In a previous RVator we discussed some of the choices and decisions we’d made when designing the RV-10 wing. We were a bit coy about the airfoil we’d chosen, implying that it was a deep, dark secret. Actually, the only secret is that it is a new airfoil that hasn’t yet flown, and is making its debut on the RV-10. It could be a disaster, or it could be revolutionary breakthrough.

In reality, it will probably be neither. Airfoil design technology has progressed to the point where results are reasonably predicable. Airfoils are designed and tailored to correspond to the performance and handling goals of an airplane. With modern tools, it is often possible to do better with a new "tailored" airfoil than with an "off-of-the-shelf" airfoil. But not much better! Considering the tools available to them, the NACA engineers of the 1920s and 1930s did a remarkable job of designing a family of general-purpose low (subsonic) speed airfoils. These are the airfoils used on all of the production airplanes you have flown, and on most homebuilts like the RVs, other than the RV-9A. There are some relatively new airfoils which have been widely acclaimed and tried in recent years. We did not choose one of these because the mission they had been designed for is not our mission.

The stall speed of an airplane is dependent on the maximum lift coefficient of the wing, so it is desirable to choose an airfoil which offers a high maximum lift coefficient. The problem is that airfoils designed for maximum lift also have very high drag. Most airfoils in common use are low drag "cruise" airfoils which we convert to higher lift coefficient airfoils by designing hinged flaps which can be deflected to approximate the shape of a true high lift airfoil.

Designers are constantly in search of the "miracle" high lift/low drag airfoil. The hard truth is that it doesn’t exist. Neil Willford, in a recent Sport Aviation article, calculates the wing area needed for a hypothetical Light Sport Airplane class design. He assumes a lift coefficient of 2.0 for a flapped wing. Interestingly, if you read my article on LSA in the 5^th 2001 RVator issue, you will note that I assumed a lift coefficient of 2.15 for my hypothetical airplane. I recently checked the specifications and performance of several production airplanes including the new Cirrus and Columbia. Based on their listed weights and stall speeds, their maximum lift coefficients calculated to be nearly identical: just over 2.00. In the real world, a lift coefficient of 2.0 is an about as high as is achievable with a good general-purpose airfoil and fixed hinge flaps. Since homebuilts and ultra-lites don’t have to verify their performance claims, very optimistic stall speeds are often listed. I am reminded of a conversation over 10 years ago with a fellow kit designer who was expressing his amazement over the 28 mph stall speed quoted for the then-new Avid Flyer prototype. He had done some simple math and concluded that its airfoil had a maximum lift coefficient of something over 3.5. I was surprised by his naiveté for assuming the stall speed was really 28 mph, rather than my position of questioning the reported stall speed based on the improbability of such a high lift coefficient. In reality, the stall speed was probably a few mph higher, and the lift coefficient was a more realistic 2.5 or so.)

All RV models prior to the RV-9A used very basic flap systems. Their relatively low wing loadings and short wings limited the advantages to be gained with more elaborate flap designs. With the RV-9A, lower landing speed was a primary goal. So, we designed a single slotted flap system, which along with the greater flap span possible with the longer wings, yielded very good results. The higher gross weight and wing loading of the RV-10 suggested that we use a good flap design, at least as good as that of the RV-9A, to keep landing speed as low as reasonably possible. While we did choose a slotted flap design, we decided to stick with a fixed hinge point rather than the more idealized and complex flap track system common to "Fowler" flaps. With a flap track, it is possible to tailor the flap movement so that the position and angle of the flap is idealized throughout its travel. For a flap with a simple displaced hinge pivot point, that pivot point is located so that the flap swings back to some idealized position at maximum deflection. For intermediate positions, exact flap position and angle are slightly less than ideal. We consider this to be a minor compromise for the simplicity of the fixed hinge point.

In an earlier RV-10 report, we talked about experimenting with side-stick controllers. After frustrating prototype efforts and discussions with knowledgeable users and designers, we have (for now) settled on conventional between-the-knees sticks for the pilot and co-pilot. The mechanism and geometry needed for a side stick controller proved too complex. Our experience entering and exiting our cabin mock-up (the Pine Pigeon) has shown that our original concerns about cabin entry difficulties with this configuration were unfounded. We find that we can step into the front seats and over the control sticks in a manner similar to entering our 2-seater’s cockpits. Actually, it is even easier because of the low door sills. Most side-by-side 4 seat production airplanes have used control wheels and columns. One reason is that for cabins with a door on only one side, entry and exit would be very difficult with floor mounted control sticks. However, we feel that the control stick is a little more natural and "sporty", plus being easier to design than a control wheel arrangement.

From the RVator.  Second Issue, 2002  posted 05/15/02

The wings and tail for the prototype RV-10 are going together in our shop. Now that the RV-9 is flying, the shop crew of Miles Towner, Scott McDaniels and Phil Duyck have more time to dedicate to the four-place airplane. We expect they will finish the Ten components they have on hand quite quickly. RV-10 wings, from the builder’s viewpoint, are very much like RV-9 wings, although the prototype flap is impressively long: an inch over eight feet. When the first assemblies are finished, we do what every builder does…hang the tail on the rafters.

    

On the engineering floor, Mike Schwarz, Ken Krueger, Van and draftsman Phil Rivall are all pursuing different aspects of the airplane…fuselage, landing gear, engine mount/cowling, etc. Ken noted "The last major milestone that we passed was, of course, the wing static test. We now are working our way through the many, many details that will define the fuselage. You won't be hearing much about this because there aren't many "milestone" type events that are associated with that part of the design. Progress on the fuselage will be slow, again because of the number of details that must be worked-out along the way." That’s the truth! For the next several months, don’t expect much in the way of progress reports. When we do have something to "show-and-tell" it will be posted on the website and appear in the RVator.

The wings for the RV-10 prototype are now sitting in our shop.Other than the size, the different airfoil and the big flap hangars extending out the back, they look like a typical RV wing…a big plank. There are a pair of spanwise stiffeners running from the root to the tip. This is a light, simple way to add some strength without going to thicker heavier skins.

The pivoting slotted flaps require 3 big hangers. These are made of heavy aluminum and riveted to the rear spar and wing ribs. They run through the bottom wing skin and establish the hinge point of the flap several inches below and aft of the bottom of the wing.