Helios: Creating a Model of NASA’s Solar-Powered High-Altitude Flying Wing

Editor’s note: This project is part of a NASA grant for which the AMA has partnered with the Children’s Museum of Indianapolis. Jeff helped the National Model Aviation Museum build a scale model of Helios, which is a solar powered flying wing designed for NASA by AeroVironment, Inc. As Jeff worked to complete the project, he posted updates on a forum called “72nd Scale Aircraft”. The information in this article is copied directly as it was written by Jeff and appears on the forum. You can view the original posting and discussion thread here.

First posted 6-28-13

Posted: Jun 28 2013, 12:52 PM

What has a 247 foot (75.3 meter) wingspan, fourteen props, and can fly higher than an SR-71? Well, it's sure not another Messerschmitt!

Helios is a solar powered flying wing designed for NASA by AeroVironment, Inc. The aircraft is the culmination of a progressive development of several designs intended to explore utilization of high-endurance, high-altitude aircraft for communications relay purposes. Details are here.

On August 13, 2001, Helios set a world altitude record for sustained level flight, reaching 96,863 feet. The previous record was 85,069 feet, set by an SR-71A in 1976. More pictures here.

The Academy of Model Aeronautics (AMA) has their national headquarters and museum in Muncie, Indiana, near my home. Link here.

The AMA is to Radio Control modelers what the IPMS is to plastic modelers. They have an on-site museum; the museum’s director is Michael Smith, who is a talented modeler himself. When Michael calls, the project is always something interesting, unusual, and big. "Want to do a model of the Helios?" Seems Michael at the AMA and Janna Bennett at the Children's Museum of Indianapolis had worked out a swap as part of their Curious Scientific Investigators: Flight Adventures exhibit  - the AMA would get NASA’s I2000 inflatable wing drop-test model in exchange for the AMA providing a Helios model for the Children's Museum. Michael would coordinate, I would build. After several discussions it was decided to build two 1/72 scale models to the HP01 configuration which set the world altitude record. There are no kits of this one!

The Helios design appears quite simple - one long constant-chord wing, five pods, and fourteen motors. Only when you look at the underside is the complexity revealed. The structure is made up of a long cylindrical main spar, with framed ribs. The wing underside is transparent to allow light to reach photocells inside the wing, which also makes the entire structure visible. The wing is quite flexible, and takes on a pronounced bow when in flight. The Children's Museum wanted to display their model representing Helios in flight, so the internal structure will be quite visible. Here are several short videos of Helios in flight, check out the "flapping" during gusty conditions starting at 1:45.

I have an Eduard "Strip Down" Fi 156 Storch in the stash. After studying that model, we decided to take a similar approach and have the rib sections photo etched. Steve Weaver of the AMA Custom Products department did the design and CAD work, and the files were sent to Fred Hultberg at Fotocut for etching. (Fred's email here: oletcherfred@yahoo.com) Each rib is attached to a runner fore and aft to ensure proper alignment, six of these sections will be needed for the wing, which will have a total span of 41.17 inches (104.6 cm) in 1/72 scale.

Keith Davidson of Red Pegasus Decals did the art and production of the decals. If you've attended IPMS shows in Region Four you have likely met Keith, he's the guy who makes the decals for the air racers, among other subjects. His rates are quite reasonable, and the decals turned out beautifully! Red Pegasus link here.

These are masters for the pods and engines. The pods are built up from 0.08" laminated Evergreen sheet. The engines and spinners are sprue turned in a Dremel tool and filed to shape. Copies of these will be cast in resin.

The main structure will be wire and a photo etch, with a vacuform upper wing surface. The underside of the wing is one gigantic clear part. The "fuselage" pods and motors will be resin castings, and the whole thing is covered in custom decals. What could go wrong?




Posted: Jul 5 2013, 11:50 AM

Figures or vehicles give the viewer a sense of scale they can relate to. This is especially important when the subject is an unusual one like the Helios. In the Dryden photo archive linked in the first post, there are several pictures of AeroVironment technicians at work with the aircraft, such as this one:

While there's not a set of stock figures for these, conversions are not terribly difficult. Preisser makes several sets in natural poses which are a good place to start, here are a couple modified and with parts from other figures. Extra belts and clothing can be made from tape or filler putty, and the safety vests are cut from wine bottle foil.

Finished figures - a tribute the AeroVironment technicians who worked on the project, several of whom were kind enough to provide information and answer questions vital to modeling the Helios. Special thanks to Adam Peltz, Bob Curtin, Wyatt Sadler, Thad Matuszeski, Casey Heninger, Kirk Flittie, and Jim Daley of AeroVironment, Inc., and John Del Frate, and Mary A. Harness of the NASA Dryden Flight Research Center.

The ribs each need to be bent 90 degrees. After some trial and error I found the best method was to bend the ribs up while keeping both the PE and the pliers flat on the workbench. The runners on each side provide a convenient way to keep the ribs aligned and properly spaced during construction.

I'm building these in two different configurations, one on the ground and one in the air. When Helios is in flight the wing flexes and it adopts a pronounced bow shape. The main spar is constructed of 1/8" (3.2 mm) Evergreen tube, with a 0.055" (1.4 mm) K&S music wire insert. We had sized the holes in the ribs for 1/8" stock, but Mr. Murphy made an appearance and all the tubes required a considerable amount of sanding to fit.

This is a section with the spar in place. Each rib is secured with superglue, applied to the top of the spar where any excess glue won't show. The trailing edge runner will remain on the finished model, but the leading edge runner will be removed after more of the internal detail is added.

Here is the flat wing assembled, the outer wing sections have a slight gull bend to them. I was surprised at how rigid the structure has already become. This picture illustrates a problem common to the real prototype - it's going to be difficult to get pictures of the whole thing and show any details properly.

Casting proceeded without any drama, spread over a dozen sessions. Altogether ten fuselage pods and twenty-eight engine assemblies were cast, plus some spares. I used MicroMark's resin and RTV mold solutions.

The plan is to perform each construction step on the "ground" model first and work out any bugs, and then perform the same process on the "in flight" model. That will allow the museum model to benefit from experience gained on the first one.

Thanks for looking!




Posted: Jul 12 2013, 10:58 AM

More progress on the Helios. There are three longitudinal structures visible in the photographs, one on either side of the main spare and another near the trailing edge. These were added with 0.020" and 0.015" music wire, respectively. Once those were in place, the leading edge PE runner was removed and the attachment points were ground smooth with a Dremel. The runner did its job - the PE ribs are consistently spaced and aligned.

Here's a shot of the main structures of both wings together, the curved in-flight model is standing on its trailing edge. There is still some additional wiring and detail work to be done inside the wings, and then they can be painted and covered. The length of the wings makes them interesting to handle, but they are quite strong.

This is the master for vacuforming the upper wing surfaces. We had a few extra PE rib assemblies made as insurance against errors, I assembled one of these spares and wrapped it in thin brass sheet. Epoxy holds the whole thing together.

The folks at the AMA have made the mother of all vacuform machines, and were kind enough to let me play with it. The wing master on the left is approximately 7" (178 mm) wide, so there's a lot of working space unused, which I blocked off with a cardboard insert. With the vacuum fan running, I used the heat gun like an airbrush and the Evergreen sheet just drew right down over the mold. Noisy as heck, but a lot of fun!

Here's the result! A dozen will be needed, but they're turning out well so no worries except for the dogs, who find it necessary to hide from the noise.

That's it for now, thanks for looking!




Posted: Jul 12 2013, 12:30 PM

Things look like they’re coming along nicely, Jeff. What did the AMA use as a vacuum source on their machine?



Posted: Jul 12 2013, 01:42 PM

What is that big perforated base plate made of? Any source?
Matter of fact I am looking for something bigger than the electronic board plates
of 10 x 15 centimeters I have been able to find.....

Thanks and good luck and a lot of fun with your project!



Posted: Jul 12 2013, 05:14 PM

My apologies, I should have provided more information on the AMA's vacuform machine!

The machine was built in the AMA shop years ago. The motor appears to be salvaged from a large vacuum cleaner, and is wired to a switch on the outside of the box. The box itself is made of pine, with wire screens for safety on the sides. The fan evacuates a chamber below the working surface. While it uses many spare parts, the quality of the workmanship is excellent.

The working surface is an 8.5" x 17" (28.6 x 43.1 cm) electronics board. The frame holding the plastic sheet is sized to fit around the electronics board so that the plastic sheet to be formed will lay down directly on the surface of the board. The frame is in two pieces and has several sharp pins glued into its surface which align with holes in the opposite piece. For 0.015" (0.4 mm) sheet like I'm using the pins are sharp enough to penetrate without the need for drilling. There is a rubber gasket to limit air leaking in from under the frame.

There you have it! This is much bigger than we would generally need for working in 72nd scale, but the basic concept is easily adapted to making a smaller version. Rolf has posted a guide to one in the Completed Project Reference section on this board which is nice because it plugs right into a household vacuum cleaner.




Posted: Jul 19 2013, 12:42 PM

A brief update on the Helios work. This one's a little shorter than usual; I'm into a time-consuming stretch of repetitive work, so there's not a lot to show in pictures. The vacuforming is complete, barring any mishaps. I have made a stack of upper wing surfaces, and I'm happy with the test fit. I still have to fabricate a small strip to cover the leading edge.

Painting the wing structure is where most of the time is going. I was able to airbrush the carbon ribs, but the orange spars and conduits all have to be brush painted, and it's detailed work. I used Floquil railroad paints here, Grimy Black and UP Light Orange. The orange took three coats to cover properly, Bob Marley repeatedly assured me that "every little thing gonna be alright" and got me through it, but was tedious!

Next is covering and detailing the wings, and I've blocked out more time at the bench. If the modeling gods stay happy there will be a bit more to show next update. Thanks for looking!




Posted: Jul 26 2013, 12:33 PM

Painting is finished on the internal structures for both wings, so it's time to attach the vacuformed upper wing skins. Tedious work. After cleaning up the skin sections, each is carefully aligned and clamped into place. They are glued with superglue, I didn't use accelerator here to allow the capillary action to flow the glue as much as possible.

After the main sections are all in place, a small sliver of wing material is slipped between the ribs to cover the seam. This reinforces the joint, and eliminates the seam from the underside. It is thin enough to be almost invisible when in place.

The opaque white covering wraps under the leading edge of the wing. Here is a sliver of Evergreen being clamped before gluing. Alignment is constantly checked using proportional dividers.

Any gaps are filled with more thin strip and layers of superglue. This is one of the uglier joints. I've used superglue to fill gaps on several builds, and I'm always a little surprised that it smooths so well when filed, given how rough it starts out. It is easier to file if accelerator is used and it is filed soon after hardening.

A general overview of the chaos on the workbench. Both wings have seams sprayed with Mr. Surfacer 1200 thinned with lacquer thinner. This fills minor imperfections, and reveals those which are more serious. Those are brushed with Mr. Surfacer 500, and then the whole thing is sanded down so the process can be repeated. It generally takes me about three passes before I'm happy, but even then there's always a few spots left to find and eliminate.

Here are both wings after the first coat of Alclad white primer. The ragged edges visible in the photo are actually masking tape applied to the underside of ribs to protect the interiors. Most of the wing surfaces will be covered with decals representing photocells, both inside and out.

Thanks for looking!




Posted: Aug 2 2013, 01:53 PM

Work on the Helios is moving along. The upper wings were coated with several layers of Alclad White primer, sanded down, and then covered with Floquil Reefer White. The white was buffed out and sealed with Testors Glosscote lacquer.

As all this was drying the resin pods were cleaned up and filled as needed. Each pod has two small wheels, on the prototype the rear wheels were taken from bicycles! This was modeled with a half loop of bronze wire and filled with putty to account for the covers.

With that underway I moved on to the photocells mounted inside the wing. There are 145 ribs, with a line of photocells mounted between each. The Red Pegasus decals were carefully cut into small blocks and trimmed to fit. Let the decal-a-thon begin!

The decals are thin, and have to be coaxed under the lateral spars. The decals performed flawlessly using MicroSol. They are quite thin, but did not tear or fold, and settled down quite nicely. Great work, Keith!

After six hours, 288 blocks of photocells are on! I'm really pleased with the metallic blue color. The upper wing surfaces are covered in these as well, but I'm waiting until later in the build to apply those decals to minimize any chance of damage during construction.

Janna Bennett and Nick Schanz of the Children's Museum of Indianapolis and Michael Smith of the AMA stopped by to check on the progress so far and discuss mounting options for the display. The weather could not have been nicer, so after a great lunch outside at a local restaurant, it was off to the secret underground workshop. Here is Janna inspecting the work.

That's it for now, thanks for looking!




Posted: Aug 12 2013, 01:22 PM

Problems with the internet connection this week, but the modeling has continued. I'm closing in on the finish, but there are a few details to add first. In the "it's there if you look for it department" there are a number of boxes buried in the leading edge of the Helios wing. AeroVironment provided a picture of a section of the wing which showed details of two types of these boxes, and the Dryden archive shows their locations across half of the span. They are tiny in our scale, so they were taped down to keep them in place while 0.008" wire leads were superglued in place.

Here are both types in place. The blue strap on the white boxes was made from strips of blue decal film, sealed in place with Future.

The pods are all painted, decalled, and sealed, ready for installation. The decals worked perfectly, although I did manage to fold one. Good thing we made spares! No two pods are marked exactly the same, and the markings evolved throughout the test program. The markings here are those carried during the record-setting flight.

These are a few of the assembled engine pods. The struts are 0.020" bronze wire, and are blended to the resin nacelles with superglue. Fourteen are needed for each model, and each will require two prop blades.

Still a lot left to do, but the end is in sight. Thanks for looking!




Posted: Aug 16 2013, 12:12 PM

The prop blades had me a bit stumped. I tried cutting some out from 0.015" sheet stock, but that was yielding inconsistent results. They just weren't all going to look the same and were taking a lot of time. If only I could copy a blade! Well, why not? I found an image with a proper blade profile, cropped it out, and duplicated it. The photo paper stock is rigid and relatively thick, and can be contoured to the proper curvature. As a bonus, only the back side needed painted.

The big job is attaching the transparent underside to the wing, along with the pods. These were made using six strips of clear 0.010" Evergreen stock, and were sized so the seams are concealed behind the pods. A thin layer of Micro LiquiTape was painted onto the ribs to hold the clear strips. Epoxy was used to join the metal pod posts to the main spar within the wing; the pods also help fasten the clear parts in place.

An overall shot of the bench, engine nacelles and props are just visible on the lower left. The models are nearing completion, and then I will move on to bases & cases. Surprisingly, the flat model is a tail-sitter at this point, if it only had a tail!

Thanks for looking!




Posted: Aug 16 2013, 12:31 PM

Jeff, it's looking great. I have a question though, are you sure that the photo paper you copied the props on won't curl at some point down the road? With the amount of work you've put into this it would suck if that happened. I mention this because I just tried to use some old photo paper and there was enough curl in it that it wouldn't feed right in my printer.



Posted: Aug 16 2013, 01:17 PM

Your point is well taken Woody, thanks. My intention is to apply a thin layer of superglue to the back of the blades when they are installed in the spinners. I'm beginning to think the model is composed mainly of superglue, with the other parts only used to give it shape and color! When everything is together, I'll shoot the model with Future to seal the detail parts and give it a uniform finish. That should help keep everything together and pointed in the right direction.

I'm hoping all that will make for a stable build over time. The museum model will have to be able to withstand some handling, and potentially periods of storage and transportation. Variations in temperature and humidity will be the main worries; the materials used have different coefficients of expansion. The model is surprisingly flexible, and I'm trying to preserve a bit of "give" between components as the build progresses.




Posted: Aug 23 2013, 12:34 PM

There's a lot of props on the Helios, fourteen in all! The nacelles were sprayed with Alclad Aluminum tinted with Bronze, and then the engine sections were masked off prior to applying a layer of white. The blades were curved over tube stock and reinforced with superglue, then fitted into holes drilled through the spinners. The assemblies are small, only 1.125" (29 mm) across. I'll wait until the end of the build to fit them to minimize the risk of damage.

These are the avionics probes which mount in the center of the wing. The main spars are 0.015" rod, everything else was fabricated from bits of Evergreen stock. Evergreen stock is another "bustoffable" that will have to wait until the end of the project. My intention is to make this removable when the model is transported.

There are 72 control surfaces on the trailing edge of the Helios wing. I fitted 72 individual elevators on the flat model first. A lot of work, and I thought the final appearance could be cleaner. On the "in flight" model, I used long strips with scribed lines representing the fine separations between the elevators. This was easier to fit, and created a neater appearance.

This is one of those builds where everything is taking twice as long as expected (or more!) but I'm almost there. Decals and details left to go. Thanks for looking!




Posted: Aug 30 2013, 07:19 PM

One done! There was a bit of drama with the undersurface, the clear styrene would lift along the edges. Close one gap, and another would appear. After trying several different adhesives, I finally tired of playing whack-a-mole and stripped the whole thing off. Michael kindly provided some archival polyester to try, which worked nicely. It is crystal clear, thin, and much more flexible than the clear styrene which allowed it to lay down much better.

Construction proceeded along quickly after that. Laying 41 inches of decals on each wing took a while, but the metallic panels produce a neat effect.

This is a closer shot of the center section, with everything in place. I'm working on a base for the second model. It will be hard-mounted to ensure that it's not a tail sitter and to make certain all ten wheels touch the ground at the same time. I will wait to add all the final assemblies and details until it's mounted to the base.

Here's an in-flight shot. For some flights the engineers attached Mylar streamers so they could see the airflow coming off the pods. I think it adds a sense of motion to the model, the Mylar strips are represented here by Mylar strips. I'd love to photograph the model outside but don't dare take the risk, so here it is indoors suspended from some very thin monofilament sewing thread.

More soon, thanks for looking!




Posted: Sep 7 2013, 02:12 PM

Finished! The second model is mounted to its base with metal pins and superglue. I had some mirrors cut at a local glass shop to fit under the wing sections so the interior detail can be easily seen. The mirrors were surprisingly cheap, only about one dollar each.

A box was constructed for the in-flight model from birch plywood, and lined with rubberized shelf liner for kitchen cabinets. Here is the model being delivered to the Children's Museum of Indianapolis. We were lucky and got a tour of the museum's shops and artifact storage areas, which was quite interesting. We signed paperwork for the delivery on a table where a skeleton prop used in Indiana Jones and the Chrystal Skull was being prepared for display. Not a typical day.

In terms of both subject and construction, these are easily the most unusual models I've ever built. Very much a team effort on several levels, I was just fortunate enough to get to be the guy who got to put it all together. Many firsts for me, but I think it's good to push personal limits and venture into new territory once in a while. Having said that, I'm definitely ready for a nice kit in a pretty box for the next build!

More finished pictures here.

A big thanks to all who offered their support and encouragement along the way!