lesson 4 - three-axis control

 

 

Connection to the Wrights: The Wrights were first to develop a fully controllable airplane; they understood that flying machines needed to have three-axis control in order for flight to be safe.

To control the pitch of their first glider, they used an elevator mounted in the front of their glider (the Wrights called it their "front rudder"). To control roll and to turn, they developed "wing-warping." To control yaw, they perfected the rudder.

Objective: To determine how the Wright brothers controlled their canard glider in three dimensions. To determine why rudders are necessary in preventing adverse yaw.

Description: In this activity, students construct inexpensive canard gliders out of straws, manila file folders, paper clips and tape. Students learn how to control their gliders by warping the wings and by adjusting the rudder and forward elevator. These simple experiments demonstrate control of roll, pitch and yaw.

 

Materials and tools:

  1. Manila file folders (1 per four students)
  2. Paper clips (or modeling clay)
  3. Straws
  4. Scissors (or razor blade)
  5. Tape
  6. Note book
  7. Pen or pencil

 

 

Background information concerning control:

Pitch is the motion of the airplane as its nose points up or down. Today, movable control surfaces called "elevators" are used to control the up and down motion of the airplane along its lateral axis.

The elevator on the Wrights' glider was in the front, what Wilbur called a "happy accident of design." In later years, Orville noted, "we retained the elevator in the front for many years because it absolutely prevented a nose dive such as that in which Lilienthal and many others since have met their deaths." Today this arrangement with a small wing ahead of a large wing is known as a "canard" configuration, the French word for duck, perhaps because of the similarity to the flying profile of the bird. Around 1910, the elevator and rudder were located together at the tail, because engines needed to be located at the front of the airplane.

Roll is the tilting motion of the airplane when one wing rises or falls in relation to the other. Wing warping was the method used by the Wrights to control roll: to raise the right wing, cables twisted the leading edge of the right wing upward, increasing the angle of attack of the wing and the amount of lift. As a result, the right wing rises. Meanwhile, the left wing drops because cables simultaneously decrease the angle of attack and lift on the left side.

Today, movable control surfaces called "ailerons" control roll. There are two ailerons responsible for banking the airplane, one mounted in the trailing edge of each wingtip. When an aircraft banks to the left, the left aileron deflects upward, and the right aileron deflects downward. The force of air on the defected control surfaces causes the left wing to drop and the right wing to rise, producing greater lift on the right wing. The lower, right aileron results in a greater angle of attack, which results in greater lift. The lift is less on the left wing, and thus, the left wing drops. The airplane banks to the left.

Yaw is the twisting motion as the nose turns left or right. Rudders control yaw.

 

Although airplanes turn due to the action of ailerons, rudders are necessary to guarantee that the airplane turns more efficiently.

The Wrights discovered a disturbing phenomenon when they were trying to turn their 1901 glider. When attempting to make a turn to the left, the leading edge of the right wing was warped up to increase angle of attack and lift. The leading edge of the left wing was warped down. As expected, the glider's right wing raised and the glider began to bank to the left. However, instead of turning to the left, the glider pivoted about the higher tip of the right wing and turned to the right! This truly dismayed the brothers. Today, this phenomenon is known as "adverse yaw." After much thought and testing, a movable rudder was added to the glider to prevent the phenomenon.

Adverse yaw may occur when the leading edge of one wing is warped upward, thus increasing the angle of attack, lift and drag. Essentially, the increased drag on this wing slows it down. Meanwhile, the other wing has relatively little drag and quickly pivots around the tip of the slow wing. The addition of a movable rudder prevents adverse yaw from occurring.

 

How to make a Wright (Canard) Soda Straw Glider

Procedure:

  1. You will need a standard straw, a manila file folder, (or 5x8-inch note card) paper clips, modeling clay and some tape.
  2. Cut out a 1 x 2-inch vertical fin out of the manila file folder. The fin should also include a base that can be inserted into the straw. Cut slits in the vertical fin to make a rudder.
  3. Split the top of the straw with a razor and insert the vertical fin.
  4. Make a wing by cutting a 2-inch strip from the manila file folder. The wing should be roughly 2 x 8 inches.
  5. Fold and crease the center of the wing. This crease will give the wing dihedral and stabilize the aircraft. In addition, fold the last inch of each wing. These extra folds will strengthen the wing, prevent it from warping and increase stability.
  6. Place the straw along the center crease of the wing and attach the wing with tape. Place the tape on the top of the wing. The wing should be slightly ahead of the vertical fin.
  7. At this point the glider should look like this:
  8. Cut out a 2 x 4-inch section from the manila folder, which will serve as the "front rudder" of your airplane. Crease the middle of the front rudder and attach it to the front of your straw.
  9. Add elevators by cutting slits in the trailing edge of the front rudder.
  10. Your glider should now look like this from the front:
  11. Balance the airplane by attaching 1 to 3 paper clips to the front of the straw. Clay may be substituted for paper clips.
  12. Adjust the elevators on the forward rudder to attain lift. If your plane stalls and lands on its tail, readjust the elevators or add more weight to the nose of the glider. Readjust the elevators or remove weight if it dives on its nose.
  13. Check the balance of your airplane by gliding and dropping it. A well-balanced glider will parachute down if dropped.

This image illustrates the relatively safe parachute-style response following a stall of the canard configuration used by the Wright brothers. The violent spin and nosedive typical of a rear-tailed configuration upon stalling is depicted at the right.

Working with canard glider

Procedure:

  1. To have your canard glider bank left, gently twist the leading edge of the right wing up and twist the leading edge of the left wing down.
  2. Adjust the rudder so that it points slightly toward the left.
  3. Fly your glider.
  4.  

  5. To have the canard glider bank to the right, gently warp the leading edge of the right wing down and warp the leading edge of the left wing up.
  6.  

  7. Adjust the rudder so that it points slightly toward the right.
  8.  

  9. Keep "tweaking" your canard glider until it flies.

 

Just for fun:

Assessment:

  1. Contest time! Quickly configure your glider so it will fly to the left in a circle. The first group to do so wins. Explain what caused it to fly to the left.
  2. Contest time again! Configure your glider so it will fly to the right in a circle. The first group to do so wins. Explain what caused to fly to the right.
  3. Evaluate! If gliders turn due to wing-warping, what is the purpose of rudders?
  4. What control surface would you modify to make your glider stall? Try it.

Extension - Building a modern straw glider

Procedure:

  1. You will need a standard straw, manila file folder, (or 5x8-inch note card) paper clips, modeling clay and some tape.
  2. Cut out a 1 x 2-inch vertical fin out of the manila file folder. The fin should also include a base that can be inserted into the straw. Cut slits in the vertical fin to make a rudder.
  3.  

     

     

  4. Split the top of the straw with a razor and insert the vertical fin.
  5.  

  6. Cut out a 2 x 4-inch horizontal stabilizer and attach it to the straw with tape.
  7. Add some weight to the front of the straw by adding 1 to 3 paper clips or clay.

 

  1. Make a wing by cutting a 2-inch strip from the manila file folder. The wing should be roughly 2 x 8 inches.
  2. Fold and crease the center of the wing. This crease will give the wing dihedral and stabilize the glider. In addition, fold the last inch of each wing. These extra folds will strengthen the wing, prevent it from warping and increase stability.
  3.  

     

  4. Balance the fuselage on your fingers. You will attach the center of the wing to this balance point.
  5. Place the center of the wing at the balance point and secure it with tape.
  6.  

  7. Cut some slits on the trailing edge of the horizontal stabilizer to create elevators. These will control the pitch of your glider. In addition, cut some slits on the trailing edge of your wing to create ailerons. These will control the roll of your glider.
  8.  

  9. Your glider should now look like this from the front:
  10.  

  11. Manipulate the ailerons, elevators and rudder and glide your airplane. Add mass to balance the glider if necessary.