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Old Jan 26, 2013, 02:30 PM
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Originally Posted by cityevader View Post
you CAN glide at different speeds with the same trim if it's balanced. It should go exactly where it is pointed.
Actually, the only way you can glide at different airspeeds with the same trim is by changing what I think you call balance, i.e. the CG position.
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Old Jan 26, 2013, 02:34 PM
Phil.T-tailer
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Devon, UK
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Originally Posted by cityevader View Post
the entire goal of a balanced plane is so that it WON'T require different trims for different speeds; that is an indication of an unbalanced plane, and it is NOT desirable to have it fly different all the time just because speeds are different (an unbalanced plane).
After 16 pages of stuff which is mostly about CofG & "decalage"
how about some clarity on "trim" - which is one of the questions you started with?

Trim IS about flying the plane at different speeds.
Start by flying your plane nice & level - then hands off the controls - it flies at a certain speed - for that state-of-trim
add a few clicks of down elevator "trim" - again hands off - it will fly faster.
likewise if you add a few clicks of up trim - again hands off - it will fly slower.

If you ever get to learn in a full-size sailplane, you will be taught to adjust the trim lever so it flies hands-off at the speed you want - say 50 knots. Want to fly faster between thermals? - adjust the trim "down" so it now flies at, say, 70 knots.

Hope that makes sense?

NOTE - all of the above is for one particular CofG position.
and of course CofG position & elevator "trim" are inter-related - move the CofG forward, and you will need to add up trim to fly hands-off at the same speed as before - move the CofG back, and you will need to add down trim.
now, if you move the CofG forward and DONT retrim the elevator - what happens? - the plane flies faster hands-off - which for small changes is pretty much the same thing as adding a bit of down trim, except you cant change the CofG in the air.

Now - I'm interested to hear what other folks mean when they are talking about "trim" ?

Phil.
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Last edited by Phil.Taylor; Jan 26, 2013 at 02:48 PM.
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Old Jan 26, 2013, 02:51 PM
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Australia, QLD, Townsville
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Thanks for the feedback gents.
Like I said, it does fly OK on the glide ,but I reckon with some more experimentation, i can get better performence.

Cityevader,i'll try what you said about gordy's suggestion.

One thing the dynamic does is drop it's nose sharply sharply when I power down. So maybe with playing around with setups ,I can eleveate this.

I bought a graupner maxisport off a fellow gliderman ( Fisherman- gliderman ?). Now that's a whole new ball game flying that bird.
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Old Jan 26, 2013, 02:57 PM
I don't want to "Switch Now"
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Originally Posted by medroller View Post
I bought a graupner maxisport off a fellow gliderman ( Fisherman- gliderman ?). Now that's a whole new ball game flying that bird.
Glider Guider

Here in Canada the CBC insists on calling them "Fishers" so as to not be gender biased. That would mean those who fly gliders are also gliders ,
Or perhaps gliderers?
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Old Jan 26, 2013, 03:01 PM
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Originally Posted by HerkS View Post
What you describe is both normal and desirable. The why is somewhat complex, but it's mainly because of the interaction between the weight/CG which is pretty much constant and the aerodynamic forces which are proportional to speed squared. The slight down elevator at your ideal soaring trim and CG doesn't cause any problem and doesn't need fixed. It's normal too for higher speed flight to require more down trim in a well balanced sailplane.
To endorse what Herk said, every plane that has been trimmed out for near-level flight will have the CG ahead of the neutral point. It can then be said that the combination of the stab and elevator must be providing some downward force to offset the fact that the CG is ahead of the neutral point. In a simplified view, this is like a balance-beam problem. You may have an elevator that is slanted downward relative to the stab, but the combination of stab and elevator will be providing a net downward force.

When a plane speeds up (e.g. steep or vertical dive), the aerodynamic forces (net effect of wing, stab and elevator) increase. The CG ahead of the neutral point does not provide enough pitch-down force to offset the increased aerodynamic forces that are pushing the nose up. Thus the nose pitches up.
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Old Jan 27, 2013, 12:24 AM
MrE
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You guys are forgetting the several posts by top pilots who fly their models with a combination of CG/trim such that they tuck when in a dive and others that dont tuck or pull out in a dive.

Also Dr Drellas notes on the Bubble dancer clearly show a CG position that results in zero force on the stab. With zero force on the stab the model can easily be trimmed to pull out in a dive OR tuck OR do nothing.

I have played with my new BD and with the CG at his suggested point, the model can indeed be trimmed to do all three - and will still fly hands off level at all three settings. Of course, this results in 3 different "trimmed" speeds.

Well, actually there are any number of speeds depending on the trim setting, but you get my point.
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Old Jan 27, 2013, 03:30 AM
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Francesco and Phil, I think my personal definition of trim is much more broad than it should be, and you are correct. I wasn't referring to a properly balanced plane that was trimmed one way or another, rather, an unbalanced (nose heavy) plane requiring "trim"; My definition of trim also includes the stick corrections that are constantly required (during the slope flying I do) as the models speeds and slows, speeds and slows...the stick itself becomes the trim because you obviously can't take your thumb off the stick in order to hit the trim during a dive/high speed pass/rolling pullout into a near-hover at the top, dead-into-the-wind. This is what I was referring to as being undesirable, rather than desirable, flight characteristic.

And a side note fwiw, there's also a lot of TD guys chiming in here with an assumption that my topic was "single speed" trim and relation to cg etc. when in fact I had been seeking a "neutral" plane at all speeds.
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Old Jan 27, 2013, 07:13 AM
Phil.T-tailer
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Quote:
Originally Posted by cityevader View Post
My definition of trim also includes the stick corrections that are constantly required (during the slope flying I do) as the models speeds and slows, speeds and slows...the stick itself becomes the trim because you obviously can't take your thumb off the stick in order to hit the trim during a dive/high speed pass/rolling pullout into a near-hover at the top, dead-into-the-wind.
LOL - I call that "flying"

You & the slope guys move the CofG back for control responsiveness/neutrality
The TD guys move the CofG back for performance & lift signalling
On my local slope, I do both! - great thermals way out over the valley in front of the slope - I need those different "trim" speeds to get out there & hang there.

Good that we got you sorted - happy flying !

Phil.
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Old Jan 27, 2013, 07:44 AM
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Phil T has summed it up nicely. Just go fly.. Don't over hype the little details, it will do you no good, proven here on RCG countless times with umpteen types of gliders. You don't need the internet to become a proficient rc flyer.
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Old Jan 27, 2013, 04:29 PM
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Originally Posted by MrE View Post
You guys are forgetting the several posts by top pilots who fly their models with a combination of CG/trim such that they tuck when in a dive and others that dont tuck or pull out in a dive.

Also Dr Drellas notes on the Bubble dancer clearly show a CG position that results in zero force on the stab. With zero force on the stab the model can easily be trimmed to pull out in a dive OR tuck OR do nothing.

I have played with my new BD and with the CG at his suggested point, the model can indeed be trimmed to do all three - and will still fly hands off level at all three settings. Of course, this results in 3 different "trimmed" speeds.

Well, actually there are any number of speeds depending on the trim setting, but you get my point.
With the CG at the neutral point, there will indeed be no force on a flying stab. Top pilots typically set the CG very close to the neutral point in order for the plane to be highly responsive. However, it takes a lot of skill to fly at this CG. If the plane goes into a dive, it continues in that dive (or tucks). This is hard to sense when the plane is high in the air. For this reason, a lot of pilots place the CG ahead of the neutral point so that the plane has more pitch stability. If the CG is placed far ahead of the neutral point, the plane is sluggish and tends to easily get into large pitch oscillations.
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Old Jan 27, 2013, 08:32 PM
MrE
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Actually, you dont need the CG ahead of the neutral point - just a tad of up trim will do exactly what you want - pull out of a dive.

We just spent several pages proving this point - Its the trim setting (decolage) that determines if the model pulls out of the dive - NOT the cg position.

You can have your cake (a plane that indicates lift as well as it possibly can) and eat it too (have pitch stability)
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Old Jan 28, 2013, 11:41 AM
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Tom and Mike and several others have commented that as the Cg is moved closer to or at the neutral point, the performance improves?

1. Can you define what is performance?

2. How can I determine I have reached the maximum performance of a plane?

3. If Cg AND elevator trim setting determines if I pull out of a dive, What should be done to determine if maximum 'performance' is reached?
How do I know I am flying a plane too slowly and should not move the Cg further back?
Isn't it possible that I chose the wrong speed to start with?
Are we trying for min. sink or best L/d speed when we are trimming?

Frank
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Old Jan 28, 2013, 02:30 PM
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Quote:
Originally Posted by Fly2High View Post
Tom and Mike and several others have commented that as the Cg is moved closer to or at the neutral point, the performance improves?

1. Can you define what is performance?

2. How can I determine I have reached the maximum performance of a plane?

3. If Cg AND elevator trim setting determines if I pull out of a dive, What should be done to determine if maximum 'performance' is reached?
How do I know I am flying a plane too slowly and should not move the Cg further back?
Isn't it possible that I chose the wrong speed to start with?
Are we trying for min. sink or best L/d speed when we are trimming?

Frank
Frank,
You have some good questions there that I too would like to see answered. In addition to your questions can some one comment on:
  • Where is the neutral point on a RP?
  • If you move the CG further and further back, will you have to add down trim to keep it from stalling?
  • How do you know if you have the CG too far back?
Dave F
>< *>
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Old Jan 28, 2013, 02:58 PM
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Quote:
Originally Posted by Fly2High View Post
Tom and Mike and several others have commented that as the Cg is moved closer to or at the neutral point, the performance improves?

1. Can you define what is performance?

2. How can I determine I have reached the maximum performance of a plane?

3. If Cg AND elevator trim setting determines if I pull out of a dive, What should be done to determine if maximum 'performance' is reached?
How do I know I am flying a plane too slowly and should not move the Cg further back?
Isn't it possible that I chose the wrong speed to start with?
Are we trying for min. sink or best L/d speed when we are trimming?

Frank
1. Performance is hard to define. This discussion leads toward one definition, namely that the pitch response of the plane indicates lift well. A plane that has the CG slightly ahead of the neutral point will have pitch oscillations (phugoid oscillations) that are so slow that they are hardly noticeable in most instances. A long tail boom does a good job of dampening these oscillations.

2. The maximum performance of a plane is very dependent on your flying style. As the CG is moved back, the response to both wind gusts and transmitter commands is greater. This means that the plane is reliant on more active control by the pilot. As said earlier, controlling a plane at long distance and/or high elevation is difficult unless that plane has a modest amount of pitch stability. A case in point is cross-country planes that fly high. For these planes, the CG is farther ahead of the neutral point than would be the case for thermal duration competition.

3. Once the CG is established, elevator trim comes in. By changing elevator trim, the plane can be moved over the full range set by the L/D curves for that plane. One trim can place the plane at minimum sink. Another trim can place the plane at the maximum L/D. Other trims can place the plane at the maximum distance covered in a head wind, etc. A great presentation of this subject is at:

http://www.5c1.net/Glider%20Performance%20Airspeeds.htm
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Old Jan 28, 2013, 03:00 PM
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Originally Posted by got-one View Post
Frank,
You have some good questions there that I too would like to see answered. In addition to your questions can some one comment on:
  • Where is the neutral point on a RP?
  • If you move the CG further and further back, will you have to add down trim to keep it from stalling?
  • How do you know if you have the CG too far back?
Dave F
>< *>
You can find the neutral point on a sailplane by moving the CG back until the plane becomes uncontrollable in pitch.

As the CG is moved back, more down trim is needed to keep the glide slope constant.
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