Sapphire Posted May 19, 2013 Posted May 19, 2013 This thread has turned into a classic, and I agree that it is very confusing for people trying to learn from the student thread. I think discussions like this is one of the reasons I got worn out with instructing.I'm pretty sure that the quote above in Sapphire's post #46 is where the confusion stems from. While Sapphire has been discussing flying around in a moving body of air of constant velocity, I think he has actually been talking about very important teachings in glider training which refer to flying from one moving body of air into another body of air, and the two of those are moving at different velocities relative to each other. The examples that Sapphire mentions describe the plane going from air travelling at say 10kts, into air travelling at say 20 kts. This situation is sought out as a normal part of soaring when entering or leaving thermals, given that the thermal is a rotating mass of air it depends on which side of the thermal you enter or leave it, to whether you enter air that is travelling toward the body of air you just left, or away from it. In the former case your aircraft plunges into the new air mass and until drag evens things out the planes inertia (momentum actually) does mean that for a short while the plane will be travelling faster through the new air than it was travelling in the old air, so there is a very noticeable increase on the ASI. And, if you were just about to stall in the old air mass, the increase in real airspeed should prevent the stall. This might not always be the case though because the rotating air mass is also ascending and the increased angle of attack experienced instantaneously by the wing might actually allow the stall to develop - but that's another discussion. If you enter the thermal on its other side you would meet air travelling away from (relatively slower than) the air you just left, and it would also be ascending, so you would most definitely stall if you were about to stall in the old air ... And if you bear in mind that thermals are surrounded by cool descending air the situation is exacerbated. Conversely, leaving the thermal you might exit it on the first side described above and meet slower descending air or you might exit it on the second side described above and meet faster descending air, the former might avoid you stalling if you were about to stall while in the thermal, and the latter would almost certainly prevent the stall. The above also applies to powered planes that are not deliberately seeking out thermals. As you fly in a straight line and hit a thermal dead centre you will first descend as you meet the cool air mass descending around the outside of the rotating hot core, then you will meet air that will push the nose of the aircraft up and to the left (because the core is rotating clockwise in our part of the world) and if you were flying close to the stall you might well stall at that stage having gone from descending air to rising air, and nose high, then you will fly level in lift but the plane will be displaced sideways, first left and then right, and then you leave the ascending air to enter the descending air where the nose gets pushed left again and also it drops, before getting back to straight and level. Some folks call it turbulence, I call it free fuel if you fly slow in the lift and fast in the sink. What happens if you don't hit the thermal dead centre, but instead hit it one quarter to the left or right side is a good mental exercise for any students who have persevered this long with this thread. Similar things happen when transiting the earth's boundary layer, aka the wind gradient, but the difference here is that you are not flying from one air mass to another, instead the part of the same air mass which is closest to the ground has its speed reduced by the roughness of the earths surface - trees, rocks etc - and so when landing into wind there is less wind on the ground than at 100ft so you experience a loss of airspeed due to the aircraft's inertia and conversely, when landing downwind you experience an increase in airspeed as you descend through the wind gradient, which is one of the very few good things that can be said about downwind landings. I hope that helps but I'm preparing myself to being Assimilated anyway, just in case ... I considered turning from into wind to to downwind was entering a new mass of air. Can't discuss further now as I am crewing on a sail boat going up the Barrier Reef for two weeks. If anyone wants to discuss sharks and seaweed, I'll be avaiable. Ask Facthunter about being assimilated, though he is still in euphoria.
motzartmerv Posted May 19, 2013 Posted May 19, 2013 I thought you would conclude more professionally. There is no useful substance there. Sorry? Ok I've concluded you have no real understanding about what you talking about, you have probably googles most of what you have posted, you have argued a null point with physics teachers, school teachers, professional pilots and others. While we have enjoyed readin your attempts to sound clever and useful, we would now appreciate you stop sprooking in the student thread. Thanks. That a better conclusion?
Sapphire Posted May 19, 2013 Posted May 19, 2013 Sorry? Ok I've concluded you have no real understanding about what you talking about, you have probably googles most of what you have posted, you have argued a null point with physics teachers, school teachers, professional pilots and others. While we have enjoyed readin your attempts to sound clever and useful, we would now appreciate you stop sprooking in the student thread. Thanks. That a better conclusion? Who is we-take a vote.
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