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

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).

• Could you elaborate on the context of your question? I hope we are not doing your homework. Feb 8, 2020 at 18:17
• I already understood the problem, as written here: link
– Stan
Feb 8, 2020 at 18:22
• Related and we have a number of other questions about sudden changes in wind direction that might be helpful. Feb 8, 2020 at 20:13
• Re case 3, it might be more instructive to consider the case of a hang glider circling at 30 degrees per second, flying at an airspeed of 20 mph, in the presence of a 25 mph wind. What do you think the glider "feels" in terms of a tendency to pitch up or or down, or to lift up or drop, or in terms of any tendency for the airspeed to change? Do you understand why? Likewise do you understand why the "dynamic soaring" technique of the albatross is entirely dependent upon the presence of a vertical wind gradient (+ an extra boost from slope lift effects)? It cannot work in a uniform airmass. Mar 9, 2020 at 17:25

## 3 Answers

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.

• For your answer no.2- regarding the Jet fighter- yes, a level turn, without bleeding airspeed (assume engine was added prior to the beginning of the turn). And still, I am having a difficulty understanding why these examples are different., just due to the reason that the jet self- positioned itself into a "different air mass, in contrary to the airliner which accidentally flew directly into it. What I can say for sure is that probably due to the immense force and overpower of the fighter, with some more engine power/slight change in pitch- the wind change goes unnoticeable.
– Stan
Feb 8, 2020 at 12:48
• But the wind does not change... Feb 8, 2020 at 13:18
• I edited the answer a bit, maybe now it's more clear? Feb 8, 2020 at 13:34
• as per your last note about the F16- yes it's about that. but it's no problem to maintain a turn at say 15,000 but I did exaggerate a bit. We can point it at 15 degrees per second, making it 6 seconds, as seen here- link. I agree with most of you answer, except the last part- If the fighter jet turns 90 degrees in 6 seconds- meaning losing a headwind component, it changes the wind for himself, by turning it by 90 degrees. Now the question is- is a wind change is also self made.
– Stan
Feb 8, 2020 at 14:38
• There is no wind change. Feb 8, 2020 at 14:48
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.

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!

• Close but no cigar. In the event of a sudden (infinite g), the wind would still not be a contributing factor. Please read the article I linked in my answer and try what is described there out yourself, on your next flight for example. Feb 8, 2020 at 15:54
• I want to correct myself- If we changed our vector completely to a different direction, meaning basically beaming 90 degrees to the right- then- we would experience a sudden change in airspeed.
– Stan
Feb 8, 2020 at 16:18
• And i couldn't spot your link.
– Stan
Feb 8, 2020 at 16:23
• Feb 8, 2020 at 16:54
• I will hold a note with your name of course :)
– Stan
Feb 8, 2020 at 17:27