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Jun 03, 2019, 07:44 PM
Lift is good
Rayven's Avatar
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Discussion

Question on Lift Indication


I've observed, and have always been told, that when your sailplane flies into lift, the tail usually rises. There has been discussion many years ago on this forum, but I don't think a conclusion was ever reached--though I think Mark Drela and Phil Barnes had some well thought out ideas. Anybody have some ideas they want to share?
Last edited by Rayven; Jun 03, 2019 at 08:02 PM.
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Jun 03, 2019, 09:00 PM
Registered User
Not all designs react the same . My Aquila will indeed raise its tail , my Quasoar will act like it's being evicted or tossed out of the lift forcing me to reenter it on a more direct route and my
Pierce 970 just goes up nice and level .
Jun 03, 2019, 10:35 PM
Lift is good
Rayven's Avatar
Thread OP
So as with so many other things about the handling and performance of sailplanes its a matter of the sailplane and the pilot and the ensuing opinions! Which is what makes this hobby challenging and fun actually.
Jun 03, 2019, 10:51 PM
Registered User
mdickey's Avatar
I think about the tail rising sometimes, but really it seems that the plane becomes much more lively and responsive in lift. Sometimes, the lift can be so strong that the plane seems out of control. When in sink, the plane is slow and sluggish while falling out of the sky much quicker. Each plane telegraphs lift and sink slightly differently, so it is important to try and pay attention to its behavior while in positive, neutral and negative air. One last part. Sometimes, it seems like the plane is showing lift when it is really just turning into the wind. Keep that in mind.
Jun 04, 2019, 03:46 AM
Balsa breaks better
Thermaler's Avatar
"Turn to the rising wingtip dummy" were the words my Dad told me at my first contest.
That is only when you don't fly straight into it.
Sometimes tail up, sometimes the whole plane up depending on strength of lift, setup and plane.

Joe
Jun 04, 2019, 07:36 AM
Registered User
Sometimes I get wing waggle, like turbulence. That's probably my most common indicator. I've seen the tail lift occasionally, or the nose bob up, or a wing tip. I've seen greatly diminished-almost total loss of rudder authority, as in the plane is going left and I'm turning right but it isn't responding to my input...never quite sure if I'm being sucked into lift to my left or pushed away from lift to my right. But I think the tail gets sucked toward the lift. Which would explain the scenario above.
And it seems at times, simply turning into the wind can produce any of those indicators to some extent.

I'm still trying to figure all this out. Big boomers are easy, it's the lighter stuff low to the ground that is a mystery to me.
Jun 04, 2019, 09:12 AM
Registered User
aeronaut999's Avatar
Quote:
Originally Posted by mdickey
I think about the tail rising sometimes, but really it seems that the plane becomes much more lively and responsive in lift. Sometimes, the lift can be so strong that the plane seems out of control. When in sink, the plane is slow and sluggish while falling out of the sky much quicker. Each plane telegraphs lift and sink slightly differently, so it is important to try and pay attention to its behavior while in positive, neutral and negative air. One last part. Sometimes, it seems like the plane is showing lift when it is really just turning into the wind. Keep that in mind.

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
Jun 04, 2019, 09:20 AM
Sagitta Fanboy
Quote:
Originally Posted by aeronaut999
Why would a plane be more lively and responsive in lift?
Lift adds energy to the plane. Sink removes it. So the plane will be livelier when it's in a higher energy state, which occurs when it's in lift.

Quote:

Why would the plane act like it is showing lift when it is really just turning into the wind?
Generally what you are seeing there is that air currents flowing into the thermal that indicate lift are hard to differentiate from a small gust of wind or turbulence. But those gusts are very often cause by thermal activity, if the lift seems missing, it's probably nearby.
Jun 04, 2019, 10:30 AM
B for Bruce
BMatthews's Avatar
Like the folks above I've had models that lift the tail and others that lift the nose. And at times each of those signals in the opposite manner. So I suspect that how the model reacts has something to do how a number of factors add up when arriving at any given thermal. It's likely somewhat related to design, somewhat related to the pitch trim/stability margin of the model and finally how the particular thermal is behaving at that particular moment in that particular portion of the thermal's formation.

So all in all I'd say with great conviction..... "it depends...."

If it helps I have found that models with somewhat shorter tail moment arms tend to raise the nose... sometimes. The models with longer tail moments seem to like to lift the tail. But again put them into a slightly odd part of a thermal and each can switch around and act the other way. But GENERALLY I found that this seems to hold true.
Jun 04, 2019, 12:17 PM
Registered User
John O'Sullivan's Avatar
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.
Jun 04, 2019, 12:21 PM
Registered User
aeronaut999's Avatar
Quote:
Originally Posted by mawz
Lift adds energy to the plane. Sink removes it. So the plane will be livelier when it's in a higher energy state, which occurs when it's in lift.
I don't think that that's a very robust statement. To me, "livelier" means things like, I get a higher roll rate, or maybe when gliding along at a given pitch attitude or elevator position (take your pick), I have more energy that I can translate into a temporary zoom climb (change in vertical speed compared to original vertical speed before beginning maneuver.) Or a high bank angle imposes less of a sink rate penalty (as seen in relation to the immediately surrounding airmass, not in relation to the ground.) I don't believe that any of this is true in lift, unless the pilot is unconsciously causing the plane to fly at a higher airspeed when it is in lift.

Maybe there is a temporary dynamic when crossing into the thermal that leads to an airspeed change that the pilot is not correcting, and the glider's static stability is relaxed enough that it is does not correct itself, or at least not very quickly. Something like that could be going on. But there's no way to support the position that the glider is intrinsically more maneuverable or in some other way "livelier" in rising air.
Last edited by aeronaut999; Jun 04, 2019 at 12:46 PM.
Jun 04, 2019, 12:27 PM
Registered User
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.

Steve
I'm not sure how much dependency on a vario to signal lift, diminishes our ability to read air. I want to hook up vario but have intentionally refrained to try and learn the visual cues. But air is fluid, much like water. To look at water in a stream it is easy to see the dynamics and flow, swirls, whirls, eddys, etc. Same with air. And in all that is air going UP which is what we are searching for. I sometimes sense my planes flying a little more lively going up, however you wish to define that but "lively" makes sense to me. But one thing is for sure in my mind, very often times a plane in sink tends to fly "mushy" for lack of a better word. I've seen mine take on a tail down posture, almost like it's tail heavy but with mushy response on the controls.

I think really good pilots only depend on the plane's visual cues to identify exactly where the lift is...but he already knew it was there before he launched!
Jun 04, 2019, 12:55 PM
Registered User
Libelle201B's Avatar
If you are paying very close attention to your glider you can tell if it is climbing, sinking or just maintaining altitude by watching the plane in relation to the horizon, if you are down low. Once at altitude it’s a bit harder to tell due to the increasing angle to the horizon. It takes time and experience to detect when you are in lift, but you will know fairly quickly if you are in sink as you quickly lose altitude. The one mantra I have always followed is if you don’t see or detect your glider climbing, don’t stay in that area, go somewhere else searching for lift, you may have to go quite a ways away till you find it, just remember to keep enough altitude to get back. At distance or altitude you will never see what the tail of your glider is doing.
Jun 04, 2019, 01:27 PM
The Mr. Rogers of RC soaring
rdwoebke's Avatar
Quote:
Originally Posted by Rayven
I've observed, and have always been told, that when your sailplane flies into lift, the tail usually rises. There has been discussion many years ago on this forum, but I don't think a conclusion was ever reached--though I think Mark Drela and Phil Barnes had some well thought out ideas. Anybody have some ideas they want to share?
I don't know why it does what it does or the math behind it but my experience is that sometimes when the plane encounters lift the tail rises. I believe that Dr. D and Phil were of the mindset that the tail doesn't rise when you are in lift that the wing does but that the tail rising meant that you had flown through the lift. Either way it indicates you would be near lift which would be a good time to start circling and evaluating which side of the circle is stronger.


Quote:
Originally Posted by Crashbound
I want to hook up vario but have intentionally refrained to try and learn the visual cues.
It might help strengthen your ability to visually read the plane. I haven't tried this yet but one of the things on my "to do list" is to spend some sessions using a vario to try and improve my ability to read lift when flying directly upwind. I struggle with reading lift when directly upwind.


Ryan
Jun 04, 2019, 02:18 PM
Registered User
GliderJim's Avatar
A lot of times lift is turbulent, especially around the edges. We're up there circling, trying to find the core. Maybe we keep entering and exiting the lift. The plane gets bounced around. You put the nose down a bit to keep your speed up. This allows the control surfaces to be more effective. You're flying with more energy. The tail will appear to be high because you're effectively flying downhill in the rising air, but hopefully you're rising faster than you're coming down.


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