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Old Jun 26, 2012, 12:00 PM
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The dreaded down-wind turn, fact or fiction?

In theory, there should be no such thing as a downwind turn. The theory states that wind is of no concern to the pilot unless he is taking off, landing, or navigating.

My question is, why do so many people get into trouble when turning downwind? Whether or not there is any logical explanation, it is apparent that many crashes have been caused by not correctly performing that maneuver.

There has been some heated discussion of this on other forums. Some people passionately believe it exists, and other people passionately believe it does not. Hopefully this discussion will not degenerate into people calling other people idiots. My purpose in starting this thread is to try to understand this phenomenon and how to avoid it.
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Old Jun 26, 2012, 12:16 PM
Tossing planes into the snow
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Reserving this space.
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Old Jun 26, 2012, 12:54 PM
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downwind turn

Hi Jovanx,

I think the problem with RC flying is that we typically don't have an airspeed indicator and judge the plane's speed relative to the ground rather than relative to the air, and it's airspeed that really matters to keep a plane flying. On the downwind leg, it's tempting to decrease the throttle since the airplane is flying a lot faster relative to the ground with the help of a strong tail wind. In this case one may have slowed the plane too much (relative to the air) and be very close to stalling, even though the ground speed may still be fairly high. As one turns, the inboard wing slows even more relative to the air and could stall.

I hope that this makes sense and helps!

Cheers,
John (originally Jovan as well)
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Old Jun 26, 2012, 01:45 PM
Tossing planes into the snow
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Thank you for your input. Your explanation makes total sense to me. There are also a couple of other things going on that affect small planes more than large ones. We are usually flying our planes fairly close to the ground, and that is a zone with a lot of turbulence. It is different from making a turn at a higher elevation where the air is moving more steadily. A sudden gust that slows it down and then dissipates will affect this little plane more than it will affect a plane that has a cruising speed of 200 mph.
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Old Jun 26, 2012, 02:58 PM
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Originally Posted by Jovanx View Post
In theory, there should be no such thing as a downwind turn. The theory states that wind is of no concern to the pilot unless he is taking off, landing, or navigating.
The problem is that the theory that you stated, while the prevailing view, is wrong. Unfortunately, its true deficiencies are known only by dead pilots and passengers.

Here is the error. The theory doesn't account for the fact that aircraft are not weightless. They have mass. As such they also have a property called momentum which means that they do not respond immediately to forces being applied to them. There is a time lag between the application of force and its noticeable effect on the massive body it is affecting.

The theory also does not account for the fact that the aircraft is not a point, that it has physical size and surfaces that respond differently to external forces. These surfaces also act as levers that dynamically alter the orientation of the aircraft with respect to external forces just applied, altering what they will then do in the future. It is a feedback path.

If the wind is ABSOLUTELY steady and the airspeed and direction speed of the glider is ABSOLUTELY constant we then have a condition where the AVERAGE wind VELOCITY vector is equal to the INSTANTANEOUS wind velocity and the AVERAGE airspeed VELOCITY vector are equal to the INSTANTANEOUS airspeed velocity vector (please note that I am using the term "vector" as it is used by a physicist meaning a quantity that has magnitude and direction).

Let us say for a moment that we are flying our glider in a perfectly homogenous air mass, ie. one with no discontinuities and therefore no gusts. We are flying with an airspeed 1 mph above stall. Everything is fine. Then we encounter a brief gust of wind. The glider is a massive body. At first its momentum carries it at the same speed (groundspeed measured with the earth as the reference frame) as before. There is no apparent change in aircraft attitude. However the INSTANTANEOUS airspeed changes causing the actual forces acting on the wing and other surfaces to change.

What these forces actually do is determined by the direction from where they come. Back to our example. If the gust comes from directly in front, the plane balloons up. If the gust comes from directly in back. Well, you guessed it...down she goes.

What is really bad with a gust from the back is the dynamics I mentioned with the surfaces. The tail gust acts to lift the tail upward levering the plane around the CG which causes the airplane to point straight down - not good when you are coming in for a landing. (not so good at altitude either as many of us have found out).

Most of the common pilot texts, (and FAA exams) were authored before we had blazing fast computers. Transient conditions were known to exist but were ignored because they could not be studied analytically (word used as a technical mathematics term) but only computationally which is impossible using pencil and paper. This is true in every field including my own (electronics).

The aviation theory books and FAA have actually admitted the deficiency when they admonish pilots to add to the normal landing approach speed an amount equal to half the gust speed as a safety factor. We now also monitor wind gusts at airports with micro burst detectors. They have saved a lot of lives.

In summary,

1 - Airplanes have mass so they don't respond instantaneously to changes in wind direction.

2 - Carry more speed when you are low.

3 - The theory you stated is for an idealized case. It is good for thought to illustrate an idea. However, for reasons stated above (non-homogenous air mass, non point size body, massive body), it's underlying suppositions do no exist in nature.

Cliff

PS One more thing I'll add for illustration. A Cessna with a stall speed of 60 may have an approach speed of 80. If it gets bumped by a 10 knot gust from the rear, it is still flying just fine.

A model glider with a stall speed of 15 is flying at 17. If it gets bumped from the rear by the same 10 knot gust, its the equivalent of the Cessna being hit with a 40 knot gust. The outcome will be different. It is all about percentages. What we fly is very sensitive to what the air is doing - think ballet.
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Old Jun 26, 2012, 04:24 PM
Tossing planes into the snow
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Thank you very much for that, cliffkot. You have confirmed many of my suspicions and stated them in a very clear and understandable manner. The traditional "theory" only works if the winds are steady and the differential between the wind speed and cruising speed of the plane is large.

This is exactly the kind of information I was hoping for when I started this thread. The theory says wind direction shouldn't matter, but experience says it does.
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Old Jun 26, 2012, 05:53 PM
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I agree in theory "downwind turn" doesn't exist but there are some weird things that happen when not all of the air is doing the same thing at the same time.

Don't forget that body of "stationary" air you're flying in on a calm day is a stiff westerly of about 1000mph, it just so happens the surface of the Earth is keeping pace.
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Old Jun 26, 2012, 05:56 PM
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Originally Posted by jcerne View Post
Hi Jovanx,

I think the problem with RC flying is that we typically don't have an airspeed indicator and judge the plane's speed relative to the ground rather than relative to the air, and it's airspeed that really matters to keep a plane flying. On the downwind leg, it's tempting to decrease the throttle since the airplane is flying a lot faster relative to the ground with the help of a strong tail wind. In this case one may have slowed the plane too much (relative to the air) and be very close to stalling, even though the ground speed may still be fairly high. As one turns, the inboard wing slows even more relative to the air and could stall.

I hope that this makes sense and helps!

Cheers,
John (originally Jovan as well)
Quote:
Originally Posted by cliffkot View Post
We now also monitor wind gusts at airports with micro burst detectors.
You are confusing stalls with a condition where a sudden downburst of air is physically pushing an aircraft body towards the ground.

http://en.wikipedia.org/wiki/Microburst

This is not the same as a stall, though granted the overall effects are the same.

Quote:
Originally Posted by wikipedia
Danger to aircraft
Further information: Downburst and Wind shear

The scale and suddenness of a microburst makes it a notorious danger to aircraft, particularly those at low altitude which are taking off and landing. The following are some fatal crashes and/or aircraft incidents that have been attributed to microbursts in the vicinity of airports:

A BOAC Canadair C-4 (G-ALHE), Kano Airport - 24 June 1956.
A MalÚv Ilyushin Il-18 (HA-MOC), Copenhagen Airport – 28 August 1971.
Eastern Air Lines Flight 66 Boeing 727-225(N8845E), John F. Kennedy International Airport – 24 June 1975[8]
Pan Am Flight 759 Boeing 727-235 (N4737), New Orleans International Airport – 9 July 1982[8]
Delta Air Lines Flight 191 Lockheed L-1011 TriStar (N726DA), Dallas/Fort Worth International Airport – 2 August 1985[8]
Martinair Flight 495 McDonnell Douglas DC-10 (PH-MBN), Faro Airport – 21 December 1992[9]
US Airways Flight 1016 Douglas DC-9 (N954VJ), Charlotte/Douglas International Airport – 2 July 1994
Goodyear Blimp GZ-20A (N1A, "Stars and Stripes"), Coral Springs, Florida – 16 June 2005
Bhoja Air Flight 213 Boeing 737-200 (AP-BKC), Islamabad International Airport, Islamabad, Pakistan- April 20 2012

A microburst often causes aircraft to crash when they are attempting to land (the above-mentioned BOAC and Pan Am flights are notable exceptions). The microburst is an extremely powerful gust of air that, once hitting the ground, spreads in all directions. As the aircraft is coming in to land, the pilots try to slow the plane to an appropriate speed. When the microburst hits, the pilots will see a large spike in their airspeed, caused by the force of the headwind created by the microburst. A pilot inexperienced with microbursts would try to decrease the speed. The plane would then travel through the microburst, and fly into the tailwind, causing a sudden decrease in the amount of air flowing across the wings. The decrease in airflow over the wings of the aircraft causes a drop in the amount of lift produced. This decrease in lift combined with a strong downward flow of air can cause the thrust required to remain at altitude to exceed what is available.[8]
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Old Jun 27, 2012, 09:26 AM
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A strong wind in the face at the field will be given notice by experienced fliers. While it certainly means cross wind landings, it also means that the transition from down wind leg to base leg needs some additional caution as air speed will suddenly drop with the turn onto base leg.

We are mostly used to the down wind leg being well er... down wind and a turn onto base leg coming cross wind so the the strong wind in the face changes our normal ball game and unless we are alert to the change... there can be danger.

Given a strong wind in the face, the modeler should fly the down wind leg higher and longer than normal so that a greater descent rate is observed during both the turn onto base and final leaving a longer final to compensate for the no head wind landing.

The condition produces a temptation to avoid a longer final in the cross wind and turn early and possibly have too shallow of descent and that can spell danger. As well, the anticipated fight of the cross wind on final might distract from attention needed to properly adjust the down leg higher to provide for steeper descents.
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Old Jun 27, 2012, 10:04 AM
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Quote:
Originally Posted by dedStik View Post
You are confusing stalls with a condition where a sudden downburst of air is physically pushing an aircraft body towards the ground.

http://en.wikipedia.org/wiki/Microburst

This is not the same as a stall, though granted the overall effects are the same.
Well actually I was talking about gusts in genera, l regardless of direction. Microbursts go down but also spread horizontally at some point in all directions as they get closer to the ground.

but getting back to the problem with the theory we are talking about is that it assumes that an air mass is perfectly homogeneous and non-accelerating. The reality is that this state exists only in a glass bottle or in the mind of the book writer.

One thing I should say that we have going for us as RC pilots is that unlike the Cessna aircraft, our models are very lightweight, so they track changes in the surrounding air mass a lot faster than the Cessna would. What really goes against us though is that we are not in the cockpit. We can't sense the changes as fast. We lack the the visual clues or the feeling in the seat of our pants that a real pilot has. When you fly a light plane, you make corrections automatically by instinct in the same way that you do when driving a car. (I've never flown an airliner so I can't say what they feel).

When you are flying RC, especially gliders at the limit of visual range, you see something and have to first figure out what happened. My first reaction is always the same, "what the XXXX?" Since I'm not a young whipper-snapper anymore, I'm not too quick with the stick and I've learned that altitude is my friend. But I'm still amazed when flying at an apparently safe speed, how quickly the air can change and send the plane straight down because of a sudden stall that shouldn't have happened.

We've all experienced it, but deny it because of the theory that states that the aircraft moving in an air mass is in effect "one with the air". It is on average, but not every instant. We need to realize that.

The theory is very useful for flight planning, and understanding in general how things work. But like the Niels Borh diagram of atomic structure that still appears in every physics textbook after 100 years, it is very incomplete and in this case deadly inaccurate.

Cliff
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Old Jun 27, 2012, 11:30 AM
Tossing planes into the snow
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I'm thinking that very tight radius turns might cause the theory to break down as well. Supposing you had a sailplane that was just hovering in the wind, and ground speed was basically zero. It is high enough to be away from ground turbulence but low enough that you can easily see what it's doing. Now you want to make one full round turn and come around to the point where it is hovering again.

If you do it gradually, making a large circle, it works ok. The airspeed is maintained all the way around, and it only loses a small amount of elevation. If you crank a hard short-radius turn, it can suddenly find itself pointing downwind with almost no air speed. We didn't allow it enough time to get moving for the downwind leg, and it stalls. No problem, because we are still fairly high and have time to recover.

Now we're at about tree top level, working a little harder to keep it in one place because of the turbulence, and we want to make one more downwind turn and then come around and land. The temptation is to crank that turn a little harder than we should, just because the field is only so big and we don't have all the room in the world to make the big circle. That's where I seem to run into trouble.
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Old Jun 27, 2012, 01:50 PM
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I'm thinking that very tight radius turns might cause the theory to break down as well. Supposing you had a sailplane that was just hovering in the wind, and ground speed was basically zero. It is high enough to be away from ground turbulence but low enough that you can easily see what it's doing. Now you want to make one full round turn and come around to the point where it is hovering again.

If you do it gradually, making a large circle, it works ok. The airspeed is maintained all the way around, and it only loses a small amount of elevation. If you crank a hard short-radius turn, it can suddenly find itself pointing downwind with almost no air speed. We didn't allow it enough time to get moving for the downwind leg, and it stalls. No problem, because we are still fairly high and have time to recover.

Now we're at about tree top level, working a little harder to keep it in one place because of the turbulence, and we want to make one more downwind turn and then come around and land. The temptation is to crank that turn a little harder than we should, just because the field is only so big and we don't have all the room in the world to make the big circle. That's where I seem to run into trouble.
The problem is not with the plane or what it is doing. The problem is that the air mass is not constant. That said, the plane does react differently depending on which direction the gust is coming from, so in that sense attitude and orientation is important.

If the the air is absolutely still and unvarying, the conventional theory works perfectly. The cause of our problem is the time delay between the change in the air and the response of the plane caused by its inertial mass. During that period the airspeed is actually changing all over the place, sometimes dropping to zero or even negative. That is what sometimes causes planes to drop out of the sky. If the airplane were weightless, it would instantly track the changes in the air, and the relative airspeed would remain constant with the actual speed of the plane referenced to earth (GS) varying as erratically as the air itself.

The theory assumes tight coupling between the air and the aircraft. The reality is that because of the mass and physical size of the plane, there is actually a lot of slop in the relationship.
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Old Jun 27, 2012, 05:31 PM
Zor
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Theory is understanding

Just finished reading this thread.

I read a lot of misunderstanding in this thread in many postings.

There is no possibility of commenting on them; it would need too much time.

Just a note for the general readers to evaluate what is written.

Zor
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Old Jun 27, 2012, 06:19 PM
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There is no possibility of commenting on them; it would need too much time.
Zor, if you can find the time, your comments would be appreciated. It is only by understanding both sides of this issue, that we might all gain from the understanding, and our planes might crash less often. This forum has 2 purposes:

1 That we treat each other with respect.
2 We try to answer the question: "Given that it is widely believed that the downwind turn does not exist, why do people continually get themselves in trouble while trying to execute this maneuver?"
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Old Jun 27, 2012, 06:45 PM
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An observed stall and lengthy rebuild is not a theory... it is a reality.
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