Yes, I'm sure this question has been raised before in this forum, and, in fact, it would be odd if it hasn't already been discussed countless times, but that doesn't mean we can't bring it up again, even if only to help settle an argument.

The question isn't about what happens when you enter or leave lift, although we can certainly discuss that too, it's about stable flight while inside a homogeneous body of air. Does a glider fly any differently if that body of air is rising than if it's sinking? Aside from directly observing that the plane is going up or down (or monitoring a variometer), is there any way to know whether you're in lift or sink based upon how your plane is behaving?

Tail position or a speed change or the combination of the two can be my first indicator.

Absolutely , tail goes up , speed goes up when my Aquila finds itself in lift.

Mike:

In the particular universe I am in at the moment, here is how things appear to me:

My gliders do not appear to fly any differently in lift, as compared to when in sink. However, I fly them differently when in lift, than I fly them when in sink.

Typically, when in lift, I am working the sticks to center the thermal. I also fly as slow as possible (without stalling!) so that the turn diameter is as small as possible. If that also requires more bank angle, there goes more stick action. A lot depends on the nature of that thermal: wide/narrow, small/large, strong/weak, rough/smooth and so on. In lift, I tend to work the airplane.

Typically when in sink (which also means: whenever I'm NOT in lift), I let the airplane fly. The airplane is my lift indicator, and I want whatever happens to the airplane to show up, and not be masked by what I might be doing with the sticks. Furthermore, I fly the airplane appreciably faster in sink, well above stall speed. If the sink is very apparent, I actively put the nose down and get the hell out of there!

Let me stress that this is how I perceive the world. Other people see the world differently (or, perhaps, see a world I am not aware of). I'm not necessarily right, other people are not necessarily wrong.

Good luck!

Yours, Greg

In sink, with basically zero input I have noticed gliders flying with their tail down, like a dog dragging its butt across the grass.

In lift, I've seen either the pilot starts turning, or the plane looks to fly the same, or starts speeding up, or slows down -- i.e., all of the above except tail down.

Thanks for all the input so far, but please keep in mind that I'm not asking what your plane does when you enter lift or leave lift, or how you fly your plane when you're in lift and how you fly it when you're in sink. I'm asking about how the plane itself behaves after it has entered lift and has been flying in that lift for a while, and how it behaves after it has entered sink and has been flying in that sink for a while. Aside from the obvious answer that it goes up or down, is there any difference in the way it flies?

That's pretty much how answered, Mike -- in Steady-State, that's what I've seen. In lift it's just hard to see steady state most of the time, because we don't want to waste the lift, so we start turning.

But basically, in lift, in steady state, I've seen a glider do all of the above (flies faster, slower, or stays the same. I think it depends on the nature of the lift and the rest of the ambient air. Turbulent, smooth, strong, etc. Pretty hard to generalize, in my opinion.

One factor that may be relevant is that thermal lift is air that is warmer and therefore less dense than surrounding air. Therefore, each surface has to work a little harder to "do it's thing". Depending on the plane, you might note the behaviour above (tail up in lift - like my DLG). But other things could be going on for the same reason.

Cheers

Sorry, can't resist. Yes, it flies completely differently. In lift it goes UP, in sink it comes DOWN. Lift is better ;-)

I fly 2 metre thermal, all I have noticed when moving from lift to sink is first a fast loss of altitude and then an increase in speed, that's when the thumbs kick in to reduce the speed to reduce the loss of altitude and search for the thermal again. Moving from sink to lift, I see a raise in altitude and a slow reduction in speed, again that's when the thumbs come into play to stop a stall and try to stay with the thermal as long as possible. That is how both my built up and foam core sheeted 2 metre wings react, It may be different for larger gliders. Anyone fly full house 3.5 to 4+ metre?

3.4 meter. When it's in lift, it does something to my brain and makes me feel like the first time I held a girl's hand. At that point, I just want the feeling to keep going. When it's in sink, it's like seeing that same girl talking to another boy.
...but I digress.

That's the one thing I hate about our 2 metre comps, 6 minute flight window, here I am 5 minutes in a thermal and one minute to spot land.
I don't want to land!

Physically: No. Absolutely no difference in the way the model flies if we are talking a body of air.

However in sink or lift the model experiences a slight change of angle in the apparent airflow and so the trim will appear to change relative to the horizon, all things being equal.

The reality is that ones perception and own responses are dominant and any discernable differences are more likely to be due to this.

Since lift (thermals) are typically limited in size, and sink is typically spread over a larger area, its is hard to discern any difference since, as already mentioned, when in lift we start circling and controlling the plane to stay in that small patch of rising air.
All that being said, I believe that there is a bit of perception involved. Think of the downwind/upwind flight issue. Relative to the air the plane flies the same speed (as long as we don't touch the sticks). To the pilot on the ground it appears that the plane flies faster downwind and slower upwind. This 'point of reference issue' may play into our perception of a plane flying in sink/lift. With the notable exception that strong sink will push the tail down creating an in unfavorable angle of attack for the wing - this increased drag will add to the loss in altitude.

My $0.02

Martin

Originally Posted by MMR

... strong sink will push the tail down creating an in unfavorable angle of attack for the wing...

And (with no correction by the pilot) will it keep holding the tail down as long as the plane remains in sink?