Source: 2018 Q4 Beartracks, Kevin Deutscher
Often we experience a flying sensation and report it as “My right wing is heavy” which is a fine statement of what you are perceiving. Observations from a flight experience have a bit of freshness and uneasiness often clouding the senses and information is stored in emotional memory. The take away is, the right wing is heavy. Are you sure? Why is the left wing not light? How do I fix that? How do I validate my feelings? How do you go about debugging an aircraft to understand what adjustments might need to be made to enhance the handling to what you would like it to be? That requirement is going to be different for each of you but similar by mission and convention. We all want a plane that is predictable and capable of flying stable in a hands off environment without quirks.
There is plenty of detailed technical commentary in various EAA, FAA, NASA, Sport Aviation, Navy and Air Force manuals that are excellent reads on the subject of Flight Stability, Handling, Stick Force Gradients, and Positive or Neutral Stability. The vast majority of that has been synthesized into the Bearhawk design and everyday conventional flight expectations. Suffice to say that for a Bearhawk and the “Normal” mission, positive stability is a plane that can remain hands off without deviating from the established trajectory and will tend to return to that condition when disturbed from that condition. Non-conventional flight responses are the primary reason autopilots have difficulty controlling some planes. All the control algorithms are predicated on conventional predictable response to a control output. Autopilot wants a left turn to track the VOR, left turn commanded, left aileron raised but nose yaws right and drops, this makes an autopilot sick. Non-conventional response to a standard input.
That said how do we identify, isolate, and correct an undesirable situation? The answer is, Flight Test. This is something that can be handled in the initial Phase 1 testing and or later as the plane ages and changes from wear or maintenance. Flight testing should be accomplished with a specific purpose and format to Investigate, identify, gather data and observations in a consistent and repeatable manner. When flight testing it will be best to have an observer to assist in gathering and recording data and keeping a visual scan for traffic allowing the flying pilot to fly. Should an observer pilot not be available a GoPro camera and a data download from an EFIS can assist. Always select a clear calm day, at generous altitude of 4000 ft AGL or so away from populated areas.
The aircraft has 3 discrete motions. Pitch, yaw, and roll. Which in a perfect world would not interact, but in reality often do and the test flight effort is to try and sort out the interactions into specific behavior that can be studied and corrected on the ground. When observing a behavior we are looking for direction, rate, force and coupling of off axis effects. OBSERVATION is key. That is all about BIG PICTURE observations, out the front windscreen and the force in your hand.
A technique I like to use is called STOP and it is a great way to move forward.
Stop, Think, Organize, Proceed, each step of the way, planning the flight, gathering the data, understanding the data, adjusting the aircraft. Stop, Think, Organize, Proceed.
It is beneficial to put some structure and organization into gathering the data and prepare some type of log to be the reviewable record. A log with 3 columns, pitch, roll, yaw. The rows are the test being performed. All that is needed is a dot in the column to reflect the direction trend. Dot in the center of the column is neutral. Off center is direction and further off is magnitude. Whatever method works for you is good, keep it simple and quick.
Let’s plan a flight, observe, gather some data, assess the data, compare to our previous geometry measurements and make a change and flight test the change.
Preference is to address the pitch performance first as pitch difficulties cloud other axis issues, interact most with the overall stability and interfere with your ability to debug the other axis if always chasing pitch and hence attitude and speed.
Reported issue: Elevator / pitch is twitchy and requires frequent trim changes:
Load the aircraft to approximately mid CG, two people, full mains, this should be the initial configuration for all testing, no loose gear floating around the aircraft. Head on out to the flight test area and establish a cruise at approximately 90 mph. Maybe 2200 rpm and 22 inches, Trim for hands free. Notice how much stick play there is before a stick input causes an aircraft motion. What is the nose doing in yaw? Is the ball centered? Is the ground track straight with the CL of the aircraft? Can you release rudder pressure without the nose yawing? How much pressure and deflection does it take to keep the nose stationary in yaw? Ailerons, same questions, get this feel into muscle memory. Are you holding altitude? Wait I thought we were working on Pitch? Yes. So now will perform pitch tests. Make the changes slowly, 5 seconds to change, 15 seconds to stabilize. Call out your observations to your data recorder for logging, both visual cues and pressures.
From trimmed stable flight with absolute minimum control input:
Increase power to normal climb. What happens? Reduce power to normal cruise. What happens? Reduce power to approximately 40%. What happens? Back to normal cruise. What happens? Repeat this sequence 5 or 6 times. You will find your observations will be broadened. The first few times you will go tunnel vision, force your focus to be very broad. What is happening is viewed out on the horizon not, on the instrument panel. Try the last one stick free, hands off. Are the results different? In what manner?
Normal cruise power, trimmed hands off pitch. Slowly apply back pressure until the nose is approximately 10 degrees up and speed falls by 15 mph. Relax back pressure. What happens? Reset and repeat 5 or 6 times. Same initial setup, this time forward pressure, relax forward pressure? What happens? Reset and repeat 5 or 6 times. Try the last one stick free, hands off. Are the results different? In what manner?
Head back to the field, let your head chill and let’s review some data.
After compiling the data look to see what patterns are discernable and if they make sense to previous measurements and support the “twitchy” concern.
Looking over the pattern of the data and examination of anticipated or desirable handling some oddities are noticed. What do they mean and how do we make sense of the issue? STOP.
Summarizing the observations from our hypothetical flight data a statement like this can be made:
When adding power the plane accelerates, remains mostly level while banking and yawing to the right. When pushing over from trimmed level flight or power reduction, the nose drops and the speed continues to accelerate, plane does not attempt to return to level flight. Banks and yaws right. Why?
Looking at some of the geometry measurements of our hypothetical aircraft there are some things that might have a bearing or explain what is going on. Horizontal tail is -3 degrees from the level datum and the right horizontal tail is about ½ inch lower at the lift strut attach strut than the left side. The right aileron measures trailing edge low by about ¼ inch compared to the wing tip and the straight line of the wing bottom. The left aileron seems correct.
Conventional wisdom, standard practice and physics dictate with application of power a correctly rigged plane should climb, yaw left, followed by a left bank which slowly transitions to a left turn. When displaced from level by pushing the nose over then releasing forward pressure the nose should rise and return to level flight at trim speed. Our theoretical plane does not perform conventionally.
Keeping in mind right now we are investigating Pitch. It appears the horizontal tail needs to have the leading edge lowered by at least -1 degree to -4 as a starting point. Lowering the leading edge will provide the tail with more down force at a given speed. Increasing the speed will cause the downward force on the tail to increase, raising the nose to get back to trim speed. More airflow, power increase over the tail causes more downward force and raises the nose causing a climb attitude. It seems like there are several data points and measurements to justify a change to the incidence of the horizontal tail surface. When the tail incidence is moved -1 deg, make sure the twist from side to side is eliminated. Making a change has a snow ball effect, now you need to install shims to lower the leading edge. Obtain longer bolts, adjust the horizontal cross tube fuselage covers, and adjust the horizontal leading edge support struts and possibly the flying wires, all the while making sure the changes do not cause binding of the elevator. There is a lot of detail work here and it needs to be done carefully and precisely. These adjustments cannot be just wished into place.
Somebody out there is thinking, that’s a lot of work and my pitch is not totally “Positive” but I can live with it. My real beef is running out of elevator in the landing flare… Well, the solution for more elevator in the flare, is exactly the same. However adjust too far and there are consequences. Like in a high powered go around configuration the nose will go up fast. Be careful. Small changes, one at a time.
So time for another test flight to evaluate the change. If pitch is noticeably improved you can perform the roll / heavy wing data gathering on the same flight.
Investigating the roll tendencies has a different set of variables. Having pitch sorted out makes the workload easier and the plane more stable. Testing the wing heavy or light syndrome will involve separating contributions of flap positioning, aileron rigging, and positioning all the while making sure that the airplane is not skidding or slipping, the ball must be kept centered. That is the job of your feet! Make the pedal pressures as light as possible, do what is needed to keep the plane from yawing. Ailerons inputs should be handled with fingertips on the stick as much as possible.
Reported issue: The left wing is heavy:
The preamble and setup for this testing is the same as the for the pitch investigation. Approach the testing in the following manner.
From trimmed stable flight with absolute minimum control input:
Normal climb power. (Rudder) What happens? Reduce power normal cruse (Rudder) What happens? Reduce power to approximately 40% (Rudder) What happens? Normal cruise (Rudder) What happens? Repeat this sequence 5 or 6 times. Did the wing heavy change as the result of power change?
Flaps up, establish a high power cruise, 120 mph straight and level, release stick, note bank and stop yaw. Reduce power to just above Idle. Slowly decelerate holding altitude, rudder adjust, no yaw.
Let the speed decay to about 70 mph, power up resume cruise. Were there substantial changes to the wing heavy condition? Did the roll tendency increase with speed? Were the stick pressures symmetric or was one direction heavy and the other light? Did the pressures change with speed? When allowing the stick to be free did the stick offset and then the bank stop? Repeat this test 5 or 6 times to be comfortable that you can predict the behavior. Look at the relationships of the aileron leading edges, do they protrude below the wing? How are the relationships of the tips to ailerons? Are ailerons and flaps aligned with each other? Constantly check heading and that the ball is centered.
Repeat the above test with 2 notches of flaps and an initial speed of about 100 mph and decay to just above the stall, recover to level flight. There is no need to enter a stall. Is the behavior identical or different? If different is it better or worse?
Head back to the field, let your head chill and let’s review some data.
After compiling the data will look to see what patterns are discernable and if they make sense to previous measurements and support the “left wing is heavy” concern.
Looking over the pattern of the data and examination of anticipated or desirable handling some oddities are noticed. What do they mean and how do we make sense of the issue? STOP.
Summarizing the observations from our hypothetical flight data a statement like this can be made:
The left wing is “heavy” only when the flaps are retracted and left rudder is constantly need to keep the nose on heading and the ball centered.
Well there are several different ways to approach this paradox. First is the flap configuration change eliminated the heavy wing with application of flaps. When the flaps are deployed they free float and balance air loads equally between the flaps via the “Y“ cable arrangement. Since the wing heavy is reduced with flaps deployed the issue is most likely flap related while in the full up position. The observation that it requires left rudder to center the ball and stop the yaw is of interesting note.
Left wing heavy and left rudder should be causing a left turn but it does not…
There is an ah-ha moment coming here. Lift produces drag. The right wing is producing more lift, raising the right wing, hence the right wing is LIGHTER than left and producing more drag. The induced drag from lift production causes the right wing to have more drag than the left therefore the drag retards the right wing causing yaw right. LEFT rudder is required to eliminate the yaw. With this condition present it is not possible to be sure the ailerons are perfect yet. One step at a time. If you reduce the right lift by raising the right flap a bit then lift will be reduced and drag will be reduced. How much to raise the right flap? Start with 2 turns inward of the threaded rod end at the flap. Yes, time for a flight test. Repeat the flight test data set and make your observations. It should be better. Now pay close attention to the resting positions of all the trailing edges and the leading edges of the ailerons relative to the bottom of the wing. If a wing is still heavy it may be time to correlate the data to the condition and adjust the ailerons. The nitty gritty of the aileron adjustment pressures, feels and differential aileron rigging will be covered in the next installment.
The yaw function of flight coordination is typically a result of pilot responsibilities being left unattended, had to get that in there. After this rigging effort the pitch and roll are better, maybe not perfect, another round of tweaking will be required but you have a system down and the tools to work with the data. There you are humming along and removing feet from the rudder pedals creates yaw and provides your feet constant entertainment. Yes! Yet another flight test is in order but first we need to make a ground installed reference system. With pilot seated in the airplane look down into the foot wells or by the seat edge and find a location where the rudder cable is visible. Establish neutral position of the rudder to the vertical fin and place a tape mark on the rudder cable and the fuselage interior to indicate neutral rudder. Are your feet in neutral on the pedals when the rudder is centered? If not fix that first on the ground.
OK, up to the sky again. Establish a normal cruise power setting and verify straight level and nearly hands off on the stick. Smile! Notice rudder position indicator with respect to centered ball. Remove feet from pedal and note the nose yaw, if any. Where does the rudder position indicator come to rest? Note the displacement of the indicator marks. Head back home.
Looking over the pattern of the data and examination of anticipated or desirable handling some oddities are noticed. What do they mean and how do we make sense of the issue? STOP.
Summarizing the observations from our hypothetical flight data a statement like this can be made:
When in flight and feet off the rudder pedals the nose yaws slightly left and stops with about 5 degrees of deflection. The mark on the rudder position indicator is off by about 3/8 of an inch, rudder deflected left.
Several possibilities here, add a rudder trim tab, rudder trim system, add more drag to the right wing tip, remove the rudder return springs and insure that they are not causing a rudder bias or add spring bias. Lubricate the rudder assembly. Check the tail wheel springs and the centering of the tail wheel and centering friction because if the tail wheel is cocked and not in trail or the springs are not equal force (Maule Setup) or unequal chains, then tail wheel will be a mini rudder deflecting the rudder causing the yaw!
Hopefully this has helped define a method to assist in debugging and rigging of your plane. This is an incremental process that is iterative and small adjustments have big change effects. It is important not to jump to conclusions and misdiagnose or apply the wrong correction as you may end up chasing your tail and adjusting something to mask an incorrect adjustment. I have walked thru this activity on several Bearhawks and always end up with a hands off stable aircraft with minimal adverse yaw that can be flown and turned rudder only, stick free or aileron turns feet on the floor.
Next installment will address your questions and cable rigging, control counterbalance and differential aileron configuration.
Fly safe with a smile!
Kevin Deutscher, Bearhawk272@Gmail.com