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Old 12-02-2013, 06:53 AM   #1
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Default COG measurements

While I am enjoying my Mini Switch, I am having a bit of an issue with cog. In low wing, cog is is measured at 68mm from the leading edge. When I do this, the plane is very nose heavy with the weights I have in. The plane likes to gain altitude quickly even at half throttle, except when banking of course. Would this be caused by being tail heavy, or something else? The plane is still very controllable, just I would like to put more throttle and not have the plane wanting to shoot up into the sky, but have a more gradual incline, and at lesser throttle have the plane keep a steady altitude.
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Old 12-02-2013, 07:02 AM   #2
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takie a line through mid leading edge to mid trailing edge and see what angle of incidence is compared to fuselage centre line.

You may have a combination of too much incidence angle and possibly not enough downthrust.

I would guess - too much wing angle of incidence. You can try this : Put a wedge or sliver between wing and fuselage at front and reduce that +ve angle .. fly and see what happens. It only takes a small amount.

You could also have a tail plane that is angled down ... nice for a high winger but possibly too much for a low winger. Not much you can do about that.

OK - CoG too far forward can lead to it ... at slow speed the weight fwd keeps the nose down, you trim and maintain a level cruise ... speed up and wing starts to overpower the fwd CoG and cause model to pitch up ... it's actually more complicated than that - but that's what you see ... so a forward CoG can lead to pitching up at increased speed. It's actually an addition to a natural tendency for climb with increased power anyway.

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Old 12-02-2013, 07:13 AM   #3
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Can to much forward cog lead to an increased altitude gain at higher throttle. Judging by the angle of incidence it is around 12 degrees or so. Just guessing here, but it's very close to that. I can't really alter the down thrust in this plane. Also, the tail has a down angle on the horizontal stabilizer, if that is what you are referring to.
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Old 12-02-2013, 07:18 AM   #4
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Originally Posted by thepiper92 View Post
Can to much forward cog lead to an increased altitude gain at higher throttle. Judging by the angle of incidence it is around 12 degrees or so. Just guessing here, but it's very close to that. I can't really alter the down thrust in this plane.
I thought I answered that ?

Yes.

And Wing incidence, aerofoil form is biggest reason for usual climb on increase of power .... greater speed means greater lift of wing ...

Stick hand at slight +ve angle flat out of car window as slow speed ... now speed up and feel what happens.

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Old 12-02-2013, 07:29 AM   #5
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Originally Posted by solentlife View Post
I thought I answered that ?

Yes.

And Wing incidence, aerofoil form is biggest reason for usual climb on increase of power .... greater speed means greater lift of wing ...

Stick hand at slight +ve angle flat out of car window as slow speed ... now speed up and feel what happens.

Nigel
Sorry I measured from the wrong spot I think to what you were referring to. I understand what you are referring to now. The angle isn't any more than a few degrees. Would increased lift from too much forward cog be caused by the angle of incidence and the weight sort of plowing into the air and being pushed up, due to the plane wanting to point down? I can't think of the issue being the angle. I'm still new to how to adjust plane properly. My understanding before is that tail heavy would cause the tail to want to drop, and with the angle changing, power would be applied to that angle.
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Old 12-02-2013, 07:42 AM   #6
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The usual result of tail heavy is extremely sensitive pitching of model ... literally kangarooing around ...

Here's a model that exhibits similar to a tail heavy model ...

https://www.youtube.com/watch?v=P-8ul4voRjc

It's actually the elevator was not aligned properly by me .. and too much movement ... but effect is same.

You can hear others in background remarking on 'twitchy, tail heavy etc.'

"SIT" of a model in normal flight is a combo of wing angle, engine downthrust and tail angle ... adjust one or other and the speed she sits at the design SIT alters. This manifests itself in each persons idea of cruise speed is different ... some like slow .. some like faster. That will alter the trim setting of same models elevators just because wing lift has changed because of speed.

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Old 12-02-2013, 07:52 AM   #7
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Originally Posted by solentlife View Post
The usual result of tail heavy is extremely sensitive pitching of model ... literally kangarooing around ...

Here's a model that exhibits similar to a tail heavy model ...

https://www.youtube.com/watch?v=P-8ul4voRjc

It's actually the elevator was not aligned properly by me .. and too much movement ... but effect is same.

You can hear others in background remarking on 'twitchy, tail heavy etc.'

"SIT" of a model in normal flight is a combo of wing angle, engine downthrust and tail angle ... adjust one or other and the speed she sits at the design SIT alters. This manifests itself in each persons idea of cruise speed is different ... some like slow .. some like faster. That will alter the trim setting of same models elevators just because wing lift has changed because of speed.

Nigel
Wow that is a really sensitive plane. Mine isn't sensitive at all, so I guess mine isn't tail heavy. So down thrust will bring the nose down and allow for a greater speed before there is altitude gain, angling of the tail down (the rear lower than front) will cause the tail to lift and angling of the main wings will cause the plane to have lift? Is that correct?
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Old 12-02-2013, 07:55 AM   #8
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If it's not tail heavy ... then it's fine ... go fly !!

Every plane has a cruise speed where all balances out ... you cannot design out that to be all speeds all balanced .. you can only compromise.

Even aerobatic planes have a balanced speed ...

Nigel

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Old 12-02-2013, 08:03 AM   #9
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Originally Posted by solentlife View Post
If it's not tail heavy ... then it's fine ... go fly !!

Every plane has a cruise speed where all balances out ... you cannot design out that to be all speeds all balanced .. you can only compromise.

Even aerobatic planes have a balanced speed ...

Nigel

Thanks for your help, I really appreciate it. I took all the weights out, and it turns out it balances perfectly without the weights (I originally had weights in it because it was tail heavy in high wing). I'll try it tomorrow and see how it goes, hopefully it will fly level at half throttle, and I won't have it trying vertical at full throttle, but rather at an incline like half throttle was today.
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Old 12-02-2013, 08:06 AM   #10
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Originally Posted by thepiper92 View Post
Thanks for your help, I really appreciate it. I took all the weights out, and it turns out it balances perfectly without the weights (I originally had weights in it because it was tail heavy in high wing). I'll try it tomorrow and see how it goes, hopefully it will fly level at half throttle, and I won't have it trying vertical at full throttle, but rather at an incline like half throttle was today.
BEFORE flying it ... reset tail trim to manuals advised figure ... you probably have UP trim in her from being too nose heavy ... that will now if you do not remove it - cause her to go nose up on launch and possibly stall.

Nigel

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Old 12-02-2013, 08:17 AM   #11
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Originally Posted by solentlife View Post
BEFORE flying it ... reset tail trim to manuals advised figure ... you probably have UP trim in her from being too nose heavy ... that will now if you do not remove it - cause her to go nose up on launch and possibly stall.

Nigel
Actually I had it an no trim at all. She was taking off perfectly fine on take offs and before half throttle, so I was just fighting with the elevator when I went higher in the throttle. Surprising inverted was perfectly fine too. I was flying around a lot inverted. Only issue is my rudder. I set it to center and when I go out I find it to the right when I turn the plane on, some what annoying having to adjust the trim to account for that. I am thinking the push rod is slipping on the rudder, so I just put a very small amount of glue, just enough to hold it a bit better. Should be perfect tomorrow, now that you have helped me out with the cog.
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Old 12-02-2013, 11:03 AM   #12
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?? How is your rudder connected to rod ?

How is the servo connected to rod ?

It is not unusual to have a servo with a part stripped gear that horn slips a few teeth ... I've had it often after a few harder landings ... you can often tell by just pushing the rod back to centre position ... it will click back to position. If it does it - change the servo - you cannot trust it anymore.

Nigel

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Old 12-02-2013, 04:18 PM   #13
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Originally Posted by thepiper92 View Post
Actually I had it an no trim at all. She was taking off perfectly fine on take offs and before half throttle, so I was just fighting with the elevator when I went higher in the throttle. Surprising inverted was perfectly fine too. I was flying around a lot inverted. Only issue is my rudder. I set it to center and when I go out I find it to the right when I turn the plane on, some what annoying having to adjust the trim to account for that. I am thinking the push rod is slipping on the rudder, so I just put a very small amount of glue, just enough to hold it a bit better. Should be perfect tomorrow, now that you have helped me out with the cog.
Regarding the rudder centering, when adjusting any control surfaces on a model, it (and your tx) should be on and the trims centered. If not, you may accidentally move a servo, or it may have been moved before you even started. With it powered on, you know it will be where you wanted. And remove the propellor when you do this! Many accidents happen like this if someone just bumps the throttle, their little brother comes along and goes "what's this? ", a while lot of scenarios. Just remove the prop. It doesn't take long.
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Old 12-02-2013, 06:08 PM   #14
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Originally Posted by solentlife View Post
?? How is your rudder connected to rod ?

How is the servo connected to rod ?

It is not unusual to have a servo with a part stripped gear that horn slips a few teeth ... I've had it often after a few harder landings ... you can often tell by just pushing the rod back to centre position ... it will click back to position. If it does it - change the servo - you cannot trust it anymore.

Nigel
It's connected to a control horn that the rod pushes through and gets tightened down with a screw. It is definitely not a servo missing teeth, the servo sounds perfectly fine. I've had a few experiences with servos snapping teeth, and this one is fine. It must be the control horn not having a good grip on the rod.
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Old 12-02-2013, 06:10 PM   #15
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Originally Posted by xmech2k View Post
Regarding the rudder centering, when adjusting any control surfaces on a model, it (and your tx) should be on and the trims centered. If not, you may accidentally move a servo, or it may have been moved before you even started. With it powered on, you know it will be where you wanted. And remove the propellor when you do this! Many accidents happen like this if someone just bumps the throttle, their little brother comes along and goes "what's this? ", a while lot of scenarios. Just remove the prop. It doesn't take long.
I always reset to zero for the trims, and then move the control horn to make it go left to center it. Then I power it on and see that it always goes back to center. When I get to the field and power it one, it always is back at a slight right position.
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