I have two Fancherized Twisters under construction.  (Boy, am I glad to be done cutting the ribs and half ribs.)  Before I get any further on the wings I want to decide what I am going to cover them with.
Since these planes are going to be used as advanced trainers they might not last very long.  That being the case, I don't want to spend the time and money necessary to cover the wings with silkspan or polyester tissue or anything else that requires a dope finish.  In short, I want to use iron-on film.  But, I know that iron-on film does not give a wing as much torsional strength as silkspan and dope, hence my questions.

1. If iron-on film is used - - which I am used to doing - - and is properly applied will the wing be sufficiently stiff for my purposes?
2. If the answer to "1" is no, would adding diagonal partial ribs from the spars to the trailing edge, thus creating something of a Warren truss behind the spars, add enough additional rigidity to be worth the trouble?
3. If the answers to "1" and "2" are both no, would I ruin an otherwise pretty good wing by using 1/20 or 1/16 over the stock structure to create a C or D tube structure, doing which would, of course, add a tenth or an eighth of an inch to the thickness of the airfoil?

Thanks in advance for your thoughts, comments, and suggestions.

 If you whant a stronger wing take the center ribs as templets and cut outer ribs down to that size by doing so youcan sheet the whole leading edge and add cap strips d- tube wing and it does not get any bigger than original wing. you can add some 1/16 shear webs out about four bays also. hope this helps you. I just did one like this works great.    Thanks John Jordan

I have a fancherized twister that I have been flying for over three years. It has at least one thousand flights on. It has seen three diferent motors and many profile contests. The wing is built stock right from the plans except for the addition of half ribs between the full ribs. the wing is covered with orange monokote and there has never been a problem. The wing is plenty strong for monokote covering. cover it with monokote and enjoy. I think you will like it

Lee, John, Mark, Mike, and Russell:

Thank you very much for your responses.  The unanimity of your responses convinces me that iron-on film will be just fine, and so it shall be.  I will probably use Ultracoat as that is what I am most familiar with.

The suggestion that I add sheer webs is great advice.  I don't know why I had not thought about that.

In retrospect I probably should have done as John did and use the center section ribs full span and made it a C-tube structure.  Isn't hindsight wonderful?

These planes were supposed to be ready for this past summer but you all know the way that life can intrude upon one's plans.  Let's see.  It's now only October.  Even I, who builds at a glacial pace, should be able to get them in the air in 2010.

Thanks, again for your help.

I think Mark has a very good point.  Stiff objects break and flexible ones flex.  Just don't let the flex get too large. 

Ed, it really doesn't take that much longer to do the silkspan on the fuselage as it takes less dope for the finish.  DOC Holliday

Ditto: Spruce Spars and Iron On film. Very good flying plane .

From the source: (Model Aviation)

The half ribs are useful since they true up the airfoil. I use balsa spars and web them with light sixteenth. Usually I reinforce the center of the trailing edge top and bottom with 1/32" ply, 2" or so. The last two mods makes for a much stronger wing IMHO. Worth the slight gain in weight. My first Twister survived a full power nose in crash without any damage. It had rained the day before and the earth was pretty soft. Still. The wings were monokoted. The fues silkspan and dope.

I have one built by someone else- monocote on wings and slikspan / dope on fuse and tail feathers.   I flew the outboard wing on mine through some tree brances in Phoenix and it kept right on trucking, a couple rips in the last bay and tip, but no real damage. I also dorked it pretty good in the grass twice and it survived with minimal damage. It does have the balsa tripler on the inboard side of the nose.

Since you are like me and using it for trainer / learning the pattern I would go with monocote.
Quicker finish = less emotional involvement ( if you don't count all those 1/2 ribs and other tweaks that take time).
Straight, aligned and smooth controls will give you a good flying plane that builds confidence.

I have a second one under construction, Going to be a Twist - Stang- modified fuse / wingtips, tail feathers to look like a P-51B. I just have to get back to Phoenix for work, it is waiting for me under my friends spare bed!