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Old Sep 21, 2012, 10:53 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,636 Posts
Now my scratch built Eagle, (based on the EPP Eagle), flies a treat, and is a similar size at a guess.

Unless the model is quite heavy, it shouldn't need the rather enthusiastic launch. Mine will climb away from a gentle launch, and will try to go vertical on full power.

Control is just 'ruddervators', very responsive.

Span - 59"
Flying weight - 20oz
Motor - BM 2408-21 (the old 'bell' motor).
Prop - 8x4
Battery - 3s 1800mAh Lipo.
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Old Sep 21, 2012, 11:08 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,636 Posts
Just did a check.

The CG is at approx 20 - 21%. Chord is about 12" average, CG is 2.5" back fro LE.
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Old Sep 21, 2012, 11:14 AM
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Old Sep 21, 2012, 12:00 PM
agnotology
kcaldwel's Avatar
Joined Jan 2007
3,652 Posts
Quote:
Originally Posted by Montag DP View Post
While this is true, if your Cm is such that your trim angle of attack is negative, it will make for a pretty "exciting" maiden flight even if the CG is far enough forward. That said, from the video it looks like the problem is static instability. The Cm of the wing could be too high as well, but until the CG is fixed it will be hard to tell if that's the case.
Yep,

Sorry for the diversion. I've had the Cm/stability discussion too many times with Bruce, and didn't need to muddy the waters here with it.

Attached the latest updates from the Draw file Joel sent me. 10% SM should be about 1.7" back of the wing/fuse junction, it will be very sensitive to the actual as-built model geometry, and the position of the tail relative to the wing wake (higher velocity above, more effective tail).

Kevin
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Old Sep 21, 2012, 01:11 PM
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Joel K. Scholz's Avatar
Kingsland, Texas USA
Joined Aug 2000
2,411 Posts
Kevin, thank you for your expertise. I will try to balance to 1.5" for a bit more stability. I am going to have to put lead in the head to get there. Perhaps I should have chosen a bird with a longer neck.
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Old Sep 21, 2012, 02:20 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,636 Posts
Going by the video of your test flights, there looks to be some fairly long grass there. Why not try some hand glides into the long grass to see just how it glides.

Another method of determining the CG, and to get some idea of how much reflex is affecting the trim, a smaller scale chuck glider is well worth considering. It has worked for me a couple of times, even using a simple flat sheet foam model, (a Pteranodon and an unusual shaped delta).

Once you have a small glider that works, then the full size model wont be much different.
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Old Sep 21, 2012, 02:47 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,527 Posts
Quote:
Originally Posted by Joel K. Scholz View Post
Kevin, thank you for your expertise. I will try to balance to 1.5" for a bit more stability. I am going to have to put lead in the head to get there. Perhaps I should have chosen a bird with a longer neck.
So do we see a Stork design in your future?

I tend to agree that the first line of attack would be to try a more forward CG. As mentioned it does seem to be suffering from a strong pitch up divergence which then produces the out of control chain of events. As is said so often you can fly poorly with a CG that is too far forward often. But you often only fly once with a CG that is too far aft.

I just hope for your sake that the farmer doesn't come along and mow off your "safety pillow" too soon...
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Old Sep 21, 2012, 02:53 PM
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Joel K. Scholz's Avatar
Kingsland, Texas USA
Joined Aug 2000
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I have tried to balance the CG to 1 1/2 inches behind the leading edge. 10 oz of lead won't do it. The only solution I can see is to cut the wing in 1/2 and sweep it backward. Can anyone confirm what my drawing indicates. Assuming the red wing dot is the CG location on the original.Will sweeping the wing back keep the CG point on the original wing the sam place on the wing or will it change?
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Old Sep 21, 2012, 03:45 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,636 Posts
Joel, maths is not exactly my strong point, so I reverted to one of my old methods, graph-paper.

I copied your last picture, printed it out onto graph-paper and started counting squares on the slightly swept forward wing.
Worked out the approx area, then tried a 20% of the area CG location. It still needs to be forward of you CG marks.

I tried 20% as that's what my Eagle flies at.

I can't give a dimension, but an educated guess would be 1" back from the LE at the side of the fuselage. This is for the 'swept forward' drawing, just under the 'Original' word.
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Old Sep 21, 2012, 04:12 PM
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Joel K. Scholz's Avatar
Kingsland, Texas USA
Joined Aug 2000
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Eflightray. I could not get the bird to balance at 1 1/2 inches even with 10 oz of lead. I have decided to cut the wing in 1/2 and insert the shaded piece into the middle. Hopefully this will sweep the wing back enough to allow me to balance it and still retain a birdlike profile
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Old Sep 21, 2012, 04:57 PM
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peterangus's Avatar
Blackpool, Great Britain (UK)
Joined Dec 2003
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Joel

With your cranked wing planform, the inner halves of the wings need to be stiff in torsion.

If not stiff, then the smallest loads can have drastic aerodynamic effects.

You could try adding a flat underside skin, to create a torsion box structure. [I am assuming that the section is thin, and high camber]

I agree with post 14, that some fin area would be beneficial [maybe essential].

When you get the model flying reliably, you can then work to progressively remove any undesirable features.
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Old Sep 21, 2012, 05:45 PM
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Joel K. Scholz's Avatar
Kingsland, Texas USA
Joined Aug 2000
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peterrangus. the wings have flat carbon spanning the top and bottom of the wing.
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Old Sep 21, 2012, 06:29 PM
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Blackpool, Great Britain (UK)
Joined Dec 2003
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Quote:
Originally Posted by Joel K. Scholz View Post
peterrangus. the wings have flat carbon spanning the top and bottom of the wing.
That helps the bending stiffness. It does nothing for the torsional stiffness.
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Old Sep 21, 2012, 07:44 PM
Texas Buzzard
Texas Buzzard's Avatar
McAllen,Texas
Joined Mar 2004
1,034 Posts
A beautiful paint job.

Joel,
I gave up hand launching after flying electrics for two years. I don't care how good you are - you just cannot hand launch well CONSISTANTLY.

MAKE A CATAPULT OR A LAUNCHER , YOU'LL LOVE IT! Spend about $20 on some 3/4" PVC pipe and some connectors + a bungee
suitable for the size of you plane. This will give a consistant speed and attitude and your wings will be level too. Google RC Catapult/Launcher for pics.

I hope you add a Fin. I think you have a Sever Duthch Roll meaning poor directional stabilit. Ya' gotta' have some Fin - and you dont need rudder with ailerons. You don't have to launch at full throttle either. Add power a couple of seconds AFTER the launch. High power when the plane is "catewampous" creates disaster. GOOD LUCK - But ADD A FIN THAT DOESN'T ACT LIKE A MOVABLE RUDDER. THAT'S MY 2 CENTS.
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Old Sep 21, 2012, 08:41 PM
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Joel K. Scholz's Avatar
Kingsland, Texas USA
Joined Aug 2000
2,411 Posts
The deed is done. I have swept the wings more to allow for balancing. I am hoping Kevin will give me an accurate balance point. I believe it to be approx 3 inches back from LE. I love EPP. It allows for quick mods. The torsional stiffness is very good. It may be helped by my filler. Combination of Elmers.woodglue and lightweight spackling.
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