Aerodynamics Discuss the concepts of aerodynamics here

Downwind Turns

Old 03-02-2009, 02:38 AM
  #76  
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Originally Posted by HX3D014 View Post
I have just realised that there are people out there that belive a downwind turn is Dangerous or that the plane will behave differently through the air when turning downwind.

For Educational purposes;
Downwind turn is when you turn to the direction of the wind. IE you nose is pointing the same direction as the wind sock (Or close to it)

It can be made in two basic ways.

You are flying cross wind and turn 90deg to the downwind direction

or

you are flying into the wind and turn 180deg to the downwind direction.

Any of us here belive the myth ?

Are there any people here who think that downwind turns cause the Aircraft to fly differently throught the air?
Nice job of goading the group into a heated arguement. Well done.

So, what is this notion of a dangerous turn? As long as the air speed exceeds stall, the plane should be in no danger. Of course an inexpeirienced pilot can mess up no matter what the wind does, or the air speed.

Last edited by AEAJR; 03-02-2009 at 12:03 PM.
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Old 03-02-2009, 02:40 AM
  #77  
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leave it to AEAJR to ruin the fun... I had popcorn and everything! just kidding AEAJR!
offtopic:btw AEAJR thanks for the great posts you have on gliders/sailplanes!

I like my opinion on wind:
it's dangerous. end of story!
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Old 03-02-2009, 02:56 AM
  #78  
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Originally Posted by pd1 View Post
I agree with your nephew, there is a difference turning in a thirty knot wind. The turns are either elongated or compressed depending on your turn direction.

But the wind will not drop your airspeed. The airspeed is unaffected by steady state wind.
Groundspeed is.
Wind shear or turbulence is another matter.

Paul
No one is talking about groundspeed or ground reference as it is irrelevant for the airplane. The comment from my nephew pilot is that upwind/downwind turns in high winds require very different control input even for what amounts to gradual turns. There obviously is a difference even for a Cesna 172.


Clint
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Old 03-02-2009, 03:01 AM
  #79  
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Yes and I just broke open a bottle of Tequila and wanted to have a toast, to keeping us all on our toes and thinking.

Peace brothers, no dissention here with me.

But the biggest danger there is in the air IMO is a pilot allows his ground instincts to take over and make him do the wrong thing at the wrong momment.
Wolfgang wrote in his book about getting rid of these ground bred instincts and how he must do what his self preserving instinct is telling every bone in his body not to. And believe me, it takes real daring and courage to push the stick forward when the ground is rushing up at you.
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Old 03-02-2009, 03:02 AM
  #80  
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Originally Posted by HX3D014 View Post
Starting from the first Fault. how do you get 0 airspeed.
Plane had zero kinetic energy and is stationary. As it turns away from the wind, it will initially have zero velocity and will have to accelerate from 0 to build up airspeed. (Wind is still coming over right wing at 20 MPH generating zero lift.)


Clint
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Old 03-02-2009, 03:03 AM
  #81  
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"Plane flying 20 MPH into a 20 MPH headwind (airspeed = 20, groundspeed = 0)
hard turn left 90 degrees
Airspeed = 0
hard turn another 90 degrees
Airspeed = -20"

The bizarreness just went ballistic!
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Old 03-02-2009, 04:07 AM
  #82  
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Originally Posted by cbatters View Post
Plane had zero kinetic energy and is stationary. As it turns away from the wind, it will initially have zero velocity and will have to accelerate from 0 to build up airspeed. (Wind is still coming over right wing at 20 MPH generating zero lift.)


Clint
In your mind, can you see that if you think it can happen in the case of;
Plane flying 20 MPH into a 20 MPH headwind (airspeed = 20, groundspeed = 0)
hard turn left 90 degrees
Airspeed = 0
hard turn another 90 degrees
Airspeed = -20
then it would be true for ;

Plane flying 20 MPH with a 20 MPH tailwind (airspeed = 20, groundspeed = 40)
hard turn left 90 degrees
Airspeed = 0
hard turn another 90 degrees
Airspeed = -20
For what ever plane can spin instantly while flying against the wind can do the same while flying with the wind.

Bryce.

PS
AEAJR;
I am proud of how well the group is handling this. I only wish we could send little Electric shocks to one another (Bzzz) that would be interesting

Howdy from the land down under .

cbatters: Just send me a couple of (Bzz Bzzzz Bzzzzzzzzzzzzzzzzzz shocks )Bryce.
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Old 03-02-2009, 04:28 AM
  #83  
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Airspeed = 0

Oh come on Clint.
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Old 03-02-2009, 08:19 AM
  #84  
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Originally Posted by Buck Rogers View Post
I think cbatters is on the money here. Yes, the plane does not know the wind but the fact remains that it has to accelerate when making the turn. It might be better to think about this problem in terms of conservation of energy.

In your example the plane is travelling at 0 mph ground speed in the up wind direction and 40 mph ground speed on the down wind leg. Therefore there is a change in the planes energy state if you consider the kinetic energy (relative to the ground).

If KE = 1/2 m x V^2

lets assume the plane weight 1kg then

The in the up wind state KE = 1/2 x 1 x 0 = 0

In the down wind state th lane is travelling at a ground speed of 40 mph or 17.9 m/s

That means now the KE = 1/2 x 1 x 17.9^2 = 160 Jules

So where did this energy come from? The planes power was set to equal the drag at 20 mph IAS and this has not changed during the manoeuvre. There could be an exchange from the planes potential energy (as a result of altitude) to the planes KE which would explain a loss in height. The only other way you can balance the equation is to add power during the turn.

Ok, a bit more explanation needed on this I think.

Since I didn't consider the up wind turn I didn't realise an important point.

In the down wind case the plane goes from 0 mph to 40 mph (ground speed) needing 160 joules of energy

In the up wind case the plane goes from 40 mph to 0 mph or +40 mph in the opposite direction so the same amount of energy is used in the up wind turn as the down wind turn.

This would seem right because, as pd1 said, you need to increase the AOA to do the turn which in turn leads to a increase in drag and therefore an increase in the power needed to maintain the same speed. This is true in real aircraft too because you have to add power in a turn to maintain speed without losing height.

But the point is that change in energy state is the same whether the turn be up wind, down wind or even if there is no wind. So in theory at least a down wind turn should be no different to an up wind turn.

In practise, I think the problem is more to do with how the turn is done - as I said earlier because it is being done with reference to the pilot on the ground the path flown in the air is not a constant rate turn.

If when turning down wind the plane is turned faster than normal and if the throttle is reduced at the same time (because the plane appears to be speeding up) this could certainly cause a stall.

So, if someone has a downwind turn crash, its probably down to pilot error.
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Old 03-02-2009, 08:51 AM
  #85  
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Originally Posted by Buck Rogers View Post
Ok, a bit more explanation needed on this I think.

Since I didn't consider the up wind turn I didn't realise an important point.

In the down wind case the plane goes from 0 mph to 40 mph (ground speed) needing 160 joules of energy

In the up wind case the plane goes from 40 mph to 0 mph or +40 mph in the opposite direction so the same amount of energy is used in the up wind turn as the down wind turn.

This would seem right because, as pd1 said, you need to increase the AOA to do the turn which in turn leads to a increase in drag and therefore an increase in the power needed to maintain the same speed. This is true in real aircraft too because you have to add power in a turn to maintain speed without losing height.

But the point is that change in energy state is the same whether the turn be up wind, down wind or even if there is no wind. So in theory at least a down wind turn should be no different to an up wind turn.

In practise, I think the problem is more to do with how the turn is done - as I said earlier because it is being done with reference to the pilot on the ground the path flown in the air is not a constant rate turn.

If when turning down wind the plane is turned faster than normal and if the throttle is reduced at the same time (because the plane appears to be speeding up) this could certainly cause a stall.

So, if someone has a downwind turn crash, its probably down to pilot error.
Very good

Its all very good


I suppose as RC Pilots we have no excuse. IE we have our Thumb/Pincers on the sticks all the time. but a Real world Fixed wing pilot dose not always have their hand on the Throttle and so can easily lose conscious thought of the Throttle setting.

Bryce.

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Old 03-02-2009, 10:24 AM
  #86  
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Originally Posted by Buck Rogers View Post
Because we are observing the plane from the ground, not the air mass and what we are interested in is the planes position relative to the ground.
Speak for yourself. I'm interested in how the plane flies in the air. I have no interest in its position relative to the ground.

But is it really your claim that the physics of model aircraft is different from that of piloted aircraft because we observe one from the ground and the other we observe from in the cockpit ?

I hope you're just having a laugh and you don't really believe all this .

Steve
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Old 03-02-2009, 10:38 AM
  #87  
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Originally Posted by HX3D014 View Post
I suppose as RC Pilots we have no excuse. IE we have our Thumb/Pincers on the sticks all the time. but a Real world Fixed wing pilot dose not always have their hand on the Throttle and so can easily lose conscious thought of the Throttle setting.
I think it's almost the other way round. R/C pilots have the throttle too easily available and the additional problem of only being able to see from their ground reference so they fiddle with the throttle to make the plane LOOK better from the ground and then have to fiddle with it again as they realise that they didn't need the original adjustment. In full-size aircraft the pilot sees the actual behaviour from inside where true airspeed is more easily available than perceived groundspeed so he knows he has no need to mess with the throttle.

As "Buck Rogers" finally realised the real RC-only downwind turn problem is when you turn downwind, the plane looks like it's going too fast and you back off the throttle, then the plane falls from the sky. You're looking at a groundspeed well over the stall speed but the airspeed, which you can't see, is far too low. It's pilot error but it's a very easy error for new RC pilots make.

Steve
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Old 03-02-2009, 11:08 AM
  #88  
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Originally Posted by slipstick View Post
Speak for yourself. I'm interested in how the plane flies in the air. I have no interest in its position relative to the ground.

But is it really your claim that the physics of model aircraft is different from that of piloted aircraft because we observe one from the ground and the other we observe from in the cockpit ?

I hope you're just having a laugh and you don't really believe all this .

Steve
Of course the physics are the same no matter where you are in relation to the plane. What I am suggesting is that the control input from the pilot on the ground can be different from a pilot in the cockpit because their perception of what is hapenning is different.

Regarding how the physics works out, it makes no difference which point of reference you use.

In the example, on the ground the plane appears to have accelerated from 0 mph to 40 mph during the turn. From the perspective of the air mass the plane has accelerated from 20 mph to -20 mph relitive to a fixed point and direction within the air mass - this is also a difference of 40 mph.
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Old 03-02-2009, 12:03 PM
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Originally Posted by slipstick View Post
I think it's almost the other way round. R/C pilots have the throttle too easily available and the additional problem of only being able to see from their ground reference so they fiddle with the throttle to make the plane LOOK better from the ground and then have to fiddle with it again as they realise that they didn't need the original adjustment. In full-size aircraft the pilot sees the actual behaviour from inside where true airspeed is more easily available than perceived groundspeed so he knows he has no need to mess with the throttle.

As "Buck Rogers" finally realised the real RC-only downwind turn problem is when you turn downwind, the plane looks like it's going too fast and you back off the throttle, then the plane falls from the sky. You're looking at a groundspeed well over the stall speed but the airspeed, which you can't see, is far too low. It's pilot error but it's a very easy error for new RC pilots make.

Steve
That is worth Pondering.

Seems to me, to be a valid View pint.

The more I think of it, The more I am beginning to think that way as well.


Of cause......... Any pilot worth their weight in popcorn would check the ASI and use the knowledge passed on regarding setting throttle correctly for a given turn rate etc. (I have never flown a Real Airplane, just helicopters and only 14hrs instruction at that, and even that was 10yrs ago)

Mmmm... I may even take that previous posts statement back soon!
This part that is
I suppose as RC Pilots we have no excuse. IE we have our Thumb/Pincers on the sticks all the time. but a Real world Fixed wing pilot dose not always have their hand on the Throttle and so can easily lose conscious thought of the Throttle setting.

Revising this statement
Give me time on that
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Old 03-02-2009, 12:07 PM
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Enjoy boys!
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Old 03-02-2009, 12:10 PM
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Originally Posted by slipstick View Post
the real RC-only downwind turn problem is when you turn downwind, the plane looks like it's going too fast and you back off the throttle, then the plane falls from the sky. You're looking at a groundspeed well over the stall speed but the airspeed, which you can't see, is far too low. It's pilot error but it's a very easy error for new RC pilots make.
Nicely Summerised!

mind if I use it ?

Last edited by HX3D014; 03-02-2009 at 12:14 PM. Reason: Summerised
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Old 03-02-2009, 12:16 PM
  #92  
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Throttle? You guys use throttles? Then where is the issue with a down wind turn? You have your own wind with you. That spinny thing takes care of the wind.

Now, if you want to have some fun, make those down wind turns in 15 mph winds with a pure glider. NOW you really get to know how a plane flies. No wind machine attached to the plane.

Motors? We don't need no stinkin motors!

If you really want to learn to fly, leave the motor home. Fling that plane off a catapult to 500 feet. Fly upwind for 1/2 mile and find some warm rising air. Drift down wind while you rise in the thermal. Then come back 1/2 mile from down wind into a 15 mph wind.

Then you can have a discussion about upwind and down wind turns and how a plane flies in the wind.

Motors? Sheese!
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Old 03-02-2009, 12:36 PM
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Originally Posted by AEAJR View Post
Throttle? You guys use throttles? Then where is the issue with a down wind turn? You have your own wind with you. That spinny thing takes care of the wind.

Now, if you want to have some fun, make those down wind turns in 15 mph winds with a pure glider. NOW you really get to know how a plane flies. No wind machine attached to the plane.

Motors? We don't need no stinkin motors!

If you really want to learn to fly, leave the motor home. Fling that plane off a catapult to 500 feet. Fly upwind for 1/2 mile and hand out in the thermal lift for 10 minutes. Drift down wind while you rise in the thermal. Then come back 1/2 mile from down wind into a 15 mph wind.

Then you can have a discussion about upwind and down wind turns and how a plane flies.

Motors? Sheese!
Dohhhh...

You got us...

We should call it an "Electric Motor Power Setting Adjuster Thingy Stick" being an Electrics only thread Or just call it the "Doodad"

hehe. Gliders are Great also eh. Never flown one myself. I think Dynamic Soaring would be challenging. I Must try it someday.

And Thermal Soaring one day

Bryce.
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Old 03-02-2009, 01:20 PM
  #94  
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Clint, As I said before I agree with your nephew, Turns are made differently upwind from downwind.

Lesson one in full sized planes.
Rectangular pattern around a point on the ground.

This teaches the student to compensate for the wind and prepares the student for flying in the traffic pattern.

Part of the lesson is to demonstrate to the student that the wind does not effect airspeed, only ground speed.
The airspeed indicator indicates air pressure supplied from a pitot tube.
It is calibrated to show "airspeed". On each of the four legs the students attention is brought to the airspeed indicator so the student can actually see the airspeed is the same whether upwind downwind or crosswind.

On each leg the student trims the plane for hands off and notes there is no change in compass heading including the crosswind legs.
The only thing that the wind does is change the track across the ground.

Although turning downwind doesn't "cause" the accident, it does contribute to the pilots visual misinformation.
This problem is hard to grasp until properly explained and demonstrated, so much so that it is an included part of every students training.

I've done that same first lesson with students for over thirty years. I've done it in hundreds of airplanes. Not one has acted differently.

I hope you get a chance to fly the rectangle with your nephew, it will make visualizing whats happening much easier.
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Old 03-02-2009, 01:37 PM
  #95  
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Too much wind, here.
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Old 03-02-2009, 01:49 PM
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Comment from Nephewewas that upwind/downwind turns are different having nothing to do with ground speed or reference.


Clint


Originally Posted by pd1 View Post
Clint, As I said before I agree with your nephew, Turns are made differently upwind from downwind.

Lesson one in full sized planes.
Rectangular pattern around a point on the ground.

This teaches the student to compensate for the wind and prepares the student for flying in the traffic pattern.

Part of the lesson is to demonstrate to the student that the wind does not effect airspeed, only ground speed.
The airspeed indicator indicates air pressure supplied from a pitot tube.
It is calibrated to show "airspeed". On each of the four legs the students attention is brought to the airspeed indicator so the student can actually see the airspeed is the same whether upwind downwind or crosswind.

On each leg the student trims the plane for hands off and notes there is no change in compass heading including the crosswind legs.
The only thing that the wind does is change the track across the ground.

Although turning downwind doesn't "cause" the accident, it does contribute to the pilots visual misinformation.
This problem is hard to grasp until properly explained and demonstrated, so much so that it is an included part of every students training.

I've done that same first lesson with students for over thirty years. I've done it in hundreds of airplanes. Not one has acted differently.

I hope you get a chance to fly the rectangle with your nephew, it will make visualizing whats happening much easier.
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Old 03-02-2009, 01:52 PM
  #97  
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So you notice a difference in upwind / downwind turns in a glider? Can you confirm that you lose more altitude when transitioning to downwind leg? (Need to drop the nose to regain airspeed when turning downwind)


Clint

Originally Posted by AEAJR View Post
Throttle? You guys use throttles? Then where is the issue with a down wind turn? You have your own wind with you. That spinny thing takes care of the wind.

Now, if you want to have some fun, make those down wind turns in 15 mph winds with a pure glider. NOW you really get to know how a plane flies. No wind machine attached to the plane.

Motors? We don't need no stinkin motors!

If you really want to learn to fly, leave the motor home. Fling that plane off a catapult to 500 feet. Fly upwind for 1/2 mile and find some warm rising air. Drift down wind while you rise in the thermal. Then come back 1/2 mile from down wind into a 15 mph wind.

Then you can have a discussion about upwind and down wind turns and how a plane flies in the wind.

Motors? Sheese!
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Old 03-02-2009, 02:02 PM
  #98  
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Of course the physics are the same no matter where you are in relation to the plane. What I am suggesting is that the control input from the pilot on the ground can be different from a pilot in the cockpit because their perception of what is hapenning is different.
At last you got there...there is no problem with the way the plane is flying which requires any action, the "problem" is purely caused by the pilot's incorrect perception of what the plane appears to be doing. And that is why it's a problem for pilots standing outside the plane flying it and not a problem for pilots sitting in and moving with the plane.

I believe we may now have achieved enlightenment .

Steve
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Old 03-02-2009, 02:13 PM
  #99  
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Originally Posted by dogsheep View Post
Too much wind, here.
]

I agree. Fortunately there is a lot of it. and it is hot air. There should be enough to thermal my Ford Taurus.

Last edited by AEAJR; 03-02-2009 at 04:13 PM.
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Old 03-02-2009, 03:08 PM
  #100  
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Originally Posted by slipstick View Post
At last you got there...there is no problem with the way the plane is flying which requires any action, the "problem" is purely caused by the pilot's incorrect perception of what the plane appears to be doing. And that is why it's a problem for pilots standing outside the plane flying it and not a problem for pilots sitting in and moving with the plane.

I believe we may now have achieved enlightenment .

Steve
Precisely, and so we must arm ourselves with the knowlege of what "is" happening up there, and ignore what our eyes are trying to tell us and fly the plane normally, turn the plane normally, climb and descend normally. For it is flying normally in spite of how it may look from down here.

In a headwind, a tailwind, or a crosswind the plane flys normally, so long the pilot flys it normally. His airspeed is the same in a headwind, tail wind, or crosswind. So he must do nothing fancy with his controls, just fly the turn normally. No increase or decrease of the throttle(unless of course he wishes to climb or descend through the turn). and use the elevator to hold altitude as he would normally.
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