History of Aviation - Chapter 8
Airplanes shipped from the manufacturer or from another field almost always suffer more or less from shipment or packing. Care must be exercised in unpacking in order not to do any more damage. Boxes should be placed with the part marked "Top" uppermost. Cables and wires must be handled carefully in order not to bend or twist them. Every bent or kinked wire or damaged turnbuckle must be replaced, or at least brought to the attention of an inspector.
The order of erection is as follows:
1. Assemble landing gear to fuselage and align landing gear before putting on main panels.
2. Assemble tail.
3. Assemble engine section and align before attaching main panels.
4. Assemble main panels.
1. Landing Gear Assembly to Fuselage.-The
landing gear is assembled by mounting the wheels on the axle, and bolting wheels in place. The fuselage should now be elevated to receive the landing gear. This may be accomplished in one of two ways-either by tackle or by shims and blocking. For either method, first connect up the tail skid.
This is accomplished by pinning up the front of the skid to the spring-fitting, and then pi in the other end to the tail-post socket.
If block and tackle are used to raise the pass a line under the engine-bed supports or just to the rear of the radiator. To this line attach hook of block. To avoid damaging or some part do not attach lifting device to any other point. With the fuselage now resting on its attached tail skid, lift the front end until the lower longeron clips clear the landing gear. When the clips on the longerons line up with the clips on the ends of the struts of the landing gear the bolts are passed down through the holes thus aligned. This places the nuts on the down side of the connection thus facilitating assemblies and inspection of connections. The castellated nuts are then put on the bolts and drawn up tight, until the drilled hole in the bolt is visible through the castle of the nut. Then insert cotter-pin and spread the two leaves backward over the nut. This locks the nut in place. When the landing gear has been completely assembled to the fuselage, the tail of the machine should be elevated and supported by a horse and blocking until the upper longoron is level. This can be determined by placing a spirit level on the upper longeron at the tail or on the two engine-bed sills in machines where these sills are parallel to the top longeron, as in Curtiss JN-4B.
2. Horizontal Stabilizer.-After the upper longeron is levelled up, the horizontal stabilizer is as-
sembled to the tail of the fuselage. The horizontal stabilizer is fastened by means of bolts in the top longeron and the tail post. The nuts are all drawn up tight and cotter-pinned. The vertical stabilizer is next erected in place.
3. Vertical Stabilizer.-The vertical stabilizer is now fastened to the horizontal stabilizer, first by means of the bolt which passes up through the forward part of the horizontal stabilizer and then by means of the flexible stay lines running from the top of the vertical stabilizer. The forward bolt passes through the clip at the lower front point of the vertical stabilizer. Draw the nuts up tight and lock with cotter-pins. Flexible wire cables are attached to vertical stabilizer, and turnbuckles are used to align and tighten cables. The vertical stabilizer is further aided in its alignment by the bolt clip at its toe and by the double clip at its heel. This rear double clip passes over the two bolts which are attached to the tail post and which hold down the horizontal stabilizer.
4. Rudder.-The control braces are first attached to the rudder. These braces are so placed that the upper tips point toward the hinge line. In this fashion the holes will match up. The rudder is mounted on the tail post and vertical stabilizer by means of the hinges. The hinge pins are inserted in the hinges, and cotter-pins passed through the drilled holes in the bottom of the pins. The cotterpins should be spread backward as usual.
5. Elevators or Flaps.-These are first equipped
with the control braces which are also arranged so that the upper tips point toward the hinge line. The elevators are mounted to the horizontal stabilizer by means of the hinges and hinge pins. The hinge pins are kept in their bearings by the cotter-pins, inserted through the drilled holes in the bottom of the hinge pins.
6. Panel Assembly. The panels are now to be
assembled. Before the main panels can be con- ~-. nected to the fuselage, the engine section panel
must be erected.
Engine Section PaneL-The engine section struts are first set into place in their sockets on the engine section. Then the whole thing is lifted up to place and the four struts are set into their sockets on the upper longeron. The bracing wires are attached and the engine section aligned by means of them (see alignment).
7. Main Panels.-The main panels are now to be assembled to the machine. There are two methods for accomplishing this: first, assemble panels, struts and wires, before attaching to fuselage; second, assemble the upper plane to the engine section, and complete assembly. The first method is the most advantageous, since it permits the setting of the main panels at the correct stagger and dihedral, and does not require as much adjustment as the second method, which will be omitted.
Assembling Panels Together Before Fastening Them to Fuselage.-All the main struts will be found to bear a number. These numbers run from
1 to 8, on Curtiss JN-4. The numbers on the Standard run from 1 to 12 including the center section struts. The method used in numbering the posts is as follows: Starting at post No. 1, with the outer post, on the left-hand side of the pilot, as he faces his direction of travel, the posts are numbered successively from No. 1 to No. 4; Nos. 1 and 2 being on the left side and Nos. 3 and 4 being on the right side. The rear posts are similarly numbered from No. 5 to No. 8, Nos. 5 and 6 being on the left and Nos. 7 and 8 being on the right. This system of numbering does not include the engine section struts. The plan shows the system graphically (see Fig. 39).
The system of marking also insures that the struts are not inverted in their sockets. This is accomplished by painting the number on the strut, so that when viewed from the pilot's seat, all numbers can be read, i.e., the numbers are painted on that side of the strut intended to face the fuselage. If a strut is inverted by mistake, it can thus be quickly detected. The procedure of assembling panels is as follows:
1. The upper left-wing panel is first equipped with mast, by inserting the mast into its socket on the upper surface of the wing. The mast wire is then connected up to the clips to the right and left of the mast. Adjust the tension in this wire, by means of turnbuckles, until the spar becomes straight.
2. Stand the upper left-wing panel and lower left-
wing panel on their "leading" or "entering" edges, properly supporting the panels in cushioned blocks to prevent damage to the nose. Space the panels apart, at a distance approximately equal to the length of the struts.
3. Next connect up the diagonal cross wires. These must be loosely connected up, to permit the easy entering of the posts into the sockets. The wires are connected before the posts or struts are sets in place, since with the latter in place, the connecting of the wires to the lugs of the sockets is accomplished only with difficulty. After these wires are thus connected, insert the posts and bolts into place.
4. Connect up closely the "landing" (single) wires, and "flying" (double) wires of the outer bay to hold the wings together as a unit. The outer bay is thus completely wired, though but loosely.
5. The posts that are used for this left side are, according to the diagram, No. 1, No. 2, No. 5, No. 6. No. us the outer front; No. 2 is the inner front; No. 5 is the outer rear; No. 6 the inner rear.
6. The wings, as above assembled, are now erected to the fuselage. Extreme care should be exercised in transferring the wings to the fuselage, not to strain or break them. In carrying the wings, use wooden boards placed under the wings, and block up under the wing beams (which can be easily located), so that these take the strain of the load. Do not attempt handling assembled wings, using the posts as carriers; or by attachments to the trailing or leading edges. The wings should be
suitably supported temporarily by suitable sling at the outer upper post point (not beyond this point) or by a horse, properly blocked under lower wing at outer lower post point (not beyond this point) during fitting of wing to machine. The wings will have the approximate stagger if assembled as above, since the posts are in place, and the tension cross wires are adjusted to almost correct length when shipped. Insert the hinge pins through the hinges as now coupled up, lower hinges first.
The machine is now ready for alignment, perhaps the most important of the rigger's duties.
Alignment of Airplanes.-The proper alignment of a machine largely determines the flying qualities of that machine.
The alignment of the fuselage should be done at the factory or in the repair shop. However, the alignment of the whole machine depends upon the correctness of the fuselage. Directions for aligning and checking fuselage are, therefore, given.
The order in which the different parts of a machine should be aligned is as follows:
1. Alignment of landing gear.
2. Alignment of center section.
3. Alignment of leading edge.
4. Getting both wings the same height.
5. Dihedral angle, if any.
6. Alignment of trailing edge (angle of incidence).
9. Tightening and safe tying all wires.
10. Length of struts, positions and fittings, warp in planes.
11. Alignment of ailerons.
12. Alignment of stabilizer.
13. Alignment of elevator flaps.
14. Alignment of rudder.
The tail of the machine should be
the fuselage is nearly horizontal before starting alignment.
1. Alignment of Landing Gear.-When a is being assembled, it is easier to align the landing gear before the wings are put on.
Take the weight off the landing gear by supporting the fuselage on sawhorses.
The axle should be parallel with the lateral axis of the machine.
The center of the axle should be directly under the center of the fuselage. This can be secured by either of two methods:
(a) By Measuring Cross Distances.-Loosen and tighten the cross wires until the cross distances are exactly the same. Take all measurements from similar points on the fittings to which the wires are attached.
(b) With Level and Plumb Bob.-Level the fuselage crosswise. Mark the exact center of the fuselage and drop a plumb bob. Mark the exact center of the axle. Adjust the cross wires until the plumb bob is over the center of the axle. Tighten the wires until fairly tight, and safety them.
2. Alignment of the Center Section.-When assembling a machine, the center section should be aligned before the wings are put on.
When a machine is already assembled, the first thing to do is to loosen all wires except the landing wires. This is very important, for if one wire is tightened against another wire, an unnecessary and possibly a dangerous strain may be put upon some member. The bracing wires connecting tops of center section struts should be tight enough to hold the shape of the center section when bracing wires are tightened up.
(a) Machines Having No Stagger.-I n machines having no stagger, the struts of the center section should be perpendicular to the propeller axis. As the upper longerons are usually parallel to the propeller axis, they may be used as a base line.
Align one side of the center section first, then the other side, and lastly the front.
From a point at the lower end of one of the front
center section struts (the center of a bolt head for example), measure forward on the longeron a certain distance. From the same point (center of bolt head) measure back on the longeron exactly the same distance.
Move the upper end of the strut forward or backward by loosening one of the bracing wires and tightening the other, until the distance from the two points on the longerons to some point on the center line at the top of the strut (center of bolt head) are exactly the same. The strut will then be perpendicular to the propeller axis. Tighten both wires evenly until fairly tight. Measure the cross distances (the diagonal distances between similar points at the upper and lower ends of the front and rear struts), and align the other side of the center section until its cross distances are the same as those on the opposite side.
Align the front of the center section by loosening one cross wire and tightening the other, until one cross distance is exactly the same as the other cross distance.
(b) Machines Having Stagger.-In machines having stagger, the shape and position of the center section strut fittings usually determines the amount of stagger the machine was designed to have (Fig. 38-a). The JN-4 has lO 5/8-in. stagger, 'i.e., a plumb line dropped from the leading edge of upper panel should be lOx-in. from leading edge of lower panel.
Adjust the wires on one side of the center section
until the struts and that side are in their correct positions as shown by the shape of the fittings. Tighten the wires, measure the cross distances, and adjust the wires on the other side of the center section until the cross distances are exactly similar -to the first set.
A more accurate method is to drop a plumb line from the leading edge of the center section and adjust until the line is at the correct distance ahead of the point on the fuselage where the leading edge of the lower wing meets it. This point may be determined by measuring the distance from the inside of the front hinge to the leading edge of the lower wing and then laying off this distance on the body from the front of the hinge on the lower longeron. Better still, if the hinges are at the same distance from the leading edge on both top and bottom wings, the plumb line may be dropped from the front side of the hinge on the center section and the stagger measured back to the hinge on the lower longeron (Fig. 38-b). This has the advantage of setting the plumb line out far enough to clear the fuselage. Also the measurements are easily made.
Next, adjust the two front wires until one cross distance is exactly the same as the other cross distance (Fig. 38-c).
3. Alignment of Leading Edge.-(a) Upper Plane.-The leading edges of the upper and lower planes of one wing should next be made perfectly straight. By standing on a step ladder, placed 15 to 20 ft. to one side, and sighting along the leading
edge of the upper plane, any bow or warp can be easily seen. This should be straightened out by loosening or tightening the front landing wires,, The edge should be brought in exact line with the
leading edge of the center section. .. -.
(b) Lower Plane.-After the leading edge of the - -upper plane has been made straight, sight along the leading edge of the lower plane. If there is no warp I in the plane, this edge should also be straight.
(c) Align the opposite wing in the same manner.
4. Getting Both Wings the Same Height.-Place a small tack exactly in the middle of the leading edge of the center panel.
Measure from this tack to similar points at the lower ends of the intermediate and outer struts (Fig. 39). Make these distances the same on each side by raising or lowering one wing or the other, or by raising one wing and lowering the other wing, all the while keeping the leading edges of both wings perfectly straight.
5. Diliedral.-The method of setting the wings of a machine at a dihedral angle is as follows:
Place two tacks in the~ leading edge of the upper plane, one tack near the tip of each wing and exactly -the same distance out from the tack in the center section. Stretch a string tightly between the two outer tacks, until there is no sag in the string.
A dihedral angle of 1780 means that each wing has been raised 10. To set the wings of a machine at a dihedral angle of 178~ for example:
(a) Find the natural sine of 10 (0.0175).
(b) Multiply this by the distance in inches between the center tack and one of the outer tacks. The result will give the rise, in inches, of the string over the tack in the center section.
Raise the wings equally, keeping the leading edges perfectly straight, until the proper rise shows over the center section.
6. Alignment of Trailing Edge (Angle of incidence).-(a) Lower Plane.-The trailing edge should be brought parallel to the leading edge. This can be done by bringing the rear spar in line with the leading edge.
Stand squarely in front of the center of the ma-
chine 15 to 20 ft. away. Sight under the leading --edge of the lower plane; move forward or backward
until the fittings under the rear spar are just visible.
Raise or lower the trailing edge by loosening or -~ tightening the rear landing wires, until all of the
fittings on the rear spar appear equally under the leading edge.
(b) Upper Plane.-After aligning the trailing edge of the lower plane, place a ladder in front of the k~ center of the machine, and sight under the leading edge of the upper plane. If there is no warp in this plane, the trailing edge should align with the leading edge.
The objection to this method is that since there are no fittings next the body on the rear spar, there is room for considerable error in the angle of incidence.
Reversing the process and finding the angle of incidence at each set of struts secures the alignment of the trailing edge and removes the liability to error. To set wings at correct angle of incidence proceed as follows (Fig. 39): Place the airplane in rigging position, i.e., level up the top longeron or engine bearers. Set the corner of the straight -edge against the center of the rear spar, level up the straight-edge, and measure from the top of the straight.edge to the center of the front spar or to the lowest point of the leading edge. This must be done next the body and under each set of struts. (It is useless to make such a measurement between the struts because of possible warping of the wings.)
Unless the wings have a washout or washirt the measurements must agree, thus making the angle of incidence the same all along the wing. Then the trailing edge must necessarily be parallel to the leading edge.
7. Stagger.-The stagger should be the same all along the wing as it is for the center section. With the machine in rigging position drop a plumb line from the leading edge of the upper wing in front of each set of struts. The distance from the plumb line to the lower edge should equal the stagger. If there is too much, tighten the diagonal wire running from the lower rear socket to the upper front socket, being sure that the other diagonal wire is loosened somewhat. For too little stagger tighten the latter and loosen the former wire.
Check up the dihedral and alignment of the trailing edges to see if these have been disturbed while setting the stagger. If not, the droop may be put in.
8. Droop.-To correct for the torque of the propeller, one wing of a machine is slightly drooped.
In single-motored tractor types, if the propeller turns to the right, when looking from the rear, the left wing is drooped, and vice versa.
The outer rear landing wire of the wing to be drooped should be loosened until the trailing edge, between the outer and intermediate struts, appears to be about an inch (for machines of not more than 100 hp.) lower than the rest of the trailing edge. The practice with the Curtiss JN-4B is to loosen the
inner rear landing wire on the left Wing ~1 in. and loosen the outer rear landing wire j~ in. after the angle of incidence and stagger have been adjusted so that corresponding wires on the right and left wings are the same length.
9. Tightening and Safetying All Wires.-(a) After the wing is drooped, all flying wires should be tightened to the same tension, and just taut enough to take out all sag.
(10 Next tighten all drift or cross wires between the front and rear struts to the same tension.
(c) Drift wires from the wings to the fuselage, and from the wings to the landing gear, if any, should be tightened last.
(d) Safety all turnbuckles. A wire too loose will vibrate when the machine is in the air.
The flying and drift wires should be so tightened that when they take the weight of the machine in the air, there will be no sag in the landing wires.
10. Length of Struts, Positions of Fittings, Warp in Planes.-The above instructions are given for machines that are true, that is, machines having no bends, warps, or bows in the spars and leading or trailing edges.
(a) Similar struts should be of the same length.
(b) Similar fittings occupying similar positions should be spaced the same. If difficulties are encountered in getting the measurements to tally, check up the lengths of the struts and the positions of the fittings.
(c) If the planes of a machine are warped,, the machine should be so aligned that the warp is equally divided between both planes.
11. Alignment of Ailerons.-Before aligning ailerons, place the shoulder yoke or wheel controlling the ailerons in the center of its path of movement.
(a) Trailing-edge Ailerons.-Trailing-edge ailerons should be set ~1 inch lower than the trailing edge of the plane to which they are attached.
(b) Inter plane Ailerons.-Interplane ailerons should be set so that they are both in the same plane, when in neutral position.
In machines having interplane ailerons, nose heaviness and tail heaviness may be corrected by setting the trailing edges of the ailerons up or down.
The proper amount to raise or lower the trailing edges can be determined only by experimenting with each particular type of machine.
(c) The control wires should be just tight enough to eliminate any lost motion.
12. Alignment of Stabilizer.-Support the weight of the tail on the tail skid.
The rear edge of the stabilizer should be perfectly straight, and should be parallel with lateral axis of the machine.
Stand behind the center of the stabilizer, and align its rear edge on the leading edge of the upper plane by sighting. Tighten wires and safety turnbuckles.
13. Alignment of Elevator Flaps.-Set the elevator control in its mid-position. Adjust the elevator