ANATOMY OF A ZENITH CH701
How does it get those STOL characteristics
The words of Chris Heintz - the designer of the 701. The full article including diagrams is available here in pdf format
POWER
Overpowering an existing aircraft is the easiest way to achieve short take-off performance (with enough power anything will take-off in a short distance!), but this requires a lot of fuel for acceptable endurance, and is an expensive, heavy, and inefficient way to obtain STOL performance, and does not provide good slow flight or payload due to the heavier engine weight and/or fuel load requirement. My experience tells me that I need 60 to 100 hp for a two-seat aircraft, or 150 to 200 hp for a four-seater capable of carrying 1,000 lbs. As an airplane designer and builder (and not an engine manufacturer), I design aircraft around existing and readily-available engines. For maximum flexibility and to keep costs low, a kit aircraft must be designed to accommodate different engine types so that owners can choose among existing (and new) powerplants.
WING
To be practical, a STOL aircraft must be able to fly at very low speeds, yet it must also offer acceptable cross-country (cruise) performance. The next big challenge is to design a wing with a high lift coefficient so that the wing area is as small as possible, while take-off / landing speeds are as low as possible. Relatively short wings make the aircraft easier to taxi, especially when operating in an off-airport environment with obstructions, and requires less space for hangaring, while being easier to build, and stronger (less weight and wing span to support).
The stall of the wing occurs at the highest lift coefficient on an airfoil, when the airflow can no longer go around the airfoil’s nose (leading edge) and separates from the upper wing surface.
To delay the stall to a higher lift coefficient, many airplanes are equipped with flaps (on the wing trailing edge), and the CH701 use slats (on the wing leading edge) to further lower the stall speed. The use of flaps and leading-edge slats can effectively double a similar wing’s lift coefficient.
LEADING EDGE SLATS
Leading edge slats prevent the stall up to approximately 30 degrees incidence (angle of attack) by picking up a lot of air from below, where the slot is large, accelerating the air in the funnel shaped slot (venturi effect) and blowing this fast air tangentially on the upper wing surface through the much smaller slot. This "pulls" the air around the leading edge, thus preventing the stall up to a much higher angle of incidence and lift coefficient. The disadvantage of the leading edge slat is that the air accelerated in the slot requires energy which means higher drag. As the high lift is needed only when flying slowly (take-off, initial climb, and final approach and landing) the temptation for the designer is to use a retractable device which closes at higher speeds to reduce drag.
But there is a simple solution: The amount of drag increase created by the slot depends on the amount of air going through the slot in the whole range of flight. In take-off and landing configurations we want maximum lift, and in cruise we want minimum drag. By equalizing the amount of air pressure on the top and bottom of the wing at the leading edge (where the slat is located) in cruise configuration, there is no air flowing through the slot, and thus no lost energy (or extra drag created). Equalizing air pressure is easily achieved in cruise configuration with a slight trailing edge upward deflection of the wing flap.
The wing with slats and flaps is the solution for slow flight where high lift is required, and also has little drag penalty in cruise. It is a light weight wing with no moving mechanical parts associated with the leading edge slats. A noticeable drawback is a relatively small low drag range, which means a narrow economical cruise speed range, but the overall configuration provides the best wing design for a STOL aircraft.
Thus, I have chosen this fixed slat configuration for the two-seat STOL CH 701 and the new four-seat STOL CH 801. The wing is lightweight, yet yields a very high lift coefficient, making it a very reliable, simple, and a low-cost high lift device for these two designs.
I have also used a relatively thick wing chord on these designs to provide high lift. The thick wing chord, combined with a relatively short wing span, also provides maximum strength and low weight. With its constant chord (as opposed to tapered) the wing is as also easy to build and assemble.
This can be done in different ways: The slats can be mounted on roller rails so that at high angles of attack they are automatically pulled out by the airstream around the leading edge, and in cruise (at lower angle of attack) they are pushed in. This is a relatively simple system and not too heavy to design, but it has one big disadvantage: in gusty weather only one wing slat may be drawn out while the other stays in, creating a potentially major problem for the pilot who now needs full aileron just to keep the airplane level...!
So the safe way is to connect the right and left wing slats mechanically to prevent asymmetric extension. However, creating such an installation is heavy and more complex. The efficiency gained by the system must be very significant to compensate for the extra weight of the device (not to mention cost and complexity). A pilot controlled slat extension system is another approach, but has the same drawbacks: weight and complexity.
WING TIPS
For a long time, I’ve said that Hoerner wing tips should be used on most light aircraft designs, since they increase the effective wing span from 8" to over one foot without having to carry any additional weight: As we all know, there is low pressure on top of the wing, and higher pressure on the bottom of the wing, with the pressure difference creating the lift that allows us to fly. Toward the tip of the wing, the high pressure ‘feels’ that there is less pressure on the top of the wing (just around the tip), and wants to go there to equalize the pressure, thus creating a secondary flow out toward the tip of the wing. This secondary outward flow generates a vortex (a circular motion) behind the wing.
With a rounded or squared wing tip, the vortex is centered around the wing tip.
With drooped or raised wing tips, the vortex is forced further out. Drooped wing tips are often seen on STOL aircraft, but they create a weight penalty since they need to be added to the wing.
If the wing tip is cut at 45-degrees with a small radius at the bottom and a relatively sharp top corner, the air from the
secondary flow travels around the rounded bottom but can’t go around the sharp top corner and is thus pushed outward.
The performance of the aircraft depends on the distance from the right to the left tip vortices (the effective wing span), and not the actual measured geometric span. Hoerner wing tips provide the largest effective span for a given geometric span or a given wing weight.
