52 NOVEMBER 2009

EAA Sport Aviation 53

Canard design considerations

NEAL WILLFORD, EAA 169108

G rowing up, my annual vacation was attending the EAA convention with my dad and brother. I consider myself

fortunate, but perhaps the biggest drawback of frequent attendance is that the memories started blending together—unless something profoundly unique stood out.

As a 12-year-old attending Oshkosh ’75, two things did just that. The first was the hot, dry weather. The second was a unique little homebuilt parked on its nose. It was Burt Rutan’s prototype VariEze. It had a canard or tail-first configuration, was powered by a pusher engine, and featured a moldless composite-construction method pioneered by Rutan. Plans for a refined version of the VariEze were made available the following year, and it became extremely popular with homebuilders. Over the next 10 years . . .

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Decades before Rutan, the Wright brothers’ 1903 Flyer was the first successful application of the canard configuration for powered flight. According to a 1986 article by Seth Anderson in a NASA publication, the Wrights selected it based on intuition and Wilbur’s concern that Otto Lilienthal’s fatal accident was associated with his glider’s aft-tail configuration. The early Wright aircraft were not stable in pitch, though, and in 1909 Orville urged Wilbur to adopt an aft-mounted tail to improve stability. Orville did add an aft tail to the canard-equipped Wright Model A in 1909, and by 1910 the brothers had dropped the canard altogether. By that time most other aircraft designers had also been migrating to this configu-ration. In the following decades, canard aircraft were designed and built by a variety of designers, but not with the level of success Rutan had with his designs. Canard: A Rev-olution in Flight, by Andy Lennon, provides an interesting historical overview of canard aircraft for those wanting a closer look.

When properly designed and built, the canard configuration can provide stall departure resistance. This can be a challenge and should not be taken lightly by the aircraft designer. With that caveat, let’s take a look at some of the design consid-erations for canard aircraft.

All naturally stable aircraft are con-sidered stable in pitch as long as the center of gravity (CG) is located ahead of the aircraft’s neutral point. The neutral point is defined as the loca-tion along the wing’s mean aerody-namic chord (MAC) where a change in angle of attack does not result in a change in pitching moment. A pilot flying with the CG at this location would find that the aircraft would not tend to return to the trimmed air-speed if disturbed in pitch, either by a gust or control movement. Instead, it would tend to stay at the new angle of attack. This would require constant pilot attention and be annoying and potentially deadly. Consequently, the aft center of gravity limit is set ahead of this location to ensure that the air-craft has what aerodynamicists call a “positive static margin.” Flight testing

by the National Advisory Commit-tee for Aeronautics during the 1940s found that aircraft with a stick fixed static margin of at least 8 percent MAC had good flying qualities. Any-thing less than 3 percent MAC was considered dangerous.

An aircraft’s neutral point is influ-enced by a variety of factors, includ-ing the size and aspect ratio of the wing, canard or horizontal tail, the distance between those surfaces, the fuselage shape, and power effects.

A stable aircraft also requires that the pilot be able to trim at a desired airspeed, or at least hold it in trim with a reasonably low force. For most conditions this requires some amount of load on an auxiliary flying surface (either a horizontal tail or canard). An exception to this is a flying wing, which can be trimmed through the careful selection of airfoil(s), elevator deflection, and/or a combination of wing sweep and twist. However, for our discussion we are going to ignore this option.

At first glance, you might think that in trimmed flight a canard sur-face with the same area and location ahead of a wing would carry a simi-lar load as a horizontal tail located the same distance behind the wing. This is not the case, as we will see in a moment. An isolated wing has a neutral point at its aerodynamic center, which is the location where the wing’s lift is applied and also the pitching moment is constant. The aerodynamic center is at 25 percent MAC in theory and doesn’t vary too far from this point in reality. Adding a horizontal tail behind the wing moves the neutral point aft of the 25 percent position, with the exact loca-tion depending on the wing’s down-wash characteristics as well as the size and location of the horizontal tail. Since stability requirements dic-tate that the CG needs to be located ahead of the neutral point, the end result is that the CG is generally close to the 25 percent MAC position for a conventionally configured aircraft.

The situation reverses when a sur-face is placed ahead of the wing, with the neutral point now moving ahead of the 25 percent MAC. Estimating the amount it moves forward is more complicated because the wing is par-tially in the canard’s downwash, and it’s best determined through wind tunnel testing or using a computer

Jim

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Due to its unique center-of-gravity configuration, the Long-EZ parks with its nose wheel retracted.

. . . the Rutan Aircraft Factory (RAF) developed several more canard designs, including the popular Long-EZ. Rutan stopped selling plans in 1985 to focus efforts on his other company, Scaled Composites. Though plans for Rutan’s canards are no longer for sale, there are a few Rutan-inspired designs still available to the homebuilder.

EAA Sport Aviation 55

aerodynamic vortex lattice or panel method program. Assuming that the canard aircraft’s neutral point loca-tion is known, placing the CG ahead of it for stability will result in a CG well forward of the 25 percent MAC.

It’s pretty straightforward to deter-mine the load sharing between the wing and horizontal tail or canard required to trim an aircraft once the CG location is known. Figures 1 and 2 show how the load sharing require-ments differ between the two con-figurations. Each figure compares the percentage of canard or horizontal tail area (in terms of the combined area of the wing plus the auxiliary surface) to the percentage of total lift that the surface needs to keep the aircraft in trim. The estimated trends in both fig-ures were generated using a computer vortex lattice program and include the following assumptions:

• CG located 10 percent MAC ahead of the neutral point.

• Wing and horizontal tail/canard have an aspect ratio of 7.5.

• Tail arm equals 2.5 times the wing MAC.

• No fuselage or power effects.

Figure 1 shows the trends for a horizontal tail configuration, and several observations can be made. The first is that the percentage of lift the horizontal tail provides goes up with increasing tail size. This is because increasing tail area moves the neutral point farther aft from the wing 25 percent MAC. And since I kept the CG a constant 10 percent MAC ahead of the neutral point, this means that the CG moves farther away, too. The farther the CG is from the wing’s 25 percent MAC, the great-er the load the auxiliary surface needs to provide. Though the trends are for a large range in tail area percentages, most “conventional” aircraft have horizontal tail areas in the 15 percent to 20 percent range. Consequently we will focus on that range for the other observations.

The second observation is that the horizontal tail load is affected by the magnitude of the wing’s pitch-ing moment. Most airfoils have a negative pitching moment coeffi-cient (CM), meaning that they cre-

Figure 1. Approximate aft horizontal tail lift requirements.

ate a moment that tends to twist the wing’s leading edge down and trailing edge up. This nose-down moment is reacted by the horizontal tail and reduces the amount of lift it provides. Most airfoils used on light aircraft have a CM between 0 and -0.1 (flaps up), which becomes more negative with flaps down.

Depending on the horizontal tail size and wing pitching moment coef-ficient, Figure 1 indicates that a con-ventional aircraft can have a slight lifting load in flight. For those cases, the CG is behind the wing’s 25 per-cent MAC, and consequently the horizontal tail has to provide a lifting

load to trim the aircraft. Moving the CG forward increases the static mar-gin and has the effect of shifting the curves down in the figure, and it may result in a zero or slight downward load on the horizontal tail.

One of my initial assumptions was that the wing and either horizontal or canard have equal aspect ratios. This is normally not the case for aft-tail configurations, but they were kept equal for this study to keep the number of variables to a minimum. Using a lower aspect ratio for the horizontal tail results in the same general trends, but with the curves shifted down some.

Figure 2. Approximate canard lift requirements.

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The last observation from Figure 1 is that the percentage of lift (either up or down) on the horizontal tail is quite a bit less than its percentage of total lifting surface area. The implica-tion is that the horizontal tail’s lift coefficient requirements are lower and historically have been met by using a symmetrical airfoil equipped with an elevator.

Figure 2 shows the estimated load-sharing trends for a canard configuration. The major difference from Figure 1 is that the canard always has a positive lifting load. As I mentioned ear-lier, this is because the canard aircraft’s neutral point (and resulting CG loca-tion for stability) is always ahead of the wing’s 25 percent MAC. Focusing on the same 15 percent to 20 percent area range we did earlier, you can see that the canard would need to provide roughly 20 percent to 30 percent of the total lift to keep the aircraft in trim. Moving the CG farther forward would result in shift-ing the curves up some, indicat-ing an increase in canard loading. The canard lift requirements are fur-ther increased if the wing’s pitching

moment coefficient becomes more negative due to deploying flaps. This is one of the reasons flaps are rarely used on canard designs. The Beech Starship was one of the few that did use wing flaps, and it incorporated a forward-swinging canard to increase distance from the wing to the canard. Other designs have used speed brakes located on the belly of the fuselage to allow steeper approaches for landing. The final observation is that unlike the aft horizontal configuration, the percentage of canard loading is always higher than its percentage of

total lifting surface area.

The canard’s higher lift require-ments mean that careful attention must be paid to the design or selec-tion of the airfoil. Designers often use a high aspect ratio planform for the canard, because it stalls at a lower

angle of attack than a comparable lower-aspect-ratio surface, and you want the canard surface to stall at a lower angle of attack than the wing. The consequence is that the canard’s chord length tends to be short com-pared to the average wing chord. An airfoil’s characteristics are subject

to scale effects, and aerodynamicists found that they can quantify those effects by examining an airfoil at dif-ferent Reynolds numbers. Reynolds number is the ratio of the inertia forces of the air flowing around an airfoil to the viscous forces of the air. The inertia forces depend on the air’s density and speed, whereas the viscous forces are a measure of the air’s “stickiness.” At sea level, stan-dard-day conditions, the following formula can be used to estimate the Reynolds number of an airfoil:

Reynolds number ≈ 9360 x airspeed x chord

…where the airspeed is in miles per hour and the wing chord is in feet. For example, the Reynolds num-ber for a wing on a light aircraft can range from 2 million at stall speed to 7 million or more at top speed. However, a canard’s Reynolds num-ber on a homebuilt can be around 500,000 or so. Normally an airfoil’s maximum lift coefficient goes down with decreasing Reynolds numbers. Rutan initially used the same airfoil (NASA’s GAW-1) for both the wing and canard on the prototype VariEze. Initial flight testing showed that the aircraft had a high stall speed, so he installed a new canard with an airfoil specifically designed to operate at low Reynolds numbers. This airfoil had a rather unwieldy designation of

While the tiny tailwheel pant/rudder of the Quickie in its original form was perfectly adequate in flight, it resulted in demanding ground handling characteristics. A taller vertical fin and a conventional rudder surface made the Quickie easier to fly and taxi.

The canard’s higher lift requirements mean that careful attention must be paid to the design or selection of the airfoil.

EAA Sport Aviation 57

GU25-5(11)8 and is shown in Figure 3. Subsequent flight testing demon-strated an 8-knot decrease in mini-mum flight speed with the canard airfoil change. More details about the development of the VariEze and Long-EZ can be found in Burt Rutan’s article “Tale of Three EZs,” published in the February 1980 edition of EAA Sport Aviation.

It is also important that the canard airfoil has gentle stall char-acteristics. NASA wind tunnel test-ing on a powered, full-scale VariEze (“Wind-tunnel investigation of a full scale canard configured general aviation airplane,” NASA TP-2382, Lon Yip, 1985.) included a con-figuration with the GU25-5(11)8 canard airfoil being replaced with a 12 percent thick symmetrical airfoil. The data showed that the “stock” VariEze had good post-canard stall characteristics. Replac-ing the original canard airfoil with a symmetrical one showed that the aircraft had a lower maximum lift coefficient (which would lead to a higher stall or minimum speed), an abrupt stall of the canard, and an undesirable pitch-up tendency in the post-stall range.

One more consideration in select-ing a canard airfoil is to under-stand what the effects of rain or other surface contamination may have on the aircraft’s flying quali-ties. Rain, bugs, or even paint stripes near the leading edge can prema-turely trip the laminar flow on the canard surface and cause an increase in minimum flying speed and a change in the trim requirements to maintain the desired speed. NASA investigated the effect of water and a fixed transition on the canard as part of its VariEze wind tunnel test. Burt Rutan wrote “Effects of Rain or Surface Contamination on Pitch Stability and Control” in EAA Sport Aviation, March 1983. The article provides some additional detailed flight test results for having an early, fixed transition on several Rutan air-craft. Sometimes rain effects require a modification to the aircraft, as was the case when vortex genera-tors were added to the canard of the around-the-world Voyager. RAF also tested several canards with differ-ent airfoils for the Long-EZ to better understand this phenomenon. The end result was a canard equipped

Figure 3. Canard airfoil used on the VariEze and Long-EZ.

Figure 4. VariEze calculated wing lift coefficient distribution during climb.

Figure 5. Drag area comparison for several canard aircraft.

with the R1145MS airfoil designed by John Roncz. According to RAF’s quarterly newsletter, the Canard Pusher, this optional canard offered a negligible trim change in rain and a 2-knot increase in stall speed under the same conditions.

Some canard designs like the Quickie, Q2, Dragonfly, and PAT-1 Pugmobile have a canard similar in

size to the wing. Though the larg-er canard results in the canard air-foil operating at a higher Reynolds number, care still needs to be exer-cised in selecting an airfoil. In his article “Quickie-Type Aircraft Design Origins” (EAA Sport Aviation, Octo-ber 1981), Burt Rutan discussed the background of the development of this tandem-wing arrangement. The

The blue curve shows the wing’s calculated CL distribution as affect-ed by the canard’s downwash. The downward shift of the distribution for the inboard 40 percent of the semi-span is caused by this por-tion of the wing operating at a lower angle of attack due to the canard downwash. This downwash reduces near the canard’s tip, caus-ing the local CL to increase until the two curves cross near the location of the canard’s tip (at 53 percent of the wing semi-span). You can also see that the canard’s influence also continues beyond its tip, as the wing’s CL distribution is now above that for the wing alone. This is caused by a vortex occur-ring at the canard’s tip created by the equalizing of the pressure differences above and below the canard. The tip vortex creates an upwash that increases the angle of attack on the outboard portion of the wing, resulting in the raised CL distribution. This increase can

be problematic if it causes prema-ture stalling of the outboard wing, and the VariEze had a segmented, outboard-wing leading-edge droop added to prevent this from occur-ring. Later, it was found that several small fences located below the lead-ing edge (called vortilons) provided the same benefit and had less drag, so they were incorporated into the VariEze and Long-EZ designs in lieu of the outboard droop.

While the canard’s influence on the CL distribution can be adjusted by varying the wing’s airfoil incidence along the wingspan, the canard air-craft designer also needs to take into account the wing-lift distribution as affected by the canard when sizing the wing spar. The altered lift distri-bution could impact the sizing of the spar web and caps.

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article “Wind-tunnel investigation of an advanced general aviation canard configuration,” NASA TM-85760, written in 1984 by Joseph Chambers, Lon Yip, and Thomas Moul, presents the wind tunnel results for a powered one-third scale model of the PAT-1.

The wing design for a canard-equipped aircraft also requires careful attention. The canard downwash has a noticeable effect on the wing’s angle of attack along its span. Figure 4 illustrates this, which is the calculated span-wise lift coefficient (CL) distri-bution for the VariEze wing during

climb. The green curve shows the CLdistribution for the wing without the presence of the canard, with the dip in the curve in the first 30 percent of the semi-span due to the wing strake. Outboard of the strake the CL distri-bution drops off toward the wingtip, but does not go to zero at the tip like it would on a typical wing. The reason is that the VariEze is equipped with winglets that are the aircraft’s vertical tail and rudder. These winglets also act like a wingtip extension and make the wing behave as though it has a longer span than it actually does.

These winglets also act like a wingtip extension and make the wing behave as though it has a longer span than it actually does.

EAA Sport Aviation 59

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Speed has always been one of the big selling points for aircraft. A fair question would then be, “How does the performance of a canard air-craft compare to a similar conven-tional aircraft?” There have been quite a few records set by canard aircraft, which is a good indication that they can do very well. AIAA Paper 84-2507, “Design and Anal-ysis of Optimally-Loaded Lifting Systems,” by Ilan Kroo is a theoreti-cal look at the big debate, and its conclusion gives the performance edge to a conventionally config-ured aircraft. Figure 5 provides a parasite-drag-area comparison of several high-performance canard and conventional homebuilt air-craft designs. The data came from David Lednicer and various CAFE Foundation flight tests. It has been adjusted to remove the estimated landing-gear drag area in order to provide a fair comparison. If we take the drag area for a particular design and divide the value by its exposed surface (wetted) area, we get its wetted drag coefficient. This coefficient is an overall indication of how clean a design is. Looking at Figure 5, we can see that the canard aircraft have a wetted drag coefficient around 0.0050 (50 “drag counts” in aerodynamic speak). This drag area is comparable to that of the T-18 and Glasair, but higher than a few of the other high-per-formance aircraft. The higher value is likely due to the higher drag of the canard airfoils used and the relatively blunt after-body on the VariEze and Long-EZ.

Depending on the designer’s goals, it is likely that the canard configura-tion will continue to be used on some future designs. As Rutan stated, “The designers’ database for these types of designs is extremely limited, and the importance of understanding their aerodynamics is great.”

An EAA member since 1981, Neal Willford learned to fly in an ultra-light in 1982 and received his pilot certificate in 1987. He has done design work on a variety of aircraft at Cessna, from the 172 to the Citation X. In recent years he has been heav-ily involved in the development of the Cessna NGP and 162 SkyCatcher. In his spare time he is finishing a Thorp T-211 Sky Scooter.

References:“A look at handling qualities of canard aircraft,” NASA TM-88354, Seth Anderson, 1986. Canard: A Revolution in Flight, Andy Lennon, Aviation Publishers, 1984.“Tale of Three EZs,” Burt Rutan, EAA Sport Aviation, February 1980.“Wind-tunnel investigation of a full scale canard configured general aviation airplane,” NASA TP-2382, Lon Yip, 1985.“Effects of Rain or Surface Contamination on Pitch Stability and Control,” Burt Rutan, EAA Sport Aviation, March 1983.“Quickie-Type Aircraft Design Origins,” Burt Rutan, EAA Sport Aviation, October 1981.“ Wind-tunnel investigation of an advanced general aviation canard configuration,” NASA TM-85760, Joseph Chambers, Lon Yip, and Thomas Moul, 1984.

“ Design and Analysis of Optimally-Loaded Lifting Systems,” AIAA Paper 84-2507, Ilan Kroo. Available online at http://Aero.Stanford.edu/Reports/MultOp/multop.html.

NASA reports are available online at http://NTRS.NASA.gov/search.jsp. EAA Sport Aviation articles are available online in the members-only section at www.Oshkosh365.org.