Spin Aerodynamics

[mhcasey]

Once again, the FAA books have let me down and I come seeking a more thorough understanding.

My thinking: In a skid, the inside wing stalls first because it's aoa is greater than the outside wing. I'm having a tough time visualizing this, though. I can see two possibilities:

1) The outside wing literally receives more air over its surface than the inside wing due to interference from the fuselage. Also, relative wind will tend to move inboard over the outside wing, but outward over the inside wing, so the inside wing will produce more induced drag as more air will escape off the edge into a vortex (though the interference drag at the fuselage on the outside wing may counteract this?).
2) The inside wing is "slower' into the wind than the outside wing, though this only seems to be the case if there is some acceleration around the vertical axis.

I know the inside wing would stall first, but I'm not convinced my explanation is thorough or even completely accurate.

Also, why the incipient and recovery phases? Does the plane's inertia just initially want to remove it from the spin until all of the forces balance?

Thanks in advance, and I look forward to hearing another crystal clear and useful explanation.

 

[tgrayson]

I know the inside wing would stall first, but I'm not convinced my explanation is thorough or even completely accurate.

The inside wing stalls first by definition; otherwise, it wouldn’t be the inside wing. So the real question is simply “why might one wing will stall before the other?”

There can be a variety of reasons, some as unusual as wing contamination, misrigging, wing damage, etc, but our interest in the question normally assumes a perfect airplane.

For unintentional spins, the condition that typically leads to entry is lack of coordination, most likely a skid. In this condition, the relative wind is coming at the airplane at an angle, so there is some crossflow over the wings. The fact that the wind is no longer taking the shortest path across the wing means that the boundary layer has a chance to get thicker, increasing the tendency to separate. On one wing, the flow will be moving towards the wingtip, so the thicker boundary layer will lie in that direction. As the AOA approaches the critical angle, the wing tip will go first, causing a roll in that direction.

Many sources also point to the downward deflected aileron as causal, as well, in a skid, since flapped wings stall at a lower AOA. However, I’ve never noticed a tendency for an aircraft to roll in that direction while the aircraft is coordinated. One of my flight testing books also is skeptical of that explanation, even though it’s plausible.

Also, why the incipient and recovery phases? Does the plane's inertia just initially want to remove it from the spin until all of the forces balance?

Spins are still regarded as the most complex aircraft behavior, one very resistant to theoretical analysis. Not only are the aerodynamic forces complex, but there are also inertial and gyroscopic forces involved, too. For instance, as the aircraft rotates around its vertical axis, the mass of the airplane tends to align itself in the plane of rotation. What this means is that an airplane in a nose down spin has a tendency to nose up, increasing the AOA and deepening the stall. Worst case scenario is a flat spin. Since the airplane is a gyroscope, any applied force has its effect 90 degrees in the direction of rotation.

So the incipient phase is when these forces are struggling for equilibrium. That may be achieved when the aircraft spontaneously recovers or becomes established in a relatively stable spin. Compare it to when you wear a pair of rollerblades. There will be times when you start to lose your balance and your arms and body flail around in a ridiculous manner as you struggle to stay upright. This is an incipient fall. You may recover your balance, or you may become established in a stabilized fall, although you will soon reach a new equilibrium, just as you would in a spin at low altitude.

 

[nosehair]

Here's how I think about it - as it was explained in good detail in the old Flight Training Handbook. Too bad the current Handbook explanation is watered down.

First, quit thinking about the vague statement that "the airplane must be uncoordnated, or have cross-controls, to spin". While that is a true statement, it is incomplete and leads to a wrong conception of the aerodynamics.

There must be YAW. If the nose is not turning, the airplane cannot enter a spin.

For instance, when you are doing a power on stall, and you think you are keeping the nose straight, but in the last second or two before it stalls, you allow some left tuning P-Factor to pull the nose left as the wings stall, you are moving into the incipient phase. If you do not allow the nose to move left, if you keep the nose straight, it will not, and cannot spin.

So, the usual pilot will allow some yaw as the AoA approches stall, and P-Factor is increasing rapidly. The nose-left yaw will cause the left wing to retreat, relatively, and lose some lift. If right aileron is not applied and the left wing descends as a result of the slight loss of lift, this descent will result in a higher AoA of the descending wing, and a decrease in AoA on the rising wing. This increase in AoA of the left wing at critical AoA results in a full stall, while the rising wing reduces AoA and develops a less-than-full stall. This causes a further drop in the left wing, a sidways sliding (slipping) which causes a weathervaning effect and further causes the nose to yaw further into the relative wind, and into a full spin.

 

[CamYZ125]

If right aileron is not applied and the left wing descends as a result of the slight loss of lift, this descent will result in a higher AoA of the descending wing, and a decrease in AoA on the rising wing. This increase in AoA of the left wing at critical AoA results in a full stall, while the rising wing reduces AoA and develops a less-than-full stall.

I teach my students to keep neutral ailerons throughout a stall and stall recovery... isn't that the general consensus? Ailerons in a spin can aggravate it as well.

Also, when I was doing my spin training for my CFI, I was always taught that both wings are stalled, but one is essentially more stalled than the other.. thus creating the forces to spin the airplane.

 

[SpiraMirabilis]

I teach my students to stall at a bank as listed in the PTS, with neutral ailerons [if possible].

 

[tgrayson]

There must be YAW. If the nose is not turning, the airplane cannot enter a spin.

What you mean here is yaw rate, of course, and this is only relevant to spin entry when a roll develops as a result of the yaw rate. This is how we normally do intentional spins and you described the mechanism accurately.

In other spin entries, it’s pretty much irrelevant because either the yaw rate is very low, resulting in a very low roll rote, or the roll rate is controlled via ailerons. For instance, during the infamous cross-controlled stall, the roll rate can be zero, since you’re preventing the roll with aileron, but you will still have an ugly entry. The cause is as I detailed before, the sideslip and perhaps the aileron too. There may be a yaw rate, but since it doesn’t result in roll, it can’t be contributory to the spin, at least via the mechanism you described.

Likewise in the power-on stall setup. Most often the aircraft seems to break to the right, because even when perfectly coordinated, the aircraft will be sideslipping to the left, due to the lift generated by the rudder. (Just as a ME aircraft will sideslip towards the dead engine.) Breaking to the left will happen with inadequate coordination, even when perfect heading is maintained (zero yaw rate).

As someone else mentioned, raising a stalled wing with the ailerons is generally a no-no, because it will only deepen the stall. Some people use this effect to their advantage during intentional spins to get an improved entry.

 

[tgrayson]

I teach my students to keep neutral ailerons throughout a stall and stall recovery... isn't that the general consensus?

Depends on what you consider the "recovery." Most CFI's teach it differently from the way the Airplane Flying Handbook specifies. It says

1) Lower the nose
2) Add full power
3) Roll level with coordinated aileron and rudder.

Most people it seems teach stomping on the rudder to raise the low wing, a technique not endorsed by the AFH.

 

[CamYZ125]

Depends on what you consider the "recovery." Most CFI's teach it differently from the way the Airplane Flying Handbook specifies. It says

1) Lower the nose
2) Add full power
3) Roll level with coordinated aileron and rudder.

Most people it seems teach stomping on the rudder to raise the low wing, a technique not endorsed by the AFH.

Yeah, I teach "Pressure, Power, Rudder, Level, Climb".. as in:
-Release the backpressure (and add forward pressure as needed)
-Add power
-Make sure you're coordinated with rudder
-Level the wings (once you've got a little bit of airspeed)
-And climb away...

 

[nosehair]

I teach my students to keep neutral ailerons throughout a stall and stall recovery... isn't that the general consensus?

Yes, that is the general consensus, which is wrong - if you stop there.

The proper way to recover depends on the actual airplane being flown. The Cessna wing is warped so that the outer portion of the wing where the ailerons are is not stalled when the inside portion is stalled. The ailerons are very effective during stall recovery. Read the Airplane Flying Handbook. That is what you should be teaching from.

The practice of using rudder-only to pick up a dropped wing is a primary teaching technique to stop the beginning student from using aileron-only, or mostly-aileron to control yaw. The beginning student will try to drive the nose with the 'steering wheel', so we have to say "rudder-rudder-rudder!" a lot.

In the beginning, we have to do a lot of stuff different than what you eventually wind up doing - let me say that differently:

In the beginning, you have to set up rote-response habit patterns, such as: rudder turns the nose, ailerons bank the wings. Period. After the student gets the automatic knee-jerk response of pushing the rudder to turn the nose, and twisting the wheel to control the bank, then you can begin to 'coordinate' the two.

Which is how you should recover from the stall. With aileron, but not over-use of the aileron, which could result in stalling the wing, but in coordinated use of aileron with rudder.

Over-use of rudder in stall recovery is also dangerous. That's what the infamous base-to-final turn is all about - too much rudder.

 

[CamYZ125]

Ok, I'll agree with coordinated use of ailerons and rudder, as long as the aileron input is not excessive. You're right that the first instinct for primary students to lift a dropped wing is to put in a lot of opposite aileron, which if an excessive amount of uncoordinated aileron is used then it will aggravate the stall and provide an adverse yaw that can lead to a spin.

 

[tgrayson]

Ok, I'll agree with coordinated use of ailerons and rudder, as long as the aileron input is not excessive.

If you get a severe wing drop, this is indication that the wing tip is stalled, which can happen regardless of washout; aileron in this situation, coordinated or not, is likely to be counter-productive. The stall must be broken first, as you said.

Stalling of the inboard areas of the wing produce milder rolling tendencies and leaves the wingtips functioning, and could probably be countered with aileron. But why? Break the stall first and then you don't have to worry about discriminating between the two situations.

 

[mhcasey]

My take on the procedures:

In most light trainers, it's going to take about 1 second to break the stall. Just learn to break it, and gradually pull up with coordinated gradual aileron inputs and you'll be fine. I think the concern should be less on a secondary stall from excessive aileron than it should be from excessive back pressure pulling out from the nose low attitude following the stall. I learned as a primary to just stick to rudder, which worked fine. After a bit of aerobatics, I began to realize that the aileron thing was not really a big deal provided that you were out of the stall.

Case in point: Do a snap roll with neutral ailerons. Now try it with full aileron into the turn. Your rate of rotation with full aileron is going to be immensely faster. The same can be said of a basic spin, which is why my aerobatics instructor taught:

PARE: Power to idle, ailerons neutral, full opposite rudder, elevator to break the stall.

Also remember, if the student ends up in some nasty banked stall that did not quite result in a spin, he's still going to want to level the wings before climbing out so as not to overload the aircraft.

Back to the aerodynamics: TGrayson, your response gave me the justification I needed to not worry too much about what's physically going on. Anytime I hear that gyroscopic forces are involved, I assume it's just a mess that's over my head.

 

[fish314]

My take on the procedures:

In most light trainers, it's going to take about 1 second to break the stall. Just learn to break it, and gradually pull up with coordinated gradual aileron inputs and you'll be fine. I think the concern should be less on a secondary stall from excessive aileron than it should be from excessive back pressure pulling out from the nose low attitude following the stall. I learned as a primary to just stick to rudder, which worked fine. After a bit of aerobatics, I began to realize that the aileron thing was not really a big deal provided that you were out of the stall.

Case in point: Do a snap roll with neutral ailerons. Now try it with full aileron into the turn. Your rate of rotation with full aileron is going to be immensely faster. The same can be said of a basic spin, which is why my aerobatics instructor taught:

PARE: Power to idle, ailerons neutral, full opposite rudder, elevator to break the stall.

Also remember, if the student ends up in some nasty banked stall that did not quite result in a spin, he's still going to want to level the wings before climbing out so as not to overload the aircraft.

Back to the aerodynamics: TGrayson, your response gave me the justification I needed to not worry too much about what's physically going on. Anytime I hear that gyroscopic forces are involved, I assume it's just a mess that's over my head.

That PARE technique is almost exactly the same as the spin recovery procedure in the T-37. In that aircraft, we taught that during the spin the ailerons would be ineffective in taking the aircraft out of the spin, and depending on their usage could actually hinder recovery. This was because during a spin in that aircraft the ailerons were stalled, so if you tried to use ailerons to roll out of the spin, that would deflect the inside wing's aileron down. This would increase the camber, which would increase the drag on the inside wing, but since this portion of the wing was past the stall angle of attack, it wouldn't help create any more lift. So aileron during the spin would just tighten the turn rate and make the spin worse. Drag would increase on the outside wing as well, BTW, but because of the aileron moving up, the wing's camber would decrease (in the area of the aileron), so induced drag would go down a little to counteract the increase in parasite drag on the outside wing.

Similarly in the T-6 (Texan II, not the old ones), they teach ailerons neutral in the spin. Ailerons towards the spinning direction accelerate the spinning tendency, and ailerons opposite the spinning direction just caused more pronounced oscillations while spinning.

As for whether to use ailerons or not in STALL recoveries, we did teach the students to step on the high wing, but we also taught them to roll the airplane towards level (in the tweet, that is).

In the T-6, with the big propeller, it seems as though more emphasis is placed on using rudder than on ailerons during stall recovery.