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Why do I ask this question? Because Wilbur Wright sent a letter to Georges Spratt (an aviation enthusiast he personally knew) saying that his plane, Flyer II, once in flight, accelerated easily to 45 mph, by itself, after reaching 30 mph. (The plane had no throttle.)

I know that once in flight, the airspeed of a plane is independent of the headwind, so it simply does not matter how fast the wind blows from in front of the apparatus. Those lulls in the wind Wilbur feared about would not have had an impact on the course of the aeroplane unless the wind suddenly dropped in intensity (this is more a theoretical case). Only then for a short time the airspeed of the plane decreases to its former airspeed minus the drop in the headwind speed, but the plane regains quickly its former airspeed.

The question is, can a low flying plane really touch the ground just because the intensity of the headwind diminishes?

We have made forty five starts with our 1904 Flyer. Unless the relative speed at starting is 27 miles in a calm and two or three miles more than that in a wind, the machine will gradually slow down till unable to fly. After the relative speed passes thirty miles the velocity accelerates till a relative speed of 45 to fifty miles is reached. We found it difficult in practice to get a speed down the track greater than 20 miles an hour, so that unless we had a wind of about 10 miles we were not sure of being able to fly for a lull in the wind would let us drop below the real flying limit. … We have now finished a starting apparatus which gives a speed at start of 27 miles an hour in a dead calm, and expect shortly to begin circling. (Wilbur Wright, “Letter to G. A. Spratt”, Dayton, Sep. 10, 1904)

enter image description here Source Small plane flying at low altitude.

Update:

The topic Does a sudden drop of the velocity of the headwind affect the airspeed by lowering it? and its explanations do not answer my question. I was already aware that a sudden drop of the headwind intensity reduces for a short time the airspeed of a plane and I have already written this thing to make it clear I knew about this phenomenon.

The plane of Wilbur flew a few meters off the ground. So, assuming Flyer II traveled at an airspeed of 30 mph against a headwind of 10 mph and suddenly the machine entered a region of perfect calm, its airspeed would have reduced to 20 mph instantly, but did the plane need to drop more than a few meters to regain its former airspeed of 30 mph?

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  • $\begingroup$ It appears that the drop in the wind speed would have created problems only for a few seconds while the plane of Wilbur Wright was still below 30 mph. After that, from what he said in the letter, I understand that the plane accelerated quickly to 45 mph. From his explanations it appears that the headwind speed added to the ground speed of the plane all the time. Flyer II got airspeed from the headwind below 30 mph and then from a reduction of the drag as the airspeed increased! $\endgroup$ Mar 27, 2021 at 8:02
  • $\begingroup$ The phenomenon described by Wright might not be related to headwind, it could have also been about aerodynamics of the Flyer. Above certain speed, the efficiency of the wings, propellers and the engine might have reached some optimal state, enabling easier flight. $\endgroup$
    – Jpe61
    Mar 27, 2021 at 8:46
  • $\begingroup$ One thing is sure, if nothing improved, after the plane reached an airspeed of 30 mph, Flyer II could not have reached 45 mph unless its power had increased (45 mph / 30 mph )^3 = 3.375 times. As the plane had no throttle and the engine run at constant power, tt is quite difficult to imagine a phenomenon that would have had the same effect as a more than three times increase in power. $\endgroup$ Mar 27, 2021 at 13:42
  • $\begingroup$ Imagine this: the backside of the power curve 🙃 $\endgroup$
    – Jpe61
    Mar 27, 2021 at 14:31
  • $\begingroup$ I have already wasted a lot of time, weeks, trying in Mathcad to simulate a Flyer II working on the backside of the Power Curve. The conclusion is that the plane could not have gained those 15 mph just due to the power curve profile. The gain would have been no more than 5 mph. However, no wind tunnel test have demonstrated that the drag of Flyer II decreased with the airspeed for a certain range of speeds, while in flight. $\endgroup$ Mar 27, 2021 at 15:51

2 Answers 2

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Yes. The theoretical concept of "moving with the airmass" essentially says that for any given constant wind or no wind, airspeed is dependent on thrust and drag and reaches a steady state.

However, in nature, especially close to the ground, obstacles such as buildings, trees and hills can make airflow anything but constant.

This is why the modern aviator adds a little airspeed when landing on windy days. A gust or a lull can momentarily change your airspeed faster than the plane can restore its thrust/drag balance.

Wind shears are an extreme example of this. Were it not for the wind, flying that Cessna would not be more difficult than driving a car.

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    $\begingroup$ Not sure that you would want to take someone from a car and throw them into a 152 and tell them "piece of cake" just because the day was windless. $\endgroup$
    – John K
    Mar 27, 2021 at 1:06
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    $\begingroup$ No wind definitely helps, as do qualified instructors. $\endgroup$ Mar 27, 2021 at 1:41
  • $\begingroup$ See also: code7700.com/rule_07.htm (a discussion of Vref, and the importance of adding airspeed when wind shear is anticipated). $\endgroup$
    – randomhead
    Mar 27, 2021 at 2:43
  • $\begingroup$ Rule of thumb: add half of gust speed to your airspeed. $\endgroup$
    – Jpe61
    Mar 27, 2021 at 8:48
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    $\begingroup$ Agree with @JohnK, basic flying is not that hard, but it still does not quite compare with driving a car. $\endgroup$
    – Jpe61
    Mar 27, 2021 at 8:52
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The only speed that matters to a fixed wing airplanes ability to keep flying is airspeed. Whether that airspeed results from acquired groundspeed or changing headwind is irrelevant. To the airplane, airspeed and any kind of wind are the same thing. No airspeed, no airlift and yes, that turns flying into falling, which makes the ground come up.

However,... there also is such a thing as ground effect, which may still save the day, should this ever happen to you. Ground effect does not result from a reduced pressure on top of the wing, but from the wing working like an upside down bulldozer, compressing the air underneath it. It actually always does that, even way up in the air, where it causes wake turbulence. In flight, any airplane is trailed by this wake turbulence, which is a set of 2 counter rotating wind vortices going straight down from where the airplane was when it generated them.

At altitude, wake turbulence is the other guys problem. Closer to the ground however, that wake starts to matter to the airplane itself. Since the ground stops these vortices from going any further down, the lower you fly, the harder it is for that air to escape anywhere, making it push back against the bottom of the airplane which works like a cushion.

How much it matters exactly is hard to tell. It obviously relates to the forward speed. The way it works however, relates to the fact air always moves at the speed of sound, which is a field of understanding common aviation aerodynamics hasn't gotten into yet.

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