Twister Wing Flex

[Joe Rice]

I completed a Twister wing using basswood spars and half ribs per some of the modifications I ran across. This wing has a lot of torsional flex compared to the D-tube cap strip wings for which I am familiar.  I plan to cover the wing with Polyspan.  Should I web the spars, or leave it up to the covering to stiffen the wing? 

Joe

[James Mills]

I completed a Twister wing using basswood spars and half ribs per some of the modifications I ran across. This wing has a lot of torsional flex compared to the D-tube cap strip wings for which I am familiar.  I plan to cover the wing with Polyspan.  Should I web the spars, or leave it up to the covering to stiffen the wing? 

Joe

Joe,

Web spars from the center out 4 or 5 bays would help (make sure the grain runs vertical).  Once the wing is doped and polyspan applied the strength will greatly increase.

James

[Joe Rice]

Joe,

Web spars from the center out 4 or 5 bays would help (make sure the grain runs vertical).  Once the wing is doped and polyspan applied the strength will greatly increase.

James

Thanks, James.  That confirms my concern.  The first web is now clamped in place...

Joe

[wwwarbird]

 I've never used basswood for spars, but I do always use spruce on my stuff anymore. The Twister wing is definitely a bit flexy as designed. The last one of them I did I added half ribs and 3/16" x 1/16" X-bracing across the entire span. It came out nice and stiff and flies great.

[Tim Wescott]

I've never used basswood for spars, but I do always use spruce on my stuff anymore. The Twister wing is definitely a bit flexy as designed. The last one of them I did I added half ribs and 3/16" x 1/16" X-bracing across the entire span. It came out nice and stiff and flies great.

I don't know that half ribs would do much, but the cross bracing would be the ticket.

[wwwarbird]

 Yeah, the half ribs don't necessarily help the rigidity any. They do help make the airfoil a little more consistent though, plus it looks cool.

[Bob Kruger]

With a doped finish, you will see a lot less flex.

Another trick I learned from Tom Morris was to sheet the last bay on both the inboard and outboard wing.  You will have to trim off 1/16" from the outside of the last two ribs.  1/16" balsa sheeting on the last two ribs of each side will really stiffen the wing.

I also recommend cross bracing between the spars and filling in the trailing edge sheeting.  I've used scrap 1/32" non contest wood to do both and the change is remarkable. 

Bob

I completed a Twister wing using basswood spars and half ribs per some of the modifications I ran across. This wing has a lot of torsional flex compared to the D-tube cap strip wings for which I am familiar.  I plan to cover the wing with Polyspan.  Should I web the spars, or leave it up to the covering to stiffen the wing? 

Joe

[Joe Rice]

With a doped finish, you will see a lot less flex.

Another trick I learned from Tom Morris was to sheet the last bay on both the inboard and outboard wing.  You will have to trim off 1/16" from the outside of the last two ribs.  1/16" balsa sheeting on the last two ribs of each side will really stiffen the wing.

I also recommend cross bracing between the spars and filling in the trailing edge sheeting.  I've used scrap 1/32" non contest wood to do both and the change is remarkable. 

Bob

Ok...  I web capped the spars out to the fourth bay with 1/16 contest balsa.  The result was a very significant improvement in resistance to flexing of the wing.  I also web capped the trailing edge sheeting out to the tip rib. This resulted in a no discernible, if minor,  resistance to flex. I would recommend to anyone building a scratch Twister to modify the ribs, spar and LE to build a D-tube cap strip wing.   

Joe

[Bob Kruger]

Ok...  I web capped the spars out to the fourth bay with 1/16 contest balsa.  The result was a very significant improvement in resistance to flexing of the wing.  I also web capped the trailing edge sheeting out to the tip rib. This resulted in a no discernible, if minor,  resistance to flex. I would recommend to anyone building a scratch Twister to modify the ribs, spar and LE to build a D-tube cap strip wing.   

Joe

Sheet the last bay, e.g. the one closest to the leadouts or the tip.  Alternate the sides of the spars between ribs you put the vertical grain webbing.  Put the vertical webbing between the sheeting at the end of the wing. 

Cap strips add little or nothing to wing rigidity, if anything.

Do this, and you will have a much more rigid wing than a straight D-Tube.

V/r

Bob

[Damian_Sheehy]

<snip>
Do this, and you will have a much more rigid wing than a straight D-Tube. 

Bob,

As a guy with an aero structures background I don't believe your suggested modification will provide more stiffness that a D-tube that runs the span. I think it may give you a good return in terms of the effort you put in to do it.

Damian

[Larry Cunningham]

My advice is to build the wing as stock, balsa spars and all, no webs, etc. When it is covered, it will be more than adequately strong. I built seven of them, and wing flex was never a problem. You can't judge the rigidity of an uncovered wing very well; if you did, you might be appalled at the general floppiness of an I-beamer..

It is the monocoque feature of the wing covering, being taut and resisting stretch, distributing loads smoothly over a large surface area which imparts the rigidity. When you're adding structure (say vertical webs between top and bottom spars), you need to ask yourself two important questions. 1) Would my webbing need to stretch if my structure flexes? and 2) does my web material resist stretch?

You also need to consider what your flexing problem is. Is it torsional (I'm assuming this is the concern with a Twister wing) or has it to do with vertical or horizontal loading? Under flight loads, have you noticed Twister wings sagging? Vertical webbing between the top and bottom spars would help that. Horizontal drag forces on the wing are almost never a problem, and vertical webs are orthogonal to such loads and not useful.

Of course, the webbing material itself might be (torsionally( rigid. If this is what you're depending on, consider using something besides balsa (say plywood) for webbing. Difficulties in achieving sheet stiffness have a much to do with the "2D" aspect of a flat sheet.

As wimpy as silkspan might seem, when it is doped and filled it becomes quite taut and strongly resists stretch (better than, say, plastic). Applied as a smooth 3D surface, it works amazingly well. That said, almost all of my Twisters had wings finished in Monokote which worked adequately well. 

L.

"Sometimes when I reflect on all the beer I drink, I feel ashamed.
Then  I look into the glass and think about the workers in the brewery
and all of their  hopes and dreams. If I didn't drink this beer, they might
be out of work and their dreams would be shattered. I think, "It is better
to drink this beer and let their  dreams come true than be selfish and
worry about my liver." -Babe Ruth

 

[Bob Kruger]

Damian;

So, a 1/8" balsa D-Tube spar provides less predeliction to twist than two 1/4" sq spars with alternating side vertical webbing between spars.

Is that your position?

V/r

Bob

Bob,

As a guy with an aero structures background I don't believe your suggested modification will provide more stiffness that a D-tube that runs the span. I think it may give you a good return in terms of the effort you put in to do it.

Damian

[Damian_Sheehy]

Hi Bob,

Yes, that's correct, and 1/16'th thickness sheeting is plenty for a Twister-sized plane.
I have emailed you an article that explains why (sorry, PM does not seem to support attachments). Basically, closes tubes are very good at resisting torsion or twisting of the wing; i.e. grabbing the plane under your left arm and twisting the right wing tip. The spars are good at resisting bending; if you have strong spars the wings will not bend into a U shape when you sharply pull out of a dive.

In a D-tube wing the shear panels do two things; obviously without them we would have a open C-section and closed D-tubes are much stronger in torsion. They also help stabilize the spars against bending failure in scenarios where a sharp pull-out could snap off a wing.

As Larry pointed out, the stock wing will be "strong enough". But you really don't get that stiffness until the covering is in place. In other words, if you are covering with dope and silkspan you can potentially get a warp because the wing was not able to resist torsion during the application of the skin. If your wing has a D-section construction there's no way that warp is going to happen during the application of dope and silkspan.

Damian

[Bob Kruger]

Ah, sheeting.  Makes a world of difference.  But that does no one any good with an original wing with a kit unless they do a lot of rib trimming.  We are talking about the Twister wing, e.g. 1/2" LE, no sheeting (less the center section), and no cap strips. 

What I was referring to was the original wing as laid out in the kit. 

Again, vertical shear webs between the spars (with the kit wing), and the same towards the trailing edge with the sheeting that leads to the TE.

Now, if I was to build a Twister wing from scratch, and used C-Tube construction to include sheeting to the LE and spar, with vertical alternating side webbing on the spar between wing panels, it will be more resistant to twist than a straight D-Tube. 

V/r

Bob

Hi Bob,

Yes, that's correct, and 1/16'th thickness sheeting is plenty for a Twister-sized plane.
I have emailed you an article that explains why (sorry, PM does not seem to support attachments). Basically, closes tubes are very good at resisting torsion or twisting of the wing; i.e. grabbing the plane under your left arm and twisting the right wing tip. The spars are good at resisting bending; if you have strong spars the wings will not bend into a U shape when you sharply pull out of a dive.

In a D-tube wing the shear panels do two things; obviously without them we would have a open C-section and closed D-tubes are much stronger in torsion. They also help stabilize the spars against bending failure in scenarios where a sharp pull-out could snap off a wing.

As Larry pointed out, the stock wing will be "strong enough". But you really don't get that stiffness until the covering is in place. In other words, if you are covering with dope and silkspan you can potentially get a warp because the wing was not able to resist torsion during the application of the skin. If your wing has a D-section construction there's no way that warp is going to happen during the application of dope and silkspan.

Damian

[Bob Reeves]

A more or less stock Twister wing covered with Monokote can last 6+ years and get you into the middle of Advanced. Add the 1/2 ribs and be done with it...

[Bob Kruger]

Bob;

This is personal experience alone.

Box stock 'Twister' wing covered with Monocoat failed in less than 20 flights.  Just personal experience.  Those built with a doped silkspan finish did not.

Later ships built with vertical webbing between the spars did not fail with Monocoat.

Just facts.  Nothing more.

BTW - This has nothing to do with the 'Twister' as a good design.  It is a great design - one that I have built a number of and have one on hand right now...

V/r

Bob

A more or less stock Twister wing covered with Monokote can last 6+ years and get you into the middle of Advanced. Add the 1/2 ribs and be done with it...

[wwwarbird]

Bob;

This is personal experience alone.

Box stock 'Twister' wing covered with Monocoat failed in less than 20 flights.  Just personal experience.  

 First, let me make it clear that I'm not trying to stir anything up here, just stating some personally proven facts.

 I don't know what defines "failed" in the above case, but if it was structural it had nothing to do with the design. Built properly and maintained properly a box stock Twister will provide countless sessions of very capable pattern and stunt flying.  
 Without going into great detail, if it was constructed properly Monokote, silkspan and dope or whatever, would have no relevance at all on any structural failure. I have built and flown multiples with each type of construction. Any structural "failure" definitely would have had to have had something to do with the construction. For an easy to build box stock design, built well, light, and straight, it's still one of the best flying planes you can get.

[Bob Reeves]

Bob;

This is personal experience alone.

Box stock 'Twister' wing covered with Monocoat failed in less than 20 flights.  Just personal experience.  Those built with a doped silkspan finish did not.

Later ships built with vertical webbing between the spars did not fail with Monocoat.

Just facts.  Nothing more.

BTW - This has nothing to do with the 'Twister' as a good design.  It is a great design - one that I have built a number of and have one on hand right now...

V/r

Bob

Hi Bob, The TwistMaster is actually 11 years old and still flying, although I no longer own it.
Got me kicked out of Intermediate, trophied at Brodaks, couple 1st place Advanced trophies at local contests, flew many patterns with several ounces of camera, batteries and transmitter strapped to it's back in 03 and last several years had a Saito 40 for power. The wing was built from the kit and the only thing I added was 1/2 ribs, actually didn't even use full 1/2 ribs. To save weight I made a cutting jig and use sliced ribs.

[Bob Kruger]

Failed was defined as the wing cracking just outboard of the center sheeting.  Kit was box stock and built that way, contructed with Tightbond for the wing ribs, 3 hour epoxy for plywood and wing attachment, Monocoat as the covering.  The covering never broke after the wing cracked, and no glue joints failed when I did the post mortem.  If was somewhat windy that day, but I didn't consider it excessive.  I did notice the wing flexed more than I was comfortable with, but the wing was not loose from the fuse, the controls had not popped loose, the center sheeting was even against the ribs and fully attached, and none of the ribs had broken loose from the glue joint.  After that, I cross braced the sparts if using Monocoat.

So the real bottom line is this:  When balsa flexes, it fails sooner than not.  Was it a soft/weak spot in one of the spar or the LE?  Who knows now.  What I didn't have was rigidity offered by a doped finish that provides either compression or tension to minimize flex.  So, since then, if its a plastic finish without sheeting, its vertical sheer webbing.  Takes very little weight, provides a bit more insurance.

I've build a boat load of Twisters, more than most - I have one sitting on my plotter that I got from a yard sale that I will build this summer.  Its a great flyer and has been the basis for a number of other designs.  That one that experienced the wing cracking outboard of the sheeting is the only one that I have do that.  Sample size of one is something that still makes me use a little more insurance, cheap insurance...

Just my experience, and I will cast no aspersions on others building ability when they describe a failure.

V/r

Bob

First, let me make it clear that I'm not trying to stir anything up here, just stating some personally proven facts.

 I don't know what defines "failed" in the above case, but if it was structural it had nothing to do with the design. Built properly and maintained properly a box stock Twister will provide countless sessions of very capable pattern and stunt flying.  
 Without going into great detail, if it was constructed properly Monokote, silkspan and dope or whatever, would have no relevance at all on any structural failure. I have built and flown multiples with each type of construction. Any structural "failure" definitely would have had to have had something to do with the construction. For an easy to build box stock design, built well, light, and straight, it's still one of the best flying planes you can get.

[Bob Kruger]

Ok...  I web capped the spars out to the fourth bay with 1/16 contest balsa.  The result was a very significant improvement in resistance to flexing of the wing.  I also web capped the trailing edge sheeting out to the tip rib. This resulted in a no discernible, if minor,  resistance to flex. I would recommend to anyone building a scratch Twister to modify the ribs, spar and LE to build a D-tube cap strip wing.   

Joe

Joe;

Do you alternate side on the bracing?  If you go down just one side of the spar or the center, it will help you resist crushing, but not do much for twisting.

Again, this is personal experience, but I have webbed using vertical grain on 1/32" on both sides, effectively creating a light box spar, that worked great for me.  If you can't do that, with the Twister alternate sides.

Yes, a sheeted scratch build wing will be more resistant to twisting.  But it you want to make a kit wing a little stiffer, alternating the sides really helps.  Of course, covering it with a doped finish really tightens it up...

Bob

[Bob Reeves]

Hard to say, maybe the one that failed had really soft spars. I've built and flown 3 over the years and every one of them was stock and Monokoted. Back in 78 or so I had just moved from the East coast to SanDiego, built one box stock (except a change in the rudder shape) on the kitchen table, drove up to a contest in Orange County and placed 3rd in Advanced. I gave the airplane away (still in one piece) when I moved to Oklahoma in 86.

Not saying it isn't a good idea to stiffen the wing, just defending my previous statement that they can work just fine built stock and covered with plastic.

[Joe Rice]

Joe;

Do you alternate side on the bracing?  If you go down just one side of the spar or the center, it will help you resist crushing, but not do much for twisting.

Again, this is personal experience, but I have webbed using vertical grain on 1/32" on both sides, effectively creating a light box spar, that worked great for me.  If you can't do that, with the Twister alternate sides.

Yes, a sheeted scratch build wing will be more resistant to twisting.  But it you want to make a kit wing a little stiffer, alternating the sides really helps.  Of course, covering it with a doped finish really tightens it up...

Bob

Bob, I ran the web strips on the TE side of the spar out to the 4th bay.  Alternating would have been a real pain as I have 1/2 ribs on the LE side of the spar.  Though I am pleased with the result, It appears that there may be a bit of overkill in this webbing process given responses from Larry and others who have significant experience with this design.  However, the exchange of ideas and experiences shared on this topic are what this forum is all about. 

Joe

[Bob Kruger]

Hard to say, maybe the one that failed had really soft spars. I've built and flown 3 over the years and every one of them was stock and Monokoted. Back in 78 or so I had just moved from the East coast to SanDiego, built one box stock (except a change in the rudder shape) on the kitchen table, drove up to a contest in Orange County and placed 3rd in Advanced. I gave the airplane away (still in one piece) when I moved to Oklahoma in 86.

Not saying it isn't a good idea to stiffen the wing, just defending my previous statement that they can work just fine built stock and covered with plastic.

Spars didn't seem soft, but who knows.

I know that your Twistmaster has performed yeoman duty for over a decade, and that is a fair testimonial.

I have meant no disrespect to SIG Manufacturing on the Twister, nor its designer.  I believe it was designed in an era of lighter, unmuffled engines (or the genesis of muffler use) and a doped finish, hence the moment arms and the simple and cost effective from a production standpoint wing platform.  Heavier engines with mufflers have forced either more tailweight or longer tail moments.  There is obvious debate on using film covering alone on the wing... :-).  Regardless, the wing generates sufficient lift to get even heavier versions through the pattern given sufficient thrust and has been the tool to get many fliers from beginner to advanced that was low cost and simple to build. 

BTW, I did the wing rib in CAD for C-Tube years ago.  The only other change was to allow for a 1/4" flap.  Worked (and works) quite well.  I have it in Adobe .pdf format, without the leadout cutouts (I usually leave that for people to do as they please).  If anyone is interested, just send me an email and I will be happy to send them a copy.

Bob