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Old Oct 30, 2012, 11:40 AM
Dance the skies...
Tom Frank's Avatar
United States, MA, Walpole
Joined Dec 2003
17,878 Posts
Quote:
Originally Posted by rgt52 View Post
I could not wait until Wednesday, I had some free time this morning and decided to test the bird again. It was very windy (BAD IDEA) … the only good thing about windy days is the extra lift for the hand launching. Anyways, I balanced the bird just ahead of the spar. It flew OK from my hand, a few clicks here and there, no pitching issues … but … it started banking side to side like crazy, the more wind, the more it was banking. I tried my best to correct the constant left and right banking, but It was impossible … it finally died in a sort of spiral dive.
Fortunately, the damage was not too bad, I just need to rebuild the nose section which is not a big deal … the question is, why is it doing this?
Hmmmm... my eagle is one of the more flyable planes in the wind I have, but it sounds like you have lost longitudinal stability, possibly due to moving the v-tail forward too far. But you flew it before without the problem, and if I understand your recent change, you only changed the angle of incidence. And I would not expect moving the CG slightly forward would worsen the stability!

The description you gave sounds like some kind of divergent "dutch roll" instability. When yawed, the eagle will also roll in the yaw direction as the forward wing tip polyhedral causes it to rise and the trailing wing tip wants to dive. Normally the eagle tail feathers can dampen out the yaw (and roll) and keep the bird flying straight and level pretty well. (NOTE: this is why I don't use rudder in turns.. gentle aileron action works much better, but I didn't look to see if you put ailerons on yours.) If you were using manual rudder input to try to control the bank, you might have gotten into the divergent motion due to the slight lag in stick movment reacting to what you were seeing. But if the bird does not dampen out the bank oscillation with your hands off the stick, you can rule out that possibility.

As a test, you could splice on a fixed vertical stab between the v-tail to see if that eliminates the condition. Or maybe move the CG further forward.

It's too bad you are experience these problems. My eagle can fly itself, hands off the sticks, and I often do that, putting it in a gentle flat turn with a click or two of aileron, and then sit back and watch it circle around over head, switching from power off gliding to power on climbs. I can get 30 min. of flight like that on a 1300 mAH 3 cell lipo. It's a great platform for shooting aerial video with one of the tiny keychain HD cameras (only 17 gms.)
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Last edited by Tom Frank; Oct 30, 2012 at 11:42 AM. Reason: spelling
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Old Oct 30, 2012, 11:57 AM
Registered User
St Catharines Canada
Joined May 2002
907 Posts
Hi All

Contemplating building a FFF eagle, lurking otherwise.

rgt52:

Try mounting the V tail upside down. I believe the close coupled nature causes the tail to act as additional dihedral so when you get a gust of wind from the side it exacerbates the dihedral of the wings and banks harder. I think this may also counter to some degree the up and to the left tendency on launch, prop torque being the source of the yaw.

Do it on the paper model and post if the anhedral mitigates the problem.

Thanks

Richard
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Old Oct 30, 2012, 02:10 PM
Registered User
Joined Mar 2009
1,217 Posts
Quote:
Originally Posted by rgt52 View Post
I could not wait until Wednesday, I had some free time this morning and decided to test the bird again. It was very windy (BAD IDEA) … the only good thing about windy days is the extra lift for the hand launching. Anyways, I balanced the bird just ahead of the spar. It flew OK from my hand, a few clicks here and there, no pitching issues … but … it started banking side to side like crazy, the more wind, the more it was banking. I tried my best to correct the constant left and right banking, but It was impossible … it finally died in a sort of spiral dive.
Fortunately, the damage was not too bad, I just need to rebuild the nose section which is not a big deal … the question is, why is it doing this?
As Tom said, it certainly sounds like a classic case of Dutch Roll (Or pilot induced oscillation to give it another common name). Models with a lot of dihedral often do this. The strong wind probably meant you were actually flying at a higher airspeed to try and make progress into wind - so the controls become more sensitive and it is very easy to get into this yaw / roll oscillation that gets progressively worse. The best way to fight it is - don't! Slow up and center the controls and it will probably sort itself out. If you are fighting it and just slightly out of phase with your control inputs, the oscillations get worse.
I also agree with Richard that an anhedral tail might help a bit with this problem. Otherwise just fly it in kinder conditions

Hope that helps.

Malcolm
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Last edited by highviz; Oct 31, 2012 at 09:40 AM.
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Old Oct 31, 2012, 04:49 AM
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Joined Nov 2006
21 Posts
I fixed the nose of the bird last night and took her out again this morning for more testing. The weather conditions were much better than yesterday. Carefully, I tried all sorts of things in different combinations, in particular different configurations of thrust line, decalage and CG location. I was out there for about two hours and what has worked best so far was adding some wing/tail decalage (raised about 5 mm the LE of the wing), thrust line parallel to the wing and CG where indicated in the original plan. It flies very slow and stable like that. I get about 30 min. out of a 1300 mah battery flying at 30-40% throttle. Ruder and Elevator authority are very strong due to the pusher configuration (control surfaces are very close to the prop down wash) . Ailerons are not effective at all. I do not need them anyways since a slight deflection of the rudder produces nice flat turns. Yet, a more aggressive use of the rudder produces the side to side banking observed yesterday, particularly under power.

I know that this thread is not attracting much attention right now. Nevertheless, I share my test results just in case someone is interested in building a pusher version of the eagle in the future.
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Old Oct 31, 2012, 04:55 AM
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Joined Nov 2006
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I just forgot to mention that the glide rate without power is still very steep ... not happy about it ... but as long as I do not have to land her dead stick, then it is tolerable.
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Old Oct 31, 2012, 09:01 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
3,767 Posts
Quote:
I know that this thread is not attracting much attention right now. Nevertheless, I share my test results just in case someone is interested in building a pusher version of the eagle in the future.
I'm lurking also. Thank you for posting.

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Old Oct 31, 2012, 11:58 AM
Lee
PERFECT LANDING !!!
Lee's Avatar
USA, UT, Orem
Joined Jul 2004
9,248 Posts
I have built at least 40 variations of birds.

I have had control problems if:
They are overpowered
CG is too far forward or back. CG is often not where I thought it should be.
There is too much movement in the elevons or ailerons. They can be very sensitive to control surfaces.
If the wing is too flat or too much dihedral or polyhedral.
If I didn't have a large enough rudder. In fact they didn't hardly fly without a rudder.
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Old Oct 31, 2012, 04:22 PM
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Joined Nov 2006
21 Posts
Thank you guys for your feed back. I was suspecting of Dutch Roll, but in this case was so violent that I concluded that it should be something else (I had Dutch Roll in a B747 which I built a while back, yet it was not this bad). I may have made the situation worst by trying to fight it as mentioned by highviz.

I like the idea of mounting the v-tail inverted. I may actually try that at some point.

For now I will continue doing some more testing in calm weather conditions. I am trying to get a nice glide out of this bird without power, but so far it is not happening
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Old Nov 01, 2012, 03:12 AM
DIY Mania from Taiwan
Taiwan
Joined Aug 2011
2,004 Posts
Spent 3 days to build a new gull wing with bluefoam in the center for easy carving work.
tested flight this morning
quite windy out there
loved the silly bird
cheers
Sam
PICT1633
PICT1666 (1 min 38 sec)
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Old Nov 01, 2012, 03:52 AM
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Joined Nov 2006
21 Posts
Quote:
Originally Posted by samwei1950 View Post
Spent 3 days to build a new gull wing with bluefoam in the center for easy carving work.
tested flight this morning
quite windy out there
loved the silly bird
cheers
Sam
PICT1633
http://www.youtube.com/watch?v=JEiOtBlJgZM
Beautiful !!! ... Congratulations !!!

Is it your own design or you got plans from someone else ?
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Old Nov 01, 2012, 05:16 AM
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G-LO's Avatar
Va.Beach VA.
Joined Aug 2007
2,177 Posts
Yeah, like it better than other gull. Very good shape in the sky*
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Old Nov 01, 2012, 06:49 AM
DIY Mania from Taiwan
Taiwan
Joined Aug 2011
2,004 Posts
Quote:
Originally Posted by rgt52 View Post
Beautiful !!! ... Congratulations !!!

Is it your own design or you got plans from someone else ?
Thanks,it's own wild design just for fun
same wing will come back with vid on the inverted "V" tail soon
regards
Sam
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Last edited by samwei1950; Nov 01, 2012 at 11:51 AM.
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Old Nov 01, 2012, 07:07 AM
Good Better Best quest.
olmod's Avatar
Australia, VIC, Cranbourne East
Joined Apr 2004
10,523 Posts
Well done what was the weight and windspeed ? not sure if some of the flight was gliding no power.
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Old Nov 01, 2012, 11:57 AM
DIY Mania from Taiwan
Taiwan
Joined Aug 2011
2,004 Posts
Quote:
Originally Posted by olmod View Post
Well done what was the weight and windspeed ? not sure if some of the flight was gliding no power.
Thanks,mate
RTF weight 750grams,did'nt know the wind speed and did some gliding during the flight.
the gull is originally also designed for slope
regards
Sam
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Old Nov 02, 2012, 06:14 AM
DIY Mania from Taiwan
Taiwan
Joined Aug 2011
2,004 Posts
Inverted "V" tail tested this AM
better gliding ratio vs "V" tail gull
well,both way goes
cheers
Sam
PICT1681
PICT1692 (1 min 10 sec)
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