Hey, Steve.

I've given some thought to the problems we were discussing and I wonder if you'd like to try and experiement and let me know how it works out.

If the stab on that airplane is "flat" with a "rounded" leading edge, why don't you fold a piece of heavy paper in half  --with a tight crease -- about an inch and a half or so wide and the length of each side of the stab.  Tape it onto the "round" leading edge to turn it into a "sharp" one and see what it does to the problem characteristic we were discussing.

I've had a theory about round/flat stabs that relates somewhat to this sort of problem.  It would be interesting to see if sharpening the LE helps out.

Ted Fancher

ps. Had this thought at work and didn't have your email address here.  That's the reason for the circumspection in the post.

Ted,

  Thanks for the idea.  I actually flew the plane in question this afternoon, maybe its ears were burning!  The stab is actually wedge shaped rather than flat/rounded, albeit the leading edge of the wedge is not exceedingly sharp.  It would certainly be possible to fabricate little leading edge extensions as you describe and see what effect they have.  I have plenty of cardstock in the shop!
  Progress, considerable progress, was made today by piling on another ounce of nose weight to the ship.  Its not perfect by a very long shot, but its flyable enough to warrant further efforts in trimming instead of hanging it up somewhere in the back of the shop...

More to follow.....

Steve

Glad to hear you've made some progress.  Nose weight can be a friend at times. 

However, I'm not generally a fan of "curing" a hunt by adding nose weight unless there is other undeniable evidence that the airplane is tail heavy. IMHO, making a ship less responsive than it has to be just to make it "feel" stable is also compromising its aerobatic potential.  Does the inside/outside turn performance seem in "balance", i.e., does itt require more control input one way than the other?  That could be a clue.  Generally speaking, if an airplane doesn't turn equally well both ways the problem will also show up as some degree of level flight hunting as well.


I'm sure you were careful assembling it, but the concept of a wedge shaped stab gives me a bit of pause.  I remember one airplane (granted, it won the nats the year the guy flew it) that had a wedge shaped stab and had been assembled with a lot of negative incidence.  I had a picture of that ship in the air for a long time (don't know whatever happened to it but were talking back in the late '70s here) and in level flight the flaps were level but the elevator was down a "lot" to compensate for the tapered wedge of the stab being pretty much glued onto the top of the fuse box.  I don't want to embarrass the guy who built/flew it so I'll not give anymore detail ... on the other hand, how do you embarrass a Walker Cup winner?  Anyhow ...

I bring it up only to suggest you insure that your airplane was assembled with the stab properly aligned with the wing/thrust lines.  Needless to say, a significant difference would require some flap/elevator tweak like my old friend's ship to get it in trim.

At the risk of upsetting my verbal sparring partner from the Lone Star State, I'd be interested in knowing where the CG was located when you balance it on your finger tips halfway out each wing panel.  Also, what percentage of the wing area the stab area is. Finally, when the engine quit (before you added the nose weight) did the ship stay under positive control in the glide or did it get light on the lines and try to pitch up going into the wind?  I'm obviously curious to know whether the CG was/is in "Ted's theoretically correct range of CG on the Mean Aerodynamic Chord".

Another trim setting that can cause hunting is a too far aft leadout location.  Without going into what is sort of a complicated reason for that (just note that it is something I've experienced and cured on my own designs by getting the leadouts in the proper location) just be aware that the proper location is really and truly a pretty predictable place unless the pilot has done something wierd with offsets (engine/RUDDER, etc. -- and,yes, rudder is in all caps to simulate shouting!)  If you find that your leadouts are anywhere much beyond an inch behind the CG at the tip (forward or aft) consider setting them at an inch and flying it there.  If the rudder is offset, please try moving it to straight ahead prior to a test flight of that WAG leadout setting.

Just some thoughts.  If any of them sound worth trying let me know what the result is.

Ted

I hope Steve & Ted forgive my intrusion... but Steve did mention my name, heh...   

Actually, both Steve and my Avanti's are well within the design param's all up weight. In all fairness to the Avanti, I think it's worth mentioning for onlooker's to these threads that both Steve & I missed the design C/G by a bit. Mine was an inch and a half behind where the plans show when I first built it. My initial flights were with a 1.5 ounce nose weight (still a half inch back from plans). I got stupid and took the weight out and retrimmed the plane.

Oh, that's another thing. Stability. I wouldn't have called what I experienced a true "hunt" but more a typical "tail heavy" instability. I know that was used for lack of a better term, but mine never porpoised, or went one way then over compensated in another direction, etc like a true hunt.

For quite a while, I pig headedly tried to make the rear C/G work, and narrowed the handle spacing to 3.25", and it was sort of OK, but it wouldn't glide well on landing, it wouldn't whip very well either, if I landed it slow it would land tail down & when the field winds got turbulent, it was harder to fly a nice level lap. Typical tail heavy symptoms. If it was a true "hunt" I don't think I would have been able to clean it up with handle spacing (Oh, and a 3blade prop, and I dialed out a little elevator).

I didn't want to mess with that setup because the KOI was looming. That rig was still good enough to place in Adv.

Since then, I've retrimmed the airplane to the config that Steve mentions in his post. I'm now within a hair of design C/G and have put my handle spacing back out to 3 3/4", etc. The plane is groovy, tracks well in the straight legs and and glides properly etc.

It bugged me a little to take my welter weight ship and turn it into a median weight ship to get the C/G right, but in hind sight it was well worth getting the better trim package. Oh, Ty - I think the wing could carry another 5 to 7 ounces with ease and still be competitive.

EricV

Busy tending to the kids at the moment, but I did take a quick snapshot of the stab for Ted to see what I was referring to as far as the shape goes....

Steve,   I made my stab 1/8 in thicker than the plans showed.I also have a flat if not airfoiled with round LE. I dont know if this helped, but it didnt seem to hurt. I tried to make it as light as possible back there. The plane grooves pretty good and seemes to track well in maneuvers. It took a little to get used to to get good flat bottoms but its coming together. The ship came out at around 61OZ ready to fly. I noticed the shape of your stab. Do you think that has something to do with it?                                                      Jim

Speed read all this input about huntin' stunt ships in the night......but I didn't see anything mentioned anywhere about the engine thrust alignment?

Joe Dill and I ---(maaany-moonies ago)test flew an old Shark that belonged to a flyin buddy who "rebuilt the plane after it had been FLIGHT CRASHED REKITED!  After regluing not only the nose section...and reattaching the aft body section...
Bottom line:
This old Shark not only "hunted" in level flight..but would turn insides tighter and easier. After checking all the wing and stab alignment....which appeared to be amazingly "reconnected" but Joe noticed that the engine alignment was considerably off with a sizable amount of UP-

N'allthatjazz?

Don,

  Yes indeed, engine thrust misalignment can cause a multitude of trouble.  My first Avanti (the wreckage of which is posted on the "Stabs and the Avanti" thread) had some upthrust built in, but this was detected and corrected before the airplane flew.  The current Avanti seems to be as straight as I can detect in terms of engine/wing/tail alignment.  I guess if I still can't fix it, I could shim some upthrust or downthrust in there to just see what happens...

Steve

Hummmm? I also remember in some article....(I think it was from Bob Gialdini) that so many stunt flyers induce waaaaaaaaaay too much engine offset....and that he advocated  LITTLE OR  NO ENGINE OFFSET if he could get away from it?
 Even the variables in prop weight and pitch can really do weirdolddo gremlins (maybe somethin' to do with the  gyro effects to both engine offset and engine thrust alignment.
Like ol'man Newton, for every induced action----there is that old opposite reaction...n'allthatjazz.

Interesting thread. 

I had a Saturn a few years back with a flat stab with rounded LE.  Had some trim troubles. 

Off to the field I go with things to add to the LE to see what happens.  Added 1/8" rod and nothing noticeable.  Added triangle 1/4" balsa giving it a sharp point!  The plane would not turn more than 75 degrees without stalling.  It would track better than any plane I have ever flown.  But it would not turn.  The tail was stalling.  I almost crashed it a few times messing with it in the corners.  Adding a very sharp point of separation to the stab can be dangerous if not fatal.  Next I knocked of the sharp point with a sanding block.  The stall was gone and the plane flew like normal again.

I proceeded to remove all the trim on the plane, TW, LO position, CG position, and intentionally messed up the pushrod length.  I completely re-bench trimmed it and went back out.  It flew good.  I worked and worked over the next several days getting the plane to fly very good.  Once I was satisfied with it, meaning I felt I could win a contest with it.  I check all the settings.  TW was the same as before, LO in the exact position, CG same exact place.....leaves only one thing....pushrod length.  It was shorter by just a small amount.  The initial trim issue I was having was the feeling of never being able to do 2 maneuvers, tricks??? dogs do tricks we do maneuvers , in a row right on top of each other.  It just wouldnt track.  It was ok in level flight and I could make it go pretty good but not great. 

The pushrod length was the culprit.  I also think that is the culprit with many planes that kind of lurk around and not actually show a full hunt.  People just "learn" the plane and overcome it not realizing there is a real issue there.  If it is close you can mask it with CG adjustments.  If the CG is forward or in the ballpark and there is still a hunt, there is something more to the issue.  If the plane is straight, meaning engine thrust is good and the alignment of the tail is at least not negative, and the LOs are in a usable area, then the pushrod is probably too long, IMHO.

Personally I follow the idea that the flat stab with the round LE is the way to go.  Simply because it is easy to get dead straight every time.  Easy to make it super strong, and still light!  It is easy to install with good alignment every time.   These things to me are more crucial to the plane flying good than the difference between the airfoiled, wedge or, flat design.  They all will work exceptionally well, the key is they must be dead straight.  What is the most reliable way to built a straight stab?  FLAT ON YOUR FLAT BUILDING BOARD.  Having to sand in an airfoil or wedge can be done and many do it all the time but a few wrong strokes and you get that little warp and it can be big trouble later.  Alignment can be a little tricky as well.  maybe I dont build as well.  But I look at it from the pros and cons when deciding where to go with a part.  BB says in most cases alignment, and structural integrity are more crucial to a good flying model than the actual design.  I tend to agree.

Sorry to ramble so, I know I didnt add much.

 

Doug,

Thanks for sharing your experience with stab L.E. configurations.  Am I correct in thinking that shortening the pushrod corrected an elevator that wasn't perfectly "in trail" with the stab or was there an issue with stab incidence?

Thanks,
Jim

Today the Avanti was flown with 2.1 oz of lead glued in under the engine to replace the 2 oz spinner weight.  It was light winds at Fentris NAS with 5 mph gusting to 10 on the base weather station.  I had switched from the wood 13x6 two blade to a bolly 12x6.5 three blade.  First flight the plane felt a thousand times worse than last week, porposing all over the place, unable to lock down any of the flats, wandering all over the place on rounds, and getting loose on the third loop of the clover.  When I landed I told the guys it was the worst plane I'd ever frigging built.  It was like none of the trim from a week before had helped at all.
  After they convinced me not to stomp on the airplane, we added another ounce nose weight, 1/4 oz tipweight, and moved the leadouts back about 3/16th".
  Big improvement, the airplane seems to have almost no detectable hunt now, doesn't get loose in overheads, and even with 3.1 ounces total weight in the nose, seems to fly good squares with pretty good locking in on the flats.  I may not stomp on it for another week!
  I do need to pitch the prop down to 6" or so, 6.5" was way too much, but I figured it wouldn't hurt to try it out first....

S

Ted,

  Right now the CG is at 19.4% of MAC.  It measures 2.25" from the leading edge at half span, with a total chord of 11.6"  The center bolt in the leadout slider is 1.25" behind the CG at the tip.

Ted,
  The leadout slider is one of the single slider units, I'll have to measure the distance apart but I guess maybe a half inch off the top of my head.

The groove seems decent, not super, but decent, in level flight now, in calm or windy, and full tank vs empty.  Thye plane seems to have alot of other little funnies that I will have to wrestle with one at a time now.  My flying session yesterday got cut short by a lost motor mount bolt, but maybe I'll try it some more tomorrow.  Right now I need more flights just to catalogue the funnies better in order to break them down......

Steve