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Downwind Turns!!

Old 08-29-2010, 12:02 AM
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HunkaJunk
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Default Downwind Turns!!

Isearched the forums but did not find much on this subject, it seems to fit best here.

I was always told by numerous pilots that "downwind turns are no different", However, a pilot flying an RC plane has only ground speed to judge his aircraft by. We are unable to read an airspeed indicator or "feel" the controls. Given this fact, how is it possible that a downwind turn would be "no different"?

The last time I was told this was a couple of years ago after I stalled a plane and crashed it after making a downwind turn, and I have overheard it numerous times, it has bugged me ever since.

what's up with this statement "downwind turns are no different"?


Old 08-29-2010, 01:02 AM
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AH1G
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Default RE: Downwind Turns!!

Well... A turn is a turn is a turn HOWEVER, When you turn downwind you need to hold a little up elevator in your turn & increase the throttle to increase your flying speed over the ground and maintain level flight. (depends on wing cross section)
If you are flying at say, 85 kts, you have a wind speed of 20 kts, you need to increase your throttle to maintain 85 kts IAS or 105 kts over the ground. If you don't, your air speed could actually be 65 kts. That could put you under your stall speed and you prang your plane.
In other words, those that crash/stall on the downwind leg have insufficient air speed to maintain flight.
Old 08-29-2010, 01:37 AM
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combatpigg
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Default RE: Downwind Turns!!

It's too bad there are beginners flying planes that have high enough wingloadings to make this a concern. A good trainer should be light enough to be very forgiving and very hard to stall while making a turn down wind. Actually, this doesn't just apply to trainers but to what I consider good all around sport planes in general.
Old 08-29-2010, 04:02 AM
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ameyam
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Default RE: Downwind Turns!!

Trainees get the hang of how much speed is sufficient for an airplane to fly with time and practice. Like many have explained above, upwind the actual air speed is an addition of ground speed and wind whereas downwind it is a subtraction of the two. You judge how much speed is enough speed by looking at the airplane and judging its "energy". Some folks refer to this as "the airplane talks to you". It may sound silly but its true and it actually means that you have sufficient understanding of how the airplane flies that you can predict what is going to happen next. Atleast thats what I feel.

If there is a very strong headwind (we fly just across from the sea and it does happen), I personally prefer not to fly or to fly as much as possible upwind. I have had occassions where my trainer had flown backward wrt the ground due to the headwind. Another is to fly sufficiently high and let the airplane gain height upwind, on the downwind section you will lose height and should endup at the same height with which you started the upwing run.

Needless to say you land upwind so you fly with sufficient throttle to reach the landing strip and they throttle down and allow the airplane to settle in easily. Trainers should be most forgiving for this type of flying.

In general, I usually see what problems others are having with the wind before I take-off. If its too gusty, I either restrict my aerobatics or dont take off at all. Its personal preference

Ameyam
Old 08-29-2010, 04:26 AM
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OzMo
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Default RE: Downwind Turns!!

Hi,
As an instructer I teach the student to watch the decent ( up or down)
in the turns. This is most important in the turns where the plane is coming at you. When you are looking at the front of the plane its near impossible to determine forward speed, wind or not. But if you are looking for it you can see decent rate. As we always say with trainers on aproach, "speed is controled by the elevater and decent is controlled with the throttle".
One of the exercises for a student is the slow low fly by. Start making passes 20 feet high then 15, 10, 5 and so on. Keep the fuse dead level with the ground, SMALL control inputs to keep it right down the runway. Often the first landing is the instructor telling NOT to increase throttle a clic or two and the plane just lands.
TIP: On the final turn watch for the trainer's wheels to be between the tail and the fuse when its pointed straight at you.
Old 08-29-2010, 09:13 AM
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noveldoc
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Default RE: Downwind Turns!!

I trained on a Kadet Senior that had a big wing and fin. In a heavier wind, it would be hard to turn because the wind would kind of blow the high wing back down some. Also, I got some weather vane effect from the big fin.

Tom
Old 08-29-2010, 12:48 PM
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OzMo
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Default RE: Downwind Turns!!

Nothing like a cross wind to make the student's knees knock
Bet you learned to use the rudder!
Old 08-29-2010, 01:12 PM
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Top_Gunn
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Default RE: Downwind Turns!!

The only reason "downwind turns" cause problems is that the pilot of an RC plane flying downwind may be fooled by the plane's high groundspeed into thinking that he has adequate airspeed. In fact, a plane that looks like it is flying fast downwind may be flying at just above stall speed. If it is, it will stall when it turns. So it's absolutely true in one sense that "downwind turns are no different": if you flew that slow in still air, or even upwind, you'd still stall if you started a turn that slow (because the control inputs for the turn increase your plane's angle of attack). The problem with the downwind turn is that the high groundspeed you get when flying downwind can fool you about your airspeed.

Notice that full scale pilots never talk about "downwind turns." They have airspeed indicators and other things to warn them of an approaching stall, so the issue doesn't come up for them.
Old 08-29-2010, 04:01 PM
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Default RE: Downwind Turns!!

Throttle and elevator usage should be identical with upwind, downwind, and crosswind turns. The problem occurs as stated before, when the person controlling the aircraft tightens the turn to keep the ground track the same as the up wind turn. This causes a steeper bank which in turn requires more elevator to maintain altitude, which in turn slows the airspeed. The slower airspeed is not noticeable as the ground speed has increased. Other than gusts and turbulence the aircraft is oblivious to wind, we only notice the wind because of the effects seen with our ground track and speed. Think of it like a boat on the Mississippi. Lets say the river is flowing at 5Kts (wind) and you are traveling at 5 Kts indicated that is a total of 10 kts down river. If you make a turn up river and don't change the throttle does the bow raise up and you suddenly are going 10kts? No you are still going 5 kts indicated and other than the fact that you are not moving in relation to the shoreline nothing has changed. You are basically in a giant mass of air all moving at a set speed and the A/C is flying in it. Wind or no wind the A/C doesn't care. Again this all excludes gusts and turbulence.
Old 08-29-2010, 07:39 PM
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Charlie P.
 
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Default RE: Downwind Turns!!

Downwind loops, too. Watched a fellow do a figure "9" into the ground with a first flight lightly powered (aka "scale powered") Tiger Moth when he tried to repeat a loop but downwind instead of upwind. It certainly makes a difference.

The problem with a downwind turn is that it is often the first one after take-off and the model is still climbing and not at a good flying speed yet. You need sufficient height and/or airspeed before that first turn.
Old 08-29-2010, 07:45 PM
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Default RE: Downwind Turns!!

ORIGINAL: HunkaJunk

The last time I was told this was a couple of years ago after I stalled a plane and crashed it after making a downwind turn, and I have overheard it numerous times, it has bugged me ever since.
Read this post:

http://www.rcuniverse.com/forum/fb.asp?m=8947745
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Old 08-29-2010, 08:26 PM
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Top_Gunn
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Default RE: Downwind Turns!!

After reading cfircav8r's useful post I'd like to modify my remarks a bit. One reason downwind turns cause problems is that the extra ground speed when flying downwind can cause pilots to think they're flying faster than they really are, and when this happens they are closer to a stall than they think (Post No. 8). Another reason is that if you are flying downwind and then turn, you'll have to turn more sharply than in calm wind to get the same turn (in relation to the ground) than in calm air. circav8r is right that if you use the same control inputs that you'd use when it's calm you wouldn't have a problem. But you're not likely to do that, so, as he says, you'll likely tighten your turn. That means you're pulling more G's and using more up elevator than otherwise, and that in turn makes you more likely to stall.

Furthermore, a lot of RC pilots don't really have a base leg in their landing approach. Instead of flying a rectangular pattern, they just fly downwind and then do a sort of semicircle and come around into the wind. That makes it even harder for them to judge speed and the attitude, as they're looking at the plane from a different angle all the time through the turn, and it means they're pretty much going straight from downwind to upwind all at once, instead of in two stages. Doing it that way also makes it easier to get your angle of attack too high than if you make 90-degree turns and fly a straight base leg. This isn't really a "downwind turn" problem, as it can be a problem even on a calm day. But on a windy day, this happens at the same place you're making the dreaded downwind turn, so the wind may get blamed.

Bottom line: turning too sharp, too slow can get you in trouble. The downwind turn condition and the rounded-off approach can both cause people's turns to be too sharp and too slow.
Old 08-29-2010, 08:51 PM
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Default RE: Downwind Turns!!

I would refer HunkaJunk to the book "Stick and Rudder: An Explanation of the Art of Flying" by Wolfgang Langewiesche. It gives great explanations on the theory and operation of flying. It is great for both full scale and models, after all they fly the same.
Old 08-29-2010, 09:07 PM
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ndb8fxe
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Default RE: Downwind Turns!!


ORIGINAL: AH1G

Well... A turn is a turn is a turn HOWEVER, When you turn downwind you need to hold a little up elevator in your turn & increase the throttle to increase your flying speed over the ground and maintain level flight. (depends on wing cross section)
If you are flying at say, 85 kts, you have a wind speed of 20 kts, you need to increase your throttle to maintain 85 kts IAS or 105 kts over the ground. If you don't, your air speed could actually be 65 kts. That could put you under your stall speed and you prang your plane.
In other words, those that crash/stall on the downwind leg have insufficient air speed to maintain flight.
No offense intended, but this makes no sense. Why would your airspeed downwind be 20kts less at a given power setting just because you are making a turn downwind? A model at a given airspeed will have the same airspeed reguardless of direction of wind unless you are not changing the power setting(and not changing altitude). You will loose a nominal amount of speed in any turn, but that's not what we are discussing here.

What you are describing would be true of a wind shear or gust situation where the wind suddenly changes speed or direction, but wouldn't apply to a aircraft changing direction.

The downwind turn problem is related to the pilots fixed position point of view and the pilots trying to judge airspeed by how fast the plane is moving over the ground.
Old 09-02-2010, 12:04 AM
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Ben-M
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Default RE: Downwind Turns!!

Fifteen years ago, a stall in the downwind turn destroyed my first RC plane - a glider - because I had no idea what I was doing. Recently I've had two incidents on the downwind turn on gusty days where airspeed has dropped low enough that a gust has stalled and then flipped the plane inverted.

Why would your airspeed downwind be 20kts less at a given power setting just because you are making a turn downwind?
I believe the problem isn't just one of ground speed / perception. The issue stems from momentum; as the longitudinal wind vector switches to a tailwind, the power setting which previously maintained airspeed now needs to increase airspeed. Once airspeed has increased to its desired level, the power setting must be returned to its previous level so as not to continue accelerating the aircraft.

I note that a lot of the guys at the club don't seem to have the issue I do - I tend to fly circuits at half throttle and close to stall speed (or even glide them for fun), whereas these guys who fly WOT all the time never come across this issue because they're always well above the stall speed as they make that turn.
Old 09-02-2010, 01:46 AM
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Default RE: Downwind Turns!!


ORIGINAL: Ben-M

Fifteen years ago, a stall in the downwind turn destroyed my first RC plane - a glider - because I had no idea what I was doing. Recently I've had two incidents on the downwind turn on gusty days where airspeed has dropped low enough that a gust has stalled and then flipped the plane inverted.
//snip//
I note that a lot of the guys at the club don't seem to have the issue I do - I tend to fly circuits at half throttle and close to stall speed (or even glide them for fun), whereas these guys who fly WOT all the time never come across this issue because they're always well above the stall speed as they make that turn.
Maybe you should ask those members that don't have your problem. [sm=confused.gif]

Now except for use of a term that rather sets a level playing ground in discussions, there jus' ain't no critter such as "stall speed".
Like so many that speak of bank angle increases stall speed. Just so much ignorance which somehow became standard back before someone figured out how to recover from a spin. In subsonic convergent airflow the term "stall speed" has no foundation because the entire situation is a function of loading - critical angle-of-attack - where the airflow departs company with the low pressure side of a lifting surface caused by that surface being loaded into such angle from another source such as a horizontal stabilizer and elevator or such.

When you make a turn from an into-wind heading to a heading downwind, maintaining a constant airspeed, LEVEL altitude, then when you bank, the lift vector tilts to perpendicular to the machine's lateral axis. Without any change on your part, the total lift vector will be the same yet the vertical vector becomes less than that needed to sustain the established level flight. Therefore you add some up-elevator which increases the Angle of Attack (AOA) and if you were as you say, close to minimum speed and near max load factor, you could have increased beyond critical AOA and thus your machine would not be responding to your commands because there is no converging airflow over the wing.

One really needs some basic understanding of the basic Lift/Drag functions to really be in control of an airplane that is flying on the wing. I say the wing because so many now simply try to make airplanes into small bladed helicopters in disguise of an airplane.
Some model pilots don't understand the High-Speed stall. You can stall an airplane at any speed if you can pull the "Gs" needed to separate the airflow. Many a pilot has found that out for the one and only time allowed.

In a near max AOA pull, turbulent/convective air masses can kick your behind, causing a snap roll or such. There are a number of factors that will shove all the still-air theory believers into some strange positions. So if you just like to cruise low and slow, well it was once said, "Be careful out there, stay slow, stay low, and in the turns use lots of top rudder." Bad advice! [X(]
Old 09-02-2010, 01:56 AM
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Ben-M
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Default RE: Downwind Turns!!

Yeah aoa was ingrained into me for my PPL. Nevertheless,

ORIGINAL: Hossfly

Now except for use of a term that rather sets a level playing ground in discussions, there jus' ain't no critter such as "stall speed".
given I was responding in part to

ORIGINAL: AH1G

That could put you under your stall speed and you prang your plane.
and the followup comment, I'm pretty sure that your 'level playing ground' exception applies.

But thanks for the lecture anyway.

Old 09-02-2010, 07:30 AM
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Default RE: Downwind Turns!!

as the longitudinal wind vector switches to a tailwind, the power setting which previously maintained airspeed now needs to increase airspeed.
Why would it need to increase? I think you are assuming that the plane is flying "against" or "with" the wind. It isn't; wind is moving air, and the plane is flying in that mass of air, not "with" it or "against" it. The settings needed to achieve a given airspeed are exactly the same whether the plane is flying upwind or downwind. People sometimes summarize this by saying that the plane doesn't feel any wind.

Somebody told me recently that a plane turning into the wind would see a sudden increase in airspeed, because after the turn the airspeed would be the plane's original speed plus the wind speed. Same error. A car turning into the wind would indeed see an increase in airspeed, but that's because a car's motion depends on how it interacts with the ground. A plane's motion depends on how it interacts with the air.

(In principle, this is the same confusion that leads to the debates about the "plane on a treadmill problem." (Please don't start that one up again.)
Old 09-02-2010, 01:15 PM
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Default RE: Downwind Turns!!

[sm=sad_smile.gif][sm=72_72.gif]Now I am confused[sm=confused.gif]

Are we all talking about the same "downwind turn" about which the OP asked?

It seems to me that some have referred to the downwind~upwind turn, while others have referred to the upwind~downwind turn.

Top or bottom turn in the diagram of Post #11?
Which one is the tricky one?[sm=drowning.gif]

For me, the dangerous turn is the one at the top, since the plane may enter the turn being underpowered.
Old 09-02-2010, 01:30 PM
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Default RE: Downwind Turns!!

Are we all talking about the same "downwind turn" about which the OP asked?

It seems to me that some have referred to the downwind~upwind turn, while others have referred to the upwind~downwind turn.
Top. No reason the bottom turn would give anybody trouble, is there?
Old 09-02-2010, 02:05 PM
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Default RE: Downwind Turns!!

Hence, the OP asked and we are discussing the turn at the end of the downwind leg (or fast leg, from the point of view of the pilot).
Old 09-02-2010, 05:28 PM
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Default RE: Downwind Turns!!


ORIGINAL: Top_Gunn

Why would it need to increase? I think you are assuming that the plane is flying "against" or "with" the wind. It isn't; wind is moving air, and the plane is flying in that mass of air, not "with" it or "against" it.
I hear what you're saying, and it's true but for one thing - momentum. The momentum of the aircraft will resist the change in velocity that a changing headwind will impose, as energy must be expended to change momentum. Thus when you turn onto the downwind leg, you will observe a loss of airspeed.

ORIGINAL: Lnewqban

Hence, the OP asked and we are discussing the turn at the end of the downwind leg (or fast leg, from the point of view of the pilot).
I couldn't find anywhere else on this page where 'end of the downwind leg' was pointed out. The OP's question was
ORIGINAL: HunkaJunk

The last time I was told this was a couple of years ago after I stalled a plane and crashed it after making a downwind turn, and I have overheard it numerous times, it has bugged me ever since.
which I interpret to be making a turn downwind - ie entering the downwind leg.
Old 09-02-2010, 07:32 PM
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Default RE: Downwind Turns!!

The momentum of the aircraft will resist the change in velocity that a changing headwind will impose, as energy must be expended to change momentum. Thus when you turn onto the downwind leg, you will observe a loss of airspeed.
You need energy to turn (i.e. "change momentum"), but no more in one direction than another. A headwind won't "impose" any change in velocity (if that means airspeed), because the plane is in the air. Same for a tailwind. And it's only airspeed that counts in figuring out how a plane flies. After all, even a small, light plane is moving at thousands of miles an hour relative to the sun, but that doesn't affect its flight. Neither does its motion relative to the ground. Imagine a cloud layer under the plane and no one on the plane could have any way of knowing what its motion relative to the ground is (absent GPS, etc., of course).

Here's an analogy I've found helpful. Suppose you've got a fish swimming in a large goldfish bowl on a train going north at 50 miles an hour. The fish is swimming north at one mile an hour, relative to the water in the bowl (groundspeed 51 mph). The fish turns around and swims south at one mile an hour (groundspeed now minus 49 mph). Does the fish lose waterspeed because it's going "upwind" now? No. In fact, there's no way the fish could even determine which direction the train was moving (assuming no windows in the car). The plane situation is exactly the same in principle, though of course the numbers are different. The plane is flying in a mass of air that's moving (on a windy day) or not moving (on a calm day). If the wind is absolutely steady, a passenger in the plane can't figure out whether it was windy or calm, unless the ground was visible. To the passenger, turns into the wind direction and away from the wind direction will look and feel exactly the same, assuming the same control inputs.

This is not my idea: It's standard information and not controversial.

As for terminology, RC people use "downwind turn" to describe the turn away from flying downwind. That's the one that causes trouble, because of high groundspeed being misleading and the need to tighten the turn to hold the same ground track.
Old 09-02-2010, 10:00 PM
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Default RE: Downwind Turns!!

For some reason even some full scale pilots have a hard time with this one. I have given many biennial flight reviews and had the pilot gain or lose altitude through a turn and blame it on the wind. We are fighting an entire lifetime of experiences with wind in relation to our ground bound activities. I really think some may never get it.
Old 09-02-2010, 11:59 PM
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Default RE: Downwind Turns!!

Kind of like Einsteins' theory of relativity...

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