Herb Clark’s (SRA # 2892) newly completed Stearman PT17 after his 5+ year restoration. Herb’s attention to detail on this airplane is outstanding. With Herb’s permission, we have used his airplane as the subject of this article.
Herb Clark’s PT-17
Rigging a biplane can be a very “rewarding experience.” At least back when you were a kid, that’s what the grown-ups always told ya’ when you were falling on your face! Remember? Well, we have been rigging biplanes for over 20 years and have developed a few tricks and tools that can make the process much easier. The following is the procedure we follow to rig a Stearman.
Rigging a Stearman can be tough if you don’t have the right tools. So, what are the right tools? A copy of the rigging instructions (Maintenance and Erection Manual), rigging board(s), a precision level, 4 plumb bobs, a measuring stick or two, a couple of ladders, a means of lifting the tail and keeping it there, and enough breeze free hangar space to do the job.
The tools of the trade. Kimball Rigging device, Digital level, plumb bobs. Yard sticks (ladders and hoist not shown)
We have developed a special rigging device that we can use on any airplane of 72” chord or less and any airfoil. This Kimball Rigging Board shown is used to measure both incidence and dihedral. Its use will be shown later. For those interested in having one of these units, we can make copies of this device available to those who wish to have one. The yard sticks we use are actually 48” long! That’s a 4 foot yard stick. These rules have a spirit level built in that aids in getting true horizontal dimensions required in rigging. We use digital level that is capable of 0.1 degree increments and the plumb bobs are standard commercially available units. The manual to use is the Stearman Erection Manual. Special care should be taken to note stagger dimensions that include stall strips. There are several configurations of stall strips on Stearmans. None, all 4 wings, lowers only, half span, etc. so you may need to do some math to determine the proper stagger dimension you need at the center section and at the N struts. Basically, the stagger is 26 13/16” +/-1/8” for the full span. If you have no stall strips or have strips on all 4 wings, use this number at all 4 stagger measurement points. If you have stall strips on the lower wings only, subtract 1/4” from stagger dimension for the outboard points.
The first thing to do is assemble the airplane. Install the wings, tail, ailerons streamline tie rods (flying and landing wires), etc.
Safety all bolts and pins etc. Set all tie rods to the lengths specific in the erection manual plus 1/8” by starting the terminal ends on one turn each then rotate the wire to reduce length to the quired dimension+1/8”. This will ensure the ends are on the wire evenly. We assemble a Stearman as follows: C/S and cabane struts, tail, upper wings and N struts, lower wings and ailerons. We have cradles, ropes and pulleys to suspend the upper wing/N struts as the lower wings are installed.
Level the airplane both longitudinally and laterally by using the leveling points in the lower forward fuselage. In some cases, these lugs have been removed or modified during the “Duster Days”. If this happens, you will need to use the upper longerons as your reference. The airplane will look tail high when it is level. Shoot for 0.0 degrees in both directions, fore and aft by raising the tail, left and right by adjusting the air pressure in the main tires.
Set Center Section:
With the airplane level, the next step is to properly locate the center section both fore and aft and left to right. This is spelled out in the rigging manual. Take care to hang the plumb bobs from exactly the same point over each end of the C/S. We tie the string to the lifting lug and let it pass over the outboard side of the forward wing attach bushing centered on it. You want the measure at the same point on each side and make sure the ruler is level. We rest our 4ft yard sicks on the top of the pulley bracket to ensure a repeatable reference point each time. Adjust the sway wires (the ones that cross aft of the firewall) until an equal measurement is achieved for the 2 sides. The target dimension is 23” +/- 1/16”. In this case, just a string width under 23”.
Plumb Bobs Set for C/S Centering
Try to keep the wires at a high tension while adjusting them. Keeping them close the tension will reduce the movement of the C/S as you set the tensions. Take note of the gap between the left and right stagger wires at the point where they cross. This gap should be equal on each side if the fuse is truly level and the C/S is perfectly centered over it.
Centering the Center Section
Both sides at 23 inches
Now set the C/S stagger by adjusting the left and right stagger wire pairs. The target dimension here is 26 13/16” +/- 1/8” for all configurations of stall strips mentioned earlier. Place a plumb bob line over the leading edge of the C/S near each butt rib.
Measure the distance from the lower wing leading edge (no walkway trim installed) to the string. Again, make sure the ruler is level and perpendicular to the wing leading edge. Adjust the stagger wires with a few turns at a time on each side until the proper C/S stagger of 26 13/16” is obtained.
Tension C/S wires:
Now that the C/S is properly located over the fuselage, the tensions of the 6 C/S wires should be set. All other rigging of the wings is based on the center section. Getting the C/S properly set is very important. The best way to do this is by using a wire tensiometer.
Gap between two wires—Same on Both Sides
Plumb bobs over leading edge for stagger set
Adjust stagger wires to get 26 13/16 in on both sides
SRA Wire Tensiometer
The SRA recently made a few of these units and made them available to club members. We have one and it is very nice. More may be available in the future if demand is there. But, for now, if you do not own one, you need to find one to borrow if you really want the set the wires properly. If you had the wires close to proper tension during the C/S rigging, you will be able the set the tensions quickly. If not, you may need to recheck the centering and stagger after you set the wire tensions.
The desired final dihedral of the lower wings is 1.5 degrees. The stainless steel tie rods will stretch a bit when under tension. Likewise, the wooden wings will compress a bit in span. Because of this, the dihedral needs to be set to 1.6 degrees with the landing wires to allow for the stretch of the wires. Set both left and right wings to 1.6 degrees dihedral measured on the front spar. Setting dihedral to 1.6 degrees with Kimball Rigging Device
As the flying wires are tensioned, the dihedral will be reduced to 1.5 degrees and the final tensions in both flying and landing wires will be very close to the specs.
Setting dihedral to 1.6º with Kimball Rigging Device
Tension Wing Wire (Watch Stagger)
Now we can tighten up the flying wires. The first step is to place a plumb bob on each upper wing near the N struts. Make sure it is in the same location on each wing. Measure the stagger at the N struts. Chances are it is not the same as the C/S stagger. If you have no stall strips orall 4 stall strips, use 26 13/16”. If you have lower wing only stall strips, use 26 13/16” – 1/4” (thickness of the strip) or 26 9/16” +/- 1/8” as your target stagger number. It is important that the wires on both left and right be tensioned equally. A few turns on the left then a few on the right. Try to keep the stagger dimension the same left and right even if it is not the target number at this point. Here is where it gets a bit tricky. The rear flying wires are larger than the front ones (Why is another story) and have a greater ability to pull the wings aft than the front ones can pull forward. So, the stagger needs to be cheated forward about ¼” at the N struts with the front flying wires so that the stagger will be correct when the proper tension is achieved in the rear flying wires. Again, work left and right wings the same amount and try to keep the stagger measurements equal. The front wires will be quite snug at this point but not yet up to tension. . It is best to have 2 people turning the wires, one at each end. Now tighten the rear flying wires left and right equally and watch the stagger dimension at the N struts come back. When the stagger is back to the desired distance, the tension in all the flying and landing wires will be close to the specs. Now is the time to take a quick tension check with the tensiometer. Typically, the front wires will be near the top of their tension range and the rear ones near the bottom. That is OK as long as they are in range. If the rear wires are too loose or the front ones too tight, tighten the rears or loosen the front ones as needed but keep and eye on the stagger at the N struts. There is 1/8” of variation allowed on the stagger and it is very easy to get the tips of the upper wings too far aft when setting the wires. If the rear wire tensions do not come into range and the stagger gets too small, start over again by loosening all the flying wires and resetting the stagger at the struts a bit less than ¼” over the desired number. Repeat the above steps until the stagger and wire tensions are all within limits. If a wire bottoms out against a fitting, the wire was not properly centered in the terminal ends or the rigging is way out of specs.
The incidence of the center section and upper wing is set by the lengths of the struts and the stagger. If the stagger is correct, the incidence will be correct if the wings are not warped. The upper wing incidence is 4 degrees. The lower wing incidence is adjustable at each tip but is fixed at the fuselage to a setting of 3 degrees. Adjust the forks in the rear N struts such that the incidence is 3 degrees at the strut area of the lower wing. These forks will be adjusted as needed to correct any wing heaviness during test flights.
The rigging manual spells out the use of a certain # of washers on each of the 4 stab attach bolts where it mounts to the fuselage. This is a starting point and washers should be added and removed as needed to get the trim and flight attitude to the desired point. We have found 7 washers under the front stab points and zero washer under the rear gives excellent trim settings and a bit more speed at cruise. This is how we set the stab an all Stearmans we build.
The horizontal tail is set level using the 2 elevator hinge lugs on each side. Set the tail wires, upper and lower as needed to get the stab level. After the stab is level, adjust the top tail wires as needed to get the fine vertical. Measure the fin at its rear spar and get the same reading on each side rather than using level on only one side. Once level and plumb, set the tail wires to the tensions in the specs.
Streamline wires and lock jam nuts:
Streamlining the wires is best done with a straight edge. Lay the straight edge across a pair of wires and set the rotation of the wire such that the gap to the leading and trailing edges are equal. Repeat for all wires. When all wires are streamlined, set all the jam nuts while holding the wire from rotating. Don’t forget those jam nuts that are inside the fuselage. Using a piece of safety wire, check all the ‘tattle-tale” holes in the terminal ends to ensure the wires have adequate thread engagement. If you find a wire end that is not threaded together far enough, check the other end of that wire and see if the terminal is threaded on too far. If it is, you will need to loosen and disconnect one end of that wire, re-center the wire in the terminal ends, and reinstall it. You will have to check all the rigging again to make sure it has not changed..
Adjust control throws, pushrods and cable tensions:
The final step in rigging a Stearman is to set the control throws and the cable tensions. Following the erection manual is the best plan. First set the neutral point of the elevator and sticks per the instructions. Follow that with setting the elevator up and down stops to get the elevator travels in the specs. Next set the aileron alignment to the wings with the stick centered. Set the aileron throws to the specs with the stop bolts that contact the horn on the torque tube. Use a cable tensiometer to set the rudder and trim cable tensions as required in the erection manual. Safety all turnbuckles with safety wire as needed with the rudder cables all getting double wrap type safety.
Checking incidence angle on a Pitts 12
Side Bar: Kimball Rigging Device.
The Kimball Rigging device is a tool that can be used to rig countless types of airplanes. The device consists of a main spine board with degree scale, leading edge plates, sliding trailing edge plates, and sensitive level mounted to an arm and pointer. The leading edge plates are cut for each airfoil and can be ordered for most any airfoil. The leading edge and trailing edge plates are precisely cut to have the ends the same distance from the spine board so that the unit can be inverted and used to measure dihedral. The tool is quite easy to use. Simply hook the leading edge plates on the wing and slide the trailing edge plate forward until it is firmly locked onto the wing. Loosen the wing nut on the pointer to allow the arm to move. Move the arm until the bubble in the level is centered and tighten the wing nut. Read the incidence angle. Conversely, you can set the device to a desired angle and adjust the wing or wires until the bubble is centered setting the desired wing incidence. To check the dihedral, invert the device and place it on the wing with the leading edge end toward the fuselage. Adjust the pointer to get a center bubble and read the dihedral.
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