Aircraft Builder’s Frequently Asked Questions
(Builder FAQ and How-To Section)
The FAQ section is intended to answer commonly-asked questions and provide a forum to share helpful techniques. While we obviously can’t cover every topic here, we would like to hear from you on anything you wish to know. If it’s of interest to others, the question and answer will likely be added here. Remember, there is no such thing as a dumb question, except the one that doesn’t get asked… we’re here to help. Send us your questions!
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Hale used to offer routed spars. Why are they no longer available?
We don’t believe they are safe. The structural analysis Hale had done on the routed spar only addressed the major axis of the spar. Indeed, the analysis shows that the spar is not significantly altered in strength along the major axis. Routing will substantially reduce the stiffness of the spar in the minor axis however, as most of the stiffness is provided by the material at the surface in skin effect. The flying wires place substantial compressive column loads on the top wing spars, and any fore-aft flexure of the spar will substantially reduce its column strength and resistance to buckling. We simply cannot approve the use of routed spars in any of our airplanes. The modest weight delta is a small price to pay for peace of mind. Remember also that the wing carries it’s own weight – the weight of the wing on the wing is somewhat free. Weight saved in the wing and added to fuselage in fancy radios is certainly not.
When we determined these things, we notified everyone who had obtained routed spars or routed spar wings from Hale and told them we didn’t consider them safe, and that the spars already in wings should be replaced. Elsewhere in these FAQs is a description of a process we use to replace damaged spars, and its fairly easy once you get the hang of it. The level of effort is a small fraction of building a new wing.
Why are laminated spars stronger than solid wood?
Any natural grain has a weakest bias and will fail along the weakest grain in a catastrophic manner. The weaknesses in the grain structure line up. With several laminations, each plank has its own weakest bias, and they are unrelated to each other. (We deliberately swap ends and sides of each plank). Destructive testing has shown that each plank fails separately, and the failure is incremental, not coincident. One-piece test sections in comparison simply explode at failure. Another way of looking at this is to envision the complex matrix of grain structure in the laminated cross-section. This lack of “grain in alignment” also explains why laminated spars remain absolutely straight over time, temperature, and moisture.
What are the tradeoffs between built-up and routed-ply wing ribs?
Our humble opinion, built-up is the way to go. Built-up ribs are lighter, stronger (not that ply ribs are unsafe), and they are the classic method of construction. Routed ribs use a bunch of expensive plywood, but routing is a quick, relatively low-labor method of producing ribs.
What are the tradeoffs between aluminum and ply leading edges?
We much prefer ply leading edges over aluminum. The expansion coefficient matches the wood wing, so slip-joints under flannel are not necessity to prevent oil-canning and dimpling on a hot day. Aluminum LE’s are also very delicate, and a small hanger bump will leave a permanent dent.Ply leading edges also stiffen and strengthen the wing. If glued-up properly, the ply LE forms a “D” box section along the front spar, which is rigid and very strong.
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I bought a project and the fuselage is an inch shorter than the plans. Is this a problem?
Not at all. This is a VERY common construction error with any of these airplanes. We’ve seen this on Pitts, Skybolts, and other similar designs. The problem arises when the side panels of the fuse are laid out. The station numbers describe the finished fuselage, not the tube locations of the side panels in the jig. When the longerons are bent inward to meet at the tailpost, the fuse shrinks an inch or so, depending on the airplane type. As far as flight performance or weight & balance is concerned, the difference is too small to matter.
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Who is Steen Aero Lab?
Depending on how you look at it, you can trace Steen Aero Lab’s roots back either to the early 1970’s when Lamar Steen was working on his Skybolt design, the 1940’s when Curtis Pitts developed the Pitts Special – synonymous with aerobatics, or even to the late 1920’s when Vernon Payne developed his hot-rod Knight Twister designs. But since the company that now supports all three of these design families really got its start with Lamar Steen and only later became officially associated with the other designs, let’s start with him.In the early 1970’s near Denver, Lamar Steen designed the Skybolt, and the prototype aircraft was built by the high school shop class he taught. Designed to fit the need for a Pitts inspired 2-place aerobatic airplane for larger pilots, the Skybolt quickly became known regionally and then nationally as a fine aircraft, both for its spacious comfort and for its performance. Lamar founded Steen Aero Lab to offer plans and components. The Skybolt soon became the most-constructed two-place experimental aircraft of all time.
In the early 1990’s, Hale Wallace bought Steen Aero and the rights to the Skybolt. In addition to offering plans and components for the Skybolt, Hale purchased the rights to the Pitts S1-C series. The fuselage plans were updated to the latest configuration, and a new symmetrical wing was designed using aerodynamically balanced technology from the Pitts S1-11B Super Stinker, which produced the Pitts S1-SS. To add to the growing list of biplanes that Steen supports, Hale then obtained rights to the Knight Twister series designed by Vernon Payne. Steen also became the North American distributor of Bruntons brace wires and Hoffmann propellers.
Hale kept Steen Aero a small, three-man operation and ran it as his retirement business. He was known for saying that he “ran off” two-thirds of the work offered him. Sadly, in 1999, Hale learned that he had terminal cancer.
Paul Goetsch and Jere Larson, two lifelong friends and aviation fanatics had been looking for a Skybolt for some time and Jere had gotten to know Hale quite well. They were developing CNC manufacturing capability and applying it to the manufacture of aircraft parts.
The relationship with Hale ultimately resulted in Paul and Jere taking on the mission of Steen Aero, and applying modern manufacturing technology including CNC and CAD/CAM to producing parts and kits for these fine aircraft.
Hale passed away long before the planned smooth transition could be accomplished, but after much hard work, Steen was moved to a modern 17,000 square foot facility in Palm Bay, FL, equipped with CNC routers, a waterjet cutter, and all the resources needed to make virtually any part of an aerobatic biplane.
Today, Steen is growing quickly and is providing many new innovations for their designs. They now have the rights to Mac MacKenzie’s Firebolt variant of the Skybolt as well as the rights to the experimental version of the Great Lakes biplane. The Skybolt has now evolved into two exciting new models, the Skybolt Delta featuring a three-section wing and hydraulic landing gear, and the Skybolt Radial which was designed for the 9-cylinder M14P radial engine. These model enhancements were designed by Mr. Curtis Pitts as integrated systems.
The latest project is well underway, and represents the biggest news in the history of Steen Aero – the development of the innovative Pitts Model 14, a clean sheet design which we believe is the finest work Curtis has done to date.
Many new component products have been developed and include –
- Vacuum-formed, laminated plywood leading edges for Skybolt, Pitts and virtually any wood wing, which are completely superior to aluminum leading edges.
- Laminated wing spars and spar blanks, which are significantly stronger than a single-piece spar but weigh virtually the same.
- Pre-cut, all-inclusive wing kits and special CNC-produced rib jigs that allow building a complete rib at once in a single gluing cycle.
- Wing and fuselage hardware kits which provide very high-quality fittings and other items that are difficult to manufacture in a typical home shop.
The homebuilt aerobatic biplane certainly has undergone a lot of changes since Curtis Pitts first built his hot-rod little biplane and Lamar Steen borrowed the inspiration, built the first Skybolt, and took it on the airshow circuit.
We are looking forward to many exciting developments and announcements in the near future.
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When will parts kits for the Pitts Model 14 be available?
Flatwork parts, fuse and tail feather jigs and pre-formed tubing kits will be available shortly. The tubing kit comes already formed for the cluster intersections and ready to tack up. Both tacked and finish-welded fuselages will be available.
When will plans for the Pitts Model 14 be available?
Steen Aero is now releasing a select few sets of plans to qualified builders who wish to share in this exciting adventure and build in parallel with the factory. We ask that you have previous experience in building wood, tube and fabric, ideally aerobatic biplanes. The fuselage plans are complete, and the wing plans are nearing completion. General unrestricted availability of plans and parts will be six to nine months.
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I understand the Skybolt Radial drawings have been done for some time. It seems you guys are holding back on them. What’s up?
We have chosen to be selective in how the drawings are released. A number of sets have been given to seasoned builders who are committed to working with us through the details. The drawings are excellent, but subject to prototype issues which must be documented. There are many little gaps which could frustrate a first-time builder. These are being addressed as construction takes place. As the plans are perfected, they will be available for general release. If you are sincere and committed to building, and have some experience, please talk to us. It may make sense to have you join the collective effort. We are not trying to be restrictive. We are in this for the long haul.
What are the historical engineering changes for the original Skybolt?
Essentially there are three, and they were all done in the early years.
1) Forward Slave Strut
2) Landing Gear Geometry
3) Stretched Fuselage
Forward Slave Strut
The original upper aileron drive had the slave strut attached to the ailerons behind the hinge line. Early on, several incidents of aileron flutter were experienced. These were determined to be induced by a combination of backlash, lack of aileron system balance, and aerodynamic oscillation of the streamlined tubing slave struts in these specific constructions. The aileron system was redesigned to make it a lot more tolerant to play and imbalance by moving the slave strut attach points forward of the hinge line, thus the term “forward slave strut”. It was also recommended that round tubing be used for the slave struts, or a spoiler ridge be added on one side if streamlined tubing is used, to prevent any periodic aerodynamic oscillations.
Landing Gear Geometry
The original Skybolt gear geometry was much lower with the wheels forward from the current location. This made full-stall landings impossible, and the forward cantilever component of the wheel position placed excessive torsion loads on the hinge bolts during hard landings. The wheels were moved aft, and the hinge bolts up-sized from 3/8 to 5/16. The new gear had superb ground handling characteristics. There is an easy way to tell if a particular Skybolt has the original gear (it is rare these days). If you are 5’10” or so, the spinner will be at chest level. With the new gear, it will be at nose level.
Another change to the gear was to up-size the wall thickness of the shock cord truss to .049. There had been instances of significant damage to the truss cross-member by the stop cables on very hard landings. Repeated dimpling and crushing of the cross tube by the stop cables eventually caused several truss failures.
Stretched Fuselage
To satisfy the 6-cylinder weight and balance envelope, the fuselage was extended three inches. The current fuselage length is 168 inches from firewall to tailpost.
We rarely see examples of these early features. Most of the existing Skybolt fleet has been modified and upgraded.
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Can the new three-piece wing be used with an existing Skybolt fuselage project or rebuild?
If yours is a project in process, and the cabane has not been welded to the fuse, you are there. If the cabane has been welded to the fuselage, rework of the cabane structure is required. (The three-section wing uses a Stearman style cabane). The surgery to the fuse must be performed with care, but should be routine for a competent welder.
Can an existing upper wing be adapted for use in the three-section wing?
Yes. The spar is shorter on the new wing, so careful cutback and removal and some rework will result in suitable wing panels. A new center section must be constructed.
Can an existing wing set be used to construct a three-section wing for the Skybolt Radial having a 2250 gross weight?
Yes, but skilled surgery is required. The front spar of both the top and bottom wings must be replaced with douglas fir. We have replaced several spars on customer wings damaged by ground loops using this technique.If the wing has ply leading edges, the toughest job is carefully removing these. The majority of the LE is cut out between the rubs with a saber saw. The remainder is carefully pealed away at the glue joint, which will part by leaving some of the ply. This is easily sanded away. Care and patience is needed to avoid breaking the rib noses.The existing front spar is cut out in sections between the ribs, as close to the ribs as possible. We use a SawsAll with a fine pitch blade. Next we clamp a steel guide over the first spar stub. The guide has a rectangular hole the same size as the spar cross-section. A right-angle drill and a 0.375 bit is used to drill out the majority of the old spar in the rib. A side cutting bit takes out the rest. Any remainder is easily broken away with duckbill pliers, and cleaned up with a small sanding block.It takes some patience, and careful fitting of each rib using a short piece of spar as a fit gauge. The new spar is installed from the wing root, and the wing aligned on a wing table, just like new construction. We tack-glue the ribs to hold the alignment, then fillet-glue the ribs to the new spar with the wing panel suspended vertically, using a ladder to access the upper ribs. You will need to do this outdoors if you don’t have high-bay overhead. Gravity and surface tension balance each other and the resulting T-88 joints are beautiful. Repeat the process for both the inboard and outboard sides of the ribs.
What are the advantages of the new three-section wing for the Skybolt?
1) The three-section wing allows construction in a small garage or basement.2) The wing tank (if used) goes from 9 gallons to 18.3) It is easier to de-mate the wings and transport the airplane over land or by sea. It will comfortably fit in a short container.4) The shorter wing panels result in much a much shorter spar span. This adds significant stiffness and strength to the wing system.5) If douglas fir front spars are used on lower and upper wing, the new wing system is capable of a 2250 lb gross weight.
Can standard Skybolt wing wire sets be used with the Skybolt Delta or Radial.
Skybolt Delta – Flying wires yes, landing wires no.
Skybolt Radial – Sorry. There are also minor differences in the drag/anti-drag wire schedules for any three-section wing. Steen Aero will allow generous credit (our cost) for new condition Bruntons Skybolt streamlined wires in trade for the required new lengths.