Loose lines on outsides

[Tim Wescott]

So, I made this plane to be a flying test bed for my timer project, without really meaning to try to build the World's Best Stunter.  I wanted something cheap, easy, quick to build, and with lots of clear space on the fuselage for mounting weird experiments.  I got all that, and more.

What surprised me is that except for one rather serious quirk, the plane is an excellent flyer, to the point where I'd happily put into the hands of anyone from rank beginner to Intermediate on the cusp of going into Advanced.

The problem is that the plane comes in on the outsides.  It flies wings level both upright and inverted, it's built straight, it has plenty of line tension at any altitude if it's pointing in a straight line or turning inside, but on outside loops and corners it loses line tension. 

I did notice that it had an ever-so-slight amount of left rudder as built (on the order of the TE of the stab cocked in 1/32"); I fixed this, and indeed over-corrected it (TE is cocked out about 3/32" now), but it doesn't seem to change the flight characteristics at all.

The last flying session I tried putting about 1/4 ounce more tip weight into the thing; this didn't seem to change the characteristics much at all -- and since it responds so differently on the insides and the outsides, it can't just be a tip weight issue.

What's going on, and how do I fix it?

[Douglas Ames]

Prop. precession maybe ? Try more vert. fin area ?
Might be a candidate for a Rabe rudder.
Just my 2c.

[Dave_Trible]

Tim sometimes an almost undetectable warp, mis-shaped leading edge or mis-aligned wing tip can cause this.  Also if your power plant produces a bunch of torque you can have this problem. The cure might be as simple as a tin trim tab dangling off the outboard trailing edge out near the tip where you can bend it a little up or down to even line tension.

Dave

[Tim Wescott]

Tim sometimes an almost undetectable warp, mis-shaped leading edge or mis-aligned wing tip can cause this.  Also if your power plant produces a bunch of torque you can have this problem. The cure might be as simple as a tin trim tab dangling off the outboard trailing edge out near the tip where you can bend it a little up or down to even line tension.

Some sort of trim tab was going to be my next thing to try, if no one came in with some "aha!" solution.

[Randy Cuberly]

Keep adding tip weight, and move the leadouts forward in 1/8 inch increments until it changes!

Looks like it has a split elevator...make sure both side are aligned straight.

Randy Cuberly
PS:  I assume the engine, wing, and Stab are 0-0-0!  The picture looks like the wing could be positive incidence...probably just the angle of the photo.

[Dave_Trible]

By looking at your side view it can be possible that if it's going loose at the tops it could also be a vertical CG issue.  Either way a tab will correct the feel at the handle.

Dave

[Tim Wescott]

By looking at your side view it can be possible that if it's going loose at the tops it could also be a vertical CG issue.  Either way a tab will correct the feel at the handle.

Dave

I checked the vertical CG before I glued in the leadout guide, and again after -- the wings hang straight down when you suspend the plane from the leadouts.

[Tim Wescott]

Keep adding tip weight, and move the leadouts forward in 1/8 inch increments until it changes!

Looks like it has a split elevator...make sure both side are aligned straight.

Randy Cuberly
PS:  I assume the engine, wing, and Stab are 0-0-0!  The picture looks like the wing could be positive incidence...probably just the angle of the photo.

I hope that's just the picture -- I checked the incidence as it was going together!  Elevator is split, halves are level, but I'll double-check that.

I feel like more tip weight will just be swamping out the problem, since it doesn't show itself in the slightest on the insides -- but I'll try that if nothing else works.

[Jim Thomerson]

I'd try moving your leadout guide up a little bit.  If that made it worse, then try moving it down.   

[Randy Cuberly]

I hope that's just the picture -- I checked the incidence as it was going together!  Elevator is split, halves are level, but I'll double-check that.

I feel like more tip weight will just be swamping out the problem, since it doesn't show itself in the slightest on the insides -- but I'll try that if nothing else works.

Nonsense...swamping out the problem is in fact fixing it!  If adding tip weight doesn't cause the wing to drop on squares then there isn't too much.  I'm quite sure that moving the leadouts forward will help or eliminate this problem after adding about 1/4 oz more tip weight!!!

Try it...if it doesn't work you can yell at me Nayah, nayah,...nayah nayah nayah!!!!  You don't want to miss a chance to do that do you!

Randy Cuberly

[Russell Shaffer]

Since it looks very much like a profile Boxcar Chief,  I agree that moving the leadouts farther forward may help.  It did on my full fuselage version. 

[Howard Rush]

This thread, despite questionable trim advice, uses both "lose" and "loose" correctly: a rarity. 

I'd go with Dave's suggestion that there's a bit of asymmetry in the wing.  Eyeball the wing from the back, centering the TE on the wing.  You can see most warps that way.  Remove the warp if you can see where it is.  If you can't see the warp,  give the wing a little twist anyhow while pouring boiling water on it.  If it's film covered, you can supply the heat with a heat gun or iron.  Don't use a tab unless you want the airplane to suck as a stunt plane, although the airplane may still serve its purpose. 

There is a more subtle effect: see the cartoon at http://aviation.stackexchange.com/questions/7980/in-a-slip-or-skid-how-does-one-wing-have-a-higher-angle-of-attack-and-more-lift .  This would make your high-wing airplane lose line tension in outsides when the airplane is downwind.  If you feel the effect increase as wind speed increases, this could be a culprit. 

[Steve Helmick]

I'd try adding a much larger vertical stabalizer (i.e., fin) area first...and a bit of right thrust, second. Also, wood (sic) read "Circular Airflow" by Frank Zaic one more time.  Steve 

[Dane Martin]

i swore my rm was completely straight. and in outside maneuvers the lines were slack. not bad, but my scores were terrible. turns out i did have a wing warp. trim tab, and that went away. also, i found that with that fixed, the tip weight was a little light. so maybe both are a possibility for you too.

[Howard Rush]

trim tab, and that went away.

for one turn radius.

[John Park]

That fin does look pretty small - does she go a bit light on the lines on the upwind side of the circle as well? 

[john e. holliday]

Thanks guys, my newest Ringmaster gets light on the inside loops.   Great on outsides.   I figured it was the lack of wind and an engine still in break in mode.  Flying by myself I can't tell if wings are level or not.  Has decent line tension for me.

[Tim Wescott]

That fin does look pretty small - does she go a bit light on the lines on the upwind side of the circle as well? 

Y'know, I'm not sure -- I haven't put in many flights.

That fin looked great on the CAD program; it only looks small in real life.  One of the changes I'm contemplating is making it significantly bigger (or whacking it off and replacing it with a bigger one).

[Brett Buck]

The problem is that the plane comes in on the outsides.  It flies wings level both upright and inverted, it's built straight, it has plenty of line tension at any altitude if it's pointing in a straight line or turning inside, but on outside loops and corners it loses line tension. 
The last flying session I tried putting about 1/4 ounce more tip weight into the thing; this didn't seem to change the characteristics much at all -- and since it responds so differently on the insides and the outsides, it can't just be a tip weight issue.

What's going on, and how do I fix it?

   If it doesn't have the same tension on insides and outsides, check for an RPM difference, if none, tweak it until you get equal tension inside and outside, then see what you have the rest of the time. If it isn't level, then investigate further.

    Brett

[Tim Wescott]

   If it doesn't have the same tension on insides and outsides, check for an RPM difference...

It sags at the tops of loops, but it does so equally upside down and upside up, on outsides and on insides.

(It's my test plane, so I could even quote RPM numbers).

if none, tweak it until you get equal tension inside and outside, then see what you have the rest of the time. If it isn't level, then investigate further.

Tweak what?  There's no flaps; I either need to put on a tab or put in some twist.

[Brett Buck]

It sags at the tops of loops, but it does so equally upside down and upside up, on outsides and on insides.

(It's my test plane, so I could even quote RPM numbers).

Tweak what?  There's no flaps; I either need to put on a tab or put in some twist.

   The wing.

     Brett

[John Leidle]

 Lack of power ,lack of power , lack of power.

[Randy Cuberly]

Or over controlling...remember No Flaps...Maybe too heavy? 

Randy C.

[Dennis Moritz]

Are you flying too slow for the design? Light planes need to fly faster to maintain line tension. That could exaggerate issues. Sounds like a warp or misalignment.

[Larry Renger]

Are you using a right or left hand prop?

[Tim Wescott]

Are you using a right or left hand prop?

Regular rotation -- it's powered by a stock a LA 46.

[Lauri Malila]

 Once you do the datalogging, have you checked that there is no rpm difference between inside- and outside loops? It's quite normal for a side mounted engine with normal running direction to slow down during outside loops. The best way to see it is to fly horizontal 8's, it's not allways so visible when you just do loops from level- or inverted flight. If that is the case, fuel accumulation in bottom end of crankcase, reversing running direction will reverse the problem. Only way to make it slower is to improve burning efficiency and/or increase rpm, but it will allways be there.
 But also, your plane looks so strange that I would not be surprised of asymmetrical yawing or hinging during maneuvres. That changes the position of fuel head in relation with venturi, and especially with your inboard tank the effect can be quite dramatic. I'd start with something more in-line, a healthier tank position and not horizontally mounted engine. Trim tabs are no good as in this case you would be balancing a static imbalance with aerodynamic device and those forces do not increase or decrease in same ratio. L

[Tim Wescott]

Once you do the datalogging, have you checked that there is no rpm difference between inside- and outside loops? It's quite normal for a side mounted engine with normal running direction to slow down during outside loops.

I posted the results of the horizontal round and square eights in a flight here: http://stunthanger.com/smf/index.php/topic,37026.msg378040.html#msg378040.  While there's a general downward trend in RPM through the eights, and while the RPM sags at the top, I don't see a specific tendency to turn different speeds in upright and inverted flight.

[Lauri Malila]

Aha.
It's not same as level/inverted speed.
By "downward trend", you don't mean inside/outside symmetry, do you?

[Paul Walker]

Tim,
I think you need the down line leadout position farther back. Your down line lead out guide is separate and in the back, isn't it Tim?

OK, I know the answer. Remove the guide, reverse it and try again. If that helps you might consister making separate LO guides and reversing the BC.

[Tim Wescott]

Aha.
It's not same as level/inverted speed.
By "downward trend", you don't mean inside/outside symmetry, do you?

If you look at the speed in the loops, the first outside is lower speed than the first inside -- but then the second inside is just as much lower, etc.  The speed doesn't go up between the first outside and the second inside, or level off, or anything -- and the second inside, which is slower than the first outside, is solid on the lines where the first outside isn't.

So I just don't think it's a speed thing.

[Tim Wescott]

Tim,
I think you need the down line leadout position farther back. Your down line lead out guide is separate and in the back, isn't it Tim?

OK, I know the answer. Remove the guide, reverse it and try again. If that helps you might consister making separate LO guides and reversing the BC.

Usually when you make this suggestion people resist because the guide is buried in the wing (whining quoted: "hard ... refinish ... rattling nuts ...").  I guess I can't do that here, since the guide couldn't be any further out in the open air and still be attached to the airplane.

[Lauri Malila]

If you look at the speed in the loops, the first outside is lower speed than the first inside -- but then the second inside is just as much lower, etc.  The speed doesn't go up between the first outside and the second inside, or level off, or anything -- and the second inside, which is slower than the first outside, is solid on the lines where the first outside isn't.

So I just don't think it's a speed thing.

Yes I see. That is same kind of phenomen when I don't find speed difference between outside- and inside loops when they are done from inverted- or level flight. The change only comes in more aggressive change of trajectory, like in intersection of horizontal eight. Sorry, this may go a little outside your original subject but I'll continue anyway;
We have gone quite deep into this phenomen and I'm quite sure that it is caused by fuel/oil accumulation in the bottom end of carter. It may be magnified by our piston scavenging and perhaps slightly too small scavenging cross area, that would explain why problem gets smaller when I reduce fuel consumption and load.
The phenomen may be less important in a profile fuselage because of vibration.
I know there will be someone saying that it's caused by centrifugal effect of crankweb but that's not so simple.

Try with lighter prop & more rpm first.

Lauri

[phil c]

Tim,  the force layout looks way different from most models.  Have you checked how the plane hangs from the leadouts?  If the wing hangs vertical and parallel to the lines the leadouts are about right vertical position.  if the leadouts are too low with respect to the CG it would roll the plane in on outsides.  I've found the vertical LO position has to be within 1/2 in. of the vertical CG.

If that checks out you've got a warp making it roll right(comes in on outsides).  Twist the wing and iron the covering, or add a lomg narrow trim tab.

The only other thing might be the side area distribution.  Most planes have the thrust line a bit above the wing  and a fin above the centerline.  With so much sidearea above the thrustline propwash may be roing the plane right.  You could try adding a subrudder to balance things out.

Phil C

[Tim Wescott]

If that checks out you've got a warp making it roll right(comes in on outsides).  Twist the wing and iron the covering, or add a lomg narrow trim tab.

Unfortunately I failed to follow the excellent instructions that come with SLC, and ended up with slack covering.  I'm pretty sure that I need to rip it off and start over again before the covering will be taut.  That's on my list.

It does hang correctly from the leadouts -- I actually used that to set the vertical position of the leadouts.

[john e. holliday]

Don't tear the SLC off.   Get your hot air gun and a straight pin.   Reshrink the covering.  Sounds like iron or gun was not hot enough on the initial covering job.  Now you say the plane keeps slowing down as it goes through maneuvers.   Change props as the plane should fly at or near a constant speed.   you also may be over controlling with a nose heavy plane.  But, in reality you need someone with the knowledge to fly the plane and see how it feels to them.   

[Tim Wescott]

Don't tear the SLC off.   Get your hot air gun and a straight pin.   Reshrink the covering.  Sounds like iron or gun was not hot enough on the initial covering job. 

Hot enough gun to shrink SLC means hot enough to melt the underlying foam.  No, thank you, I don't want to melt my wings.  Phil has very specific instructions to successfully use SLC, which I thought I was following but on seeing what I did and re-reading the instructions saw that no, I had not.

[Tim Wescott]

Unfortunately I failed to follow the excellent instructions that come with SLC, and ended up with slack covering.  I'm pretty sure that I need to rip it off and start over again before the covering will be taut.  That's on my list.

I just went and re-did the covering, actually following Phil's instructions, and not only do the wings look a lot better, but I found the problem (or at least a problem, and one that's certainly bad enough to be the problem:

The @#$% airframe isn't straight.  The stab is tilted.  The inside tip of the stabilizer is over 1/4 inch lower than the outside tip.  I noticed this after I stripped the wings and recovered them.  I went to look for any visible twist in the wings, and noticed that the stab is tilted.  I hate it when this sort of thing happens -- I certainly checked the airplane every which way when I built it.

This is entirely consistent with how it was acting, because it seemed to be yawing to the inside more than rolling.  The direction of tilt on the stab means that up elevator should yaw the tail out, while down elevator will yaw it in.

So, three good things came out of tonight's work: first, I have a problem that I can fix; second, the wings now look way better, and are far more torsionally stiff, and finally, if I do need to twist the wings, I can actually do it and they'll stay put.

[Motorman]

I checked the vertical CG before I glued in the leadout guide, and again after -- the wings hang straight down when you suspend the plane from the leadouts.

Yes, but the vertical CG is below the wing.

MM

[Tim Wescott]

Yes, but the vertical CG is below the wing.

So is the leadout guide.

[Tim Wescott]

Flew it today.  Even though the wing is now badly warped, I have good tension on both insides and outsides.

The wing is warped badly -- the outside tip is at least an inch, maybe two higher than the inside tip (I couldn't say -- it just looked Really Bad). 

Moreover, the tank location (there's pictures of the plane someplace) is right where you want to grab the plane.  This meant that the tank had been knocked so high that flying at 45 degrees inverted made the engine quit.

I need to unwarp the wing, but the airplane went onto the field in better trim than it had last flown, and left the field in better trim than it went on, with a clear prescription for making it better yet -- so I'm happy.

[Tim Wescott]

I flew today, after forgetting at least some of what had been going on (I should take notes!).  I added area to the fin, which seems to help the thing recover from being loose on the lines -- before, it would come up tight, bounce, then repeat.  Now it just recovers gracefully.

Clearly, I need to do more to my "area tab".

[Dane Martin]

for one turn radius.

Tim, since you renewed this post with the update, i wondered what Mr. Rush meant by this. It was in response to my trim tab statement. So Howard, if you see this, could you elaborate for a rookie learning how to trim a plane correctly?

[Tim Wescott]

Howard likes being cryptic.

He feels that the effect of a trim tab will not be as consistent for all angles of attack compared to (best) taking the warp out of the wing or (distance second best) tweaking flaps.  So the roll correction you'd get from a trim tab will, in his opinion, only be right for one angle of attack and, hence, turn radius.

He's not the only one, but he does seem to be the most prone to making comments about the effect.  If you just can't fix a problem any way else, a trim tab is probably better than stomping the airplane.  But it should be far from your first choice.

[Dane Martin]

Ok. Totally makes sense. And on that particular model, the trim tab masked the real problem. The tail plane was not as straight as i originally thought. So when the elevator was applied upright, the plane rolled out, tighter lines. Inverted, it rolled in. Pretty obvious problem and solution now, but last year i was even greener than today! Lol.
Thanks Tim