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Sylvestre.........holy cow. So planes with propeller are more stable......than what? Stability in any direction has nothing to do with the method of thrust production. And I am not sure what happened or what you did wrong during your turn while retracting flaps, but "absolute airspeed" has nothing to do with it. My guess is your feet were flat on the floor.
Hey, ever noticed how your airspeed seems to drop rapidly when doing an uncoordinated turn in a certain direction- either left or right? Know what causes that? It's an interesting phenomenon that can lead to some misconceptions about airspeed.
Planes with a propeller are more stable in indicated speed than jets and gliders, or if you prefer are quite easier to conduct at a constant airspeed because the output of the propeller is a power, and a power is speedXforce; so when the speed changes the thrust changes, and that helps. That is why a piston or turboprop is always rated in HPs or kWs and a jet engine is always rated in kNs, kLbs of thrust.
Flying a holding pattern inside a jetstream near your maximum altitude is something I would not recommand because your ability to accelerate is usually very weak even if with a jet at altitude the time to perform a 360° at a commercial bank angle could be more than five minutes.
The NTSB has not invented the dangers of the downwind turn, they are real. They are serious guys, believe me.
Sylvestre........I'm not really sure what to say to that. I fly airliners and there is no danger of turning inside a fast moving mass of air. At all.......other than it possibly being a bit turbulent. And stability is simply an aircraft's ability to remain in or return to a given state after disturbance. Thrust will equal drag in level unaccelerated flight. The method of thrust might determine how quickly equilibrium is returned, but this still has absolutely nothing to do with wind or ground speed. I'm sorry, but you are wrong.
And your ability to accelerate at high altitude is simply due to a lack of excess thrust. Aircraft weight and air temperature determine cruise altitude, based on the thrust available.
Turn rate is a function of bank angle and true airspeed. Bank should be limited at high altitude, but that has NOTHING to do with wind. Bank increases load factor (read: weight) of the aircraft, and therefore AOA and the thrust required to maintain altitude. Transonic airflow over the wing is also a factor.
Long story short, wind is only a factor if you want to figure out how long it will take you to get from A to B and how much fuel you will burn.
You need to research this stuff before you start preaching the dangers of turning downwind. People do not need to be afraid of things that do not exist.
One last time, it is way simpler than you think. Read this from 1941.
Quoting from 'Win Your Wings' copyright 1941, authored by Colonel Roscoe Turner and Jean DuBuque, page 230. "Reverting to my earlier interposition, regarding airspeed relative to groundspeed, it will be observed, during downwind turns close to the ground, that when the plane is approximately crosswind, it seems to suspend momentarily without speed and as the turn is completed to the downwind course, the ship seems to gain speed with a rush, resulting in misapprehension of the actual airspeed on part of the novice pilot. Bear in mind that these seeming actions are purely optical illusions caused by watching the ground even though the pilot may not be directing his full attention toward it. Only the speed relative to the ground is affected, whereas the airspeed remains constant. For example, assume you are flying "on instruments," that is, without visual reference to any ground object. How do you know which way is upwind, downwind, sidewind, or what have you? The airspeed, naturally, will not vary during a normal turn because the plane moves, when free of the ground, in relation to the air masses and not the earth.
The authors continue; “Don’t misunderstand me, as I am not advocating use of the downwind turn, but I am endeavoring to clarify for you the principals involved, and to belittle the prevalent fallacy regarding this maneuver”.
These authors seem to think a downwind turn can be a cause for an accident as does the NTSB, but for reasons of human factors not physics. Turner and DuBuque finish the discourse this way on page 231;
“ Since the optical illusion, resulting from indirect observation of the ground, after the completion of the downwind turn, has a definite bearing on the reactions of the in-expert pilot or poorly trained pilot, let me admonish you to refrain from believing your airspeed has increased and from attempting to force the plane into a steeper climb”.
Dear Maddogdriver, the next time you make a turn at altitude with a say 100 kts wind ahead or tail ask atc to do a 30° heading change or wait for a waypoint with a turn and look at your airspeed and autothrust, you will be convinced. the important thing is the wind change related to time and also the ability of your aircraft to accelerate or decelerate. At altitude we have often strong wind but poor power margin so it is easy to see it.
Sylvestre, I have good news for you:
You are right: Airplanes with propellers are more stable in speed than jets.
As for the rest, oh, man, I feel sorry for you. You are so convinced (enough to call Peter Garrison stupid) and so wrong at the same time.
You invoke Newton without understanding it's law of motion:
- You mention an "absolute speed", when Newton himself established that there was no absolute frame of reference, and hence no absolute speed.
- You say that that absolute speed would be about the stars, but the Earth was a good approximation because it moves so steadily. The speed at which the Earth moves about any star (including the Sun) is so high that little difference would make a tailwind.
- Then you talk about the second law of motion without understanding it.
I beg you, try to read what follows without blocking your mind.
Newton's second law of motion is F = m * a, correct?
Correct.
In a turn without wind (to make it easier) the airspeed and groundspeed remain constant and there is no acceleration, correct?
W-R-O-N-G ! ! ! (Maddogriver, you have also been saying this. I note that you have the idea right, but you are explaining it wrongly).
THE SECOND LAW OF MOTION IS A "V-E-C-T-O-R-I-A-L" EQUATION.
FORCE IS A VECTOR. ACCELERATION IS A VECTOR.
Acceleration is the rate of change of speed over time.
AND SPEED IS A VECTOR TOO.
If you don't agree, don't understand, or at least won't believe that, don't keep reading. It's worthless.
So, an airplane is flying East at 100kts (no wind, so that airspeed and groundspeed).
It makes a 180° until it's flying at 100kts West.
Did the airspeed change? Did the groundspeed chage?
Y-E-S. While the MAGNITUDE of the speed VECTOR is the same, the DIRECTION of the speed VECTOR has changed, and hence, the speed VECOTR, as a whole, has changed.
I know that one talks about speed, airspeed, etc. as a scalar magnitude. And that's ok. If you ask me "what's the speed limit in this highway"? I will not say "70 mph South".
But when dealing with Mechanics, and the second law of motion in particular, it is necessary, indispensable, to treat the force, acceleration, and speed as vectors.
Let me give you a very clear example: I throw a stone straight up at 50 fps. The stone goes up at ever diminishing speed and then, after being still for an instant at the summit, starts to fall at increasing speed. By when it reaches my hand again, it's falling at 50fps (neglecting the drag). And you will tell me that its speed is the same, and hence that there was no acceleration?
No, sir. The speed changed. In fact, it changed 100 fps in the vertical axis: It was 50 fps up, and now it's 50 fps down. Not only that, but the direction of the change was "down". If the speed is a vector, the change of speed is also a vector.
And there was an acceleration which, on average (although in this example was constant), was 100 ft/s divided by the number of seconds that the stone took to return to my hand. And it took exactly 3.11 seconds. I know because I know the acceleration, 32.174 ft/s2 DOWN, which is the acceleration due to the gravity.
Let's return to our airplane. It was flying 100kts East, made a 180, and now is flying 100kts West. Did the speed VECTOR change?
I hope, pray, that by now you have clear that the answer is YES: it changed 200 kts West.
Does that means that there was acceleration? Well, since acceleration is change of speed VECTOR in time, and the speed VECTOR did change, there must have been acceleration there. How much? Well, if it was a standard turn, it took 1 minute to make the turn, so the acceleration, on average, was 200kts per minute West.
Wait a minute, if there was an acceleration, as per the second law of motion there MUST have been a force that provided that acceleration. What's that force?
The lift. Or better, part of it. When you turn the lift VECTOR is tilted sideways in the same angle than the bank angle. The vertical component of the lift is cancelled by the wight, but the lateral component is "free" to provide the acceleration. And God knows that it does.
So let's follow the plane step by step during the turn.
You are flying East and then you bank left.
Now you have the lateral component of the lift aiming... North.
North? But haven't I just said that the acceleration aimed West? Shouldn't force and acceleration aim in the same direction? Yes. They must, and they do. But I've said that the AVERAGE acceleration was West. Please, follow me for another few minutes.
So, the force initially aims North, and so does the acceleration. This makes speed vector of the plane starts to deviate from East to North. That is, the plane starts to turn left.
As it does, the lateral component of the lift vector also starts to turn left. That is, it starts to deviate from North no West.
For example, by when you have turned 45 degrees and your speed vector (and heading) are aiming North-East, the lateral component of the lift aims North-West. I am sure you can imagine the plane seen from above describing the circular turn with the lateral component of the lift always aiming to the center of the circle.
As you see, now the acceleration still is pushing the plane North, but also starts to push it to the West, although initially that's only enough to reduce it's Easterly motion.
By when the turn reaches 90° and your speed vector and your plane are heading North, the lateral component of the lift vector is aiming due East and the plane has already "lost" all of its initial East motion. But on the other hand, the North component of the lateral lift has increases the plane's speed in the North direction from zero to 100 kts. In fact, what the lateral component of the lift has done in the first 90° of the turn was to accelerate the plane, that is, to change it's speed vector, in a way that the magnitude of said vector didn't change but the direction of said vector changed from East to North.
The second and last 90° of the turn will do the same thing: The lateral lift, and hence the acceleration, will be continuously changing it's direction from West to South, as the speed vector and airplane's heading changes from North to West.
In the end, the North component of the lateral lift and the acceleration during the first 90° of the turn are cancelled by the South component of the lateral lift and the acceleration, with the end result that the final speed in the North / South direction is the same than in the beginning: Zero.
But the West direction of the lateral force and acceleration is not cancelled, so the net average effect is a change of the speed vector in the East/West axis: It started being 100kts East and finished 100kts West, a change of 200kts West for an acceleration of 200kts/minute West (if it was a standard turn, that took one minute).
Let's make a table:
Initial airspeed vector: 100kts East.
Final airspeed vector: 1o0kts West.
Change in the airspeed vector: 200kts West.
Acceleration: 200kts/minute West.
Initial groundspeed vector: 100kts East
Final groundspeed vector: 1o0kts West.
Change in the groundspeed vector: 200kts West.
Acceleration: 200kts/minute West.
If you understood the above, it should be easy to understand what follows.
The part where the turn is explained par by part doesn't change if there is a steady wind: The plane flies Eest, banks left, a lateral component of the lift appears. The plane turns from East to North to West as the lateral component of the lift turns from North to West to South. The loft is the same and the bank angle is the same, so the magnitude of the lateral component of the lift is the same. The airplane mass is the same, so since F = m * a (VECTORIAL) the acceleration is the same that without wind: 200kts/minute West (on average).
So, let's add an 80kts steady wind that blows from East to West to make a dramatic downwind turn.
I hope you agree that the groundspeed of an airplane flying 100kts East will be 20 kts and that of one flying 100kts West wiould be 180kts.
And see how magically this turns out when out plane turns 180° from East to West:
Initial airspeed vector: 100kts East.
Final airspeed vector: 1o0kts West.
Change in the airspeed vector: 200kts West.
Acceleration: 200kts/minute West.
Initial groundspeed vector: 20kts East
Final groundspeed vector: 180kts West.
Change in the groundspeed vector: 200kts West.
Acceleration: 200kts/minute West.
A key point of your mistake was to think that while the airspeed can change rapidly, the groundspeed can't. With a steady wind, neither the airspeed or groundspeed can change very rapidly. They are both "speeds VECTORS" and both need an acceleration to act over a period of time to change. That, together with you thought that a turn at constant speed was constant speed in Newtonian terms (for the last time, it is not: if the direction changes, then the speed vector changes and there MUST be an acceleration) is what led you in the wrong direction.
I hope that the above at least made you more insecure about your initial statements.
If it didn't, don't bother to insist. You are either unable or unwilling to understand, and I can't help that. I will not reply.
If you understood, then great.
If you have doubts, then ask and I will reply.
RandyL: the authors are saying that the only cause for an accident on the down wind turn is an OPTICAL ILLUSION, nothing to do with any sort of inertial change or acceleration. There are many optical illusions out there, but they have no physical effect on the plane.
Sylvestre: Please read up, or better yet ask someone else to review your ideas and see what they think. I can't keep saying the same thing again and again.
Gabriel: I have absolutely no idea how any of what you posted above has any relevance to this article or our discussion. Thrust vectors, headings turns, fpm, all this is irrelevant. Here are the facts;
A plane in a turn, regardless of wind speed or direction, will always experience the same acceleration for a given airspeed and bank. NOTHING ELSE CAUSES AN ACCELERATION, DROP IN AIRSPEED OR ANYTHING OF THE SORT. ESPECIALLY THE WIND.
Talking about what is going on relative to the ground serves only to affect time and fuel burn to a point in space. THAT IS IT.
There is no point in talking about a wind vector, tailwind component, turn direction, heading change, speed tables, angle of attack.
All of your speed components and heading change examples, while impressive in their convolution and circuity, are nonsense. And I mean this with the utmost respect for your desire to get this all straight.
THERE IS NOTHING DIFFERENT BETWEEN A DOWNWIND TURN AND AN UPWIND TURN, OTHER THAN A CHANGE IN GROUND SPEED.
If I could say this in a more simple way, I would.
maddogdriver: The authors ARE saying exactly that. Of course the OPTICAL ILLUSION has no physical effect on the plane. The OPTICAL ILLUSION has the effect on the PILOT. In the case of the downwind turn, causing him to "attempt to force the plane into a steeper climb".
OPTICAL ILLUSIONS are very common and they have been the cause of many accidents. Many types are referenced in FAA handbooks.
Think of a Black Hole approach. Assuming a functional airplane, it is the OPTICAL ILLUSION that is the threat.
Maddogdriver: I fully agree with you. The only issue is that you explain the what and only skimming the why. That's not enough for somebody that thinks that his (wrong) version is supported by his (wrong) understanding of the Newton's laws of motion. So what I tried to do was explain how Syvestre was using them wrongly and how using them right leads to the result you describe: The airspeed doesn't care about the direction of the wind: turn or no turn.
And a small correction. In a turn (level, constant speed), an airplane will experience always the same acceleration for a given bank angle. Speed is not a factor in that acceleration, while it is a factor for turn rate and turn radius.
RandyL: I agree, optical illusions do cause a lot of accidents. I was under the impression after reading your post that you were stating that these optical illusions were what the NTSB was talking about in regards to the downwind turn. While optical illusions can cause a pilot to act incorrectly, the NTSB was stating that a drop in actual airspeed when turning downwind caused the accidents. Shooting a visual approach at night with no VASI or surrounding airport lights makes it a bit harder than normal due to the black hole effect. I can vouch for that as I do a lot of them.
Gabriel: I am not sure what you mean by the "what" and the "why." This is physics, and the what and the why are one and the same; simple laws of physics. We can use analogies or metaphors, but in the end what the law is and why it works the way it does is the same explanation, although we can explain it in different ways to make it easier to understand.
And you are 100% right in correcting me about acceleration in turns. I have no idea why I included airspeed as a factor in turning acceleration, as it only affects turn rate/radius as you stated. My guess is that my brain has turned to mush after reading all of sylvestre's posts......
