Dihedral questions

Some use it most don't. Semi-scale models look better with it. but, is it necessary? How do you measure how much dihedral is enough, or too much? How does it affect stunt performance, especially on inverted flight when D is negative?

Use dihedral to align the leadout guides with the model's vertical c.g. position for semiscale low-wingers or any model whose tank position or similar issue forces lower wing placement. Apparently the line tension stabilizes the roll of the wing in inverted flight, since those who know (like Al and Ron) report no problems.

I don't think that you're likely to be able to calculate vertical c.g. all that accurately, but you can probably make a pretty accurate educated guess by looking at wing position on other successful stunters and taking into account that the lower placed wing root does lower the c.g. some from its position on standard straight-wing designs. Then when the plane is close to completed, suspend it from its leadouts to see whether the tips line up in a vertical plane, and adjust to the extent possible with finish, l.g. and other variable or exchangable masses.

For low wing semi scale stunters, the wing placement might be part of the SEMI-scale aspect - higher than true scale dictates, but with a dihedral to give scale-like appearance and align the leadouts vertically.

SK

LAST EDITED ON Dec-02-04 AT 08:45 AM (CST)
 
This my first and only attempt at a dihedral-wing control line airplane. It's a Yak-11 racer built for profile scale and Jim Walker event in Canada. I built it to scale dihedral.

The first time I tried a figure 8, it barrel-rolled across the circle and almost crashed. However, with a more liberal dose of tip weight, it stunts just fine. I can hardly tell it has dihedral.

To answer a previous question, "hingeing" is defined as the opposite of "eating the plane".

One of the older and wiser CL sages told me (belatedly) that the proper amount of dihedral is just enough that people can see that it has dihedral.

>This my first and only attempt at a dihedral-wing control
>line airplane. It's a Yak-11 racer built for profile scale
>and Jim Walker event in Canada. I built it to scale
>dihedral.
>
>The first time I tried a figure 8, it barrel-rolled across
>the circle and almost crashed. However, with a more liberal
>dose of tip weight, it stunts just fine. I can hardly tell
>it has dihedral.
>
>To answer a previous question, "hingeing" is defined as
>the opposite of "eating the plane".
>
>One of the older and wiser CL sages told me (belatedly) that
>the proper amount of dihedral is just enough that people can
>see that it has dihedral.

Ron B.
F4Fguy

I don't know who your sage was, but he's either not old enough or not wise enough. See Serge's post above and mine below. I'm as sure as I can be that your barrell roll had nothing to do with it having dihedral. Much more likely: not enough tip weight; a warp in the wing; not enough dihedral; misaligned stab; any combination of two or more of the above.

Nice airplane, by the way. I hope you got it sorted out.

LAST EDITED ON Dec-03-04 AT 05:57 AM (CST)
 
Man.. that about hurts your eyes, doesn't it?

LAST EDITED ON Dec-04-04 AT 08:39 AM (CST)
 
>Man.. that about hurts your eyes, doesn't it?
>

When this article appeared in AAM it jumped out of the page at me. I happened to have the CHROME and Day-Go Green in my MonoCote barrel, so I HAD to design and build the plane. On top of that, the actual owner, Bob Yancy, sent me roll of photos.

Only downside, when it barrel-rolled across the circle, there was only one witness, but he keeps reminding people of it. Just the barrel-roll, not the save.

>Use dihedral to align the leadout guides with the model's
>vertical c.g. position for semiscale low-wingers or any
>model whose tank position or similar issue forces lower wing
>placement. Apparently the line tension stabilizes the roll
>of the wing in inverted flight, since those who know (like
>Al and Ron) report no problems.
>
>I don't think that you're likely to be able to calculate
>vertical c.g. all that accurately, but you can probably make
>a pretty accurate educated guess by looking at wing position
>on other successful stunters and taking into account that
>the lower placed wing root does lower the c.g. some from its
>position on standard straight-wing designs. Then when the
>plane is close to completed, suspend it from its leadouts to
>see whether the tips line up in a vertical plane, and adjust
>to the extent possible with finish, l.g. and other variable
>or exchangable masses.
>
>For low wing semi scale stunters, the wing placement might
>be part of the SEMI-scale aspect - higher than true scale
>dictates, but with a dihedral to give scale-like appearance
>and align the leadouts vertically.
>
>SK

Ron B.
F4Fguy

Serge:

It's not a matter of no problems using dihedral, it's that there are great problems in not using it in a low wing airplane(or any airplane with the wing root below the vertical center of mass).The requirement has little to do with "scale",or looks.The lead out guide MUST be at or near the vertical center of mass to avoid a large roll couple,and to make the airplane fly in the plane of the lines. but it is interesting that scale dihedral will usually be acceptably close.

I've watched Pat Johnston build a series (a long series) of semi-scale stunters and every one has dihedral. Never have seen any adverse affect. In fact, I think they may be a bit more stable, but that's just my opinion.

Many years ago, I rebuilt a slightly damaged Magician 35 into a profile F4U, for slow combat. It was too slow but that is another story. I rebuilt the wing so it had the "bent wing" bird look of the F4U. It did a little hingeing, but was very stable in level flight and real stable inverted. At the time I had never heard of tip weight boxes nor moving lead outs. With better trim, it would have flown even better, but was typical of the Magician from the kit. I flew it for three years until a stupid priming mistake, with the glow plug still hooked up on an K&B Greeen head .35 burned to ashes.

I have used dihedral on several low wing models and found it has improved performance by remeoving the dreaded hingeing. I have used it on Sterling Yaks and Mustangs and my own Twin Mustang had dihedral. I have never seen anything in their looping or inverted flying characteristics.
Bare

I built two planes with dihedral (cathedral) wings. The Sterling Mustang and my Little Cherokee.
The Cherokee, with 1" per tip flies perfectly upright or inverted. Powered with a SAITO .30
The Mustang has 1" total and everyone who tried it said it was the best flying Sterling Mustang they ever flew. Powered with an OS Surpass .26
Bob Z.

Related thread link: http://www.clstunt.com/cgi-bin/dcforum/dcboard.cgi?az=read_count&om=3123&forum=DCForumID1

I have built two Yates Dragons that have diehedral, and they are very solid flyers.

So is it agreed that the vertical CG should be horizontal to the wingtip? How do we determine the location of the VCG? I assume it's along the thrustline.

I remember someone (I can't remember who it was) wrote in a magazine (can't remember which one, or when exactly, either.. ) coming up with a theory as to why Al Rabe's moveable rudder and dihedral worked well together.. and it brought a reply from Al, who hadn't been heard of for years, at the time...

>I assume it's along the thrustline.

That would definitely not be true for your average inverted engine stunter, where the thrust line is usually above the wing chord. For determining where a vertical C.G. position will be, I stand by what I said in my first post. For a completed plane, hang the plane by its leadouts and use a plumb bob. Where the line on the plumb bob crosses the center plane of the fuselage is the c.g. position, minus effects of line stiffness (only possibly relevant for very light models). The process will doubtless be problematic, and you will only be able to get an approximation. About the only usefulness in this determination is that you could then calculate (estimate) the restoring moment needed to move the c.g. to the plane of the leadouts; that would allow you to come up with a ballpark determination of mhere to put how much weight.

But it would just be easier to put the ballast wherever you can until things are as close to correct as you can get them, bearing in mind that the longitudinal c.g. position may be affected too.

As Ron hinted in his response, you don't want that c.g. much above the leadout guides, since it will be below them when the plane is inverted. Hinging is only a part of the problem. High c.g.'s are a recipe for coziness with your model at circle's center, when you turn it over.

SK

Here's Pat's Bearcat with about an inch and a quarter of dihedral. Flew very well.