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Old Jun 01, 2012, 11:52 AM
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discus.fly's Avatar
United Kingdom, Central Bedfordshire
Joined Jul 2008
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Pitch Stability

Having built several bird like designs all have exhibited a similar flight characteristic which I would like to cure.

All my designs suffer from pitch stability with a tendency to put the nose down and not recover from a dive.

Latest design is a 72inch eagle. Flat bottom wing with the TE angled upwards to provide reflex.

Some flights would suggest the cog is to far forward but brining it back seems to make the tendency of pitchg stability worse.

What am I doing wrong?
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Old Jun 01, 2012, 11:59 AM
Lee
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I flew some of the Peregrines with elevons rather than aileron elevator. The fact they would fly this way made me look at them as flying wings more than conventional airplanes. Flying wings have the CG at about 23% of the total wing area which is farther forward than most try to fly them. If you look at the Q-Plane we did years ago it has some of the same characteristics.

http://www.rcgroups.com/forums/showthread.php?t=931080
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Old Jun 01, 2012, 01:01 PM
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United Kingdom, Central Bedfordshire
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Pitch Stability

So treating it like a flying wing do you find a nose heavy cog position is better with more reflex of a more rearward position?

As I say 95% of the flight is fine then she puts the nose down and acclerates with little to no effect applying up elevator.

Only managed to save her from impact tonight by applying power and climbing out of the dive.
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Old Jun 01, 2012, 01:12 PM
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United States, MA, Walpole
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Quote:
Originally Posted by discus.fly View Post
Having built several bird like designs all have exhibited a similar flight characteristic which I would like to cure.

All my designs suffer from pitch stability with a tendency to put the nose down and not recover from a dive.

Latest design is a 72inch eagle. Flat bottom wing with the TE angled upwards to provide reflex.

Some flights would suggest the cog is to far forward but brining it back seems to make the tendency of pitchg stability worse.

What am I doing wrong?
Your picture looks like a larger version of the 52 in. eagle I built from plans posted very early in this thread. Mine has no wing reflex and none needed... it flies very stable with the flat wing. Have you check the inclination of your V-tail? You could be getting some instability from the reflex and tail interaction.

FWIW, I tried adding some flaperon action, but being more like a flying wing, a little down flaperon cause major diving tendency. But it flies so slow, there is really no need for flaperons anyway... it was just an experiment.
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Old Jun 01, 2012, 02:14 PM
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United Kingdom, Central Bedfordshire
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Pitch Stability

Design is a 160% increase on the standard EPP eagle.

Tail is aligned to the bottom of the wing. Should it be inclined?
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Old Jun 01, 2012, 02:39 PM
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Quote:
Originally Posted by discus.fly View Post
Design is a 160% increase on the standard EPP eagle.

Tail is aligned to the bottom of the wing. Should it be inclined?
It should be aligned with the wing as you have it, but with the reflex in the wing I would expect the plane to pitch up, requiring some down elevator based on the way mine reacted (pitch down with down flaperons). Is your reflex done with ailerons or is it fixed? I'd get everything in alignment if you can and go from there. Mine climbed up and to the left by itself with everything in alignment... had to add a bit of down-right motor offset to counter that and get straight away flight at mid power. And when accelerating under full power, it would again climb to the left, so I added some v-tail mix to the throttle stick to offset the higher powered climb tendency.

Once everything was dialed in, it is a hands-off flyer if I choose to just let it cruise around on it's own with barely enough power to maintain altitude. I get flat gentle turns using just the ailerons... I actually call them warperons because I did not cut the outer end of the ailerons free, so they actually twist when they deflect, with the most deflection is near the fuse with virtually none at the tip (emulating the way the real bird's wings act in flight). I'd do that again on a rebuild. There's some pictures and vids in the front of this thread I posted.
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Last edited by Tom Frank; Jun 01, 2012 at 02:48 PM.
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Old Jun 01, 2012, 03:42 PM
Lee
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Quote:
Originally Posted by discus.fly View Post
So treating it like a flying wing do you find a nose heavy cog position is better with more reflex of a more rearward position?

As I say 95% of the flight is fine then she puts the nose down and acclerates with little to no effect applying up elevator.

Only managed to save her from impact tonight by applying power and climbing out of the dive.
Check your linkages. You may be nose heavy if you can't flair the plane with the power off.
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Old Jun 01, 2012, 04:22 PM
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uk/england
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Well built my eagle ish!!!!!!!!!!
Ok so i dont know how to paint! (should look alright from 1000meters) just need to install servos and rx .
with the motor only installed it balances 2.3/4" from le, seems its a good un
using rons and ruddervators i have a cf 2822/830kv motor installed, hope it has enough umph on 3s/1200mah?
pics to follow
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Old Jun 01, 2012, 08:50 PM
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United States, CA, San Rafael
Joined Mar 2012
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tried flying my seagull again today with a little bit of breeze. moved the CG further forward and rounded the wing tips as well as bent the tips up. was able to get a little further this time before kissing dirt and busting the nose!

I think the CG still needs to go further forward, but at least I'm not getting those crazy snap rolls anymore. I've hit my patience limit with this bird and am going to put it down and work on my F22 again. This is just too temperamental for me and I'm tired of rebuilding the dang nose...which after 2 maybe 3 crashes basically requires an entire rebuild.

Maybe I'll get tired of the F22 and come back to this someday, but for now I'm over it!

Thanks to everyone for the helpful suggestions and tips. I'm very jealous of you all that have successfully built and flown a foam bird!
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Old Jun 02, 2012, 10:33 AM
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United Kingdom, Central Bedfordshire
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Pitch Stability

Tail is aligned to the wing with reflex provided by a fixed angled trailking edge on the wing.

Have checked the linkage as this certainly was the reason I lost the first build at this size.

Have used far bigger servoes and a more stiff push rod system but think there still could be some deflexion due to pressure.
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Old Jun 02, 2012, 11:05 AM
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St Catharines Canada
Joined May 2002
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Hi discus.fly

I've only recently found this thread and have been lurking herein. I can offer another idea on the issue you have. "All my designs suffer from pitch stability with a tendency to put the nose down and not recover from a dive. Some flights would suggest the cog is to far forward but brining it back seems to make the tendency of pitchg stability worse." That is an effect called 'tuck' and is made worse by moving the CG back. Search 'dive test' for more info.

What I think is needed is to add more down thrust. My experience is that if the thrust line is too up or even zero in many cases the plane will pitch up under power as a result it gets trimmed for level flight that way. Cutting the power then causes a pitch down which is aggravated by tuck from too aft a CG. Solution; add perhaps 10* down thrust and trim for level powered flight that way. That should remedy the issue to some degree. Adjustment should be made to fine tune. Add some right thrust [~2*] if there is any left tendency on launch or powering up.

Hope this helps.

Richard
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Old Jun 02, 2012, 12:48 PM
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United Kingdom, Central Bedfordshire
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dive test

Do have down thrust but possibly more could be added although this will be impossible on this build without major work as the motor is fully concealed.

Problem does appear to get worse moving the cog further back so you could be right about the effect.
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Old Jun 02, 2012, 01:07 PM
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United Kingdom, Central Bedfordshire
Joined Jul 2008
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Tuck effect

Just read several posts on this topic and it would appear that indeed I have the cog too far back.

Will fly again to prove the point when the rain and wind here in England decides to abate!
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Old Jun 02, 2012, 01:14 PM
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uk/england
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can we have some pics of your eagle!

Iain
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Old Jun 03, 2012, 01:39 AM
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Ashford. Kent. England
Joined Feb 2005
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tucking pitch stability
Is normally due to a rearward cog setup trim... Get some weight forward so it needs a bit of up trim to fly and glide.. then it should not tuck

Ive been working on a Pod and Boom construction for powered gliders and having built birds before I decided it should get the treatment ...lightweight pod and booms with thin KFm9 wings have been giving me very good results..
here is as far as Ive got..
I am using a KFm section, thin wing and a less bulky body to try to improve the glide...
the build log is starting here.. http://www.rcgroups.com/forums/showt...1401748&page=4
I based it on the fffeagle http://www.rcgroups.com/forums/showp...5&postcount=46
but lengthened the tail feathers and stretched the wing a touch using sketchup..

Its all done at 253gms...9oz...wing and tail ares is just over 3sqft so 3ozsqft ...lightly loaded
the motor is an emax1806/25 with a gws 7x3.5 prop and a 500 3 cell lipo.

EDIT.... I didnt like the long tail so went back to the shorter fat look, more suited for a hawk type
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Last edited by davereap; Jun 04, 2012 at 03:07 AM.
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