Read Stick and Rudder. Still seeking clarification.

[schmoo]

On the advice of one or two people in this forum, I have just finished reading, Stick and Rudder.

As promised, it has really helped clear up a number of things that I was having trouble fully comprehending. Not sure if it is just because the book is over 60 years old, but the language used by the author was very straightforward and easy to understand.

That being said, I still have a number of things that are bothering me a little. I have made notes of them somewhere, but I do remember a couple of things off the top of my head :

I have heard and read many times that the plane "does not feel the wind" once it is in the air. It is only experiencing the relative wind. Examples such as a man walking on a moving train or on a moving ferry help to illustrate this. Okay that's fine. But, why does one increase final approach speed when it is gusty ? Is it because gusts are special types of wind that due to their intensity can actually affect the plane temporarily ?

In picturing myself trying to swim in a fast moving river, I can accept the fact that if I just relaxed in the river, I would soon start floating downstream at the speed of the current. But in a river, the water is not all moving at the same speed. Shallow areas move faster, areas behind rocks might slow down, etc. So, is it the same with a plane ? Even though the air is moving as a unit, are there not some parts of the air mass that are miving faster or slower than others ? Could the plane not react differently within the air mass in the same way that I would find it harder to swim upstream in a shallow, rapids-filled area ?

Related to the above questions, the other day I really tried to monitor my approach speed on final since I have a tendancy to come in too fast. It was a windy day but it was straight down the runway. I noticed that on final, I had the opposite problem in that my speed dropped very quickly and I needed a blast of power to get myself back onto a god glide path. So, what is going on here ? Did my speed drop because the fast moving air mass that the plane was in was moving away from the airport and making me automatically decrease my glide slope angle and ultimately made my speed drop more quickly than normal ? I hope so, because this I could understand. Otherwise, I am left still thinking that the plane does feel the wind and is reacting to it.

Unless I am misunderstanding what is meant by "the plane doesn't feel the wind" , I am having trouble fully accepting this fact. It doesn;t seem to me that all objects yield to an airmass or a moving current in the same way. If I drop a penny off the CN tower in a 60 mph wind, will it end up hundreds of feet from the tower ? Maybe. But how about a 10 pound weight ? It doesn't seem to me that it would drift with the wind the same way as the penny. Similarly, I feel that a plane wouldn't necessarily yield 100% to the motion of the airmass, but would at least partially resist it, and in this sense, would "feel" it.

I dunno.

[slam525i]

I have heard and read many times that the plane "does not feel the wind" once it is in the air. It is only experiencing the relative wind. Examples such as a man walking on a moving train or on a moving ferry help to illustrate this. Okay that's fine. But, why does one increase final approach speed when it is gusty ? Is it because gusts are special types of wind that due to their intensity can actually affect the plane temporarily ?

That's because gusts are CHANGES in the wind. Say you're on a ferry, you can walk just fine and you don't feel the ferry's movement. But say the ferry starts rocking, you will feel the rocking. So, yes, the airplane doesn't feel the wind, but if the wind changes (i.e. gusts), then the airplane feels it. The reason you add speed on gusty approaches is that while the airplane isn't affected by constant wind, a sudden change can drop your AIRspeed, putting you close-to/below stall.

Even though the air is moving as a unit, are there not some parts of the air mass that are miving faster or slower than others ? Could the plane not react differently within the air mass in the same way that I would find it harder to swim upstream in a shallow, rapids-filled area

Yes. If there are trees, hangers, other buildings nearby, it will affect the wind. Surface wind is often different from wind higher up. The airplane will "feel" it when it changes.

Unless I am misunderstanding what is meant by "the plane doesn't feel the wind" , I am having trouble fully accepting this fact. It doesn;t seem to me that all objects yield to an airmass or a moving current in the same way.

You're right. Not all objects yield to an airmass the same way, just like a barn door is different from a pebble. BUT, that's because they're not "floating" in the air. A penny and a 10 pound weight, dropped from a sufficient height, will land in about the same place. Sufficient height being something ridiculous because it takes a lot of time to accelerate the 10-pound weight by wind while a penny, with big flat surfaces and not much weight, will accelerate quickly. But, once both are "stable" in flight (er... in fall), both will be moving at the same lateral speed. Off the CN tower, the penny will accelerated by the wind much faster than a 10 pound weight, and hence the difference in landing point.

(I hope I haven't made this worse!)

[iflyforpie]

You are right to a certain extent that not all airplanes yield to the wind the same way, but this is accounted for in their no-wind performance. A 100HP 150 will only go 90 knots while a 180HP Mooney will go 160 knots. But put each into a 30 knot headwind, and the ground speed on each will be reduced by 30 knots. As far as the airplanes go, they don't know the difference.

Add gust into it, and you will have different results. The light and draggy 150 will be pushed and bounced around much more easily than the heavier and sleeker Mooney.

[old_man]

I have heard and read many times that the plane "does not feel the wind" once it is in the air. It is only experiencing the relative wind. Examples such as a man walking on a moving train or on a moving ferry help to illustrate this. Okay that's fine. But, why does one increase final approach speed when it is gusty ? Is it because gusts are special types of wind that due to their intensity can actually affect the plane temporarily ?

You should read up on 'wind shear'. The wind may change very suddenly in intensity but it takes time for the airplane to accelerate back to equilibrium.

In picturing myself trying to swim in a fast moving river, I can accept the fact that if I just relaxed in the river, I would soon start floating downstream at the speed of the current. But in a river, the water is not all moving at the same speed. Shallow areas move faster, areas behind rocks might slow down, etc. So, is it the same with a plane ? Even though the air is moving as a unit, are there not some parts of the air mass that are miving faster or slower than others ? Could the plane not react differently within the air mass in the same way that I would find it harder to swim upstream in a shallow, rapids-filled area ?

I think what you are describing is mechanical turbulence.

Related to the above questions, the other day I really tried to monitor my approach speed on final since I have a tendancy to come in too fast. It was a windy day but it was straight down the runway. I noticed that on final, I had the opposite problem in that my speed dropped very quickly and I needed a blast of power to get myself back onto a god glide path. So, what is going on here ? Did my speed drop because the fast moving air mass that the plane was in was moving away from the airport and making me automatically decrease my glide slope angle and ultimately made my speed drop more quickly than normal ? I hope so, because this I could understand. Otherwise, I am left still thinking that the plane does feel the wind and is reacting to it.

I wasn't in the plane with you nor am I a qualified instructor so I won't guess as to what caused this to happen to you. However, I will say that wind will change in both intensity and direction with altitude. But the way you are describing airspeed and glide path leads me to believe you may still be learning how to master the stabilized approach. Pilots having been blaming 'the wind' for years..................

Unless I am misunderstanding what is meant by "the plane doesn't feel the wind" , I am having trouble fully accepting this fact. It doesn;t seem to me that all objects yield to an airmass or a moving current in the same way. If I drop a penny off the CN tower in a 60 mph wind, will it end up hundreds of feet from the tower ? Maybe. But how about a 10 pound weight ? It doesn't seem to me that it would drift with the wind the same way as the penny. Similarly, I feel that a plane wouldn't necessarily yield 100% to the motion of the airmass, but would at least partially resist it, and in this sense, would "feel" it.

I dunno.

Airplanes generally don't plummet to the ground at terminal velocity. (Unless something really bad has happened). You are missing the point of the example though. Both objects don't know their is a wind out, they only know that the speed at which the air is moving over them is at terminal velocity. Only you know that there is a wind out because insist on describing their movement relative to the ground and not the air mass they are in.

Clear as mud?

[schmoo]

So it's not 100% true that a plan in flight does not feel the wind, because during a gust it takes the plane a few moments to succumb to the wind, so for a second or two, the plane is actually resisting ( so to speak ) the wind.

So if the gust was a crosswind from the left, does it have any kind of weathercocking effect ~ meaning, would the nose of the plane momentarily point into the wind ~ or would the gust simply push the entire plane or does it depend upon a number of factors ? I'm trying to get a handle on what is happening when I am on final and am continually adjusting rudder pressure to keep the nose lined up with the rnwy.

I am getting closer to understanding this, I promise.

[Tim]

gusts by nature are turbulent eddies of air, they swirl around (high to low pressure of course) so it really just depends on how each gust hits you. sorta like the winds in a low pressure system change depending on where you are in it, the gust will effect you in different ways depending on which way and how fast that little patch of air is moving. you can get a 2 dimensional idea of this from watching gusts of wind on relatively flat water. the gusts spread out and dart all around, well the same thing happens in 3 dimensions in the air. thats why youre always working the stick and rudders on a gusty day, constantly changing airflow.

for the plane to weathercock there has to be lateral movement of air WRT to the plane, i.e. uncoordinated flight - take a look at the ball next time youre coming in on a gusty day. you`re right, the plane isnt keeping up with the air when a gust hits, this is because the plane has inertia. if youd like an analogy you can think of driving from pavement to gravel without slowing down. when you first hit the gravel your going start decelerating from the extra resistance but its not instantaneous. youll probably also start to skid around a bit while moving the wheel to keep pointing down the road.

[Beefitarian]

Your swimming example is a good one Schmoo. If you float down steam you won't feel the current because you are riding it. If you try to stop and stand up on the bottom the water goes past you, then you feel the current and may not even be able to stay standing.

The wind is basically just the current of the air. So on take off with a crosswind there will be some shaking but once the wheels are off the plane will just ride that current. If you don't correct for it the plane will drift smoothly in the direction of that crosswind.

[old_man]

So it's not 100% true that a plan in flight does not feel the wind, because during a gust it takes the plane a few moments to succumb to the wind, so for a second or two, the plane is actually resisting ( so to speak ) the wind.

Again, read up on wind shear. Airplanes take time to reach their new equilibrium. Yes, if you are in gusty turbulent conditions you are going to 'feel' it. I had a really interesting experience once, flying formation in turbulent conditions. You could definitely feel the bumps but holding position wasn't hard at all because both planes were being bumped the exact same way. Same parcel of air so to speak.

So if the gust was a crosswind from the left, does it have any kind of weathercocking effect ~ meaning, would the nose of the plane momentarily point into the wind ~ or would the gust simply push the entire plane or does it depend upon a number of factors ?

The term 'weathercocking' referes to how the airplane is moving, or looks like it is moving, relative to the ground. Same thing with 'crabbing'. A gust just means the wind has changed quickly. You now need to adjust/update your cross wind inputs to stay on centerline.

I'm trying to get a handle on what is happening when I am on final and am continually adjusting rudder pressure to keep the nose lined up with the rnwy.

In gusty conditions one has to always continually adjust pressures to make the airplane do what one wants it to do. I do believe they refer to this as 'piloting'

[cgzro]

So if the gust was a crosswind from the left, does it have any kind of weathercocking effect

Not usually because the plane experiences the gust evenly over its entire length. Youd need a very narrow gust and/or a large surface to experience that king of localized effect. However if I remember correctly you may see this with Gliders and rising hot air where a glider will turn away from lift as one wing is lifted more than the other.

[Gannet167]

In a stable mass of air, moving uniformly at a constant rate, the plane doesn't feel anything. The notion that the plane doesn't feel the wind is a great way to think of it in general terms and it really helps people understand concepts of airspeed vs ground speed, drift, and the never ever ending argument that persists about "Plane on a Conveyor Belt" which really misleads many people, a few pilots included (who probably should not be pilots, but that's another thread.)

However, the plane still has some inertia. It takes some time for the mass of the plane, considering its thrust vs its drag, to adjust speed when the airflow changes rapidly. If you're flying at 200 kts, with a 50 kt headwind, you've got 150 kts ground speed and 200 kts of relative airflow over the wing. If that 50 kt headwind were to change suddenly to say, 10 kts, then in some length of time the aircraft will adjust to a ground speed of 190 kts, and 200 kts indicated airspeed (and 200 kts of relative airflow over the wing). But this isn't instantaneous. If it were, everyone would have a broken neck from an immediate acceleration of 40 kts in ground speed. Rather, initially the aircraft will experience some sort of a drop in indicated airspeed, and lift until everything stabilizes and a new equilibrium is reached. If you're on approach, (or anytime) you will have a noticeable change in how the aircraft flies. This very extreme and theoretical example of changes in wind speed is wind shear.

This is why your airspeed indicator can bounce around when you're on an approach. Those rapid changes in wind speed and/or direction are having an effect on the airflow over the wing faster than the overall aircraft's mass adjusts to the change - which would show up as a change in ground speed. So, if the wind has a gust factor of 10 kts (say 270/10 Gust 20 kts) and you're landing into wind, in the final stages of the approach there's 20 kts of wind, then suddenly as you enter the flare it drops to 10 kts, you have all of a sudden (or as quickly as the wind velocity changes, which can be pretty instantaneous) experienced a 10 kt drop in airspeed and the associated lift. This can be disastrous. Given a few seconds for everything to settle out and find a new equilibrium, that change in wind speed would be reflected in a new ground speed and a return to the original airspeed. But if it happens fast and at the wrong part of the approach/landing, you crash (or, potentially float down the runway if the change in wind speed is positive.)

A common method of mitigating this is to add a gust factor to your approach speed. If your approach speed is 120 kts, and the winds are gusting 15 kts, you would fly 135 indicated. This way, if the gust causes the relative airflow to suddenly drop and the wing initially loses some lift, you're theoretically still have at least the normal 120 of airspeed and corresponding amount of lift. On the other hand, if the gust causes the relative airflow to suddenly increase, you initially gain a bit of lift and could end up floating down the runway until you bleed off the extra airspeed. On these days, a "greaser" landing is probably less desirable than a more positive touchdown - as it allows better accuracy on the touchdown point and vertical rate.