From the archives: Leighton Collins on angle of attack, 1965

Editor’s note: This article, originally published in the May 1965 issue of Air Facts, is a companion to Richard Collins’s recent article on The three keys to flying safelyHere, Richard’s father considers the history of angle of attack as both a concept and an instrument, which offers important lessons for pilots of any airplane. It also proves that, contrary to some headlines you may read, angle of attack is hardly a new concept.

Monitair Angle of Attack Indicator

The needle that knows

By Leighton Collins

When the public first started flying in this country, i.e., after Lindbergh’s safe arrival in Paris on May 21, 1927, the phrase “angle of attack” was unheard around airports. Engineers, of course, understood and used them regularly. The Wright brothers had understood this inner secret of the wing even before that, and like the sailor with his small pendant atop the mast to show relative wind they had their string out front, even though it was not too articulate.

Wright glider
The Wrights understood angle of attack – “the secret of the wing.”

It was a long time after May, 1927, before the term angle of attack appeared in the popular text books and instructors started using it. This is surprising because angle of attack carries the secret of maximum performance as well as safety. Even without a proper formal knowledge of angle of attack, though, anyone, then, or now, who flies an airplane long without wrinkling it most certainly has at least a subconscious understanding of it.

Angle of attack measurement, though not recognized as such, started out in terms of attitude, or the position of the nose with reference to the horizon. The attitude of an airplane with reference to the earth plane is not angle of attack, of course, but in many situations a proper judgment of attitude does keep angle of attack within limits. In the Fleet we learned on we had one rocker-box which we’d never let get above the horizon in a full power climb, then a higher one on another cylinder which would be held on the horizon in a cruise power climb. There was a level sight, of course, and finally one of the boxes on the top cylinder was carried a few inches below the horizon for a power-off glide.

Our instructor would mention airspeed on the rare occasions when he did talk, but since there wasn’t an airspeed indicator in the airplane this had to do mainly with judgment of speed in slow flight: the sounds changed, the wind beat less on your helmet. A stall was spoken of only in terms of too low an airspeed, or loss of airspeed. And there was also some stress on the feel of the controls: the slower you got the lighter and less responsive they got, so this was another speed gauge. Speed was important. It was the Main Thing. It was especially important to keep the speed up in a steep turn, we gathered.

New Day

Cardinal
AOA is a little harder to get a feel for when you’re not sitting out in the wind, as was the case with the Cardinal.

Our first cabin airplane, a Cardinal (two-place side-by-side, 90 h.p. 7 cyl. Le Blond) brought on a readjustment of our attitude-sound-feel concept of speed control. In a cabin the sound effects were missing and in addition the airplane’s having less drag than an open biplane caused it to glide farther and with the nose less down and as a result an overshoot was easy to get suckered into simply by carrying the nose a little too low. It had no airspeed indicator. In our first demonstration flight in a cabin airplane, a Velie Monocoupe, one of the outstanding pros of the day, Vernon Omlie, nearly rolled into the fence after landing long at a large size airport (3000′ runway, sod). So everybody was having too-high, too-fast troubles.

We soon had our climb and glide marks determined for our Cardinal but the real solution against the too-fast-glide problem came in discovering that if the airplane were just held level, power off, it wouldn’t stall and would finally slow down until it began to settle or mush. So on base we’d hold it level and feel for the settling speed and then lower the nose just a hair after thus determining that we had no excess speed. Then on final, when we still had a couple of hundred feet or more we’d check for the settling attitude again if there was any doubt at all about being too fast. A too high reading on the tach, power-off, was a sort of backstop on too much speed in the glide. Nobody had yet said anything to us nor had we read anything about angle of attack.

Lucky Star

152 in a spin
The view can get very interesting if you lose awareness of AOA.

If you’re in an airplane you’re in the Angle of Attack business, though, and this came to us, or at least we came to realize rather suddenly that our knowledge of how an airplane flies had a gap or two in it. The usual ingredient of being in a hurry was present. And it was hot and moderately gusty. With a long way to go before dark that afternoon we agreed to a once-around-the-field flight for the fiancee of a friend, who was as daring as she was lovely. Right after take-off she asked if we wouldn’t do some aerobatics. We told her that we didn’t know any but that in our first turn we’d tighten it up a bit and that would give her a sample of g-load, which was usually the first big impression a person got out of aerobatics.

Just out of the field in our climb-out and with no more than 600′ we levelled off for a short interval and then rolled into a steep right turn, intended to be 90 degrees. Next thing we knew we were looking over the nose at a spot on the ground around which the rest of the world was rotating – and wondering what was going on. After what seemed like a long time (three-quarters of a turn to be exact) it dawned on us that we were in a spin.

In this account of the roundabout ways there were (and still are) of judging angle of attack, maybe it would not be amiss to say that luck helps. Our instructor had had air show aspirations, and on most of our flights in the Fleet at some point, with little notice, he’d attempt a double snap roll. Invariably at the one and one-half point he’d run out of steam and go from his horizontal spin into a normal straight-down one. Apparently, by a process of osmosis, we had learned from these gyrations that it is imperative never to be distracted by the nearness of the ground and thus overlook the need for getting the stick forward a bit, the nose even lower, and then, after a slight pause, start a pull-out that is neither too fast nor too slow. In our loss of control at low altitude in a steep turn shortly after take-off, with the city’s leading damage suit lawyer’s daughter as a passenger, we missed the ground by only about 10′ in our mushing, settling, pull-out. Her reaction was a gleeful “Do it again!” When we got around and down the local operator, a fellow named Ranney, told us, “We’ve got an inspector comes through here and if he ever saw you do a thing like that you’d be walking the rest of your life.” We thanked him, sincerely, because we sure need to thank somebody for something right then.

In the Public Interest

Airspeed indicator
It may seem impossible, but pilots flew for a long time before the airspeed indicator was mandated.

By the middle thirties the then FAA thought hard and reasoned that if everybody had airspeed indicators in their airplanes they wouldn’t be stalling so frequently nor would they be coming in so fast they couldn’t land and get stopped. Airspeed indicators became mandatory. Fortunately they were not too expensive and they had a high degree of mechanical reliability. And, of course, they had other valuable uses, such as in cruise control, and maximum rate of climb and maximum angle of climb speeds (if you had an altimeter and thermometer and the necessary graphs). One of the airspeed indicator’s greatest virtues is that in unaccelerated flight conditions the airplane in which it is installed always stalls with the same indicated airspeed, regardless of altitude or temperature.

Circa 1940

Before long, though, pilots began to become aware of some of the eccentricities and limitations of airspeed indicators. For one thing they were not easy to control, except with the help of some attitude reference. If the airspeed needle was on 80 and you wanted it on 120 and held forward pressure on the stick until it got there you’d be pointing almost straight down, or in going from 120 to 80 in this manner you could get pointed straight up. So it was necessary to learn to fly the trend of the needle, holding a little forward pressure to stop an increasing speed indication and back pressure to stop a decreasing speed indication. Somewhere along here Wolfgang Langewiesche pointed out that the airplane always wants to fly right, that if flying slower than its trim speed it would try to speed up and vice versa. And also about this time the idea of thinking solely in terms of attitude, even in needle, ball, and airspeed instrument flying, became accepted.

Stall speed chart
Pilots soon learned that “stall speed” is not a constant number.

Meanwhile, the CAA, and inventors and even aircraft manufacturers were aware that airspeed indicators were not doing much of a job on reducing stall/spin accidents, or even the overshoots. Nobody had told the pilots that when it comes to turning flight the indicated stalling speed can come up to right where you’re flying, or in other words that if you get enough g-load on an airplane it can stall at any indicated airspeed, which can be a long way from the “60” a pilot may be carrying in his head as his stall speed. The explanation of the continuing overshoots was more obscure. Either pilots weren’t looking at the indicator coming in, or else they didn’t believe it, or feel comfortable at any lower indicated, or something. (It still happens. We saw a high performance airplane recently with the nose-wheel knocked off. On a second too-fast pass at a small field the pilot had pushed the nose down at midfield trying to fly it on.)

As Time Ran

Even so, the airspeed indicator was not falling too low in the esteem of pilots. As the state of the art improved pilots were taught about the vagaries of stalling in other than straight flight, and also during accelerated flight conditions in other than turning flight, such as in gust conditions and in turbulence. Adding half the reported gust velocity to the normal approach speed became a rule of thumb, or simply an extra ten or even fifteen if the air was real rough. Still another state-of-the-art improvement was that they put the drag back in the airplanes, if you wanted it. As flap travel increased an airplane wouldn’t float very far even when brought in ten miles too fast. On the debit side, performance had taken a lot of the feel out of the controls with higher stick forces, downsprings for increased stability, and so on.

In Quest of Safety

Stall warning light
The airspeed indicator wasn’t enough, so the CAA added a stall light.

By the mid-forties the CAA decided to try again. There were still too many stall accidents. This time they’d fix things right. There’d be not only a light which would light but a horn which would blow when a pilot approached a stall, not just in straight flight but in any situation. They awarded a development contract for a one-angle angle of attack indicator. There was public discussion as to how to close the stall horn and light should be set. Some thought quite close, others that the warning should come well ahead of the stall point. Trouble with the latter idea, which we favored, was that the horn would blow on every landing, and if a cut-off switch were provided then the whole purpose of the instrumentation would be defeated.

At any rate, this is how our stall warners came about, and as in the case of the airspeed indicator FAA exercised their mandatory prerogative, unless the aircraft manufacturer wanted to build in buffet characteristics which tended to raise the stall speed and consequently the landing speed and which were consequently objectionable. In some military installations instead of a horn there was a stick shaker. As of now, Piper has been able to talk the FAA out of the horn.

It would be difficult to establish how effective stall warners have been in reducing stall accidents because there has also been improvement in aileron effectiveness at the stall, improved stall characteristics, and improvement in the teaching of stall recognition. It may be that the stall warner has not done as well as expected. After being apprised of the imminence of stall the pilot still has to be relied on to perform properly. All too often he still tries to manhandle the airplane back to a level attitude instead of let it go momentarily so the wing can get back in business.

A Third Try

Stall warning horn
The basic stall warning horn is really a one-angle of attack device.

The one-angle of attack indicator had not long enjoyed mandatory status before the idea of expanding its capability to cover a wider range of measurement became current. As a matter of fact, some of the aircraft manufacturers had asked instrument makers if they couldn’t come up with a lower-cost, extremely reliable instrument for this purpose.

We flew one of these instruments (not, however, low cost) for several years in a Cessna 180 and Skylane and during an instructional experiment in a Cessna 150 and thought highly of it. We did not, however, use it as the manufacturer recommended as we had it set for a happy operating speed rather than maximum performance. With the needle centered we got the maximum rate of climb speed, which was about 15 m.p.h. above the actual stall speed in stable conditions. In short, we were using the instrument for a neither too fast and neither too slow climb and approach speed; in other words, a good working speed. We wanted it to keep us comfortably away from the stall area, not as close to it as feasible.

Unfortunately this instrument, even with a 50% price cut, did not find a market and its manufacturer carried the development into the military and airline field where landing speeds were going up but carriers and airports weren’t getting any longer. We will not go into those applications except to mention the next thing coming, super-sophisticated. In seeking to go from 200 & 1/2 instrument minimums to 100 & 1/4 with the jets the question of abandoning an approach or a missed approach becomes critical. Some of the pilots have argued that if they go to 100′ they are committed to a landing so they shouldn’t go that low until they are equipped to make a blind landing and blind roll-out.

The way this is being worked out, at least by one airline, is that as the pilot flies his flight director down the glide slope by keeping “the little airplane” superimposed on the horizon bar, which means he does exactly all things necessary to say on the glide slope – when he gets to 100′ and the co-pilot says there is nothing there, the captain simply pours on the coal and pushes a button which disconnects his flight director bar and its computer with its glide slope signal input and connects it to an angle of attack indicator. The computed signal he gets on the bar is what it takes to rotate to exactly maximum rate of climb angle of attack. The altitude loss from the time the go-around is started is 10′. There’s nothing like angle of attack, if you need to know what it is when you need to know.

The Basics

Angle of attack graphic
A simple concept, but not always easy to visualize.

Before reporting on the Monitair instrument let’s review briefly what angle of attack is. And the fact that it is where you find it. As the books say, and the instructors nowadays, angle of attack is the angle between the chord line of the wing (an imaginary line from the leading edge to trailing edge of the wing, not counting flaps if they’re down) and the relative wind. The catch is that the airplane makes its own relative wind and knows no other kind, except gusts. As one of your early contributors, Soderlind of Northwest Airlines says (yes they still deal in basics and this was in a recent classic multigraphed Flight Standards Bulletin which he issued to Northwest pilots) “Relative wind blows back along the path being traveled.” And, since we fly “over the nose” and he is a practical man he continues, “Angle of attack is the angle between the airplane’s longitudinal centerline and the direction of the relative wind.” And if we may paraphrase a bit, he goes on to point out that pilots must have a clear distinction in their mind between angle of attack and pitch attitude as related to the horizon: only in straight flight at a constant altitude are they the same. And he points out that the relative wind blows “uphill” in a descent, “level” in cruise, and “downhill” in a climb.

In short, the relative wind is coming from somewhere in front of the airplane, wherever it is pointed, and we need always to think in terms of a reduction in angle of attack coming only from the nose being pitched down from where it is. The angles are small. An airplane cruises at around a 3 degree angle of attack. A ten degree change in attitude (not the right word, but good visually) is mighty little but add it and you’re way down the string towards the stall, as most wings stall at 16-18 degrees. And by stall we don’t mean quiver, but that the nose falls down or else the airplane rolls off into a steep bank, or both.

Where the real problem comes is in turning flight. What’s the angle of attack then? In “Aerodynamics for Naval Aviators” where they depart the straight and level they say, “Regardless of the condition of flight, the instantaneous flight path of the surface determines the direction of the oncoming relative wind and the angle of attack is the angle between the instantaneous relative wind and the chord line. To respect the definition of angle of attack, visualize the flight path of the aircraft during a loop and appreciate that the relative wind is defined by the flight path at any point during the maneuver.” We like that “instantaneous relative wind.” It is important mathematically. In pilot terms “momentary” might be just as important.

Will-o-the-wisp

Monitair AOA probe
The Monitair angle of attack indicator used this vane as the sensor.

We are not trying to sell angle of attack indicators, but only a clear concept of angle of attack under the varying conditions the pilot encounters, one of the most varying being in gust effects. Most of us think in terms of gusts being bumps and when, flying straight, we encounter one we can readily visualize the rising column of air into which we’ve flown and its lowering the direction from which the relative wind is coming and thus increasing momentarily our angle of attack and lift (which produced the bump).

What about a horizontal gust, a puff of wind coming from straight ahead? In straight flight it will increase our airspeed momentarily, and our lift, and give a bump too. But think also of flying along and making a turn in front of a gust and flying into it with the bottom of the wing facing it. The gust would increase the already high angle of attack which a turn required. There could be no attitude judgment. The controls would feel lively because the speed would be high. Well, as we learned early, most of our stall troubles come in turns, at low altitude. And today we still have little to go by in judging when they’re near. The stall horn blows pretty late, the airspeed reads high, the controls still feel good.

Straightforward

The Monitair instrumentation is designed to measure and indicate the full range of angle of attack from 1 degree to 18 degrees or thereabout. They get their information from a floating vane mounted ahead of the leading edge. It moves, surprisingly, through about a 60 degree range, which fortunately has a linear relationship to actual angle of attack. This gives them a built-in 3:1 gear ratio as it were and simplifies the panel presentation. The vane position is sent, via a potentiometer, to the panel indicator, mounted atop the instrument panel so that it will be close to your line of sight in an approach or take-off. The indicator has a needle which moves from left (stall area) to right and the scale over which it moves is specially designed for each airplane.

Debonair AOA gauge
Each airplane has its own angle of attack scale – here a Debonair shows best approach at 6 degrees.

Starting at the right hand end of the scale (since we’d rather stall last than first) there is a green band which runs from 1 degree angle of attack leftward to a 6 degree point. (We’re talking about a Twin Comanche installation.) This left end of the green band is also marked as the angle of attack for maximum rate of climb for this airplane (both engines operative). The next band to the left covers from 6 degree to 9 degree angle of attack and is white. Near the middle of it is a blue line, and it carries the legend App and R/S/C, which means to fly with the needle on the blue line in approaches and also for maximum single-engine rate of climb. The next band, still to the left, is yellow and runs from 10 degrees to 12 degrees. This is the slow band and a dot at about the center of it at the top is marked A/C, which means that 11 degrees angle of attack will get you maximum angle of climb performance. The last band, red, on the left runs from 12 degrees to the end of the scale (probably about 18 degrees) and any time the needle is in that area it shouldn’t be. As you can see, in the white and yellow bands, which are the maximum performance areas, the numbers are mighty small (only 5 degrees between maximum rate and maximum angle of climb!) and while the scale length is generous enough and the change in nose attitude with reference to the ground or the artificial horizon may be large as between power on and power off conditions, what is actually being dealt with is in fairly short supply.

Hold Tight

Evangelist
AOA advocates “are impelled by intense missionary zeal.”

The people who get deeply into the angle of attack realm are purists and they also are impelled by intense missionary zeal. They can save your soul and want terribly to do so. And in the trying they literally scare the pants off you. The reason is that precise angle of attack measurement permits flying the airplane safely in its maximum performance areas. Our first flight was in a Debonair. It was a cold winter day and there was hardly a breath of air stirring. We know that a Debonair performs well, but with only half the gas and two aboard even putting the Monitair needle on the maximum rate of climb mark gave a startling attitude because of the high rate of climb, around 1500 f.p.m. At about 100’ Al (Heinsohn) told us, “Now fly it on the maximum angle of climb dot,” which is the middle of the yellow sector. Which we did, but not at all happily, for this put the nose so high that little of the ground was to be seen. (After all, your chair tilts back too). We could have flown a Debonair a long time before finding out that it would climb this fast or this steeply.

It Really Talks

They show you a number of things, but we will mention only a few of the more significant ones. For one thing, the amount of movement of that vane is surprisingly high. In choppy air (on a later flight) it bobs up and down a lot which makes you realize that with the damping provided in the needle, what you get is an average value. In other words, in turbulence angle of attack is a rather rapidly fluctuating thing. It is also impressive, in smooth air, to see how much you can move the elevator and thus the angle of attack needle without any immediate change in airspeed for the simple reason that angle of attack can be changed rapidly while it takes time for an airplane to slow down much or accelerate to an appreciably higher speed. In level flight you make a 30 degree bank, and boy does that needle slide from way over in the green sector well into the white sector, or even yellow if you add a little too much back pressure.

Revelation

Cessna landing
Flying strictly by AOA, you will approach at a far higher pitch attitude than you thought possible.

If they scared you in the take-off you must steel yourself for the approach because they’ll ask you to put the needle on the blue mark and keep it there. You’d better start on this pretty far out on final because gear and flaps down and power quite low it takes a while to lose enough speed to get the needle down to the blue line. Then, power off, with the nose higher than you’ve probably ever carried it, and the airspeed too low to bear looking at, you glide in in a soft, moderately steep mushy descent, nose position adjusted as necessary to hold the needle on the blue line. Somehow you aren’t too worried about stalling in the approach, probably because it feels good enough, but the nearer you get to the end of the runway the more positive you’re likely to be that there just isn’t a chance in the world of being able to flare and get enough extra lift to stop the rate of descent. But when the time comes to flare you really do a job on this because of the way the relative runway is coming at you, and in consequence you have the tail really low on contact. There is a softness, a sort of weightlessness about the flare and touchdown and at the same time plenty of control and a cute little float that is longer than you’d think it possibly could be after coming in so slowly. Due to the tail being down about as far as it can be gotten the landing is at an uncommonly low speed, consequently the distance to a stop is also less than usual.

The Airplane Has It

This is the way they want you to fly with a Monitair. They have led you back into a performance parameter which most pilots aren’t very much aware of today with their airspeed indicators and heavily flapped airplanes which take so much out of the float of a fast approach. They take you back to the way most everybody flew and had to fly at the beginning because nearly all airports were small. And the way people who live on small airports still fly. And the way the city slicker who seldom lands on a short field has to fly too unless he wants to roll of the end of the runway, which he does more often than you might think.

Twin Comanche

AoA Gauge in ICON
Modern angle of attack instruments show the same thing they always have – “the bird-sense part of flying.”

In our flight in Van Dusen’s Twin Comanche, with Curt Erickson who operates a top grade instrument flight school in Minneapolis, some new considerations were introduced. The Monitair’s purpose in life is to enable you to get the full performance of which your wing is capable in take-offs and approaches, i.e. maximum rate of climb, maximum angle, and an approach speed which will be safe yet which will give you the shortest distance from flare to touchdown. In doing these things it takes care of variables the effect of which a pilot can only guess about. That is, it takes care of variations in weight of the airplane, temperature, altitude, and turbulence. All it knows is to tell you what to do with the elevators to keep that vane where it has to be for the needle to indicate say, 7 degree angle of attack. As the variables mentioned vary from day to day it will have you flying at twenty different airspeeds, over a range of about 20 m.p.h., but still with 7 degree angle of attack. But it can’t tell whether you have two engines or one or none. It’s interested only in the bird-sense part of flying.

Exception Reserved

In the Twin Comanche in delivering The Most we feel it overlooks too much the minimum directional control speed. Normally we climb out in the Twin Comanche with 120 indicated. That keeps the nose down to where you can see, provides good cooling, and at 75% power 8-900 f.p.m. climb. For best rate of climb the Monitair calls for a 6 degree indication. In standard air at sea level at gross weight this is supposed to and probably does produce the 112 m.p.h. indicated which Piper says is the maximum rate of climb speed on both engines. Our take-off showed how the only thing constant about a given indicated airspeed is the numerals. On this day, light and cold and smooth, with the Monitair needle on the R/C 6 degree dot the rate of climb was 1500 f.p.m. approximately. The indicated was around 105. We weren’t curious about the angle of climb dot, but did put it on the 7 degree Approach and maximum single rate of climb blue line, and the indicated went down to about 98 and we expected momentarily a call from the tower to calm down.

Man!

Piper Twin Comanche
Is it really a good idea to fly a twin so close to Vmc?

In the approach, we couldn’t get up the courage the first time, but on a second try did come in with the needle on the Blue or Approach line, full flaps, indicated airspeed 82 m.p.h. It sure seemed mighty slow and actually we do not feel it is healthy to be power gliding in that close to a Vme in case one stopped whispering and a roar was needed from the other. This time we felt even more strongly than we did in the Debonair that we’d surely punch a hole in the runway when we started to flare. But live and learn. Like the Debonair, it not only flared but held off a reasonable time and landed softly with the nose way up. At this low speed, too, none of the ground cushion effects developed which you get from going into the ground cushion too fast and too flat in a Twin Comanche which results in a fast, level landing before you can get the tail down.

Why?

Now, we might as well talk frankly. An angle of attack indicator is difficult to sell. Price, of course, is a factor, but they’d probably be difficult to sell even at a low price. People have to want something first no matter what the price, and most pilots don’t feel they really need an angle of attack indicator. How many, even, would pay extra for a stall warner, if it were offered on an optional basis?

It is easy to see why this sales resistance exists. A person flying a Debonair on a 3-4000’ field who climbs out at 100 indicated and approaches at 90 certainly feels he’s well on the safe side and he’s happy, so what’s an angle of attack indicator going to do for him? By the same token a Twin Comanche pilot using a 120 for climb and who approaches 90 flaps down and 100 flaps up in average conditions is likely to feel the same way—maybe even more so, because he has Vme on his mind.

While we do not feel that an angle of attack indicator is a cure-all, and certainly here, as always, the proponents of a new instrument seem to reach too far and demand too much of it, we’d still like to have one. In a sense it is a special-circumstance or special-use instrument and even with it we’d still at times fall back on the primitive method of attitude. Or at other times knowing airspeed would be the main thing of interest. And certainly no matter what’s in an airplane any time we feel like it is starting to fall out from under us we’re going to gun it for we believe that is valid, basic, and essential until a seat of the pants indicator is developed. And when things began to get a little windy and wild we’d add maybe half a degree to whatever Monitair setting we normally used. What would we want one for then?

For a lot of reasons, and not necessarily in the order of their importance.

At Night

Night flying
An AOA indicator could be a very helpful thing at night.

It has always been a wonder to us that more pilots don’t fly into the ground in night take-offs, or in missed approaches in instrument flying under zero-zero conditions. In a slow flight condition it is easy to have the artificial horizon showing a nose-up attitude, say with the little airplane 1/8” above the horizon, and still fly into the ground, even with full power. To get a good full-power climb after breaking a glide at a normal approach speed the artificial horizon has to have the little airplane almost at the top of the window. Afraid of stalling, the pressure is to be conservative and keep it well down. It’s not just theory. Even KLM flew into the River Shannon after a night take-off.

Under such conditions with a Monitair angle of attack indicator all we’d look at on a night take-off or go around on instruments would be the Monitair needle and the DG. With the needle on the R/C dot (maximum rate-of-climb angle of attack) and full power it can only go up no matter what the horizon, the airspeed, or anything else is indicating. The one exception might be the IVSI. When it shows up you’re going up.

For On Edge Work

Saba Airport runway
Landing on a short field? AOA will get you stopped sooner.

Now this thing of small fields. We all do it at one time or another, and the less frequently the more critical it is with airspeed and feel. In the instance cited earlier of a too-high, too-fast approach and a knocked off nosewheel, someone might say that this fellow could have handled his situation by looking at his airspeed, but he didn’t look, so why assume that he’d look at a Monitair either?

We feel there’s an answer to that. A person flying fixed airspeeds, say 90 for an approach, will soon learn from watching a Monitair needle that as far as angle of attack is concerned he’s been playing a slide trombone without realizing it. An airspeed indicator can bring you in too fast as well as too slow. One of the biggest early impressions flying the angle of attack needle will make on you is that in flying any fixed angle of attack you seldom see the airspeed the same twice. It brings home that what you’ve been doing is flying constant airspeed with highly variable angle of attack, when what you really want is constant angle of attack. While the fellow who knocked his nosewheel off should have gone around or off somewhere and thought it over, it is still entirely possible that he did look at his airspeed and that it read just about what he always used in an approach. But, due to light load and temperature and smooth air, had he been able to set up the angle of attack that normally goes with 90 his indicated might have been 75. As mentioned before, if you pick a climb angle of attack and one for approach you’ll see, from day to day, up to a 20 m.p.h. variation in indicated airspeed.

Better Landing

With a Monitair we’d pick a take-off angle probably just a hair to the right of the R/C dot and always use it, and on approaches our mark would likely be just a bit to the left of the R/C dot, and we’d always use that, ignoring the indicated airspeed entirely. This should give us better landings for they would always be made starting with the same angle of attack and consequently the time from starting the flare to touch down would always be the same. When you come in at all different angles of attack you have to deal with floats of all different lengths and that makes it harder. Like trying to waltz when the governor on the music box is surging.

On One Engine

Maybe we’re overworking the fixed airspeed concept, but it is the main point. You can’t fly an airplane with the Owner’s Manual in one hand. In the Twin Comanche we have four figures in mind, straws as it were: Vme 80 m.p.h.; stall, flaps and gear down, 70; stall, flaps up and gear down, 75; maximum single engine rate of climb speed, 105. All of these speeds are at sea level, in standard air, and at gross weight. Additionally, the Vme is with gear up; the stall speeds are with power off; the single engine 105 clean, of course, with one feathered, cowl flap on that one closed, and cowl flap on the other open. Now, quick, what are the figures for stall, power on, or three hundred pounds light, or with moderate turbulence, or at 2000’ above sea level with an outside air temperature of 80 degrees?

Aztec on one engine
With one engine shut down, the AOA indicator really shows its value.

The Monitair solves all these riddles. If 6 degrees is maximum R/C angle just put it on 6 degrees and you’ll get all there is no matter what the altitude, temperature, load, or turbulence situation. In case of engine out (remember, no matter what the load, etc.) just put it on the blue line. We tried this on the Van Dusen Twin Comanche, at 2000’ with the left one throttled back to 10”, prop control left on its 2300 r.p.m. setting. At 105 indicated, with the right one full throttle and flat pitch, two aboard and half gas, the rate of climb showed 150-200 f.p.m. With the needle on the blue line the climb went up to 350-400 f.p.m. Interestingly enough, the indicated was around 95 with this climb. Surely one of the most important capabilities of the Monitair is in engine-out situations in a twin. It can tell the pilot the exactly correct thing to do (put it on the blue line) when there’s no time for cut and try methods.

A final value of an angle of attack indicator, which comes with time, is that it creates an atmosphere of confidence in the cockpit. It is true that you do not look at the instrument very often, but in the long run in the low speed situations a pilot knows full well that in whatever roundabout way he is attempting to judge his situation at the moment, what he really needs to know is angle of attack. It is a comfort to be able to look and see exactly what it is at any time. And when on-edge flying becomes necessary it tells you exactly where the edge is without your having to nibble into it now and then just to be sure you aren’t getting too far away from it. In short, as well as performance capabilities, it has great value not in telling you you’re stalling but that you are as close to a stall as you should get.

Although an angle of attack indicator may not cure all of everybody’s problems, it can certainly take care of some of everybody’s problems, and tough ones at that. Besides it is comforting to know what every bird knows, i.e. the exact angle at which the wing is set to the relative wind, no matter where it’s coming from. That’s what counts.

4 Comments

  • Nice treatment of AOA. It really hones in on the fact that by flying the airplane by AOA you’re flying the wing by how it performs regardless of all the variable a pilot otherwise has to manage in his head.

    No instrument or concept can prevent all human error, whether errors in understanding or errors in concentration or execution .. but it surely seems rather obvious that learning to fly your airplane’s wing is the most fundamental concept in aviation. We pilots all like to speak fondly of the importance of stick and rudder, yet most of us remain unaware of how these controls affect the performance of the wing at a detailed, precise level.

    As for the “true believer” criticism of perhaps some AOA advocates that Mr. Collins references, I think that is simply not something to concern ourselves with. Who among us would not benefit from knowing how to get the most from our airplanes, and fly them with the best skill we can apply? That is not a fanatical pursuit – it’s just about being better pilots.

  • This is a really good article on the practical uses of AOA indicators. The first AOA article I read some 15 years ago talked about how the US Navy adopted AOA for carrier landings at the start of the jet age. The first year cut landing accidents in half, and the second year cut them in half again. It is a great testament to the power an AOA meter can bring to any pilot who is willing to study and train with one. Just remember that training with an AOA meter is necessary for the pilot to get the most use from the device and the aircraft it is installed on.

    I would add that the only instrument on the Wright Flier was a rudimentary AOA indicator and they taught themselves and thecrest of the world how to fly.

Leave a Reply

Your email address will not be published. Required fields are marked *