Downwind turn Myth
11-23-2010, 09:37 AM
#176
RE: Downwind turn Myth
ORIGINAL: Sport_Pilot
The problem here is the laws of relativity. F=MA on earth and F=MA from a fixed point, and F=MA relative to the wind is not the same. We are always moving on spaceship earth. The Sun is pulling us in, but we are circling around the sun to cancel out the pull of gravity, we are pulled toward the earth, and the earth is rotating trying to sling us back out to space. To say that the turn is causing acceleration relative to the earth is not a valid point, because the plane is flying relative to the air. The acceleration of the turn is affecting the stall speed in the positive G in the turn, but has no effect in the down wind turn. It is like saying we are accelerating faster when we run to the west than we do to the east. But we do not feel a differance in G force because we are moving relative to the earths surface.
The problem here is the laws of relativity. F=MA on earth and F=MA from a fixed point, and F=MA relative to the wind is not the same. We are always moving on spaceship earth. The Sun is pulling us in, but we are circling around the sun to cancel out the pull of gravity, we are pulled toward the earth, and the earth is rotating trying to sling us back out to space. To say that the turn is causing acceleration relative to the earth is not a valid point, because the plane is flying relative to the air. The acceleration of the turn is affecting the stall speed in the positive G in the turn, but has no effect in the down wind turn. It is like saying we are accelerating faster when we run to the west than we do to the east. But we do not feel a differance in G force because we are moving relative to the earths surface.
I'm guessing that we won't be seeing you on the podium in Stockholm to join your contemporaries (Planck, Einstein, and Fermi) anytime soon.
Cheers,
Jim
Edit: Changed "Oslo" to "Stockholm". I guess I won't be there either if only for the reason that I went to the wrong city
11-23-2010, 09:55 AM
#177
RE: Downwind turn Myth
You keep saying all you have to do is observe what the plane is doing. All you ever see is what the pilot is telling/allowing the plane to do. The plane is dumb, it has no will of its own. If it climbs or descends in a turn it is because the pilot, either intentionally or unintentionally told/allowed it to do so. Go out on a calm day and see how many turns are nice and level. You will find the same number if issues with or without wind.
11-23-2010, 10:14 AM
#178
RE: Downwind turn Myth
David,
In your original example you turned a helicopter through 180 degrees whilst maintaining 0 groundspeed. Effectively, as far as the airframe is concerned, you are now flying backwards. The increased power requirements aren't due to the turn or the air over the rotor disc but the fact that the fuselage is now 'downwind'. The same additional power demand is required if you lift into the hover downwind and this area of manoeuvering does require caution for a number of reasons, including restricted power. As I mentioned before, the rotor disc will still experience the same airspeed (and translational lift) when downwind but this is offset by extra power needed to hold the tail against weathercocking and issues caused by the horizontal stabiliser and possible tail rotor interference. If you were to maintain 20 kts forwards airspeed (i.e. forward flight) whilst turning then there is no such limitation on handling and you can be relatively 'care-free'. Also, if you allow the helicopter to drift with the wind you can pedal turn without any problem as the airspeed will be low. The one area that can catch people out when doing this very close to the ground is the visual illusion issue caused by varying groundspeed which will cause inadvertant airspeed changes. This is now back to the 'downwind turn' myth.
p.s. Retreating blade stall is a high speed issue and can be disregarded in hover manoeuvres.
In your original example you turned a helicopter through 180 degrees whilst maintaining 0 groundspeed. Effectively, as far as the airframe is concerned, you are now flying backwards. The increased power requirements aren't due to the turn or the air over the rotor disc but the fact that the fuselage is now 'downwind'. The same additional power demand is required if you lift into the hover downwind and this area of manoeuvering does require caution for a number of reasons, including restricted power. As I mentioned before, the rotor disc will still experience the same airspeed (and translational lift) when downwind but this is offset by extra power needed to hold the tail against weathercocking and issues caused by the horizontal stabiliser and possible tail rotor interference. If you were to maintain 20 kts forwards airspeed (i.e. forward flight) whilst turning then there is no such limitation on handling and you can be relatively 'care-free'. Also, if you allow the helicopter to drift with the wind you can pedal turn without any problem as the airspeed will be low. The one area that can catch people out when doing this very close to the ground is the visual illusion issue caused by varying groundspeed which will cause inadvertant airspeed changes. This is now back to the 'downwind turn' myth.
p.s. Retreating blade stall is a high speed issue and can be disregarded in hover manoeuvres.
11-23-2010, 10:17 AM
#179
RE: Downwind turn Myth
Here's the bottom line...
If the your aircraft control inputs don't cause the plane to stall, snap, or lose altitude in calm air, the exact same inputs won't cause you plane to stall, snap, or lose altitude in ANY steady wind.
PERIOD!!!
If the your aircraft control inputs don't cause the plane to stall, snap, or lose altitude in calm air, the exact same inputs won't cause you plane to stall, snap, or lose altitude in ANY steady wind.
PERIOD!!!
11-23-2010, 10:56 AM
#180
RE: Downwind turn Myth
ORIGINAL: rjbob
Here's the bottom line...
If the your aircraft control inputs don't cause the plane to stall, snap, or lose altitude in calm air, the exact same inputs won't cause you plane to stall, snap, or lose altitude in ANY steady wind.
PERIOD!!!
Here's the bottom line...
If the your aircraft control inputs don't cause the plane to stall, snap, or lose altitude in calm air, the exact same inputs won't cause you plane to stall, snap, or lose altitude in ANY steady wind.
PERIOD!!!
Are we done yet?
Jim
11-23-2010, 10:59 AM
#181
RE: Downwind turn Myth
ORIGINAL: rjbob
You are incorrect.
The pitch number on a propeller is NOT an indication of blade angle. It is a measurement, in inches, of how far the the propeller will travel forward in one revolution if the propeller is 100% efficient. Both numbers describing a propeller size are in inches...not degrees. In other words, a 10-6 propeller is 10 inches in diameter and will travel 6 inches forward per revolution.
Since, as pointed out in an earlier post, there are many variables to consider, the pitch is used as an approximate value. A highly efficient propeller, such as an APC, will come pretty close, however.
BTW...Where did you get that ''one inch...no more, no less'' measurement from?
ORIGINAL: FalconWings
A 10x6 prop travels exactly one inch, no more no less, on earth as well as Mars. It ia called pitch and has nothing to do with air, mass, density or whatever factor you want to pitch in. It has nothing to do with Fluid Mechanics either. It is just a simple measurement of angle, similar to threads on a bolt. Or did you think a model with a 4-40 bolt instead of a prop would travel 4'' every 40 revolutions?
Screw Jetcat I want a 4-40 bolt on my Bandit!
And furthermore, yes it would travel 1'' if it was incompressible fluid. ;-)
ORIGINAL: p51Dpony
Well, I think you're applying aerodynamic principles in an oversimplified manner or worse yet in an out of context manner, the same way Jehova's Witness's quote out of the bible. If we can ignore drag, slippage, aircraft front to aft asymmetry, and skin slipperiness in an aircrafts design and performance it's like saying a 10-6 propeller advances 6'' with every revolution - not only it doesn't advance 6'' but the speed and efficiency and actual advance are effected by numerous variables. While to real pilots of FS I bet we're splitting hairs of things that don't matter much, in our academic discussion these things matter greatly. In summary, 2-3 equations have been applied to a 5-6 equation problem, you have an ok applied result but an absolute failure to prove the original premise.
I appreciate that some persons arguments were based on ground-reference to the pilot and agree that those arguments don't prove or disprove anything of the original premise as that's another subject entirely.
Well, I think you're applying aerodynamic principles in an oversimplified manner or worse yet in an out of context manner, the same way Jehova's Witness's quote out of the bible. If we can ignore drag, slippage, aircraft front to aft asymmetry, and skin slipperiness in an aircrafts design and performance it's like saying a 10-6 propeller advances 6'' with every revolution - not only it doesn't advance 6'' but the speed and efficiency and actual advance are effected by numerous variables. While to real pilots of FS I bet we're splitting hairs of things that don't matter much, in our academic discussion these things matter greatly. In summary, 2-3 equations have been applied to a 5-6 equation problem, you have an ok applied result but an absolute failure to prove the original premise.
I appreciate that some persons arguments were based on ground-reference to the pilot and agree that those arguments don't prove or disprove anything of the original premise as that's another subject entirely.
Screw Jetcat I want a 4-40 bolt on my Bandit!
And furthermore, yes it would travel 1'' if it was incompressible fluid. ;-)
The pitch number on a propeller is NOT an indication of blade angle. It is a measurement, in inches, of how far the the propeller will travel forward in one revolution if the propeller is 100% efficient. Both numbers describing a propeller size are in inches...not degrees. In other words, a 10-6 propeller is 10 inches in diameter and will travel 6 inches forward per revolution.
Since, as pointed out in an earlier post, there are many variables to consider, the pitch is used as an approximate value. A highly efficient propeller, such as an APC, will come pretty close, however.
BTW...Where did you get that ''one inch...no more, no less'' measurement from?
1. You can call it a measurement of angle becasue it is logarithmically proportional, and assumes 100% efficiency. (FOR A GIVEN PROPELLER SHAPE, AIRFOIL............)
2. 100% efficiency = incompressible fluid (which would make the prop close to 100% efficient). This is where fluid density comes into play.
3. One inch no more no less IS the definition of pitch of 1"( one inch for every revolution), (for a 10x6 would be 6" and so forth...I typed too fast on that one). On a model airplane prop, the losses are taken into accout when the pitch number is defined. Different airfoils = different performance = final pitch value.
11-23-2010, 11:05 AM
#182
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RE: Downwind turn Myth
Pretty simple really.
You expect your downwind loop to "look" the same as your upwind loop with the same control inputs. It can't! If it does, then you are making corrections and the inputs arent really the same are they? The mass of air it is flying within is moving, so by default it can't look the same with the same control inputs. By trying to make it look the same, you are flying it differently upwind than downwind. Your .40 size trainer sees EXACTLY the same airspeed no matter what the wind is doing. YOU see a difference, but the plane doesn't.
It's all about your perspective from the ground, the way the plane "looks". If there were a way to program your control inputs for a loop with the flip of the switch, you could easily see this by simply flying one loop upwind and one down. The loops would look quite different in shape to you on the ground, but the plane (or a person in the plane) would see/feel exactly the same. The plane doesn't care about wind, YOU do on the ground by trying to make it look the same!
Many folks here are confusing what the airplane sees (IAS) and what you see (GS). These folks just wont get it no matter what.
You expect your downwind loop to "look" the same as your upwind loop with the same control inputs. It can't! If it does, then you are making corrections and the inputs arent really the same are they? The mass of air it is flying within is moving, so by default it can't look the same with the same control inputs. By trying to make it look the same, you are flying it differently upwind than downwind. Your .40 size trainer sees EXACTLY the same airspeed no matter what the wind is doing. YOU see a difference, but the plane doesn't.
It's all about your perspective from the ground, the way the plane "looks". If there were a way to program your control inputs for a loop with the flip of the switch, you could easily see this by simply flying one loop upwind and one down. The loops would look quite different in shape to you on the ground, but the plane (or a person in the plane) would see/feel exactly the same. The plane doesn't care about wind, YOU do on the ground by trying to make it look the same!
Many folks here are confusing what the airplane sees (IAS) and what you see (GS). These folks just wont get it no matter what.
Nice and straight explanation.. agree 100%
What surprises me is that many people in this thread with presumingly professional experience seems to have clashing opinions and can´t agree in a consensus.. no matter who is right, there will be many more "misterious" crash videos on youtube on years to come.
Enrique
11-23-2010, 11:41 AM
#183
Senior Member
RE: Downwind turn Myth
ORIGINAL: p51Dpony
Right on, that's what I'm talking about TimT2000. My worst experience w/ this was 20 yr ago was a ST .45 powered high wing float plane, lots of drag, did a slow speed loop into the wind (no problems of calm days) and as it hit top of loop going downwind it just started falling, one would think 300' would be plenty of altitude to get airflow going again, one would be wrong.
Right on, that's what I'm talking about TimT2000. My worst experience w/ this was 20 yr ago was a ST .45 powered high wing float plane, lots of drag, did a slow speed loop into the wind (no problems of calm days) and as it hit top of loop going downwind it just started falling, one would think 300' would be plenty of altitude to get airflow going again, one would be wrong.
Elevator controls airspeed.
To climb, add power.
Merely pulling back on the stick will cause the airplane to start to climb -and- lose airspeed. The plane can't do anything else.
Pattern fliers will add power when looping, and cut it back on the downside.
It takes experience to get to know to do this.
11-23-2010, 11:50 AM
#184
RE: Downwind turn Myth
ORIGINAL: erbroens
Nice and straight explanation.. agree 100%
What surprises me is that many people in this thread with presumingly professional experience seems to have clashing opinions and can´t agree in a consensus.. no matter who is right, there will be many more ''misterious'' crash videos on youtube on years to come.
Enrique
Pretty simple really.
You expect your downwind loop to ''look'' the same as your upwind loop with the same control inputs. It can't! If it does, then you are making corrections and the inputs arent really the same are they? The mass of air it is flying within is moving, so by default it can't look the same with the same control inputs. By trying to make it look the same, you are flying it differently upwind than downwind. Your .40 size trainer sees EXACTLY the same airspeed no matter what the wind is doing. YOU see a difference, but the plane doesn't.
It's all about your perspective from the ground, the way the plane ''looks''. If there were a way to program your control inputs for a loop with the flip of the switch, you could easily see this by simply flying one loop upwind and one down. The loops would look quite different in shape to you on the ground, but the plane (or a person in the plane) would see/feel exactly the same. The plane doesn't care about wind, YOU do on the ground by trying to make it look the same!
Many folks here are confusing what the airplane sees (IAS) and what you see (GS). These folks just wont get it no matter what.
You expect your downwind loop to ''look'' the same as your upwind loop with the same control inputs. It can't! If it does, then you are making corrections and the inputs arent really the same are they? The mass of air it is flying within is moving, so by default it can't look the same with the same control inputs. By trying to make it look the same, you are flying it differently upwind than downwind. Your .40 size trainer sees EXACTLY the same airspeed no matter what the wind is doing. YOU see a difference, but the plane doesn't.
It's all about your perspective from the ground, the way the plane ''looks''. If there were a way to program your control inputs for a loop with the flip of the switch, you could easily see this by simply flying one loop upwind and one down. The loops would look quite different in shape to you on the ground, but the plane (or a person in the plane) would see/feel exactly the same. The plane doesn't care about wind, YOU do on the ground by trying to make it look the same!
Many folks here are confusing what the airplane sees (IAS) and what you see (GS). These folks just wont get it no matter what.
Nice and straight explanation.. agree 100%
What surprises me is that many people in this thread with presumingly professional experience seems to have clashing opinions and can´t agree in a consensus.. no matter who is right, there will be many more ''misterious'' crash videos on youtube on years to come.
Enrique
11-23-2010, 12:56 PM
#185
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RE: Downwind turn Myth
Forget the loops...go back to original simple question: does turning DW cause any effect re altitude?
Trim/power for straight and level flying into a significant wind. Hands off. Quickly initiate a 180 turn/let the model turn itself. Then make no control inputs, no compensations. What happens? After the 180 is complete and leveled, the model keeps losing altitude until it has fully caught up and exceeded that steady sea of air to return to the original AS. The amount of time that takes obviously proportional to the height loss....and, yeah, going pretty damn fast, now, relative to the Tx....and definitely lower in the sky.
Greater the WL/wind/the closer the AS is to the wind speed, the greater the effect. A powered feather be less noticeable; only a model w/ no mass would truly show no DW altitude loss.
Myth of the myth exploded. Period. But I know it won't be for some.
Ray
Trim/power for straight and level flying into a significant wind. Hands off. Quickly initiate a 180 turn/let the model turn itself. Then make no control inputs, no compensations. What happens? After the 180 is complete and leveled, the model keeps losing altitude until it has fully caught up and exceeded that steady sea of air to return to the original AS. The amount of time that takes obviously proportional to the height loss....and, yeah, going pretty damn fast, now, relative to the Tx....and definitely lower in the sky.
Greater the WL/wind/the closer the AS is to the wind speed, the greater the effect. A powered feather be less noticeable; only a model w/ no mass would truly show no DW altitude loss.
Myth of the myth exploded. Period. But I know it won't be for some.
Ray
11-23-2010, 12:56 PM
#186
RE: Downwind turn Myth
states that the laws of physics are the same for all observers in uniform motion relative to one another.
In other words F=MA (force = mass x acceleration) from Newton's Second Law of Motion on Earth, from a fixed point, and relative to the wind are indeed all the same.
11-23-2010, 01:24 PM
#187
RE: Downwind turn Myth
ORIGINAL: Sport_Pilot
Yes and true.
Incorrect and contradicts the former statement. F=MA from a moving body of air, and F=MA from a fixed point, and from the earth are not the same. The differ by the amounts the bodies are moving. Therefore the inertia of a plane turning in the air is completly resolved by the G-Force created from the bank and does not make any differance in headwind. F=MA relative to the ground is differant, the differance being the windspeed. Differances from inertia of wind gusts was conceeded long ago.
states that the laws of physics are the same for all observers in uniform motion relative to one another.
In other words F=MA (force = mass x acceleration) from Newton's Second Law of Motion on Earth, from a fixed point, and relative to the wind are indeed all the same.
What are you talking about? Inertia of the plane, inertia of the wind gust? Inertia isn't resolved by G-Forces. G-Forces aren't created by the bank. And we're not even talking about wind gusts. You're throwing around terms as if you actually understand them. Try applying them correctly.
Jim
11-23-2010, 01:31 PM
#188
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RE: Downwind turn Myth
I confess I have not read each post here and indeed the subject has been looked at from a variety of angles. However with fear of prolonging an already lengthy post, here is my two cents..
Indeed the airplane is traveling in an airmass and knows not whether it travels upwind, downwind or crosswind. Gusts, sheer, turbulence aside of course.
The issue which must concern us is what is described as"Illusions Created by Drift". This is simply theillusion of increased or decreased airspeed and also of skidding or slipping during turns when turning from upwind to downwind or vice versa. This illusion results from taking visual cues from outside the aircraft, rather than the flight instruments. In full sized aircraft this condition is at its most noticable when flying low and in high winds. It is taught to develope recognition and avoidance of the possible repurcussions.
In RC aircraft we observe the aircraft from outside at all times and are subject to the same illusions, if not even more so. It seems important to manage airspeed and attitude with this in mind.
Now on the subject of aviations great questions....
If a 747 has a cargo of chickens which weighs 20,000 pounds, and should those chickens all a once start to fly around in the cargo hold, does the aircraft weigh 20000 pounds less.
pjp01
Indeed the airplane is traveling in an airmass and knows not whether it travels upwind, downwind or crosswind. Gusts, sheer, turbulence aside of course.
The issue which must concern us is what is described as"Illusions Created by Drift". This is simply theillusion of increased or decreased airspeed and also of skidding or slipping during turns when turning from upwind to downwind or vice versa. This illusion results from taking visual cues from outside the aircraft, rather than the flight instruments. In full sized aircraft this condition is at its most noticable when flying low and in high winds. It is taught to develope recognition and avoidance of the possible repurcussions.
In RC aircraft we observe the aircraft from outside at all times and are subject to the same illusions, if not even more so. It seems important to manage airspeed and attitude with this in mind.
Now on the subject of aviations great questions....
If a 747 has a cargo of chickens which weighs 20,000 pounds, and should those chickens all a once start to fly around in the cargo hold, does the aircraft weigh 20000 pounds less.
pjp01
11-23-2010, 01:32 PM
#189
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RE: Downwind turn Myth
Ok guys I ve read all the post on this some I understand others not so much. I ve been flying R/C 20 plus years mostly sport planes some scale planes but never a jet or pusher type and I do have an indoor type heli which is a blast to fly as long as you keep it away from the ceiling fan....lol. I have flown in all types of wind over the years and do believe there is a definite difference when you turn downwind but also I ve learned it depends on the airfoil of the wing you are flying . Alot of people kept reffering to trainers and their flat bottom wings usually in a loop condition but how about pointing it into a headwind cutting the throttle and just "hovering " in front of you. Another point no one brings up is the prop wash over the control surfaces, the thread started out talking about the crash of a jet which I watched the video of and believed he stalled unfortunatley. Does not having any prop blast in a downwind turn come into play ? Any time I was doing a downwind turn I would just cut the throttle keep a little more altitude and make a larger than usual turn keeping the wing level and use rudder then start adding throttle keeping a good eye on altitude and work my way back to the runway, Hell its fun playing around with the headwind imo. Like I stated i ve never flown a jet but do think airfoil and having the thrust from the rear play a big part in this case. Let me know if Im off on this one, always wanting to learn more about things like this
11-23-2010, 01:41 PM
#190
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RE: Downwind turn Myth
Forget the loops...go back to original simple question: does turning DW cause any effect re altitude?
Trim/power for straight and level flying into a significant wind. Hands off. Quickly initiate a 180 turn/let the model turn itself. Then make no control inputs, no compensations. What happens? After the 180 is complete and leveled, the model keeps losing altitude until it has fully caught up and exceeded that steady sea of air to return to the original AS. The amount of time that takes obviously proportional to the height loss....and, yeah, going pretty damn fast, now, relative to the Tx....and definitely lower in the sky.
Greater the WL/wind/the closer the AS is to the wind speed, the greater the effect. A powered feather be less noticeable; only a model w/ no mass would truly show no DW altitude loss.
Myth of the myth exploded. Period. But I know it won't be for some.
Trim/power for straight and level flying into a significant wind. Hands off. Quickly initiate a 180 turn/let the model turn itself. Then make no control inputs, no compensations. What happens? After the 180 is complete and leveled, the model keeps losing altitude until it has fully caught up and exceeded that steady sea of air to return to the original AS. The amount of time that takes obviously proportional to the height loss....and, yeah, going pretty damn fast, now, relative to the Tx....and definitely lower in the sky.
Greater the WL/wind/the closer the AS is to the wind speed, the greater the effect. A powered feather be less noticeable; only a model w/ no mass would truly show no DW altitude loss.
Myth of the myth exploded. Period. But I know it won't be for some.
Sorry Ray, drop the 'quickly' from the statement and it will/won't work? It should work whether controls are moved quickly or not in your example, which it won't. By moving the controls 'quickly', you're inducing more drag, slowing the plane rapidly. Define 'significantly' and 'quickly'. The myth abides.
I still have an old VHS Bill Evans sent me of him and his crazed pals flying one of his overpowered Simitar flying wings in the desert on an extremely windy day, I'd say 40 mph minimum. At one point they bring the airplane into a hover about 10 feet from the camera. Then they 'quickly' turn down-wind - without raising power or pitching up - and come back in a huuuuge oval. As soon as the plane starts to bank, it starts to get blown downwind. Moving with the ambient air. They had to use full power to get back to the starting point. At the end, they land out of a hover and Bill has to jump on the plane to keep it on the ground.
11-23-2010, 01:41 PM
#191
RE: Downwind turn Myth
What are you talking about? Inertia of the plane, inertia of the wind gust? Inertia isn't resolved by G-Forces.
G-Forces aren't created by the bank.
11-23-2010, 01:45 PM
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RE: Downwind turn Myth
Lift generated by a wing going into a headwind will be less when you turn down wind unless you increase the trottle to make-up the difference. If the throttle setting does not change something must to keep the plane at the same elevation. Add more elevator, if you can without stalling or loose altitude and gain airspeed.
Jeff
Jeff
11-23-2010, 01:54 PM
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RE: Downwind turn Myth
ORIGINAL: highhorse
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?
Don.
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?
Don.
Your analogy is incorrect. The boat doesn't need to move through the water at a certain speed to stay afloat like a plane needs to move through the air to stay in the air.
A closer analogy might be a water skier skiing against the flow of the river with the boat pulling him just barely fast enough to stay on the surface, then turn the skiier around to ski at the same speed (throttle setting on the boat) with the flow of the river. He will sink. Because in comparison to the water around him he is not moving fast enough to stay on the surface. This same idea can be applied to a plane in the flow of moving air.
11-23-2010, 01:58 PM
#194
RE: Downwind turn Myth
He will sink. Because in comparison to the water around him he is not moving fast enough to stay on the surface. This same idea can be applied to a plane in the flow of moving air.
11-23-2010, 02:12 PM
#195
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RE: Downwind turn Myth
Even if we somehow magically cut out the turn and the skiier, instantly, was facing the opposite direction and going at the same speed on a surface of water that was moving with him instead of against him.... he would sink. This was the bases to Einstein's theory of relativity. lets take it a step further and say he wasn't moving at all compared to the rushing water. he would sink. Yet from the perspective of someone standing on the bank of the river, he is moving quite fast with the river. According to you he's moving so he should still be skiing.
11-23-2010, 02:18 PM
#196
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RE: Downwind turn Myth
Seems to me that a water skier travels upon the water not through it. In terms of comparing water and air, only a displacement hull or type of movement would be similar.
Something that get up on plane is not traveling in the water.
Something that get up on plane is not traveling in the water.
11-23-2010, 02:31 PM
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RE: Downwind turn Myth
Good analogy with the water skiier! Air or water it's the same principle if you don't generate enough lift you sink. The hull of a boat is not a good comparison since it's using boyancy (like a blimp). The skiier is similar in that he/she needs some amount of force (water pushing against the bottome of the ski) to stay above the water and ski. If that force falls below the weight of the skier they sink. Same as in an airplance except the wing generates some lift which a ski does not.
Jeff
Jeff
11-23-2010, 03:01 PM
#199
RE: Downwind turn Myth
ORIGINAL: Sport_Pilot
He sinks because now he must bank his ski to stay afloat. The ski loses lift in a bank just as a wing does. It is after all why we call them planes.
He will sink. Because in comparison to the water around him he is not moving fast enough to stay on the surface. This same idea can be applied to a plane in the flow of moving air.
[sm=thumbs_up.gif] [sm=thumbs_up.gif] [sm=thumbs_up.gif]
What makes planes (water skies or wings) turn in (or on) a fluid?
Why airplanes stall easier while performing a turn (upwind, downwind or crosswind), if the pilot is not careful (or skilful enough)?
Only the posters that can answer those questions correctly will be able to untangle the myth of the downwind stall and spin.
11-23-2010, 03:03 PM
#200
RE: Downwind turn Myth
No, you missed my point. In the example I gave the idea was to MAINTAIN 20 k IAS, , ie effectively hovering into wind and then (with the same forward cyclic position which gave 20 kIAS ) allowing the heli to accelerate when turning it downwind to try and maintain that 20 K IAS The IAS WILL fall as the machine accelerates to a groundspeed of 40 K whilst recovering that 20 K IAS, with some loss of translational lift, and resultant sink if power/torque is not added ! The sink and some loss of some (translational)lift is caused by turning downwind.
The retreating blade IS a high speed phenomenon but contrary to what someone else said a heli. is not always, at all parts of the disc, away from the stall, (or at least loss of lift because the blade angle may not be at a value at which it stalls, it is merely too slow to produce significant lift or may even suffer from reverse flow if speed is high enough) .
Of course my CFS(H) instructors at Tern Hill back in the '60s COULD have been teaching a load of rubbish or my memory really has gone to pot, but as I seem to recall that no one seems to have told the Bell 47 (Sioux in RAF speak) that the downward turn was a myth, ! !
On fixed wing is not something I have noticed !
Regards, David.
The retreating blade IS a high speed phenomenon but contrary to what someone else said a heli. is not always, at all parts of the disc, away from the stall, (or at least loss of lift because the blade angle may not be at a value at which it stalls, it is merely too slow to produce significant lift or may even suffer from reverse flow if speed is high enough) .
Of course my CFS(H) instructors at Tern Hill back in the '60s COULD have been teaching a load of rubbish or my memory really has gone to pot, but as I seem to recall that no one seems to have told the Bell 47 (Sioux in RAF speak) that the downward turn was a myth, ! !
On fixed wing is not something I have noticed !
Regards, David.