Carl Goldberg's "Valkyrie"

(More Photos at the bottom of the page. Download the maiden flight video Here or see it on YouTube: Here)

Well, after building the Cumulus I set my sights on one of the more complex and unusual Old-Timers: The 1938 Carl Goldberg "Valkyrie". It is a large model with a wing span of ten feet and a fuselage length of six feet but, that isn't the most interesting feature of this model. What is really unusual is the design and construction of the wing. It is a double elliptical plan form that consists of 58 built up ribs and 17 spars. Each rib is built as a truss and the spars lock the ribs together at the truss connection points. The original construction article (1938 Original Air Trials Article ZIP) describes a process called "slicing" to build this unusual design. The first step is to draw out the wing 1:1 scale using the formulas given in the article. Then, using 1/8" balsa sheet, draw out the outline of the first rib top. Cut this shape and then cut again 1/8" below your first cut. This is repeated until all rib tops are "sliced" out of the sheet. Then, with the leading and trailing edges flat on the plans, each rib top half is glued in place at the correct location. The wing is then removed from the board and stood on edge on the root rib. Then the bottom halves of the ribs are sliced and added to the wing. The spars are then slid in and glued in place and all the 1000 truss members are carefully cut and inserted locking the entire structure together. Well,... It took me a few times reading through the article to figure all this out and, I have to admit, it did have me a bit intimidated. I then found a full size set of plans online that had most of the ribs and their structure all laid out. This meant that I could lay out and build each rib ahead of time and then assemble the wing in a more conventional manner. Since the wing was the most difficult part I started with that. It took about six weeks to complete all the ribs and a about three more for each wing panel, but once I finished the first panel I knew I would be able to complete the project. My total building time was six months.


Since the original was designed for free-flight and not radio control, I knew I would have to add control surfaces. Also, since this monster is 10'X6', I knew I would never be able to transport it unless the wing halves could be separated and the tail was removable. All this required a bit of engineering... I also wanted the wing attachment, tail attachment, and all control linkages to not be visible. I'm not a big fan of screw attachments so I built a pinned lock-down mechanism for both the wing and tail. With a fuselage this large I didn't want to mess with long control linkages so I placed the servos actually in the removable tail. That way all I have to do to remove it is disconnect the servo connections.


I also decided on an electrical power plant for ease of use and to allow the most streamlined nose possible.


I have broken up the construction into groups with photos in Flash slide shows. They may take a bit to load, but just start reading and they'll pop in eventually...


The Wing: As I mentioned above, there are 58 ribs in the wing and Even with the 10 that were to remain solid balsa, that left 48 that had to built up from 1/8" balsa stock. I started with a small balsa nose piece and then, using pins and scrap balsa, laid out the top and bottom curves. Then I cut all the diagonal truss pieces and glued them into place. Each rib took between 20 and 30 minutes. One of the problems with the plans is that the ribs are drawn in such a way that you use the same drawing for groups of ribs instead of there being an individual drawing for each rib. So, the wing didn't quite taper as smoothly from root to tip as I had liked and I ended up having to shim some of the ribs after initial sanding. The trailing edge was cut out of multiple pieces of flat stock and pieced together. The leading edge had to be sliced to get it to bend around the elliptical shape. Another tricky part was the construction of the the dihedral braces. Essentially the entire wing is cantilevered off the third rib and I didn't want the braces to extend into the wing any further than that or they would show as the sheeting only extends to the third rib. I was a little worried about strength and opted to slice in bass wood for the first 2/3s of the spars (all 17 of them!). It all worked out in the end and the wing is quite strong. When the covering was applied it got even stronger.

The Stabilizer: Next up was the horizontal stabilizer. It is really just a smaller (4' span) version of the wing without any dihedral. It has the same built up ribs so I started in on them first. The big challenge was to cut in the elevators and find some way to get them to work together without adding much weight. Also, I had no idea just how much control would be needed so I wanted to err on the side of bigger control surfaces. Another problem I encountered was that, since the elevators were cut out of the structure, that left the stab a bit weak. I decided to use the same sort of bracing scheme that the wing uses to beef it up a bit. Finally, I had to come up with was a way to easily remove it from the fuselage. With a 6' fuselage and a 4' stab, it would not fit in my car if it was permanently attached... Down below you will see my idea to make that all happen.


The elevators are controlled much like "strip" ailerons are with torsion tubes. The tubes were selected from Carbon fiber and aluminum. The control tabs were epoxied into place. Hinge boxes were built to except the flat, pinned hinges.

The Vertical Fin: After the stab it was time to tackle the fin and sub-fin. For the fin I had the same issue with cutting in a rudder. Control was worked through the same type of torsion tube system as the elevators. The nice thing about these control methods is that there are no exposed control horn or linkages. Everything is internal. The sub-fin below also had to be modified. This time it was to add the steerable tail wheel. Now, a steerable tail wheel may seem a bit silly and really won't allow you to taxi out of the pits or in a tight area due to the fuselage length and its distance from the main gear. Rather, since the East field at our park is concrete, it helps tame the dreaded ground-loop. Since there are no ailerons, these big rudder-only airplanes can be a little tricky to get lined up on the runway with the wings level. Add to that the tendency of the airplane to always want to fly into the wind and you are almost guarantied that it will be landing cribbed at some angle to the flight direction. When it touches down, the main wheels start to take over but are far forward of the center of mass and the tail will then try to swing around. The rubber tail wheel provides a bit of stick-em to reduce that and allow the airplane to roll somewhat straight. The tail wheel "drive dog" had to be positioned just so, that it would connect and slip over the bottom of the rudder control post. It also had to separate easily when the tail group was removed as the sub-fin is permanently attached to the fuselage. All a bit complicated, and took some extra time to sort it all out.

The Fuselage: The fuselage construction is a little a bit unique in that the supporting structure is not in the center, but outside the main internal box. This allows for a very light "monocoque" structure with a high degree of "away-ness-of-mass" or "product of inertia". Think of the construction of a cathedral. All those flying buttresses allow the internal structure to be open. On the Valkyrie fuselage, the main load will be in bending and then in torsion. The more mass you can move away from the center (like and I-beam), the stronger it will be. Since torsion is carried on the surface of a tube, it also is beneficial to have the external surface as large as possible. This has to balance with drag, so, to reduce parasitic drag, Goldberg designed a circular cross section. The fuselage came out looking a bit like a submarine and that would make sense as they deal with similar loads.


The internal structure is a simple extruded box made by laying out identical right and left sides and then gluing in identical length cross members. Then, 14 circular formers are cut at the correct diameters. It is really hard to just cut out large disks of balsa and get them exactly round. But, if they aren't just so, the fuselage skin will show the irregularities. My solution was to rough cut them as best I could just a tad larger than they needed to be and then to spin them with my Dremel and sand the edge to the correct diameter. That also gave me the exact center and I was able to then measure the cutout from their station on the box structure to cut out the square opening in the center to allow them to be fitted over the box. It sounds a bit complicated, but once I did the first few, the rest went pretty fast. Then, I glued all the circular formers in place and started in on the 12, 1/8" stringers that go from nose to tail. At this point it was time to start thinking of the main landing gear mounts, the pylon mounts, the firewall and motor mounts, and the tail group mounts. I wanted to have the landing gear removable if it was damaged in an accident and also to make the final fuselage prep and painting easier. The pylon mounts from the top of the fuselage, and needs a shelf to mount to. The tail was still a mystery to me but I knew I would at least need some kind of platform. For the motor, I actually mounted it one bay back from where the engine would be on a ply former and fitted a 3" spinner. I think this improved the look but would affect my balance a bit. It all worked out in the end. The one area I neglected to see as a potential failure point was the final bulkhead back at the leading edge of the stabilizer. I did in fact have an issue there and it will explain a little later. The fuselage is 6' long and I struggled to keep it straight and all the parts attached to it aligned. After sections were finished, I sheeted it in 1/16" balsa sheet and not 1/4" planks as per the plans. The planks would have been stronger, but all the extra glue would have added a lot of weight aft of the CG. Also, when it was sanded it would invariably have little glue ridges running down the length of the fuselage. I also had visions of varnishing a large window along the sides so I wanted there to be no glue lines. The sheeting went pretty well, but it was hard to keep it from warping and it has to be applied in opposite panels which makes fitting closed in panels a bear. Finally I fashioned two hatches where the originals were to go so I could access the motor and battery compartment. Once again I didn't want any fasteners showing so I opted for magnets. They hold the hatches on really well and are easy to slide off. I was a bit amazed at the lightness and strength once the fuselage was finally finished.

The Pylon and Wing mount: The pylon was quite new in 1938 and had not even been tagged with the name yet. Up to then most models were designed to look like their full-scale counterparts. Without the need for a pilot however, models don't really need a cockpit so Goldberg designed the Valkyrie not to include one and, instead, focus on only the necessary aerodynamic and structural components. Still, he needed to come up with a way to mount the wing above the fuselage to increase stability and position the wing above the thrust line. The pylon was invented to fulfill this purpose.


The construction started by laying out the top wing saddle platform across the center of the bottom of the wing. In Goldberg's original model the wing was constructed as a single 10' foot piece. Since my wing had to be removable and the wing halves had to be separateable, I built in ply tabs with alignment and connection holes in the center section of the wing halves. The saddle had to have slots to accept these tabs. The saddle also had to follow the under-camber of the bottom of the wing airfoil and the dihedral angle as well. It was constructed out of 2" wide, 1/16" thick strips actually laid over the wing as a form. It was an interesting compound curve structure when it was finished.


The main structure of the pylon was constructed similarly to the rudder but with reinforcements and two ply web attachment points for the wing that would anchor into the shelf installed in the top of the fuselage. I wanted an aerodynamic and invisible attachment scheme that would not only hold the wing on, but pull the two halves together as well. I came up with two carbon-fiber tubes that run down the length of the pylon on each side capturing the holes in the pylon supports and the tabs extending from the bottom of the wings. Once again they are secured with magnets once installed. The final pylon was sheeted and a large fillet created to give a smooth transition from the pylon to the saddle above.

Assembly: With all the major components built I could turn to the assembly of all those parts. I had toyed a bit with pull-pull control lines as I used on the Cumulus, but an easier method was to just mount the elevator and rudder servo in the tail group. That way only the servo connections would need to be attached and detached instead of control lines. I picked small metal gear servos with over 50 oz-in of torque to provide the power for controlling those large surfaces but still kept the weight down. I fashioned ball-link push rods and attached them to my toque tube control arms. I also had to come up with a mounting scheme for the entire tail group. As with all the other mounting schemes I went with a carbon fiber rod inserted from the rear that captures a ply tab extending down from the center of the stab. There are two carbon fiber dowel pins to lock the tail group in from the leading edge of the stab.


I worked on the elevator and rudder to accept their torque rod control arms and fitted small fairings to the top, front of the stab to carry the fuselage couture.


With all this finalized I was now ready to position and mount the wing pylon. The reason for the delay was that I had no idea where the center of gravity would fall and, in fact, Goldberg even stated in his instructions to wait until this point to position the pylon to get the CG where it should be: 45% (or 9") from the leading edge of the wing. It turned out that, by luck, the wing's mass was distributed such that its center of mass was also 45% back from the leading edge rendering it irrelevant to the balance. That made things a whole lot easier not to have to deal with a 10' wing while positioning the pylon back and forth trying to get the balance just so. Also, the motor had to be mounted, the landing gear installed and wheels mounted, the battery installed, the tail attached, the servo wires and radio installed and the vast majority of the sheeting applied to the fuselage to come close to an accurate position. I still could not account for the weight of the paint so I set the pylon position just a bit nose heavy. In the end it as very close to the correct CG position and moving the battery would take care of any deviance.


With the pylon epoxied in place, I added the forward and rear fairings and the rest of the sheeting. I also added a fillet to the top of the fuselage and finished up the rest of the fuselage sheeting. Now what was left was final shaping/sanding and covering/finishing!

In the "Bones": With all the intricate design work on this model I wanted to take several photos of it "in the bones" before it was covered and painted. It really is something to see - over 1000 pieces of wood in the wing alone!


Finishing: For covering, I went with a translucent white Ultracote Lite. The fuselage was varnished in the open sections and then sprayed with the white and metallic green enamel. The green trim on the wing and tail was also sprayed right over the covering which adhered quite well and was much easier than tying to replicate with heat-shrink covering. Plus the color is a perfect match! The black trim is Ultracote trim tape helped through the curves with a trim iron.


Next: "So, enough of all this building... How does it fly?"

Flight Photos: These photos were taken across a couple evening close to dusk when the wind was calm. The take off roll is about 10' and the landing is about 15'. It seems to fly less than 10mph and slower. Very graceful and majestic in the air.


Maneuverability is quite good. I did mix in a bit of down elevator for full throttle to ensure a straight climb and a little exponential due to those huge surfaces.


Just a absolute joy to fly!


Download the maiden flight video Here: Big and Small

Ah, Oh!: Just before our yearly Old-Timers contest this year, I took the Valkyrie out for some addition flights to lock down the trim and get it ready for the big day. My flight went really well and I was up about 15 minutes, but I was there to adjust climb-out trim so I set up for a nice slow approach on our newly surfaced West runway.


A word about the new surface: The West runway has always been gravel (the East is concrete) which can be pretty hard on airframes. Down in Arizona and New Mexico there are some clubs that have been experimenting with "Geotex" as a surface. Geotex is a loose weave nylon fabric in three or four foot widths that can be seemed and pined down to create a nice flat surface that will stand up to the blazing sun we get here. One thing to note is that it will shrink a bit as it is exposed to the sun so it is applied somewhat loosely initially and allowed to shrink. The guys at our club did just that and seemed all the edges well. The problem was that it actually shrank a bit more than expected causing the seemed edges to pull apart a bit and roll up.


Back to my landing... The wind was a bit fickle that day and switched direction a bit just as I was setting down. The tail started to weathervane into the wind and started a slight ground-loop. That wouldn't have been a problem except that my tail wheel and then the leading edge of the stab caught a section of the rolled up Geotex edge and tried to stop the airplane dead. The airplane kept moving a bit more and that pulled the entire tail completely off! It separated right at the bulkhead just in front of the stab leading edge with a fairly clean break except for a few dangling stringers.


I learned a few things: 1.) Maybe the West field is a little small for big rudder-only airplanes. and, more importantly, 2.) The rear of the fuselage needs further reinforcement if the tail group is to be made removable.


My repair consisted of adding several carbon fiber flat spars along side the original spars and then re-attaching the tail with epoxy. It was a bit stressful and tricky to get the tail group aligned again but it all came out well in the end and several of the flying photos from above are after the tail was repaired. I got it done and trimmed a week before the contest and was still able to compete. I had hoped for a little better results, but the Valkyrie maxed out (30 sec motor run, 5min glide) easily on two of the three heats and then, on the last flight, took off into some bad, bad sink. I watched as the tail dropped and the nose went up, but it just couldn't climb out enough. The total time for that flight was a dismal 2:45 minutes securing a third place overall due to pilot error and not the airplane. Well, there is always next year... Not all was lost as my Cumulus took first place and first in the mass launch - last down competition.


Below are some photos of the repair. I would suggest that you incorporate these mods during construction if you plan to build with a removable tail. It sure would have been easier to do before the sheeting was in place!

Here are the specs for the Valkyrie :

Span: 10 feet, double elliptical taper, 20" root cord, "Goldberg" airfoil (under-camber).  58 ribs and 17 spars.
Fuselage Length: 6 feet, circular cross section. 8.125" max diameter
Stab: 4 feet, same design as the wing
Wing area just about 1870sqin
Initial CG point: 45% back from LE (9")
Flying Weight: 5 lbs, 13oz which is about one pound over Goldberg's free-flight weight. Not too bad considering the conversion to RC and removable stab and wing
So, with the smaller 10oz battery, and a final weight of 93oz that gives a wing loading of 7.16 oz/sq in and a "cubic" loading of 1.99. With that loading, it stay up for quite a while. The Cumulus, by comparison has an 8' span at 76oz, but a wing loading of 9.29oz/sq in and a cubic loading of 3.25 and it won't come down either...

The motor is a Turnigy TR42-60C 500KV motor capable of about 900W and runs very smoothly and heat is very manageable even with little direct airflow. The ESC is 60A and measured current is below 35A.
Measured static power draw is 510W on a fresh battery with a 16X7 wood prop and the 30C, 2450mAh 4S Lipo. That drops to a more steady 475W (~80W/lb) and then a final 425W. The battery won't last too long at those power levels, but it is enough for two or three full climb outs. I can move up to the 3000 mAh pack if I want more climb outs, but two should be fine. I'm a big fan of light v. s. higher power.

The thrust is strong and should pull quite well. The prop should do even better in dynamic flight loading.


There is a complete build log here: Build Log on RCGroups


Below is a collection of lots of the photos in the above movies that you can look through. If you click on a photo it will bring up a larger image size.


I hope you enjoyed my build of Goldberg's Valkyrie!! If you have any questions or comments, please leave me a note on my contact page: Contact Franny