Tail wiggles Clarification! LOL!!

LAST EDITED ON Sep-30-03 AT 09:20 AM (CDT)
 
I have an original design profile with OS 40 FP. 48" span, reduced Impact airfoil. This plane has great characteristics except for one problem. In a hard corner, the rear half of the fuselage wiggles like a bull dog!
What can I do to correct his on a "built" plane? Ain't got no stinkin' time to build a new one. (Plus I lost all my original drawings for it.)
Thanks!
Bill

EDIT: The rear half of the fuselage (from the TE back) DEFLECTS, or actually bends, itself, back and forth. ie.: the front of the plane goes straight, the rear oscilattes (sp). Lack of rigidity. Need ideas as to a fix. Cut a "core" out of the rear fuse and install geo braces, etc.. Thanks!

I assume you mean that the tail "wags" sideways. Sounds like it is due to the gyroscopic effect of the prop. Here are a couple of suggestions:
1) Go to a lighter prop.
2)Lower the diameter of the prop.
3)Lower the rpm of the engine.

I know that the 3 items are probably not simultaneously possible when looking at the complete picture. However all three will decrease the angular momentum of the prop, and this will decrease the effect.

A fourth possiblilty is to install a Rabe rudder, which adds an opposing kick to the one from the prop. A fifth possibility is to stop hitting that corner so hard!

>I assume you mean that the tail "wags" sideways. Sounds like
>it is due to the gyroscopic effect of the prop. Here are a
>couple of suggestions:
>1) Go to a lighter prop.
>2)Lower the diameter of the prop.
>3)Lower the rpm of the engine.
>
>I know that the 3 items are probably not simultaneously
>possible when looking at the complete picture. However all
>three will decrease the angular momentum of the prop, and
>this will decrease the effect.
>
>A fourth possiblilty is to install a Rabe rudder, which adds
>an opposing kick to the one from the prop. A fifth
>possibility is to stop hitting that corner so hard!

An interesting follow up here, Alan: I asked about the prop with that in the back of my mind, but the truth if we have never expereicned this as the cause of "wiggle", and Matt flies with probably one of the biggest props around (except for the 4-strokes). And he is headed bigger. We were using a 12 1/2 inch prop with 35 powered 570 square inch plane and never experienced this "wiggling" problem. Now the new plane did exoerience a wiggling problem, but it wasn't caused by the prop. Part of the problem was the Rabe rudder itself that wasn't adjusted properly. We got the wiggle out by going back to "square one", and setting up the basics. Never touched the prop or the engine.

Although the large prop can cause a "softness" in some of the outside corners, we have yet to experience any wiggling problems, or any inability of the plane to turn as a result. As a matter of fact, Matt's new plane turns much sharper than his previous ones, due, in part, to the fact that he has increased the elevator throw. (Read that as less flap, more elevator, and nothing over maybe 25 degrees.) In fact, what we are working on now is a much improved, but still persistent, "banging" in the corner. And it is early yet. It is a rare plane that flies off the workbench in perfect trim. In fact, I would think that plane is non-existent.

Len,
If indeed the problem was due to gyroscopic precession, my answer is correct. However as you and Crist point out, there are a lot of other things going on which can cause the same effect.

Actually Bill's question seems to be more about a flexible tail end, so I completely misunderstood the question!!

>Len,
>If indeed the problem was due to gyroscopic precession, my
>answer is correct. However as you and Crist point out, there
>are a lot of other things going on which can cause the same
>effect.
>
>Actually Bill's question seems to be more about a flexible
>tail end, so I completely misunderstood the question!!

Didn't we all? Words. There's what I said, what I intended to say, what I thought I said, what I meant to say, what you heard me say, what you thought you heard me say, what you thought I meant to say from what you thought you heard me say, and what...

What'd you say?

We have been wrestling with Matt's new plane at the moment with a similar situation. First off, how much side area do you have in the back (rudder included) and how big of a prop are you using? Does it wiggle in outsides as well as insides, or both outsides and insides?

Now, basics: Check for alignment problems. Is the stabilizer absolutely in line with the wing horizontally (no tilt?) Is the stab hingeline absolutely parallel with the wing hingeline? Both of these errors will throw your off, and no amount of trimming will change this until this is fixed.

Check the fuselage to see that is is vertical to the wing. Same with the rudder. If this is tilted (wing tilted?) it will steer the plane when you do your maneuvers.

If you have a lot of offset on either the enginen or rudder, get rid of it. Get rid of all of it. You can always add some if you really need it, but, for now, get rid of all of it. If anything, make the rudder adjustable so you can see the effect this has on the flight. You will be surprised.

Where is your CG? If it is too far back, move it forward (slightly). Make sure the wings are level when upright and unverted.

OK, that is done. Now, is the wing tip dropping, or is the tail just wiggling? If the wing tip is dropping (or rising) take out or add tip weight. Do you have equal or unequal panel wings? If you have a lot of assymetry (or even none) you may need to add more flap area to the outside flap. Try adding a "wart" to the flap. Add a couple of square inches to the tip of the flap. When you do this, you will probably need to add more tip weight. This will translate into more tension which is good. (On the "wart" thing, you can just take a couple of pieces of 2 inch by 2 inch 1/64 plywood or 1/32 balsa and using silicon sealer, "glue" one on top and one on the bottom of the outboard edge of the flap with the edges touching and extending about an inch beyond the flap. This is easily removed again if it proves to not be helpful.)

Check your leadout location. The leadouts may be wrong and jerking the plane in/out as you gain and lose tension. (This is something many people overlook).

Oh (this is for Ted) and you may also want to check your hinge line gaps. Just to be safe, try sealing all of the hingelines with tape to eliminate this as a problem.

It looks like a common problem I had with my stunters. I cured it by reducing rudder offset to 0 and moved the LO aft a bit. What happens with rudder offset is that the LO end up too far forward to counteract the rudder. Then in a hard corner the up line ends up having more tension and actually yaws the nose of the plane in. This yaw then creates a rolling motion too. All happens fast. Brett has a paper on this and really helps to understand what is going on.

Crist Rigotti
"A driver trying to be a pilot."

Hey Bill,

The tail of my rebuilt and modified Banshee wiggles a lot. Ironically caused by sanding it too much, thereby making it very weak, AND NOSE HEAVY, thereby causing me to add tailweight.

Uh, if something like this is causing your problem, maybe adding ply doublers, be sure to sand the edges to a feather to avoid a stress riser (easily a problem with a tail that is too weak to begin with), doping on some carbon veil, something like that.

I wouldn't dig into the structure, it'll most likely only get weaker. An external fix won't be as pretty for sure, but stronger.

You didn't mention what type of covering it has. If it's an iron on covering, as you deflect it, look for the wrinkles, that's where you need it stronger.

Watch out for really calm days, when my Banshee flies through it's own turbulence, it really gets funky!

Tools

LAST EDITED ON Sep-30-03 AT 10:17 PM (CDT)
 
I thought Bill had diagnosed the aft fuselage as too soft, the problem being how to correct this. I would clean it very well, multiple times, sand down to the primer with coarse paper, and apply a layer of .5 oz carbon vail to both sides, probably with epoxy, possibly with dope or whatever is compatible with existing finish. On bare wood, Polycrylic. Depends on what you have under the finish. If possible, I would prefer to wrap the CF around the top and bottom edges, giving double layers there, for ding protection, but this could be difficult with dorsal fins, etc. Always, ALWAYS, put the strong stuff on the outside. Steve

One way to do a profile fuse is to drill it lengthwise and insert a round hollow K&S aluminum tube. I've done this with good results. The tubing is featherweight and stiff. For really thin sheeting I use hardwood dowels from the hobby shop or skewers from the grocery store. The skewers are also tapered at one end and easier to insert.

I suppose a hardwood dowel or strip of basswood set along the bottom and top edge of the fuse would help but I haven't tried it.

How do you keep the drill bit centered?

LAST EDITED ON Sep-30-03 AT 11:24 PM (CDT)
 
Very good suggestions! Might I add another??
A good friend of mine had a similar situation which was the result from using too soft a piece of wood for the profile fuse. It was obvious, even in flight, that the tail was indeed "wiggling"!!
Not wanting to carve into the body, he simply installed another horn and pushrod to the other side of the fuse. Now there is one on each side of the body, and the problem was solved.
Wes

LAST EDITED ON Oct-01-03 AT 04:14 PM (CDT)
 
Howard,
I made hardwood handles (1/2"sq x 5"L) for several of my long bits (18")so drilling could be done slowly by hand and eyeball. Alignment is easy to see with the long bits and really isn't a problem. I pilot with a 16th dia bit and go bigger from there. Once the original hole is in you can "saw" with the bit to straighten it up or change direction.
Bill P.