will answer your question.
   2.  The vertical stabilizer is not causing the heading indicator to change headings.  It is responsible for the inclinometer working.
   3.  Back pressure creates the load factor on the airplane.  Remember at 60 degrees of bank is 2G’s only in level flight.
   4. Aerobatic pilots can do snap rolls by quickly applying full aileron and opposite rudder.  Hopefully the nose remains constant with respect to the horizon and the heading indicator will not change.  Practically speaking the heading indicator will oscillate about 5 degrees because few pilots can keep the nose in the same attitude.

   0 Votes 2 Votes  2 Votes

5. 
   

   
   

   Steve Pomroy on Dec 30, 2010

   To start answering your question, consider two extreme examples.  Neither example is realistic, but we can use them to illustrate a point.
    
   First example:  Consider an aircraft conducting a flat turn — all rudder, no bank.  There are a bunch of reasons why we don’t do this in practice, but it can be done.  To maintain the flat turn, we will need lots of rudder and lots of opposite aileron.  The aircraft will then continuously yaw in the direction of the turn.  The heading will change, and the flight path will change direction.
    
   Second example:  Now consider an aircraft with 90 degrees of bank.  Let’s pretend for the sake of the thought experiment that the vertical component of lift is provided by magic, so we don’t fall out of the sky.  If we pull back on the stick, the aircraft will pitch continuously in the direction of the turn.  So the heading will change , and the flight path will change direction.
    
   Now let’s look at what happens in a real, coordinated turn.  We’re at some intermediate angle of bank, and the aircraft is changing heading, and the flight path is changing direction.  What’s happening?  Well, for starters, the aircraft is both pitching and yawing into the turn.  If the though experiment above doesn’t help to visualize this, try a different one:  Get a model airplane and put it into a banked attitude.  Yaw the airplane (i.e. – rotate it aroud the vertical axis, which is inclined because of the bank), and you will note that the direction changes and the nose drops.  Now pitch the airplane (i.e. – rotate it around the lateral axis, which is also inclined because of the bank), and the nose will return to the original pitch attitude, but the direction will be changes.  Now imagine executing the yaw and the pitch simultaneously — the result is a steady coordinated turn with a constant pitch attitude and changing direction.
    
   Ok, so far so good.  Now back to the original question.  WHY is the aircraft turning?  The horizontal component of lift (HCL) contributes, as noted above.  The purpose of the HCL is to accelerate the aircraft into the turn — it is an unbalanced force, and as per Sir Isaac, we change direction as a result.  But this only changes out flight path, not our heading.  Once the flight path changes, we are flying sideways, and we are in a slip.  The vertical stabilizer reacts to this slip by yawing us into it in order to keep the aircraft coordinated.  At the same time, because of the bank angle, the sideways motion causes a reduction in AOA.  The aircraft responds to this via it’s pitch stability by pitching into the turn.
    
   Note that the pitch into the turn is a stability response, and we don’t have to actively produce it.  The extra pitch input (i.e. – back pressure) that we need in the turn is becasue of the increased AOA required by the increase in lift, which in turn is called for by inclining the lift away from the vertical.
    
   QT:  “Aerodynamically is the vertical fin (not the rudder) generating horizontal lift creating the heading change? Or does the use of back pressure (elevator) create the turn?”
   Bottom line:  the vertical and horizontal stabilizers work together to turn the aircraft.  The balance of labor depends on the bank angle, with the V.Stab. doing most of the work at low AOB, and the H.Stab. doing most of the work at high AOB.
    
   Wow, that took a lot of words.  It’s so much easier when I’m sitting in front of someone with a whiteboard and toy airplane!
    
   Cheers,
   Steve
   http://www.flightwriter.com

   +3 Votes 4 Votes  1 Votes