(PART 5 OF 5 PARTS)

The By Robert W. Bushby, EAA 26 (EAA Designee 20) 848 Westwood Dr., Glenwood, 111. STABILIZER CONSTRUCTION

The primary structure of the stabilizer is the rear spar assembly. This consists of the two spar sections and the two spar reinforcement pieces. Cut the two rear spar sections from .032 in. 2024-T3 aluminum according to the

drawings. The top and bottom spar flanges are bent through an angle of 93 deg. so that they will align with

the slope of the stabilizer ribs at that point. The right and left spar sections are identical. The two stabilizer rear spar reinforcements are cut from .125 in. thick 2024-T3 aluminum. In addition to carrying the stabilizer loads, these reinforcements are also used to splice the two spar sections together. Position the two stabilizer rear spar sections on the work bench, being certain that the spar centerlines are in

alignment. This alignment can easily be checked by stretching a string above the spar and sighting to be certain that the spar centerlines at each end and at the center are directly beneath the string. Position the two spar reinforcements securely to the spars so that they will not shift, and proceed to drill the rivet holes through both the reinforcements and spar in one operation. As the rivets we are using are Vs in. dia., a No. 30 drill should be used. Locate and drill the six holes for the stabilizer-attaching bolts at this time, using a No. 12 drill. At

the time of assembly, these holes will be reamed, if necessary, for the AN3 attaching bolts.

The vertical and horizontal stabilizers are fitted to the fuselage at this stage.

Locate the stabilizer outboard hinge bolt holes, position the .032 in. aluminum reinforcing pieces on the front side of the spar, and drill the required holes through all of the pieces in one operation. The AN366-1032 plate nuts are installed, as described in a previous article. The spar pieces should now be disassembled and all burrs and chips removed prior to riveting of the assembly. Rivets used in assembling the stabilizer spar can be either AN430AD4 or AN442AD4 rivets. The stabilizer front spars are fabricated from .032 in. thick 2024-T3 aluminum. Use a bending brake to form all flanges 90 deg. The stabilizer butt and tip ribs, of .025 in. 2024-T3 aluminum, are formed in the same manner as was used in forming the wing ribs. The two stabilizer forward attachment fittings are fabricated from .065 in. thick 4130 steel sheet. These steel fittings should be thoroughly painted so that on assembly there will be no direct contact between the steel and aluminum parts. As the location of the forward attaching brackets must be in alignment with the plate nuts which have previously been attached to the fuselage bulkhead, they will not be riveted to the ribs at this time. The two stabilizer skins are cut from .032 in. thick aluminum sheet and are preformed in the same manner as was used for forming of the wing leading edge skins. With all of the components now complete, we can proceed with the assembly of the stabilizer. To eliminate the need for exacting hole location and alignment, to insure proper fit, and to eliminate the need for any fixtures, the stabilizer will be assembled directly on the fuselage. To facilitate this alignment, the fuselage should be in the fuselage assembly jig and positioned in a level attitude, both laterally and longitudinally. The positioning and alignment of the stabilizer rear spar has been described previously with the fuselage construction, and will have been accomplished at the time of fuselage construction. The first step in assembly of the stabilizer is to attach the tv.-o butt ribs and the two tip ribs to the stabilizer rear spar. The butt ribs are attached with four Vs in. dia. structural type rivets, and the tip rib by one similar rivet. The stabilizer front spar can now be riveted in place on the two ribs. This spar and rib assembly is now bolted in position on the fuselage at bulkhead Station 177.5. Since the stabilizer will be removed later, and to prevent wearing of the nylon insert of the plate nuts attached to bulkhead 177.5, it is recommended that short bolts with washers be used for this initial attachment of the stabilizer spar. Using the previously mentioned short bolts, the stabilizer forward attach fittings can be fastened in place on fuselage bulkhead 163. Clamp the stabilizer butt ribs in place on these fittings and drill the four No. 30 holes (Continued on next page)

SPORT AVIATION

29

MIDGET MUSTANG . . . (Continued from preceding page)

through both the rib and fitting. Only the rivet holes located in the web of the rib are drilled at this time. The fitting can now be unbolted from the fuselage and the rib sprung away far enough to allow these rivets to be installed. After installation of these rivets, the fitting is again bolted to the fuselage and we are ready for the attachment of the stabilizer skins. Position the desired stabilizer skin (either right or left) on the structure. Check to determine that the tip rib is longitudinally level and clamp the skin securely to the rear spar. When clamped in position, the skin will maintain the tip rib in position. However, to guard against accidental misalignment, it is advisable to clamp a support from the floor to the leading edge of the rib. Lay out all rivet locations and proceed to drill the holes. All rivets used for attaching the stabilizer skin are AN426AD4-4.

Rivet spacing is 2 in. throughout except that three rivets should be used through each flange of the forward attach fittings. Location of the rivets in relation to the rib and spar centerlines is determined in the same manner as was previously described for the wing leading edge installation. After all the rivet holes have been drilled, the skin should be removed for deburring and dimpling of the rivet holes. Following the deburring and dimpling operations, the skin is positioned on the stabilizer structure and attached with skin fasteners. We are now ready to begin riveting. The following riveting sequence will prevent misalignment of the structure and eliminate any hard to get at rivets. As the stabilizer rear spar flanges face to the rear, the riveting will be easily accessible here. Therefore, we will install all rivets in the top flange of the rear spar first. The next rivets to be installed will be those in the tip rib top and bottom flanges forward of the stabilizer front spar. With the previously mentioned

floor to tip rib support in place, the skin fasteners attaching the skin to the rear spar bottom flange, and to the butt and tip rib lower flanges aft of the front spar, are removed. This will allow the bottom skin to be bent down sufficiently for riveting to the front spar. Install all rivets in the front spar top flange first, then the rivets in the bottom flange. Reattach the lower skin to the structure with the skin fasteners, and complete the riveting of the

A common rib-blank layout is accomplished by using a form block and marking guide.

tip rib and rear spar lower flange. The opposite stabilizer skin is attached in the same manner. Fabricate the two elevator stops from .125 in. 2024-T3 aluminum. These stops are riveted to the outside of the stabilizer rear spar top and bottom flanges. At the time of final assembly, the width of the stops is adjusted to

The small squares of paper aid in locating blind holes.

regulate the elevator travel. The stabilizer assembly is now removed from the fuselage for installation of the rivets in the butt rib. The riveting done prior to removing the stabilizer is sufficient to prevent any change of alignment while installing the final rivets in the butt rib. VERTICAL FIN CONSTRUCTION

The fin assembly consists of only six parts, and its construction is identical to constructing one-half of the stabilizer.

This is how the airfoil profile templates and straight-edge are used in aligning control-surface trailing edges. 30

AUGUST I960

CONTROL SURFACE CONSTRUCTION All control surfaces of the "Midget Mustang" are of similar construction. Each surface consists of one spar made of .032 in. 2024-T3 aluminum, end ribs of .025 in. 2024-T3 aluminum, two skin pieces of the appropriate thickness, several stiffening angles, and an edge reinforcement. Hinge and control fittings are fabricated of 4130 sheet steel.

control surface skins with AN426AD3-4 rivets with 2 in. spacing . . . this is a 3/32 in. dia. rivet requiring a No. 40 drill. The control surface edge stiffener (Piper Part No. 17058-04) is a bulb section similar to one-half of a piano hinge. The joggle formed at the trailing edge of the control surface is to accommodate the rolled portion of the stiffener with its leg extending between the control surface skins at the trailing edge and tip. Because the stiffen-

er member cannot be bent to the corner radius, it is necessary to install it in two sections . . . a trailing edge and a tip piece. To stiffen the corner area, a 4 in. length of 3/16 in. dia. wire is bent to match the bend radius and installed so that its ends extend into the two stiffening members. To provide a uniform riveting surface at the corner, a filler piece made of .032 in. aluminum is inserted between the skins at this point. ELEVATOR CONSTRUCTION

These skin-stiffening angles are

The right and left elevators are identical and interchangeable. Cut the four elevator skins from .020 in. 2024-T3 aluminum. The skin leading edges ahead of the spar are bent inward 40 deg., this being performed on a

installed on the top flap skin.

The skin stiffening angles are used instead of ribs to reduce the total weight of the assembly and to simplify the construction of the control surface. These skin stiffening angles arc M> by y> in. angles of appropriate length made up from the same material as used for the skins. To prevent chafing or interference with the opposite

skin, the aft 4 in. of the vertical leg of the angle must be tapered off. These stiffening angles are riveted to the The elevator-control fitting is shown in detail.

The control-surface trailing-edge s t i f •

fcner corner detail is illustrated here

. . . note the piece of wire which joins

the two stiffening members.

bending brake. It is necessary to form a joggle at the skin trailing edge to accommodate the edge stiffener. This joggle is formed by passing the metal through the dies of a hand rolling machine. This hand roller is available at any sheet metal or furnace shop. The direction of the trailing edge joggle and the leading edge flange will determine whether or not the skin will be for top or bottom installation. The elevator spar is cut from .032 in. 2024-T3 alumi-

num and is fabricated on a bending brake by bending the flanges through an arc of 87 deg. Attach the plate nuts and backing plates for attachment of the elevator outboard hinges in the same manner previously described for (Continued on next page)

The fin and rudder lower portion details include the tai (wheel arm.

steering

This method of clamping the elevator trailing edge will maintain alignment during the riveting operation. SPORT AVIATION

31

MIDGET MUSTANG . . .

FLAP CONSTRUCTION

(Continued from preceding page)

Cut the flap skins from .020 in. 2024-T3 aluminum and form the trailing edge joggle. As this joggle does not extend around a corner as was the case with the rudder and elevators, it may be formed in a bending brake. To eliminate a gap between the wing skin and flap when in the down position, the leading edge of the top flap skin is rolled inward. To form this roll, place the flap skin on the bench with the leading edge protruding past the edge of the bench. Clamp the skin to the bench by means of a board placed with its edge at the spar flange position. Clamp another board at the skin forward edge and form the roll by exerting pressure on this board. Attachment of the skin stiffening angles and the two corner gussets will complete the skin assemblies. Attach the flap control fitting to the inboard end of the spar. This fitting also serves as a corner reinforcement and attaching member for the butt rib. The outboard flap rib is attached with an angle formed from .032 in. aluminum. Position the lower flap skin and piano hinge section in place on the flap spar. The hinge section is installed between the spar and skin with its bulb section inward and just clear of the spar flange. Check to be certain that the piano hinge is properly indexed with the hinge section attached to the wing so that when assembled the flap will be in proper position. The flap skin and

plate nut installation. The elevator rib, of .025 in. alumi-

num,

is also formed on a bending brake. Following completion of the component parts, the first step in assembling the elevator is to lay out the position of the spar on the inside of one of the skins. The locations of the skin stiffeners are laid out next, and their length measured. The stiffener should extend from Va in. aft of the spar to Vfe in. ahead of the trailing edge joggle. These skin stiffeners are fabricated from the .020 in. scrap remaining from the skin material. Position the stiffeners on the skin and drill the rivet holes through both the skin and stiffener in one operation, using a No. 40 drill. Rivet spacing is 2 in., with one rivet located Vi in.,

from each end of the stiffeners. The pieces can now be deburred and dimpled in the usual manner. The stiffeners can now be attached to the skin with AN426AD3-4 rivets. The elevator control attachment assembly is now riveted in position on the inboard end of the spar. The tubular member of the control fitting must be in alignment with the elevator spar centerline. The fitting is attached to the spar with four AN430AD4 rivets on each edge. This fitting is also the attaching member for the elevator rib. Position the rib so that it is perpendicular to the elevator spar, with the spar and rib centerlines indexed, and attach it to the control fitting with eight AN430AD4 rivets. The elevator skins can now be attached to the elevator spar and rib assembly, using AN426AD4-4 rivets with 2 in. spacing. Riveting up of the trailing edge and incorporating the edge stiffener will complete the elevator. Rivets used at the trailing edge are universalhead AN470AD3-4 with 2 in. spacing. The simplest method of assuring a straight trailing edge is to position the skins by clamping a board parallel to the trailing edge. This board should be just far enough forward so that it does not interfere with the riveting operation.

rivets spaced 2 in. apart. When attaching the skin in the area of the corner gusset, place one rivet through the skin and gusset \'2 in. from the end of the gusset. This will locate the next rivet 1% in. from the end of the gusset and eliminate any unsightly joggle that might otherwise occur. The top flap skin is attached in a similar manner. Do not rivet the flap skins to the ribs, or rivet the trailing edge at this time. To assure proper alignment of the trailing edge, the flaps and ailerons are completed on the wing. This procedure is described following the section detailing aileron construction.

RUDDER CONSTRUCTION

AILERON CONSTRUCTION

Construction of the rudder is the same as that of the elevator except for the bottom end detail. The rudderspar bottom end extension, which contains the attachment fittings for the rudder cables, lower hinge and tailwheel steering arms is fabricated from .050 in. 4130 steel sheet. The flange of this part can be formed without too much difficulty by heating the area to a dull red heat and forming over a steel forming block. A steel hammer will be used in this forming operation, and the excess material will be removed by stretching it beyond the % in. flange width. After cooling, the excess material will be trimmed off and the flange dressed down to the proper width. An alternate method of constructing the piece is to weld a '2 in. wide strip of steel to the edge of the web member. The lower hinge and rudder cable brackets are welded in place on the spar bottom extension. The tailwheel steering arm is fabricated of .090 in. steel and attached with three AN3-3 bolts. After thoroughly painting the assembly with zinc-chromate or other primer, it is

The right and left ailerons of the "Midget Mustang" are identical, and therefore interchangeable. Their construction is similar to that of the flap and will not be detailed here. All weight aft of the aileron hinge line must be balanced by weight ahead of the hinge line. To keep this weight at a minimum, the aileron skins are made of .016 in. aluminum. The aileron counter-weight assembly is simply a 28 in. length of 3/i in. dia. steel tubing welded to

hinge section are attached to the spar using AN426AD4-4

the two brackets fabricated of .090 in. 4130 steel. Attachment of the counterweight brackets to the aileron spar is

done with four AN23-8 clevis bolts and AN366-1032 plate nuts. A doubler piece of .032 in. aluminum is used on the aft side of the aileron spar when installing the plate nuts. Aileron balance is obtained by filling the counterweight tube with lead until the desired balance is reached. Although tests have shown that the "Midget Mustang"

riveted to the rudder spar with AN430AD4 rivets. Only

those attaching rivets located in the spar web should be installed at this time. The flanges of the rudder spar and bottom extension are riveted at the time of skin attachment. Because of its compound curvature, the rudder lower skin section is most easily fabricated of fiberglas. Two layers of 7-oz. glass cloth will be adequate. This fiberglas section is attached to rudder rib and lower spar section with NAS 395-16 Tinnerman nuts and sheet metal

screws. 32

AUGUST 1966

Aileron will balance in this position

is free from flutter at speeds up to 240 mph with 75 percent balance, it is recommended that the ailerons be balanced to 100 percent. To check the balance, the aileron must be mounted in a level attitude on a set of knife edges or bearing blocks so that it is free to rotate about the mounting bolts. When using a pair of knife edges for balancing, they must be mounted absolutely level and parallel. Pour the estimated amount of lead into the tube, mount the aileron in the balancing fixture, and observe its static position. When 100 percent balance is obtained, the aileron will balance with its chord line in a horizontal position. Ailerons to be painted must be balanced after painting. When painting the aileron, a minimum

amount of paint should be applied to keep the weight down. On aircraft operating in the higher speed ranges because of high horsepower engines, it may be desirable to balance the ailerons to 110 percent . . . slightly nose

heavy. Although this style of aileron balance requires a greater weight than a balance mounted on a longer external arm, it has the advantages of more uniform weight distribution and is aerodynamically clean as the entire balance assembly is contained within the wing. AILERON AND FLAP TRAILING EDGE ALIGNMENT

Because of the 2V» deg. washout of the "Midget Mustang" wing, the ailerons and flaps must be constructed with this same amount of "twist" so that they will form a straight trailing edge. The following method is a simple way to obtain the desired trailing edge alignment. Construct a pair of airfoil profile templates by cutting the wing root and tip airfoil shapes from a piece of plywood. Mount the aileron and flap in position on the wing and slip the profile boards in place. This will properly position the flap root and the aileron tip. A board or other suitable straight edge is now clamped in place spanwise just forward of the aileron and flap trailing edges. As the control surface trailing edges will be riveted while held in position in this manner, it is necessary to place the straight edge approximately 2 in. ahead of the trailing edge so as not to interfere with the riveting operation. With the aileron and flap trailing edges thus held in proper alignment, the trailing edges can be riveted up while incorporating the trailing edge stiffener. The rivets used here are AN470AD3-4 with 2 in. spacing. After

completion of the trailing edge riveting, the surfaces will be rigid and can be removed for the wing to complete the riveting of the skin to the end ribs. The riveting of the control surface trailing edges can be performed with the wing mounted in the construction jig, or with the wing lying flat on a pair of horses. If

they are applicable, with slight modification, to the construction of any aircraft of all-metal design. The construction of such items as the engine mount, landing gear and control system components are the same for the all-metal airplane as for airplanes of other type construction, and therefore have not been included here. CONCLUSION

There are several labor saving techniques which, due to their generalized nature, were not included in the previous articles. This series will, therefore, be concluded by describing some of these techniques. The first deals with the layout of the metal blanks from which the ribs and bulkheads are formed. Nearly all of the drawings available to the homebuilder provide a full-size layout of the wing rib and bulkhead sections. The form blocks for these items are most easily constructed by gluing the drawing directly to the form block material, then cutting and sanding the piece to the line. To lay out the metal blank, a marking jig is built from a scrap of heavy gauge material. I have found that a V» in. thick piece works fine. Cut a square from this material, with a side length equal to twice the desired flange width. In the center of this piece, drill a hole the diameter of which will accept the point of the pen or scribe that is to be used in marking the metal blank. (A scribe can be used for this layout as the line is a "cut" line. The scribe must not be used to mark other lines). Lay the completed

form block on the aluminum sheet and, with the marker pen inserted in the hole of the marker jig, run the jig around the form block, thus marking the trim line of the metal blank. By constructing a square marking jig, you need not be concerned which edge is toward the form block when using it. To obtain a tight edge when riveting up a skin

seam, such as the fuselage skins or wing spar joint, it is helpful to break the edge of the sheet slightly. A break of 5 to 10 deg. and 3/32 in. deep is sufficient for this purpose. Such a narrow bend is difficult to make on a bending brake, but can easily be made by forming the sheet over the edge of a straight board with a mallet. Light taps with the mallet should be used for this operation so that the aluminum will not be stretched. The edges of all aluminum pieces should be dressed with a file or abrasive paper to remove all nicks and tool marks. Corners of all cut-outs should have at least a 3/32 in. radius. This is especially important where the metal is to be bent or formed. A small-diameter chain saw file and a round-edge mill file work very well for dressing these corners and notches.

An inexpensive rivet cutter can be made from a pair of common slip-joint pliers. With the pliers partially

the operation is done with the wing lying flat, it is suggested that the straight edge piece be clamped to the underside of the wing so that any sag can be removed by means of a prop from the floor. The profile templates used here will again be found useful for positioning the ailerons and flaps when rigging the controls at final assembly. SUMMARY

The primary purpose of this series of articles has been to acquaint the homebuilder with the techniques involved in the construction of an all-aluminum airplane.

These techniques and procedures, developed and proven during the 14 years that I have been associated with the development and construction of the "Midget Mustang", will make the all-metal aircraft the simplest type for the

opened, simply drill a hole of the desired rivet diameter

inexperienced builder. Although the methods described

the same length, washers can be placed under the rivet

herein have dealt specifically with the "Midget Mustang",

a short distance behind the pivot screw. The shearing action when the pliers are closed will produce a square cut of the rivet. When it is required to cut several rivets of (Continued on bottom of next page) SPORT AVIATION

33

From The . . .

Designee File HE FOLLOWING comes from Cecil Goddard, Designee T No. 63, of Alida, Saskatchewan, Canada: Many builders have found that sash and door factories use Sitka spruce, so they pick this over. Some of the spruce is O.K., but much of it has the serious fault as shown in the accompanying diagram. Even an expert can't detect this fault by sight. The best way to tell how the grain was cut is to purchase an extra length of it and perform a two-way split test to see how it breaks. Also, squirting ink from a fountain pen onto the corner and watching how it soaks in will help determine the grain's direction. If this slant cut wood is used in a spar, the builder is asking for deep trouble. The best insurance is simply to purchase all spruce from an aircraft wood supply source. It's worth it. Your airplane, and your life, depends upon the structural soundness of the lumber inside.

Gilbert Hausler, Designee No. 126 of Phoenix, Ariz., warns that using surplus instruments as is, without having them checked out in a shop is a dangerous practice. In struments should always be calibrated before use! Richard R. Blair, Designee No. 29, of Vincentown, N.J., has these suggestions for welding: When welding bushings or similar items that require bolting during tacking or welding, the problem always exists of removing the alignment bolt. This problem can be solved by using "All-Thread", a continuously threaded rod available at your local hardware store in 2-foot lengths for about 8J cents for a % in. diameter piece. With a nut on each end, it aligns as good as a bolt, and when welding is completed it can be driven out very easily, or screwed out. Another suggestion from Designee Blair is: When welding the many brackets and fittings, it is good to clean, inspect and chromate them as soon as possible. The life of the small chromate pressure cans is usually limited, due in most part to clogging from not being cleaned or being cleaned so many times the propellant is exhausted before the paint. The answer here is to fill the paint can cap with dope thinner and store the can

MIDGET MUSTANG . . . (Continued from preceding page)

head as spacers to regulate the length. The use of diagonal cutting pliers to cut rivets is undesirable because of the uneven end left on the rivet shank. When the aluminum sheet must be cut out with tin

snips and a straight edge free from waviness and distortion is desired, the piece should initially be cut out approximately '/4 in. oversize. Upon cutting to the final shape, it will be easier to follow the line. The narrow piece being removed will curl easily, thereby eliminating any stress or distortion in the finished part. When installing new cowlings, fairings, etc., it is sometimes necessary to transfer the location of an existing screw hole in the airframe to the piece being installed.

The following technique will eliminate the need for exact34

AUGUST 1966

AN AOOIDENT IN

THE.

MAKING

BOARD CUT ON SLANT FROM LOG

ALL GRAIN LOOKS PERFECT.

BY

O.E.BROWNE EAA 2349

upside down by inserting the can into the filled cap. Thb is better for cleaning the nozzle than spraying whi'.e th? can is upside down, as it makes the life of the can much longer. In a pinch, water would do for the cap as its main purpose is to keep air from drying out the nozzle.

Fourth Western EAA Fly-In To Be Held October 1 and 2 HE EAA WESTERN Fly-In Association is sponsoring

its Fourth Western Fly-In which will be held the first T weekend in October, the 1st and 2nd. It will take place at Fox Field, Lancaster, Calif. The purpose of the fly-in is for the education and entertainment of all members of the EAA and anyone interested in the "grass roots" of flying. The program, under the direction of Pat Day (Chapter 40) will include spot landing contests, balloon bursting, toilet paper cutting, navigation contest and many more for those flying their homebuilts. Invitations have been extended to the Rotary Wing Associations, Soaring and Antique enthusiasts. The date listed is firm and will not be changed. Further information can be obtained by contacting Russ Earnhart, 925 E. Whittier Blvd., La Habra, Calif. 90631, phone AC 213-697-3758 or 213-723-8851.

ing measurements which may be difficult to obtain on some of the curved parts. Tape a piece of paper to the structure approximately one inch above the hole, with

the paper hanging down to cover the hole. The hole is then located on the paper by punching with a sharp pencil or punch. The piece to be marked is then positioned

on the structure, with the paper flaps outside. The hole location can now be marked from the paper templates. So, there you have it! These techniques, if followed from layout to final assembly, will assure you of the simplest and most durable of aircraft construction. The beautiful thing about all this is that when the piece is removed from its jig, it's done! It requires no further covering or painting as do other types of construction,

thereby further simplifying the process and reducing the costs. ®