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Old Feb 06, 2008, 08:53 AM
Texas Buzzard
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The Downwind Turn -killed Two Here

Last summer a pilot working for the Border Patrol was killed during a low altitude downwind turn near here.
Last night 3 were killed in a helicopter crash in the Laguna Madre South of Corpus Christi, Texas.

In both cases the wind was blowing at a rate of 25 to 35 mph.

About 7 years ago there was a conversation/debate in RCM (mag) about the effects of the downwind turn on RC aircraft. This debate went on for several months. Various types contributed to the debate.

Just a week ago my adult son lost a small prop jet which had a wing loading of close to 12 oz/sq ft. He was about to land w/ half power headed downwind in a 10 to 15 mph wind. He made a tight aileron turn to the left when the nose dropped, it rolled left and from 20 feet altitude he couldn't recover. The Park Jet hit vertically crushing the nose.

We know the K.E. (and momentum) of the small Park Jet is significantly less than a 7 pound glow ship, thus making the electrics more susceptible to the FATAL DOWNWIND TURN- right?

I've been flying bigger glow powered ships for over 30 years and have noticed the effects of making a tight downwind turn - a loss of speed and altitude. It's common for winds blow here at 10 to 20mph during the day.

*** In RCM some college professors w/ a private pilots license + full-scale instructors continually talked about K.E., relative velocities, etc.

Can any of the readers talk about the FATAL DOWNWIND TURN?
We are NOT talking about a wide sweeping turn - we are talking about a TIGHT TURN while moving DOWNWIND.
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Old Feb 06, 2008, 09:38 AM
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The plane is flying in an air mass. It could not care less whether it is flying down wind or up wind and will behave the same in any direction if the air speed is constant. What confuses many RC pilots is that they relate the airplane speed to ground speed and slow down to much (lose airspeed) when they make that downwind turn as they feel the plane is moving to fast and cut the power still more to decrease the ground speed and stall the plane out by losing air speed.
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Old Feb 06, 2008, 10:32 AM
Wait...Say again?
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Oh no, it's back. It's the myth that refuses to die. No amount of vector diagrams can kill it. No explanation of the actual physics involved can drive it back to its lair. It's the dreaded DOWNWIND TURN!!!
Run, I tell you, run for your lives!!

::Runs from room, screaming...::

Joe
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Old Feb 06, 2008, 10:49 AM
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>>>>"""Last summer a pilot working for the Border Patrol was killed during a low altitude downwind turn near here.
Last night 3 were killed in a helicopter crash in the Laguna Madre South of Corpus Christi, Texas.

In both cases the wind was blowing at a rate of 25 to 35 mph."""
>>>>>>

I guarantee that NTSB and/or FAA have NEVER heard of a "killer downwind turn". Aside from mechanical failure there is only poor piloting and/or lack of situational awareness that caused those crashes not some mythical "wind effect".
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Old Feb 06, 2008, 11:03 AM
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It's a myth, it doesn't happen and can't.

You're talking about ground speed versus indicated and/or true airspeed that the plane sees.

I can't believe this topic keeps rearing it's ugly head.

Curtis
Montana
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Old Feb 06, 2008, 11:08 AM
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So maybe people are going too slow to make the turn because they misjudge the true airspeed due to the velocity added by the tailwind?
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Old Feb 06, 2008, 11:10 AM
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Quote:
Originally Posted by Texas Buzzard
Last summer a pilot working for the Border Patrol was killed during a low altitude downwind turn near here...

In both cases the wind was blowing at a rate of 25 to 35 mph.
Pilot error. He failed to watch his airspeed
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Old Feb 06, 2008, 11:41 AM
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Quote:
Originally Posted by Texas Buzzard
Last night 3 were killed in a helicopter crash in the Laguna Madre South of Corpus Christi, Texas.

In both cases the wind was blowing at a rate of 25 to 35 mph.
Helicopters don't stall, so it seems unlikely that anybody could attribute that crash to the "Killer downwind turn".

Quote:
Originally Posted by Texas Buzzard

Just a week ago my adult son lost a small prop jet which had a wing loading of close to 12 oz/sq ft. He was about to land w/ half power headed downwind in a 10 to 15 mph wind. He made a tight aileron turn to the left when the nose dropped, it rolled left and from 20 feet altitude he couldn't recover. The Park Jet hit vertically crushing the nose.
That was an upwind turn, not a downwind turn.


Once an RC airplane gets above it's stall speed and leaves the ground, it's only tie to the ground is the pilot, but that relationship can cause the "killer downwind turn". In high winds the airplane will speed up considerably relative to the pilot when it turns downwind, so the normal response is to pull back on the stick, sometimes causing a stall. It can take quite a bit of stick time flying in high winds to suppress that tendency.

Dan
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Old Feb 06, 2008, 01:31 PM
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Yep... I'll add another supporting voice to the 'aint no such thing as a downwind turn' argument.

'Wind' is relative to a ground datum only. Once the model leaves the ground it travels with the moving air mass and the wind you feel while standing on the ground has no effect on it.
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Old Feb 06, 2008, 01:48 PM
Cessna Skyhawk Pilot
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Those of us who are private pilots (or even Commercials and ATPs ) recognize the scenario. In a full size plane we have this little gauge called the ASI – the air speed indicator. It keeps us from stalling out when turning downwind to base, and much more frequently, base to final. I’m not aware of the “downwind” turn as being nearly the killer that the base-to-final turn is.

But our RC models lack such an instantaneous readout of airspeed. So we rely on ground speed. If the wind speed is 15 mph we need to fly the downwind leg 15 mph faster than we would in calm air. This seems too fast. We adjust the ground speed to what we are accustomed to in calmer air and our margin above stall speed narrows.

You describe a “tight aileron” turn. Could you comment on the bank angle? By aileron turn, do you mean uncoordinated, that is, aileron-only, without rudder input?

A 60 degree coordinated turn causes the airplane to weigh twice as much, so stall speed is increased by 40%. Combined with an already too slow air speed due to our improper interpretation of the fast ground speed, the model stalls.

When the air speed falls to below stall speed, the model stalls. The wing doesn’t care how much kinetic energy the model has, whether the engine is electric, gas or no engine at all; it doesn’t care about relative speeds (which are, however, getting the pilot into trouble!). The wing only knows the angle of attack is too great for the present conditions and it stalls.

There is considerable misunderstanding about stalls in the landing pattern, sadly among full scale pilots who should know better, and have a stall warning horn and ASI to keep them out of trouble. So it’s no wonder we RC pilots have problems with this critical phase of flight as well.

I think I’ve managed to say in several hundred words what several others so succinctly pointed out with far fewer words in their messages.

Good landings,

Dennis
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Old Feb 06, 2008, 02:25 PM
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N9DP,
Very nicely stated.
-Mike
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Old Feb 06, 2008, 02:38 PM
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It's back... Okay, who's going to email Myth Busters to finally put this one to rest.
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Old Feb 06, 2008, 02:52 PM
Texas Buzzard
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Downwind turn - MORE

Later today I heard the report that last night's helicopter crash, at first glance could from the radio conversation of the pilot and the EMS which was on Padre Island, there was a directional shift in the direction of the wind. I have seen the wind shift from SE to directly N when a Norther hits down here in South Texas.

The same night a 70 passenger jet landed very hot in a turbulent crosswind. I talked to a neighbor who was on that plane. He said the plane was really rocking from side to side when they came down through the clouds. Weather reports said the wind here was 25 gusting to 40 mph. That was SOP for a crosswind,i.e., lower upwind wind wing a bit and play with the rudder to stay on line in a crosswind.

But my primary reason for posting this was to stimulate and possibly make us more aware of how the RC plane handles when turning quickly from downwind in a 180 and back into the wind.

How many of you have flown a 7 to 8 lb RC in winds over 15 mph?.... Then have the same pilots flown a 14 oz. Electric plane in the same 15 mph + wind?

If some of you do have experience with a 7 lb RC and also a <1 lb RC did you notice a difference?

Back in my initial post in this thread I wrote,"Can any of the readers talk about the FATAL DOWNWIND TURN?
We are NOT talking about a wide sweeping turn - we are talking about a TIGHT TURN while moving DOWNWIND
.

As you can see I am talking about a tight turn. One poster asked if the rudder was coordinating the turn. NO, in a Park Jet like ny son was flying it cannot use rudder control. It has no rudder.

If some of you have sailed a small boat like a HobieCat 16 in 20 mph winds then later sailed in a 30 ft. heavy sailboat, then you certainly noticed a big difference in how they handled. The small (16 ft) Hobie bounces around much more than the 200 lb sailboat.

You guys a much like the guys who fly in my club. When faced with an unusual topic - each guy has a ready answer even tho' he might not really know much about the question. My guys (20 of them)would probably fall into 3 different groups. RC guys are very opinionated and independant - right?

For those of you who dispute the "downwind turn syndrome" ( YES THAT IS IN THE LITERATURE OF THE FAA)

Four of you posters fall into the "There ain't no such thing" group : see posts # 3, 4, , 5 and 9. Posters # 7 & 8 (NMasters and Dan Baldwin ) showed some experience with the topic. N9DP says he is a private pilot. (I too was a private pilot who let his license lapse 4 years ago ( eyes). I took my private in a 7AC Champ.

So if any of you think a light plane will handle in gusty & high winds when making a tight 180 left turn will be effected the very same as a Boeing 747 ; explane why this is true.

Addendum: The Border Patrol pilot's accident was ruled pilot error. Furthermore spectator said," he was following illegal aliens in Mesquite country. He was headed North in a wind of maybe 30 mph from the South. He suddenly banked left (steep????what does that mean??) Then the plane rolled to it's right and went into a spin down low. He crashed while spinning to his right.

As a student pilot my ex-USAF instructor had me practice stalls from a climbing left turn. The 7AC has no "stall warning" device. The 7AC would stall and FALL OFF TO THE RIGHT. It's easy to recover if you stick the nose down and add some power - catch it quick - right?Can any of the readers talk about the FATAL DOWNWIND TURN?

We are NOT talking about a wide sweeping turn - we are talking about a TIGHT TURN while moving DOWNWIND.
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Old Feb 06, 2008, 03:46 PM
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[QUOTE #Buzzard........(snip).............."topic - each guy has a ready answer even tho' he might not really know much about the question".......(snip)........."group : see posts # 3, 4, , 5 and 9. Posters # 7 & 8 (NMasters and Dan Baldwin ) showed some experience with the topic."
[QUOTE]

What a bunch of condescending Bull........highly likely that quite a few of us that have posted have MORE experience than your over inflated self esteem can imagine. Just that we likely didn't build rockets back in the Eisenhower Administration or whatever you claim to have done.
And, just what class standing did you get out of UPT/Air Cadets if you even graduated? Nah, I don't care.

This thread sounds like a really bored person trying to get attention.

I'm outta here -and Buzzard is on my ignore list.


-Mike
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Old Feb 06, 2008, 03:56 PM
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I'm attaching the text of an article I'm writing for a hang gliding magazine on this subject. I apologize that the figures aren't finished, but maybe the words will give you some idea of what is happening when turning near the ground in a steady wind. This article does not address turbulence, gusts, wind gradients, etc.. The main reason I wrote it is to help solve the exact conflict that is occurring here:

1) The academics claim there is no problem, just watch your airspeed.
2) The practitioners know there is a problem because other sensory cues confuse the situation.

The unfortunate result of these discussions is a lot of name calling and no useful rules to fly by that reduce accidents. In my article I try to show how the visual cues and flight planning (from a hang glider pilot's perspective) must be re-assessed to fly safely near the ground in wind. RC pilots can adapt these results and also learn to be safer.

The bottom line is that downwind turns are a danger and people need to know why and how to deal with them. The academic answers rarely provide this information.

Steve Morris
---------------------------------------------------------
(Long article attached here)



Downwind Turns: What Your Instructor Never Told You

Steve Morris


Introduction
How an aircraft behaves when turning downwind has often resulted in heated arguments amongst pilots, instructors, and scientists who often have conflicting ideas on the physics of this maneuver. A carefully study of the various arguments from a scientific perspective shows that the theories which satisfy the laws of physics all tell the same story for this maneuver, but the interpretation of these results still leads to pilot confusion and accidents. This article will not attempt to revive these age old arguments (many of which use incorrect physical aurguments) but instead will try to explain what a pilot needs to know to stay out of trouble when flying near the terrain in wind. The situation we are most concerned with is the influence of a steady wind on the way we maneuver near the terrain. The influence of wind gradients and turbulence from thermals or rotors are very important when flying near the terrain, but will not be considered here since they do not change the basic issues we hope to explain.

It can be mathematically proven using Newton’s laws of motion or conservation of energy principles that:

The rate of descent of a gliding aircraft turning at fixed airspeed and bank angle will not be affected by the amount of wind or the direction it is blowing from.

This statement is the same as saying that when the air mass is in uniform motion and you are maneuvering relative to the air (i.e. fixed airspeed and bank angle) the aircraft feels no influence from the steady motion of the air mass. Of course, your trajectory over ground is distorted by the wind (figure 1), but if you ignore the ground and just fly relative to the air the wind has no influence. Flying in the wind can be problematic when we focus on the ground as a visual reference or when we want to maneuver relative to obstacles on the ground to avoid colliding with them. In these critical cases the influence of the wind on our motion over the ground can confuse us and result in piloting errors that can be fatal. The old adage that a steady wind can’t affect a turning aircraft is irrelevant when you are concerned with terrain obstacles in the environment. Instead, the pilot needs to know exactly how wind affects his or her perception of motion over the ground and how he or she should maneuver the glider to safely control their flight path. Unfortunately, this topic is rarely taught to students because it is tricky to explain, yet when we fly we need to deal with it at the most critical phases of our flight; launch, landing, and soaring near terrain.

Beginner pilots are more susceptible to the dangers of flying downwind near the terrain because most of their training takes place flying upwind and away from the hill. The first time they fly towards the hill or directly downwind, they may not be adequately prepared for what they will see and feel. Many accidents occur regularly because of confusing perceptions when flying near terrain in a moving air mass and it is important to study this problem in the “classroom” so that we can be prepared when we encounter it in the air.

What is a Downwind Turn?
A downwind turn occurs when the velocity of the aircraft (with respect to the ground) changes from 90 degrees crosswind, to straight downwind, and then 90 degrees crosswind again, but now heading in the opposite direction from which the glider originally started (figure 2). The remaining portion of the 360 degree turn that restores the aircraft to its original heading is called an upwind turn. Please note that the downwind turn is defined by the heading angle of the velocity vector over the ground (not relative to the air) and that the aircraft will be crabbed into the wind at the beginning and end of the downwind turn maneuver. I’ve chosen this definition of a downwind turn because it defines the region where extra care must be taken to avoid perceptual confusion or stalling of the glider. Some may prefer to think of a turn from straight downwind to straight upwind as a downwind turn but, as we shall soon see, many of the confusing effects caused by the wind cancel out during this portion of maneuver.

Turning with fixed bank angle in a steady wind

Figure 3 shows the ground path of a glider flying a 360 degree turn at a fixed bank angle in a steady wind equal to ˝ the glider’s airspeed (both wind and airspeed are assumed constant). The portion of this maneuver that meets our definition of a downwind turn is highlighted in red and the upwind portion is shown in blue. The triangles drawn along the path show the position of the glider at equal time intervals and from this one can gain a sense of the velocity over the ground during the maneuver. Keep in mind that the motion of the glider relative to the air is a perfect circle for this fixed bank angle maneuver and therefore figure 3 shows the distortion of the path over the ground due to a steady wind.

If you study figure 3 closely you should be able to verify these conclusions.

Fixed bank angle turn in wind
1) The amount of time spent in the downwind portion of the turn is longer than spent in the upwind portion.
2) The rate at which the glider’s velocity vector changes heading (relative to the ground) is lower on the downwind portion of the turn than the upwind portion.

Considering the pilot’s perspective, we can reduce these conclusions to an even simpler result. If you’re flying straight downwind and bank for a turn, it will feel like you aren’t turning very fast relative to obstacles on the ground. It will actually take longer to get your velocity over the ground to change direction than if you performed the same turn while flying directly upwind. This is an important thing to know if you’re trying to avoid trees or a hillside! Most glider pilots spend little of their flight time heading straight downwind near the terrain and don’t get to study this phenomenon much. What we experience most often is the opposite effect, i.e. turning with a strong headwind, where we experience an enhanced sense of maneuverability relative to the terrain. Anyone who has soared a ridge in strong wind knows that only a slight deviation in heading is needed for the glider to make a big change in direction relative to the ridge. If the wind speed is nearly equal to your flight speed, you need barely move the nose relative to the horizon and like magic your traversing 90 degrees down the ridge! Move the nose a few degrees the other way and you traversing the opposite direction along the ridge, or 180 degrees of heading change in your ground velocity. Unfortunately, the opposite effect occurs when we have our nose pointed straight downwind and now we need to swing the nose of the glider more than 90 degrees in order to get our velocity over the ground to change 90 degrees. This overlooked effect is responsible for many accidents where pilots hit the terrain, not knowing why the glider “felt like it stopped turning”. Beginners are especially susceptible to confusion when turning from straight downwind flight because they have almost all of their flying experience has been with the nose into the wind which enhances the sensation of maneuverability.


Turning with fixed radius over the ground in a steady wind
Now imagine that we choose to fly a perfect circle relative to the ground instead of the air. This is the scenario we find ourselves in when we want to avoid the hill or other obstructions. Let me stress that you should always plan your flight to avoid being in this situation and this article is not encouraging pilots to fly downwind close to the terrain. It is useful, however, to study the ground-fixed radius maneuver as a basic element of all maneuvers which avoid the terrain when flying in wind.

Assuming constant airspeed, we can solve for the bank angle required to maintain constant turn radius (as seen from the ground). Figure 4 shows the resulting trajectory and figures 5&6 show the bank angle and g forces during the maneuver. The red portion is the downwind turn and the blue is the upwind, based on our previously mentioned definitions. I’ve drawn a cliff directly downwind of the circle to be representative of the situation pilot’s sometimes find themselves in when soaring.

This maneuver feels quite different to the pilot than flying a constant bank angle 360 in the wind. The downwind portion of the turn happens quickly and at much greater bank angle and ‘g’ force than the upwind portion. The glider feels like its “whipping” around the downwind side of the turn and then un-banking to crawl along the upwind segment. The pilot’s motion over the ground is also quite different. Instead of drifting a large distance downwind, the glider carves a perfect circle over the ground and changes direction most quickly on the downwind portion of the maneuver, exactly opposite of the constant bank angle case! Here are a few more important observations for the maneuver:

Fixed ground radius turn in wind
1) The amount of time spent in the upwind portion of the turn is longer than spent in the downwind portion.
2) The rate at which the glider’s velocity vector changes heading (relative to the ground) is lower on the upwind portion of the turn than the downwind portion.
3) Maximum bank angle (and ‘g’) occurs when the gliders ground velocity vector is pointed straight downwind
4) This maneuver takes longer to complete than fixed a bank angle 360 of the same radius and airspeed.


In a constant radius turn over the ground, the glider spends a lot of time at low bank angle on the upwind portion of the turn and experienced pilots will use the slow portion of the upwind segment to gauge the wind and to establish proper clearance from the terrain before turning in and “whipping it around” on the downwind side. Turning in or banking too soon will sacrifice terrain clearance and lead to disaster. The “whipping” portion of the turn requires aggressive pilot action, precise control and timing. If the wind speed is ˝ the airspeed of the glider, the maximum bank angle required will be over 45 degrees while on the upwind portion the glider is banked less than 10 degrees. The maximum bank angle occurs when the glider is flying straight downwind (i.e. directly at the cliff in figure 4) and not when the glider is parallel to the cliff. An experienced pilot can perform this maneuver in a very fluid motion, often masking the skill and timing necessary to get it right. If he or she banks too soon (or not enough), the circle will distort and possibly intersect the terrain. Many beginners can understand this aspect of the maneuver, but they often forget to reduce the bank angle on the upwind portion which is vital for establishing clearance from the hill and for setting up the downwind turn. Beginners may also have little familiarity with steep-banked turns and the phenomenon of high-speed stall, often entering the maneuver with too little airspeed to safely pull ‘g’ on the downwind side.


What your instructor never told you

So far we have studied two types of 360 deg turns in wind; fixed bank angle and fixed radius over the ground. These maneuvers are the basic pieces of the downwind turn puzzle and show us why downwind turns feel different when we are near the terrain. Now let’s consider an approach into a confined landing area with a hill and trees on the downwind side and a significant breeze blowing. A standard landing approach would consist of a downwind, base, and final approach leg, but this field is so small that only a downwind and final will be possible. An experienced pilot would fly the downwind leg with excess speed and initiate an aggressive high-bank turn at the downwind end of the field (with appropriate terrain clearance), leveling out with enough room upwind to land inside the field. At the beginning of the 180 deg onto final, the pilot knows that initially he or she will experience a slow turn rate w.r.t. the ground and this is why a high bank angle is needed. As the glider comes around past 90 degrees from straight downwind, it will have a higher turn rate over the ground as it encounters a headwind and the pilot reduces the bank angle accordingly. The excess airspeed at the beginning of the maneuver is needed to avoid a high-g stall during the downwind portion of the turn.

A less experienced pilot in the same situation might do things in a disastrously different manner. While flying downwind the terrain appears to be moving quickly so the pilot subconsciously slows to a lower airspeed to keep the visual cues looking the same. At the downwind end of the field a moderate, constant bank angle turn is initiated and the glider slowly comes around while drifting downwind off the edge of the field. Even though it is banked up, the glider doesn’t feel like its turning because the scenery isn’t rotating as fast as the pilot expects for this bank angle and speed. In these crucial moments the glider’s ground track is pushed towards the trees by the wind and the lack of sufficient bank angle. Sensory overload occurs because the “bad” things are happening fast: the trees are getting closer, the glider feels “funny”, like it won’t turn. The pilot may freeze at the controls in a state of brain-lock or push out to accelerate the turn and stall into the trees because the airspeed is already low.

Each year many accidents like this occur and they are often blamed on the pilot’s inability to maintain airspeed while turning, when in fact the root cause is a lack of familiarity with downwind flight techniques near the terrain. Here are some key points to remember:

Important points to remember

1) When flying straight downwind a glider has a slower turn rate w.r.t. the ground and will take longer to change heading. This can confuse the inexperienced pilot and lead to “brain-lock” or the sensation that the glider won’t turn when flying downwind.
2) Carry extra airspeed when flying downwind and be prepared to turn aggressively with high bank angle to avoid terrain. Understand and avoid high speed stalls when turning aggressively.
3) A properly executed aggressive downwind turn will feel like you are whipping through the turn, where as a turn with lower bank angle and airspeed will seem to take forever and cause you to drift far downwind.
4) If you are 360’ing near terrain your maximum bank angle will occur when the glider is flying directly downwind. Reducing bank angle on the upwind side of the 360 is essential to “open up the circle”, establishing terrain clearance and giving the pilot time to prepare for the downwind “whip”.


To better appreciate the effects of wind on turning maneuvers it is best to practice 360 degree turns about a point fixed with plenty of terrain clearance and altitude. You should have a minimum of 500 ft. altitude and 500 ft. terrain clearance on all sides of the maneuver. Try to execute a fixed radius circle about a point on the ground and notice how the bank angle and turn rate change. Practice increasing your airspeed after flying directly upwind and then roll the glider so that the maximum bank angle occurs when straight downwind. Remember to reduce the bank angle smoothly so that when you are flying directly upwind the glider is almost level. Use this portion of the turn to relax, gauge your drift, and prepare for the downwind “whip”.

When you are soaring, do not maneuver downwind close to the terrain if you can avoid it. For example, fly linked 180’s when thermalling near the hill instead of 360’s until you have sufficient altitude. Every time you begin a landing approach, mentally prepare yourself for the turn from downwind to base, since this turn must be started early and requires more bank angle to avoid drifting off course. If you are making a 180 onto final and the field is restricted, carry extra airspeed and prepare to execute the downwind “whip” onto final.
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