How changes in Air masses, Air flow direction and intensity affect lift?

Question

I have a question regarding the airflow an aircraft experiences during 3 distinct scenarios:

1. An aircraft during landing, experiencing a sudden gust of 40 knots from the tail. What would be the immediate consequance- Airspeed, lift? All this if the aircraft does not correct the situation with pitch/ engine thrust.
2. An airliner cruising at 270 IAS, suddenly experiencing into an area of tailwind- meaning- the tailwind component of the wind aloft changed and increased by 40 knots. How would the airspeed look- just the following moment, and from there- lift? Regard to the situation without any compensating action by the pilot,
3. A fighter jet, at 20,000 feet, turning from at 25 degrees per second, meaning doing 90 degrees in under 4 seconds, from a situation of 50 knots headwind to no headwind. If the air mass is stable and blowing at a constant 50 knots, would there be any change in the characteristics of flight to the jet, meaning deteriorating airspeed which he/she would have to compensate for using pitch/thrust adjustment? In this situation, we assume that the pilot doesn't care for maintaining ground speed or track, just keeping the turn at a constant rate of 25 degrees per second.

My question is (and we discussed it in the previous question)- is there any diminishing effect to the fast change of airflow that was just ahead and changed to sideways/behind- when the change is caused by the aircraft itself, just as witnessed (by me recently) in an airliner when suddenly encountering a different air mass (I witnessed increasing Ground speed but decreasing IAS, until the Autothroottles commanded a slight increase in thurst).

Answer 1

For sake of simplicity, lets state that lift is produced by airflow over and under the wings (the rest of the airframe also contributes, but in the scope of this question we can neglect that).

Lift for any given airfoil is basically dependent on airspeed and angle of attack. Up to a certain limit any increase in airspeed or angle of attack will amount to increasing lift and vice versa. So, the question:

Case 1. and 2. The situation is the same. Sudden tailwind of 40kts will lead to a sudden 40kt drop of airspeed and sudden decrease of lift. The groundspeed will start to increase (slowly, because of inertia of the plane). If immediate corrective action is not taken, some altitude will be lost. Near the ground this would be catastrophic.

Case 3. I'm a assuming a level turn here: During or after the turn the wind will not have any effect on the airspeed or lift of the aircraft.

The aircraft is flying embedded in an airmass. If we are considering aerodynamics (lift and stuff), all changes made in the flightpath of the plane are made in relation to the airmass. Not the wind, not the ground. The wind is simply the movement of the airmass in relation to the ground.

If the wind does not change, nothing changes for the plane (overly simplified, of course, there's temp, pressure etc, but that's not on subject here). In cases 1 and 2, the wind changes, as the velocity is momentarily different (gust). In case 3 there is no change in the wind.

Your example of the event you've witnessed, with increasing groundspeed and decreasing airspeed is a simple case of flying into a region of tailwind. Tailwind would simultaneously reduce your airspeed and push you to go faster in relation to the ground, increasing your groundspeed. This event might happen as a result of flying somewhat perpendicularily through top or bottom part of a jetstream.

Since this subject has proven to be somewhat difficult to comprehend (here and in this question too), let us end this answer in a humouresque article on Flying Magazine website, that might help understand the misconception of turning in wind: The Last Word on Downwind Turns, Really.

As a sidenote, in case 3 you have described a rather steep turn without increase in engine power, the aircraft will bleed energy during the turn resulting in loss of airspeed, if airspeed is not maintained with power or glide. I'm also pretty sure that a constant 25°/second rate of turn is just too much to sustain for most fighters, even with full afterburners, and it is definitely too much for any pilot to handle for long. F-16 has a max rate of turn of about 27°/sec at about 380kts and 9g's.

Answer 2

As mentioned in Why do gusts change the airspeed during landing, but turning (which changes where the wind is coming from) does not?

1. The immediate effect will be a loss in airspeed and lift. Groundspeed will stay the same. The aerodynamic forces of thrust and drag will catch up causing the airspeed and vertical speed to return and the groundspeed to increase. If not recognized, this could be a dangerous situation. A sustained tailwind will cause your landing to be long and flat. You will be moving along the ground at a greater rate than desired unless you reduce thrust and airspeed. Since you can only reduce airspeed so much, alternatively, you can increase vertical speed in your descent rate to maintain your 3-4° glideslope. Either way, you have to watch your horizontal (ground) speed, ground roll, and Landing Distance Available to not overshoot the runway nor float due to the reduction in wind velocity near the surface caused by surface friction with the ground. Because of this, a 40 knot gust factor or tail wind may be too much for most GA airplanes.

2. In an airliner flying at cruise altitude and speed would not be affected by the change in wind velocity. In cruise, the gust factor is a smaller percentage of speed than it would be when landing. The instruments would indicate a momentary loss of airspeed and an increase in groundspeed and negative (downward) vertical speed. This may go unnoticed by the pilots, because it will happen and recover quickly unless the change is sustained. It will instead be experienced as turbulence.

3. The practical effect on the fighter jet would depend on its airspeed and thrust. That quick of a turn would require a lot of bank. If constant bank were maintained in a level turn at constant thrust, the aircraft’s airspeed would remain constant. But, it’s ground track would not remain constant. The airplane would drift downwind. The slower the airspeed, the more the aircraft would drift off of its constant radius. The faster the airspeed, the less apparent the drift. The ground speed would be determined by the aircraft’s heading in relation to the wind vector.

Answer 3

Ok, I was wrong. In order to better understand what is happening, other than saying that the plane flies embedded in air mass, it is easier to understand that the turn, no matter how fast it is- maintains equilibrium. meaning- it is not abrupt in no time, but a collection and a gradual turn into the tailwind or vice versa. This is easier understood by looking at each particle of air in which the jet fighter is flying. every second millisecond etc.- that the plane turns, the particles travel with it by the same velocity in relation to the ground, making it move altogether. The situation would be different it the aircraft turn 90 degrees in no time, making the air around it change direction in no time- like happens in a windshear or inversion.

Whooa that was interesting!