# Why is a canard configuration more Structurally efficient for Rutan's Voyager?

Firstly, I’d like to kindly request not to mark this question as a duplicate of previous SE question without first reading this, because the approach is different and the previously given answers are merely incorrect.

Background

I found this information in VoyagerEssay.pdf from Burt Rutans website.

EDIT: I am putting this possibly redundant info with the hope it might help better clarify the problem as suggested.

From Burt's own notes.

The range of an aircraft is determined by three basic criteria: its propulsion efficiency, its weight and its aerodynamic efficiency. I had to make major improvements in one or more of these areas in order to build an aircraft that could fly more than twice as far as any previous flight.

Please see "breguet-range-equation" for more info. And this range equation suggests us the range of an airplane is exponentially sensitive to the initial and final weight ratios; and linearly sensitive to L/D and total propulsive efficiency eta.

And as the per the same source, this configuration was chosen to achieve higher structural efficiency.

The primary reason we were able to double the old record related to our success in weight control. By using a new, unusual configuration we could place a large amount of fuel at three span-wise locations: the fuselage and two large booms at 30% of the distance out to the wing-tips. A very light main wing and canard wing provided just the amount of structural support for this large fuel mass. The two wings supported the fuel-laden booms via their bending stiffness without depending on the torsional stiffness of a single slender wing. This was Voyager’s secret to success. Its graphite composite structure weighed only nine percent of the take-off weight. The fuel consisted of 73% of the take-off weight. This phenomenal weight performance was the main reason we were able to achieve our goal of true global range.

And of course in return traded off aerodynamic efficiency.

Regarding aerodynamic efficiency, I was unable to achieve a result as high as a typical sailplane since I was forced to use the unusual configuration

In this John Roncz presentation, he mentioned this configuration was to solve the problems of having high inertia fuel booms, [~18 mins] but without any specific note on how it resolves into a canard. As per Mr. Roncz, he appears to suggest this minimizes the bending stresses of the booms. (Please correct me if Im wrong) and note from Burt appear to suggest the config was selected to minimize wing torsional loads.

Question

Can any of you please provide me with an explanation as to why the canard configuration ideally could be more beneficial for this design?

My argument is if he had chosen the second config shown below, he would have gained BOTH structural benefits of canard plus aerodynamic benefits of conventional config.

In my eyes, this is an even better win-win solution but most certainly too good to be true; hence my question.

If possible I would love to see some quantitative answer rather than qualitative but any constructive answer is more than very welcome.

• You may edit your question to be more specific (e.g. focusing on inertial fuel boom). Otherwise, comments on this answer provide enough answer to the main part of your question and I'm tempted to mark your question as a dupe for this reason. – Manu H Jan 8 '20 at 12:45
• It is a very thoroughly composed and pertinent question, which considers earlier questions and answers and presents points not considered earlier. Not a dupe. – Koyovis Jan 8 '20 at 20:22
• This answer to a related question explains that canards are not actually efficient on stable aircraft. But then I believe I've seen a mention somewhere that Voyager is not stable. – Jan Hudec Jan 8 '20 at 22:20
• Thanks for all the input guys. (especially Koyovis for understanding the question). Most people did not appear to understand what was asked at all, so I put more background hopefully to get less cluttered answers. – user46017 Jan 8 '20 at 22:30
• Would a look at the "White Knight" help? Instead of a canard/wing "box" the two "fuselages" each have their own aerodynamic stabilizers. The center section is strengthened to carry the Spaceship. Combining the three fuses into one may have gotten closer to the sailplane, had structural strength allowed it. – Robert DiGiovanni Jan 9 '20 at 2:07

## 2 Answers

I listened to John Roncz explanation more than a couple of times and figured the reason he was explaining.

The first thing to understand is any aerodynamic surface placed in front of the CG is a DE-stabilizer. The aft wing is the stabilizing surface.

The high inertia of the fuel filled booms demand either a very torsionally stiff wing structure or else a very big stabilizing power characterized by higher tail volume. But torsional strength is very expensive in terms of wing weight because higher torsional loads imply a bigger secondary spar and thicker wing skins.

Considering the fact that the Breguet range equation favors higher structural efficiency over the other parameters, It appears that Burt chose the second approach. This adds to the second benefit that wing and canard both subjects almost only to the bending loads.

Quick number-crunching shows that with 10% static margin canard configuration leads to ~7 Times statically stable airplane than the conventionally configured one.

EDIT: As of why GlobalFlyer with very similar mission requirements used conventional configuration could be tracked down to the following reasons.

1. Due to the nature of the Voyager project, Burt's responsibility was only to design the airplane. The materialization of the design was on Dick and Jeana, he opted for the most simple structural solution with the minimal analysis he could come up with at the time.

2. As per the previous analysis on tail volume comparison, it is clear that conventional configuration provides only minimal "P" gain for the longitudinal transfer function; meaning that dampening out a gust response, for example, could result in diverging oscillations or even a stall. At the time, I believe completely failsafe autopilot is not a feasible solution given the additional weight it put and/or risks it brings onboard.

3. Also, Global Flyer demands even a higher fuel fraction due to fuel thirst jet engines, so losing the aerodynamic benefits of conventional configuration was not a feasible one. Instead, they ended up utilizing a fulltime autopilot.

• Best to stop looking at it as "main wing" and "tail". It's really 2 wings, fore and aft. Stability depends on placement of CG. A "canard design" is little more than a plane with a tiny wing and a giant tail. Rutan needed a gentle stall from the tiny wing in front. Check out sailing rigs. With 3 masts, you can drop the mainsail in high winds and sail along on the jib and mizzen sails. Or, if you want, use the jib as a slat to the mainsail for more lift into the wind. – Robert DiGiovanni Jan 15 '20 at 11:04

You are right that a conventional, glider-like design would had have at least the same if not better aerodynamic quality. Canards do have more disadvantages than advantages if you compare them directly with the conventional layout.

However, that advantage is not large and will shrink if the design is optimized for one specific task. By only considering the polar point of longest range, the disadvantages of the canard configuration disappear and the performance will be very similar to that of a conventional design.

Rutan is right about the torsional stiffness of a slender wing with huge tanks at 30% of span. However, if you look at the optimum slenderness to achieve the largest volume over drag ratio, the Voyager tanks are way too slim, so I see some potential for improvement right there. Also, a conventional tail at the end of the tanks will provide aerodynamic damping just as well as a canard does, providing aerodynamic stiffness to compensate for the lack of structural stiffness (which only would be needed when the tail is mounted to the fuselage and the damping forces need to be transmitted through it, then the wings to keep the tanks from wobbling).

So I'd call it a wash in that particular case but see other areas than configuration choice to give the design even better structural and aerodynamic efficiency.