Rebuilding a Vintage Biplane with Alibre Design Professional
German Designers Use 3D CAD To Meld Outdated Engineering With Modern Technology
It’s not unusual for enthusiasts of vintage aircraft to go down to their local hobby store and buy a 1:72 scale model fighter to glue together. But very few, like Alibre Design user Patrik Schlemmer, would go so far as to build the actual plane at 1:1 scale.
“I’m not using Alibre Design Professional in a commercial capacity. I’m not the owner of a company. I’m using it for my hobby, which might sound a little bit strange for you -- it’s designing parts for airplanes. In fact, I’ve designed an entire airplane with Alibre, which is about 80 or 85 percent completed right now. And I’ve just made landing gear for a new project, the Heinkel HE51.”
Schlemmer’s free time is spent creating replicas of antique airplanes. When complete, you can climb into the cockpit, put on your goggles, throw your scarf around your neck, and take off into the wild blue yonder.
The Heinkel HE51 is a 1935 German biplane, a fighter that was in service between the two world wars. The Heinkel company created a series of numbered HE models, which are numbered sporadically from 1 to 280, spanning much of the early history of aviation. The HE51 represents the pinnacle of the evolution of biplane design, and is one of its last examples. After seeing some action in the Spanish Civil War, the HE51 -- and biplanes in general -- became obsolete. The higher performing monoplane took over as a fighter, playing its central role in the Second World War.
Schlemmer’s extracurricular pursuit may be unusual, but he is not the only one who works on a bigger scale. He joins two other aviation enthusiasts, Peter Davies Gartner and Swen Jansen, in the Heinkel re-creation. Schlemmer linked up with the team after reading about the project in a magazine article.
“I called this gentleman up, who is now a friend of mine, Peter Davies Garner, and I started chatting with him. This turned out to be an hour-and-an-half conversation,” he recalls. “We came to the conclusion that there is a lot of design and stress analysis to work out. He actually is an architect, and his specialty was to re-design things, but not to do the mechanical engineering. Since we have no old calculations or stress analysis, much less complete drawings, we have to do everything from scratch. So I offered him my support.”
The first challenge for the HE51 team was to create the building plans for the biplane. This was problematic, since not only are there no longer any of the 70- year-old airplanes in existence, there are also no original construction documents.
“We have some minor drawings that survived the war. The only information source so far is a couple of old military manuals on how to do the maintenance on the airplane,” explains Schlemmer. “The problem with these is that there were no dimensions, so we essentially have to guess. To do this, we have to have a good overall appearance of the parts. This is something that you can only do when you have 3D models on hand.”
When he came into the project, Schlemmer suggested that the team re-draw plans not in 2D CAD, but as a solid model assembly in Alibre Design Professional. Using 3D models permitted Schlemmer to analyze the stresses on the structure by importing the 3D data into ALGOR, a finite element analysis program.
The Alibre model also aided the team in keeping the assembly in proportion, even without the original dimensions. “2D drawings do not allow us to see the interference and clearances of the parts, and how they relate to each other. By using 3D models in Alibre, we can judge the appearance from different perspectives.”
International Part Hunting
Much of the structure of the HE51 could be inferred from the traditional construction of biplanes. Unlike the rigid metal or composite material construction we associate with today’s aircraft, the outer shell of a biplane was soft. The fuselage is built as a truss system of steel tubing, covered with a lightweight fabric. Similarly, the wings are made out of a trusswork of wooden members and spars, covered by either fabric or thin sheets of spruce.
This kite-like construction might seem delicate, but the biplane carries a substantial load. Once the engine is in place, the weight of the plane will be about two tons. Thus the ability to handle stress and loads is of particular importance in the re-design, especially in the landing gear, the first area where Schlemmer applied his analytical expertise.
“We had this basic information from this old maintenance handbook -- instructions of how to maintain the brakes and the tires and so forth -- but it was only intended for the mechanics. There were a couple of pictures inside from which we could understand the structure of the landing gear. From that we could see that they were actually quite complicated. We decided to re-design quite a bit of the structure using equipment that’s available today.”
To ensure safety, the team took a slight departure from the tradition methods of biplane construction. They elected instead to use modern manufactured alternatives that would closely resemble the antique mechanics. After completing all necessary landing load calculations Schlemmer found himself on an international scavenger hunt for components. After checking all over Europe, the only shop that would build shock absorbers to their specifications at an affordable cost was in the Ukraine. The solution for the brakes and wheels was found farther west, in America.
“I was talking to some of the engineers from Parker-Cleveland in Cleveland, Ohio, who were quite nice and very friendly, and they helped me understand what components from today’s product line would work for an alternative to the original brake system as well as the wheeland- tire assembly. We ended up using the brake and wheel system from a twin-engine Beech Bonanza. Our landing gear design is largely based on 2D drawings we received from Parker-Cleveland.”
No Small Project
As far as using high-technology tools to build an old-fashioned plane, Schlemmer says the teams found many benefits in going 3D.
“We are enjoying using Alibre Design, because it helps us tie the entire project together. I was actually using Alibre for my own personal use because of cost. The price/performance ratio with Alibre is very high, so you get a lot of capabilities for just a little money. That’s a real consideration when you’re working on your own projects.” The Alibre Design Professional package costs only a fifth of most commercial mechanical design programs.
“It’s a program we can use for even complex systems. The engine will probably be the next challenge. We’ll be modeling the entire engine in Alibre Design Professional.” The original 750 HP engine on the HE51 was a BMW VI, only a few physical examples of which are left in Germany. Like the landing gear design, the engine re-build is expected to be a combination of locating contemporary equivalents for some parts and wholesale re-design of others.
While it might be rare these days to want to take your software’s technical support reseller out for a beer, for Schlemmer, it’s a standing offer. “I’ve had excellent support from my Alibre dealer in Germany, Ralf Schrödter at O-Punkt CAD. He did more than I could have expected from a standard support line.”
The downside of choosing such a massive hobby is that it takes a while before one gets to the finished product. “Since we are not doing this on a professional basis and entirely outside our normal work, it’s going to be a long project.” With three years invested in the airplane so far, there may be several years more before Schlemmer, Davies, and Jansen get to take turns flying their vintage warplane.
“We expect that the plane will be very easy to handle, as was the original. The designs at that time were excellent. We know from a few reports we’ve found that this plane was very easy to handle and very easy to fly. All of us, I think, would be more than keen to fly it,” says Schlemmer. Until that point, the trio will simply take their pleasure in 3D modeling and truss building. “It’s a lot fun and a lot of work. We absolutely love it.”