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Old Nov 01, 2012, 08:23 AM
Chuck 'Em and Chase 'Em
Fly2High's Avatar
United States, NY, Plainview
Joined Aug 2005
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How should you fly a sailplane? positive or neutrally stable

guys,

I am reading again on trimming a plane. In doing so, I ask, what it the better way to st one up?

Ignoring turbulence and wind.


Which to choose:

1. Neutral stability. - plane is pointed in the direction you want to go. It will continue on whatever path you set it. Plane will hold path and speed until instructed otherwise even when sticks are returned to neutral.

2. Positive stability. Cg is a little forward. Plane will desire one speed to fly. Any disturbance will cause it to try to return to that setting. If flying too slow, it will drop a nose. If flying too fast, it will pitch up. To alter its path one must HOLD control input. once released, the plane will attempt to regain trim attitude and speed.

Can you give me pros and cons to each and why you prefer to fly a plane one way over the other?

To me, I think I can fly both but not sure what is better. I looked at it from the standpoint of flying long distance and positive stability seems good.

If I look to make all stick inputs do about the same thing for pitch, yaw and roll, I think (and could be wrong) that when you enter roll or yaw input and release, depending on the plaen, it might remain with a banked wing or will remained yawed (to some degree) Do they return to level and striaght flight if control input is returned to neutral sticks? Not sure.

Any help to understand better would be greatly appreciated

thanks
Frank
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Old Nov 01, 2012, 08:30 AM
Hey was that a Thermal ????
Joined Nov 2009
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WOW Frank !!! You really are looking to stir the pot !!

I like both for different reasons but over-all I like my planes more neutral these days based on your definition.A neutral plane is more tolerant of different conditions, speeds and trim.

Nest week I may change my mind though.
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Old Nov 01, 2012, 10:06 AM
Thermal, where art thou?
BavarianCharles's Avatar
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Quote:
Originally Posted by Fly2High View Post
guys,

I am reading again on trimming a plane. In doing so, I ask, what it the better way to st one up?

Frank
Quote:
Originally Posted by The_Builder View Post
WOW Frank !!! You really are looking to stir the pot !!
Keep stirring - this topic has been discussed many times before, but is buried in older threads. Rehashing stuff keeps discussions going and provides renewed visibilty to newbies, which is good.

I like almost neutral, and prefer a slightly squirrily plane to one that is solid but lethargic. The defining moment is the landing: If it comes in and just seems to glide forever, without being overly sensitive, then the setup is good. If the nose suddenly drops to the ground, then no good. I don't use the dive test, since it only indicates trim, unless the CG is way off.

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Old Nov 01, 2012, 10:08 AM
G_T
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Joined Apr 2009
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Frank,

On pitch, you want at least some level of positive stability. These are sailplanes; the goal is to stay airborn! Flying off optimal speeds for any given camber setting is just a faster way to the ground.

For yaw/roll (they are coupled in DLGs) the plane may exhibit some small tendency to level itself, if the bank angle produced by the disturbance is small.

But really, it is the pilot's job to fly the plane. Just set it up so it can fly the longest without intervention. That lets the pilot do the bigger job, which is deciding where the heck he should be flying!

Gerald
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Old Nov 01, 2012, 10:41 AM
Launch high. Fly low.
United States, CA, Lake Elsinore
Joined Aug 2003
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+1
What Gerald said.
I probably fly juuuust a little more positive than 'some level of positive stability'.
On a StobelV3, for example, 76 mm might be considered neutral. I've heard some guys fly it at 74 and thought it was positive. I fly mine at 70-72 all around flying and 68 to 70 in turbulent/windier conditions.

So, definitely positive.

Soar!
Jun

Quote:
Originally Posted by G_T View Post
Frank,

On pitch, you want at least some level of positive stability. These are sailplanes; the goal is to stay airborn! Flying off optimal speeds for any given camber setting is just a faster way to the ground.

For yaw/roll (they are coupled in DLGs) the plane may exhibit some small tendency to level itself, if the bank angle produced by the disturbance is small.

But really, it is the pilot's job to fly the plane. Just set it up so it can fly the longest without intervention. That lets the pilot do the bigger job, which is deciding where the heck he should be flying!

Gerald
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Old Nov 01, 2012, 11:20 AM
Will fly for food
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Bellevue WA,
Joined Dec 2003
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It also depends on the type of plane and where you fly it. For F3J ships I like mine very near neutral or even a little behind. For my slope rockets they have to be neutral. My DLG's tend to be forward of neutral. My 3D foamy's are way back from neutral.
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Old Nov 01, 2012, 11:39 AM
Chuck 'Em and Chase 'Em
Fly2High's Avatar
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SO David, when you set them up a so little positive it barely will self correct or at least it will take lots more time to self correct. Is this true?

With so little, what benefit do you gain?

So you prefer to ahve your planes be guided all the time.
Do you still feel like you can take your eyes off the plane to scope out lift or that is a plane dependent thing?

I am not sure how I want my planes. I know I do not like the plane constantly pitching up on launch but that might be that I have my trim set too slow even for speed mode. Being neutral will help with that but I do liek to search the sky and know my plane will be moving at a certain speed. Not sure how much i like the idea that some disturbance could cause the plaen to slow up or pick up speed without my input. Obviously this is relative.

Thanks for the input guys. Keep it coming.....

Frank
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Old Nov 01, 2012, 11:44 AM
Chuck 'Em and Chase 'Em
Fly2High's Avatar
United States, NY, Plainview
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Jun,

So what does it feel like to you when you fly sicne you are the opposet of David.

Do you find that you need to hold control input while running home from downwind to get the plane to run? I would think that david would give an input of down, eventually release it when thepitch desired is reached and the plane will continue at that attitude until something (conditions or himself) changes it. He then only needs to enter slight direction control when a peturbance alters his direction.

SO I guess I should look at this as 3 possibilities:
1. Lots of positive stability
2. slightly positively stable
3. Neutral

Jun you are a strong thrower. How do you overcome the tendency to pitch up on launch since I would suspect that your speed mode is still slower than your launch speed?

Thanks guys. This is great.

Frank
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Old Nov 01, 2012, 12:00 PM
Yes..ok..maybe..lol.....
Tucson Avra Valley, Arizona, United States
Joined Jul 2004
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Positive...........definitely.........much easier to control when ranging out at great distances.
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Old Nov 01, 2012, 12:11 PM
Will fly for food
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Bellevue WA,
Joined Dec 2003
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Quote:
Originally Posted by Fly2High View Post
SO David, when you set them up a so little positive it barely will self correct or at least it will take lots more time to self correct. Is this true?

With so little, what benefit do you gain?

So you prefer to ahve your planes be guided all the time.
Do you still feel like you can take your eyes off the plane to scope out lift or that is a plane dependent thing?

I am not sure how I want my planes. I know I do not like the plane constantly pitching up on launch but that might be that I have my trim set too slow even for speed mode. Being neutral will help with that but I do liek to search the sky and know my plane will be moving at a certain speed. Not sure how much i like the idea that some disturbance could cause the plaen to slow up or pick up speed without my input. Obviously this is relative.

Thanks for the input guys. Keep it coming.....

Frank
Again it depends on the plane. If we are talking DLG's here, the 5 DLG's I have had a lot of time flying all ended up on the positive side. Some more so than others. All DLG's fly very smoothly compared to other glider types and have a positive stability (due to the light wing loading) that just cannot compare to most any other gliding platform. The relative differences in CG placement can be felt by the pilot when the CG is moved but they are nowhere near as extreme as with other types of gliders. The pitching up of a DLG that has the horiz stab in the correct angle to the wing, usually is a neutral to negative condition. Don't be afraid to grab a quarter (or two) and tape them to the nose and "prove" to yourself what a really nose heavy DLG feels like. Then do the same to the rear but use a dime on the tail and feel what a tail heavy DLG feels like. You will not know how you like your flying style until you experience both. You should do this to EVERY airframe you have. Each will respond differently
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Old Nov 01, 2012, 12:15 PM
Hey was that a Thermal ????
Joined Nov 2009
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Originally Posted by BavarianCharles View Post
I like almost neutral, and prefer a slightly squirrily plane to one that is solid but lethargic. The defining moment is the landing: If it comes in and just seems to glide forever, without being overly sensitive, then the setup is good. If the nose suddenly drops to the ground, then no good. I don't use the dive test, since it only indicates trim, unless the CG is way off.


Yah like that for me !!
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Old Nov 01, 2012, 12:41 PM
Launch high. Fly low.
United States, CA, Lake Elsinore
Joined Aug 2003
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Actually not so different from David.
My DLG's are positive but my open class Xplorer is pretty neutral.
We need to set CG as personal preference and then trim for straight flight. I have some down trim on all my DLGs flight modes. Being very familiar and comfortable with my planes, I know the speeds they will fly best on every flight mode. The plane will get a down blip when it starts pitching up at speed, definitely. Although, sometimes I tend to stall the plane. LOL. just ask jfinch. Coming home at speed, I like to 'stair step?' the plane, instead of a straight run with controls held. Feels like I get more distance. Being positive helps with this.

I'm not really a strong thrower, thanks, but how do I overcome the tendency of the plane to pitch up at launch? Throw above the horizon- around 20ish degrees; well-timed launch preset release (just before the plane reaches desired attitude; depends on winds, too, and this happens almost instantly); my speed mode has plenty of down trim to fly straight as long as possible.
I think this is what I do... LOL!!! I'm so comfortable with my Stobels that I don't think about it anymore.

To keep it simple, positive CG and compensate with trims and manual input. The plane is stable and really doesn't need much input. I'll never fly sailplanes with aft CGs. I've tried it many times and I don't want to babysit the plane because it reacts to every single puff or gust. Paul Anderson taught me this a few years ago.

This is how I like my planes to fly. To each, his own, of course. Again, for those who are new or want to try something different-- It never hurts to try.

Soar!
Jun
Quote:
Originally Posted by Fly2High View Post
Jun,

So what does it feel like to you when you fly sicne you are the opposet of David.

Do you find that you need to hold control input while running home from downwind to get the plane to run? I would think that david would give an input of down, eventually release it when thepitch desired is reached and the plane will continue at that attitude until something (conditions or himself) changes it. He then only needs to enter slight direction control when a peturbance alters his direction.

SO I guess I should look at this as 3 possibilities:
1. Lots of positive stability
2. slightly positively stable
3. Neutral

Jun you are a strong thrower. How do you overcome the tendency to pitch up on launch since I would suspect that your speed mode is still slower than your launch speed?

Thanks guys. This is great.

Frank
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Old Nov 01, 2012, 01:02 PM
hot air rises...
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Pleasant Grove, UT
Joined Jul 2005
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Originally Posted by jcats View Post
... Although, sometimes I tend to stall the plane. LOL. just ask jfinch. ...
Mum's the word here ... sure with you were going to AZ this weekend!

So while we're here making confessions, I think I've been flying my Polaris a bit too tail heavy. I've added a bit more nose weight (more positive stability) and it hasn't really affected the air reading of the plane but the tail seems to "drag" around the sky less.
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Old Nov 01, 2012, 01:15 PM
a.k.a. Bob Parks
Glendale, AZ
Joined Jun 2008
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Originally Posted by BavarianCharles View Post

I like almost neutral, and prefer a slightly squirrily plane to one that is solid but lethargic. The defining moment is the landing: If it comes in and just seems to glide forever, without being overly sensitive, then the setup is good. If the nose suddenly drops to the ground, then no good.
I am with Charles, but like Ryan says about himself, I am not a good DLG pilot!

It takes a bit of testing to get flaps and elevator balanced for a smooth landing - tip catch approach. I am finally getting my plane to come in steady enough to attempt tip catches!

BP
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Old Nov 01, 2012, 05:12 PM
Will fly for food
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Bellevue WA,
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Is it just my opinion or do others agree that the Zone 2 wing tends to tollerate a more positive CG placement?
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