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Jun 05, 2019, 02:10 PM
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"greater mass means less upward acceleration " all the rest being equal yes , not per se. Among a zilion factors, wing loading surely plays a key factor in translating vertical speed changes in an air mass into vertical speed changes of an airframe,

All this IMHO obviously, Cheers
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Jun 05, 2019, 02:59 PM
Registered User

Different thermals, different shapes


For many years I worked in a location where the road to lunch restaurants was close to a garbage dump. Several times a week, this attracted 50 or more gulls. The gulls in the thermals let me see what shape the thermals were and there were a lot of different shapes. Globular, like a ball was pretty common. So was a shape I call "the ice cream cone" and sometimes the "rubber ice cream cone". Cone shaped, small at the botttom, larger at the top, the "rubber" one would wave the bottom of the cone around. Columns, either fairly stationary or erratic were also seen.

I guess this means that since thermals can vary, and models can vary in their response to a thermal that there are a lot of possibilities and no definite rules to how a model will behave in a thermal.
Jun 05, 2019, 03:02 PM
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Gratter's Avatar
I have heard the same thing that the plane speeds up in a thermal but my experience is similar to mikes. “See the Dr. Drela quote above in my post #22 or view his post directly. My experience has been that my planes pitch up slightly when entering lift, and my immediate reflex is to push a little down elevator to keep the plane level.”
I do not see a pitch up but a noticeable slow down which I am assuming is the nose rising thus slowing down the plane. If I start to circle most of the time I start to climb.
Jun 05, 2019, 03:09 PM
Registered User
Maybe it was Dr Drela, maybe it was Dave Thornburg, who helped me see it first. From the perspective of a glider (motor runs and dive recoveries excepted), the plane is always going down relative to the air in which it is flying. It is up to the pilot to figure out if the plane is going through air that is rising faster than the plane is descending.

Hence the discussion on ‘how can I tell whether’s in lift or not’. Vario’s, altimeter recorders, and chicken entrails will all help to varying degrees - but nothing works better than getting out there and flying *your* plane in various conditions to figure out how it signals lift.

Now, I need to take my own advice and get back out there too
-Keith
Jun 05, 2019, 04:09 PM
Registered User
Libelle201B's Avatar
I have only used a vario in one of my rc sailplanes and that was long ago, back I think in the late 80’s, it was in my Sailair. We had a great soaring site west of town back then, (Wellington, FL), it was wide open and we had established a X country course of about 11 sm out and back. The Ace vario came in real handy particularly when at high altitude where it is really hard to tell if you are climbing or sinking. The real hazard of flying way up high is that it is hard to judge your airspeed, you may think you are flying slow when in fact you may be close to vne. I got kinda lucky during one event and made it past the halfway point but had to land about a half mile later, the vario in a constant moan
Jun 05, 2019, 05:37 PM
turn, turn, turn.
Quote:
Originally Posted by John O'Sullivan
This “rising tail on entering a thermal” issue is a thing that has irritated me for a long time.

Imagine a sailplane entering a thermal at 20 ft/second (6m/sec) airspeed. With an approximate 1 metre between wing and tail, this would make 1/6 second between wing entering the thermal and the tail ( assuming the plane is flying forward). In other words the wing enters the upflow before the tail. How then would this cause the tail to rise? Maybe if the tail entered the upcurrent before the wing ( Model flying Backwards???) it could happen. This red herring has about as much credibility as the “downwind turn”.

I flew competetive Free Flight from the mid 50s to the 1980’s and still fly and design Free flight models in addition to RC sailplanes which I have flown since early 70’s. Never once did I hear this theory put forward by free flight fliers. Free flight models are trimmed to fly in loose circles very close to the stall and will very easily indicate turbulent air. Observation of their “non human controlled” situation gives a much more accurate evaluation of the thermal activity. Too often when an RC flier sees his model bump around his instinctive response is to give control input which may in fact only appear to be caused by the thermal.

Thermals are not clearcut items where it is an instant change from flat air to rising air. There is a swirling around of air at the thermal’s edge and this alone is liable to buffet the sailplane in unpredictable ways. Sometimes it will buck up or dive or wallow dependent on the “swirrillyness” of the thermal’s boundary. Just because you saw your tail rise on occasions does not make it a proven fact.
The tail is lighter so it reacts before the main wing... Even though the main wing
encountered lift first.

The plane will also pick up speed, which makes it seem livelier.
Jun 05, 2019, 05:43 PM
turn, turn, turn.
Quote:
Originally Posted by aeronaut999
Why would a plane be more lively and responsive in lift?
Why would the plane act like it is showing lift when it is really just turning into the wind?

I'm not a contest-level rc soaring pilot, I'm just some guy who flies his Radian to tiny-speck altitudes using a vario. So, I don't know anything really. But at least SOME of this has got to be due to human factors (psychology and perceptions.)

I've never seen my plane behave differently turning upwind versus downwind. Yes, sometimes the plane does seem to be more "buoyant" when entering lift but it's hard for me to say more precisely exactly what this means to me.

If the glider were in an enclosed box (smooth air) and the pilot were also there in the enclosed box and he couldn't see out, he couldn't tell by the way the glider responded to control inputs whether the enclosed box, glider, and pilot were shooting up at 2000 feet per minute or sinking at 2000 feet per minute or stationary. Likewise if the enclosed box were translating horizontally at 200 mph, the pilot couldn't tell by how the glider responded while turning "upwind" or "downwind" which direction the box was moving.

Just thought I'd point this out. If anyone really wants to get too deep into it, better copy this post over to the "Modelling Science" forum. Now back to your regularly scheduled soaring discussion--

Steve
You might wanna think about not specking .your glider out so much, and start flying very low and when you reach 200' come on back down and catch another thermal.
Jun 05, 2019, 10:23 PM
Registered User
aeronaut999's Avatar
Quote:
Originally Posted by Kenny Sharp
You might wanna think about not specking .your glider out so much, and start flying very low and when you reach 200' come on back down and catch another thermal.
And a time to every purpose under heaven.
Jun 06, 2019, 09:50 AM
Save the ALES.
Miami Mike's Avatar
Turn! Turn! Turn!
Latest blog entry: Some Thoughts on ALES vs. F5J
Jun 12, 2019, 01:48 PM
mostly gliders
liukku's Avatar
Quote:
Originally Posted by Miami Mike
In response:
This is a lame attempt by mawz to explain a phenomenon that doesn't exist.

aeronaut999 is right, a glider behaves exactly the same whether it's flying in lift, sink, or neutral air, except of course that it's rising, descending, or maintaining altitude. It's the transition into air with a different vertical velocity that can cause a glider to temporarily change attitude and provide a visual signal, but it will then settle down and fly as it normally does. This is due to a principle called Galilean invariance.

Over 15 years ago I offered this simple thought experiment to show that it doesn't make sense to think a glider would fly any differently in rising or descending air:

My post won the approval of a highly respected authority:
Quote:
Originally Posted by markdrela
Mike's exactly right with the elevator analogy. In physics there's something called Galilean Invariance:

A uniform overall velocity by any dynamical system (airplane+airmass in this case), is undetectable from within the system. So the airplane's orientation and flight behavior cannot be affected by a uniform upward motion of the airmass around it. The airmass can be the core of a big thermal or the air inside the huge elevator -- same thing.

The behavior just after the glider's entry into the thermal is another matter. I've done a bunch of flight simulations where a glider flies into a thermal, trying different thermal profiles and different stability margins (CG locations). What the sims show and what I see in reality is that the glider initially pitches up and "balloons" as it meets the rising air, as though a sharp up-elevator impulse was applied during the entry.

What happens after the initial nose-up depends on a number of things. Consider some particular cases:

1) The glider is fairly stable with a relatively foward CG, and the pilot is alseep as the sticks.
At the top of the initial pullup, the stable glider will nose over and start the usual slowly-decaying porpoising flight (phugoid motion). The first nose-over and short dive after the initial pullup is perhaps what some people perceive as a "tail up" signal. But in reality it's a late symptom. The initial pullup is the earliest signal of the thermal entry.

2) Glider is almost neutrally stable with a relatively aft CG. Pilot is still asleep.
In this case the glider has little tendency to return to level flight, and the pullup will continue for a longer time, likely leading to a stall. The nose-over from the stall will result in a long steady dive with no porpoising, like in a Dive Test. The dive is of course a "tail up" orientation. But again it's a late symptom. The initial pullup and stall is the first signal.

3) The pilot is alert and fast on the sticks.
The pilot will see and quickly correct the initial pullup with a touch of down-elevator. This will prevent the nose-over and possible stall. Ideally the elevator correction will bring the glider back to its original level orientation, but it will now be translating upward with the rising airmass.


In practice these are idealized situations, since a real thermal is not uniform. But the general rule is:

"When flying into air with increased vertical air velocity (into lift or out of sink), the glider acts as though it was given a touch of up-elevator. Flying out of lift, or into sink, produces the same effect as a touch of down-elevator.

A good thermal pilot is highly tuned to these "uncommanded" pitch inputs. The father aft the CG is, the stronger the glider's response to any vertical velocity change, but the greater the workload needed to correct it before a stall or dive occurs. It's a tradeoff.


I get happy every time I read Mark Drela's explanation. It is good that we are reminded of it from time to time. So, thank you Miami Mike for the reminder!

/Ville
Jun 12, 2019, 02:22 PM
mostly gliders
liukku's Avatar
Quote:
Originally Posted by Miami Mike
Varios and visible lift indications from gliders are not alternative ways of getting the same information.

When a glider signals lift or sink with a change in attitude, that informs you that you're entering air with a different vertical speed. But because your plane has mass, its vertical speed won't instantly change in step with the air that you've entered. That takes time, and it's the very reason that the plane's attitude changes! It changes because vertical forces on the wing and stab are unequal while you're in the process of enter different air, and that causes the fuse to pivot vertically about the plane's center of gravity (i.e. change pitch).

A variometer does a different job. It comes into play after the plane has vertically accelerated to its new positive or negative sink rate relative to the ground, and it provides a running indication of your vertical speed. When you're in a thermal and it breaks up, your plane won't signal that, but a vario will.

In other words the two are not alternatives, they complement each other.
Quote:
Originally Posted by Miami Mike
See the Dr. Drela quote above in my post #22 or view his post directly. My experience has been that my planes pitch up slightly when entering lift, and my immediate reflex is to push a little down elevator to keep the plane level.There may be some delay caused by the electronics, but there's always a delay between the plane signaling lift and actually beginning to rise because, as I mentioned above, the plane has mass.

Force = Mass X Acceleration, so Acceleration = Force / Mass. Acceleration in this case is increase in upward speed, and upward speed is what a vario measures. Faster rising air means more upward force and greater upward acceleration, but greater mass means less upward acceleration and slower response to newly encountered lift.

Yes, especially when it's overhead.
Hello!
Well written Miami Mike, as so often. I often use vario, for example when I fly a GPS-Triangle flight, then it is invaluable, because it is not easy to fly a predetermined triangle track on order, especially if each lap must be terminated with the same or higher height as the starting height, I fly according to the OLC rules, https://www.onlinecontest.org/olc-3.0/rc/index.html. In addition, varion is valuable just as mentioned when flying with the aircraft right above myself.
I also find that my gliders get livelier in a thermal bladder, and I do not need any scientific evidence to make it clear that this is just the case. Anyway, I think so.

/Ville
Jun 15, 2019, 04:02 PM
Registered User
Libelle201B's Avatar
Quote:
Originally Posted by liukku
Hello!
Well written Miami Mike, as so often. I often use vario, for example when I fly a GPS-Triangle flight, then it is invaluable, because it is not easy to fly a predetermined triangle track on order, especially if each lap must be terminated with the same or higher height as the starting height, I fly according to the OLC rules, https://www.onlinecontest.org/olc-3.0/rc/index.html. In addition, varion is valuable just as mentioned when flying with the aircraft right above myself.
I also find that my gliders get livelier in a thermal bladder, and I do not need any scientific evidence to make it clear that this is just the case. Anyway, I think so.

/Ville
I’m curious about these rc scale glider competitions based on full scale type competitions. Vario and gps telemetry allows you to go out of visual sight of your glider, with turnpoints and final glides?
Jun 15, 2019, 05:47 PM
mostly gliders
liukku's Avatar
Quote:
Originally Posted by Libelle201B
I’m curious about these rc scale glider competitions based on full scale type competitions. Vario and gps telemetry allows you to go out of visual sight of your glider, with turnpoints and final glides?
Hello Libelle201B.
No, I never fly out of sight. The 3-d distance is usually around 700-800 meters, (765-874 yards). I usually fly with my F5J models (3.5 meters) in a 1-1.5 kilometer long triangle track, but never out of sight. I do not fly GPS-Triangle with my scale models. Next year i will fly my Tangent Vortex, (5 meter) Sport type model in OLC and GPS-Triangle Sport Class.

/Ville
Jun 15, 2019, 06:31 PM
Registered User
Libelle201B's Avatar
Quote:
Originally Posted by liukku
Hello Libelle201B.
No, I never fly out of sight. The 3-d distance is usually around 700-800 meters, (765-874 yards). I usually fly with my F5J models (3.5 meters) in a 1-1.5 kilometer long triangle track, but never out of sight. I do not fly GPS-Triangle with my scale models. Next year i will fly my Tangent Vortex, (5 meter) Sport type model in OLC and GPS-Triangle Sport Class.

/Ville
I have been quite interested in all this new technology and the 4 meter plus semi scale models being used in a full scale triangle soaring format. Is the task one time around or several times around, the winner being who is fastest?
Jun 16, 2019, 10:24 AM
mostly gliders
liukku's Avatar
Quote:
Originally Posted by Libelle201B
I have been quite interested in all this new technology and the 4 meter plus semi scale models being used in a full scale triangle soaring format. Is the task one time around or several times around, the winner being who is fastest?
Hello Libelle201B.
Totally short, cut from GPS-Triangle rules, https://gps-triangle.net/gps-triangl...ons-documents/

1 Definition of GPS Triangle soaring for Scale Gliders
1.1 Purpose and Goals
GPS Triangle competitions are meant to build a bridge between model soaring and full-size soaring competitions. Its goal is to display cross country soaring competitions of full size soaring in the scale of our model aircraft. The main task when participating a GPS Triangle contest is to fly around a virtual triangle (perimeter ~2.4 km) as often as possible in a period of 30 minutes. In order to obtain comparable results, the maximum starting altitude (usually 500 m) and the maximum starting speed (usually 120 km/h) when crossing the starting line are equal for all pilots. In addition to the before mentioned task, GPS Triangle also provides a special and more exciting task. In the so-called Speed Heat, pilots have to fly around the virtual triangle only one time as fast as possible.


When it comes to OLC:
You can read the OLC-RC- HowTo here; https://www.onlinecontest.org/olc-3....url=OLCRCHowTo
Use Google translate for English version. Below you can read cut from the OLC-RC- HowTo ,

1) What is it about the RC-OLC?

The RC-OLC is a decentralized compared for model gliders. Unlike central competitions with pure time rating (eg F3J or F5J), the RC-OLC is a haul flight comparison. It applies just as in the OLC of passenger-carrying gliders to find thermals to exploit and implement in line. When RC-OLC there, using the slogan "Let's see what's going on today!", Neither a fixed flight time, nor a fixed route. Instead, creative flying and staying up long announced. Anyone can participate, regardless of time and place - there is no fixed dates yet venues. We fly on a flying field or on the "green field".

Each participant performs in the model with a GPS logger that records the flight in tamperproof IGC format. Via SD memory card or cable the flight the pilot is read out on the PC and then uploaded to the OLC. The OLC server analyzes the flight and calculated according to the applicable rules, the achieved during the flight points. All submitted flights are displayed in a score. So participants can measure the world with other model pilot, learn from them and constantly improve their skills.
...

4) What is my flight task?

When RC-OLC, it is taking advantage of thermal updrafts, in the form of variable rounds not predetermined triangles to erfliegen. A round (it includes the free and flat triangular one) is at the start-finish circuit ( SCC ) starts and ends in the start-finish circuit (SCC). The final height must be greater or equal to the starting height. Since the thermal changes constantly in the course of a competition flight, it is optimally adapted to the course with each new round. Since the pilot determines its flight path free, each round may result in a completely different triangle.
The flight will be analyzed by the reporting OLC server and the best possible triangles are automatically calculated and points are awarded. Ideally, at least 10 valid rounds come about in a competition flight, but must not be flown at a time. A competition flight with 10 valid rounds can thereby also take an hour or longer.
Calculation of Points: The fastest 10 rounds of a competition flight will be added together and the sum divided by velocity 10th
Important: For a valid competition flight not need to come together still 10 rounds. Is the weather not here, is enough one round to (but then little according) to get points.


Good luck!

/Ville


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