pitch=airspeed; power=altitude
- Thread starter js0305
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I am not sure either. You say pitch is changing from upward moving air but then you related that to wind shear which is horizontal moving air. So now I don't know which you are talking about.
Why wouldn't you assume a stable start to an approach?
Well of course it pitches up. But the increase in airspeed is dependent on which scenario you are talking about. Like I mentioned in that first quote: are you talking about an updraft or wind shear?
In my comment on this you said, "the updraft on final." So for now I will assume you meant updraft since you said updraft.
An updraft does not increase your AS. The air flowing up, since you are angled down in a descent, might have some effect on the airspeed indicator. However, your aircraft speed, regardless of what the ASI tells you, will not change in upward flowing air.
Your AOA changes, increasing in an updraft, because a portion of RW is now directed from the ground up, which caused your pitch change.
Again it sounds like you are referring to an updraft so I will stick with that. However, let's call them thermals since that is what they really are. Maybe there are other types of updrafts, but to my knowledge a thermal is the only thing that causes air to flow vertically.
You might think you are controlling altitude with pitch, but are we really? Let's look at a picture of airflow around a thermal:
Entering the thermal you hit downward flowing air, controlling the reduction in AOA by pitching back to keep the same AOA. Next the thermal and you get lifted. Again you control AOA with pitch, this time reducing it. Finally you exit the thermal, sinking again and again raising AOA with pitch.
The whole time you are putting your AOA back to where it started and keeping it their. That action controls your airspeed. The thermal controlled your altitude.
Is there any chance you live in the NJ area?
tgrayson
New Member
If the updraft is not perpendicular to the flight path of the airplane, there is a component of the velocity that is aligned with the flight path and will increase the velocity of the airplane through the air.
ppragman
FLIPY FLAPS!
I take great pleasure in these debates. Not because they really answer any pertinent questions, or have any useful bearing on the aviation world in any way. Really, they don't, because no one (I repeat no one) thinks about one side of the lift equation or another when they're doing a power off 180. I find these debates incredibly interesting because they really show us how people think about aviation in general. You can also glean a little bit of knowledge about the level of experience of the participants in the debate based on how and what they post. Frankly, I say, keep em' coming, if not for the content, then at least for the anthropological look at what and how pilots think.
Here's my take. You can think about flying in terms of control systems. By that I mean that you can think about what you want to control at a given time, and how then you plan on introducing the desired change. Basically there are only 3 things that you can actively change to successfully achieve the desired outcome. You can either change your angle of attack, or you can change your power setting, or you can change both.
That gives you
Change AoA
Change Power Setting
Change Power Setting then AoA
Change AoA then Power Setting
Minor loss of altitude in cruise? We change AoA
Too fast on final? We change Power Setting, then subsequently Change AoA to slow down
Too high on final? Dive and Pull the power back, or add flaps and reduce power, or reduce power then add flaps...
The possibilities are endless, that said, its next to impossible to say that "AoA controls airspeed, and power controls altitude" without specifically describing which situation you're in, and what kind of airplane you're flying. Some of the argument in this thread occurs over "what is the primary mover and shaker of airspeed and altitude," but really, you can't have one without the other in powered flying. In a practical sense, you can't have airspeed without power, and you can't have altitude without airspeed. To try to separate the two from one another is unrealistic.
As for shdw, having flown light airplanes in all sorts of goofy turbulent windy conditions, I'd posit that there are times when updrafts and downdrafts will do things that don't change your airspeed but do change your pitch, or will physically move your pitch because a gust will catch the airplane asymmetrically, or something to that effect. I've also flown from calm winds into a 30kt headwind at about 100-200' AGL, and seen my pitch remain unchanged and watched my airspeed jump up. It was just a sudden deceleration. No noticeable pitch change. Also, think about windshear, I don't think I'd call that a false notion.
Here's my take. You can think about flying in terms of control systems. By that I mean that you can think about what you want to control at a given time, and how then you plan on introducing the desired change. Basically there are only 3 things that you can actively change to successfully achieve the desired outcome. You can either change your angle of attack, or you can change your power setting, or you can change both.
That gives you
Change AoA
Change Power Setting
Change Power Setting then AoA
Change AoA then Power Setting
Minor loss of altitude in cruise? We change AoA
Too fast on final? We change Power Setting, then subsequently Change AoA to slow down
Too high on final? Dive and Pull the power back, or add flaps and reduce power, or reduce power then add flaps...
The possibilities are endless, that said, its next to impossible to say that "AoA controls airspeed, and power controls altitude" without specifically describing which situation you're in, and what kind of airplane you're flying. Some of the argument in this thread occurs over "what is the primary mover and shaker of airspeed and altitude," but really, you can't have one without the other in powered flying. In a practical sense, you can't have airspeed without power, and you can't have altitude without airspeed. To try to separate the two from one another is unrealistic.
As for shdw, having flown light airplanes in all sorts of goofy turbulent windy conditions, I'd posit that there are times when updrafts and downdrafts will do things that don't change your airspeed but do change your pitch, or will physically move your pitch because a gust will catch the airplane asymmetrically, or something to that effect. I've also flown from calm winds into a 30kt headwind at about 100-200' AGL, and seen my pitch remain unchanged and watched my airspeed jump up. It was just a sudden deceleration. No noticeable pitch change. Also, think about windshear, I don't think I'd call that a false notion.
True, but the only reason it won't be perpendicular is because of a wind sheer or it is moving steadily with the airmass. In the later it will have no effect on airspeed. A thermal, in and of itself, will never have a velocity component.
tgrayson
New Member
I don't know what you're saying, but an updraft, thermal, wind shear, all the same thing. If the flight path is angled downwards, then one of those things will alter the airspeed of the airplane.
Not really. Here, paraphrased from my meteorology book:
Wind shear is wind that changes in speed, direction, or both abruptly. It may or may not be a continued occurrence in the same location.
Thermals are rising columns (bubbles) of air. Since a thermal is a steady, sustained, flow of air it cannot be classified as wind shear, it is not "abrupt."
Updraft, not surprisingly, isn't in this text or any meteorology text I have seen. It isn't a meteorological term, it merely defines the direction of movement of a meteorological effect. You might argue that an updraft can be a thermal or wind shear because wind shear can have any direction.
A thermal is nothing more than taking a helium balloon and letting it go. If there is no airmass movement it rises straight up. If there is a steady 10 knot wind it will still rise straight up, except it will now drift at 10 knots. Since the aircraft also is drifting at 10 knots, IE they both move equally with the airmass, the balloons(thermal) would have no effect on the aircrafts airspeed.
The point is a thermal alone will not change an aircrafts speed. Sure if it is angled downward, as I said in my original post, it will have a small, negligible, effect on the airspeed indicator. But it will not have an effect on the aircraft speed as a thermal never has a horizontal component because it and the aircraft are effected equally by the airmass.
Keep it simple:
Pitch + Power = Performance.
When you're slow, pitch for airspeed, and adjust power for altitude.
When you're fast, you can get away with pitching for altitude (to an extent).
If I had to choose just one, I'd say pitch for airspeed, power for altitude (I don't think using this would get anyone killed).
My $0.02
Pitch + Power = Performance.
When you're slow, pitch for airspeed, and adjust power for altitude.
When you're fast, you can get away with pitching for altitude (to an extent).
If I had to choose just one, I'd say pitch for airspeed, power for altitude (I don't think using this would get anyone killed).
My $0.02
tgrayson
New Member
shdw said:
You're neglecting the fact that an airplane is moving and often flies into a thermal. This qualifies as wind shear.
You're thinking like a landlubber. You need to rotate your coordinate system. We don't care about the updraft with respect to the horizon, we care about it with respect to the flight path of the airplane. The vector component that lies along the flight path is given by
updraft velocity * cos(angle between updraft and flight path)
Assuming that we have a descent angle of 3 degrees, the interior angle between the updraft and flight path is 87 degrees. If we assume an updraft velocity of, say, 2000 fpm, that gives us
(2000 * 60/6076) * cos(87) = 20 knots * .052 = 1 knot
Obviously steeper descents would provide a greater amount of airspeed change, as would faster updrafts, but it looks like the vertical windshear is only a small issue with airspeed changes. Even so, the vertical updrafts is unlikely to be laminar and will contain eddies and vortices, which would tend to change the direction from which the air is flowing.
Regardless, I think your assertion that the A/S changes are solely limited to the indicator and do not reflect the relative wind velocity is unfounded.
Or about 20 knots.
shdw said:
I should remove indicator from that sentence. My overall point is the ASI is relatively unreliable for what is actually happening. This one knot increase, in your scenario is the actual AS increase. The ASI would probably indicate ~5, guessing of course.
Not neglecting, defining what it is in a meteorological sense. By your definition everything we encounter is wind sheer, regardless of what it actually is or the properties it inherits.
You are right, I screwed up in my initial post on this. I didn't consider 1 knot, even a 9 degree glide path being 3 knots, to really be worth discussing.
I resent that, I said "if it is angled downward." Indicating that I was considering the descent angle and not basing it on relative to the horizon!
tgrayson
New Member
Why do people feel compelled to make inane comments about threads that don't interest them? Just skip it and move on.
Its been my experience that one a short field landing you are aiming for a point with your pitch and using power to control airspeed. You are in essence following an imaginary glideslope that will lead you to the runway and clear the obstacle. If you hit an updraft on final while on your "glideslope" you may not gain altitude but your rate of descent would probably decrease, therefore in my mind its easiest to lower the nose to keep it on my aiming point and adjust the power to maintain the desired airspeed.
Heres another "absurd" scenario for chew on. In a descent at say 1000 fpm and 110 knots (172in' it) you're coming up on you're level off altitude. Are you going to add power or start to raise the nose first to stop the descent? You're not worried about being at a certain speed when you level off, so would you keep adding power while gaining airspeed and hold true to the saying "my cfi told me power for altitude" and drive it into the ground? At a certain pitch down you can add all the power you want and that nose isn't going to rise up.
Say you're in something that burns Jet A. In cruise, you get a 30 knot speed reduction for in-trail spacing. Are you going to click off the autopilot and pitch up and pull the power back to get to slower or leave the autopilot on and pull the power back only and let the autopilot level it out. Think about what makes most sense. BTW 30 knots in a fast plane would be a very minimal change in pitch.
As I said before, do what works for you.
Heres another "absurd" scenario for chew on. In a descent at say 1000 fpm and 110 knots (172in' it) you're coming up on you're level off altitude. Are you going to add power or start to raise the nose first to stop the descent? You're not worried about being at a certain speed when you level off, so would you keep adding power while gaining airspeed and hold true to the saying "my cfi told me power for altitude" and drive it into the ground? At a certain pitch down you can add all the power you want and that nose isn't going to rise up.
Say you're in something that burns Jet A. In cruise, you get a 30 knot speed reduction for in-trail spacing. Are you going to click off the autopilot and pitch up and pull the power back to get to slower or leave the autopilot on and pull the power back only and let the autopilot level it out. Think about what makes most sense. BTW 30 knots in a fast plane would be a very minimal change in pitch.
As I said before, do what works for you.
nosehair
Well-Known Member
Let me expand on the idea of the "updraft".
When I first started flying in a Cub, and reading about the updraft, I could relate the simple idea, along with the drawing, of a vertical rising column of air, and I would 'feel' it, and see it on the altimeter, so I thought of it simply as a vertical column, as you do.
At first, as the cool morning air heats and begins to rise, this imagined scenario is fairly accurate, and a 'bump' is pretty much only an altitude increase as the airplane rises along with the column of air.
But as the velocity of rising air increases, the 'friction' with the surrounding stable air causes that air to 'roll' upward causing little eddies of forward moving air both with and against the airplane.
Did you ever stand, or sit, by the bank of a strong fast moving river and notice along the edges of the bank how the water may be swirling, and at points, be actually moving upsteam? That's how turbulent air is; in small 'eddies', or 'swirls', the air is moving the light airplane in any and all directions.
So the pilot tries to hold a stable attitude, or keep the nose pointed towards the spot, or pitching towards the glideslope needle with the yoke, and also,by the way, keep the nose pointed laterally towards the spot, or localizer needle, with the rudder, and keep the wings level, or in a controlled bank, with the ailerons.
This technique is called 'flying the airplane'. It uses the fundamental concept of controlling the 3 axies of flight with the flight controls designed for that purpose: Pitching with the Elevator, Steering with the Rudder, and Banking with the Ailerons.
The power control may be added to any of those control movements which changes the effect, which is why we learn to 'fly the airplane' with a variety of combinations of power added or subtracted, or changed during the control(s) movement.
wjmiller3
Well-Known Member
Cause its like a train wreck, you know how it will turn out but you can't help but watch..
Wouldn't those 3 words have been easier to type and sum up everything you just said?
As for the thermal discussion:
The picture I linked showed the eddies, so obviously I knew of their existence. Consider the river again, 10 feet wide with maybe 6 inches of eddies on either side. The eddies are a small percentage of the overall thermal and relatively weak on thermals caused by parking lots we encounter on final.
A more subsequent issue is the cap of the thermal (refer to picture). Most of the little bumps you hit on final are not large thermals, they likely are capped below pattern altitude on all but the most unstable days. For proof of this fly the final approach at pattern altitude and watch the bumps go away.
If you are flying near the top of the thermal eddies are replaced with downdrafts.
I was in a very similar situation in 1988 when I started teaching for a not so large school where the owner insisted that we teach the pitch=flight path, power=airspeed technique. I had learned, flown, and taught the pitch=airspeed, power= flight path technique up to that point and was convinced that it was the only way that an airplane could be safely flown.
The first thing that you must realize is that these two technique are just that--techniques. Neither is a law of physics, neither is right, and neither is wrong. They both work very well when applied correctly; and, when applied correctly, a pilot using each technique will apply the exact same control inputs when presented with the same situation. The only difference is in the logical process used to determine those control inputs.
That is true. You are trimmed for a particular airspeed. The error that you're making is that you are assuming that trim is the same as pitch, it is not. In your example, the aircraft's pitch attitude prior to the power change will be higher than the pitch attitude after the power change. Pitch attitude changed though trim and airspeed remained constant.
A pilot using the pitch=A/S technique would reduce power to initiate the descent while lowering the nose (pitch) to maintain 90 kts. Trim is unchanged.
A pilot using the pitch=flight path technique would lower the nose to initiate the descent while reducing power to maintain 90 kts. Trim is unchanged.
The control inputs, and result, is the same.
I wasn't convinced either, at first, but it was my job to go out and teach it the way my boss told me to teach it. After a couple of weeks of mentally "converting" my instructions from the "right" way to the "wrong" way before I spoke them to my students, I suddenly realized that the pitch=flight path technique made sense and worked. Since that day I can fly or teach either technique.
Now, with almost 30 years of flying and nearly 10,000 hours total time, I believe that a pilot doesn't fully understand the relationship between pitch and power unless they understand and can fly using either method.
Is that how you explain the relationship between the four-forces of flight?
The elevator does not control AoA. It has an effect on AoA but it only controls half of the AoA equation--cord line position. When pitch angle changes the flight path will also change. AoA is the angle between the cord line and relative wind (flight path) so you have to first determine what the change is in the flight path before you can decide if, or how much, the AoA has changed.
Regardless of the technique a pilot uses, the parameter that he controls with pitch will be the parameter which is controlled the most precisely. If you really want to nail an altitude, glideslope, vertical speed, airspeed, etc. then use the pitch=[whatever parameter you won't to nail] technique.
tgrayson
New Member
Actually, the error he makes is the error that pretty much everyone makes, which is to associate pitch with AoA. The elevator, even though it's referred to as the pitching control, doesn't determine pitch, it determines AoA. Pitch is really the addition of AoA + flight path. The trim wheel adjusts the elevator position and hence the AoA.
The mathematics in the stability and control sections of aerodynamics books makes it clear that the elevator is the primary controller of the AoA. There are some secondary effects, of course: prop wash over the horizontal stabilizer and downwash from the main wing all affect the lift coefficient of the horizontal stabilizer. In the end, it's the lift coefficient of the horizontal stabilizer that controls the AoA. The way it works is that any change in flight path will alter the lift coefficient of the main wing and horizontal stabilizer, producing a pitching moment until they are in rotational equilibrium once again. In effect, they weathervane the aircraft into any new relative wind.
The flight path only has a trivial effect, and it's due to the fact that thrust or drag can support some of the weight of the airplane during a climb or descent, reducing the required lift from the main wing. But that's considered to be negligible at flight paths smaller than 15 degrees with respect to the horizon.
There are formulae that connect the elevator position to equibrium airspeed and AoA, so this is basic stability and control subject matter. I can post them if you're interested.