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Low Speed Flight

Aero

When considering low speed flight, a pilot needs to think more than about stalls, spins, high lift devices, and ground effect. The region of reversed command is far more likely to bite you when you least expect it. No, this isn't a reference to some hokey British flight flick about approaching the speed of sound, this is something that can happen to you in holding, during a climb, or even during an ocean crossing.


 

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Stalls

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We tend to think of the stall as the point the wing simply runs out of speed and the nose drops predictably to prompt us to recover. But is that really what happens? The FAA, 14 CFR 25.103 (a), defines stall speed thusly:

VSR VCLMAX nZW

Where VSR  =  Reference stall speed
  VCLMAX  =  Calibrated airspeed when the load factor-corrected lift coefficient is first a maximum.
  nZW  =  Load factor normal to the flight path at VCLMAX.

And the regulation goes on to stipulate: "In addition, when the maneuver is limited by a device that abruptly pushes the nose down at a selected angle of attack (e.g., a stick pusher), VCLMAX may not be less than the speed existing at the instant the device operates."

All of that reinforces with us as we train in simulators that the wing stops producing lift at stall warning and if we don't do anything about it, the nose is going to drop of its own accord, stick pusher notwithstanding. But this clearly isn't true for a swept wing aircraft, where the wing continues to produce lift well beyond CCLMAX as shown by the coefficient of lift graph. The T-38 I flew years ago did not have a stick pusher and when it stalled, the nose didn't pitch at all. (Your only indication was wing buffet and a plummeting VVI.)

In fact, a swept wing, T-tail airplane actually pitches up during the stall. (More on this at Stalls.)

And all of this is the reason most modern swept wing, T-tailed jet aircraft are equipped with stick pushers: to make sure the angle of attack is reduced at or prior to CLMAX to give you an added margin over the aerodynamic stall. From this you may wish to take away a few points:

  • Since you aren't allowed to practice the full stall in most aircraft or even simulators, it is critically important that you initiate the recovery when the stall warning system activates.
  • If your stall warning system is inoperative and you venture into the stall, the nose of your aircraft may pitch up. You must reduce the angle of attack aggressively to break the stall.
  • If you are in a Controlled Flight Into Terrain or Windshear escape maneuver in a swept wing aircraft, you still have more lift to extract from the wings after the stall warning at CLMAX.

Spins

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The first requirement for a spin is to be at or near an aerodynamic stall, the second requirement is for yaw. As we saw in How Wing Shape Affects Stall Warning and Roll Control, a stall begins at the roots for straight wing aircraft and at the tips for swept wing aircraft.

That means a straight wing aircraft has lots of warning that a stall is about to occur and continued aileron control to keep the aircraft in coordinated flight.

Conversely, a swept wing aircraft has less warning and once a stall is reached, less lateral control. A spin is more likely in a swept wing aircraft.

As we saw in the previous discussion about stalls, a swept wing, T-tail aircraft actually pitches up during a stall, making spin entry even more likely if the angle of attack isn't reduced immediately.

Spin Characteristics

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Figure: Spin Characteristics, from [Hurt, pg. 310].

The accompanying chart on top is for a straight wing aircraft ("conventional configuration"), and on bottom is for a swept wing aircraft ("high speed configuration").

[Dole, pg. 149] The straight wing aircraft's spin is characterized primarily by the rolling motion with moderate yaw. The attitude of the spin is about 40° or more, nose down. In a straight wing aircraft, a stalled condition must exist before a spin can develop, but this is not true for a swept wing aircraft.

The swept wing aircraft has CL and CD curves as shown [in the figure]. Note that the CL does not have a well-defined maximum lift point. When this type of aircraft is rolled at high angles of attack, only small changes in CL take place. There is no definite stall, and the wing autorotation contribution will be quite weak. The change in CD that occurs between the two wings, however, is substantial and a strong yawing moment is developed.

Modern, low aspect ratio, swept wing aircraft have the mass of the aircraft distributed along the longitudinal axis of the plane rather than in the wings. As the yaw develops, during the spin, this mass contributes to the inertial moments and tends to flatten the spin. This results in extremely high angles of attack and high sink rates.

Spin Recovery

To get out of a spin you must stop the rotation and lower the angle of attack. The problem in a straight wing aircraft is that your rudder and ailerons may be ineffective during the spin itself. The problem in a swept wing aircraft is that the gyrations may be so extreme that you may not have the time or ability to manipulate the controls quickly enough.

The U.S. Air Force once required all its pilots be well versed in spins and required its pilots master the "Single Spin Recovery" in the T-37:

  1. Throttles - Idle
  2. Rudder & Ailerons - Neutral
  3. Stick - Abruptly full aft & hold
  4. Rudder - Abruptly apply full rudder opposite spin direction (opposite turn needle) & hold
  5. Stick - Abruptly full forward 1 turn after applying rudder
  6. Controls - Neutral after spinning stops & recover from dive

The procedure is called the "Single Spin Recovery" because, in theory, it will get you out of any spin. We heard war stories of F-4 fighters and large bombers using the procedure to success.

Video: T-37 Spins.

High Lift Devices

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Figure: Flap Configurations, from [Hurt, pg. 40].

We can increase the coefficient of lift of a wing by increasing its camber, decreasing the boundary layer separation, or by adding to the kinetic energy of the airflow above the wing:

  • Camber Changers — The camber of the wing can be changed by adding leading or training edge flaps, slotted flaps, or slats to the wing. This, in effect, gives you a new wing on demand which produces more lift, albeit with an increase in drag.
  • Boundary Layer Separation Control — The boundary layer can be made to remain closer to the wing by creating a thin layer of turbulence which increases the air friction. Vortex generators are a common method of achieving this. Another method is to introduce a vacuum over the top of the wing using holes with vacuum sources to pull the boundary layer in.
  • Kinetic Energy Adders — The kinetic energy of the air above the wing can be increased by blowing air over the wing from an external source. A wing mounted engine with exhaust over the wing can achieve this.

For the purpose of this discussion, it will suffice to say these high lift devices serve to replace the existing coefficient of lift curves. In the case of trailing edge flaps, the new curves tend to the higher and displaced to the left, to lower angles of attack. To the pilot this means the angle of attack required for any given amount of lift is reduced.

Ground Effect

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Figure: Ground Effect, from [Hurt, pg. 380].

As we saw in our discussion about Induced Drag, high pressure from the bottom of the wing spills over to the top from wing tip vortices. Because the aircraft is moving forward, this air spirals downward to depress the down wash of the wing and pulls the aerodynamic force of the wing aft. The aft component of aerodynamic force is induced drag.

The vortices spiral into a growing circle until they hit the fuselage, at which point they end. That is one-half of the wing span. So too, the vortices end as the aircraft nears the runway or any other ground or water surface. When this happens the deflection of the down wash decreases and therefore so does the induced drag.

A similar thing happens to the horizontal stabilizer on low tailed aircraft. As the tail enters ground effect it becomes more effective and may cause a nose-down pitching moment.

Ground Effect on Takeoff

When the aircraft is first rotated the wings are in ground effect. As the aircraft climbs to one-half the wing span, some aircraft may have a tendency to settle until more speed is developed. We could definitely notice this on older KC-135A aircraft at heavy weights. The tail can create a nose-up pitching moment when leaving ground effect. On lower power aircraft this can require the odd nose forward pitch input as the aircraft settles. On higher powered aircraft, such as the Boeing 747 or just about any Gulfstream, I've never noticed either issue on takeoff.

For more about this: Ground Effect.

Ground Effect on Landing

When the aircraft enters ground effect during landing, the decrease in induced drag means more of the aerodynamic force is directed parallel to the relative wind. The wing becomes more efficient and the aircraft may have a tendency to float. We noticed this in the Boeing 747, hence the need to "fly the airplane onto the runway" and the warning not to attempt to flare the aircraft to zero sink. A low tailed airplane may also experience a nose-down pitching moment.

The Region of Reversed Command

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Figure: Region of Reversed Command 1, from [Hurt, pg. 354].

As we've seen from our discussions about thrust required curves and L/DMAX, you can plot the thrust required to keep the airplane in unaccelerated flight versus velocity. Where this chart dips and is at a minimum, the lift to drag ratio is at a minimum. At this point drag is at a minimum and the aircraft is flying at its best endurance angle of attack.

Using the first chart as an example, you can see that at Point 2 the airplane is in what an aeronautical engineer calls the region of normal command and at the "available power setting." If you were operating at any point between the bottom of the curve and this point, you would notice the following:

  • Reducing power allows you to reduce velocity and then maintain that velocity by increasing the power setting to some point less than the original power setting. This relationship holds true for any point between the bottom of the curve and the available power setting.
  • Adding power allows you to increase velocity and then maintain that velocity by reducing the power setting to some point more than the original power setting. This relationship hold true for any point between the bottom of the curve and the available power setting.

In other words, the thrust relationship to velocity is as you would expect it to be. In should be intuitively obvious to any pilot should always want the airplane in this region.

Notice, however, that Point 1 requires the same thrust but is valid for a lower velocity. This is the region of reversed command, the airplane flies just fine in this region, but your actions are different and your margin for error is smaller.

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Figure: Region of Reversed Command 2, from [Hurt, pg. 354].

It might seem an unlikely place to be, but you can find yourself there in several situations, here are two for example.

  • Low altitude, excess power available. If you are in holding pattern flying at Maximum Endurance speed, which happens to be at L/DMAX, and select a slower speed, you will see the power requirement go up. Let's say L/DMAX was attained at 160 knots and 92% RPM on both engines held it beautifully. If you retard the throttles to decelerate to 150 knots the airplane will comply. But when it comes time to push the power up to maintain the lower speed, you will need more than 92% RPM. If you then want to accelerate beyond the original 160 knots, you will need a lot more power to get "over the hump" and back on the correct side of the power curve. Point B of the second chart shows this point graphically. You will have to add more power than what will hold the same speed in the region of normal command.
  • High Altitude, no extra power available. If you climb to altitude over Nova Scotia on your way to Europe, you often find the outside air temperature climbing as you coast out over the water. If you leveled off at L/DMAX and the temperature increases, you are now on an entirely new page where your current thrust setting may be behind L/DMAX, placing you in the region of reversed command. Since you don't have any excess thrust available, your only choice is to dive in an attempt to increase your velocity to the region of normal command.

Book Notes

Portions of this page can be found in the book Flight Lessons 1: Basic Flight, Chapters 15 and 16.

References

14 CFR 25, Title 14: Aeronautics and Space, Federal Aviation Administration, Department of Transportation

Air Training Command Manual 51-3, Aerodynamics for Pilots, 15 November 1963

Connolly, Thomas F., Dommasch, Daniel 0., and Sheryby, Sydney S., Airplane Aerodynamics, Pitman Publishing Corporation, New York, NY, 1951.

Davies, D. P., Handling the Big Jets, Civil Aviation Authority, Kingsway, London, 1985.

Dole, Charles E., Flight Theory and Aerodynamics, 1981, John Wiley & Sons, Inc, New York, NY, 1981.

FAA-H-8083-15, Instrument Flying Handbook, U.S. Department of Transportation, Flight Standards Service, 2001.

Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013.

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

Hage, Robert E. and Perkins, Courtland D., Airplane Peformance Stability and Control, John Wiley & Sons, Inc., 1949.

Hurt, H. H., Jr., Aerodynamics for Naval Aviators, Skyhorse Publishing, Inc., New York NY, 2012.

Technical Order 1T-38A-1, T-38A/B Flight Manual, USAF Series, 1 July 1978.

Revision: 20131005
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