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Dec 08, 2016, 11:37 AM
MMR
MMR
Martian
Correct - hence the advise: when in sink drop the nose and get out of there!

Martin
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Dec 08, 2016, 11:39 AM
Registered User
I spent some time a few years ago experimenting with my F3K airplanes, and developed a procedure that allowed me to evaluate relative performance. In the process, I also learned how important pilot input was, and what the price was for each and every movement of the transmitter sticks.

The procedure emphasized minimum control inputs, and taught me to watch the airplane ever so carefully. I also learned cues for where on its performance curve the airplane was flying. For instance, if heading took active rudder to maintain, the airplane was flying too slow, and I gave it a click or so of nose-down trim. A click or two of nose-down trim, and flight times increased. A few too many clicks, and flight times started to decrease again. I found minimum sink was significantly faster than expected, since I was timing each flight.

When I flew in the early morning, it was easy to see the thermals developing: the airplane would show increasing rates and magnitudes of altitude change, both up and down. Typically, without stopping to turn in thermals, flight times steadily increased.

And of course, by keeping my hands off the sticks, I was able to observe the airplane in both lift and sink.

My observation was that lift and sink affects "how" an airplane flies in exactly the same way as wind: there is an apparent affect from the standpoint of an observer on the ground, but within the frame of reference of the moving air, the airplane itself does not know anything about it. That is backed up by my flight time in full-scale gliders, and my experiences as a flight test engineer (among other things, testing navigation systems).

It is common wisdom that an airplane in sink will fly tail down. In my opinion (and I have no objective data to back this up), any airplane flying tail down is flying too slow for conditions. As it transitions from air mass to air mass, and crosses boundaries, I fully expect an airplane to pitch up or pitch down; but it should recover its best glide attitude if trimmed properly.

By the way, in conditions of minimum sink flying in calm (pretty much! It's never actually totally calm), I found that my DLGs paid these prices for control inputs: move the elevator, and lose about half a meter. Same price for moving ailerons or changing camber. Move the rudder, and expect a penalty of about a quarter meter. Lesson: small stick movements!

Yours, Greg
Dec 08, 2016, 02:07 PM
Registered User
There is no simple answer. It depends on how the glider is trimmed as to what appears to happen. In a thermal a the stab will go up and the glider will speed up. In down air the stab will go down and the glider will slow down. If the glider has the cg forward of the neutral point not much will may be observed. If the glider is trimmed with more nose down than really needed then not much will be seen. I had a light weight gilder that was trimmed to close to the neutral point for early morning. The speed change was the best indication of what type of air it was flying in. Flying over head I could not tell the stab went up but it did. As the wind went up I move the cg then it became much harder to tell what type air I was in but I could fly the glider smoother and the glider ratio was better over all.

In down air I normally add several click of trim to speed up the glider and to reduce the sink rate as to slow increases sink rate ( I do that with mode changes on my radio).

So all I am saying some say they see no real difference may be right because of the way the glider is trimmed. Not knowing how the glider is trimmed there is hard to say what can be given as to what is observed. The glider will always lift the tail and speed up in a thermal and slow down in down draft sum even if it is not observed.

Art
Dec 08, 2016, 02:26 PM
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Miami Mike's Avatar
Thread OP
Great write-up, glidermang, but let me focus on your second paragraph from the end:

Quote:
Originally Posted by glidermang
It is common wisdom that an airplane in sink will fly tail down.
Does that mean you believe it's true?
Quote:
In my opinion (and I have no objective data to back this up), any airplane flying tail down is flying too slow for conditions.
At a given speed, under what conditions would a plane fly tail-down, and under what conditions would it not fly tail-down? And couldn't there be other reasons for flying tail-down?
Quote:
As it transitions from air mass to air mass, and crosses boundaries, I fully expect an airplane to pitch up or pitch down; but it should recover its best glide attitude if trimmed properly.
This statement appears to contradict the "common wisdom" you mentioned above, implying that you don't believe it's true, but I wonder, does it really matter whether or not the plane is trimmed properly? If a plane is trimmed improperly but at least achieves some sort of stable flight with hands off the sticks, is it possible for it to change pitch in lift and then remain that way until it eventually crosses another boundary into air with a different vertical speed, or while it's still in the lift will it eventually return to the same flying attitude it had before it entered that air?

I apologize for what probably sounds like a hostile cross-examination.
Dec 08, 2016, 03:05 PM
IT'S NOSE HEAVY!!!!
cityevader's Avatar
I've noticed that it sure is a lot harder to turn when in heavy sink, losing lots more altitude in the process.
Dec 08, 2016, 05:58 PM
Registered User
slowmatch's Avatar
Does this help? The wings angle of attack remains the same but the angle of the oncoming air changes and so the model looks different with reference to the horizon.

(This applies to how a stable trimmed model flies within a big blob of steady air - flying between lift and sink usually changes the wings aoa and so can have different effects depending on it's pitch stability, CG etc etc)

EDIT: This opinion has been revised!
Last edited by slowmatch; Dec 11, 2016 at 12:21 PM.
Dec 08, 2016, 06:58 PM
Sagitta Fanboy
Quote:
Originally Posted by slowmatch
Does this help? The wings angle of attack remains the same but the angle of the oncoming air changes and so the model looks different with reference to the horizon.

(This applies to how a stable trimmed model flies within a big blob of steady air - flying between lift and sink usually changes the wings aoa and so can have different effects depending on it's pitch stability, CG etc etc)
There's another caveat,

Often when transitioning between lift & sink you are moving between airmasses of different speeds, and your glider will retain its basic energy (inertia) which is to some approximation also its ground speed, so you may see an angle of attack change because the change in speed of the surrounding air results in an airspeed change and a shift in angle of attack to account for that.

That is likely what both Greg and Miami Mike are getting at about 'tail down in sink'. It's commonly accepted, actually incorrect, but does describe what the pilot sees from the ground initially.

Also Mike, I think that what Greg was getting at about 'properly trimmed gliders recovering their best glide' is more a commentary that improperly trimmed gliders will recover their trimmed but not optimum glide path, any improperly trimmed glider settling on their best glide path is either going to be accidental due to current conditions or temporary.
Dec 08, 2016, 07:57 PM
Registered User
rrcdoug's Avatar
I do n't think the pitch change is permanent. I think it is a transient result of moving into lift or sink. I think the downwind upwind analogy is very pertinent. Silent-8AVR said it all, it definitely flies different, but I think it should handle the same. Can you tell the difference when your plane is 2000 ft up and the whole field is lifting?
Dec 08, 2016, 08:20 PM
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Miami Mike's Avatar
Thread OP
Quote:
Originally Posted by slowmatch
Does this help? The wings angle of attack remains the same but the angle of the oncoming air changes and so the model looks different with reference to the horizon.
Sorry, but no, I don't think that drawing is legitimate. Here's why:

Imagine flying a small model plane around inside a very large elevator. While the elevator is stopped, your model flies normally. When the elevator starts going up, you feel the upward acceleration and your model plane "feels" it too, responding in some manner. But after the elevator reaches speed and the acceleration ends, the model recovers and flies exactly as it did when the elevator wasn't moving. The elevator is an inertial frame of reference. And if it was a glass elevator, an observer on the ground would not see the plane's pitch to be any different than you'd see it from inside the elevator. You'd both see it flying with the same pitch it normally has, but of course the observer on the ground would see that the plane is going up.

Dec 08, 2016, 08:50 PM
Registered User
Mike:

I'll respond to your questions in order.

It is common wisdom... I've seen my airplane fly by with tail down plenty of times. The only way it can do that, is if I've changed the position of the elevator or flaps, such that the airplane is now trimmed too much nose up for the conditions. I have to re-trim to get the nose down. I do not believe the tail settles (or the nose rises) because of lift or sink. Like others above have stated, the airplane is in an inertial frame moving with the air mass. So, short answer: no, I don't believe it. But, maybe I'm wrong?

...any airplane flying tail down... Yeah, I believe this one. If an airplane thought to be in good trim starts flying (in a steady manner) tail down, it is because something changed. In my experience, that something is initiated at the transmitter: either a change in trim setting or a hand on the pitch control. I can only speak for myself, here. I work hard on NOT touching the controls.

As it transitions.... This statement is based on my long ago education in aeronautics. A gust was defined then (for purposes of analysis) as an "instantaneous change in perceived wind". It was depicted as a step change in air speed (velocity, direction, or both). Airplanes are trimmed for a given speed, and will pitch up or down to return to that set speed. That is the basis for pitch stability, in the simplest sense.

That's my rationale, but it has no bearing on whether or not I am right or wrong. I see little in any of the posts above that show anyone of us to be in either the "wrong" or the "right". We are all struggling to understand what is going on, and these kinds of questions help us. All of us, even me.

So, bring it on!

Yours, Greg
Dec 09, 2016, 04:35 AM
Registered User
slowmatch's Avatar
Quote:
Originally Posted by Miami Mike
Sorry, but no, I don't think that drawing is legitimate....
Hmm. I can see what you are saying. This is as bad as the downwind turn (except I understand that one )

I think where the drawing goes wrong is that it doesn't take into account the vertical movement of the model. If you plotted it all out you could probably prove that the attitude remains the same as you say?
Dec 09, 2016, 11:32 AM
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Miami Mike's Avatar
Thread OP
Quote:
Originally Posted by slowmatch
I think where the drawing goes wrong is that it doesn't take into account the vertical movement of the model. If you plotted it all out you could probably prove that the attitude remains the same as you say?
Not exactly. I don't believe the attitude remains the same, I believe a glider responds to upward or downward acceleration when it enters rising or sinking air, but the acceleration ends as the glider reaches its new vertical velocity, and it then returns to its normal pitch attitude as it flies within its new inertial frame of reference.

My own conjecture is that the vertically moving air acts more strongly on the stabilizer than the nose because the stabilizer has more drag in the vertical direction. If I'm right then that leads to some interesting predictions that could actually be tested:
  1. If a glider is positioned on the ground with one wingtip straight up and the other straight down, and allowed to swivel horizontally about its CG (or perhaps about the wing's center of lift, which is almost the same thing), it will act like a weather vane. The nose will point into the wind. (Call this "Experiment #1.")
  2. If a second stabilizer is attached to that glider, out in front of the nose, designed to counteract and neutralize the vertical drag from the rear stabilizer so that the plane no longer behaves as a weather vane in Experiment #1, that glider will no longer signal lift while in the air. When it encounters lift its pitch attitude will not change.
  3. Since acceleration is what causes a pitch change, a glider could actually exhibit temporary downward pitch when passing from bad sink into not-so-bad sink, or from sink to neutral air, misleading the pilot into thinking that he's found lift.
  4. Similarly, a glider could exhibit temporary upward pitch when passing from strong lift to weaker lift, misleading the pilot into thinking that he's found sink.

Here's an afterthought: Some folks complain that they wouldn't have to put so much lead in the nose if they'd only make the nose longer, but maybe they don't because the glider wouldn't signal lift as well.
Dec 09, 2016, 11:54 AM
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Miami Mike's Avatar
Thread OP
Here are a couple more afterthoughts based upon my conjecture above, which, of course, could be wrong:
  1. It may be better for a long ballast bar to be placed horizontally inside the wing instead of longitudinally inside the fuselage because the reduction in the moment of inertia along the pitch axis should result in more dramatic signaling of lift.
  2. A variometer indication will lag behind a change in pitch because the pitch change happens as soon as the plane enters new air, while the variometer indication doesn't happen until the vertical speed of the glider actually changes. This is caused by the mass of the glider and its consequential inertia.
Dec 09, 2016, 12:52 PM
IT'S NOSE HEAVY!!!!
cityevader's Avatar
Here is my thought on tail lift/drop entering lift (still technically off topic, sorry).

I see lots and lots of this within a single flight, exactly at eye level (hitting thermals while slope soaring). The tail rises during that transition into lift (contrary to what lots of neck-craning thermal guys say).

My theory? A trick of the eye. Just as I can wig-wag a solid pencil just right to make it look like a floppy rubber one, when the plane is in the process of "re-orienting" to lift, the plane is simultaneously changing altitude AND and attitude, in a trajectory foreign to what the brain is used to seeing.

How many brain teaser picture puzzles need to be made, to know that the eye/brain can easily get dis-comboobulated?
Dec 09, 2016, 01:52 PM
R2R
R2R
Registered User
R2R's Avatar
Here's a possible wrench thrown into the discussion so far.... if the glider has nuetral stability, i.e., aft CG, the model may not easily regain normal flight pitch after the transition into sink; therefore, the tail will continue to fly lower until the pilot initiates some down elevator.

Like wise, into lift, the tail will remain high and the model will continue to remain in the pitch-down position and speed up until the pilot corrects. Been there, done that with an aft CG balanced sailplane.


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