Mirage wing questions -- Ollie, are your there?

[Joe Minton]

I have decided to build the Mirage Sailplane but with three changes:

I'll break the wing in four places instead of the planned two. I have no aerodynamic reason for this. Rather, I like the appearance of the Bubble Dancer & Comet Sailplane wings and dislike two-break wings.
Two questions:
1) If I maintain the average dihedral, will the ship handle about the same? I'll break the tip panels in their middle.
2) How much joiner strength do I need in the outer dihedral breaks?

The other three changes?:
I'll strengthen the center section with CF (see other thread).
I'll fit a spoiler (see other, other thread)

BTW: I do not intend to launch this plane very hard. I will use fusible links to ensure that I do not break the wing (see the other, other, other thread).

Joe

[Ollie]

For equivqlent dihedral calculations see:
http://www.rc-soar.com/tech/spiral_eda.htm

A single ply of 0.007 carbon from CST or ACP will increase the compression strength of the top 1/8 X3/8 spruce spar cap by a factor of 3.4 or so. The bottom spar cap is already about twice as strong as it needs to be because wood is much stronger in tension than in compression. If you want to strengthen and stiffen with carbon top and bottom, then put twice the thickness on top. The bending load decreases exponentially as you move away from the wing root. At 30 % of the way from root to tip the bending moment is 49% or less than at the root end. At 50% of the way to the tip, the bending moment is less than 25% of the load at the root. The carbon reinforcement only has to extend about 40% of the way to the tip. Beyond that, the carbon would just be along for an expensive ride.

For a joiner I would use a piece of 1/8 X 1/2 rectangular carbon rod from Aerospace Composit products.

I would build a joiner box from one piece of 1/8 birch ply and two laminated outer pieces of 3mm (1/8) lite ply. The over all shape to be as a dihedral brace between the spar caps. Make a snug cut out in the birch ply for the carbon bar. Leave the bar out when laminating the three pieces. After assembly, cut the ply joiner box in half. Glue the boxes between the spar caps and wrap the sparcaps to the boxes at each end of each box with about two dozen turns of kevlar thread. After wrapping, the kevlar can be CA'd in place with thin CA. The birch ply's hardness is required to take the bearing loads of the joiner bar without crushing.

For the outer dihedral break, its strength requirement depends on its distance from the wing tip. Assuming it is 18 inches from the wing tip, the outer break only has to be about 1/10 as strong as the wing center. It is important to keep this joint light by not using birch ply dihedral braces. Use 1/8 sq. spruce carry-across front and back, top and bottom. Taper the spruce carry across in both width and thickness to a point at each end to save weight and reduce stress risers. You can also taper the spruce spar caps in the outer panels to 1/8 sq. to save weight where the strength isn't needed.

[Ollie]

Badger makes a couple of good points.

Spruce and carbon don't share load efficiently because of differences in their stiffness. I should have taken that into account. The conclusion is that the spruce does little good from a load carrying point of view.

I agree that it is very difficult to hold a thin carbon spar cap in column so that it will carry compression loads.

If you want a moderately stronger wing, go to a larger wood spar cap crossection at the wing root and taper the spar caps in width and thickness to the tip. This saves money on carbon and puts the strength where it is needed.

A stronger spar will allow you to tow faster without breaking the spar but it exposes the wing to flutter because of the lack of torsional stiffness in the open rib bay structure. Strengthening just the weakest link in the structural chain exposes the next weakest link to possible failure.

[Mark Miller]

Joe,

I understand the desire to add dihedral to the center of the wing. It's an asthetic thing. I like the flat center section for one reason. For it's weight, straight spars will be stronger than ones with dihedral built in. There is a built in stress point and you have to add weight to make it strong enough.

That being said, reccomendations from Mark Drela on the Mirage spar in the center section is to have a .021 carbon cap on the top and .014 on the bottom. As Ollie said, the spruce will be along for the ride and act as a "shim to take up space. You can use thinner spruce spar caps or bass wood for this to reduce weight. Double the width of the shear webs and wrap the joiner boxs with Kevlar thread or tow. I have had good luck laminating a joiner blade from two pieces of 1/16" ply with 2 oz. glass in between. You can substitute carbon too. A full carbon joiner blade is over kill.

Leave the tip panels stock except for wrapping the joiner boxs with Kevlar. According to Dr. Drela's calculations this should allow a 80 lb. pull on the winch line which is adequate and it will be no heavier than a stock spar. Stronger at the same weight is a good thing. This is all assuming a flat center section of course.

One other suggestion is to replace the 1/8" square balsa turbulators with basswood. It will still be pretty light but will be much more durable. They have a tendancy to have compression failures on hard landings and hanger rash.

Check out some construction photos with comentary and an article on the Mirage on my website. Look in the sailplane section under Mirage. The links are on the left.

Mark Miller
http://www.isthmusmodels.com

[markdrela]

Originally posted by Badger
The modulus of Spruce is listed as 1.76 10^6 psi (Popov). The modulus of Carbon Fiber is 33 X 10^6 psi(CST). When the CF fails, the spruce will be carrying 5.3% of the stress level of the carbon, ignoring the beam depth stress profile for now.

The end result is that the spar is only as strong as the carbon fiber, the spruce adds no strength.

If you want to blend two materials, just looking at the modulus doesn't tell you if the blending will be structually efficient. What really matters is the ratio max_stress/modulus , which is the maximum strain before failure.

For CF in compression the max strain is about 0.007, and for spruce it is about 0.004, so the spruce will in fact fail in compression before the CF, assuming the CF doesn't delaminate and buckle. But the difference isn't huge, so neither material is getting a free ride.

[Ollie]

Just before failure in compression, the 1/8 X 3/8 spruce will be carrying a compression load of 0.046875 square inches times 5,610 pounds per square inch equals 263 pounds at a strain of 5610 PSI divided by 1.56 X 10^6 equals 0.0036 inches per inch. The carbom strip will have a maximum strain of 275,000 PSI divided by 33 X 10^6 equals 0.0083 inches per inch. At the load just before the spruce fails, the carbon will be stressed at 0.0036/ 0.0083 = 0.43 of its maximum stress or 118,250 PSI. The compression load that the carbon strip will carry just before the spruce fails will be 310 pounds. Therefore, the combined compression strength of the spruce plus carbon spar cap will be 263 plus 310 equals 573 pounds. Therefore, the combination spar cap will be 2.18 times stronger than the spruce alone.

It is interesting to note that the carbon element only uses about 43% of its potential strength when the spruce fails. You would do better to increase the size of the upper spruce spar cap to 3/16 X 1/2 than to add a 0.007 X 0.375 carbon strip. The larger all wood, upper spar cap would not only cost a lot less but it would be easier to build and be almost as strong as the combination spar cap. If you tapered the spar caps in width and thickness from root to a minimum crossection at the tip and used 3/32 X1/2 for the bottom spar cap at the root, you would have a lighter spar that had less maneuver robbing inertia at the wing tip. The half size bottom spar cap works because wood is about twice as strong in tension as in compression. The only down side is the work to taper the spar caps and to modify the rib notches.

Thank you Dr. Drela for setting me straight yet again.

[Joe Minton]

Thank you all for your replies, thank you. Good advice and I shall use it all. However before I answer each of you, I'd like to describe what I intend in a little more detail:

I merely wish to strengthen my Mirage's center panel spar in a simple, practical manner (and gain some experience for later projects).

Unless Y'all change my mind I will build my center panel spar from the bottom-up like this :
1) 1/8x3/8 spruce
2) .007 CF
3) shear web (full width in center, 1/2 width at dihedral break)
4) .014 CF
5) 1/8x3/8 spruce
6) Kevlar wrap the entire spar, in a progresive manner.

I fully realize that placing the CF on the bottom and top would make the spar much stronger. However, I'll settle for a 30% ultimate strength reduction in favor of ease of finishing.

Besides --- By trapping the thin CF between the web and spruce, together with the kevlar, the chance of a buckling failure should be reduced significantly?

Joe Minton

[Joe Minton]

Mark: Thank you for your comments.

I don't intent to change the center panel. I shall add a dihedral break in the middle of slightly longer outer panels (36" instead of 33). It is just for the 'looks."

I don't intend to launch my Mirage very hard. In fact, I'll use fusible links when launching on a winch. I just want it to be stronger than spruce and glue alone can make it.

I'll take your advice on the shear webs and joiner blade (CF between ply). I had already decided to kevlar wrap the joiner boxes. I'll also make sure that the blade is a tight, sliding fit in the box.

I have selected & bought spruce for the turbulator spars. I would have used Bass but couldn't find any that was long enough. May still use hard balsa in the outer panels.

I have read everything on your website and that information has had a lot to do with my building the Mirage in the first place -- Thanks!

I have your short kit and intend to purchase another rib set. Nice parts ---.

Joe Minton

[Joe Minton]

Thank you Dr. Drela for your input -- I am honored to elicit comments from a genuine guru ;)

I have observed that wings tend to fail by buckling the upper spar cap, sheeting, TE or whatever. Further, glue joint failure or glue joint voids seem to accompany many of these failures. The tensile strength of glue joints must be important. But -- I hear little about this. What is the tensile strength of various glues anyway?

By capturing the very stiff and strong CF between end-grain shear webbing and a 'big hunk' of Spruce with its superior sectional modulus, I should do much to reduce the chance of delamination and consequent buckling. The kevlar wrap won't hurt either, I'm sure.

Thank you for all the work you have done and shared with your fellow modelers. You have changed our sport in a fundamental manner. I intend to build a Bubble Dancer. My Mirage is a first step along the path to the skills I need to do so.

Joe Minton

[Joe Minton]

Ollie: thank you yet again for your considered and intelligent observations and advice.

I'll use the things you said more for the next project than for my Mirage. I plan to build an electric-powered sailplane patterned after one of Ed Slobod's E-Gliders. It will be about 2.5 meters in wingspan -- I'll use a Phasor 30/3 for power.
I will build the wing as a compromise design: It will have a CF/balsa spar, D-tube sheeting, rib caps & other features that will place it between Ed' Paragon and Mark's Bubble Dancer.
I'll taper the spar stiffness/strength, as you suggest, along the span. This shall necessitate some study -- fun!

Are you familiar with Carl Goldberg's "Comet Sailplane"? I think it is one of the most beautiful model airplanes ever designed. Wouldn't it be grand to create a model that is (approximately) as beautiful but built with modern materials and techniques!?

Thank you, again, for your valuable and valued comments.

Joe Minton

[Joe Minton]

Badger:
Thank you for your comments. I think that capturing the CF between the balsa shear web and spruce spar cap will help with the buckling problem.
I think that most of us tend to overlook the importance of glue and glue-voids. Any bubble near a stressed surface is a deadly defect! I intend to do my best to eliminate any voids in my Mirage's spar.
Thin CF has no significant column strength and it must be controlled, in column, if its superior stiffness and strength are to be taken advantage of.

Thank you for bringing-up the glue/void issue. I have seen very little about this.

Joe Minton

[Mark Miller]

Joe and all,

Ahhhh...Extra break in the tip panel. I get it now. Sorry for the confusion. That should look cool.

Speaking of an e-glider, I am attaching some photos of a project one of my customers did with my short kit. He stretched the tip panels 6" on each side. Used the same number of ribs but spaced further apart. I have the details here of the power etc. but cannot find it right now. Obviously he changed to a V-Tail arrangement. Used carbon on the center panel. It is a test rig for a unmaned UAV that will have a GPS for navigating a pre installed flight path. Not sure who he works for but it's a cool project and I plan on kitting it in the hopefully near future. It will be called the El Mirage. I have some other projects including my Zephyr II HLG and Dr. Drela's Bubble Dancer.

Mark Miller
Isthmus Model Co.
http://www.isthmusmodels.com

[Mark Miller]

One more photo of the El Mirage.

[sierra-gold]

Mark-

Your friend and you read my mind. I have been making up an Xmas list for my wife and was thinking of a Mirage EP.

I had a Mirage back in the early 80's that I purchased from one of my friends. It was a really nice flying glider. It met it's end at the thumb of my wife. I had it specked out and let her have some stick time. I was talking with the guys and my wife screamed "it's diving down!" I looked over and saw a terminal dive and yelled " put some up in it!" I was thinking trim... she jerked in much too much elevator. I watched the wing tips kiss and the parts fall. :(

I figure she owes me this one... 20 years later.

I would be interested in the motor system system used in the El Mirage.

Now, do I wait on your kit, or do I buy your short kit? Your kit would come with the plans?

Sierra Gold

[Mark Miller]

The El Mirage kit will be a full kit. The one pictured uses a Hacker B40 with 3.4ah batteries. I do not know how many cells and all that. I will be working on my own power system for it when the time comes.

Mark