If your plane has a tendency to turn either left or right while flying straight ahead, there are some things you can do to figure out what the problem is. A properly rigged Challenger can be flown hands off in smooth air if it's trimmed correctly.
1. Inadvertent Yaw can cause a Turning Tendency
The first thing a pilot needs to do is make sure that he is not allowing the plane to yaw either left or right due to improper rudder use. The best way to determine if any yaw is present is to refer to a yaw string on the center of the windshield.
If you are unfamiliar with a yaw string and how to interpret it, see these articles on this site:
- A. The Yaw String by George Hurt under Piloting and Navigation / Short Piloting Tips
B. Ground Reference, Part 2 by Doc Green under Piloting and Navigation / Piloting Part 4: Flying the Yaw String.
Take your plane up for a flight, settle down at level cruise, and see what the yaw string is doing.
With the yaw string straight up and down you will not have any rudder induced turn in the plane and it should now fly with wings level and not try to turn in either direction. If you still have a turning tendency with the yaw string at neutral, then one of the wings is producing more lift than the other and we need to make some checks for that on the ground.
2. Are the Strut Fairings in Proper Alignment?
If you have the streamlined strut fairings installed on the lift struts, that's the first item to check out. The strut fairings are actually symmetrical airfoils and they must all be aligned at the same angle on all four struts. If one of the fairings is misaligned compared to the others, it will either try to pull that side down or lift that side up, depending on the angle at which it's set.
There is no exact angle specified for the fairings, but they must all be set at the same angle. It is suggested that if you want the least drag from the fairings at cruise speed or higher, then set the angle parallel to the bottom of the wing. Do not set them up with the front of the fairings at a higher angle than parallel to the wing bottom. However, you can put a couple of degrees of down angle in them if you wish, but setting them parallel to the wing bottom is best for high speed.
Checking the angle against a straightedge
There are a couple of ways to quickly check the angles of the strut fairings. One way is to use a straightedge laid across them, front to rear, parallel with the longitudinal axis of the plane. This will put the straightedge very nearly at right angles to the struts.
Click to enlarge.
Then carefully measure from the bottom of the straightedge down to the leading edge and trailing edge of each fairing. If you keep both those measurements the same, the fairings should then be at the same angle.
Checking the angle using a template
Another way is to make a template out of cardboard. A good stiff cardboard is best. Make sure one edge is perfectly straight and about 12" or more long. It also needs to be about 5" wide, giving a rectangle of cardboard 12" X 5", minimum.
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Measure down about 3" from the long edge, the straightest edge, and draw a line across the face that is parallel to the long edge. Then, using a small piece of the fairing material itself, place the end of the fairing on that line so that the front and rear of the fairing is exactly on the line. Then trace a line around the fairing where it contacts the cardboard. |
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Cut the trace of the fairing cross section out of the cardboard, as shown in the picture. This will produce an opening that matches the cross section of the strut fairing. Then, make vertical cuts straight down from the leading edge and trailing edge of the fairing outline. This will leave a slot in the cardboard that is exactly as wide as the fairing and which matches the upper curve of it as well. Slide the cardboard down over the installed fairing until it seats along the top curve of the fairing.
Once the template is fitted around the strut fairing on the plane, adjust it as necessary to get the line at the front and the line at the rear to intersect the front leading edge and rear trailing edge of the fairing. The top straight edge of the template will then be parallel to the line going directly through the center of the fairing. The top edge can then be used to set whatever angle you want for the fairing.
By sliding the template up to the upper part of the strut, you can rest the top edge lightly against the bottom of the wing and see if the fairing is installed at the correct angle. In this case, the resulting angle would be parallel to the wing bottom. All four fairings can be checked this way and they all must be the same. If a fairing is not aligned properly, you will have to drill out the rivets that hold it in place, rotate the fairing to the proper angle, and then install new rivets.
Smart Level
Because the cardboard template provides a surface on which a level may be placed, you can use a Smart Level to check the angle of the fairings and then set them to whatever angle you wish. Just place the Smart Level along the top of the template. You can check the fairing angles at more than one place along the strut if you wish.
A picture of one type of Smart Level is included, and a link is provided that shows a similar model at the manufacturer's site. A Google search with the words "Smart Level" in it will uncover tool companies that sell these online. Or you can go to Home Depot or a similar full hardware store and see one there. A Smart Level has many advantages, including a digital readout to within 1/10th degree. However, one costs about $90-$100. Smart Level Web Site!
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(Alternatively, the angle finder described below in Section 6 can be used in lieu of a Smart Level, but with reduced accuracy. The angle finder is accurate only to about one-half of a degree.)
At any rate, make sure all the strut fairings are at exactly the same angle before doing anything else. If they need correction, do it. Then go up for another flight and with the yaw string centered, see if the turning tendency is still present. You may find that your turning problem has disappeared as a result of all the strut fairings being at the same angle. However, if the plane still wants to turn, the next step is to check each wing for washout.
3. Correcting Washout or Wing Warp.
The factory does a great job by using special jigs to insure that all dimensions and holes for the strut attachments are as close to perfect as they can possibly deliver. Still, some slight errors can creep in. It only takes a difference of 1/16" -1/8" of total strut length to introduce a little bit of twist in the outer wing section. Even rough handling by the trucking company during fuselage shipment can cause a small change in dimensions. However, it's relatively easy to check for any wing warp after the wings are installed, either during your building process or later when the wings are covered and painted.
Basically, you want to see if there is any change of wing angle from the root end to outboard tip. The root ends are not going to be the problem in most cases simply because both root ends are attached to the black U-shaped Rony brackets that bolt together straight through the root tube. The angle of the wing is set by the angle of the root tube, and we want that same angle to continue all the way out to the wing tips. If one wing tip is at a slightly different angle compared to the other, it will create a slight difference of lift. That difference in lift will continuously try to bank the plane.
DISCLAIMER: Now, before proceeding with any adjustments to the strut length, I want to make it clear that this is NOT a factory approved fix for wing warp corrections on the rear Rony bracket. While the factory has said that shimming the front Rony bracket is permissable, they do not approve of shimming of the rear Rony, because the rear Rony meets the strut at an angle. The factory approved shimming of the front Rony can reduce any washout for that wing, but it will not correct any washin, which is the more undesireable problem to deal with during flight and stalls.
With this DISCLAIMER and caution in mind, it can still be said that many builders have used the shim method on the rear Rony without encountering any later safety issues, as long as the work is done in a careful manner and proper AN bolts are used. So, I am merely passing on how some builders have corrected wing tip washin problems using shims under the rear Rony brackets.
The angle of the wing tip is controlled by the length of each lift strut. When both lift struts on one side are exactly right, according to original factory design, there will be no change in the wing angle, no wing warp, from root to tip. But, if one of the struts, either front or rear, is slightly too short or too long, it will introduce a slight warp in the wing. The correction will consist of adding spacer shims under either the front or rear Rony bracket to increase the effective length of that strut by 1/16" to 1/4", according to how many spacers we use. The spacers are actually large washers with a 1/4" hole in the center.
Each spacer will be 1/16" thick, so we can make adjustments in 1/16" increments. Some folks just go out and buy common fender washers from a hardware store. However, make sure they have a 1/4" hole in the center and are 1-1/2" diameter, not the more common 1-1/4" diameter carried in most stores.
Another option is to order some from the factory. You will need the same large washers that are used on the aileron bellcrank mechanism. These are stainless steel, with 1/4" holes, and a diameter of 1-1/2". The factory stainless steel washers provide a proper fit under the Rony bracket because the base of the Rony bracket is 1-1/2" wide.
The factory attempts to send out the wings and struts so there is zero washout at the tips. Washout refers to a situation where the wind tips have a tiny bit less angle than the root angle. This is OK to have, and in fact is designed into many production aircraft intentionally. There are some benefits to having a little washout. One is that the wing tip will stall just a little later than the wing root and allow more aileron control during a stall.
What we don't want, on any airplane, is Washin. Washin refers to the situation where there is more wing angle at the tip than at the root. In this case, the wing tip will stall prior to the root, which is not desirable at all.
So, how do we check for differences in wing angle, or wing warp? Start with the plane on the ground, on a level surface, or as close to a level surface as you can find. Prop up the tail so the nose wheel is just touching the ground. There are at least four different methods that can be used to determine if there is any wing warp on one side compared to the other. These are described in the following sections.
4. Eyeball Method for Checking Wing Warp
The first is the "eyeball" method. This is the least time consuming, and will get you into the ballpark, although it's not quite as precise as the next two methods.
After getting the nosewheel on the ground, crank the flaperons up "out of the way." The objective is to prevent them from interfering with your seeing the trailing edge of the wing. If flaperons aren't installed on your plane, set the ailerons as level with the bottom of the wings as possible.
Place a 6-foot ladder out in front of the plane about 30 feet or so. Climb up on the ladder to a height that will allow your eyes to evenly see the bottom of one of the wings. When you think you're "there," bob your head up & down slightly while looking at that (right or left) wing.
The bottom of that wing will barely disappear at some point so all you can see is it's leading edge. Keeping your head in the same place, look over at the other wing. If you can see the bottom of that wing along with it's trailing edge, the incidence is different between the two wings. If you can't see the bottom of the other wing, bob your head slightly again until you can. Then look over at the first wing so you can see any difference.
If you find a difference, you can correct it by shimming under the front or rear Rony brackets, as appropriate, to make both wing bottoms appear the same.
5. Bubble Level Method for Checking Wing Warp
This method utilizes a common bubble-type carpenter's level and a ruler to actually measure the difference in elevation between the rear spar and a point on the underside of the wing near the front spar. A long level gives better accuracy than a short one, but if your level is on the short side, you can simply tape a straight strip of wood, to the top of the level and then use the top surface of the wooden strip as the "top of the level."
First, position the plane so that it's very nearly level, front to back. Having the plane level from side to side is not essential as long as one wing is not obviously a lot higher than the other.
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At the position along the wing where you wish to make the measurement, gently press the back end of the level up against the rear spar. Then, on the other end of the level, pinch the ruler against the side of it with about 3 inches of the ruler sticking up above the top of the level. Then raise the ruler and level together until the ruler contacts the bottom of the fabric. Be sure the ruler stays at right angles to the level. |
Check the bubble to see what adjustment must be made to get it centered. Slide the ruler up or down accordingly, and try it again. Keep doing this until a position is found for the ruler that results in the bubble being centered when the top of the ruler is in contact with the fabric, with the top rear of the level in contact with the rear spar. Then note the inches indicated on the ruler between the top of the level and the bottom of the fabric.
If you do this a couple times and can repeat the result to within 1/16 of an inch or so, you can be confident you have gotten a good measurement. Repeat this procedure to check the wing at the root, midway, and at the tip.
This procedure might be done more easily if two people are involved, but it is not impossible for one individual to do it working alone.
On a zero washout wing, the ruler measurement will be the same in all three places. If there is any washout, the measurement will become less toward the tip because the tip is twisting down and getting closer to the front of the level. If there is any washin, the measurement will get larger. A little bit of washout is acceptable. No washin at all is acceptable.
6. Using an Angle Finder to Check Wing Warp
This method utilizes a common angle finder tool. At the position along the wing where you wish to make the measurement, gently press a 5 1/2 ft. long straightedge against the bottom of the front and rear spars approximately parallel to the fuselage centerline. The tool is held at the middle and along the bottom of the straightedge. It is not necessary to position the plane in the flight attitude or that it be level from side to side as long as one wing is not obviously a lot lower than the other. We are looking for a change in angle, not so much the magnitude of the angle. The tool can be read to about ½ degree of accuracy. This corresponds to a difference in height of about 1/2 inch between the front and rear spars. Make an angle measurement near the root, midpoint, and tip of the wing. On a zero washout wing, the angle measured will be the same in all three places. If there is any washout, the angle will become progressively less moving outward from the root toward the tip. This is because the tip is twisting down and getting closer to a level position. If there is any wash-in, the measurement will get larger. A washout of 2 degrees, root to tip, is acceptable. No wash-in at all is acceptable. |
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7. Using a Smart Level to Check Wing Warp
This method utilizes a Smart Level. It yields the most exact information, being able to read changes of angle as small as 1/10th degree. However, this method requires a tool you might not have and might not want to buy.
Start with the plane on level ground and with the tail propped up as noted before. Place the Smart Level under the wing as close to the root end as possible and read the angle. It should be same on each side. Let's say for discussion that it reads 5 degrees.
It may be slightly more or less than that, depending in part upon how the plane is sitting on the ground. However, whatever it is, that's the angle we want the wing to have all the way out to the tip. Go out to the tip and place the Smart Level under the wing as close to the tip as practical and read that angle. If the tip is also 5 degrees you have a zero washout wing. But if the angle is greater than the 5 degree root angle, you have Washin, which needs to be corrected.
If the angle is less than the 5 degree root angle, you have Washout. This is OK as long as it's not excessive. Generally speaking, a washout of 1 or 2 degrees is acceptable. In this example, that corresponds to a tip angle of 3 or 4 degrees, versus the root angle of 5 degrees. Write all this down and then check the other wing the same way. In the end, both wings should have the same angles, with no Washin allowed.
8. Correcting Wing Warp
You can correct a slight wing warp by increasing the effective length of one of the struts on either side. We can't shorten a strut, practically speaking, but we can increase it slightly by adding washer shims under a Rony bracket where it bolts onto the fuselage longeron.
To remove some of the washout, you need to raise the front of the wing. This is done by adding shims under the front Rony bracket on that wing. This will push the front wing spar up slightly and cause the wing tip to increase its angle a bit.
In other words, if you found the wing tip had an angle of 3 degrees while the root had 5, shimming the front Rony bracket will push the tip up more toward 5 degrees. You will have to experiment to determine how many washers you need to use.
You can do this initially with the original bolt that holds the Rony bracket to the longeron. But for use in flight, you should put in a slightly longer bolt. (See the article on Decoding AN Bolts for a description of how to determine what the replacement bolt will need to be for 1/8" or 1/4" of shim change.)
To continue, if you want to reduce the wing tip angle relative to that of the root tube (to tilt the tip downward), you will need to shim the rear Rony bracket. That will push the rear spar up which has the same effect as pulling the front spar down.
When you are done, you will have shimmed either one strut on one side only, or one strut on each side, to make both wings have the same angles from root end to tip. This should cause the wings to produce equal lift. It is likely that the undesirable turning will be significantly reduced, if not eliminated entirely.
As noted at the beginning of this article, numerous Challenger owners have found that they can trim out any pitch change using the flaperon mechanism and then fly their plane, straight and level, hands off the stick, without having any undesirable turning tendencies at all in smooth air.