fprintf

Well I am certainly making much progress on the automatic foam cutting wing. Today I cut my first *perfect* core and I am totally psyched. Perfect leading edges, feathered trailing edges. No bumps or bruises (well, until I dropped the core and dinged the trailing edge) at all.

One question though: the core came out of the beds perfectly. When I look at the beds everything appears straight. When I look at the new core it bends downward. If I hold it flat on the table it all looks fine, but that is just the eyeball method.

Will this bend be ok after I sheet the wing with 1/32 balsa or have I done something wrong?

By the way, thanks to everyone who suggested I build a feathercut knockoff - for $10 in parts, 7 of which were for the rollers, this has made wing cutting *so easy*, especially if my wing is ok.

joebob

My Feedback: (14)

The bend is because of tension in the foam. When you cut it from the core, sometimes the tension is released on one side more tan the other. Sort of like when you cut a strip of balsa from a larger sheet and it bows.

There's absolutely no problem with it like that. Just make sure you use the cores to hold it straight when you sheet it and all will be fine.

patternwannabee

My Feedback: (1)

Before you apply the sheeting, check the beds and verify that they will provide a flat surface for the core when sheeting.

Weight down the beds so they lie flat on the surface you will use during the sheeting. If these are bowed, then the surface you used for the foam cutting was not flat (been there, done that). You may still be able to produce a straight wing if you use the very same surface for sheeting that you used for the cutting - in the same orientation, of course.

If, on the other hand, the bed and the core lie flat when weighted down (check with a straightedge along the trailing edge), then whatever bow you have in the core will be inconsequential. After the adhesive cures, your balsa will keep the wing straight.

BTW, are you using white or blue foam? I've had some pieces of blue foam that had a wicked tendency to bow after cutting.

fprintf

I believe my table is very flat and have checked it with a straightedge - only 36" at a time, though, cause that is my longest metal edge.

I am using Pink Foam - I know it is dense, but it is for a handlaunch glider for a beginner, so I need a little ding and dent resistance, as well as bending resistance on launch.

Thanks for the answers!

turbines

My Feedback: (9)

One way to check for flatness of your table is to find 2 small pieces of hard wood that are the same thickness (find one piece of wood and saw it into 2 pieces). Get some very small diameter string or thread. Place the 2 pieces of wood at each end of your table or any place (lengthwise or widthwise or diagonal) use all of these places to really get a picture of your table as long as the string is supported ( on top of the 2 pieces of wood) at the edges of your table. Weight the string ( weight is hanging freely) on one end of the table and tie it off securely down from the table top say down low to the floor. If you have the string very close to the top of the tabel you can see the dips and bows of the table top surface. One can use a small accurate ruler/scale to measure the irregularities in the surface also. This is telling you where to shim the table to bring in into a flat plane
It's easy and fairly accurate for your tasks at hand ie foam cutting

GW-inactive

My Feedback: (4)

Great idea Turbines

Jaymom

fprintf....Well I see your doing alright with your foam cutter power supply. Wish I could say the same. I finally got a chance to build my bow and try mine out. It a 30 inch bow. It starts to heat up and then the power supply overheats. It gets so hot that the 2 long white resistors unsolder themselves. Im in the process of rebuilding the whole thing right now. I've made the circuit board both ways, etched and as the instructions show it, doesn't matter, both ways overheat but the "island" method keeps things much cooler. I'm not giving up though.

fprintf

That's really too bad. I almost gave up on my powersupply after the rheostat didn't work. I just put a regular household dimmer in the power cord and it works just great. There is a real sweet spot where the power supply just starts to hum - any higher and it actually starts to shut down for some reason. I put masking tape on the flat parts of the dimmer and have lines in magic marker for cutting temp and "full" temp, which is somewhere just less than full on.

BTW, make sure your triac is pointing the right way. I checked mine and it was incorrect (figures!). Luckily I caught it before I burnt it out.

I am having fun with this. I wish I was more of an electrical person to help you out. Now my challenge is getting a perfect taper cut on my 7" root chord and 5" tip chord handlaunch glider wing.

Jaymom

fprintf.....

Well I finally did it ! Everything is working just great. Havn't cut a core yet but it goes thru foam like butter with the switch in the "long" position on a 30" bow.

I had a bad switch (old thing) that I was using for the "short/long" switch . Replaced both switches with new ones and all my problems went away. Once again Im glad to see you have yours working so good. Hope you post some pic's of the finished product sometime. I'm in the process right now of building another one for my son.

Jaymom

fprintf

Funny thing is, I am using the "short" setting for my 36" bow. I must have screwed something up, but since it cuts so well I shall not bother fixing it.

Gotta get a digital camera. I cut a single core this afternoon. Unfortunately I think I need to weight the ends of my bow as right at the very end of the travel the bow decided to pick up off of the template. Dang it! Oh well, I might be able to salvage it if I build some junky wings just for practicing handlaunch. Fortunately I cut enough square foam blocks to make two or more sets of wings just cause I knew I'd screw something up!

Jaymom

fprintf... Have you seen this yet?

http://members.fortunecity.co.uk/slmohr/rcinterest2.htm

Jaymom

fprintf

Yes, I had seen that one. I built a hands-free drop arm cutter in one evening when waiting for some answers to my questions here and on RCGroups. For me the difficulty in cutting the taper is that the two ends of the bow move at different speeds and therefore the "strings" need to be placed on my drop arm at different points. For now I am just putting the strings on and moving the bow without any foam until I get it just right. Surely there is a more scientific way, but I am pretty close.

If you haven't already built a drop arm cutter, I really recommend it. My first core with it was perfect.

patternwannabee

My Feedback: (1)

Weighting the ends of the bow may do the trick, but also consider this:

If the rollers are positioned higher than the ends of the templates, that may be pulling the bow off the templates. To be sure that doesn't happen, try positioning the rollers below, so that throughout the travel, the drop arm is always pulling the bow slightly downward.

Are you suspending the bow from above, or using a wheel attached to the bow? There are things to watch for in both methods.

fprintf

Pattern:

I am suspending the bow from above, which I suspect is causing the lift off.

In my jury-rigged apparatus I have two strings running parallel to the direction of travel for the bow. This was to suspend both ends of the bow independently so it wasn't so wobbly. On these strings are zip ties tied in a loop that slide somewhat along the strings (although they have yet to really slide since the amount of travel is very small).

String hangs down from the zip tie loops and is attached to elastic bands which are in turn attached to zip ties and the bow. The bow hangs at a 45 degree angle tilted so the wire goes into the foam first and the top of the bow is towards the back of the table.

I suspect my problem is that most of the travel, I think, is taken up by the elastic bands (and not by sliding along the strings) and when they get somewhat stretched their spring rate starts to exceed the weight of the bow and it lifts up. Hence, more weight on the bow might help, although it might make the bow harder to move along the template.

Now that you know my setup am I making any classic mistakes in the list of things to watch for?

edit: p.s. the rollers are fastened flat on the table (screwed right in) and far enough away from the bow that the angle of the string running to the rollers is not too great.

patternwannabee

My Feedback: (1)

Fprintf

I’m having trouble visualizing your setup. See the diagram I attached. Is this what you mean? Please excuse the crude picture.

If so, I would think that you want to eliminate all sources of friction in your setup. It looks like the zip ties on the string could occasionally get snagged on the string.. maybe this has already happened?

I’m not sure why you need the string. If you’re using the right length bow for your foam piece, it shouldn’t be wobbly. If eliminating the string results in too much bow weight on the templates, try moving the bow suspension point closer to the CG of the bow. If you attach right at its CG, then you can control precisely how much bow weight will rest on the templates by selectively adding weights to the tip of the bow.

Some things to watch for using the bow suspended from above method.

By suspending the bow from above, you have a pivot on the ceiling which allows the bow to move in an arc rather than a straight line. The bow will always want to return to equilibrium which is where the attachment to the bow is at its lowest (directly below the pivot on the ceiling). The farther the bow is moved away from this equilibrium point, the more force is required to move it yet farther away.

If your work is positioned too far forward of the suspension point on the ceiling, the drop arm may have trouble pulling the bow towards the end of the cut unless it has enough weight on it.

If on the other hand, your work is behind the pivot on the ceiling, the bow will have a natural tendency to move forward rather than being neutral. This means that the initial part of the cut is driven by the bow trying to reach equilibrium rather than the drop arm pulling the bow. For a tapered wing, this is bad because the shorter chord (tip) might cut too fast until the bow reaches the equilibrium point, then stall as the drop arm catches up. This results in a diagonal groove in the foam from the middle of the tip towards but not reaching the root.

I position my work so the string suspending the bow hangs straight down. I also adjust the length of the suspending line so that when the hotwire rests on the templates, the bow arms are parallel to the table (I’m not sure what a 45 degree angle would do so this is just a preference).

The bow on a wheel method eliminates these geometry problems but introduces its own set of problems. For one, the wheel is a source of friction, so you have to make sure the wheel spins freely on its axis and that it can caster freely as well. It also needs to be able to tilt freely from side to side, so the wheels should be rounded. On a significantly tapered planform such as a delta, if the castering wheel isn’t pointed in the correct direction prior to the cut, the bow could move from side to side as it travels forward. This is bad if the drop arm strings are attached to the hot wire close to the templates. It needs a lot of trial and error to get the perfect cut.

As you can see, I’ve wasted lots of foam.

A couple of questions for you:

Do you cut from LE to TE or the other way around?
Do you use two templates per side (top male cut and bottom female cut) or do you use one template the shape of the airfoil?
What about lead-ins and lead-outs on your templates?

fprintf

Pattern: your picture was excellent. Where you have marked the line "elastic" is actually string for the most part, which is tied to a simple elastic band, which is attached to a zip tie.

Also the attachment point for the zip tie to the bow is at the crossbeam end, not at the wire end, so it is probably actually very close to the CG. I am now thinking that perhaps the two string idea is introducing too much friction into the system and that I should try one string and perhaps more elastic. I also need to push the work back towards the bow suspension point as I think that is causing the liftoff as well. As the wire moves forward the bow should be tilting up but not lifting up.

And answers to your questions:

- I always cut from LE to TE.
- I use two patterns for each end of the foam. One for the bottom cut, which I do first, and then a second one for the top cut. Both templates have holes drilled through them at the same location and are attached to the foam block with 1 1/4" drywall screws.

glidaguy28

hi guys new to foam cutting myself...if you want to cut tappered wings go to WWW.sea-gull.demon.co.uk and look at his cutting set ups....he also has a free program for calculating your tapper ...what lenght and width your drop pulleys should be...

fprintf

Glida:

Does that software work on leading edge tapers only? Or do you have to have both LE and TE tapers?

I sent an email to the author to double check. I suppose if I had broken down and bought the feathercut the book that comes with the system would have explained it all!

glidaguy28

f.printf
havent tried this yet...still in the set up side of foam cutting at the moment.....looks like it should let you cut tappers any way you like....as long as the timing of the wire enters and exits at the same time you should get a clean cut...you will have to waste some foam for me and let me know how you get on....p.s how does the fourm know to highlight a web link?I didnt do it...makes me look cleaver...Glida

fprintf

I emailed the author of the software and got my answer - the software does work for single tapers as the foam is cut from LE to TE anyway, pretty much as you described.

As for the forum, pretty neat, eh? There is a little option at the bottom "automatically parse URLs" that adds the little tags every time is sees a complete www.type message.

glidaguy28

cool have fun cutting wings...glida

steve-kerry

Like most things, it's simple once you know how

Whatever ratio your taper is, use the same ratio on the drop arm. If you have a 10" root chord and a 7" tip chord, then the string to the root attaches to the end of the drop arm and the string to the tip goes at 70% of that. Doesn't matter if you work in metric or imperial, if the tip chord is 2/3 of the root chord then the string goes 2/3 of the way along the drop arm.