Most pilots don’t really understand the relationship between airspeed and angle of attack. If they did, we would not have the loss of control accidents that we do. We fly strictly by numbers because we were taught that way. Very few flight instructors have any experience or knowledge in this area.
What really matters here: airspeed or angle of attack?
For every flight operation where we use a certain airspeed to achieve maximum performance, there is one correct AOA. That AOA never changes, but the airspeed to achieve it does. All the time. Unfortunately, we are usually only given airspeeds that apply to a very specific set of circumstances. These speeds are only correct at gross weight and 1G.
Guess what? These airspeeds are never correct because we are only at gross weight and 1G sitting on the ramp. If you happen to be loaded to gross weight, as soon as you start your engine, you are burning fuel and your weight is going down. And if you are anything but straight and level in the air, you are not at 1G.
So what’s the big deal? Wouldn’t using the POH numbers provide an extra margin of safety? The answer is NO. We have many fatal accidents every year because pilots were flying too fast on approach. Or too slow because they were not at 1G. They were all flying at the wrong AOA!
We use ten basic airspeeds to fly airplanes. That’s all the manufacturers give us. The five that are marked on the airspeed indicator are aerodynamic or structural limits. They are fixed and do not vary with weight. Here they are starting at the bottom of the indicator:
- Vso, bottom of the white, stall speed dirty, at gross weight and 1G
- Vs1, bottom of the green, stall speed clean, at gross weight and 1G
- Vfe, top of the white, max flap extended speed
- Vno, top of the green, max normal operating speed
- Vne, top of the yellow, redline, never exceed speed
The other five, the ones that we use as often as the ones on the indicator, are not there because they vary, primarily with weight. Here they are starting at the bottom:
- Vref, approach landing speed, 1.3 Vso
- Vx, best angle of climb speed
- Vy, best rate of climb speed
- Vbg, best glide speed
- Va, minimum maneuvering speed
Here’s the problem. Using the POH numbers (gross weight and 1G), results in using many different angles of attack as our weight changes, not the one AOA that is correct. For example, if you are flying an airplane that is in the normal category, the limit load factor is 3.8G. Let’s say your clean stall speed, Vs1, is 60. Your weight varies with changes in fuel load, payload and G load. Your load changes whenever you add or remove fuel, pax, or cargo. Your G load changes whenever you move the elevator with the stick or yoke, at a fixed power setting.
Standard equipment some day?
So how do you know what your stall speed really is? If you had an AOA indicator, it’s easy. Whenever your AOA indicator says you are close to stalling, read the airspeed indicator to see what your stall speed is, if you really care! At a fixed AOA, your stall speed varies directly with weight. Reduce weight (or G), stall speed goes down. Increase weight (or G), stall speed goes up.
How much can it vary? You, the pilot, control your weight, so you control your stall speed. You control your weight on the ground with fuel load and payload. You control your weight in the air with G load. Your airplane doesn’t know the difference, so you can weigh whatever you want! You can vary your G load from zero to 3.8.
Yes, we’ve all done it, you can push to zero G, meaning your weight is zero. Guess what your stall speed is at zero G? Yep, ZERO! Just like the 727 Vomit Comet, ZERO! OK, what is your stall speed at 3.8G? It varies with the square root of the G. The square root of 3.8 is 1.95. Multiply that by your 1G stall speed, 60, and your new stall speed is 117! So, you can vary your stall speed from 0 to 117 by moving the stick/yoke fore and aft, without overstressing your airplane.
So, at the first indication that you are running out of lift (i.e,. sloppy controls, buffet, stall warning, or uncommanded roll), PUSH! This pushes your stall speed down. All the way to zero if you want! Remember, if you don’t stall, you can’t spin.
Don’t go out and try to teach yourself, though. Find an instructor with an aerobatic airplane, who is experienced in all-attitude flying. Ideally, do it in an airplane with a Lift Reserve Indicator (shows lift remaining, using airspeed and AOA). I’m certain that many lives would be saved if all pilots received this training. And new airplanes came with AOA/LRI’s installed. Hopefully, someday it will be the norm, not the exception.
Be safe, and keep learning.
- Airspeed vs. angle of attack – what pilots don’t understand - March 25, 2015
>And if you are anything but straight and level in the air, you are not at 1G.
Wrong. A stabilized climb, descent, or some turns can also be 1G. Don’t believe me? Go find a plane with a G-meter and fly until you do.
I think the point was that it can never be *exactly* one G, and even the slightest deviation will give a different indicated stall speed.
Depending on your definition of “exactly”, almost nothing is ever “exactly” 1 G. But there are plenty of maneuvers that are executed between 0.9 and 1.1 G, and will therefore result in a pretty consistent stall speed.
Well, technically a coordinated turn still means you’re pulling >1g, since you are constantly changing your velocity vector.
I think the author is pointing out that it is perfectly possible to stall before the end of the white tape, contrary to what some think and are taught.
Also untrue!
You’re conflating a coordinated turn and a coordinated turn in level flight. You can absolutely make a coordinated turn at 1G, but don’t expect to maintain altitude.
Your kind of thinking scares me a little. It’s how people end up in graveyard spirals!
JPR,
What you say is true – “You can absolutely make a coordinated turn at 1G, but don’t expect to maintain altitude”.
But practically speaking, most stall-spin accidents occur precisely because the pilot is either trying to maintain altitude in the turn with insufficient airspeed/power, or even if not maintaining altitude, such as in a descending base-to-final turn, is not maintaining coordinated controls.
The real issue here is that AOA is much more complex and variable, depending upon real world conditions, in controlling the aircraft than monitoring airspeed alone can account for.
The AOA indicator is the only direct means, aside from monitoring “seat of the pants” feeling by the pilot, to tell the pilot how close the aircraft is to a stall in real world maneuvering flight where weight, G-force, center of gravity, and airspeed are constantly varying.
That is essentially why we still have so many stall-spin accidents in GA, even by highly experienced pilots (in terms of logged hours, but not necessarily seat of the pants flying).
Oh, I don’t disagree with anything you’re saying, but I think it’s pretty telling that an article that presumes to speak on the topic of precise flying makes a statement this horribly inaccurate – that’s all.
That said, yes, the main point (however egregiously he misses the details) is true – stall speed is far from constant.
JPR, very useful comments, thank you. Author should be happy with any input which helps readers better understand the desired message. It’s healthy to keep the record 100% accurate, especially when we talking about precise flying.
A properly executed barrel roll is a 1G manuever. Just watch Bob Hoover perform it while pouring a glass of tea without spilling a drop.
I suspect Bob Hover might disagree that 1g only occurs straight and level! How else could he pour his tea while doing a complete roll?
I would say that a properly executed barrel roll is a “coordinated flight” maneuver (the slip/skid ball remains centered), rather than a “1G” maneuver, as I’m pretty sure that you experience more than 1G during the initial climbing turn of the maneuver. The “coordinated flight” aspect is the main reason Bob Hoover doesn’t spill the tea. You can pour tea as long as the G loading is greater than zero, and you can even pour tea if the G loading is greater than 1G as long as you are strong enough to lift the pitcher.
It’s true that AOA is the actual parameter that the wing cares about, but to expect that many lives would be saved if all planes had AOA indicators; I’m not so sure. My guess is that people that stall and spin in are fixated on something outside the airplane and not on the airspeed, AOA or anything else on the panel. From my experience instructing I know that once the airplane is in a fully developed spin the fixation is on the ground view out the nose of the airplane; and the nature tendency is to pull up.
“Well, we’ll put it right in front of the pilot you say”. Perhaps, but I believe it’s easy to see right past any gizmos when fixated on a perceived threat.
Stephen,
You’re right that pilots can ignore an AOA indicator just as easily as we can any other panel indicator (light or horn).
The benefit of having a direct AOA indicator comes from learning to fly your aircraft using it and becoming much more intimately familiar with the seat of the pants feel that the aircraft provides in various flight regimes (airspeeds, weights, G forces, etc that create the infinite variety of AOAs that the aircraft is responding to).
Those pilots who have AOA indicators and use them routinely report learning much more about how their aircraft actually flies. Airspeeds tell the pilot little about how the aircraft is responding to all the factors that result in a given AOP.
Learning to fly by the seat of one’s pants, and how to make instant corrections to AOA (such as the author suggests, say by relaxing back pressure on the stick in a turn) will make any pilot much less susceptible to getting distracted and failing to pay attention to what the aircraft is telling him/her.
If your students are fixated outside during a spin, you have some work to do (instruction). I tell mine not to worry, the ground will still be there when you recover. Look INSIDE, check altitude (if too low, bail out), check the turn direction, double check the turn direction, check airspeed and AOA, then recover (usually neutral controls or full opposite rudder)…THEN look outside and roll to the horizon and pull the nose to it. Guess what, the ground is still there!
Hmmm, what you are describing is the teaching of spin recovery, not accident prevention. When your highly trained student buzzes the family house he will, in fact, be fixated on the admiring throngs below. When the airplane stalls and rolls, I maintain that it will not be his natural reaction to “look inside, Check altitude, check the turn direction, double check the turn direction, check the airspeed and AOA, then recover”. Even if he did follow that trained pattern, the ground impact would interfere between about “double check the turn direction and check the airspeed”.
I always that awful sounding horn in my rented Cessna was warning enough
This has got to be one of the very worst pieces of “Journalism” from anyone let alone some group with “FACTS” listed in the title. Can you please forward me the objective studies used to determine that:
“Most pilots don’t really understand the relationship between airspeed and angle of attack. If they did, we would not have the loss of control accidents that we do. We fly strictly by numbers because we were taught that way. Very few flight instructors have any experience or knowledge in this area.”
Additionally if you think that “And if you are anything but straight and level in the air, you are not at 1G.” you should all turn in your certificates now.
Actually, Marshall, your last sentence is incorrect. Any turn in space (vertical or horizontal) experienced by any object that has mass constitutes “acceleration” that is typically measured in multiples of the acceleration due to gravity (“G”).
If your aircraft is turning in any direction at constant altitude (i.e., there is no vertical acceleration) the vertical component of acceleration may be only 1 G, but that is only one component of the total force due to acceleration that your body and your aircraft’s wing are experiencing. In a 60 degree bank at constant altitude the horizontal component of forces on the aircraft is 2 G, which is a load on the wing and which increases the resulting stall speed. So Chuck is right in his statement that 1 G is only experienced in straight and level flight (i.e., no acceleration or turns in any axis).
The reality is that aircraft and their occupants constantly experience acceleration, both minor and major … in a combination of vertical and horizontal components, due to variations in groundspeed, altitude, lateral position, and due to accelerations of the air mass in which the airplane is moving. Developing a feel for these accelerations is what is meant by developing “seat of the pants” sensing
Chuck’s overall point is that the aircraft stall speed can be varied throughout a flight by many factors, some of which are under the pilot’s control and some are not, and that airspeed is just one of those many factors. Gs can be reduced by applying forward stick, and/or by reducing the bank angle. Either type of pilot action is what is referred to as “unloading the wing”.
Actually, I should correct my second para. above as follows, to be precise:
“In a 60 degree bank at constant altitude, the magnitude of force due to the turning acceleration, and acting opposite to the force of lift (both forces acting normal to the plane of the wing), is 2 G, which is an equivalent load on the wing, and which therefore increases the resulting stall speed.”
Duane
The force diagram of an airplane turning in flight is not that simple. There are 6 vectors at work. Not easy to put into words, that is why they say a picture is worth a thousand words. You are correct that the total lift has to equal and act opposite to the total force resulting from gravity and centripetal acceleration.
As for the OP, I like how he advocates unloading the wing. I would not call it a “push” as that can get you into negative G’s. I would say relax all of the back pressure on the stick. This is definitely apropos to the base to final turn. Here you see the pilot pulling hard to fight the crosswind trying to blow him/her through the final. The pilot does not want to bank too much because he/she has heard all of the warnings about banking too much in the pattern. But darn it! The nose is not coming around fast enough. If the pilot cheats with rudder, the nose comes around faster. But hold on, that increases the bank angle. So the pilot adds opposite aileron to keep from over banking. At the same time he/she tries to keep the airplane on track to intercept the final and adds some more back pressure. Just as the turn has been made, the pilot tries to unbank by adding more opposite aileron. Big surprise, the low wing just dropped out from underneath. The low wing stalled. Notice that no attention has been paid to airspeed or attitude. All attention is outside trying to line up on a geographical feature on the ground. No instrument by itself is going to save this pilot.
Until pilots stop cross controlling to save a base to final turn, you will continue to have stall/spin here. Until pilots give themselves enough space to make the base to final turn safely, you will continue to have stall/spin here. Until pilots go around for unsafe/unstabilized approaches, you will continue to have stall/spin here. The only place I purposely cross control is during a crosswind landing in the flare, or in a slip to lose altitude on final.
David – Thanks … you provided a very good detailed description of the infamous base to final skidding turn that turns into a stall-spin accident.
Cross controlling is not a problem per se, as long as the pilot understands that he’s doing it (I suspect in many of the base-final skidding turns the pilot isn’t thinking in terms of “crossed controls” so much as he is simply trying to make the nose of the aircraft point back at the runway), AND as long as the wing doesn’t stall. You can’t spin an aircraft unless it stalls first.
Another factor that I alluded to above: pilots often don’t take into account the relationship of the aircraft to the air mass through which it moves, and the temporary effects of wind accelerations on airspeed.
This comes into play, again, in the skidding base to final turn. These accidents usually happen when there is a fairly strong cross-wind in the same direction as the aircraft turning onto base. The aircraft was probably flying too close to the runway on the downwind leg, often due to the pilot not accounting for the crosswind that pushes the aircraft toward the runway on downwind.
The instant the aircraft turns to base, it momentarily loses airspeed, since the aircraft suddenly experiences a tailwind (this is a “wind shear” type of event). It takes awhile for the aircraft (and its wing) to accelerate with the change in wind direction, but in this scenario, the pilot doesn’t have time to let the aircraft accelerate with the tailwind. Given the short offset from the runway at the beginning of the turn to base, the pilot will often immediately sense he’s blowing past the runway, and so he immediately turns to final right after turning base … yet as the base to final turn is made, the aircraft is still at a relatively low airspeed.
The setup as described above, plus the stick and rudder errors you described take place to produce a very bad outcome.
The proper technique is to offset your downwind leg further than usual from the runway, and also maintain a proper crab into the wind on the downwind leg, thus giving a longer than usual base leg. Turn normally from base to final, with fully coordinated controls, and then crab into the wind after settling upon the runway alignment AND assuring that airspeed is good (add at least five knots airspeed above normal final approach speed in strong crosswinds). Cross-control only on the last few feet of descent as the aircraft settles onto the runway in the flare, keeping the upwind wing low and the nose pointed down the runway.
If these steps aren’t accomplished properly, then go around and try it again.
Minor point of clarification:
>So Chuck is right in his statement that 1 G is only experienced in straight and level flight (i.e., no acceleration or turns in any axis).
This is not quite accurate. 1 G is experienced in straight (not necessarily level) flight. This is true of cruise (level) as well as straight, constant-rate climbs and descents. It is also true of a falling left or right turn, but those are fairly uncommon maneuvers since they involve constant vertical acceleration, not constant vertical speed.
Regardless, the rest of your post (and for that matter, the main article) is correct – understanding wing loading and how it affects stall speed is important to pilots and should be well understood.
JPR – yep, you’re right .. a constant descent or ascent in straight ahead flight CAN be a 1 G maneuver, more or less. Practically speaking, however, between pilot manipulations of the stick and the behavior of the air mass (thermals, wind shear, and general turbulence) through which the aircraft is moving it’s often difficult to remain in a 1 G situation.
Given that most stall-spin accidents occur in the traffic pattern, and given that windless and therefore windshear-less conditions are fairly uncommon, especially down low in the pattern, it’s practically impossible to maintain 1 G at all times. The infamous skidding base to final turn, which has led to so many fatal stall-skid accidents since the dawn of human flight, is a direct result of pilots not managing stick and rudder and the effects of varying G forces on the aircraft stall speed. The aircraft stalls because of the low airspeed/high G load on the wing, and the stall turns into a fatal spin because of the crossed controls.
I have an AOA meter installed in my Cherokee and I train with it. It is the most valuable to me in learning how the airplane flies at the very bottom edge of the flight envelope, and how to control it there. I back up this flight training by re-reading sections of “Stick and Rudder” and “Flying the Edge”.
One of the interesting things I learned from my AOA meter was that near the bottom of the flight envelope the AOA changes instantly with any movement of the control stick while the airspeed and the feel of the aircraft remains unchanged. This can be seen clearly as you fly and control the aircraft during maneuvers. I learned this while following the curves if the river near my home field. I kept learning to fly slower and slower until I could follow that curvy river (including steep turns) while keeping tha AOA needle at the top of the red for the whole distance of a few miles.
Flying maneuvers at the bottom edge of the envelope takes practice to do well and I get rusty pretty fast because I don’t fly as much as I would like. So every few hours of flying I go out and do low speed AOA practice. It is exhilarating and fun. I fly the curves of the river just east of EZF. I get slow enough to do full deflection aileron rolls from steep turns right and left. I fly along the riverbank at 750-1000 feet and get the airplane as slow as possible so as to fly the entire course with the AOA meter on the top of red edge. If I get in trouble or feel uncomfortable all I have to do is push the stick forward slightly and I have complete control.
I think you can learn a lot by training with an AOA meter, but you need at least three hours with it to understand how it works with the airplane. The Wright brothers knew all of this. The only instrument they had was a rudimentary AOA meter and they taught themselves and the rest of the world how to fly. That feat speaks for itself.
You found an error in my article JPR. Thank you. What I should have written was “if you are anything but straight and LEVEL you are not at 1G. If you are straight, and climbing you are more than opposing the force of gravity.If you are descending you are not quite opposing the force of gravity. Of course you couldn’t read this on a standard G meter.
Unfortunately, that’s also not true.
The reason a stable climb and a stable descent read 1 G on a G-meter is that they ARE at 1 G!
Recall from Physics 101 that force produces an acceleration, not a motion. F = m * a. So, in a climb, initially you will experience more than 1G as you start the climb. But stabilize out, and you are no longer accelerating – the airplane’s upward force is *equal* to that of gravity and you are leaving the earth behind at a constant vertical speed. No acceleration.
The same logic goes for a descent. This one’s somewhat easier to intuit. How does simulated microgravity work? It’s certainly not a stabilized descent! Ask the pilots of the Vomit Comet. They accelerate the airplane down so the weightless flight path looks roughly like a parabola. Constant acceleration, which is certainly not a stable descent. If they descended at a nice constant rate, their passengers would remain firmly seated at – you guessed it – one G.
No, the upward force is equal to gravity only in level flight. If you are climbing, then you have applied additional force that exceeds the force of gravity; else, you would not climb. Only in level flight has your upward acceleration exactly balanced the downward acceleration imposed by gravity.
>then you have applied additional force that exceeds the force of gravity; else, you would not climb.
The tense “have applied” is correct here. The transition from straight and level to a climb cannot happen at 1G – there must be a force to produce this momentary acceleration from 0 vertical speed to >0 vertical speed. However, in a stable climb (that is, your VSI is constant at some value), you are at 1G.
I will continue the remedial lesson in physics I was providing. Recall Newton’s first law of motion – that an object in motion stays in motion unless acted upon by some unbalanced force. It effectively means that if an object is going in some direction with some speed, it will take some force to change that velocity; an object’s natural acceleration is zero. This law is true about any axis – vertical, horizontal, or lateral.
Newton’s second law can be derived from the first, and is expressed by the simple formula “force equals mass times acceleration”, F = m*a. We are not accelerating upward, we are at a constant vertical speed. So the value of a is zero. Thus, the sum of the external forces must also be zero – this is true at any mass. But wait, gravity is pulling us down, so we are pushed up by the airplane with an equal amount to maintain this zero acceleration. What is equal to the force of gravity? By definition, 1G.
An observer to this thread may conclude that I am nitpicking here. Nothing could be further from the truth. Why else would it be true, then, that you can stall pulling out of a dive with your nose 45 degrees below the horizon? Why else would it be true that you can climb (with sufficient power) with your nose high and yet un-stalled? These are critical concepts for the safe pilot to understand.
Sorry, no sale.
Within the bounds of normal GA (ie, relatively close to the earth), when you are flying straight and level you are at 1G. Do we agree on that?
Despite the truth (in the macro world) of Newton’s laws, it is also true that if you shut down your plane’s engine, you would eventually fall to the ground, even absent wind resistance. Do we agree on that?
Consequently, it must be true that to continue to oppose gravity we must continue to apply an upward acceleration which — as you noted — balances the downward acceleration due to G. Net: balanced forces, defined as 1G.
To continuously increase our distance from the center of the earth, we must apply greater upward acceleration — not just once, but continuously — resulting in >1G on the ass. Unbalanced forces.
Of course, this ignores the (miniscule) decrease in the forces due to the increase in R in the equation F=g(m1*m2)/r^2 as well as any relativistic effects.
And, since I’m tired of your snide comments about remedial lessons in a subject you obviously don’t grasp nearly as well as you think, I’m outa here.
I provide these comments with the intent of making safer and more understanding pilots out of my fellow aviators. Nothing more, and nothing less.
Acceleration is a change in velocity. If velocity in any direction is constant, there is no acceleration. So, whether your velocity is 0, 500, or a thousand, if it is not increasing or decreasing, your acceleration is zero and all forces must balance to exactly zero as well. And therefore, a stable climb or descent is a zero-acceleration, 1 G maneuver, precipitated by a brief acceleration at greater or lesser than 1 G.
If you don’t believe me (and to be fair, why should you? I’m just some text on the internet) please ask someone with verifiable physics credentials. University professors are usually happy to talk about their subject and are usually much better than I at explaining the concepts they spend their career studying and teaching.
Hi Michael,
At the risk of stepping in the middle of a flame war and getting hit by both sides, JPR is right.
One confusion that’s happening here is the reference frame (butt relative to the plane vs. plane relative to the Earth). The key thing to remember, though, is that what acceleration does is causes a change in velocity. In any maneuver where the velocity vector isn’t changing, there’s no net acceleration.
For straight & level, or constant rate & airspeed climbs and descents, velocity (vertical and horizontal) doesn’t change. So we know that net acceleration (from the reference frame of the plane) is zero. Note that that doesn’t not mean that you’ll feel “weightless,” as the force of your seat is pushing up on your butt at the same rate that your butt is being drawn earthward. So your butt isn’t accelerating either.
That said, I’m not sure JPR has the most delicate way of explaining this issue.
Warm regards,
Chris
Thanks, Chris. Your comment about “frame of reference” caused me to re-think this and realize that I was confounding several issues:
– The extra thrust required to maintain a steady climb represents work being used to increase potential energy; it’s not generating an acceleration
– And from the POV of the original discussion — AOA instruments and their utility — the relevant point is not the presence of acceleration but the angle between the wing chord and the relative wind. In a climb, that angle increases from the angle in straight-and-level flight — thus, AOA increases.
Michael
Michael,
Thanks for the feedback; I’m glad it was helpful. I think your way of reframing it (no pun intended0 in terms of work/PE is helpful.
I almost didn’t comment because I was feeling a little reluctant to hijack an interesting argument about the effectiveness of AoA on a point of minutiae. But this makes me glad that I did.
To the initial point of the article: I think it’s a very good one. In many (aviation and non-aviation) accidents, we see evidence after the fact that operators (pilots, in this case) have constructed a mental model that is not 100% to reality because the indicators that they had available were, themselves, abstracted models of reality. The article makes a good point that the ASI is just such an indicator, while the AoA indicator gives a more direct measure of what is, ultimately, a very important variable.
I think that the effort to equip more a/c with AoAs is net positive. Not, perhaps, because they will be used in the moment to correct from an unsafe AoA, but rather to provide better, more direct feedback during training that can develop into intuition. This is essentially the same idea that @Pete Hodges mentions above. In his case, while training, the AoA gives pilots a more direct indication of where the aircraft is on the spectrum of its capabilities at any given time.
Thanks,
Chris
Chris, I started to disagree with your last paragraph, and then realized that once again it could be viewed as a “frame of reference” issue. To wit: we’re all supposed to be training regularly and routinely. Now that I have AOA, I use it to give a ready visualization of the reserve lift available as I go through various maneuvers when doing air work; I try to put myself through the stick-and-rudder aspects of the practical test standards at least twice a year. And this gives a richer context for my usage during normal operations.
Michael
Michael, I think that’s an awesome practice. Good for you.
–Chris
To change your flight path, you have to make an Input to loading (g force)…positive or negative. A change to loading implies a change to AOA. Getting an AOA past critical will stall the wing regardless of speed. Period. The essence of the article is that pilots must understand this fact…: AOA no matter speed or attitude, is what stalls a flying surface and that unloading is the only path to recovery.
I won’t disagree with anyone but will just offer the way I use to understand the difference between “direction of flight”, or the direction the chord of the wings are pointing, and “angle of attack”. I never quite got it until I did this mental experiment.
Consider that you are in a steep dive. Then, you pull up somewhat abruptly, within limits of course. The chord of your wings will then point more toward level, or perhaps level but that direction will NOT BE your angle of attack.
Why?
Because, at least for a bit, the airplane, and it’s wings, will still be falling. So your angle of attack will be quite high during that time until the airplane “pulls out” of the dive.
That’s the way I use to picture the difference between direction of flight and angle of attack. Of course, normal flight circumstances wouldn’t produce these dramatic circumstances. However, it’s easy to see how abrupt control movements, especially during approach, could produce similar effects. Hence, the accidents.
Pay attention to angle of attack. It’s the only thing that causes a stall & spin.
From reading the comments here, I’ve come to a couple of conclusions. 1: No matter what one says, even though it’s true, there is always someone who knows it better (obvious here) and 2: When you listen to a bunch of pilots sitting around the (digital) table, it’s not much different than listening to a bunch of politicians.
Unless you are in a wind tunnel you not only have the fuel amount changing but you also have wind currents. Both of which change the 1G situation and the AOA. I agree with the author, you may pass through 1G, but you cannot maintain any G factor exactly.
My takeaway is that you need to consider the actual weight of the plane in reference to the published figures – it does affect you flight characteristics.
The 1G argument is just a sideshow and has very little to do with the content of the article. The point is that at 1G full weight the published v speeds are correct – the rest of the time they are not.
A great article Chuck…thanks! As you say, the devil here is *not* in the details but in the HUGE misunderstandings of most pilots. As CFIs we are guilty…nose always high for a stall, wings level (why not in a turn to demonstrate the problem with skidding, or the stability of a slip?) Most pilots do not know where the CG falls in relation to lift (and the tail provides what force?) This ignorance is killing pilots…we need aerodynamics education desperately.
Most pilots (and many CFIs) fear any discussion of aerodynamics because of; math, Greek letters, and fear of appearing stupid. It’s better to be humble, open and eager to learn. Your article is a great help! I have found AOPA’s “Essential Aerodynamics” pretty helpful also: http://flash.aopa.org/asf/aerodynamics/ I would appreciate your opinion here.
Thank you, David. I agree with your comments. Yes, the AOPA online course is excellent. I Recommend all of my students take the course, and to study the Pilots Handbook of Aeronautical Knowledge, Ch 5, the aerodynamics section of the Commercial Pilots Oral Exam Guide, and read Stick and Rudder. I also like John Denker’s, av8n.com. I don’t like to dwell on too many formulas and graphs, but I think manufacturers should include a Vg diagram in the POH.
I’ve been flying with an AOA indicator in my airplane for a little more than 5 years, and it has changed the way I fly when lightly loaded–well, more the way I approach to land–flying is much the same except in slow speed turns, which I’ll discuss in a moment. Even in a lowly 172 (mine’s a 63 P172D), the difference in approach speeds is significant. That makes a big difference in touch down speed, roll out, use of brakes, etc. When running at close to gross, the speeds mimic the book pretty closely.
The real safety aspect occurs in turns at slower airspeeds, such as in the pattern, or while turning around in a canyon. I don’t like to fly at the red/yellow junction (the so-called “alpha angle”), which is really quite slow, as any turbulence may cause a stall–not a nice thing when close to the ground. But staying in the lower half of the yellow is plenty safe, and gives plenty of indication of the AOA in a slow speed turn. If I see it moving more toward the red, I can either reduce the bank angle or allow the nose to drop and lose some altitude. With the AOA indicator mounted on the top of the panel, it’s easy to glance at while maintaining vigilance outside.
There are those who think an AOA indicator is nothing but fluff in small GA aircraft, but after using mine for more than 5 years and roughly 325 hours, I’m a real believer. Would it eliminate stall/spin accidents if all airplanes had them? Probably not, but it might reduce the numbers significantly. Were I to buy another airplane, I would install an AOA indicator very soon after purchase.
Cary – you make good points here. There has been a common straw man argument made by AOA indicator critics that the devices can’t (and won’t) “prevent stalls”. No – only a pilot can prevent stalls, and nobody would argue otherwise. But AOA indicators most definitely provide much more precise measurements, and feedback to the pilot, in all flight regimes, of how close he/she is flying the wing to a stall.
Think of the AOA indicator as a training aid. It’s not a magic pill, and nobody has every claimed it is. But anything that helps pilots get better trained on how to fly their aircraft precisely is a net positive. Indirectly speaking, training more pilots to fly AOA rather than to fly airspeed will likely result in fewer stall-spin accidents.
“So, at the first indication that you are running out of lift (i.e,. sloppy controls, buffet, stall warning, or uncommanded roll), PUSH!”? On final, this could cause a collision with an obstacle. If no obstacle, it could result in diving below the glide slope and destabilizing the approach. Assuming the engine is operating, just getting slow should be corrected with an appropriate adjustment in thrust. That would allow for a correction in airspeed with throttle while the stabilized glideslope is maintained with the elevator. I think a major issue in these situations is improper coordination of pitch and power, and unfortunately this article is a perfect example of an incomplete discussion of how pilots should be coordinating the airplane’s controls.
What if more power is not available to you, as it appears was the case in the accident at Orange airport that claimed the life of a 16 year old student pilot? Eye Wirnesses saw brown smoke out of the exhaust and heard the engine running rough during the climbout after inutial takeoff. Just after that the airplane went nose up before turning sharply and dropping the nose.
While it is true an AOA meter doesn’t prevent a pilot from stalling an airplane, trainig with one makes unintentional stalls much less likely. Pushing the stick forward to maintain AOA and airspeed energy to keep flying is the only way to prevent a stall/spin accident. You don’t need an AOA meter to do that, but training with one better cements the proper action into your reflexes.
I don’t disagree that if power is available, use it. But if it’s not, it’s a whole lot better to push, keep the wing flying, and land under control, even if short of the runway, than to stall and crash. For many small GA aircraft, a push will still be required, even with full power available. An AOA indicator, if the pilot has been trained to use it and understands what it is saying, is an asset to keeping the wing flying.
The problem here is that we often get complacent after flying hundreds, maybe thousands of hours, without ever having an engine glitch. But they do glitch. There are many NTSB reports that prove that. Although it didn’t result in an NTSB report, 11 years ago last month, mine threw a rod through the top of the case, and I successfully landed without any power in a field. The last seconds of that event are relevant to this discussion.
The engine had stopped at about 800′ AGL, and I was descending on close downwind to a country road I’d chosen. I had already dropped 20 flaps when I realized that a power line was so close to that road that I’d likely clip a wing tip on the poles, so I looked to the right, but that field looked too rough. I looked to the left, and that field looked acceptable, but the power line was between me and the field. As I turned toward the field, it looked like I would not clear the power line. If I pulled the nose up, I’d likely stall, so instead I dropped the nose, retracted the flaps, and allowed the speed to build so I could pull up to clear the power line. As soon as I’d cleared it, I pulled on all 40 flaps and landed, probably the best soft field landing in my life. The training from many power off landings my first instructor had “forced” on me back in 1972-73 came back, I did what he’d taught me, and it worked.
But it only worked because I kept the wing flying by dropping the nose when I had no power to work with. If I had had the AOA indicator at that time, I would have known exactly how much to drop the nose instead of guessing at it.
As in all things aviation, finesse is the name of the game. Pitch and power are intricately intertwined and in this discussion, power is not the sole answer to getting out of high angle of attack situation. It is the application of a judicious amount of power coupled with lowering the pitch just enough to get out of tight situation. To know how much of either to apply takes a good feel for the aircraft. Not flying that much? You can never go wrong by adding all the power you have and get the heck out of there. This is called a go around. That is a good thing to practice too. We could go on all day with side shows from the original subject which was the usefulness of AOA.
Thanks to Chuck for venturing out onto this tricky aerodynamic limb (I feel your pain Chuck). We got a lot of heat and maybe some light, certainly many passionate pilots!
Just my two cents, but I would hope we all (try to) maintain a calm demeanor when discussing aviation “religion and politics” (pitch, power, AOA, stalls) or this subject will become “taboo” and the deafening silence is not conducive to enhancing safety…our ultimate mission? I had a retired (supposedly mature) 737 airline captain *screaming* at me about pitch and power (in a public forum) a week ago at our school…embarrassing and unproductive!
An excellent book which puts all this theory into motion (the only place it counts) is Rich Stowell’s Emergency Maneuver Training http://www.richstowell.com/store/books/book-emergency-maneuver-training/ The cost of a good book is inconsequential compared to “blue juice”…lets keep the learning going!
Here is a great “PSM” https://www.youtube.com/watch?v=0QPkhdUC3mQ
Love the comments, food for thought in bigger aircraft and aircraft that develop fuel weight imbalances quickly, it’s common to calculate your landing speed each pattern. This obviously doesn’t apply as much for most GA aircraft, but understanding the relationship in the final turn is important for any pilot, and all physics aside, it’s a great skill to hone if you want to learn how to max perform an aircraft, and not kill yourself in the process.
Warren,Michael, and Chuck. Please go back and study some aerodynamic physics before you kill yourselves or others. JPR, Marshall, and Chris have it right. 1G of acceleration can be experienced in many orientations, including stable climbs, stable descents, and turns with increasing rates of descent. This is precisely why “seat-of-the-pants” flying is impossible in IMC. The “death-spiral” can feel indistinguishable from straight and level (until you hit ground, of course).
Bill, ok I guess a clean record while accumulating more than 15,700 hours and I suppose somewhere around 20,000 landings was just good luck. Thank you for letting me know that I’m actually a menace to society. I’ll be sure and get some additional studies done.
I think everyone has made their point and defended their gentlemanly honor. Time to bury this overly personal debate everybody. We love the passion, but we don’t need the name-calling and nasty tone.
Listen, buddy. I am never going to kill anyone! I am a highly trained and experienced pilot. Get a life. I made one little comment that was wrong. I’m over it. You guys need to do the same.
> I am never going to kill anyone! I am a highly trained and experienced pilot.
Is this a textbook example of the “invulnerability” hazardous attitude? Food for thought.
I think birds are quite likely very much aware of their AOA, they can feel and hear it. An AOA indicator is a human-relevant analogue of this; how could it be anything but a good idea?
Since visual saturation is a potential issue, how else could we receive the AOA info? Imagine that the airframe is one giant ear; don’t you think that the sound of the air over the airframe would provide this info? Possibly mount various transducers on the wings and tail, and learn what the various AOAs sound like?
I admit this is a far-out idea, but as a musician it but it has always intrigued me.
Some, but not all, AOA indicators are also equipped with the capability of giving aural warnings, some with beeps or horns and some with voice commands. However, as we all know, pilots have proven themselves capable of ignoring all warnings, both visual and aural, so that there’s no 100% guaranteed way.
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A light aeroplane (eg <1100 kg) has relatively low inertia (ie it slows rapidly going up and increases speed slowly going down). At low AIS and high AOA a pilot has little time to take action to correct a rapidly deteriorating situation, especially below 800' AGL. If I ever feel the controls light because something is distracting me, I instinctively and immediately push the control column forward to unload, check the ASI, level the wings, add power if available and sort it out from there. I am not sure an AOA indicator will help because by the time I look at it, the airflow will have been re-established over the wing from my instinctive (course) actions to reduce the AOA and stop turning. I may lose 100' or more (if I am not inverted), but I will have avoided the killer "wing down", high AOA and low IAS situation. Just my first thoughts. On the contrary, an AOA indicator for a high inertia aeroplane is essential, in my humble opinion.
Well I’m an old Crop-dusting Pilot,with around 20.000 hrs.A lot of the Crop-duster planes I flew did not have a working Airspeed indicator.So I flew by attitude and seat of the pant.And I did find until I crash crop-dusting in a left turn around,loaded and 200 ft. off the ground.My aileron cable broke in a steep bank turn around,I could not pick up my left wing,but I push the nose down and hit on my left wing and nose.I woke up staggering across the field.
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Thank you for this article. It was just what I needed. I recently purchased a plane and when practicing slow flight was five to ten MPH below the white arc and still not stalling. I was wondering if I had an airspeed indicator problem. However now I realize, that the plane has a generous useful load and I was light with just me and fuel to the tabs. I was also in very stable air and took my time setting up the slow flight, so all my control inputs were tiny. I guess I experienced the real difference in stall speed based on weight and gravity.
Just a technical point.
I read comments in this thread during a constant indicated airspeed climb you are not accelerating. As you climb at a constant IAS your true airspeed is increasing, therefor you are accelerating.
Recently, I attended an FAA “WINGS” seminar on approach stalls supposedly giving instruction on AOA effect on stall speed. In the demo of an automated warning system, it kept saying “Increase speed” as the stall approached. I pointed out that the instruction was incorrect. It should say, “Get the nose down” or “Push”. The FAA guy was perplexed. Apparently did not grasp (even with diagrams) that it is the angle of attack that kills, not the speed, and to decrease the AOA you have to get the nose down. I think that is the point of this blog, not a discussion of 1G vs 4g, which is fairly irrelevant to the main issue, which is always, “Get the nose down” if your AOA is too high. The other point being that when someone hotdogs a turn to final, cranking it over because the turn is wide, then that sets up a low and slow. And then that AOA eats them alive and they spin in. Probably happens in 50% of GA crashes. Also rears its head with engine out on TO and pilot tries to crank it around to get back to the airfield, only to pick up a tail wind and a stall- and of course a tombstone.