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  #71  
Old 02-15-2019, 07:18 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by rongawer View Post
Speaking relative to the wind, regardless of how the wind speed slows or increases, the aircraft speed within the volume of that wind will be unchanged. Relative to the ground, it has changed.

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Sorry for the thread drift, but, Ron, the above quote is just not correct.
Imagine that you're landing to the west, and above 20' the wind is out of the west at 40 knots. Below 19' the wind is calm. At 21' your airspeed is 60 knots (20 knots over the ground). If you lose 3' of altitude, your ground speed is still 20 knots (imagine an observer on the ground - he won't see you suddenly accelerate). Your airspeed will be 20 knots, too, until the plane can accelerate back to 60 - which it will do, if it doesn't hit the runway first.
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  #72  
Old 02-15-2019, 07:58 PM
NinerBikes NinerBikes is offline
 
Join Date: Dec 2018
Location: Granada Hills
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When's the last time all of you went out and flew your 3x touch and goes to stay current?

Don't get rusty, and keep in mind, rongawer flies pretty much close to an hour, daily, during the work week, weather permitting, commuting to and from work.

Stay sharp and proficient! Is it just me, or is anyone else getting cabin fever too, and needs to go fly some?
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  #73  
Old 02-15-2019, 08:06 PM
rongawer rongawer is offline
 
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Location: Brentwood, CA
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Quote:
Originally Posted by BobTurner View Post
Sorry for the thread drift, but, Ron, the above quote is just not correct.
Imagine that you're landing to the west, and above 20' the wind is out of the west at 40 knots. Below 19' the wind is calm. At 21' your airspeed is 60 knots (20 knots over the ground). If you lose 3' of altitude, your ground speed is still 20 knots (imagine an observer on the ground - he won't see you suddenly accelerate). Your airspeed will be 20 knots, too, until the plane can accelerate back to 60 - which it will do, if it doesn't hit the runway first.
Bob, in your example, you’ve defined a tremendous transition air layer boundary change of 40 knots that would likely present itself as wind shear for the landing pilot. But let’s use your example. Noting that airspeed is based on the wind relative to the airframe and measured by a pitot tube, if the wind speed above 20’ increased in speed to 45 knots, assuming you stayed level and made no power changes, how fast would you now being going in KIAS? What would your ground speed now be?

In reference to my original post of landing previously this week, I spoke of landing with 30KT ground speed, and that discussion has now morphed into landing in wind sheer (more than one volume of air interfacing at differing speeds and generally right angles) and compensating for gust (when one volume of air changes directions or roils within another), a diversion from the point of the my post. However, you’re speaking of multiple volumes of air and the transition from one volume to another and effects on the airframe when doing so - which is different than my original post as well.

What I said is accurate for a specific volume of wind and for the example I provided. If it were possible, imagine flying inside a very large box of air, now assume someone picks up the box and carefully carried it and then placed it in another location - did your speed inside the box change? No, it did not, however the speed and direction of the body of fluid you were flying in changed dramatically.

Understanding that wind sheer and wind gusts are a large part of landing, especially with crosswinds, they certainly belong in this discussion - they simply were not part of the actual landing I relayed.

EDIT: thanks for the support guys
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Last edited by rongawer : 02-15-2019 at 08:15 PM.
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  #74  
Old 02-15-2019, 09:52 PM
AirHound AirHound is offline
 
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Location: OFallon IL now, everywhere before
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Quote:
Originally Posted by rongawer View Post
Bob, in your example, you’ve defined a tremendous transition air layer boundary change of 40 knots that would likely present itself as wind shear for the landing pilot. But let’s use your example. Noting that airspeed is based on the wind relative to the airframe and measured by a pitot tube, if the wind speed above 20’ increased in speed to 45 knots, assuming you stayed level and made no power changes, how fast would you now being going in KIAS? What would your ground speed now be?

In reference to my original post of landing previously this week, I spoke of landing with 30KT ground speed, and that discussion has now morphed into landing in wind sheer (more than one volume of air interfacing at differing speeds and generally right angles) and compensating for gust (when one volume of air changes directions or roils within another), a diversion from the point of the my post. However, you’re speaking of multiple volumes of air and the transition from one volume to another and effects on the airframe when doing so - which is different than my original post as well.

What I said is accurate for a specific volume of wind and for the example I provided. If it were possible, imagine flying inside a very large box of air, now assume someone picks up the box and carefully carried it and then placed it in another location - did your speed inside the box change? No, it did not, however the speed and direction of the body of fluid you were flying in changed dramatically.

Understanding that wind sheer and wind gusts are a large part of landing, especially with crosswinds, they certainly belong in this discussion - they simply were not part of the actual landing I relayed.

EDIT: thanks for the support guys
Say, how does one discern wind sheer vs gusts unaided when planning a short/local flight around the patch, other than stepping outside and saying, “dang it’s pretty breezy out” ! I think sheer falls into the ‘expect the unexpected risk category or stay on the ground’. Sorry, .....but the foregoing thread discussion has this astronaut wanting to go back to school.
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  #75  
Old 02-15-2019, 10:56 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by rongawer View Post
If it were possible, imagine flying inside a very large box of air, now assume someone picks up the box and carefully carried it and then placed it in another location - did your speed inside the box change? No, it did not, however the speed and direction of the body of fluid you were flying in changed dramatically.
ys
Okay, lets say you’re flying trimmed at 70 kias, west, inside the box, which is at rest. Your ground speed is obviously 70 knots. At time 0, you rapidly accelerate (lets say over 0.1 sec) the box to 10 knots toward the west. If we treat the air as incompressible it accelerates to 10 knots as well. The airplane, having finite mass, cannot accelerate instantaneously. At time 0.1 it is still very close to 70 knots ground speed, but now close to 60 kias. Over the next 5 - 10 seconds the plane will accelerate to 70 kias; its ground speed is now 80 knots. At time 100, the box is stopped, and by 100.1, the air in the box is at rest with respect to the outside. However, the airplane continues west at nearly 80 knots, at 80 kias. Over the next ten or so seconds, the plane slows to 70 knots ground speed and kias. Did your speeds inside the box change? Yes they did.
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  #76  
Old 02-16-2019, 01:46 AM
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Piper J3 Piper J3 is offline
 
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For a while there I thought this discussion was going to reveal the secret of the dreaded "downwind turn"...
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  #77  
Old 02-16-2019, 07:56 AM
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DaleB DaleB is offline
 
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I've had landings where there's a significant headwind, until there isn't -- yes, airspeed drops sharply (of course), as does the airplane. It's why we add 1/2 the reported gust speed to our speed on final, right? And how do I know that I can expect the wind at the airport to be, say, 12G20? Because I get the weather via AWOS/ATIS and know the reported conditions. On top of that, I'm a little bit pessimistic and assume the wind is going to try to kill me. I would assume that Ron was aware of the reported conditions at the destination and planned his speed and approach accordingly.

I'm a 200-something hour, strictly VFR guy; I essentially know just enough to be dangerous. Nearly everyone here has far more flying experience than I do, and are far better at it than I am or probably ever will be. I'm certainly not going to plan to fly somewhere that I know will have a 30 knot crosswind. If I get where I'm going and there is a 30 K crosswind, I'm going to make an attempt or two before calling it off and going somewhere else. I'm always willing to try to incrementally expand my personal envelope through experience, as long as I can do it safely, and stay within what I'm confident I can do. The fact that Ron was able to land in those conditions doesn't mean I'm going to just blithely assume that I can, too. That said, given the limited experience I do have in the RV-12, I'm not surprised that he was able to do it.
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  #78  
Old 02-16-2019, 08:31 AM
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N941WR N941WR is offline
 
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Quote:
Originally Posted by Ed_Wischmeyer View Post
Uh, no...

If you're flying at 65 KIAS, 35 knots headwind component and hence 30 knots ground speed, and the wind abruptly stops, you're suddenly at 30 KIAS. Oops!
Ed, you are spot on and I've had it happen, during a BFR, too boot. Stabilized approach, speed spot on, dropped below the tree line and the bottom fell out.

The power came up just as the mains touched the ground and we bounced hard on the mains (Thank GOD I don't have a nose wheel!) and flew away.

The AOA went from nothing to full scale stall instantly. (I was already pushing the throttle in before the AOA went off.)
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  #79  
Old 02-16-2019, 09:31 AM
rongawer rongawer is offline
 
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Quote:
Originally Posted by BobTurner View Post
Okay, lets say you’re flying trimmed at 70 kias, west, inside the box, which is at rest. Your ground speed is obviously 70 knots. At time 0, you rapidly accelerate (lets say over 0.1 sec) the box to 10 knots toward the west. If we treat the air as incompressible it accelerates to 10 knots as well. The airplane, having finite mass, cannot accelerate instantaneously. At time 0.1 it is still very close to 70 knots ground speed, but now close to 60 kias. Over the next 5 - 10 seconds the plane will accelerate to 70 kias; its ground speed is now 80 knots. At time 100, the box is stopped, and by 100.1, the air in the box is at rest with respect to the outside. However, the airplane continues west at nearly 80 knots, at 80 kias. Over the next ten or so seconds, the plane slows to 70 knots ground speed and kias. Did your speeds inside the box change? Yes they did.
Bob, noting that this is far afield from the original post, I was trying to make a simplified example of the mass of the air, that the aircraft is flying in, is a volume that has its own course and speed. Your post attempts to explain one of the forces, acceleration, on the mass of aircraft, but if you're going to do that, then you should also talk about the air's mass and density and the reaction it as on the airfoil. The air volume (the wind) changes the acceleration of the aircraft, not the other way around, which is why I was trying to explain the effect of currents on a boat - which has the same physics as an airframe in air. If the fluid was truly incompressible and under pressure, then the airframe's speed would be exactly the same as the fluids as it is fully entrained. But air is not incompressible, so for the fluid (air) to accelerate and have a greater speed than the aircraft in that fluid would essentially mean the aircraft is going in reverse relative to the fluid and substantial cavitation forward of the leading edge of the wing (that would result in substantial buffeting and likely a stall).

However, you've pretty well answered what I was trying explain earlier - so, to tie this back into the example discussed earlier - if the box came to an immediate stop, as in "the wind went from 30KT to 0KT immediately", what would be the airspeed of the aircraft within that volume of air?

To recap, I was flying into wind with a 35KT headwind component and just over 20KT crosswind. I was not flying in a 30KT crosswind or into a 30KT gust. I do not for a moment believe the 12 will handle a 30KT crosswind, nor do I advocate that it will.

Dale, I agree with you - do go explore your aircraft and make "an attempt or two". This is why we do stall practice and slow flight and steep turns. Doing crosswind landings is just another skill that expands our knowledge of the aircraft and makes us better pilots. Just don't forget to account for half that gust...
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- Several others that are now just great memories for me.

Last edited by rongawer : 02-16-2019 at 09:37 AM.
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