AMA Cub Teaching Notes – Trimming for Flight

AMA Cub Teaching Notes

Trimming for Flight

At this point the planes have been built and first test flights have been attempted.  If the planes were built according to the instructions in my AMA Cub Master Class, all of them should fly at least reasonably well.  Sometimes instructions are not followed exactly right or other things can go wrong and a plane will not fly as well as we would like.  There are also things that can happen to a plane after it has been built and flown that will make it fly poorly.  I will talk about some of the more common problems and their cures.

The first thing to do if an airplane is misbehaving is to take a close look at it. Check that everything is symmetrical and nothing is out of alignment.  If something is not quite right, that is probably what is causing the problem.  Start by fixing that and test flying again.

Stall.  This is what you get if you follow the instructions that come with the kit.  In a stall, the plane will zoom upward with the nose pointed high, but there will not be enough power to keep it flying with the nose pointed that high.  So the airplane will slow, losing lift, and it will begin to fall.  As it falls, the oncoming air is no longer striking the wing at an angle that produces lift.  The angle between the oncoming air and the wing is called the angle of attack, or attack angle.  Lift is controlled by the attack angle.  It is best at a moderate angle.  At a steep angle, the airflow breaks away from the top of the wing and lift is lost.  With no countering torque from the wings, the torque of the motor may make the plane roll to the left.  As the plane falls, the tailplane will push up, bringing the nose down.  The plane will nose down and gain speed.  If it is high enough, it may recover and continue flying, maybe repeating the stall over and over.  A severe stall will result in the plane hitting the floor.  A mild up and down motion in which the wing does not suffer a serious loss of lift is called a phugoid.  It is impossible to completely eliminate a phugoid, but it wasts energy, so it should be held to a minimum.

This flight pattern is the result of a longitudinal imbalance, an imbalance in a front to back direction.  It takes the form of an undesirable pitching movement, a rotation around the pitch axis, a line drawn from wingtip to wingtip.  The imbalance consists of the center of lift being ahead of the center of gravity.  The lift pulls up in front and the weight pulls down in back, making the nose go up.  There are several ways to restore the balance.  The best way is to put the wing in the correct position in relation to the center of gravity and use the correct decalage, the angle between the wing and the tailplane, according to my instructions.  Fixing this could require soaking the glue to remove and relocate the wing or tailplane.  That can be difficult to do.  If the stall is mild, it can be cured by pointing the propeller axis to the left.  Grip the prop bearing and bend it slightly to the left.  This produces left thrust that pulls the nose to the left and tightens the left turn.  With the lift pointing more to the side, it will pull the plane into a left circle and reduce the upward component of lift.  Try another test flight to see whether that fixes the problem.  This can get the plane flying without a stall, but it has the undesirable side effect that it makes the plane bank and sideslip to the left.  A banked sideslip is a downward motion.  It will detract from flight duration.  For more severe stalls, the instruction sheet recommends adding clay weight to the nose of the plane.  Work the clay in your hand to warm and soften it.  Break off a small piece and squeeze it around the nose of the plane.  Be careful it does not interfere with the rotation of the propeller or shaft.  Adjust the amount of clay added until the stall is cured.  The undesirable side effect of adding ballast weight is that it makes the plane go up slower and come down faster,  detracting from flight duration.  The nose can be held down by lifting the tail.  That can be accomplished by giving the tail a greater angle to the airstream. Soak the glue holding the shim between the tailplane and the motor stick and add another or a thicker shim.  This will force the wing to fly at a  lower attack angle than is best for flight duration, but at least it will not stall.  Another way to increase lift near the tail is to glue a strip of paper along the trailing edge of the tailplane and deflect it down slightly, to act as an elevator.  Cut a strip of stiff paper about 1/2″ wide and 4″ long and glue it to the underside of the trailing edge of the tailplane.  You can cut the paper from the instruction sheet, but that paper is a bit flimsy and will not hold the adjustments.  Card paper, such as is used for postcards, business cards and advertising cards will hold adjustments better.  Loose paper on the tailplane can also cause a stall if the paper sags downwards.  The sagging paper acts like an upside down cambered airfoil, producing a downward lift.  The effect may be reversed by pushing the paper up against the bottom of the motor stick and gluing it in place.  Start near the front of the tailplane and glue it a little at a time until the problem is corrected.  Gluing too much of it at once may cause the opposite problem.  The best solution for saggy tailplane paper is to remove the tailplane, insert a friction fit temporary strut to keep the paper taught, then reglue the tailplane in place.  Once the glue is dry, you can remove the temporary strut.

Spiral Dive.  A spiral dive is a lateral instability in which the plane banks to one side and sideslips.  The sideslip produces a turning force on the fin, which accelerates the turn and bank.  The plane will dive to the side in a tightening spiral.  You will not see a true spiral dive with the AMA Cub, unless you seriously reduce the dihedral of the wings.  You may see something that looks very like it.  What you may see with the AMA Cub is a torque roll dive.  You will see this when the motor is tightly wound, especially with the thicker motors.  The tightly wound motor will roll the plane to the left, causing a sideslip to the left.  If the torque is strong enough to overcome the correcting roll of the dihedral, the downward sideslip will overcome the the upward lift of the wing and the plane will sideslip into the ground.  The plane may bounce along on the ground for a while before the torque comes down into balance with the roll produced by the wing dihedral.  The plane may then level the wings and climb to the ceiling!  The plane will not tighten the circle, but you will not see that launching close to the floor, because it will hit the floor before completing a single circle.  Since this is caused by motor torque greater than can be balanced by wing dihedral, the obvious solution is to reduce torque by reducing the number of turns put into the motor, or, preferably, using a thinner motor.  This is why I prefer to make the motor from a 24″ strand of 3/32″ wide rubber strip, rather than the 18″ length of 1/8″ rubber provided in the kit.  Both motors have the same weight, but the thinner motor takes many more turns without producing the torque roll sideslip and still has enough torque to make long flights.  The spiral dive may be countered somewhat by bending the prop bearing slightly to the right.  This will reduce the tightness of the left turn and bank.  It has the disadvantage that it will still act when the torque comes down, and can open the flight circle enough that the plane will hit the walls.  The left roll can be balanced by adding clay weight to the right wingtip.  Fortunately this is aligned with the center of gravity and will not affect longitudinal balance.  Unfortunately, it will still be there when the torque comes down and can produce a right roll, larger circle or right turn later.  The left roll can be countered by gluing strips of card to the trailing edges of the wings near the tips, forming ailerons.  The left aileron may be bent down slightly and the right aileron may be bent up slightly.  This has the disadvantage that the right roll will not be balanced when the torque comes down, with the result that the plane may roll right into a right circle and hit the wall.  A cardstock rudder can be used to similar effect, with a similar disadvantage.

Dives or Won’t Climb.  Sometimes the plane will dive to the floor or just won’t climb, no matter how much power you give it.  If it is  new plane, this is probably the opposite longitudinal trim problem to the Stall.  The wing is too far back relative to the center of gravity.  the lift in back is pulling up, the weight in front is pulling down.  The nose points down and the propeller pulls the plane into the ground.  The best solution is to position the wing and set the angle between the wing and the tailplane correctly.  Other solutions are similar to, but opposite to those used to correct the stall.  Bending the prop bearing to the right might help in a mild case, but also risks opening the flight circle to hit a wall.  Add clay to the tail end of the stick.  Reduce the thickness of the tailplane shim.  Put a shim between the motor stick and the paper of the tailplane to give it an inverse camber.  When you get it right, glue it in place.  Add a cardstock elevator to the trailing edge of the tailplane and bend it up slightly.

If the plane was flying well at first and later became unable to climb, the prop hanger has become loose and the prop is pointing down.  See if you can wiggle it up and down.  This especially happens with very soft wood.  Soft wood is desirable because it is also light.  But when the AMA Cub hits the floor, the propeller pushes back on the prop hanger, forcing it into the wood on the bottom of the motor stick and crushing the top corner.  The tension in the rubber motor pulls the hanger back, making the prop point down more than it should.  Soaking the front end of the stick in water will often restore its size, but it will still be soft.  I recommend soaking the front end of the stick in dilute glue. The glue soak should extend a bit beyond the back of the prop hanger.   The water will make the wood expand and the glue will harden the wood.  If you did not use water soluble glue, soak the nose in water and let it dry first, then soak it in glue.  You can paint full strength glue on the top and bottom of the stick for additional hardening.  If that does not do the trick, gluing thin strips of card along the top and bottom of the stick may work.  With very soft wood, it may be necessary to slice 1/16″ off the top and bottom of the stick and glue strips of hard balsa into the cuts.  Be sure to get them square and continuous with the top and bottom of the stick.

Another thing that can happen that will prevent your plane from climbing is that the paper on the tailplane can shrink when it dries out.  The best way to prevent this is to build only on days with very low humidity.  You don’t always have a choice, especially with a class that is scheduled in advance.  When this happens, the tension in the paper will pull the tailplane tips up.  That in turn will pull up on the paper across the center, producing a curve that acts like a cambered airfoil, giving more lift to the tailplane.  A solution is to cut out the center area of the tailplane paper about 1/16″ inside the sticks.  Bend the sticks back to flat if necessary.  Glue 1/8″ strips of paper along the edges of the cutout, so they extend 1/16″ into the opening,  then glue the center part of the paper back in place.  It is best to do this on a very dry day.

Poor Flight Circle. The airplane will not circle properly within the walls of the room.  It will fly straight or even in a right circle.  What you do here depends on the severity of the problem.  If it flies in a circle that is too large for the room, bend the prop bearing slightly to the left, add a bit of clay to the left wingtip, use rudder or ailerons to roll or yaw left.  Sometimes tilting the tailplane can produce a turn.  If the wing is off center or canted off centerline, the plane will fly in a poor circle.  The fix requires removing and repositioning the wing, not and easy thing to do. If the plane circles to the right within the walls of the room, smile!  You don’t need to do anything.