Get The Angle?

WHAT'S THE ANOLC ? BY

TCRRCNC.E1

O'NJCILL

tAA 5572

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By Terrence O'Neill, EAA 5572 7517 Knightswood Dr., Fort Wayne, Ind. F YOU think your airspeed indicator will tell you if you're close to stalling, you are "asking for a pine Ibox!" Airspeed doesn't necessarily have anything to do with a wing's stall. A wing doesn't stall at a consistent speed . . . a wing does stall at a consistent angle! What you need is an angle-of-attack indicator. Make one yourself according to the acccompanying sketch or buy one, but get one for insurance for a long life. Many military aircraft now have angle-of-attack indicators, as do research and development aircraft. The truth is that you can't tell what your airplane is doing unless you can "see the air." An angle-of-attack indicator lets you see the air. Anyone can make a simple angle-of-attack indicator or have one made. Mount it on your craft outside of the prop-wash . . . four feet out on a lift strut or N strut is ideal. The leading edge of the wing is also good, and it should be at least 20 percent of the chord ahead of the leading edge. This location will keep it out of the wavy flow near the wing. A commercial and expensive angle-of-attack indicator mounts only a few inches ahead of the wing but, reportedly, it oscillates a lot in flight, and swings through 60 deg. from high speed to stall, while the wing swings through only about 15 deg. If you put your angle-of-attack indicator out about a foot and one-half, you will get the stall-angle reading somewhere between 15 and 25 deg., and a very steady indication. My indicator is mounted out about four feet

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from the fuselage on the wing leading edge. I've also put them on biplane N struts about two-thirds of the distance up from the lower wing and this is an ideal location for small biplanes. The mounts are very easy to make, and the airflow at this point is nearly unaffected by the wing wash. This little instrument will add years to your enjoyment of flying. ®

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13

INDICATOR A Fail-Safe Instrument By Terrence O'Neill (EAA 5572) 791 Livingston Street Carlyle, Illinois (Illustrated by Bill Blake)

W

OULD YOU like to fly with no fear of blundering into the invisible, dangerous stall that is responsible for at least 40 percent of flying fatalities? To do so you need

662; 1,070; 453; 853; 402; and 600 (Fig. 1). This was obviously not a group of student pilots; with but two exceptions out of 15 it can be said that all

only an air-angle indicator, by means

were certainly experienced pilots and two in fact were very experienced ones! Why did they stall? Careless?

of which you can keep tabs on the relationship between the attitude of your airplane and the angle at which the air is flowing over its wings. With such an instrument, approach to the stall becomes visible; you can relax and enjoy flying, always firmly under control. Without such a device, you cannot see the air and a stall can sneak up on you. Obviously when you can see and judge the angle at which air is flowing past your airplane, you can avoid stalls. Some pilots think experience protects them and some think that adequate airspeed is a guarantee against stalling, but they are wrong and statistics prove it! A recent FAA statement on air safe-

Certainly not! Somehow each got into a bind or let their ships slow down a little more than they should have and a dozen people lost their lives in these 15 stall accidents alone — primarily because the pilots involved couldn't see the air's angle to their planes. (Continued on next page)

ty revealed that more than 50 percent of all fatal crashes were preceded by stalls or spins!

In looking into the matter of stalls, I selected at random more than 50 crash reports and, upon analyzing them, discovered that nearly 40 percent of them were caused by, or were immediately preceded by, a stall. A consideration of the flying experience of the pilots involved in the reports I studied proves that experience as a safeguard against stalling is clearly a myth. To illustrate the point vividly, I took a sheaf of 15 reports out of the pile at random and noted the pilot hours on them just as they happened to come out of the pile: 51; 554; 150; 807; 12,000; 1,300; 16,000; 1,400; 4,000;

PUZZLE:

He WONDERED where the wind was coming from?

ANSWER:

With a wind vane he would have SEEM wind at 12°.

SPORT AVIATION

37

at least 25 percent resulted from pure, unintentional stalls. Another ten percent were caused by stalling out of aerobatic maneuvers. So that totals nearly 35 percent of homebuilt aircraft crashes being stall-caused. What is lacking is something to tell otherwise-capable pilots directly and understandably what the air angle is at any given moment. Here are two examples to illustrate the menace created by the invisibility of stalls. A Cougar was making an approach to a California airport at two o'clock in the afternoon. The pilot had over 1,000 hours flight experience including 80 hours in the type. He found himself sandwiched between an Ercoupe ahead and an Aztec behind. He turned into final in nose-high attitude and continued turning as if to make a 360 degree turn for better sequencing in this traffic situation — and stalled! The Cougar crashed into a house about a mile short of the runway with fatal results to pilot and passenger. This plane had not been slowed down inordinately — yet it stalled. To see why, let's turn to another example. Consider a heavier racing plane of the Goodyear Race type. In calling it "heavier" we mean it was heavy in proportion to its overall size. Say its empty weight is 720 lbs., its wing has

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13 Pilots over ItOO hrs. 2 Pilots under UOO hrs.

Air Angle (of attack)

Oncoming Wind

FIG. 2

AIR ANGLE INDICATOR .. .

(Continued from preceding page)

This angle varies in flight, depending on throttle setting, airspeed, use of the flight controls, air turbulence, maneuvering, etc. Fig. 2 illustrated the relationship between a plane's flight path and the air angle as it would exist when flying level at reduced speed. Obviously in this — and in many other — situations, an airplane is edged toward a stall. Sometimes, such as when deliberately flying slowly, a pilot knows it and is very much on guard. Other times, such as when his attention is diverted, he lets his ship edge toward a stall without realizing what is happening. A review in SPORT AVIATION of 101 homebuilt aircraft crashes showed 38

JULY 1970

a meager 66 sq. ft. of area and has a thin high-speed airfoil that features low drag at the expense of a very modest lift coefficient, and it is loaded with a 180 lb. pilot, with a 20 lb. parachute plus 80 lbs. of gas, giving a gross weight of 1,000 lbs. Now a race is on, and this ship is doing 170 mph on the straightaway trying to catch up with the pack ahead. In this flight condition only one-third of the available wing lift is being used. A pylon is reached and this ship's pilot rolls up the wing and comes back on the stick. The G-meter on the instrument panel goes up and up. Suddenly at about 3V2 G's the seat seems to drop out from under the pilot, the horizon flips to the other side of the windshield and keeps on spin-

ning. Mother Earth comes up to givethe ship a wallop for stalling at 170 mph. Or maybe the pilot is able to recover in time. Everyone on the ground says: "He hit the propwash of the ships ahead of him and it flipped him!" But any aeronautical engineer who might have been watching would say: "Oh, oh! a high-speed stall!" And he would be right! The racing pilot trying to make a sharp, fast turn pulled back on the control stick too far and too fast and forced his wing to meet the rushing air past its stalling angle. Speed didn't save him from stalling. Any wing will stall at a certain angle regardless of speed. All depending on the shape and characteristics of its airfoil, the flow of air over it changes from steady to turbulent at some particular angle and there's your stall regardless of your speed!

You've heard of stall-warning indicators. But do you know exactly what they are telling you when their lights start flashing and their horns blow? Only that you are somewhere between five and zero degrees of stalling. Not exactly how many degrees you are from the actual stall. This is all right under normal circumstances — but normal circumstances don't stall you! You stall when unusual circumstances gang up on you. Your engine stops hitting on one cylinder during a climb-out. Or, the air is hotter than you thought. There you are, going up slower than you're accustomed to and the trees at the end of the runway are approaching fast. Right then the stall-warning buzzer starts howling. That just adds to your panic! All it tells you is that you are somewhere between five and zero degrees of stalling, where what you want desperately to know is — how far do you dare pull back on the elevator control without losing everything? No buzzer can tell you that. Angle is the only constant flight indicator. Your plane's stalling angle is the only unchanging stall warning. While flying, it is hard to remember speeds, turn degrees, and G's. It's much easier to glance at an indicator which gives you precisely, the information you need—an air-angle indicator. It lets you see the angle at which your plane is riding relative to the wind. It lets you see exactly how close you are to the stalling angle. An air-angle indicator is a fail-safe instrument and in my own view is the only instrument that should really be required in an airplane for it is the way to avoid stalling, and if you avoid stalling you avoid a lot of crashes and

minimize the seriousness of the rest of them. How does an air-angle indicator work? Let's explain by giving an actual example. Several years ago I

mounted such an instrument on Gilbert "Art" Whitacre's little biplane, Old Hen Crow, and let him try it out.

flying at a safe margin below the

stalling point. The top sketch in Fig. 3 illustrates this point. Its center

sketch depicts a typical homebuilt

monoplane and the figures given bring out how slowly down only seven mph can put the ship on the verge of

stalling during an approach. Its lower

angle at which the wind was meeting his wings. So he flew safely!

It boils down to this. Air angle is fixed and dependable. Your plane flies as long as you keep its wing or wings pointed into the airflow within the region of about 20 degrees positive and 15 degrees negative. Fig. 4 illus-

He made some turns around Ed

Shenk's field at Lattto, Ind., and land-

ed. "How did you like it?" I asked. "Oh, fine!" Art replied. That didn't satisfy me. "Did you stall it? What did it read?" I persisted. "Well," he added, "I noticed that it showed about 20 degrees when the ship was getting ready to stall. I also noticed that when I made a normal turn at cruising speed, the angle went up! That

Oncoming Wind

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ANY AIRCRAFT ON FINAL

kinda surprised me!" A year later the indicator was still on his ship and I sounded him out

about his further impressions of it.

Now he used it for steep turns and "lazy eights", in which he held it at about 15 degrees. He had learned that his plane stalled at 20 degrees sharp, without warning, and so he flew approaches and cruised at about seven to eight degrees and climbed at ten degrees which he had found was the angle for best climb. At one point his airspeed indicator wasn't working 80 mph Approach final, Ig, at lU° air angle 73 mph Approach final, Ig stall at 17° air angle for a week or two and he flew by the air-angle indicator instead with no problems. 80 mph Air angle required to ROLL TO AIR ANGLE The air-angle indicator was on his 10° - - 15° ———— ship three years later, too, I found one day when I called to inquire about 20° - - 16° it again. He was sold on it. It had

been his experience that endless questions about it were asked of him when he appeared at fly-ins. "I tell them I use it for checking the angles

30° - - 17° 1*0° --18°

FIG. 3

during turns — the indicator shows the angle that the wing makes with the wind it flies into. For example,

sketch illustrates the relationship between a plane's actual angle of bank

riding at about 15 degrees and thus

You do get very close to the stall point during steeply banked turns! I asked Art if he thought his airangle indicator had kept him out of trouble. He replied that he uses it, at least subconsciously, all the time, to keep track of the air angle. One time, for example, he was trying some knife-edge maneuvers and noticed the air-angle indicator was getting into

my plane flies with the indicator showing seven degrees in straight and level flight and, as I said before, in stalls at 20 degrees. Knowing this, I use the indicator regularly when landing on short runways; I keep the ship

make it approach with appropriate steepness of glide without ever getting dangerously near the 20 degree point. I use is for checking air angles during turns — few pilots realize how much it increases with the angle

of bank."

and the increase in air angle necessary to make it execute a banked turn.

the red even though the airspeed wa.s staying the same. He thus realized he

The angle at which the air meets a wing can change momentarily, such

was getting close to a high-speed stall and eased off before it hit him.

wash. For this reason, safety requires

Although Art has no formal training in aerodynamics, he spoke correctly in evaluating the story his air-angle indicator was telling him. It taught him to relate flight conditions to the

as when flying in turbulent air, encountering an updraft or downdraft during a landing approach, or passing through another aircraft's propeller

\

tratcs this as it applies to normal and inverted flight. Since they are present-

ing themselves to the wind upside down and their shape relative to the airflow is then much different, most airfoils will stall at a lesser angle when being forced into the wind in this manner during inverted flight. Why not make you own air-angle indicator and try it out? Perhaps the EAA can show the aviation world the better path to air safety! As shown in Fig. 5, there's really not much to it. It amounts to only a little balanced weathervane mounted on a horizontal rather than a vertical pivot. Make

yours well enough to operate smoothly and show no undesirable effects

from vibration.

Mount your air-angle indicator so that it rides in undisturbed air outside of the propeller blast and ahead (Continued on next page) SPORT AVIATION

33

AIR ANGLE INDICATOR . . .

(Continued from Preceding Page)

of the wing airflow. In general, about

one-sixth of the wing chord ahead of the wing's leading edge is about right. On biplanes, a location about one-third the way down one of the interplane struts will put it at a point where it will register minimum inaccuracy caused by interference airflow effects between upper and lower wings. Keep it away from the immediate region of points where wing struts meet strutbraced monoplane wings.

+20° 'tnfl J-Oncoming Wind

Take your plane up and run through

some stalls. Now you can see the ship's stall angle! Mark the stall point on the indicator with some red tape or paint. Once you've got the indicator marked to show your ship's stall angle accurately, you will be able to avoid stalls because you can see them coming on, regardless of the plane's speed or attitude. You'll always be able to tell just how far back you can safely haul the stick in emergencies. You'll know how sharply and steeply you can safely turn. You can make steep landing approaches without danger of having the bottom fall out from under you. You'll be able to locate and hold your plane's best climbing angle. Fly air angles for more fun and safety! ®

APPROXIMATE NORMAL AND INVERTED STALL ANGLES

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4

minimum rod or tube. Attach through leading edge to spar, on left wing, five feet from fuselage.

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FIG. 40

J U L Y 1970

5

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^^

INDICATOR A Fail-Safe Instrument By Terrencc O'Neill(EAA 5572)

This air-angle indicator was first offered

to EAA members in a July,

1970 SPORT AVIATION article by Terrence O'Neill, EAA #5572, and later in CUSTOM AIRCRAFT BUILDING TIPS, Volume 2 (no longer available). It is a simple, useful instrument for homebuilts and ultralights that will provide information important to the operators of these craft. The use of such a device could have a positive effect on their safe operation. KOST AJWOIt-S

Statistics illustrate the stalls are the cause of a large percentage of aircraft accidents. The common myth is that stalls are the result of flying too slowly. The stall speed prominently displayed on the airspeed indicator doesn't tell the whole story, because an airfoil doesn't stall at a given speed . . . it stalls at a specific angle of attack. Common stall warnings only indicate you are between five and zero degrees of the angle of attack that will result in a stall. THE AIRCRAFT TECHNICAL DICTIONARY defines angle of attack as: "The acute angle formed between the relative wind striking an airfoil and the zero lift line of the airfoil. The chord line of the airfoil is often substituted for the zero-lift line." This relative wind cannot be directly observed, so a device such as this air-angle indicator is needed to show the relationship between the attitude of your airplane and the angle at which the air is flowing over its wings. • inimra rod or tube. Attach through leading edge to spar, jocate on left wing, five feet froa fuselage.

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