Aerodynamics Discuss the concepts of aerodynamics here

Downwind Turns

Old 10-09-2009, 01:22 AM
  #226  
Octavius
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"I'm gonna hit the brakes and he'll fly right by." See, that proves he was still on the ground. See. A hu you know what I'm talking about. Can't argue with that logic.

Or to quote the greatest pilot of all times... Bug Bunny, "Good thing this has air brakes."
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Old 10-09-2009, 05:32 AM
  #227  
starcad
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OMG this debate is still going on! I thought it died in usenet years ago when Orville died.
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Old 10-09-2009, 06:56 PM
  #228  
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It has to do with a lack of comprehension of the forces involved, and the terminology.
I'm minded of one of our U-2 test flights, where the Test Director asked the pilot where he was currently..
"Heading east, over Palmdale", was the reply.
About 20 minutes later, when asked again, he said the same thing.
He'd managed to fly into a headwind which matched his airspeed, and with power and trim kept the plane in one spot at Flight Level 600.
Groundspeed 0.
Obviously, no energy involved.
This same thing is done all the time with slopers... hovering in one spot over the slope.
And with power planes and a good non-turbulent constant wind, it's possible to takeoff without the wheels rotating at all, hover into the wind, and land on the takeoff spot without the wheels rolling.
A local flyer with his Fiesler Storch wins many spot landing contests with his plane's slow flying and handling capabilities.
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Old 10-09-2009, 08:21 PM
  #229  
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Sweet. Now downwind turns are exactly like hovering. If only I knew.
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Old 11-02-2009, 12:05 PM
  #230  
HX3D014
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Originally Posted by cbatters View Post
CASE 1
Flying downwind with windspeed of 20 and airspeed of 20.
We all agree that groundspeed (40) is irrelevant because plane only cares about airspeed over the wing.
Turn abruptly 90 degrees. Airspeed over wing is still 20.

CASE 2
Flying upwind with windspeed of 20 and airspeed of 20.
We all agree that groundspeed (0) is irrelevant because plane only cares about airspeed over the wing.
Turn abruptly 90 degrees. Airspeed over wing is 0.

PS: I too subscribe to physics including the energy required to accelerate a stationary object.
Probably because you did not do the same bank rate as the other example.

what is Abruptly?
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Old 11-02-2009, 01:43 PM
  #231  
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It occured to me what might be going on last time I was flying in windy conditions. I think there is good evidence to prove a plane will not feel the wind but despite that I was doing down wind turn then the plane dropped suddenly.

I think the reason was actually a gust of wind which had caught up with the plane. Maybe we should be considering gust response rather than the plane flying in a body of air moving at a constant speed.
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Old 11-30-2009, 08:07 AM
  #232  
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Originally Posted by Octavius View Post
You're diagram is correct but you think you've finished and really haven't. You need to continue and break it down into energy. Hint: If the ground speed changed then energy had to be added or taken out. Where did that come from and what was the result of not adding/reducing power to compensate.

I previously agreed that it was simple but it really is not. Regardless of which side, most people will look at only the major components and dwell on those. Without the fine details there can't be a complete understanding.

However I do agree it's probably pointless to take a neutral approach to enlightenment. Most will just want to fight about it and win without regards to truth. Continue...
Just wondering if that was a Question (no question mark)

PS I purchased "Super Colossal" Listened to it twice
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Old 11-30-2009, 05:58 PM
  #233  
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Originally Posted by cbatters View Post
However, I maintain that the laws of aerodynamics and various books written about aviation do not trump physics and the laws regarding Conservation of Energy. The energy state of a plane flying with the wind is higher than a plane flying into the wind and the stored energy has an impact on how much altitude is gained or lost when turning.
Remember that kinetic energy is dependent on your frame of reference. Is there any particular reason you've chosen to do your energy analysis from a reference point on the ground? Why not the center of the Earth, instead? After all, even when stationary on the ground, the plane has significant velocity due to the rotation of the Earth - instead of 0 MPH upwind and 40 MPH downwind, you could use ~1000 MPH upwind and 1040 MPH downwind, or 1000 MPH upwind and 960 MPH downwind.

If you want to do an energy analysis, you need to choose a frame of reference which is relevant to the airplane. There is only one choice, really - an arbitrary parcel of air traveling with the wind. You could also use a point on the plane itself, but then KE will always be zero and you'd need a different approach to the problem.

I quoted CBatters because it was easy, but this applies to everyone who wants to use energy, acceleration, and instantaneous direction changes to make downwind turns dangerous. The only reason they can be is the pilot's likelihood of turning much more tightly without realizing it in order to make the downwind turn the same size and shape as the upwind turn.
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Old 11-30-2009, 07:30 PM
  #234  
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Cbatters,

Aramid is right, if you want to keep the math simple then choose a convenient reference to do your energy balance analysis from.. i.e. the air surrounding the plane...

However if you a glutton for hard math you can choose any other reference point and the overall energy in your system will still be found to balance

Take the example of the plane circling in a wind using the RC pilot stood on the ground as a reference. On the downwind leg the plane is flying faster relative to the ground than on the upwind leg. The plane does indeed gain and loose kinetic energy relative to the ground as it circles and at first it's hard to see how energy can be in balance, where is the energy to accelerate the plane coming from, surely the plane must dive to gain this extra kinetic energy??

The answer to this 'problem' is to be found by also considering the air around the plane in the analysis. Air has mass and velocity therefore it too has kinetic energy. To do a proper analysis you need to consider all the 'objects' within your system that have mass, so air needs to be considered along with the plane. It's the overall combined energy of plane and air mass that must balance, not just the plane alone.

A plane in flight is constantly accelerating air downward from it's wings to produce lift, this is called downwash. When the plane is banked over this air is accelerated sideways opposite to the direction that the plane turns. This downwash, directed sideways, causes a localised 'wind' relative to the ground. This downwash wind may add or subtract from any 'normal' wind, depending on which was the plane is banked. This is hard to visualise for a model plane but if you consider a Boeing 747 banking hard around you then you can perhaps imagine the 'wind' that it will produce as it turns?

If you do an analysis of the energy of the circling plane relative to the ground you will find that the kinetic energy of the plane increases as the plane accelerates downwind however at the same time the kinetic energy of the surrounding air mass decreases because the localised wind speed drops due to downwash. With a model this would be virtually impossible to detect, but imagine the circling Jumbo jet. Proper analysis would show that the increase in kinetic energy of the plane was exactly matched by the decrease in Kinetic energy of the air mass, so overall energy is in perfect balance throughout the turn, it's just exchanged between the air and the plane. If you consider Newton's 3rd law of motion 'for every action there is equal and opposite reaction' then you will see that the force pushing the plane in one direction must be exactly balanced by a force pushing the air in the other. There is therefore no need for the plane to climb or descand as it turns in order for the plane/air system energy to remain constant, it's already constant.

i hope I've done a decent job of explaining this? There is a web site somewhere with diagrams explaining it all but I don’t have the link anymore so my words will have to suffice.

Steve

Last edited by JetPlaneFlyer; 11-30-2009 at 09:20 PM.
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Old 11-30-2009, 09:40 PM
  #235  
CHELLIE
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My Planes dont know the difference between a down wind turn or a up wind turn, they just do a Split S and be done with it LOL
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Old 11-30-2009, 09:54 PM
  #236  
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This thread needs to die.
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Old 11-30-2009, 10:14 PM
  #237  
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Originally Posted by Octavius View Post
This thread needs to die.
Dont read the thread it if you dont like it, and most of all if you dont like it dont post in it

There are countless threads over in 'off topic chit chat' that I'm sure I'd consider as pure drivel if ever took the time to read them. For pity's sake the thread with the most posts in this whole forum is titled 'Dont' post in this thread'..
The answer for me is simple, i dont read threads i dont like.. problem solved. Give it a try.


Steve
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Old 11-30-2009, 11:32 PM
  #238  
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Please spare me the zen philosophy. It's really been over done.

I have posted here and I do like it. I don't like that this threads and ones like it have absolutely nothing to do with the topic. It's only about being right, winning, presumption, trying to appear smarter than the next person, and fighting until death without an once of listening or search for truth. That's why it needs to die.
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Old 12-01-2009, 12:07 AM
  #239  
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Originally Posted by Octavius View Post
Please spare me the zen philosophy. It's really been over done.

I have posted here and I do like it. I don't like that this threads and ones like it have absolutely nothing to do with the topic. It's only about being right, winning, presumption, trying to appear smarter than the next person, and fighting until death without an once of listening or search for truth. That's why it needs to die.


Please point out what in my reply to Cbatters was not squarely 'on topic' ? Did you actually read it? if you think I'm wrong say so, i wont get upset.

I take it by 'the truth' you are in fact referring to your own opinion ? (which to be honest i have no recollection of)

Steve
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Old 12-01-2009, 12:54 AM
  #240  
Octavius
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No, actually I do not mean that. You're mistaken.
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Old 12-01-2009, 12:56 AM
  #241  
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Now were "offtopic" for sure,,it's about toy airplanes,, Nothing else, Back to windy fly'in,, bubsteve
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Old 12-01-2009, 04:12 AM
  #242  
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Originally Posted by Octavius View Post
This thread needs to die.
best point so far in this thread.
i vote for closing this thread and banning the topic from ever being mentioned again.
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Old 12-02-2009, 07:30 PM
  #243  
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it is a discussion.

Discuss or disappear.

Octavius, I re read the thread and was stumped at your post. thus the quote.

If you do not wish to discuss it, then just ignore it or tell us you are not up to it or what ever, what you asked (without question marks) is relevant. I thought you might be up for some information.

PS

Are they still teaching addition at schools. I mean, I already know it and it has been done to death already (I already know my addition so why are they still talking about it)

Best point in this forum so far (of those in this thread)
"Don't read the thread it if you don't like it, and most of all if you don't like it don't post in it

There are countless threads over in 'off topic chit chat' that I'm sure I'd consider as pure drivel if ever took the time to read them. For pity's sake the thread with the most posts in this whole forum is titled 'Don't' post in this thread'..
The answer for me is simple, I don't read threads I don't like.. problem solved. Give it a try.


Steve "
and lastly;
"Without the fine details there can't be a complete understanding
Octavius
"

It is a bloody good point and the reason the thread exists.

Bryce.
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