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Author Topic: Molded Leading Edge Caps for foam core wings  (Read 21431 times)

Offline Bob Hunt

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Molded Leading Edge Caps for foam core wings
« on: December 30, 2007, 12:44:16 PM »
One of the most overlooked aspects of stunt model construction is the accuracy of the leading edge radius. If the top and bottom radii of each of the leading edges is not absolutely perfect, the result will be a model which will have trim issues to one degree or another.

Bill Werwage had a few models that should have flown well, but didn’t. He studied the models in detail and concluded that the common denominator in the ones that didn’t fly well was inconsistent leading edge profiles. In several cases he stripped the leading edges from the models and glued on new ones and then carefully shaped them to insure complete accuracy and instantly the models few much better! This is what led us to develop molded leading edges for our Lost-Foam built-up wings.

Molding leading edge shells around a mold buck - that is accurately cut with a hot wire - yields absolutely repeatable results that insure perfectly shaped pieces. If these pieces are in turn accurately attached to the wing frames, the result is a perfectly shaped leading edge.

This has been working just fine on the built-up wings for quite a while now. It seems that adapting this technology to foam core wings has been somewhat of an issue, however. The truth is that it is extremely easy to use this procedure on a foam core wing as well.

The key again to accuracy is the ability to hot wire cut a perfect radius on the leading edge of the foam core and then attach a molded cap, which is formed over an accurately cut L.E. mold buck.
I’ve worked up a photo essay that will (hopefully) explain the procedure:

Photo No. 1 shows the 1/16 inch thick balsa LE cap already formed around the foam mold buck. There are a number of things to talk about here: The first step is to make/obtain the foam LE mold buck. You can either make your own if you have the required equipment, or order one from one of the foam wing cutting services.
Note that the rear edge of the LE mold buck is reinforced with a piece of plywood. This stiffens the mold buck so that it will not “bow” during the wrapping process, insuring a straight, accurate molded piece. The balsa is wrapped onto the mold buck in exactly the same manner as you would use to mold a balsa LE shell for a built-up wing (This entire process is described and shown in detail in Robin’s View Productions’ Lost-Foam Wing Building System DVD set).

Before the LE cap sheeting is wetted and molded, it must be edge trimmed. I do this with a straightedge and a #11 blade knife. After the edge is trimmed with the knife, I sand the edge perfectly square. I use a 24 inch long Permagrit sanding bar for this operation. The Permagrit sanding bar is made from an aluminum extrusion that can be set on its side to allow the balsa sheet to be slid against it and achieve a 90 degree sanded edge (This process is also detailed in the Lost-Foam DVD set).

I prefer to make my foam core leading edge caps from a piece of 3-inch wide balsa. Trimmed and sanded this piece will be just shy of 3 inches wide ready to mold. After the balsa cap is trimmed and sanded, a centerline should be drawn down the entire length of the piece on the side that will ultimately seat against the wing core. Do this using a ball point pen.

Thoroughly wet the balsa blanks in a bath of hot water and then position the blank on the LE mold buck. Align the centerline on the blank with the centerlines on the ends of the mold buck at either end, and hold the blank in place by inserting a straight pin through the blank and into the mold buck. Now wrap the blank down around the mold buck fairly tightly using an elastic Ace bandage. Tape off the end of the wrap and let the molded LE cap dry thoroughly (Usually 24 hours at least!) See Photo #2.

When dry, unwrap the Ace bandage and the results should look like Photo #3. Note be certain to carefully remove the the pins that hold the molded cap onto the mold buck to prevent marring the surface of the molded cap.

Now it’s time to prepare to install the molded LE caps onto the foam cores. Photo #4 shows all the materials necessary to begin this process. The foam core should be thoroughly cleaned with a vacuum cleaner to remove any foam dust from sanding. The glue will not stick effectively to a dusty surface! You will need a good laminating resin. I use and highly recommend Z-Poxy’s Finishing Resin for this process. It has adequate working time (pot life), is easy to apply and it cures up fairly quickly. It also cures very hard, insuring added strength. Some epoxy resins never get completely hard, and I’m quite certain they do not yield as strong a bond as the Z-Poxy material. You will also need a masking tape dispenser and an acid brush.

Mix up about a half ounce of Z-Poxy Finishing Resin in a cup and then begin brushing it onto the inside face of one of the molded LE caps as shown in Photo #5. Only a very thin application of the resin is required. Too much resin will only add unnecessary weight! Be sure that all the balsa surface that will contact the core is coated before proceeding.

Position the molded balsa LE cap onto the leading edge of the corresponding core and align the centerline on the cap to the centerlines on either end of the core. Pin the blank to the core accurately as shown in Photo #6. Next, tape the molded LE cap to the surface of the core in several places along the span as shown in Photo #7.
The next step is critical. The core must be positioned in the cradles (pieces of foam from which the core was cut) accurately, and in such a manner that the core is supported equidistant from the surface of the cradle. This requires placing a piece of balsa in the aft end of the cradle that is the same thickness as the balsa you used for the molded LE cap. See Photo #8.

Position the core in the lower cradle and then place another piece of balsa on top of the core, as shown in Photo #9, to support the top cradle equidistant from the surface of the core.

Double check to insure that the core and the two cradles are in perfect alignment, and then weight down the leading edge as shown in Photo 10. It is imperative that the bench on which this assembly is placed is absolutely flat. Note that we are weighting it down against a piece of ¾ inch thick granite. I highly recommend investing in a piece of this material.

Photo #11 shows the leading edge cap securely and accurately glued to the core. Repeat this process to attach the other LE cap to the other core.

At this point all that’s required is to make up top and bottom skins and graft them to the edges of the LE cap during the sheeting process. I’ll work up another how-to that shows all of that in detail. There are a few neat tips that I can pass along on laying out the skins and joining them.

Again, this method produces absolutely accurate leading edges and in turn better flying models!
Those of you who have your own foam cutting equipment can try the procedure described right away. Those of you who do not have the required equipment can obtain cores that are precision cut with the round nose leading edge and the LE Cap Mold Bucks from Robin’s View Productions. We also offer cores with the molded leading edges already installed for you and even fully sheeted and sanded wings with or without landing gear systems installed. In fact we offer a full range of foam cutting and wing building services that are far too extensive to list here. Please email or call for details and pricing. My phone number is: 610-746-0106 and my email address is: robinhunt@rcn.com.

Please note that all of the photos may not be able to be posted along with the text in one posting. I’ll post it in two or three separate pieces if necessary.
I hope you have found this interesting and informative.

Bob Hunt
President, Robin’s View Productions      
« Last Edit: June 28, 2016, 08:07:15 PM by RC Storick »

Offline Steve Fitton

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Re: Molded Leading Edge Caps for foam core wings
« Reply #1 on: December 30, 2007, 12:56:05 PM »
Thanks for posting that Bob!  It is indeed some good informative reading!

Steve

ps I don't suppose you happen to make Genesis I cores?   y1
Steve

Offline Bob Hunt

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Re: Molded Leading Edge Caps for foam core wings
« Reply #2 on: December 30, 2007, 01:01:55 PM »
Hi Steve:

Certainly I make cores for the .35 size Genesis! I'll make cores for any straight taper or constant chord wing design.

Bob

Offline Wayne J. Buran

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Re: Molded Leading Edge Caps for foam core wings
« Reply #3 on: December 31, 2007, 03:53:07 AM »
Bob, thanks for the update. I appreciate your information very much and posting it on this forum.
Wayne Buran
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USAF Veteran 35 TAC GP/ 6236 CSG, DonMuang RTAFB, Bangkok, Thailand 65-66 North Coast Controliners   "A fine is a tax for doing wrong. A tax is a fine for doing well!

Offline PatRobinson

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Re: Molded Leading Edge Caps for foam core wings
« Reply #4 on: December 31, 2007, 09:06:16 AM »
HI Bob,
I have some questions for you. The foam core you show in your photo "seems" to have the complete leading edge radius already cut into the foam rather than a typical foam core that has a flat surface to which you glue 1/4" balsa LE, which you then shape.  This is quite a departure from the "norm" in CLPA foam wing design. Obviously, a shaped LE is a more accurate and better option, because it only takes a few passes too many with sandpaper on a balsa LE to create a flawed LE shape.

(I said "seems" because when I zoomed in on the photo for a closer look at the LE, the detail was fuzzy.) I apologize, if I missed a balsa LE in the photo
but, if it is a total foam shaped LE radius then it is a "big deal" that should not be glossed over or overlooked in passing but should be recognized as an example of new and excellent foam craftmanship.

So, Bob, Do you now provide cores with the LE already shaped in the foam ?or is this a special order option?  Is cutting the core with a shaped LE , a lot harder to do?  Does the shaped balsa LE cap provide greater strength because of the monocoque effect created by the curved balsa when compared to a regular ,shaped, 1/4" balsa LE  ?

Bob, Interesting stuff ,as usual and I would like to add my thanks to everyone elses for your excellent post.

                                                      Pat Robinson


Offline Bob Hunt

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Re: Molded Leading Edge Caps for foam core wings
« Reply #5 on: December 31, 2007, 10:29:52 AM »
>The key again to accuracy is the ability to hot wire cut a perfect radius on the leading edge of the foam core and then attach a molded cap, which is formed over an accurately cut L.E. mold buck.<

Hi Pat:

Sorry about that. I thought I'd made it clear that this technique was for use with foam cores that were cut with fully rounded (hot wire cut...) leading edges.

To make things clear for all: There are no blocks on or anywhere near the leading edge of the cores on which I use this technique. I've been cutting foam cores with fully rounded leading edges for several years, but mainly for the Lost-Foam fixture sets. It dawned on me not too long back that the exact same cutting technique could easily be employed to cut cores with round foam leading edges. Too soon old, too late smart I guess... Just about the time I'm ready to leave this orb I should have this whole foam wing thing fully figured out.:)

The absence of the block on the leading edge not only insures accuracy, but it also eliminates some dead weight. Also the time it takes to carve and sand the balsa block type leading edge is eliminated, as is the vast majority of the sanding dust and all of the shavings associated with the old technique. In all ways the rounded nose foam core with a molded cap is easier, cleaner, more accurate and more repeatable. As they say: "Try it, you'll like it!"  

Yes, I do offer this option for sale through Robin's View Productions. It requires a set of leading edge cap mold bucks to be cut and sent along with the foam cores, but that's not difficult. There is a slight increase in total cost if a core is ordered with this option, but the accuracy is worth far more than the slight price increase. If you order a core in the future of the same design, just tell me that you already have the LE mold bucks for that wing and there will be no additional charge as you can use the mold bucks over and over many times.

If you opt to order a fully covered and sanding wing (with or without landing gear mount system installed), it will come with molded leading edge caps installed. I will cut foam cores for sale with the old "square nose" option that requires a block leading edge, but all sheeted and sanded foam wings will from now on feature the molded cap option. I can't see any reason to do it the old way ever again.

Again, if you have any specific questions or want to place an order please call me at: 610-746-0106. I'm back doing this full-time now and can process orders quickly. My email address is: robinhunt@rcn.com
  
Thanks - Bob Hunt
« Last Edit: June 28, 2016, 08:18:41 PM by RC Storick »

Offline Larry Renger

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Re: Molded Leading Edge Caps for foam core wings
« Reply #6 on: January 02, 2008, 09:55:57 AM »
Unless you are cutting the cores by hand, with tons of experience, skill and practice, how do you cut the LE radius?  I use a TEKOA machine, and it won't make true vertical cuts that would be needed. 

I suppose I could cut it so that only a tiny piece of foam need be sanded, perhaps less than 1/8" deep and 1/4" high.  Shouldn't be much room for error there and the balsa ought to fill in any errors.

Great show and tell on a very good idea, by the way!  Keep up the good work. 

Are you coming to the AMA Convention?

Think S.M.A.L.L. y'all and, it's all good, CL, FF and RC!

DesignMan
 BTW, Dracula Sucks!  A closed mouth gathers no feet!

Offline Bob Hunt

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Re: Molded Leading Edge Caps for foam core wings
« Reply #7 on: January 02, 2008, 12:00:00 PM »
Hi Larry:

As a matter of fact I am cutting them by hand, and yes, I guess I do have the requisite experience, but believe me, it ain't rocket science. I believe that anyone who can fly a CL model can learn the rudiments of foam cutting well and quick enough to make this simple cut. And, if not... order them from me!  H^^

On the other matter, no, I'm sorry to say that I won't be at Ontario this year :( I'll miss you guys, and especially Warren's great party! Say hi to everyone for me and tell them that I'll try to make it out next year.

Thanks for the kind words... Bob

Offline Bob Furr

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Re: Molded Leading Edge Caps for foam core wings
« Reply #8 on: February 02, 2008, 11:47:11 AM »
For anyone being put off by not knowing how to cut your own foam drop me an email at robear1053 at aol dot com.   I have copies of the Good To Go Models foam cutting video available for sale. 
Bob Furr

Offline Neville Legg

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Re: Molded Leading Edge Caps for foam core wings
« Reply #9 on: August 14, 2008, 11:51:34 AM »
Bob,
Can you tell me how you get such a neat foam cut?  What dia. and type of wire are you using, and how much current/voltage?. I've cut many a foam wing but can't get that really neat smooth finish on the foam!!

Cheers  Neville
"I think, therefore I have problems"

(not) Descartes

Offline Wayne J. Buran

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Re: Molded Leading Edge Caps for foam core wings
« Reply #10 on: August 14, 2008, 02:01:33 PM »
Neville, one of the best things I ever did was to marry my wife the next best was to buy Bob Hunts VHS (at that time), lost foam jig instructions. Tells you the whole story from buying the foam and up.
Wayne Buran.
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Offline Neville Legg

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Re: Molded Leading Edge Caps for foam core wings
« Reply #11 on: August 16, 2008, 02:32:11 AM »
Wayne,
Thanks for your reply, but where can I get a copy on DVD? I'm in England, can anyone over there sell me a copy, or trade for some English stunt plans?

Cheers    Neville
"I think, therefore I have problems"

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Offline Wayne J. Buran

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Re: Molded Leading Edge Caps for foam core wings
« Reply #12 on: August 16, 2008, 04:29:19 AM »
Bob, has them available. Contact him via thia forum.
Thanks
Wayne
Wayne Buran
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Offline Matt Colan

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Re: Molded Leading Edge Caps for foam core wings
« Reply #13 on: March 30, 2009, 04:34:08 PM »
I followed this article when I molded my leading edge.  Now that the foam wings are joined and the whole wing is done except wing tips and flaps, it looks a lot cleaner even before it flies.  It is very constant.

This really is I think the way to go for any wing, a molded leading edge.

Matt Colan

Offline Bruce Perry

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Re: Molded Leading Edge Caps for foam core wings
« Reply #14 on: December 26, 2009, 06:09:31 PM »
Bubba!

Great post!  I've been cutting my own cores for a couple of airplanes now.  I'm pretty satisfied with the results but I will try the rounded LE next time. 

I watched lots of airplanes at the Nats last year and I'm convinced that the wing gear lands better.  There were some lots of really good pilots bouncing what was a great approach, with fuse gear.  Now I realize that you could "trim" the gear for shock absorption, or use what works... I'd rather not start a long development program of Carbon gear that doesn't bounce. 

So, to the question.  I've built one foam wing with gear clips in the wing.  I flew off of grass the whole time the plane was in service and the clips survived nicely.  The finish on that plane shrunk a bit and caused the clips to show.  Certainly, the finish type was part, if not all of the problem.  I'm wondering the approach you use for gear clips.  Is it on vid? 

Lastly, do you have a vid on the leading edge radius cuts?  I also have a tekoa and use two piece templates.  I'm pretty sure you use one piece right?

Anyhow, missed you two at the nats this year.  Hopefully next year?

Bruce

Offline john e. holliday

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Re: Molded Leading Edge Caps for foam core wings
« Reply #15 on: July 03, 2011, 07:57:41 AM »
The fuse gear is probably set up for grass.  On pavement it does not need to be forward so much.  With the plane sitting level, the center of the wheel should be approximately 15 degrees ahead of the center of gravity.   H^^
John E. "DOC" Holliday
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Offline Avaiojet

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Re: Molded Leading Edge Caps for foam core wings
« Reply #16 on: October 18, 2011, 06:56:50 PM »
Quote
it ain't rocket science. I believe that anyone who can fly a CL model can learn the rudiments of foam cutting well and quick enough to make this simple cut. And, if not... order them from me!

Bob,

Correct anyone can do that. Well not really.

I cut foam wings in the late 80's for RC pattern ships. Not perfect, but did a "not all that bad" a job, then ruined the wing without realizing it, by applying that 3/8" leading and trailing edge. Never could get that ink center line correct.

You do great work and you make a great point. Can't thank you enough for taking the time to put that informative presentation together.

I appreciate it. I'd like to know a bit more about your methods of attaching gear and bellcrank mounts?

Don't be surprised if you hear from me, in the future, for a project. I would be glad to have you do a wing.

I don't like making wings.

Thanks again!

CB

Trump Derangement Syndrome. TDS. 
Avaiojet Derangement Syndrome. ADS.
Amazing how ignorance can get in the way of the learning process.
If you're Trolled, you know you're doing something right.  Alpha Mike Foxtrot. "No one has ever made a difference by being like everyone else."  Marcus Cordeiro, The "Mark of Excellence," you will not be forgotten. "No amount of evidence will ever persuade an idiot."- Mark Twain. I look at the Forum as a place to contribute and make friends, some view it as a Realm where they could be King.   Proverb 11.9  "With his mouth the Godless destroys his neighbor..."  "Perhaps the greatest challenge in modeling is to build a competitive control line stunter that looks like a real airplane." David McCellan, 1980.

Offline Trostle

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Re: Molded Leading Edge Caps for foam core wings
« Reply #17 on: July 16, 2012, 06:36:32 PM »
Bob,

(Clip)

I appreciate it. I'd like to know a bit more about your methods of attaching gear and bellcrank mounts?

(Clip)

CB



Charles,

It is good to see that you recognize what a real contribution to this activity can be.  Bob Hunt has been giving us nuggets of innovation for years and there are more to come.  Now, look back on this thread and heed what others have already posted.  Get his videos.  They are great and will answere many of the questions you seem to have.

And I think I am being polite.

Keith


Offline Glenn (Gravitywell) Reach

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Re: Molded Leading Edge Caps for foam core wings
« Reply #18 on: July 16, 2012, 09:29:04 PM »
Yes, its so easy to be "polite" when theres an obviouse overtone of sarcasm.  Good for you Keith, well done. H^^
Glenn Reach
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gravitywell2011 @ gmail . com

Dwayne

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Re: Molded Leading Edge Caps for foam core wings
« Reply #19 on: June 11, 2016, 01:10:39 PM »
I'm going to assume the pics are gone because of the sever issue, can they be restored?


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