Below is an Email that I sent to an old friend of my fathers. As always when I get involved in a new design (previously a KR2) he becomes concerned about my safety. What got him concerned about the CT was my description that I avoided a nose high pitch attitude on landings to remain in a positive eneryg state. His thinking has always been to do full stall landings that mean a 3 point attitude in aircraft that he has flown.

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The arodynamics - limited ability to raise the nose above the horizontal on landings.

Don,

Now that I have a focus, which is raising the nose above the horizontal [on landings] I can look to aerodynamics to explain that this behavior is to be expected and does not indicate a lack of safety.

A bit of the design history and intent are a good place to start. I may have already mentioned that the design was originally a thesis for 1 or more (not sure) German engineering students. They accomplished something remarkable by combining a slippery aerodynamically efficient design, non-typical variable wing geometry, and an attractive power to weight ratio. The utilization of carbon fiber and Kevlar as well as a more efficient non-certified aircraft engine helped make it possible.

European micro-lights are operated somewhat outside the system much like part 103 operations in the United States but are far more capable and far less restricted. The design intent was to provide a relatively fast aircraft while maximizing useful load and economy that still worked well from small, private grass and dirt fields. The CTSW can demonstrate controlled level flight as slow as 40kts and as fast as 140kts IAS which is quite and achievement given a rate of fuel consumption typically less than 5 gallons per hour.

The CTSW is the current implementation of this design originally called the Pegasus with an interim version called the CT2K. For the purpose of explaining the limited ability to raise the nose above the horizontal on landings my focus will be the variable wing geometry and its effect on pitch attitude.

Most pilots realize that an aircraft's wing can be stalled at any speed and in any attitude. Stalling a wing is actually about exceeding the critical angle of attack which is the angle where the relative wind no longer conforms to the airfoil and the lift provided by the airfoil is lost resulting in only impact lift remaining. It is important to realize that stalling at any speed or attitude is a concept that changes when flying very close to the ground and level due to the fact that the relative wind is now only variable in the horizontal (crosswinds) but not the vertical. In this configuration the relative wind can no longer have much if any vertical component and now the concept of stall speed has real meaning.

The CTSW's wing design is unremarkable when considering its planform, chord, dihedral, and span. The variable airfoil shape is where it differs from typical light aircraft in a meaningful way. Consider 2 aspects, first the degree to which the airfoil shape can be changed is exaggerated by the use of a reflex position for both flaps and flaperons, second the full span effect of a combination of both flaps and flaperons. The control surfaces on the CTSW's wings are commonly referred to as ailerons but this is a misnomer. The flap and flaperon settings on the CTSW in degrees are 40, 30, 15, 0 and -6 [or -12]

My first aircraft utilized flaperons but no flaps and resulted in a variable geometry that performed similarly to a small Cessna. When adding both flaps and a reflex position the speed range increases in both directions (slower and faster.) Of course every design and even every design aspect is a compromise. This design achieves a pronounced low pitch attitude when 30 or 40 degrees of flaps and flaperons are deployed even if it wasn't obvious to you in the photos. The photos failed to demonstrate how close to the ground or how short the final approach was making it harder to see the difference between its pitch attitude without meaningful power and a small Cessna's. At the same relative position a 172 would already be rotating and appear more level if not pitched up and presenting its mains to the runway.

Understanding this discrepancy is easy when you realize that as when you deploy flaps you get a better look at the runway because the angle of attack is reduced and the amount of drag is increased due to changing the shape of the airfoil. The CTSW takes this concept to a greater degree and the result is a more limited ability raise the nose above the horizontal without stalling the wing when flying level and very close to the runway as in the final stage of a landing.

I can land and stop my CT in less than 500'. My takeoff roll is less than 300' and I can achieve TAS in cruise up to 130kts. The landing can be a bit unconventional but the performance envelope is impressive.

Ed


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Reply

The aerodynamic presentation is cogent and familiar. From this I gather that in a CT, and in a landing configuration at about 40kts, or lower, if you raise the nose to horizontal, you are already stalled (or stalling). Then the definition of a "full stall" landing for a CT becomes a "2 point" landing. That is going to fool a lot of general aviation pilots! Apparently you can't do a 3 point at all unless you obviate or minimize the use of flaps.