[Courtesy of Dick Williamson, williamson "at" ll.mit.edu, September 2001]
Suggestions for Building the MAD Aircraft Highlander
Table of Contents
1) Placement of battery and servos
I placed the battery pack (four AA 600 mAh NiCd batteries in a square pack) as far forward as I could while still leaving some EPP on all sides. There is probably less than 1/4 in. of EPP left along the side near the front corners of the battery pack.
The two standard servos are mounted on top of each other directly behind the battery pack. There is about 1/2 in. of EPP between the battery pack and the servos. The servos are glued to each other along the large flat sides. Servos arms were placed on the servos and the side-to-side alignment of the servos relative to each other was set such that the distance from the top of one servo arm to the top of the other servo arm was about 1/4 in. less that the width of the fuselage at the position of the servo arms. This placement allows the servos to be mounted in the fuselage such that the top of each servo arm is about 1/8 in. underneath the covering. The hole for the servos was cut slightly undersized so that the servo assembly could be nicely held by the EPP. Small pieces of EPP were glued in on the sides of the servo assembly to keep it from sliding side to side.
By placing the battery pack and the servos far forward, my Highlander came out with a center of gravity very close to the specified position. If you don't place things forward, you will end up needing lead in the nose. For a similar reason, there is no benefit in using microservos for the elevator and rudder.
2) Routing the Nyrods to the tail feathers
Run the Nyrod for the elevator straight back to the elevator horn. For the rudder, run the Nyrod from the lower servo up at a gradual angle so that the Nyrod exits near or above the front edge of the stabilizer. This way, you will need a minimal bend in the metal push rod going from the end of the Nyrod to the rudder horn. When you cut the groove in the EPP, angle the groove to the outside of the EPP as you approach the tail feathers.
Slots and cavities can be cut with a knife or melted with an iron. Instead, I used my Ryobi/Dremel with a cut-out kit (sort of a router arrangement) which did a great job. (ed: for more info on using a rotary tool for EPP routing, see the CRRC article Routing EPP Foam Cutouts) The tool I use is a fluted metal piece with an egg-shaped cross section. I ground this tool to make the fluted tip nearly spherical and with a dimension slightly smaller than the outside diameter of the outer Nyrod. The spherical part of the tool was set to run just under the surface of the EPP. The Nyrod snaps nicely into the resulting circular channel.
For gluing pieces of EPP to each other or for bonding wood to EPP, Goop works fine, but is quite thick and difficult to spread. Liquid Nails (available at Home Depot and other places) comes in a tube for small projects and is less viscous thus making it easier to use. The clear version of Liquid Nails (not the "original" version) seems to be the same basic material as Goop.
Spraying the surfaces with 3M77 before taping and again before covering can make an awful mess. If you spray the whole plane at once, you have sticky surfaces to handle as you move from section to section. While covering the top of the wing, the bottom of the wing picks up junk from the work bench. On the other hand, if you spray each section in turn before you cover it, you get 3M77 all over the sections you have already covered. The solution to this dilemma seems to be the citrus-based cleaner Goo Gone. I sprayed and covered the plane in sections and then easily removed the excess spray on the top of the covering with the Goo Gone.
When I finished covering the plane with Ultracote using an iron, the results were underwhelming. The surface looked lumpy and wrinkled. I then went over the surfaces carefully using a heat gun on the wrinkles. The result was great. It is now hard to tell from the covered surfaces that my Highlander is an EPP plane.
Recent Highlander kits seem to have been slightly modified from my earlier one. I still like my modification of the wing hold down. The nylon hold-down bolt sits tangent to the rear of the spar. What I did was to glue a piece of wood (depth approximately equal to that of the spar and spanwise dimension of about 1.5 in.) to the back of the spar. Some EPP has to be cut away to provide room for this piece of wood. The thickness (fore-aft) of the wood was 3/8 inch which is larger than the diameter of the bolt. After this piece of wood was glued to the spar, I drilled a vertical clearance hole for the bolt such that the front of the hole was tangent to the spar. This provides a solid wing hold down.
I used the standard configuration with a dowel in front and a hold-down bolt. This combination makes for a wing that rotates rather easily. This is both good and bad. The bad part is that you need to repeatedly rotate the wing until it is in the proper position, usually after each flight. To do this, I lined up the wing precisely on the bench and then made adjacent marks with an indelible marker on the rear center of the wing and the center of the fuselage behind the wing. Subsequent adjustments meant merely lining up the marks. The good part is that the wing has a lot of give during a crash.
5) Center of gravity
The CG was set at 3.25 in. behind the leading edge which is the mid range suggested in the plans. I have not experimented with moving the CG, but I do find that the plane is quite responsive to elevator at the initial CG.
6) Tow-hook location
My instructions indicated a position that is much too far forward. A position that is about 1/2 in. ahead of the CG seems about right. The extra wood block for the tow hook needs to be moved to the proper position.
7) Winch-proof wing
When properly built, the Highlander wing is nearly winch proof. The only times that I have seen a Highlander wing fail, the break occurred exactly at the center where the two spars butt up against each other. To strengthen the wing, glue an additional plywood joiner, about 6 inches long to the front of the spars astride the center of the wing.
8) Tail flex
Be sure and use adequate strapping tape everywhere, especially from the wing back to the tail. The narrow fuselage behind the wing can have a lot of flex. Some people have gone so far as to embed an arrow shaft in the fuselage from the under the wing back to the tail. This may add some unwanted weight to the tail and necessitate more lead up front. I didn't use anything other than tape to stiffen the fuselage. I have only encountered minor problems. When doing a zoom launch with a winch, the plane seems to occasionally have a hard time pulling out of a dive. Also, the plane sometimes acts funny when pulling G's in an acrobatic maneuver. The cause is probably tail flex.
9) Overall weight
The complete covered and balanced full-house plane with radio gear, two standard servos and four microservos servos came out at about 38 ounces. The polyhedral version weighs less. Various messages posted on RCSE have quoted weights ranging from 34 to 44 oz.
10) Tail feathers
To save weight and maintain straight surfaces, I built balsa tail feathers instead of using the supplied Coroplast. I can send a plan for the construction of the balsa tail feathers.
The tail feathers have the same outer dimensions as the supplied Coroplast. However, the elevator is made out of 1 1/2-in.-wide sheet balsa and is narrower than the Coroplast elevator. Correspondingly, the fore-aft dimension of the stabilizer is increased. The elevator/stabilizer hinge line is placed at the rear of the fuselage. This means that the stabilizer is moved slightly forward as compared to the Coroplast configuration shown in the instructions.
The elevator is made out of a piece of sheet balsa and tapered toward the rear. The stabilizer, fin and rudder are all framed out of balsa with the same 3/16-in. thickness as the Coroplast. The rudder is tapered toward the rear. The front edge of both the fin and stabilizer are rounded. The basic frames are made out of 3/8-in.-wide balsa. Diagonal pieces 1/8 in. wide run between the outer frame pieces much like shown in the instructions. The front and rear framing pieces for the fin run from the top to the bottom edge. The bottom of the fin (between the vertical framing pieces) is made out of a wider piece of balsa with the grain running longitudinally. The sheet balsa runs from the bottom of the fin to about 1/2 in. above the top of the stabilizer. The central 1 1/2-in. width of the stabilizer is made out of sheet balsa with the grain running transversely.
The front edge of the rudder and elevator are beveled at 45 deg. and are hinged (after covering) with hinging tape.
The fin and stabilizer are glued to each other and the top right-angle joints are filled with a fillet of 3/8 x 3/8-in. triangular balsa to hold the fin and stabilizer together. The tail feathers were covered and then glued into a slot cut in the EPP fuselage much like as is done with Coroplast tail feathers.
11) Wings with dihedral
The recommended version of the kit for beginners is the polyhedral version shown in the plans. I have also built a wing with 5-deg. dihedral that is equipped with flaps and ailerons of equal length. The instructions say that if you are building a simple dihedral, do not cut the spar and do run it in one piece from the center to the wing tip. However, this doesn't quite work. The slot for the spar runs full depth to the tip. However, the airfoil thickness at the tip is less than that for the center section. As a consequence, a straight spar fitting into the slot will cause the bottom surface at the tip to rise up slightly from the level of the center section. This will cause a slight upward bend in the trailing edge at what used to be the polyhedral break. Unfortunately, this bend occurs in the region where the ailerons go. To avoid this bend, cut the spar at the joint between the center section and the tip. Install the spar sections at full depth into the slots in the center and tip sections. Glue in the two sections of the spar with the bottom of the wing flat. The kit provides some straight pieces to span the joint at the polyhedral break. Use these same pieces in the precut slots to strengthen the spar joint where you cut the spar. The result is a nice straight trailing edge.
Tape was used to hinge the flaps on the bottom and the ailerons on the top. The ailerons and elevator are beveled at 45-deg. on the front bottom in order to allow for downward deflection. There is no bevel on the flaps because they only go down.
For control horns, I used pieces of 1/16 plywood glued into to slots that go through the control surfaces. The horns are mounted about one inch out from the inner end of the control surfaces. Another good material for control horns is fiber-reinforced PC board. I can supply drawings of the profiles of the control horns.
Microservos for ailerons and flaps fit nicely into the wing. Place the servos behind the spar rather than in front as indicated in the plans. I cut out a pocket in the fuselage right behind the bolt in order to provide a cavity to accommodate the four servo connectors.
12) Highlander web sites:
Dave Cole: http://msiNOW.com/rc/highlander2.htm
J. P. Morere: http://members.home.net/aeronut/index.htm
Dennis Hevener: http://hosenose.com/hevener/rcflight/highlander.htm
Cliff Schwinger: http://members.home.net/cliff/h/highlander.htm