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Old Jan 22, 2013, 02:06 PM
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Enlarging a micro jet CG?

Hello,

I have a 16" span scale FJ-1 Fury. Even with the CG almost on the LE it is still a little tail-heavy in flight.

My question is this.................. if I build a 48" span version, will the CG still be so far forward? So would it be safe to maiden the big one using the little one's CG?

The section used on the micro is Jedelski.

Thanks Steve
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Old Jan 22, 2013, 10:07 PM
agnotology
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Why do you think it is still tail heavy with the CG so far forward?

Kevin
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Old Jan 23, 2013, 04:09 AM
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Kevin

The model is unflyable with the CG any further back..... As I found out the hard way!

With the CG where it is the elevator is still very sensitive. This improves with noseweight.


Steve
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Old Jan 23, 2013, 09:22 AM
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Was the micro Fury rather under powered ?

As I can see that giving a similar flight to a rearward CG.
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Old Jan 23, 2013, 10:05 AM
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The tail surfaces have a rather high aspect ratio. Especially at small scales, they should be in a Reynolds number range significantly different than that of the wings. I think that the larger version will not be as extreme as the original, and will have a greater allowable CG range.
[edit:] Specifically, airfoils at lower Reynold numbers have a smaller critical angle (http://digital.library.unt.edu/ark:/...adc57640/m1/3/) so there's the possibility that the tail will stall before the wing (if the camber or AOA is unchanged) making stalls a bit harsher.
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Old Jan 23, 2013, 11:07 AM
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There is plenty of power and there has been a power upgrade since this video.

FJ-1 Fury Micro R/C (0 min 39 sec)


The high aspect ratio tail with low Reynolds number sounds interesting! The stall is not an issue and the Fury is a joy to fly.............. that's why I want to build another, but bigger.

Steve
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Old Jan 23, 2013, 11:58 AM
agnotology
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Originally Posted by stegla View Post
Kevin

The model is unflyable with the CG any further back..... As I found out the hard way!

With the CG where it is the elevator is still very sensitive. This improves with noseweight.


Steve
How much throw do you have on the elevator? Is the elevator linkage solid and free? How good is the elevator servo? Did you try reducing the throw?

I've built lots of small chuck gliders, some with high AR tail surfaces operating at way lower Re, and I've never had to use a CG that far forward. In fact, the predicted neutral point always seems to be very, very close.

How torsionally stiff is the Jedelski wing?

Something just doesn't seem right. The high AR stab will make the elevator more sensitive.

Kevin
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Old Jan 23, 2013, 12:53 PM
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Originally Posted by stegla View Post
Hello,

I have a 16" span scale FJ-1 Fury. Even with the CG almost on the LE it is still a little tail-heavy in flight.

My question is this.................. if I build a 48" span version, will the CG still be so far forward? So would it be safe to maiden the big one using the little one's CG?

The section used on the micro is Jedelski.

Thanks Steve
Simply put - the sensitivity will lessen as the size of the model increases- all other factors being equal-
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Old Jan 23, 2013, 01:35 PM
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Indeed lessening the elevator throw has helped a lot and I'm sure It could be reduced some more. But as it is the elevator is now just lively and I'm used to flying it that way. Take the cg back and you get a white- knuckle roller-coaster and a model to repair.

The model uses the onboard servos on a 6400 brick, one servo for each elevator half. There is about 4mm throw each way with 40% expo. The wing is reasonably stiff, soft 3mm balsa front section and .8mm medium balsa rear. Probably much better than foam.

Steve
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Old Jan 23, 2013, 01:39 PM
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From the CG location and the fact that you still get issues in flight and the way it flies in the video I'm thinking along the same lines as Kevin in thinking that the model has some other issue at work which you're confusing with being "unstable".

At high speeds or G loads that wing looks like it might be able to flex and thus cause the model to trip over its own shoelaces. Or are the issues you have occuring at low speeds when gliding?

More description of how it acts and the flight mode at the time might help a lot. Because as it stands now I agree with Kevin that there's something else at work. There's simply no reason why the CG needs to be located virtually at the leading edge like you're showing.
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Old Jan 23, 2013, 03:56 PM
agnotology
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Originally Posted by stegla View Post
Indeed lessening the elevator throw has helped a lot and I'm sure It could be reduced some more. But as it is the elevator is now just lively and I'm used to flying it that way. Take the cg back and you get a white- knuckle roller-coaster and a model to repair.

The model uses the onboard servos on a 6400 brick, one servo for each elevator half. There is about 4mm throw each way with 40% expo. The wing is reasonably stiff, soft 3mm balsa front section and .8mm medium balsa rear. Probably much better than foam.

Steve
The high aspect ratio stab will make the elevator sensitive, plus it looks like the elevator is a big % of the stab chord? I think you are mistaking elevator sensitivity for instability.

That wing airfoil will also have a very high Cm, far more than even a flat bottom section. It is possible that the wing is twisting in flight. That could make it prone to tucking at higher speeds.

You can check the CG by rolling inverted. Doing it in a 45 degree climb is best, if it has enough power - the drag of that wing section will be very high inverted. If it takes more than a smidge of down elevator to hold the 45 degree angle when inverted, then the CG is forward. Even the amount of down stick required for level flight inverted is a reasonable test. If the CG is too far back, it won't need any down elevator, or even up to maintain level inverted flight.

The fuse is very big and quite far forward, so it will be slightly destabilizing in both pitch and yaw. It looks like you might have used Dan's CG calculator spreadsheet, in which case the fuse should have been taken into account when calculating the neutral point.

Using a different wing section on a bigger model will remove the big wing Cm, and require less up elevator or negative stab incidence, whichever you have. But the high aspect ratio stab will still make it more sensitive to elevator, especially if it also has a large % elevator.

Kevin
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Old Jan 24, 2013, 10:03 AM
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When flown with an unstable CG position the model misbehaves in the glide making it very difficult to get back down without crashing. A bigger model would not have survived the maiden.

The Jedelski section worked flawlessly on a slightly larger Gnat that could go very quick indeed without twisting.

As is, the little Fury flares nicely with good elevator control when landing, also it glides on and on and you have to fly an aproach like a larger model. This tells me that the trim can't be too far off.

I shall try flying inverted next outing.

What do you guys think might happen if I balanced a big Fury, with a regular aerofoil, in the same place as the little one? Might the big fury be unflyable due to being nose heavy?

Thanks, I appreciate your input.

Steve
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Old Jan 24, 2013, 11:47 AM
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When flown with an unstable CG position the model misbehaves in the glide making it very difficult to get back down without crashing. A bigger model would not have survived the maiden.

The Jedelski section worked flawlessly on a slightly larger Gnat that could go very quick indeed without twisting.

As is, the little Fury flares nicely with good elevator control when landing, also it glides on and on and you have to fly an aproach like a larger model. This tells me that the trim can't be too far off.

I shall try flying inverted next outing.

What do you guys think might happen if I balanced a big Fury, with a regular aerofoil, in the same place as the little one? Might the big fury be unflyable due to being nose heavy?

Thanks, I appreciate your input.

Steve
Nope-- being nose heavy will likely just require more UP elevator to get desired glide. The stability will increase not decrease .
The bigger it gets, the easier it will be to sort out.
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Old Jan 24, 2013, 12:12 PM
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Nope-- being nose heavy will likely just require more UP elevator to get desired glide. The stability will increase not decrease .
The bigger it gets, the easier it will be to sort out.
Thanks Richard.................it's time to start building that big Fury.

Steve
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Old Jan 24, 2013, 12:15 PM
agnotology
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Originally Posted by stegla View Post
When flown with an unstable CG position the model misbehaves in the glide making it very difficult to get back down without crashing. A bigger model would not have survived the maiden.
...

What do you guys think might happen if I balanced a big Fury, with a regular aerofoil, in the same place as the little one? Might the big fury be unflyable due to being nose heavy?

Thanks, I appreciate your input.

Steve
Did you reduce the elevator throw on the TX, rather than mechanically? The resolution of those little servos is not that great. You should use the full throw and mechanically reduce the deflection for the best resolution.

A larger model with a "regular" airfoil will just have less nose down moment from the airfoil. It should fly with the CG where you have it easier than the little one does.

Try making a little chuck model of the airplane to convince yourself the CG will work back closer to the neutral point, maybe 10 to 15% static margin.

Moving the CG forward would make the elevator less sensitive. 4mm each way is still quite a lot on a little tail surface with a big elevator like that on a short coupled airplane. I fly my 62" pattern airplane with less deflection than that, on an elevator with 4x the width. It is always better to measure deflection in degrees, since that is consistent across sizes. 4mm each way is still a lot on a little model with a high aspect ratio stab.

With less elevator throw, you would likely find the CG can be placed at a more normal location, and the handling would improve. Especially for a bigger model that you might be rolling more often, a more rearward CG will make the rolls far more axial, and inverted flight easier.

Kevin
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