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Netto Vario Question


Lep

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Have been flying the Discus for weeks and liking it more each day.

I'm now putting in more time in the D-4350 model, which has an additional Netto Vario in it. I read up on that and understand it shows the actual air mass up/down movement, minus away the glider's own sink rate.

However, when NV shows a rising air mass, I see that the "winter" vario shows an even higher climb rate (eg. NV shows 450, winter shows 800). Shouldn't NV always shows a higher reading that the winter gauge, since it already has the glider's sink rate subtracted? The NV gauge shows a "m/s" on its face, but my FS is set to feet unit. So is the NV in m/s or feet/min? I understand the winter is in feet/min.

Also, when I get into a quick descent, the NV starts to show a high sink reading too. Shouldn't it be just measuring the air mass sink/rise rate? (The air mass isn't sinking during the descent). Hope to gain a better understanding of this wonderful gauge. Thanks.

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Hi "Lep",

It can be, that the netto variometer is in m/s only, but this doesn't matter, because the needle of the vario will not change if you change settings. only the face, so please look only at the angle of the needle.

It can be, that the netto vario showes less climb and vise versa, because they have all different technics behind. The Winter is very fast for example, the netto a bit slower.

The "problem" with the quick descent is no problem also, because the netto vario is a theoretical instrument. It will never stuck at 0, you can fool it.

See we SIMULATE the instruments as they would behave in real life. We do not only take one variable "vertical movement of air mass around" wich would be much easier, but calculating the value with the options the real plane offers.

You see, a matter of theoretical / practical experience.

Cheers Joachim

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Hi "Lep",

It can be, that the netto variometer is in m/s only, but this doesn't matter, because the needle of the vario will not change if you change settings. only the face, so please look only at the angle of the needle.

It can be, that the netto vario showes less climb and vise versa, because they have all different technics behind. The Winter is very fast for example, the netto a bit slower.

The "problem" with the quick descent is no problem also, because the netto vario is a theoretical instrument. It will never stuck at 0, you can fool it.

See we SIMULATE the instruments as they would behave in real life. We do not only take one variable "vertical movement of air mass around" wich would be much easier, but calculating the value with the options the real plane offers.

You see, a matter of theoretical / practical experience.

Cheers Joachim

Thanks, Joachim.

Okay will try to understand that.

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when I get into a quick descent

Let's look at this situation...

The Netto Vario is, as you correctly said, displaying the calculation of the vertical movement of the external airmass. As Joachim says, the gauge in the Aerosoft Discus is actually calculating the original analogue pressure readings and then performing the calculation done inside a real glider netto gauge. It is *not* 'cheating' by using some internal FSX variable containing 'user vertical wind speed' or some such, so this means the simulated netto gauge really behaves in a variety of circumstances as the real gauge would, and that's not always what you'd think...

So here's how it works:

Basically the gauge is trying to measure the actual sink rate of the glider, and compute the rate the glider *would* be sinking in still air, so that can be netted off the actual sink rate to arrive at the assumed vertical movement of the airmass.

Example:

if the rate-of-change of the altimeter pressure transducer suggests the glider is sinking at 5 knots,

and the glider is flying at 100 knots,

and the glider polar says that the sink rate at 100 knots in still air would be 4 knots (i.e. the glider has a glider ratio of 25:1 at 100 knots)

then the netto vario should display 1 knot down.

So the vario is reading the 'pitot' pressure reading to assess forward speed to include in the calculation. Follow that? That's the basic calculation, and life would be pretty simple if that was all but there are a couple of additional factors:

1. As with the TE vario, the netto vario has to factor out acceleration and deceleration of the glider. Otherwise putting the nose down would immediately register as heavy sink. Fortunately this is the exact same formula as is already used in the TE vario.

2. The "100 knots airspeed means 4 knots sink" really is a huge simplification and the gauge has to be more complex. Sink rate at speed is fundamentally affected by the amount of water ballast carried, the degree of buggyness of the wings, and the max glide ratio of the glider, in addition to the defined curve of the glider polar (i.e. the graph of sink rates with speed).

But you are absolutely right to compare TE with Netto because they will generally behave the way you expect relative to each other, although as Joachim says the 'time constant' affecting the movement of the needles is also different between the gauges. I think the scale of the gauges is different as well - maybe this is the simple answer.

Now regarding 'descending rapidly': first of all, if you opened airbrakes or left the wheel down, the netto vario would register heavy sink - the gauge doesn't know about the additional drag, so all it sees is you're flying at a certain speed but descending at a high rate, so there must be sink. No offence but there could be a possibility you were flying in a high-drag configuration somehow (i.e. wheel down?). Neither the netto or the TE vario are great in high acceleration situations - e.g. try a push-over or a pull-up. Basically the formulae are working with first-derivative effects and hav second-derivative errors (exactly as the real instruments). So as you pull up, the gauge shows sink as the glider pulls +ve G, but then the TE calculation starts working more-or-less correctly as you climb-and-decelerate. The netto vario will also show external sink if you do anything to fly less efficiently than expected in the polar, e.g. try yawing / sideslipping.

B21

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Thanks so much for the detailed explanation, B21. Really appreciate it. I've been flying all flyable planes/helos (from small prop to Boeing) in FS for a long time, except for glider until now. And I must say I'm amazed (and surprised) at the complexity (and simplicity) of the ideas behind the gauges that allow us humans to glide around like the way an eagle is able to do so naturally.

I'm now grasping the concept better I think. I see that in still air, the netto reads zero while the TE reads a small sink (the glider's intrinsic sink). And I learn to look closely at the netto for signs of any uplift present and make use of it.

Now just for one more thing -- you use knots (lift and sink) as measurement for the netto, but netto is also shown to use m/sec. So is it in knots, or m/sec? Would be glad to clarify this up. I take it that the TE is in feet/min. Thanks.

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the netto vario remains in m/s instead of changing to ft/min (knots, wich is the same) but 5m/s are 10 ft/min ;-) so no problem at all, just take the value x2. :-)

Cheers Joachim

Thanks, Joachim. But I think you mean to say 5 m/s approximately = 1,000 ft/min (x2 x100), right? Quite a difference here :)

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5m/s are 10 ft/min

lol yes Lep, that was a typo - 5m/s = 10 knots = 1000 feet/min (roughly).

The C4 calculations in the Aerosoft Discus are probably a bit more accurate than your average real installation in a glider, because the simulator enables you (i.e. me) to fly and test the accuracy of the polar until you get a good fit. (As opposed to real C4's get jammed into real gliders with fairly inaccurate tuning).

But make no mistake about it, the calculation of the netto reading is fundamentally the accurately measured sink rate MINUS the flight computer's approxiamate estimate of how fast the glider SHOULD be sinking given its current airspeed, flap setting (not in Discus), buggyness, waterballast, and pilot's 'best L/D' hint. The flight computer calculation is never going to 100% reflect what the glider is really doing (it doesn't know too many things) and it's calculation is going to become less accurate anywhere towards the limit of the flight envelope, particularly where the pressure sensors just read what they read even though you know that is probably a bit wrong, e.g. near the stall, at high or low G (e.g. pull-ups, tight turning), flying with yaw, forgetting to raise the wheel or properly close the airbrakes.

But your comment you fly along with the netto reading somewhere near zero and the TE vario saying you're coming down a bit is right on the money. The netto really kicks ass when you're flying along a ridge - in that scenario the TE vario will vary around zero (assuming you're maintaining height along the ridgetop and that's level) while the netto vario will show varying amounts of UP as you steam along the terrain - kind of the mirror usage to your example between thermals...

B21

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Thanks for all that. Haven't really tried the netto for ridge runs. Will give it a go this weekend.

Happy flying and have fun.

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I've done ridge runs with the netto and am getting good kicks out of it.

A question I have on the "Speed To Fly" feature of the C4 -- I understand when in strong sink, fly the speed such that the C4 "petals" are horizontal. But this way I have to practically go into a steep descent and tend to lose lots of altitude (more than a 1,000 ft sometimes). Is it wise always to use Speed To Fly? Would it be better to sit out the fast descent and emerge into lift again soon? Seems to lose less height that way.

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Hi Lep,

the faster you fly the faster you will escape the sink region. Shortening the time in sink compensates the additional sink of the aircraft by far, until a certain speed is exceeded which is the best speed-to-fly in this situation.

Vice versa, if you enter a lift region, it's good to slow down to get the most out of it. Because at very low speed sink of the aircraft increases again, there is also a minimum speed below which the effect is again turning worse.

That is in short the principle. If you like more theory and mathematics try to find some article about Paul McCready's theory which explains all of it in full detail.

Of course, if you are too low you don't have a choice ...

best regards,

Peter

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Hi Lep,

the faster you fly the faster you will escape the sink region. Shortening the time in sink compensates the additional sink of the aircraft by far, until a certain speed is exceeded which is the best speed-to-fly in this situation.

Vice versa, if you enter a lift region, it's good to slow down to get the most out of it. Because at very low speed sink of the aircraft increases again, there is also a minimum speed below which the effect is again turning worse.

That is in short the principle. If you like more theory and mathematics try to find some article about Paul McCready's theory which explains all of it in full detail.

Of course, if you are too low you don't have a choice ...

best regards,

Peter

Thanks, Peter. Will look up on that.

Lep

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