Anatomy of a STOL Aircraft:
Designing a Modern Short Take-Off and Landing Aircraft.
"Form follows function"
By Chris Heintz
The world truly seems to be smaller today, thanks in large part to aviation. This has created a renewed interest in many of us to see what is around us, and not just to dash as quickly as possible to a new destination. While recreational aviation certainly has its share of high-performance (fast) aircraft, I think that what continues to draw most of us to flying is the shear excitement, enjoyment and freedom of being at the controls of our own aircraft. We want aircraft to give us the ability to fly cross-country, but we want to be able to see and visit the country we’re flying over.
The popularity of aircraft like the Piper Cub has endured and grown over the years, not only on account of nostalgia, but because these aircraft are just plain fun and easy to fly and provide good grass field capability (most classic aircraft were developed in a time when paved runways were rare). However, because of their age, many of these older designs do not offer modern improvements that most of us take for granted, such as electrical systems, side-by-side seating, all-metal construction, steerable nosewheel, etc. And of course, classic airplanes are becoming scarce and require significant maintenance just to keep them airworthy.
For most of us recreational pilots, we’re already where we want to be when we’re in the air, and we therefore get the most enjoyment from flying an airplane that’s easy and fun to fly, that provides good comfort and visibility, and that has low operating costs (who cares about miles per gallon – we want low hourly operating costs). When we do fly cross-country, the trip is as important (if not more) as arriving to the destination. A STOL (short take-off and landing) airplane gives us the ability to go to more places, especially in remote areas, where the world becomes your runway (this is an important safety feature too). With good payload, we have the ability to haul all the bags we want (camping equipment), or amphibious floats can give us the added capability and freedom to operate from water. Of course, a STOL airplane also allows us the opportunity to operate the aircraft out of our own "back yard." Just as sport utility vehicles (SUVs) have become very popular in the automotive world, many recreational pilots are also seeking maximum utility from their aircraft
S T O L C H 8 0 1
Ultralight aircraft provide an easy and inexpensive way to experience STOL performance, and the popularity of ultralights and other light kit aircraft has proven the demand for ‘low and slow’ flying, but ultralights, by their very definition, have many limitations – low speed, low payload, low comfort level, and wind limitations, to name a few of their inherent limitations.
Today, with the knowledge accumulated for over a century on aerodynamics, structural strength, on their relation in aerolasticity (flutter), on ergonomics and with the ongoing development of modern, efficient, reliable and lightweight engines, it is relatively easy for almost anyone curious enough to seriously study the above fields to design a light aircraft capable of carrying two to four occupants.
As a professional light aircraft designer and engineer I have done just that … quite a few times.
In the mid-eighties, I decided to design a light kit aircraft that combined the advantages of an ultralight aircraft with the characteristics of a modern ‘real’ airplane. Thus I designed the STOL CH 701 aircraft: It needed to offer outstanding short and rough field performance, acceptable cruise performance, good cross-wind capability, excellent visibility, comfortable side-by-side seating, and a durable all-metal airframe - that was easy to build and maintain. The STOL CH 701 design proved to be very successful (more than 400 STOL CH 701 aircraft flying) and I subsequently designed a 4-seat utility version, the STOL CH 801 (introduced in 1998).
My STOL designs have sometimes been called ‘ugly’ because of their unconventional shape. However, with form following function, a study of the unique shapes shows the inherent beauty of these aircraft in their interesting, unique and highly effective aerodynamic and design features. Following is an explanation of the basic design concepts that I have applied in designing my STOL aircraft:
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.
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
Figure 1 – Stalled Airfoil
To delay the stall to a higher lift coefficient, many airplanes are equipped with flaps (on the wing trailing edge), and a few designs use slats (on the wing leading edge) to further lower the stall speed. The following diagram illustrates the use of flaps and leading-edge slats to increase a wing’s lift coefficient.
Figure 2 – Lift Coefficient vs. Airfoil Angle of Attack
The lift coefficient can thus be effectively doubled with relatively simple devices (flaps and slats) if used on the full span of the wing.
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 (Figure 3), 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
Figure 3 – Leading Edge Wing Slats
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.
Figure 4 – Fixed Leading Edge Slat Lift vs. 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. Figure 4 illustrates the lift coefficient and drag of such a wing design.
The illustration clearly shows that 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.
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, as illustrated below
Figure 5 – Wing Tip VorticesWith a rounded or squared wing tip, the vortex is centered around the wing tip, as shown above
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.
Figure 6 – Drooped / Raised Wing Tips 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.
Figure 7 – Hoerner Wing Tips
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.
Because a STOL airplane can fly at very low speeds, and is developed to operate in unimproved areas (often with obstacles), controllability of the aircraft at slow speeds is essential. This is one area that I’ve found to be lacking in many high-lift light aircraft designs – while many of these planes have a low stall speed, the pilot needs to fly the aircraft at a much higher speed in order to maintain control.