Plane always "falls" into the turn, turn gets tighter

[fprintf]

This is probably a very common occurrence but my plane (a highlander) seems to fall into turns or staighten up- no matter what I can't get it into a static, nicely trimmed thermal turn. It seems to require active flying all the time.

For instance, when I put it into a turn it always needs some amount of up elevator to keep it in the turn, otherwise it'll start spiraling downward. If I make a shallow turn the plane always wants to open up the turn wider, therefore requiring constant rudder input to keep turning at the rate I want it. There is probably a nice balance I have yet to find, but as soon as I give more than 30 degrees bank angle the plane likes to fall into the turns and requires significant up elevator and opposite rudder to stay in the turn and not begin spiraling.

This is a polyhedral plane with the CG well back, but not too far back that I get popoffs or anything bad happening during launch. It launches on a highstart straight up unless I launch cross wind, in which case it veers to the side until it is directly downwind and then goes up. It kites very nicely.

Could my inability to find the sweet spot in turning this plane be due to inaccurate (non matching) wash out on one of the wings? Or is this more likely one of experience and perhaps a little overeager on the thumbs/fingers?

One final question, I am having difficulty keeping the plane straight and level. It seems that one click of trim takes the plane from mushing to diving. Whenever the plane is overhead I *always* lose altitude as the plane starts to dive before I realize it. I had similar problems with my friends Ascent.

[markdrela]

Originally posted by fprintf
For instance, when I put it into a turn it always needs some amount of up elevator to keep it in the turn, otherwise it'll start spiraling downward.

Every airplane requires some up elevator in a turn. There's no way to avoid it. Light gliders with long tails need the most. A DLG requires nearly full up-elevator in a very tight turn.


If I make a shallow turn the plane always wants to open up the turn wider, therefore requiring constant rudder input to keep turning at the rate I want it. There is probably a nice balance I have yet to find, but as soon as I give more than 30 degrees bank angle the plane likes to fall into the turns and requires significant up elevator and opposite rudder to stay in the turn and not begin spiraling.

A spirally-stable r/e glider will need a small amount of rudder held into the turn, otherwise it will tend to straighten out by itself (this is one symptom of spiral stability). The "fall into the turns" behavior may simply be the result of not holding enough up elevator. For spiral stability you need some minimum amount of Equivalent Dihedral Angle, usually 10-12 degrees on each side for most gliders.

Do a dive test to be sure you have some minimal amount of pitch stability. Provided you're stable, the precise CG location will not affect the behavior you're concerned with.

[BMatthews]

"but as soon as I give more than 30 degrees bank angle the plane likes to fall into the turns and requires significant up elevator and opposite rudder to stay in the turn and not begin spiraling"

This bit tells me that you're on the verge of spiral instability. The low banked stable flight transistioning to this higher banked pro turn instability shows that it's on the knife edge.

Also your one click critical pitch trim is not helping your performance any. I once had a model set up that way as everyone told me the flight was more efficient. Coincedentally the turn response was just as you describe your model. Finally I had enough and moved the CG forward about 1/4 inch on the wing of my Top Flite Metric. This effect of this small change was to deliver me a whole new model. The pitch trim was now about 5 clicks to critical so I was able to actually TRIM for a change. And to boot the "falling in" in the steeper turns was now gone or greatly reduced with the model only getting some pro turn in at the very steepest banking angles. Strangely enough, or perhaps not so strange, the "reduction" in the model's theoretical performance was more than made up for by the model's and my "team" performance and "we" started kicking some serious butt in the novice class I was flying in at the time.

So my advice is to move the CG forward until you have a 5 or 6 click EFFECTIVE ( no stalling or tucking under, just speed variations) elevator trim range and see what happens to the turns. If it's still showing some falling in then chop off a couple of square inches from the vertical tail area and that should fix 'er up just fine. Or add a few degrees more dihedral which is probably much harder to do.

I'm assuming here that you're perhaps a better pilot and that Mark's comments aren't valid. If they do hit home then more practice may be what you need. Certainly you always need more up elevator in steeper turns than for shallow ones. And depending on how much travel you have it's quite possible that you're running out of elevator movement. Having the CG back so critical is probably part of that as the model gets so sensitive our reaction is to limit the elevator travel to desensitise the effect.

[thelocust]

I am flying a Spirit Elite at the moment, with the CG "aggresively" aft, and I find the same behaviors. One other thing is that my aileron differential I *know* is not trimmed, and so rolling left causes the plane to dip and yaw left , which could also be part of the problem (maybe?). I think that perhaps stalling could also be part of the problem, tho' too... I'd know more this afternoon, but it appears that a thunderstorm has rolled into town :(

[fprintf]

Thanks guys. I think marks comments are valid for me probably a few months ago. Now bmatthews comments seems to be hitting home for me. I'll try to add some more weight to the front and see what happens.

[Jack Hyde]

How did you determine your cg was aft but not too far? Whether it pops off or not is not a good indication of correct cg. When the plane is being towed up at a steep angle the tow line force is greater than the weight and approximately perpendicular to the direction of flight while the weight is less than the tow force and more in line with the direction of flight. The tow hook location , elevator setting, wing camber setting, line tension, how hard and at what angle the plane is thrown all mostlly determine whether the plane pops off or veers on launch. We usually move the cg back until we run into maginal pitch stability, then move it fwd until it is 'comfortable' to fly. There has been a lot written about the benefits of an aft cg - increased response to pitch inputs. Unless you are in live or die competition, you may be a lot happier with a little more fwd cg and the associated stability margin. You can move the cg aft if you get bored later.
You say the plane goes into a dive if you give a click of down. If it is pitch stable the nose will drop when you add down trim, then it will speed up and level out. If it keeps diving it is not pitch stable and needs the cg moved aft, unless for some reason you want to fly a plane that is not pitch stable.

[solo6796]

The remark about only one click making a huge difference makes me think perhaps the linkage is set for higher rates. Try connecting your servo link on a hole closer to the servo shaft and/or the control surface link on an outer hole of the control horn. You MAY be over controlling in your turns. Are you using low rates? I have a Spectra and it was is also very sensitive to elevator input. This made it handle much better. Also, a dive test for correct nose weight may be in order.

AJ

[BMatthews]

Originally posted by Jack Hyde
..... If it keeps diving it is not pitch stable and needs the cg moved aft,.....

Ummm, I think you got that one backwards there Jack. All the rest makes sense so I'm assuming this was a mixup.

[Jack Hyde]

OOps. I meant forward, as you say.

[fprintf]

So is there any easy source (perhaps locally) that carries sheet lead? I can tape some quarters on there to get the CG right, but ultimately once I get it I'd like to bury some lead in the nose.

edit: wow, I have had to edit my posts like crazy this week - all kinds of misspellings and missed keystrokes!

[Ollie]

You should be able to find sheet lead in the form of roof flashing at your local DIY outlet like Home Depot or Lowes. Also your local hobby shop should carry thin sticks of lead with foam tape backing.

[BMatthews]

Lead is much easier to come by and may even be free. Check out the local tire shops. They often have buckets of old stick on tire weights that they peel off before changing tires. Use as is or scrape off the double sided tape residue and melt down into the shape you want using a large copper plumbing end cap and a propace torch. Use a VERY DRY wood hollow form as the mould. If in doubt about how dry the wood is bake it at 180 or so in the oven for an hour first. Or you can microwave it but the wife better be out first. Many woods STINK when microwaved.

DO THIS OUTDOORS AND KEEP YOUR FACE UPWIND OF THE FUMES ! ! ! !

The fumes from melted lead can get you well on your way to a nice case of lead poisoning. It's accumulative and you only have so much your body will tolerate in one lifetime.

[Hostage-46]

Split shot sinkers work well also ....

[fprintf]

Hey guys,

Thank you so much for all the help. I just posted about my first 45 minute flight today at http://www.rcgroups.com/forums/showthread.php?s=&threadid=146729

It turned out that my problems with the plane were two fold. The first is that I was overcontrolling the plane - giving too much rudder and not enough elevator, or too much elevator causing the plane to turn too fast. I did put a quarter on the nose and the plane flew horribly - turns out I had the CG is just the right place all along.

So I put dual rates on at 50% rudder and elevator. That was awful as I couldn't initiate a turn. So I changed it to 75%, 70% respectively and that made a big difference. There is still only a 1 - 2 click difference between level flight, mushing and diving but I think I can fly around this. At least it seemed that way today! Smooooooothness is the key I guess.

Thanks to everyone on this thread, and others, for your help. This hobby is cool!!!!!

[planeman]

As an old full-size sailplane pilot - I don’t fly model sailplanes – the flight characteristics you mention are quite familiar. The tendency for a long-winged sailplane to fall into a steeper turn when you reach a high bank angle is caused by the outer wing moving faster through air than the inner wing. This problem isn’t noticed on powered planes with relatively short wings. The small additional bit of lift on the outer wing makes the plane roll into the turn. Pilots of full-size sailplanes always have to apply some opposite aileron to counteract this. This is a cross-control situation, always a no-no in standard short-winged aircraft, that is necessary in these long-winged aircraft.

One thing you can do to minimize this is to use a computer-type transmitter or control linkage setup to set the ailerons for more down-aileron to the outer wing and less up-aileron to the inner wing during the turn. This increases the drag on the outer wing and reduces the drag on the inner wing thus counteracting the tendency to roll into the turn.

I hope this helps.

Planeman

[Daemon]

Originally posted by planeman

One thing you can do to minimize this is to use a computer-type transmitter or control linkage setup to set the ailerons for more down-aileron to the outer wing and less up-aileron to the inner wing during the turn.
Of course the problem here is that he doesn't have any ailerons.
With discus launch gliders that have full span flaperons and only moderate dihedral we see the same symptoms, but solve it by turning with the rudder and elevator, and applying some opposite aileron to keep from spiraling in. The slight opposite aileron has the effect of giving the slower moving inside wing a little camber, and the outside faster moving wing a little reflex which allows it to hold a steady bank angle. I've been told this is pretty much the same technique used by full scale pilots.

I have to admit that diagosing spiral problems with a poly ship is unknown territory to me, as my first thermal bird had ailerons. I do know that you shouldn't have to hold opposite rudder as that indicates that you're sliding sideways in the turn which is draggy. How much poly does your highlander actually have?

ian

[fprintf]

Hi Ian,

It turns out most of the problems are with the pilot on this one. I have to hold significant up to stay in the turn and avoid spiraling down, and there is a very fine balance between enough up to make a nice even turn and not enough and spiraling down, or too much and stalling the plane.

Some extra practice with fingers and thumbs getting used to the plane the past two weekends has made most of the difference.

[gustabmo]

Mark Drela recently posted an article on an online soaring magazine (can't recall the name, sorry) explaining exactly how rudder and aileron should be used during a turn.

Edit: thanks thelocust for the article link (see message below), that's the article I mentioned, but I had seen it elsewhere

If I recall it correctly the turn should be initiated by the ailerons, immediately followed by increasing rudder. As soon as you have enough bank you should level out the ailerons, or maybe use a little opposite aileron so not to spiral in, and hold the rudder. Then exit the turn with opposite aileron and decreasing rudder until level, when you should level off the ailerons.

What I understood from the article is that you have to hold the rudder proportional to the radius of the turn at all times, and you also need to apply aileron correctly to maintain a bank angle that's compatible with the turn at all times. Failing at either task will cause the fuselage to be dragged sideways (slip? skid? not sure...) adding drag and loosing altitude. The article had some very well done graphics that were very helpful in explaining all that... Even I understood it! :)

There might be something on the article about how to use the elevator but that would have been too much for me... I only got the rudder/aileron part.

BTW, he pointed out very clearly that coupling aileron and rudder is the wrong way to go... It's an oversimplification of the mechanics of a turn.

On a poly ship I think all this is self correcting... whenever the fuselage skids, the poly angle acts, correcting the bank to the correct angle.

My $0.02...
Gustavo

[thelocust]

Originally posted by gustabmo
Mark Drela recently posted an article on an online soaring magazine (can't recall the name, sorry) explaining exactly how rudder and aileron should be used during a turn.

There is this article over at PoleCat Aero...
http://www.polecataero.com/articles/rudder/index.html

There are some interesting CG-testing articles there as well...

[fprintf]

Well, I figured it out. And all it took was a heat-gun that I purchased at the WRAM show.

Being a foam plane, I had not even thought too much about the wing or fuse alignment, even though the plane had been dorked quite a bit. It turns out that there was/is all kinds of warpage in the plane.

The fuse had a slight bow in it that increased the decalage (made it more negative). One wing tip had a slight wash in vs. the other wing tip with wash out. Finally the coroplast rudder is warping ever so slightly to the right when viewed from behind.

So I used the heat gun to straighten everything out, smooth out the wrinkles from 2 years of newbiey flying and tried, unsuccessfully, to get the warp out of the rudder. But the plane flies *much* better now. It still doesn't signal lift very well forward and aft, but it does pretty well by lifting a wingtip.

Most importantly, either my flying has gotten better with respect to using the elevator or the plane has improved, but it doesn't fall into the turns as much.