Heintz' Textbook

Airfoils - An Application

By Chris Heintz

[This article is part 4 of a series, where aeronautical engineer Chris Heintz continues his discussion on airfoil design with a look at the application on the STOL CH 701, and how the airfoil contributes to the STOL characteristics of an aircraft.]

In the last series of articles we have tried to grasp an understanding of airfoils - the sections of the wings which keep our airplanes airborne due to the lift provided by the airfoil geometry and the speed at which they move through the air.

Figure 1: In this illustration, we assume the landing from right to left.We have seen that without undue sophistication we can design an airfoil which provides a very high lift which will allow us to fly surprisingly slow. In other words. with a relatively small wing area and within a given weight, we can take off and land in a very short distance because we do not need to accelerate or decelerate to or from a high speed. We have also seen that the proposed airfoil has a favorable lift to drag ratio at high lift; this allows the aircraft to climb out or come in at a very steep angle allowing following performance such as shown in figure 1.

Keep in mind, however, that to obtain full advantage from the airfoil, both flaps and slats should extend full span over the whole wing.

The first idea will be to build one of those fantastic wings, design a mechanism to allow full span flaperons (flaps and ailerons combined in one full span unit and controllable as flaps with the flap control and as ailerons with the classic aileron control through a so called "mixer") ... and install this new wing on an existing aircraft. Although the idea sounds good, this is too simplistic of an approach. Remember, I have already warned that the addition of slats on any given wing will bring the aircraft "out of balance." In the present case, although the balance will be Figure 2: The horizontal tail will be "upside down."correct, we have forgotten that the high lift is achieved at an unusually high angle of attack (25 to 30 degrees versus the classic 15 to 18 degrees.)

To achieve this high angle of attack, the fuselage geometry must allow ground clearance at take off and on landing (see figure 2). From this standpoint, a tricycle landing gear is a big plus - apart from the fact that in normal ground position the wing has a smaller incidence with a tricycle gear than a taildragger and will be less sensitive to high (cross) wind taxiing.

But correct fuselage geometry is not all that is needed. Pitch control must be able to effectively control the incidence even at the high angles associated with slow flying. That means the horizontal tail must be able to push the tail down in order to lift the nose up and keep the angle of incidence high for those sustained high angle climbs.

Of course, I hear some of you talking canard, but don't forget that the downwash behind the wing will help a classic tail - but it will have no effect in the front!

No, what we need is a horizontal tail with a high negative lift; that is why it is "upside down." The elevator is designed to provide a certain "funnel effect" when deflected - see figure 3. The Zenair STOL CH-701 horizontal tail has a negative maximum lift coefficient of 2.8 when the elevator is deflected 40 degrees. But, for all this to be effective, the tail must be set so that in cruise the drag is minimized by having the elevator in line with the stabilator, and the whole tail airfoil at an incidence compatible with the general airflow at this location within the downwash of the wing.

Figure 3: Airfoil 1 CL Max =1.5; Airfoil 2 CL Max = 2.2; Airfoil 3 CL Max = 3.4And a few more details that are important to remember when designing a STOL aircraft are as follows:

  1. The landing gear must be stronger than average. Human nature being what it is, the pilot, knowing he has the ability to land in a very short distance may take advantage of this desirable feature and "plunk" his aircraft down in an unprepared 'pea patch,' resulting in some hard knocks for the landing gear to absorb.
  2. Visibility must be even better than with other aircraft designs because of the large variations in incidence. Slow flying may bring you in the way of faster aircraft, but remember with a STOL aircraft you are more maneuverable and have an advantage to give way – see figure 4 for an illustration of the visibility available in the STOL CH 701.
  3. Excellent controllability at very low flying speed is absolute necessity. Pitch control is automatically good due to the required horizontal tail design (see again figure 3). Roll control is not bad with full span ailerons by, again, use of the 'funnel effect' ' Instead of plain flaperons, carefully designed "Junker" type flaperons provide excellent roll control at low speeds without much drag penalty in cruise. (See figure 5)

Good yaw control can easily be achieved with an "all flying rudder" (see again figure 2). This relatively large moving vertical surface gives plenty of control in a crosswind, in the air as well as on the ground.

Figure 4And, finally, a strong airframe (6 g) is necessary so that the pilot does not have to fear losing a wing, which might be associated with the very light structure (430 lbs. for the Zenair STOL CH 701) used so that a low horsepower engine (50 to 100 BHP) can be used to obtain outstanding take off performance.

These are the trade marks of a good two-seat design.

As already discussed in a previous article, pilot comfort, crashworthiness and an easy-to-build, long lasting airframe are simply routine items required when designing a new generation aircraft, for which there is apparently a need as shown by the success of the Zenair STOL CH 701.

One question which frequently crops up is:
How can we increase the cruise speed of the aircraft?
With the 65-hp Rotax 582, the Zenair STOL CH-701 at full gross stalls at 28 mph, cruises at 74 mph (with a top speed of 82 mph). Considering that the cruise speed is 2-1/2 times the stall speed, this is not bad at all. Even retracting the slats would not increase the cruise very much. The apparatus needed to make this change work are complicated and heavy and, above all, would require an additional control. Considering the 50 or so hours most of us recreational pilots fly per year, we already have our hands full without this additional control. It would be catastrophic to pull "in" at the wrong time . . . and an automatic device is even more complicated.

"Keep it simple and it works" is my motto. If you want to fly faster, there are other good designs. For example, the two seat Zenair Zodiac stalls at 44 mph and cruises at 105 mph, with the same horsepower as the STOL CH 701.

Next time we will take an in-depth look at wing tips and how they contribute to airfoil performance.

Figure 5

 

This article is presented as part of a series, where aeronautical engineer Chris Heintz discusses the technical aspects of his light aircraft designs in laymen terms.

This article is part 4 of his discussion series on airfoil design for light aircraft, and was first published in EAA Experimenter magazine (June 1987). 1987, Chris Heintz.

See also: 
"Anatomy of a STOL Aircraft: Designing a Modern Short Take-Off and Landing Aircraft." by Chris Heintz

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Zenith Aircraft Company: 2006-03-30