Aileron Hinge Design

[wsimpso1]

We haven't done any specific calculations on the exact magnitude of the aerodynamic forces acting upon the aileron but I imagine that they will be pretty minor as the top speed will only be 54 knots. Most of our current dimensions are purely conceptual and are either based on similar aircraft, trends in design, or some basic calculations. Since I'm still a junior in high school, my math knowledge (and that of my team) is limited to that offered by a precalc class but I'd be more than willing to jump ahead and learn some new things if needed. Teaching myself calc (or other advanced math) though would be pretty inefficient and I was hoping to be able to get a closer estimate with some fluid analysis (still need to figure out how that works). Either way though, if you know any equations or can refer me to a textbook which goes over such calculations that would be great. I'll also look into using diagonal ribs or wires to improve torsional rigidity. Thanks for the suggestions!

Hmm, the basis for FEA and CFD are both integral calculus in two and three dimensions. Failure to understand both the basis for these solvers and how they work easily becomes a GIGO exercise. Many's the time in my career where I had advanced degree engineers doing FEA/CFD in support of my product and gotten answers that were clearly wrong. Got to the point where I would rarely do a new product without having them also do a current product in exactly the same way for their own comparison. Picked up a lot of factor of 2 and factor of 10 errors that way. Reliance upon them with understanding the basis of the analysis is likely to be troublesome.

Much of airplane design can be done without much more sophisticated work than Excel. You can get spanwise loading, shear, bending, and torsion, brace reactions, etc through Excel. Theory Of Wing Sections by Abbott and von Doenhoff will give you chordwise airloads and control surface lift and pitching moment increments for deflected surfaces. I suspect emulation (yeah, Monkey-see, Monkey-do engineering) of successful designs in your desired speed and wing loading range will go a long ways. It will also serve as a check on the sizes of your spar tubes - compare to similar UL wings.

Two issues you do want to make certain you are OK on is bending strength and compression strength of the wing. If you start with your inboard anchor and strut mount as fixed points, and then start accumulating lift from the tip in a piece wise fashion, you can build up your shear curve and bending moment curve. The vertical reactions at the inboard anchor and strut mount will be equal to the lift that the wing makes on that side. Since the inboard anchor is just a couple bolts aligned fore-and-aft, no moment is carried at the very anchor, only shear. If you get this far, you have one more thing to check out. The braces are angled in, so go to that pre-calc class (mostly trigonometry, right?) and you will figure out that load in the strut is the vertical reaction divided by the angle of the strut from horizontal. Then you can figure out that the load in the strut times the sine of the angle from horizontal is the compression that the spar that strut is attached to will see. Elastic buckling of that spar must be prevented - that is usually picked up in a Junior year mechanical engineering design class. MSMD engineering will give a check here too. Look up Euler Buckling for basics.

Billski

 

[Aerowerx]

Reality check.......you do not 3D print hinges.....You make them from steel, aluminum, pieces of aircraft piano hinge material, heim bearings or other mounted uni-ball standard bearings. Some of the advice here is coming from people who have never designed anything.

He did not say 3D print hinges. He said 3D print the bushings for the torque tube.

And as others mentioned, it would be better to machine them out of solid stock.

 

[Ollie Krause]

What could possibly be so unique about these bushings that they must be 3D printed?

Ollie, I applaud your work but this premise is not true. As soon as you look at some actual aileron hinges you will see that this is not true (esp. for ultralights). Download the Falco plans if nothing else and take a look.

They just have very specific IDs to fit around our torque tube perfectly to accommodate for any bending and also have a little flange at either end. Also, they all must have the same OD because I want to make all the ribs identical if possible and that means the bushing may change but the OD must remain constant. I'll do some more looking online to see if I can find some PTFE bushings that would be close enough in dimension. Do you have a recommended bushing supplier/site I should check out? Thanks!

 

[Ollie Krause]

I could not get into Onshape. I did look over your sketches. I have a few comments:

Bushings with grease? Probably work a lot better with Delrin or Teflon (they are self lubricating and way less messy than with white lubes), if you can not buy bushings to your size, you can certainly buy the plastic and spin it in a lathe. You will want a slight press fit into the wing structure, a slightly loose and trumpet shaped inside fit to the tube inside it to accommodate wing and aileron deflecting differently under aero loads.

The wing will deflect in one curve and the aileron will try to deflect in a completely different shape. Since they will try to deflect differently with only the bearing points to constrain them, the easiest scheme to make work is two points of support for the aileron, a firm connection between torque tube and aileron, and one bushing at the bellcrank for control input to the torque tube. When you put in a mid-aileron hinge point, it will carry about half the lift of the aileron plus a huge fraction of the load from the deflected wing bending the aileron to follow those three points. The bending load will be trying to bind the aileron, and may add weight and complexity to aileron design. The binding may be minor indeed in an airplane of this speed range, with control surfaces and linkages that are pretty soft in bending, so binding may be a minor issue. You will have to either estimate the deflection curves of wing and aileron and then check if the aileron can stand it.

Keeping in mind that this is a low speed airplane and does not need much stiffness for aeroelasticity effects, you will usually allow quite soft structures as long as they are strong enough.

Your rib spacing is curious to me. Normally the spacing is smallest at the root, and as wing loading drops off as you go outboard, the space between ribs can be increased. In ultralights, they usually stick to one spacing to commonize the diagonal drag/antidrag bracing. Placing an extra rib at each hinge point for the aileron is probably a good idea, but then you resume your ribs spacing scheme (whichever it is until the next interruption. You seem to have extras in the span of the aileron - why? Perhaps you have a good reason that is worth the weight...

Billski

Thank you so much for all the insightful advice! I totally didn't think of just turning the bushings on a lathe. When you say "trumpet inside" do you mean a conical taper like is done on some pipe threads? I was planning to fiberglass some sheets of house insulation foam board and then mill out the ribs as 2D shapes so orientation wouldn't matter during installation but I could mill a precise taper as well if you think it would afford a significant advantage.

The rib spacing is a bit odd because the entire wing structure is controlled by 26 independent variables. Based on some simple calculations, I can change any one variable and have the entire wing reconfigure itself instantaneously. For example, I have a variable called DragStrutMultiplier which I can change and it automatically adjusts and arranges the number of drag struts along the leading and aft spars. I thought this was the best way to approach designing a wing for the first time as it allows me to quickly make revisions when I mess up and not have to redesign any major parts. The way I designed the ailerons is with a variable called AileronLengthCoefficient and AileronRootCoefficient where AileronLengthCoefficient is the length of the aileron as a % of the wingspan and AileronRootCoefficient is the location of the root of the aileron also as a % of the wingspan. I also have a variable called RibMultiplier which is the total number of non aileron support ribs. Based on this variable, I can evenly distribute the ribs across the length of the wing. Hence, the exact span of the aileron and the number of ribs are independent of each other so additional aileron support ribs are added where required by the AileronLenghCoefficient variable. If I were to make the aileron length related to the RibMultiplier, it would have to be a multiple of the interval between ribs and would therefore be limited in its adaptability. Now that you mention it though, I realize that's an inefficient way to do it which lacks control and creates an inefficient wing design. Going forward, I will create three rib spacing "zones." The first one will be at the root of the wing between the bulkhead rib and the first aileron support rib and will have the shortest spacing between ribs. The second zone will be directly in front of the aileron and between the two aileron support ribs. The third zone will be between the last aileron support rib and the wingtip rib and will have the longest spacing between ribs. I don't think this would affect the adaptability of the wing at all and should save a substantial amount of weight.

To access the Onshape document, you might have to enable webGL on your browser. If that doesn't work though let me know and I'll see if I can fix it or just share it with you directly. It's really hard to understand the variable setup without looking at the document itself but I've attached some screenshots of the variables below. Note that they all show up in feet but are actually in inches if you were to open them up on Onshape.

 

[Ollie Krause]

Hmm, the basis for FEA and CFD are both integral calculus in two and three dimensions. Failure to understand both the basis for these solvers and how they work easily becomes a GIGO exercise. Many's the time in my career where I had advanced degree engineers doing FEA/CFD in support of my product and gotten answers that were clearly wrong. Got to the point where I would rarely do a new product without having them also do a current product in exactly the same way for their own comparison. Picked up a lot of factor of 2 and factor of 10 errors that way. Reliance upon them with understanding the basis of the analysis is likely to be troublesome.

Much of airplane design can be done without much more sophisticated work than Excel. You can get spanwise loading, shear, bending, and torsion, brace reactions, etc through Excel. Theory Of Wing Sections by Abbott and von Doenhoff will give you chordwise airloads and control surface lift and pitching moment increments for deflected surfaces. I suspect emulation (yeah, Monkey-see, Monkey-do engineering) of successful designs in your desired speed and wing loading range will go a long ways. It will also serve as a check on the sizes of your spar tubes - compare to similar UL wings.

Two issues you do want to make certain you are OK on is bending strength and compression strength of the wing. If you start with your inboard anchor and strut mount as fixed points, and then start accumulating lift from the tip in a piece wise fashion, you can build up your shear curve and bending moment curve. The vertical reactions at the inboard anchor and strut mount will be equal to the lift that the wing makes on that side. Since the inboard anchor is just a couple bolts aligned fore-and-aft, no moment is carried at the very anchor, only shear. If you get this far, you have one more thing to check out. The braces are angled in, so go to that pre-calc class (mostly trigonometry, right?) and you will figure out that load in the strut is the vertical reaction divided by the angle of the strut from horizontal. Then you can figure out that the load in the strut times the sine of the angle from horizontal is the compression that the spar that strut is attached to will see. Elastic buckling of that spar must be prevented - that is usually picked up in a Junior year mechanical engineering design class. MSMD engineering will give a check here too. Look up Euler Buckling for basics.

Billski

Thanks again for all of the information! I'll start looking all that over with our physics team and will let y'all know if we run into any issues. Thanks!

 

[wsimpso1]

They just have very specific IDs to fit around our torque tube perfectly to accommodate for any bending and also have a little flange at either end. Also, they all must have the same OD because I want to make all the ribs identical if possible and that means the bushing may change but the OD must remain constant. I'll do some more looking online to see if I can find some PTFE bushings that would be close enough in dimension. Do you have a recommended bushing supplier/site I should check out? Thanks!

The simple way to make the bushings is to turn them out of Delrin on a lathe.

The ID of the bushings will have to be a little loose and somewhat flared (trumpet bell shaped) to accommodate wing and aileron deflection - the wing will be bowed one way, while the aileron will sag under airloads with a curve of opposite sense. Precision fit here will bind your ailerons, making the airplane unpleasant/difficult/dangerous to fly.

This will make the tube run through the bushing at angle from both curves. If you know the max g angle between them, and are willing to trumpet shape the bore, the min diameter on the bushing will be a tiny bit larger than the max diameter of the tube divided by the cosine of max angle. Usually, the running clearance of the bushing will be almost enough and you can check it manually at assembly. If you just bore them straight through, you will will need a lot more clearance.

I have included a quick sketchup with all the dimensions exaggerated. Let's see if it goes, it did. At the more modest angles you will actually have maybe 2-3 degrees the difference between min bore and tube will be much smaller. You have to run the shear>moment>curvature>deflection curves to know. In faster airplanes, where loads are much higher and we can not stand the slop for reasons of aeroelastic flutter, we use self aligning bearings instead.

Billski

 

[Dana]

I'll do some more looking online to see if I can find some PTFE bushings that would be close enough in dimension. Do you have a recommended bushing supplier/site I should check out? Thanks!

You probably don't want PTFE either; although it's slippery it's very soft. Acetal or an Acetal/PTFE blend would probably be better. I use bearings from Igus a lot in my day job, I usually use their J-series bearings though another type may be more suitable (I didn't check) for your relatively soft (compared to the steel shafts I'm using) aluminum tubes. (Hint: manufacturers love to give advice and donate free stuff to student projects.)

The rib spacing is a bit odd because the entire wing structure is controlled by 26 independent variables. Based on some simple calculations, I can change any one variable and have the entire wing reconfigure itself instantaneously. For example, I have a variable called DragStrutMultiplier which I can change and it automatically adjusts and arranges the number of drag struts...

Sounds like you've been drinking the parametric design kool-aid. No offense, it sounds wonderful and the CAD software vendors make it look great, but in the real world nobody designs that way, except to a very limited extent. Well, I don't know what Boeing is doing nowadays, but I do know that it nearly always takes a lot longer to set up and debug all the variables and constraints than it does to do a few quick back of the envelope sketches and calculations to get the basic design and dimensions and then zero in on that particular design with only minor tweaks to make things fit.

 

[proppastie]

I hate the newer parametric stuff,....I design top down, (assembly down to details)and as I understand it the newer software is bottom up, (design the details first then assemble it).....that is not the way we did it on the drawing board, before cad, and when cad first came out the 2d drawing packages would mimic the drawing board. Just being able to manipulate software is not engineering or often good design......That looks about right (TLAR).....well how strong do you need it? how strong is it? Would a different geometry or material be a better compromise....all design is a compromise.

 

[Ollie Krause]

The simple way to make the bushings is to turn them out of Delrin on a lathe.

The ID of the bushings will have to be a little loose and somewhat flared (trumpet bell shaped) to accommodate wing and aileron deflection - the wing will be bowed one way, while the aileron will sag under airloads with a curve of opposite sense. Precision fit here will bind your ailerons, making the airplane unpleasant/difficult/dangerous to fly.

This will make the tube run through the bushing at angle from both curves. If you know the max g angle between them, and are willing to trumpet shape the bore, the min diameter on the bushing will be a tiny bit larger than the max diameter of the tube divided by the cosine of max angle. Usually, the running clearance of the bushing will be almost enough and you can check it manually at assembly. If you just bore them straight through, you will will need a lot more clearance.

I have included a quick sketchup with all the dimensions exaggerated. Let's see if it goes, it did. At the more modest angles you will actually have maybe 2-3 degrees the difference between min bore and tube will be much smaller. You have to run the shear>moment>curvature>deflection curves to know. In faster airplanes, where loads are much higher and we can not stand the slop for reasons of aeroelastic flutter, we use self aligning bearings instead.

Billski

Oh sorry I misunderstood your previous message and thought you wanted me to taper the OUTSIDE of the bushing and INSIDE of its respective hole in the rib which was why I was a little confused. I'll calculate the span wise bending and then update my bushing design accordingly. Thanks for the help!

 

[Ollie Krause]

(Hint: manufacturers love to give advice and donate free stuff to student projects.)

Thanks for the advice! I'll reach out to them about a potential sponsorship. We are already sponsored by our speed controller and motor suppliers but we still gotta lock down some hardware sponsorships.

Sounds like you've been drinking the parametric design kool-aid. No offense, it sounds wonderful and the CAD software vendors make it look great, but in the real world nobody designs that way, except to a very limited extent. Well, I don't know what Boeing is doing nowadays, but I do know that it nearly always takes a lot longer to set up and debug all the variables and constraints than it does to do a few quick back of the envelope sketches and calculations to get the basic design and dimensions and then zero in on that particular design with only minor tweaks to make things fit.

I'm pretty fast at Onshape but I'll try experimenting with different design techniques as saving time is always a big plus!

 

[Aerowerx]

Remember that with software you are limited to what the program allows.

With pencil and paper you are only limited by your imagination.

 

[Ollie Krause]

Remember that with software you are limited to what the program allows.

With pencil and paper you are only limited by your imagination.

True but modern CAD has very few actual limitations (inefficiencies but not limitations). I've never had an idea that I couldn't design in Onshape, Rhino, or Grasshopper or a combination of the three. I also suck at drawing by hand soooo...

 

[Ollie Krause]

Oki doki so our school just got canceled because of COVID-19 so I'm gonna have a LOT more time to work on this wing! Based on y'all's suggestions, I updated the rib distribution but I still have a couple more questions regarding the aileron torque tube assembly. I currently have the torque tube protruding 1" from the outermost bushing but I'm unsure how I should secure it in place so it doesn't slide back and forth (normal to the rib) and instead just rotates (like an aileron should). I was thinking of turning a short Delrin sleeve and then bolting it to the outside of the 1" torque tube protrusion but I imagine that it would either need to be perfectly rounded on the outside or have a lot of slop as it would jam if the wing bent much at all. I could also insert spacers between the aileron and either adjacent rib but I assume that would have similar issues. I did some research online and couldn't find much on how this sliding is typically prevented. Thanks in advance for any help!

-Ollie



On a completely separate note, does anyone know where I can find the CAD files for all the different AN bolt sizes? I typically use McMaster Carr but their selection of AN bolts is (surprisingly) limited and is missing some sizes that I will need. Aircraft Spruce has all the bolts but no files and I didn't have any luck on GrabCAD either...

 

[Hot Wings]

Here is what I use for AN bolt hardware - in SW *.sldprt file.
They aren't strictly accurate but are good enough for place holders.

Double check as you go against the standards in the PDF file

 

[Dana]

You need thrust washers between the ribs and the ailerons.

Once you have one AN bolt properly modeled, you should be able to adjust the parameters for any other size.

 

[Victor Bravo]

Ollie, I would absolutely allow a small amount of spanwise play or movement (the aileron tube sliding in and out of the end rib). Maybe 1/4 inch or something. As the wings flex up and down a little, the ailerons flex a little less, or a little differently as has been mentioned.

The effect of this is that the ends of the aileron will try to slide inboard and outboard through those rib bushings a little. You may have a valid need to prevent any movement at one end of the aileron (likely the root end), but do not completely restrain it at the other (outboard) end. Maybe somebody here can calculate the actual amount it needs to move, I can't. I just know it will need to be able to move a little bit.

You can have a stop collar, or a bolt, or a snap ring, or whatever... just make sure it can flide a little bit before it "bottoms out" or contacts that stop.

 

[proppastie]

typically use McMaster Carr but their selection of AN bolts is (surprisingly) limited

What Dana said......I use the stretch command
You need an accurate drawing to know how much to adjust
Mil handbook 5 has those doccuments or google.

 

[Ollie Krause]

Ollie, I would absolutely allow a small amount of spanwise play or movement (the aileron tube sliding in and out of the end rib). Maybe 1/4 inch or something. As the wings flex up and down a little, the ailerons flex a little less, or a little differently as has been mentioned.

The effect of this is that the ends of the aileron will try to slide inboard and outboard through those rib bushings a little. You may have a valid need to prevent any movement at one end of the aileron (likely the root end), but do not completely restrain it at the other (outboard) end. Maybe somebody here can calculate the actual amount it needs to move, I can't. I just know it will need to be able to move a little bit.

You can have a stop collar, or a bolt, or a snap ring, or whatever... just make sure it can flide a little bit before it "bottoms out" or contacts that stop.

Thanks for the advice! I would have never thought of that. Once we calculate our exact wing bending, we can easily calculate the required wiggle room. In the meantime, I'll design in some thrust washers with a little wiggle room that can be easily adjusted. Thanks again!

 

[Ollie Krause]

Here is what I use for AN bolt hardware - in SW *.sldprt file.
They aren't strictly accurate but are good enough for place holders.

Double check as you go against the standards in the PDF file

Thanks for the treasure trove of files! I'll also look at Mil handbook 5 to modify them to my requirements when needed.

 

[wsimpso1]

Isn't your torque tube constrained spanwise at the inner end? If so, then the aileron can be fixed simply by the joint between torque tube and aileron. Set the clearances and position so that wing and aileron deflections will not jam the ailerons on ribs and you are all set.