Aerodynamics Discuss the concepts of aerodynamics here
View Poll Results: Are the Josh & Josh right?
Nope.. they are clueless
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64.52%
Yes.. they have is spot on
8
25.81%
I dont know /dont care
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9.68%
Voters: 31. You may not vote on this poll

Flitetest join the downwind turn debate!

Old 10-30-2011, 04:27 PM
  #1  
JetPlaneFlyer
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Default Flitetest join the downwind turn debate!

Here's what Josh and Josh have to say about flying into wind vs. flying away from wind: http://www.youtube.com/user/flitetest#p/u/4/328de9cscNw

In a nutshell the Joshes are claiming that because of the wind we feel on the ground then in flight the 'wind' that the plane feels, measured by an anemometer fixed to the plane (aka 'airspeed') is higher when flying into the wind than it is when flying downwind. They then warn us that due to this difference in airspeed the plane will have different flying characteristics when going into wind vs. downwind.

Comments?

I added a poll for fun

Steve
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Old 10-31-2011, 09:37 AM
  #2  
Larry3215
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LOL

Good one Steve!

I wasnt too surprised to see the vote was going their way until I made it even up. It will be interesting to see how this goes over time
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Old 10-31-2011, 11:42 AM
  #3  
slipstick
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There's physics and then there's opinion (educated or not).

In a true democracy opinion will always win and the universe should just get on with it and do what "the people" think is obviously right .

Steve
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Old 10-31-2011, 12:14 PM
  #4  
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I didn't get much out of that episode... I got bored.
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Old 10-31-2011, 12:34 PM
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Well I don't know about physics, nor do I pretend to but I do know from bitter personal experience that when I was flying my high wing loading ( carved solid balsa wing) glow powered fun fighters and they went dead stick while flying downwind I would lose aileron response far more quickly than if the same thing happened while flying into wind. I know from these experiences that my planes ( don't know about anyone else) do behave differently under these conditions.
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Old 10-31-2011, 03:25 PM
  #6  
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By definition failure to maintain a consistent airspeed is pilot failure.

If the windspeed is constant and the pilot is flying properly airspeed upwind and downwind will be the same.

Now, flying upwind and downwind are often a bit different if there is any gustiness or turbulence because an increase in windspeed while flying into the wind is an airspeed increase, and vice versa. You can see this even on a sub-2-oz micro which accelerates with the wind very quickly, and I have a lot of time on micros in the wind.

If you are flying downwind and the ailerons get mushy it's because you have reduced throttle or increased attitude and slowed.

One of my favorite things to do with the micros is to fly into a strong wind, full throttle, bank suddenly and fly with the wind, then turn very sharply back into the wind, the plane snaps around on both legs but the downwind turn feels very very different because the plane decelerates instantly and then hovers (according to groundspeed).


Dave
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Old 10-31-2011, 05:10 PM
  #7  
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Dave's comments above sum up my thoughts perfectly, that's why I voted that Josh and Josh were had it wrong this time (maybe 'clueless' was a bit harsh).

Not taking anything away from their other work, some of which I think is very well done.

Baz,
Here's asome thought experiments for you (and anyone else who agrees with Josh & Josh): http://aeroexperiments.org/brainteasers.shtml
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Old 10-31-2011, 05:28 PM
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At the bottom of that list, I have to dispute that downwind turns are not dangerous- if people screw up during a particular phenomena, it doesn't matter if it's the wind or the pilots reaction, an increased event rate indicates an increased risk rate.
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Old 10-31-2011, 06:04 PM
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"I dont know /dont care"

For me it's a no brainer, that given a particular airfoil/Aircraft type/weight etc. each will fly just as you wanted... given the proper thrust in the transition of said turn will keep you from stalling.

Hell, I'm not concered about it at all. As long as I have the right altitude and speed into the turn, I know I will fly again!

but, frankly I prefer to stay within a certain wind conditions of under 10mph.

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Old 10-31-2011, 06:47 PM
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Larry3215
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Originally Posted by CNY_Dave View Post
At the bottom of that list, I have to dispute that downwind turns are not dangerous- if people screw up during a particular phenomena, it doesn't matter if it's the wind or the pilots reaction, an increased event rate indicates an increased risk rate.
Very good point. I think thats why this particular 'myth" persists the way it does. Especially for RC pilots.

We are not used to or trained to be thinking about air speed vrs ground speed. The very strong perception we have of ground speed makes it difficult for most to let the model accelerate relative to the ground and fly "faster" down wind.
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Old 10-31-2011, 06:55 PM
  #11  
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Just for fun I would love to install a stall warning, a nice loud piezo buzzer. Would be great for showing folks what the *plane* thinks is going on...

The concept is fer sure simple enough.

Dave
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Old 11-01-2011, 01:35 AM
  #12  
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These guys just don't understand the difference betweeen airspeed and ground speed and are in love with hearing themselves talk. And yes, it was boring.
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Old 11-01-2011, 02:12 AM
  #13  
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Originally Posted by JackM View Post
These guys just don't understand the difference betweeen airspeed and ground speed and are in love with hearing themselves talk. And yes, it was boring.
Correct
and Correct
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Old 11-01-2011, 03:42 AM
  #14  
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Conceptually, they are right. Apparent wind speed is greater when flying into the wind at a given throttle setting than when flying downwind at the same throttle setting (for a heavy aircraft, immediately after turning downwind).

Many of the airplanes that we fly are so light that, upon turning downwind, they basically become aerial tumbleweeds. They quickly match the speed of the wind. Add the power from the motor, and apparent wind speed almost as quickly becomes equal to upwind.

A heavier airplane won't react as quickly. The difference between upwind and downwind airspeed is more pronounced and needs to be considered. (But only if flying very near stall)

We really need to worry about upwind/downwind because of ground speed. Land and take off upwind to minimize ground speed.
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Old 11-01-2011, 08:29 AM
  #15  
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Originally Posted by earthsciteach View Post
Conceptually, they are right. Apparent wind speed is greater when flying into the wind at a given throttle setting than when flying downwind at the same throttle setting (for a heavy aircraft, immediately after turning downwind).
Earth,
No, on this one we disagree, what you describe is the classic downwind turn misapprehension.
The plane's momentum is relative only to the air it flies in. It has no 'inertial connection' to the ground therefore it doesn't respond in any way to it's apparent acceleration over the ground when it makes a downwind turn.

This can be quite hard for us ground dwelling humans to grasp because our brains are wired for working a ground based frame of reference and it's hard to brake away from it. It's this human error factor that makes the downwind turn dangerous, not physics.

Read and ponder on the first thought experiment here: http://aeroexperiments.org/brainteasers.shtml

Steve
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Old 11-01-2011, 08:53 AM
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Originally Posted by JetPlaneFlyer View Post
Dave's comments above sum up my thoughts perfectly, that's why I voted that Josh and Josh were had it wrong this time (maybe 'clueless' was a bit harsh).

Not taking anything away from their other work, some of which I think is very well done.

Baz,
Here's asome thought experiments for you (and anyone else who agrees with Josh & Josh): http://aeroexperiments.org/brainteasers.shtml
I didn't say I agreed with them I haven't even viewed the thread. What I did say was that in the past I flew very high wing loading 36" span solid balsa models with 25 glow motors, 4oz tanks, brick servos, 500 nicads and receivers the size of a small radio in them.
The resulting models flew like a brick and would only fly with any authority on full power. If the motor cut you only had seconds to shove the nose down and land fast. The difference between doing this into wind as opposed to downwind was the increased over the ground speed that was needed to maintain control when forced to land downwind.

Baz
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Old 11-01-2011, 04:20 PM
  #17  
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I think one reason this is a little confusing has to do with the energy involved.

A plane flying in a 20mph headwind, flying at an airspeed of 20mph, has how much kinetic energy?

To answer that, the question has to be re-specified to ask 'kinetic energy relative to what?' The ground? The air? Two swallows heading south carrying a coconut?

Unless you plan on contacting something on the ground, or landing, or taking off, in smooth wind/air your kinetic energy relative to the ground doesn't matter.


Dave
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Old 11-01-2011, 07:40 PM
  #18  
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Originally Posted by CNY_Dave View Post
To answer that, the question has to be re-specified to ask 'kinetic energy relative to what?' The ground? The air? Two swallows heading south carrying a coconut?

Dave
African or European?

http://youtu.be/H4_9kDO3q0w

After gripping it by the husks would the swallow be able to make a downwind turn?

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Old 11-07-2011, 12:34 AM
  #19  
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The test is flawed.

If I were to do such a test, I would ;

a/ pick a very open flat area
b/ indicate the Prevailing wind direction and make clear to the viewer which way it is coming from
c/ Make sure the Anemometer can be clearly seen by the camera (LOL) polarized ?

d/ let the viewer know that I have a Set throttle . IE Switched to a setting via a Switch. so as to be clear there were no throttle changes during the turn.
c/ use a aircraft with more stability. IE probably a big piper cub. or some thing of equal Slowness.
d/ Indicate to the viewer that the turn may be done however, (that's not the topic in this one) but once the Air speed and ground speed reach a constant setting, no more Accelerating etc. then and only then will the figures be read.
e/ Have two up in the air at the same time doing opposite runs to each other at the same time so that any change in wind speed may be better seen.
f/ have an anemometer positioned stationary some where in the flight level to give further indication to "e/"

but why do all that.

Just take a real aircraft up on a windy day (not gusty) use similar GPS and point the camera at the IAS.



On another point.

When they were trying to get the brick to fly, it never got out of Ground effect , for long, so it was not really a good test to the theory, especially when you take into consideration the change in wind speed as you get close to the ground.
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Old 11-07-2011, 01:13 AM
  #20  
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My point, earlier in the thread was, this is ONLY a concern when flying very near stall speed for a heavy aircraft (one that will not be easily moved by the wind). Add to that, it only matters if the headwind is significant. Significance depends on the wing loading of the aircraft and its proximity to stall.

Lets say that an airplane is flying into the wind (granted, into the wind is a relative thing because, in order to stay in the air, the airspeed over the wing must be above stall) at 10 mph above its stall speed. Lets just say that its stall speed is 50 mph IAS and the wind speed relative to ground is 20 mph. If it is 10 mph above stall, it is flying at 60 mph IAS and 40 mph ground speed.

Now, lets say the pilot kicks rudder and turns downwind without adjusting throttle setting. (We'll ignore any banking because that will only exacerbate things) and swings the nose around, pointing downwind. If it happens quickly, and the throttle is not adjusted so that the power of the motor doesn't increase airspeed (say its at idle,why not) the airplane is now 10 mph below stall speed (60 mph IAS-20 mph due to wind from tail=40 mph) and will stall.

Granted, this is a very specific situation and no pilot would ever allow themselves to get into this position knowingly. Ever land an airplane downwind? If you are piloting it, you will feel even a moderate wind push you down onto the runway when you flare. It is palpable.

At cruising speed, the whole discussion is pointless.
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Old 11-07-2011, 02:08 AM
  #21  
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Originally Posted by rcers View Post
Correct
and Correct
+1

The FT crew puts out a quality product, but yeah occasionally the Joshes run the wrong direction with their understanding. Its tough to put out quality content, and be right all the time.

Originally Posted by rcers View Post
African or European?

http://youtu.be/H4_9kDO3q0w

After gripping it by the husks would the swallow be able to make a downwind turn?

I almost spit my coffee. Timeless. Thanks for linking that.
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Old 11-07-2011, 02:14 AM
  #22  
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Ok, I clicked on the link that RCers posted and have to give it to him! He definitely wins the debate!

Who are you, who are so wise in the ways of science?
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Old 11-07-2011, 08:04 AM
  #23  
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Originally Posted by earthsciteach View Post
My point, earlier in the thread was, this is ONLY a concern when flying very near stall speed for a heavy aircraft (one that will not be easily moved by the wind). Add to that, it only matters if the headwind is significant. Significance depends on the wing loading of the aircraft and its proximity to stall.
Earth,
The plane might stall when it turns because turning increases g loading, but it would meke no difference which way the plane turned relative to wind measured on the ground. All that matters to a plane is airspeed and airspeed is measured relative to the air. the relative movement of the ground below makes absolutely do difference to the dynamics of the plane in flight. The plane has to perform exactly the same acceleration regardless of what way it turns. the acceleration the plane experiences in the turn is related ONLY to it's flying speed and radius of turn. The ground doesnt come into it.

Formula for acceleration in turn: a = V^2 / r

Radius of turn is measured relative to the air mass the plane is flying through, not the ground.

Steve

Last edited by JetPlaneFlyer; 11-07-2011 at 07:38 PM.
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Old 11-07-2011, 01:55 PM
  #24  
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the smaller the radius with same speed, the greater the acceleration ?

so then that would mean that the V^2 would need to be divided by the Radius?
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Old 11-07-2011, 03:24 PM
  #25  
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Originally Posted by Nitro Blast View Post
+1

The FT crew puts out a quality product, but yeah occasionally the Joshes run the wrong direction with their understanding. Its tough to put out quality content, and be right all the time.



I almost spit my coffee. Timeless. Thanks for linking that.

Never any glory for the straight man who does the setup... sigh...

Dave
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