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A Look at Stall Warning Devices
By Tony Bingelis (originally published in Sport Builder, September 1983)
ANY AIRPLANE THAT does not give the pilot unmistakable warning (buffeting, shaking, etc.) that a complete stall is developing is a dangerous airplane to fly . . . no matter how delightful its other flight characteristics might be. An airplane like that is dangerous to any pilot in the traffic pattern where a sudden distraction or a miscalculation due to a momentary lapse of attention can lead to a stall, especially while "turning final" (an ominous term, isn't it?).
One moment the pilot is in complete control and the next he is staring at the ground in shocked disbelief. He realizes, too late, that he allowed the airplane to stall! In spite of his frantic effort to raise the nose by yanking back on the control stick so hard the elevator hits the stops, the nose down pitching remains uncontrollable, as uncontrollable as the sudden wing drop. The accompanying rapid loss of altitude becomes excessive. If the airplane's altitude is too low and its recovery is too slow, too bad! (That's no way to recover from a stall, is it?)
Sitting comfortably in your easy chair you know that trying to raise the nose of a stalled aircraft is a suicidal act. The correct action is to pop the nose down, isn't it? It seems we all know that except for the guys who have spun in.
Aircraft manufacturers are very much concerned over inadvertent stalls and go to great lengths to build into their aircraft well announced and gentle stall characteristics. Consequently, many of their aircraft models have two different types of stall warning devices. One is the stall strip and the other an aural warning horn that is often augmented by a light to also alert the pilot's visual sense. Piper's 1983 Saratoga model even has two separate stall warning horn systems installed, one for a flaps up condition and the other for flaps down.
However important such devices might be, a newly certificated homebuilt is not likely to be similarly equipped. It's not that the homebuilders have anything against stall warning devices; it is just that they assume the things cost too much and that they cannot be accurately installed . . . for the initial flight test anyway. Well, although the aural stall warning horn devices are moderately expensive, the stall strips are not. Certainly, one such device is better than none - if needed. As for the argument that the stall warning device may not be accurately installed for the first flight, that is a logical enough conclusion. After all homebuilts do feature a wide range of airfoil types and wing configurations. And, since plans rarely, if ever, detail the installation of stall warning devices, the builder is on his own . . . with a number of questions to resolve.
Just where should the stall warning device be located? How far from the wing root? How high or how low on the leading edge should it be? Is it safe to cut a hole in the leading edge and to gouge out a rectangular opening large enough for the stall warning horn relay? With such seemingly unanswerable questions and the unattractive price of a new stall warning unit, it is not surprising that the installation of an aural stall warning unit is not a high priority item with homebuilders.
Stall Warning Devices? Who Needs Them?
Hold on now, let's not dismiss the subject too casually. Nobody can tell whether or not a particular airplane will need some sort of stall warning device until after the airplane's stall characteristics have been experienced in flight tests.
Here's some guidance. If your airplane exhibits a marked buffeting or a shaking sensation felt through the airframe or the control system, you don't really need a stall warning device. If your airplane requires a very pronounced and deliberate rearward pull force on the control stick to make it stall, you don't need a stall warning device. On the other hand, if your airplane, without warning, characteristically tucks its nose and/or exhibits a very pronounced rolling tendency anytime you let your airspeed get a bit low, you have an airplane that must be considered to have dangerous stall characteristics. Such an airplane should be re-rigged and/or equipped with stall strips or at least a stall warning horn.
Actually, the installation of stall strips or a stall warning horn need not be limited to aircraft that exhibit undesirable stall characteristics. Airplanes can be stalled under a variety of conditions of speed and attitude, therefore, even an airplane with normally docile stall characteristics could, under some conditions of flight, develop a beastly behavior.
Stall Strips Vs. Stall Warning Horns
A stall warning horn, correctly installed and adjusted, will start its unearthly raucous blare anytime the aircraft wing approaches within 5 to 10 mph of a stalled condition. This type of warning device does not change the airplane's stall characteristics at all. It does not, cannot, make the stall more gentle. All it can do is announce with its strident sound that if you don't do something pronto, the airplane is going to stall.
Stall strips, on the other hand, when properly fabricated and installed can serve to warn you of an impending stall and, to a degree, may even alter the stall characteristics of the airplane. So, although stall strips are less expensive and easier to install, believe it or not, they are, in my estimation, more functional than are the aural warning horn systems. This may explain why manufacturers often equip their aircraft with both systems.
The Lift Detector Stall Warning Horn
This is the device that has been around a long time. It consists of a horn mounted in the cockpit (usually behind the instrument panel) and an electrical relay unit operated mechanically by changes in the slipstream. This unit requires an electrical system (12 volts or 24 volts DC).
These units may be purchased from any of the larger homebuilt supply outlets. If you prefer, you can save a lot of money by buying only the lift detector unit and get a door buzzer from Radio Shack as a substitute. Of course, it is not FAA sanctioned but is about $70 cheaper. It won't be loud enough, you say? You could always use two buzzers for a total cost of less than $8.00 (1983 prices). For that matter, you could go all the way and wire in a warning light if you want one . . . again Radio Shack.
The lift detector unit is mounted in the leading edge of the wing, usually in an area of undisturbed air flow. This, of course, means outside the propeller disc area. The lift detector will not react to the normal airflow over the wing, regardless of the airspeed. However, anytime the airflow is altered, as when the angle of attack increases in an approach to a stall, the disrupted airflow will force the lift detector upwards and activate the relay to which it is attached. This sends an electrical current to the warning buzzer and/or light.
Well, how useful is this unit? It will warn you of an approaching stall provided it is properly installed. This warning will come regardless of the aircraft's speed or type of maneuver in progress. If you deliberately continue to invite a stall, the warning horn will continue its irritating blare as long as the wing is in an abnormally high angle of attack and the flow of air over the device is disrupted.
Obviously, the unit cannot warn you of an impending stall if it is not working. You can minimize this risk if you include the check of the stall warning horn in your preflight inspection - you do make 'em, of course. Simply turn on your Master Switch and walk over to the lift detector and raise that little tab and listen for the horn to go off. If you hear it you know that the unit is working. You also learn that the battery is still alive.
Incidentally, this same check can be used for another purpose. Just before you close and lock the hangar door, go over to your trusty stall warning lift detector and gently raise the little tab. If nothing happens (no horn noise) you know positively that your Master Switch if OFF. Isn't that much easier than climbing back on the airplane for a look at the switch?
How do you determine where to locate that warning horn lift detector? One thing is certain, the stalling characteristics of an airplane cannot be precisely predicted by mathematical methods. As you know, the stalling characteristics of most any airplane are markedly different under different flight conditions of power application and flap setting. The best guidance I can offer is to suggest that the photos accompanying this article are quite representative of such installations. A trip to the airport for a first hand viewing of lift detector installations would also be helpful.
An aircraft's wing (airfoil) will influence the lift detector location more than any one other factor. Ordinarily, you will find the device positioned somewhat lower on the leading edge than you might expect. Since the unit has slotted screw holes in the attachment plate a degree of vertical adjustment is possible after the initial installation. This feature may not be as good as you would believe as any adjustment of the lift detector up or down from the original placement will probably require the enlargement of the hole in the wing. Another thing . . . since that lift detector unit's plate must be screwed onto the leading edge, its installation will seem crude to the homebuilder who has a nice slick airplane with no drag producing bumps on it. That could, indeed, be the real reason we don't see more of these units on homebuilts.
The Reed Horn Stall Warning System
Here is an improvement over the lift detector type warning horn. This system will be found on Cessnas manufactured after the late 1970's. It, too, is operated by a change in airflow over the wing's leading edge. This device does look like the older lift detector warning horn installation except for the impression it gives that some vandal has pulled the flapper (lift detector) out leaving only a vacant rectangular opening. This appearance is misleading. Actually, that vacant port is a tip-off that a simplified and more economical stall warning reed horn warning system is installed.
The simple construction of the device and the equally simple installation is very much evident. The external part of this lightweight system fastens to the leading edge of the wing by means of an adjustable slotted plate that can be adjusted up or down about 3/16". Behind the adjustable plate is a scoop-like piece to which a bug screen is affixed. This, in turn, is attached to an adapter to which a plastic tube is fitted. The plastic tube, being very flexible, is easily routed up to the wing root where a plastic reed horn is stuck into the other end of the plastic tube. That is all there is to the system. No electric wiring or power is needed. As for the stall warning horn, it is nothing more than a simple reed type horn . . . something like a party noise maker.
The horn actuates just before the wing stalls, very much like the other type of stall warning horn - 5-10 mph above the stalling speed. The horn sounds off when a negative air pressure is induced at the wing's leading edge. This causes a reverse airflow through the horn.
The adjustable plate mentioned earlier controls the speed at which the horn blowing takes place. Moving the plate upward will cause the horn to actuate at a higher airspeed. Moving the plate downward conversely causes it to blow at a slower speed. The reed horn stall warning systems adjustable plate is installed in approximately the same position on the leading edge as the other model would be.
Unless you luck out on the first try, final adjustment can only be obtained after one or more flight tests.
You can check out the horn operation on the ground by covering the opening in the adjustable plate with a cloth, putting your mouth to it, and inhaling to create a slight suction in the system. This intimate action causes the horn to make its characteristically audible raspberry.
A reed horn warning system is better suited for a high wing airplane where it is possible to locate the horn at ear level in the cockpit. It probably wouldn't be as audible in a low wing airplane where the horn would most likely be situated somewhere behind the instrument panel. At any rate, the reed horn warning system is lightweight, easy to install and to adjust.
Stall strips are located where they are on the wing because the builder (manufacturer) has determined that that is where they work best . . . for a particular airplane. The sharp edge of a stall strip is intended to disturb the smooth flow of air over the wing surface in the area where it is installed . . . and do it just before the rest of the wing begins to stall. This disturbed flow or burbling will cause the airplane to shake or shudder slightly, a fair enough warning for any pilot that a stalled attitude is being approached. If set properly, the stall strips will do their thing about 5-10 mph above the normal stall speed of the aircraft.
Stall strips are usually located near the inboard leading edge area of the wing. Their purpose being (supposedly) to cause the inboard area of the wing to begin to stall first. This, theoretically, provides a more gradual stall and enables the ailerons to be effective longer. However, don't be surprised to find stall strips located almost anywhere on the leading edge . . . even out toward the wing tips on some aircraft. Since the purpose of stall strips is to announce the impending stall, their function is best served by creating a disturbance in airflow where it will be most noticeable to the pilot. Apparently, sometimes this is most effective at the ailerons. Because of this non-standardized use of stall strips, you can only conclude that you too will have to experiment with your own stall strip requirement.
Stall strips are triangular lengths of aluminum angle, plastic or wood. Wood strips are more practical to use in running preliminary tests.
A typical stall strip is either 3/8" x 3/8" or 5/8" x 5/8" in width and has an average length of about 12". The lengths observed varied from 6 3/4" to 20".
When installing stall strips, a cardboard template duplicating the first 6-8" of the leading edge portion of your wing can be most useful. Cut a triangular notch for the stall strip where you want it positioned. The template will help assure a symmetrical installation on both wings.
Attach your stall strips initially with masking tape or gray duct tape but do not fair the strips into the wing. Your objective is to maintain a sharp triangular cross section. Next, make a flight check of the newly installed strips. The result you want is a slight buffeting or vibration in the airplane or the controls. This should ideally occur about 5-10 mph before the stall occurs.
If you aren't satisfied with your first efforts try different locations along the wing and place the stall strips higher or lower in an attempt to obtain the best results. Taping the strips on will permit such experimentation without hurting the finish on your airplane. After you obtain the pre-stall response you want, secure the strips to both wings with screws or pop rivets.
The installation of stall strips makes an interesting experiment, particularly if your aircraft is bashful about displaying any pre-stall shake or shimmy.
I would assume that all stall tests would be done at a safe altitude and not in the traffic pattern. That's what I would assume, wouldn't you?