Jaybird

Does the bell mean we have a winner???


Jaybird

rgburrill

I don't argue with that. My arguement is onlly with what happens DURING THE TURN ITSELF. That's when the external forces and momentum come into play. And that's when the leading edge of the wing is no longer an inertial reference frme.

cfircav8r

My Feedback: (1)

It does matter, when you are interacting with the ground, but in the air you are not interacting with the ground. the ground is merely a bystander. Try looking at it this way. You are flying against a 20mph wind going 60mph airspeed and you hit a tree. your plane hits with the force of 40mph. now you are going with the wind with the same numbers and hit a tree. you hit with 80mph of force. what changed to make the difference. did the energy or direction of the wind change? no the direction of the plane changed. so to get that change you have to change the momentum of the plane by 180 degrees. you do this in the turn, but you have to do that with no wind as well. so if the plane makes the change necessary for that difference and it is exactly the same change as without wind, why would there be a need to account for the wind? the wind is the only constant and therefore makes no difference to the control of the aircraft beyond the need to compensate for navigation purposes.

cfircav8r

My Feedback: (1)

Again the same with or without wind.

cfircav8r

My Feedback: (1)

look up momentum in wikipedia and read the section entitled "Dependence on reference frame" and that might help.

FenderBean

62 pages, and im completely lost as to what the question is you guys are debating

kmeyers

Isn't this the downwind turn myth thread?

The myth that something different happens when you turn downwind as to what happens in all other turns.

I always think of it like this:

Two 360 degree turns.

One of them is a simple constant rate from beginning to end. This turn ends up headed in the original direction but downstream over the ground in fluid of the air. This is also the one that dispels the myth in that nothing happens in the change from any relation to wind direction during the turn.

The second turn is a 360 degree turn around a point fixed on the ground. In this turn the pilot must change the rate of the turn to make the resulting vector maintain a constant radius around the point. This is the one where the limits of the flight envelope are often exceeded (by ground based RC pilots) when parts of this turn (the turn to downwind) is part of the landing approach. Due to the lack of height to recover from the mistake, we have the birth of the myth.

The normal changing of kinetic to potential energy (up and down) and momentum based on collisions with ground objects (relative velocity) is not part of this discussion.

Just was looking around and saw this thought it is off track, but, still a good learning tool.

chip_MG

My Feedback: (14)

This is what I have experienced about 3 times in the past. Say I am flying a high wing low powered J3 cub like airplane 72" F.S 70 about 60 feet high. I am flying in a wind of around 10 Mph gusting to say 15 Mph. As I fly "into the wind" the plane is making very little forward ground speed seems very slow about 5-7 mph or so, I have power wide open too, engines running good. As soon as I make my downwind turn the plane falls out of the sky straight to the ground engine still wide open, controls seem to have no effect. I have also seen this happen at least 6 times by others too. So what gives?

cfircav8r

My Feedback: (1)

a likely cause is as you turn, the plane appears to drag the tail in the turn due to the ground track caused by the wind so you add too much rudder increasing the bank and needing more elevator, creating a cross control situation with a higher than normal G load, add to that an under powered plane and a 5mph gust at the wrong time and you get a stall spin.

Lownverted

My Feedback: (4)

What gives? This one is easy! You stalled the plane.

You tried to make the down wind speed look the same to you as the upwind speed (5-7mph wide open lol). There is no mysterious gotcha that caused the crash. You simply flew too slow, stalled and crashed. Period. If you've done this 3 times, I suggest some more buddy box time with a qualified instructor flying in windy conditions.

RZielin

My Feedback: (1)

Have you read any of this thread. I know it's long and miserable, but its about EXACTLY the scenario you describe, and this thread does illuminate "what gives". So I'll summarize this thread in terms of your question.

The "myth believers" say that the wind at your tail robbed you of airspeed when you turned downwind. That is bogus. The rest of us believe that your control inputs were the culprit, not the wind. You fell prey to the visual illusion that your plane was maintaining good airspeed or even gaining airspeed as you turned downwind. It wasn't. Because it was moving fast from your perspective, you felt safe tightening the turn, and indeed thought you HAD to add control inputs to make that turn come around before you blew too far downwind. You may have even cut the throttle when you saw the plane rocketing around. But you weren't really making as much airspeed as you thought. You were seeing groundspeed but your airspeed was the same as when you were headed upwind "making very little forward ground speed….about 5-7 mph". You (quite naturally) yanked the sticks on that downwind turn much more than you would have yanked them on an upwind turn when you were "making very little forward ground speed". Too much control input does rob airspeed and often leads to a stall. That's what happened to you and the other 6 events you witnessed.

So that's the lesson of this thread (besides maybe understanding the the actual physics and perspective problem). Make gentle control inputs when you turn downwind, just like you do upwind. The turn will be more "drawn out" visually, and cover a lot more ground as the plane is blown downwind. It's actually turning just fine and at the same yaw/turn rate as an upwind turn. It just looks deceiving because that turn is stretched out over a longer ground track when you turn downwind. Be patient. Don't reduce throttle just because the plane looks faster. Let the plane fly around it's turn gently as the wind blows it down the field. It's OK. If you panic and try to tighten the turn, you risk a stall, just as you would if you tightened the turn too much going upwind. The physics of how the control inputs affect the planes flight are identical turning upwind and downwind. That's why the "downwind turn myth" is a myth. The visual appearance of the turns is indeed very different. Our reaction to that different appearance causes crashes, and that's no myth.

drac1

My Feedback: (4)

Stalled.

dirtybird

My Feedback: (5)

Interesting. Someone finally described what the problem is. To the modeler a downwind turn is a problem because he is not in the aircraft and cannot perceive what is happening.This leads to over correction and loss of altitude.
Maybe we should just call it the modelers downwind turn problem and not a myth.

Jaybird

Even in the full size plane you can fall victim to the same scenario. You are on downwind abeam the runway threshold and bring the power back and drop flaps (if available) and slow the plane for descent for landing. You find yourself drifting further downwind than you intended so you bank steeper than usual to turn base to compensate. Well you are still further downwind than you planned so you tighten your turn and maybe add rudder to swing the tail around even faster. If you haven't added power or adjusted flaps your airspeed will decay and the plane will descend further. If you input elevator to pull the nose up instead of adding power to maintain altitude the nose will rise momentarely but the plane will slow even further getting closer and closer to the stall speed. If this continues then the classic stall-spin crash is soon to follow. Same thing happens in a model only quicker and with no warnings or guages to look at or the seat of pants to say "Hey! we aren't aren't going to be flying much longer!"

Jaybird
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Lownverted

My Feedback: (4)

Finally? It's been stated over and over from the beginning.....

FenderBean

Ok so we are talking about a stall that occurs when you go from a headwind to a tailwind. Simple stuff, just remember actual speed of the aircraft over the ground has no bearing on its ability to fly. Its all about the speed of the air over the lifting surface(wing or rotor blade) This why you can have plane sit completely still and level while maintaining altitude with a strong head wind. take that same plane and change the wind direction and it tumbles out of the sky because it no longer has the wind blowing over the lifting surface. This has cause many crashes in planes and helicopters because you can have a lower stall/ETL with a headwind and just by turning your stall/ETL speed doubles.

rhklenke

My Feedback: (24)

One thing that guys who say that wind has no effect on airspeed keep forgetting is that aircraft have MASS, and anything with MASS has inertia. Inertia is with respect to the earth (i.e., the ground) NOT the air.

Read this: http://macsblog.com/2014/02/turns-wind-and-airspeed/

To quote from it:

"But there are many pilots who simply refuse to believe that turning an airplane in a breeze also changes inertia and thus causes at least momentary change in airspeed."
...
"In a rapid turn an airplane can change the wind from a headwind to a tailwind and inertia demands an adjustment to bring the airspeed back to equilibrium. The adjustment can be a brief loss of airspeed, or an exchange of altitude to regain airspeed when the turn converts headwind into tailwind."

BTW, J Mac McClellan probably has more flying experience than everyone on RCU - he definitely has more experience writing about flying and issues therein...

Now who here thinks that a model Piper Cub flying into the wind with zero ground speed doesn't have a change in inertia when executing a 180 degree turn to downwind?

Again quoting Mac:

"So does it matter if you turn quickly from a headwind to a tailwind? Yes. At least a little. Inertia requires the airplane to accelerate relative to the ground to regain the previous airspeed it had when flying into a headwind. The mass of the airplane, the rate of turn, and the strength of the wind all are factors in how much the turning airplane must accelerate to maintain airspeed in the new wind. For most airplanes the wind caused change of airspeed in a turn will be too small to notice. But it’s there, and if an airplane is operating too near its minimum airspeed the change may be enough to matter."

Bob

Jaybird

From the first part of the article come this...

In the classic wind shear encounter a strong wind changes velocity or direction, or both, suddenly robbing the airplane of lift. Why does an airplane lose lift when the wind suddenly stops blowing or shears from a headwind to a tailwind? Because of inertia.

Key word here is "wind shear". It's been stated many times that this downwind turn myth discussion excludes that condition.

Jaybird

dirtybird

My Feedback: (5)

[h=2]Here in the first post. It took years and 62 posts to come to the the conclusion there is a problem



Downwind turn Myth[/h]

Another thread contains a postulation that a model crashed due to the wind direction relative to the models flight path as it topped a loop. In other words, the model might not have crashed if the nose had been pointed into the wind over the top. (sigh)

Most of you learned better a long time ago, but this myth just keeps hanging in there. It's persists (sadly) even in the lower rungs of full scale aviation, and among pilots who have had enough training to know better. Lets set the record straight once and for all please.

It does seem counter-intuitive, but here is the truth:

Once an a/c has broken ground, steady state winds have no effect on airspeed (hence, lift) whatsoever, and airspeed does not change simply because one is flying upwind, downwind, crosswind, or even when alternating between any combinations of the above. Period. That is the beginning, middle, and end of the story.

As an aid to understanding this seemingly counter-intuitive fact, imagine yourself boating in a wide river with a 10 knot currrent. If you are putting along at 5 knots indicated speed, the water does not come crashing over the side simply because you are traveling from one bank to the other perpendicular to the current. It does not wash over the stern when headed down stream, as if you were suddenly traveling at a speed of five knots negative.

Or go for a swim in the ocean where the current is flowing parallel to the beach.

Or go scuba diving.

You will be carried along with the current, but not feel it, no matter which way you face or swim.

Or note that airliners don't fall out of the sky when making a 180 degree turn from a 150 kt headwind to a 150 tailwind, even though that net 300 kt difference in the wind is TEN TIMES the typical stall speed margin at the altitudes where such winds are encountered.

Birds fly just fine downwind, and don't suddenly crash into the trees when turning in that direction.

We could go on and on...but hopefully that's not necessary?

JackD

My Feedback: (4)

Sorry Bob, but no, that is not correct. Inertia is always around the frame of referece. If the frame of reference is moving at a constant speed, then there is no need to adjust to the earth.

Remember, we are moving all the time. Earth rotates, air is moving at 1000miles per hour at the equator, same as the surface of the air. We happen to see it steady because the surface of the earth happens to be moving at that speed too. You dont need a second to catch up to speed on the ground riding a bicicle from east to west and suddenly doing a 180.

If you write the equations of momentum for a plane going upwind and then downwind, you will see that the windspeed, being a constant, gets cancelled. So no.. no change. I used to fly paragliders at competitive levels. We would turn in thermals in strong winds. while hanging 20 feet from your wing. is a huge pendulum, and still, you could circle on the thermals in 20 knots of wind without swinging. why? cause it is constant speed.

Very counter intuitive, but do the math, you will surprise yourself (I did!!!, I was sure it was wrong haha)

jack

dirtybird

My Feedback: (5)

If you write the equations of momentum for a plane going upwind and then downwind, you will see that the windspeed, being a constant, gets cancelled. So no.. no change. I used to fly paragliders at competitive levels. We would turn in thermals in strong winds. while hanging 20 feet from your wing. is a huge pendulum, and still, you could circle on the thermals in 20 knots of wind without swinging. why? cause it is constant speed.

I don't know what kind of math you use but if what you say is true bank and turn instruments would not work. The little ball would just out of the bottom.
My guess is you are not turning fast enough to create a significant force. Next time try taking an instrument along. I bet you will find you are leaning a bit

JackD

My Feedback: (4)

Fun read

http://www.flyingmag.com/pilots-plac...d-turns-really

JackD

My Feedback: (4)

Of course you swing to the outside of the turn. WHat I mean is that you are not swinging back and forth like kids swing.... it is all constant, independently of wind or no wind

JackD

My Feedback: (4)

Bob, here is a very well written article disproving Mac's explanation. Check it out, a little long but well and simply written http://masterflight.aero/2014/02/11/...downwind-turn/

HoundDog

My Feedback: (49)

All right. Down Wind turns happen where? Shortly after take off when the plane is at full power And climbing, Usually with reduced Air Speed because of the climb and slow take off speed. So If the down wind turn thing were true, then most stall spin accidents would happen when making a 180 degree turn to the down wind leg of the pattern,shortly after take off.This is when we have full power on so as to climb to altitude.
Now lets be honest. Think about your field, Not counting Take Off & Landing accidents, Just where do most of the stall accidents happen? Not on the down wind turn but the UP WIND turn ie. the turn from Down Wind to Base or Base To final. Just the opposite place in the pattern for a down wind turn is where most stalls accidents occur. The plane has been traveling at a higher than normal Ground Speed (Which looks too fast to the inexperienced or Just POOR Pilot). So what does he do but pull the power to slow the plane so it appears normal to him and what he is used to in a now wind condition. Now he's has a normal Ground Speed but a slow air speed. Now he starts a 180 degree (or 2 90's) into a high relative head wind at a slow air speed and it appears to slow way up and start to descend rapidly. So what does the Inexperienced / Poor Pilod do? My guess he doesn't use rudder in the turn and applies a little up Elevator While in the turn and "BAM" He Stalls and Spins into the Corn/Beans, Ect.
The fact of the matter is the air plane doesn't know if it's in a high wind condition or not. It's the person on the ground that fails to keep up the proper Air speed. Not anything to do with wind across the Ground either UP Wind or Down Wind. Like all my instructors have always said, AIR SPEED, Air Speed, Air Speed & "FLY THE AIRPLANE" were their most frequently used phrases and 3th & 4th were "HEADING" Watch your heading & Step on the Ball.
Other than that the Down Wind Turn Myth is just that a MYTH.