Homebuilt Project — Construction Article Extras — de Havilland DH90 Dragonfly

Jul 22, 2014 No Comments by

As a bonus for the upcoming construction article in the November issue of Electric Flight, here is the full-length construction notes from designer Ivan Pettigrew. With limited pages in the print magazine, we are pleased to offer this fully detailed version of Ivan’s article for our online readers to enjoy. All the construction photos are included in the Gallery section.

Plans will be available soon at: www.airagestore.com

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Text by & Ivan Pettigrew Scott Copeland

The De Havilland Aircraft Company built a series of multi engine biplanes in the nineteen thirties and the D.H. 90 Dragonfly was the last of this series.  The first design was the DH 84 Dragon which used two four cylinder 130 HP Gypsy Major inline inverted engines.  These engines were built by a division of the De Havilland Company, and used in many of the “Moth” series of single engine planes built by De Havilland during that era (including the much loved DH 82 Tiger Moth).  The DH 86 Express was next, powered by four Gypsy Six engines.  The same cylinders and pistons were used as in the Gypsy Major engine, but a longer crankcase was built to produce a six cylinder engine rated at 200 H.P.  The DH 89 Rapide was the third multi engine biplane, utilizing two six cylinder Gypsy Queen engines, later versions of the Gypsy Six.  Having tapered wings and more horse power than the DH 84 Dragon, the Rapide was much faster.  More Rapides were built than all the other D.H. multi engine biplanes combined.  Last but not least was the Dragonfly.  It was in some ways like a scaled down Rapide.  It used the four cylinder Gypsy Major engines and carried four passengers in addition to the pilot.  Sixty eight Dragonflys were built, starting in 1935. Unlike earlier D.H. biplanes, the Dragonfly had a longer top wing than lower one.  Ailerons were only fitted on the top wing.  The centre section of the bottom wing had a thicker airfoil than the outer panels.  The thicker wing section added strength to this critical area which carries the stress of the undercarriage, and also provided more space for the fuel tanks in the full scale plane.

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Two are still flying today, G-AEDU in England with a red fuselage and engine nacelles. The wings and tail are silver.  ZK-AYR in New Zealand is blue and silver.  G-AEDU has the possible distinction of being the oldest plane to fly the Atlantic.  In 1995 it flew from the UK to Oshkosh, and the following year marked its sixtieth anniversary making the return flight to England.

Gallery > DH90 Dragonfly

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FUSELAGE

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In most construction, I have found regular carpenter glue to be the most satisfactory adhesive.  It gives time to set the parts in the correct position, and when building a box structure as used in a fuselage like this, carpenter glue is much stronger than CA glues for attaching crosspieces where the end grain is being glued.  The primary section of the fuselage of the Dragonfly is of traditional “box” construction.  The two sides of this structure are built over the shaded sticks shown on the plan, and then joined together.  Notice that the cockpit area is detachable to give access to the motor batteries.  It is best built last, when the rest of the fuselage has been completed.  The top and bottom bulkheads are added to the crosspieces in the primary box section of the fuselage, then sheeting is applied forward of fuselage station #5.  Aft of the trailing edges of the wings, the square section of the fuselage transitions to having a rounded top and bottom.  The radius of the curve at the corners is very small in the first two bays behind the wing.  Instead of sheeting these sections, on option is to fill in with solid balsa shaped to fit.  Other additions are the doublers that add strength inside the wing saddles, and the stringers inside the fuselage sides that support the cross pieces which hold the servo mounts.  The platform for the motor battery is shown.  It is made longer than required to provide for movement of the battery fore and aft to achieve the correct CG location.    The exact location of the battery will depend on several factors such as the weight of the battery and motors used, and also the weight of the tail. The removable cockpit area can be built in place, pinned to the fuselage to get a good fit.  It should be separated with very thin plastic like saran wrap so that the cockpit section does not get glued to the fuselage.  The lower horizontal section of the removable cockpit section has a compound curve.  To get a good fit, it is best to carve this from an oversize balsa stick. The tail cone is built up from scrap balsa and cannot be completed until after the tail surfaces are attached.

TAIL SECTION.

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Be careful not to overbuild the tail surfaces.  With a long moment arm, weight of the tail surfaces is critical and although the motor battery may be moved forward for proper CG, lighter flies better!  Notice that the spars for the stabilizer and fin are tapered, being quite wide at the root in order to give added strength.  Having a relatively thick cross section like this means that the construction can be very light, but remains strong.  Start construction by first making the spars and fitting the hinges.  Build the surfaces over the plan in the conventional manner.  Although the stabilizer spar is built in one piece, it is necessary to build just half of the stab at a time due to the taper of the spar.  After building the first half, prop the tip up a little so that the bottom surface of the spar on the other half is flat on the building board.  Be aware that because the spars and ribs are tapered, the tail units cannot be built with the lower surfaces pinned right down to the building board, or else they will end up warped.  Check this during assembly because it may be necessary to slide some of the parts up the pins slightly so that no warp is built into the surface.  The section that has to be slid up the pin the most is the tip of the leading or trailing edge where is joins the last rib.  Tail surfaces built in this manner with a deep chord are resistant to twisting.  Do not rely on the shrinkage of the covering to reset the surface if it has been build with a warp in it.  If a tail surface is warped, dampen the structure and weight it down while drying overnight.  A couple of attempts at getting it straight may be necessary

The horizontal stabilizer is held in place by inserting it into the tapered slot formed in the construction of the fuselage sides.  It must be glued in place before the fin is attached.  The spar of the fin continues down to the bottom of the elevator.  To give it maximum strength, the base of the fin spar should be glued to the beams on the side of the fuselage that support the elevator.  A shim or wedge may be necessary to close any gap between them.  The rudder should not be attached to the fin until the wire connecting it to the tail wheel has been installed.  This step requires some De Havilland ingenuity.  After bending the tail wheel wire to shape, insert it from the bottom, threading it through the rudder control horn which is made of brass sheet.  It can be soldered to the wire arm when everything is in place.  It may be necessary to leave the top end of the wire straight until it is in place, then make the 90 degree bend at the top after it has been installed.  The final step is sliding the rudder on to the hinges and the wire steering arm.  If using nyrods, be certain to brace them well at the stations along the fuselage so there is no “bowing.”  Check that there is clearance for the full movement of the tail surfaces without the trailing edges clashing with adjacent surfaces.

 UPPER WING

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The top wing is the easier one to build because the spar is straight all the way from the centre of the wing to the tip.  Cut the main spar for the top wing from 3/32 inch sheet balsa.  Glue spruce strips to the top and bottom edges of the wing spar as shown in the plan.  If spruce is not available for these strips, bass or other softwoods that are stronger than balsa would be suitable.  Join together over the plan so that the sweep back and dihedral angle are correct.  This joint is very important both for accuracy and strength.  The spar must be held perpendicular to the building board while measuring the dihedral angle or it will not be correct.   Tapered spruce or basswood doublers must overlap the joins in these strips at the center line.

Basically all ribs are cut from 1/16 inch sheet unless otherwise indicated.  It is good to cut ribs first to the outside line shown on the plan.  Where are ailerons in the upper wing, cut the ribs full length (the ailerons will be cut out once the wing structure is built).  Cut all the ribs into two pieces at the point where they will be glued to the spar.  Trim 1/16” off the top and bottom edge of nose ribs to allow for the sheet covering between the main spar and leading edge, a task that is easily accomplished with a balsa stripper.  Aft of the main spar, trim 1/16 inch from the edge of the ribs where sheeting is inset, such as at the wing center section, and where the plates are located that hold the strut mounts in place for securing the interplane wing struts.

Assemble half of the upper wing (e.g. left half) flat on the workbench with the other half of the spar blocked up a little to give the correct dihedral angle.  Start by pinning the spar in place on the plan, but use 1/16 inch shims to keep it a little clear of the building board and allow for the 1/16” sheeting that will be attached to the bottom of the spar.  Attach the rear section of the ribs to the rear face of the main spar and glue all of the trailing edge in place.  Now attach the nose ribs to the front surface of the spar, making sure that a space of 1/16 inch is left at the bottom of each rib to allow for the sheeting on the lower surface.  At this point the 1/8 inch inner strip of the leading edge can be attached to the front of the ribs.  The wings tips can now be built.  When this half of the wing panel is complete, block up the tip so that the spar of the other half is flat on the workbench, and assemble the second half of the wing.

Sheeting is now applied from the leading edge to the main spar, but ONLY to the lower surface of the wing.  There is no washout in the inner part of the wing.  It only starts from the point where the ailerons start.  When the wing is weighted down with a 3/16 inch block under the trailing edge at the tip to give the correct washout to the outer section, apply sheet covering to the top of the wing from the leading edge to the spar.  It is very important to weigh each wing panel down on a surface that is perfectly flat while applying the sheeting to the top surface.  Once the top sheeting is in place, the wing will be quite rigid and warps will be difficult to correct.

After applying this sheeting to the top surface, the remaining leading edge strip is cut from ¼ inch sheet balsa and glued to the 1/8 inch strip already in place. It is then contoured to shape.  Notice that this outer strip does not pass through the area of the center section where the wing joins the fuselage. The leading edge of the outer section of each wing should be well sanded to give a blunt rounded curve.  Check that there is no twist in the wings except for the wash out, and that each side has the same angle of incidence.  If there is any undesired warp in the wing, it is important to correct it at this stage before it is covered.  Dampen the sheeting between the spar and leading edge, and weight the wing down overnight so that it sets without any twist.

AILERONS

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The aileron design used in this model is known as “Frise.”   This design provides good control balance at slow speed and helps avoid the bad effects of adverse yaw that results from poor aileron design.  To complete the ailerons, insert the secondary spars in the top and bottom surfaces of the wing where the ailerons will be attached.  Make a diagonal cut in each rib where the aileron will be separated.  Also cut through the trailing edge at the end point of each aileron so that is can be removed.  A strip of 1/16 inch sheet balsa is now attached to the rear surfaces of the secondary spars and ribs where they were cut.

To get a good fit for the ailerons, they are best built in place.  Cut a piece of 1/8 balsa that will form the leading edge (spar) of the aileron and hinge it in place to the upper secondary spar.  It may be best to make this strip a little wider than estimated, and later trim to the right width so that it matches up with the surfaces of the wing.   If the wood of the secondary spars is on the flimsy side where the aileron hinges are inserted, add a small doubler to the front of the spar.  Be sure to pin the aileron hinges with toothpicks.  This is done after the tongue of the hinge is inserted in the balsa slot by drilling a 1/16” hole in the middle of the tongue area.  Then push a toothpick through and apply a drop of glue.  Cut off the ends of the tooth pick.  Pinning the hinges this way will ensure that they don’t come lose.  “Hinge gap” covering on the top surface of the wing will further ensure that hinges do not come out.

Now the aileron trailing edge with attached ribs is glued to the 1/8’ spar that forms the leading edge of the aileron.  It helps to pin this leading edge securely in place to provide the correct angle.  A good option is to cut two or three small triangles and temporarily glue them between the two surfaces that are hinged together.  The ribs that are attached to the trailing edge of the aileron will need to be trimmed to allow the aileron to match up with the 1/8 inch balsa spar.  An additional rib has to be made for the end of each aileron.  Notice that these end ribs are made from thicker wood to withstand the stress resulting from shrinkage of the covering material.  The trailing edge is slightly tapered from rib #10 to the tip.  When this strip is cut off, it leaves a flat surface on the rear edge of the aileron.  The bottom surface of the aileron should be trimmed in this area so that the trailing edge is reflexed slightly and comes to a point where it meets the upper surface.  It adds a little washout and makes the tip less prone to tip stall.

Aileron differential is the other thing incorporated in the design of the DH 90 that helps to counter adverse drag.  It is important to angle the arm of the aileron servo forward as shown on the plan.  The distance the aileron horn is back from the leading edge of the aileron is another factor in aileron differential, so watch that the aileron horns are mounted where shown on the plan.  Placing the aileron horn slightly further back will increase the differential.

BOTTOM WING

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The lower wing is built in one piece, but has to be done in three stages.  Construction is similar to the top wing, but the centre section which extends out to Rib #2 is flat and not tapered.  First build the spar which has two compound joins where the dihedral and sweep back both start.  Again it is very important to get these angles correct.  First build the central section directly over the plan on the building board.  Then block up one end of the completed section so that the spar of the outer panel at the opposite end is flat on the building board while it is being built.  Build the other outer panel in a similar way.  It is best to build all three sections before sheeting is started.  Proceed with the sheeting as for the top wing.  It is wise to install the wiring for the motors before applying the leading edge sheeting to the top surface of the central section.  Note that the outer leading edge strip does no pass through the area of the engine nacelles, so it is best left until the nacelles have been built.   Extra sheeting aft of the spar  is applied to the wing in areas where the engine nacelles are located and should be inset.  This sheeting is necessary to support the nacelles and the adjoining wing covering material, so it should extend beyond the edges of the nacelles.  Washout is the same for the bottom wing as the top one.  There is no twist as far out as rib B5, but a gradual twist throughout the outer panel to give 3/16 inch washout at the tip, this being measured at the trailing edge at the last rib location.

ENGINE NACELLES, MOTOR MOUNTS AND UNDERCARRIAGE

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The engine nacelles are built by first gluing the ¼ inch spruce beams (P1) in place, securing them firmly to the lower surface of the wing and the main spar.  Then the pedestal that supports the undercarriage is built.  Cut the beams P2, P3 and P4 from ¼ x 1/8 inch spruce and glue them in place on the triangular piece of 1/16 inch ply where the ends all come together.  These joints must be of good quality.  Using carpenter glue and clamping the joints will be sufficient.  One of these assemblies is now glued to each outer surface of the first beams, (P1).  See front view of the pedestal.  Again, the joints should be clamped.  Where the top end of P3 meets up with the leading edge of the wing, a thick gusset (not shown on the plan) should be glued on the inner face of the P3 to strengthen the attachment to the leading edge.  The 1/8 inch plywood plates (1½ x 1 inch) that the undercarriage legs are attached to are now glued in place at the bottom end of P2.  Epoxy can be used for these attachments if it is not possible to clamp them well with carpenter glue.  Now glue the triangular pieces of ¼ inch sheet balsa to the top surfaces of P1 at the front where the motor mounts will be attached.  The 1½ x 1½ inch squares of 1/8 ply that form the motor mounts are now glued in place on the front of these assembly.  Adjustments may have to be made to accommodate motors of a different size.

The undercarriage is designed so that the lateral section of the U/C leg that passes through the two nylon mounting clamps serves as a torsion bar.  The swept back nature of the U/C leg means that the wheels arc both backwards and upwards when a bump is hit.  This results in very good absorption of bumps when operating from a rough surface.  Complete the construction and mounting of the undercarriage legs.

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Next the nacelle bulkheads are glued in place.  Start by gluing cross pieces of ¼ inch square balsa 2¼ inches long under the “P1” beams just ahead of the leading edge of the wing. The ¼ inch bulkheads (N1) are then attached to these cross pieces.  Glue N-3 bulkheads in place supported by a thin cross piece where they are glued to the “P1” beams.  After this, N-4 is attached to the bottom of N-1 and N-3, then a curved strip of 1/8 sheet balsa is attached between the rear point of N4 and the trailing edge of the wing.  This serves as a keel for the trailing edge of the nacelle.  The nacelles are now planked with 1/16” sheet balsa.  Notice that bulkheads N-1  at the front of the engine nacelles are cut from ¼” balsa, but the planking of the nacelle just comes to the mid point of that bulkhead.  This leaves a 1/8 inch shoulder at the front of this bulkhead which serves for holding the cowling in place.  When the planking is completed the motors should be installed.  After fitting the motors, check that they are all perfectly aligned.  Use shims to make any fine adjustments that may be necessary. The plans show the motor mount designed for the commonly used outrunner brushless motors.  However, the prototype model used geared inrunner motors, so any pictures shown of the motor installation will be different for this reason.

The engine cowlings are built in place with the motors installed.  Each cowling will be held in place by two screws at the aft end of the cowling, abeam the “P1” bearers.  These screws do not hold well if screwed into the 1/8” shoulder of the balsa bulkhead.  So two short strips of 3/16 inch square spruce should be glued to the front face of “N1” at that outer edge (at the level of P1) so that the mounting screws go into these blocks and hold well.

Next the nose blocks are made, but the hole for the propeller shaft should at this point be made just large enough for the nose block to be a snug fit over motor.  This will hold the nose blocks in place while the cowlings are built.  After the cowlings are completed, the hole for motor clearance can be enlarged.  With the nose blocks held in place on the motor, start making the cowls by cutting out the side panels of each cowling.  These are glued at the front edge to the nose block, but only attached to the “N1” bulkhead by the #2 screws at the rear edge.  Next apply the top curved sections of the cowling, gluing these to the nose block and adjacent side panels, but not of course to N-1.  The curved sheets forming the top of the cowling are easily shaped if the outer surface is moistened.  Attach these to the upper edge of the side panels by having doublers of 1/16 x 1/8 balsa along the inside of the joins.  The bottom of the cowlings, (“C2” are now shaped and glued to the nose block and lower edge of the side pieces, but not of course to “N1.”)  When the glue is dry, take out the screws and remove the cowlings.  Now the hole in the nose block for the motor can be enlarged to give adequate clearance.  Before covering the cowlings, small washers should be made from 1/64 ply and placed under the heads of the #2 screws that hold the cowlings in place.  These ply washers are glued to the sheet covering.

The wheel fairings are each held in place with two screws.  They can be built in place so as to ensure a good fit and provide clearance for the undercarriage legs and wheels.  Cut out F1 and screw in place at this point with just the rear retaining screw and a pin at the front.  Separate it from the nacelle and cowling with a piece of saran wrap to that the sections are not glued together.  As in other applications, blocks of spruce or ply should be glued to the inside surface of “C2” and “N4” so that the screws are held firmly.  F2 and the wheel wheel fairing side panels of 1/16 inch sheet are now attached to F1.  The nose piece of the wheel fairings is made at this point with a single thickness of 1/16 balsa and attached with the grain running vertically.  The joins between the nose section of the fairings and the side panels should be reinforced with doublers on the inside.  Laminate the nose section with a second smaller piece of 1/16” sheet glued to the inside in order to increase strength.   Be sure to allow clearance for the rearward movement of the U/C legs so that the wheels do not hit F-2 when the model hits a bump and the gear legs flex backwards.  The mounting lug at the front of the fairing is now attached with epoxy and the front screw inserted.

The wing struts are constructed as shown on the plan.  Where necessary, 1/16 sheet balsa plates are glued between adjacent ribs to provide for attaching the strut mounts.  Small rare earth magnets inserted into these tabs as shown will keep the wire at the end of the strut from coming out.  The struts are not functional, so can be built from balsa.  Two methods are used to attach the wire ends.  Where the wire end is straight, such as at the bottom end of the inner struts, a hole is drilled in the bottom end of the strut and a straight piece of wire is inserted.  If a drop of CA glue is put in the hole first, the wire sets up firmly and is not likely to come out.  For the other fittings, the wire is fitted into a shallow slot cut in the side of the strut.  Strength is improved if the end of the wire is bent to a right angle and inserted into a hole drilled through the strut where the end of the wire is to be located.  After the wire is glued in place, it can be bound with a few turns of thread.  The exact location of the bottom ends of the inner struts where they are inserted into the engine nacelles is shown on the plan.  When assembling the model, attach the lower wing first, then fit the inner struts into the engine nacelles before attaching the top wing.

Covering is conventional with Solar film, Monokote or other films of a similar nature. The struts are covered with film covering, using the same film as used for covering the  airframe.  Clear transparent MonoKote is great for covering all the windows.  It is best to do these first, before applying the rest of the covering on the fuselage.

Flying

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Very little needs to be said about flying. Flight characteristics are excellent.  If the wings are built as indicated, and have the correct washout, there should be no danger of tip stall. Efficiency comes with the high aspect ratio tapered wings using the thin Selig 3010 airfoil. But as in sailplanes, they make for a challenge in accurate construction.  Likewise, the undercarriage is a tight fit in the slim nacelles and wheel fairings, but these add to the streamlining of the model. The torsion bar suspension used on the main gear provides for good operation on rough fields. Unlike the full scale aircraft, the model is easy to keep straight on take off, and slow three point landings are easy to do without the use of the optional flaps.  In the full scale plane, wheel landings were “mandatory.”   With the model it is better to do a wheel landing if using flap, but this is subject to the pilot’s preference.  Aerobatics are not a scale manoeuvre for the Dragonfly, but if you want the occasional change from scale-like flying, the model is quite capable of doing very large graceful loops, stall turns and Cuban eights.  Rolls are on the slow side but can be done.  Spins depend on the C of G location and tail surface throws.  Because the interplane wing struts are held in place with small rare earth magnets, assembly at the field is fast.  It is a robust model that can be thrown about and enjoyed. Good Luck with your Dragonfly!

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Specifications

DH 90 Dragonfly

Designed in 2012 by Ivan Pettigrew

Scale: 1/7

Span, upper wing : 74 inches

Lower wing:  66 inches

Wing area:  1,026 square inches.  (80% biplane equivalent is 820 sq.ins.)

Length:  54 inches

Flying weight:  78 ounces

Wing loading:  13.7 ounces / square foot (allowing for 80% efficiency of biplane wings.)

Primary airfoil:  Selig 3010 modified towards the tips

Motors:  brushless 450 or 480

Battery: one 3S 3.6 AH li-po. (Or one 4S 3.0 AH li-po, depending on motor choice.)

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About the author

Senior Technical Editor About Me: I have a lifelong passion for all things scale, and I love to design, build and fly scale RC airplanes. With 20 plus years as part of the Air Age family of magazines, I love producing Model Airplane News and Electric Flight.
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