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Old Dec 04, 2012, 11:52 AM
Red Merle ALES
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Quote:
Originally Posted by AMBeck View Post
Something to consider is that there are several different kinds of pitch stability. Each one requires a different approach to arrive at good handling, and they interact. Three in particular will make life interesting.

The one everybody knows is static pitch stability. It is the ability of the aircraft to hold a trimmed angle of attack. This is controlled by CG position and tail volume. Moving the CG back, or making the tail smaller, gets you less and less static pitch stability.

The other pitch stability you are sure to have to deal with sooner or later is dynamic pitch stability. This is proportional to tail length squared. It is the aircraft's ability to return to a trimmed AoA after a disturbance. A dynamically unstable plane will fly normally until you stall or dive, then it will tend to stall-dive-zoom-stall-dive-zoom...until you get on the stick and force it to fly level, at which point it will behave nicely, until it stalls (or dives) again. You can have a plane that is statically stable and dynamically unstable. Unless the dynamic instability is bad, it's more of an annoyance than a problem, but it can make speck-out climbs interesting. Moving the CG forward tends to make dynamic problems worse. The best way to increase it is to lengthen the tailboom.

The third of the big three is phugoid instability. This is the tendency of a plane to hunt in pitch around the trim attitude. Most planes have phugoid instability to some degree, but it is rarely an issue and the pilot doesn't even notice. It is more of a curiosity than a problem, but it can be going unnoticed in the background and increase the pilot's workload. You see it occasioanlly in a long hands off glides in very smooth air. The plane will gently pitch the nose up and down, and your straight glide will turn into a gentle sine wave flight path. Airplanes with large horizontal tails seem to be more suceptable to this. P-51s reportedly had some interesting phugoid stability issues, especially with the long range tanks full.

The spreadheets from Curtis (CloudyIFR) work great. If you use those and set up carefully, the first flight will not be too stressful. Last spring I flew a competely new design and it worked right off the board, needding only minor trim tweaks. The only adjustment I make to the spreadsheet is to increase the tail efficincy to 80-90% for airfoiled stablizers. (Leave it at the initial setting for flat surfaces.)
Right on!
Thanks for the kind words on my spreadsheet.

Curtis
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Old Dec 04, 2012, 12:33 PM
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In order to gain clarification (or up the entertainment value of this thread) I'm going to argue with Gordy.

Quote:
Originally Posted by Gordysoar View Post
IF decalage isn't important WHY did we all go to full flying stabs?...because fixed stabs fixed us into a certain incidence, and the only way to compensate for the wrong decalage was to hobble the model with nose lead.
Or adjust the elevator trim (which is what I thought Dr. Drela was saying). And since that means having an elevator that doesn't line up with the stab (heaven forbid!) there would be this desire to either fix the incidence (go for it) or change the nose weight (wrong answer). But like Dr. Drela said, You just set the elevator trim to where it needs to be (regardless of whether or not it lines up with the stab).

Quote:
Originally Posted by Gordysoar View Post
Why would you test a model at a speed it won't ever be flown at during thermaling?
To exaggerate the effect of the tail, so that you can tell if it's doing a lot of work to keep the nose up at thermal speeds. This is all assuming that you've trimmed the model for level flight at thermal speeds.

Quote:
Originally Posted by Gordysoar View Post
And lets go on record for one other point...you offered this fella advice yet never asked if his model had a fixed stab or full flying?
Does it matter? Airspeed empowers tail feathers...full flying or stab/elevator.

Quote:
Originally Posted by Gordysoar View Post
Airspeed empowers tailfeathers....and by the way it's the tail that directs the nose. IF you load up enough lead in the nose a model, the only way it will fly level would be to add some up ELEVATOR trim. THEN when its put into a dive, the tail gets more powerful and can force that excess nose lead away from gravity's grasp. (the tail gets stronger and can direct the nose up).

Task sailplanes have zero incidence...why? Because their pilots need them to tell them about lift and sink...not airspeed changes! They definitely should pull out of a dive! See Mike Smiths explanation of the dive test on Radio Carbon Art's Masters of Soaring video...he starts his dive at about 1,000 feet and if it pulls out before it hits the ground, its balanced properly.

Airspeed empowers tail feathers and if the tail is being forced to carry extra nose weight, then it will be set with up trim....a crooked sailplane...that when sped up, raises its nose and when slowed down drops its nose....that means the pilot can't tell if the model is indicating conditions or airspeed. He can't trust the information.
I couldn't agree more. So isn't that why people do dive tests? So they can tell if they're carrying too much nose weight?

Quote:
Originally Posted by Gordysoar View Post
Change the CG on a plane with a fixed stab and you have an unflyable plane.
If you don't adjust the elevator trim too.

Quote:
Originally Posted by Gordysoar View Post
We all dream of lighter sailplanes and sailplanes so clean that they glide forever on the lightest thermals...yet you have no qualms about advising guys to make their models crooked and heavier....remember airspeed empowers tail feathers and the only way a sailplane will pull out of a high speed dive is if the tail directs the nose to do so...a crooked airframe.
Ok, you've lost me a bit here. Earlier you said they definitely should pull out of a dive when referencing Mike Smith and the video.

The dive test is just that, a test, that gives you information about your plane's setup. If it's trimmed properly for thermal speed and it pulls out very quickly, then you've probably got your CG too far forward. Or is this not true?

Jim
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Old Dec 04, 2012, 03:43 PM
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When I first started sailing, I was stuggling with all the settings and forces involved in unpowered flight, they are more critical and have more influence than a screw job.

I found Gordy's writings and it all became crystal clear. The very first article I read was this one....hope you don't mind me posting this here Mr. Stahl, everyone who is confused should read it.
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Old Dec 04, 2012, 06:29 PM
MrE
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Ah! Those two articles answered my questions to Gordy - especially the first one.

Thanks!
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Old Dec 04, 2012, 07:11 PM
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Geez, what's next, the downwind turn, again?

Sal's out of business, who's gonna get bashed this Winter?
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Old Dec 04, 2012, 07:28 PM
Red Merle ALES
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Geez, what's next, the downwind turn, again?

Sal's out of business, who's gonna get bashed this Winter?
What does this have to do with the nice learning conversation we are having?

Happy Holiday's from Montana!
Curtis
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Old Dec 04, 2012, 07:40 PM
MrE
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I think he is referring to the dive test discussion - which has been done more than a few times - do a search for "dive test" in titles only and there are a bunch right there.

I have read most of them, but this time it finally clicked - thanks to the last two articles of Gordies that were posted above. Ive read those before too, but for some reason it finally got through my thick skull
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Old Dec 04, 2012, 08:14 PM
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I don't really disagree with anything Gordy had to say, except the part about a dive test being dumb. It's only dumb if you don't understand all of the other stuff he was talking about. The point is, if your plane is out of balance/has the CG too far forward/is nose heavy or whatever you want to call it, it's going to exhibit specific behaviors that are counterproductive to the task at hand. There are different ways to expose those unwanted behaviors/conditions. Brian Agnew has his way, Gordy has his way, and Mike Smith has his way. They all work...as long as you understand what's going on.
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Old Dec 04, 2012, 08:37 PM
Red Merle ALES
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Originally Posted by GliderJim View Post
I don't really disagree with anything Gordy had to say, except the part about a dive test being dumb. It's only dumb if you don't understand all of the other stuff he was talking about. The point is, if your plane is out of balance/has the CG too far forward/is nose heavy or whatever you want to call it, it's going to exhibit specific behaviors that are counterproductive to the task at hand. There are different ways to expose those unwanted behaviors/conditions. Brian Agnew has his way, Gordy has his way, and Mike Smith has his way. They all work...as long as you understand what's going on.
Well said! Wish I had said it. There are too many smart folks using the Dive test method for it to not be effective.

Curtis
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Old Dec 04, 2012, 09:34 PM
MrE
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Im not so sure he was saying a dive test was useless or dumb as much as he was saying it shouldnt be the primary, be all, end all test. Also that it doesnt really test CG at all - it tests decolage or trim setting. I think he was mostly saying that the dive test is mis-used and mis-understood by most folks. of course, he was saying t in a Gordy sort of way

As glider Jim - and Gordy said - you need to take more things into account.
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Old Dec 04, 2012, 09:37 PM
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Ive read all of Gordy's papers before in RCSoaring Digest but Ive never actually tried his balance/trimming technique. I will do that now and see where i end up compare to my present settings

It might end up a little too neutral for my skill level, but we will see.
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Old Dec 05, 2012, 12:38 AM
IT'S NOSE HEAVY!!!!!!!
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I'm soooo hoping there's flyable conditions this weekend. Haven't flown in a month!

It'll also be soooo nice to be able to make some high speed low passes on the slope and not be constantly inputting different amounts of elevator depending if it's left-to-right vs right-to-left or faster or slower or upwind/downwind... Whew!! It'll be sooo nice to be able to finally just steer it around!

Thanks folks!!

Edited: Oh yeah, and on the worst plane-with cg going the wrong way, I did cut back all throws after adding much needed nose weight and that helped it even more. It just needed a bit more wind since it's a couple ounces heavier than most Slope Monkeys.
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Old Dec 07, 2012, 09:42 AM
Red Merle ALES
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You know I've been thinking about this more and a friend has given me a bit of guidance. You see in my work I have lots of time just sitting and staring straight ahead thus giving me time to think. I respect Gordy as he's been around a long time, no offense, that's a good thing but I think we're all accomplishing the same thing but just differently. In other words, both methods work.

In Gordy's post he says that:
Quote:
"When you put a sailplane in to a dive, you are putting into a test environment that it WON"T BE FLYING IN during its task. Why would you test a model at a speed it won't ever be flown at during thermaling? It is thermaling that we are after right?"
In his article he says:
Quote:
Next step, launch the plane and get it trimmed in the air for flat and level flight. Then flip it over inverted, your goal is to pull lead until almost no (that like almost NONE) down elevator is required to hold the plane inverted in level fight.
I don't fly my thermal duration sailplanes inverted but I do speed them up once in awhile to zip from one thermal to another, but never quite to the speed that I obtain during the dive test, close but not quite.

However, If I'm flying a Fun Cub or my E3D aerobatic plane that has an electric motor in the nose, I DO NOT perform a dive test to check the balance but I DO perform the inverted test. It works well.

So as everyone else has said, it's apples and oranges as to which test is performed.

Curtis
Happy Holidays
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Old Dec 07, 2012, 09:49 AM
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The inverted test seems to pretty much get my planes right where I like them.
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Old Dec 07, 2012, 10:47 AM
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We don't fly Thermal planes inverted!

But the inverted test is one that is done in flat and level flight....that's the speed that our models spend most of their task time.

IF there is one dot more lead in the nose of a sailplane, then it has to have up elevator in order to maintain 'normal' flat and level glide speed.

"Normal" ? Yup...in my article I explain that airspeed empowers tail feathers, that without enough airspeed for the tail to hold a nose with too much nose weight, gravity will reach up and pull the nose to the ground when the airspeed slows enough. Properly balanced (approx zero incidence) when the model slow to near zero airspeed the model will settle on to its tow hook.

So why the inverted test? Well mostly because its just something else to do for the heck of it, but aside from that, if the model has UP trim in its elevator, when inverted it will now have down trim. That is exaggerated by the pitching moment of the wing inverted (its why we flick to reflex when doing those high speed inverted passes:-).

If your model fly's nearly hands off, no down elevator having to be held in the stick when inverted, its pretty darn close enough to perfect. (If none is needed, put a dab of lead back in, so that your model doesn't indicate every single ground fart during landings! That's called good hobbling :-)

Curtis, you have some really good stuff on your website, but its okay to weigh it against current information. Our hobby continues to evovle, not so much cuz we know more but because we have been learning and because of design changes. Stab airfoils, airframe materials, the lack of aero-elasticity in airframe and radio component precision, strength and resolution.

We went from fixed articulated stabs to full flying stabs, to fixed articulated Vtails to Xtail full flying to Tipper Full flying and now back to fixed articulated horizontal and V's. That didn't happen because of performance or preference its happened because of global changes in our equipment and knowledge.

We want cheaper, more durable, lighter and better in that order. The changes don't always improve in all those categories. Vtails died because of transportability, not because they were good or bad.

We went from spans of 112" to a standard of 156", airframe weight was closer to 80ozs to now closer to 60ozs. Tip weight is half what it used to be.

Articles about dive testing and balancing to preference only cause guys to apply ideas that applied to models and equipment of their publication era.

Today pilots learn to fly models that fly straight and level regardless of airspeed...imagine a DLG with a dot too much lead in the nose, violently thrown, it would loop behind into the pilot's head :-(, BUT because there are still guys trimming for preference we now have radios we can set with a switch to compensate for up incidence settings on launch. (I did not say for incorrect balance).

Things have changed, we couldn't fly models set the way they can be now ...then.
Gordy
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