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Author Topic: Stiffening Sheet Flaps  (Read 2323 times)

Offline Motorman

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Stiffening Sheet Flaps
« on: May 30, 2018, 12:39:44 PM »
I've got some fairly hard 1/4" balsa to make a set of flaps but, I was wondering if I cut some strips of polyspan and dope it top and bottom would it stiffen them up enough to justify the added weight?

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Motorman 8)
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Online TDM

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Re: Stiffening Sheet Flaps
« Reply #1 on: May 31, 2018, 07:19:26 AM »
Use the lightest balsa you can find and insert this between the LE (1/4-3/8) and the TE section. Think of the tube as a torsion tube.   https://alofthobbies.com/carbon-fiber-tubes.html there is a 6mm OD with 5MM ID in that list.
The build will be solid balsa LE tube solid balsa TE. Shape to suit after that.
Don't worry if you have to notch through to hinge flaps to the wing. You can add some tow in the hinge area to stiffen if you are really worried about slicing through the tube. 
This will be as light if not lighter than the plain sheet will never warp and will be consistent.
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Online Tim Wescott

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Re: Stiffening Sheet Flaps
« Reply #2 on: May 31, 2018, 11:08:59 AM »
Or use this stuff from Rock West.  Woven is stiffer than longitudinal fibers, by a bit.  You'd have to work around the fact that it's bigger, though (Howard Rush makes his).
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Offline Dennis Toth

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Re: Stiffening Sheet Flaps
« Reply #3 on: May 31, 2018, 05:09:00 PM »
MM,
Using medium balsa cover with 3/4 oz glass cloth using epoxy finishing resin and the card scrap/toilet paper roll method for two coats. Will give you light stiff flaps.

Best,    DennisT

Offline phil c

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Re: Stiffening Sheet Flaps
« Reply #4 on: May 31, 2018, 06:37:57 PM »
First, leave the flaps as a sheet, don't taper it towards the trailing edge.

Apply a coat of dope top and bottom to seal the wood.

Cover it top and bottom with the lightest carbon fiber mat you can find and squeegee most of it off, leaving just enough to make the surface slightly wet.  Use the thinnest epoxy you can find(West Systems 206 is pretty good- 2 hr working time) or warm the epoxy with a heat gun to make it wet the CF more evenly.  Use at least a 30min working time and do each flap one at a time.

Clamp each flap between two very smooth,  straight, flat planks(almost anything including a granite counter top,  MDF is cheaper) with 6 mill or so polyethylene sheet top and bottom.  Make sure it lays reasonably flat.  Don't want to make a warped flap.  We don't want to glue the flap to the planks.

Keeping the flap the same thickness all over essentially makes it into a box beam, which is stiffer to twisting than a triangle shaped beam.
The carbon fiber, even the <.4 oz/sq yd stuff, is much stronger and stiffer than anything else.

You can also use polyethylene to spread a fixed volume of  epoxy over a fixed area to make all four sides similar in weight and thickness.

You can get pretty good results with dope instead of epoxy.  It will be a bit lighter.  The dope won't be as stiff as epoxy, but the carbon fiber is MUCH stiffer than silkspan.
phil Cartier

Offline Curare

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Re: Stiffening Sheet Flaps
« Reply #5 on: May 31, 2018, 07:53:49 PM »
But everyone hates square trailing edges! :'(~

Also, don't forget that 206 is the hardener, the resin is 105.
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Online Ken Culbertson

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Re: Stiffening Sheet Flaps
« Reply #6 on: June 04, 2018, 09:18:34 AM »
I use a fairly large Robart hinge points.  The hinge itself keeps the flaps from warping. They will still twist if the wood isn't straight.

Ken
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Online Norm Furutani

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Re: Stiffening Sheet Flaps
« Reply #7 on: June 04, 2018, 11:49:43 AM »
At least, start with quarter grain ("C" grain) balsa. If tapering the stock, orient the rays so the thin part would be towards the center of the tree.

http://www.rocketryforum.com/attachment.php?attachmentid=291435&d=1262959632

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Offline Serge_Krauss

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Re: Stiffening Sheet Flaps
« Reply #8 on: June 09, 2018, 08:48:55 AM »
How about biased .56-oz fiberglass with epoxy? I have found that the FG absorbs enough less resin to give a comparable, sometimes lighter result than with carbon veil. The 45obiased cloth creates great torsional resistance, and the epoxy is significantly stiffer than dope. Be sure to use a very thin, tacky first surface coat to adhere the cloth (you can use a roller), followed by a 2nd epoxy filler coat, after the cloth is anchored by the partially-cured first coat, so that it cannot float and cause added weight.

SK

Online Trostle

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Re: Stiffening Sheet Flaps
« Reply #9 on: June 09, 2018, 01:27:41 PM »
I know a well known designer, builder and flier (several times Nats winner) who has stated there is no such thing as a flap that is too stiff.  He usesd 3 layers of light fiberglass cloth at 45o bias on his flaps.  He felt the improved performance was worth the weight penalty.

Keith

Online Brett Buck

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Re: Stiffening Sheet Flaps
« Reply #10 on: June 09, 2018, 03:09:39 PM »
How about biased .56-oz fiberglass with epoxy? I have found that the FG absorbs enough less resin to give a comparable, sometimes lighter result than with carbon veil. The 45obiased cloth creates great torsional resistance, and the epoxy is significantly stiffer than dope. Be sure to use a very thin, tacky first surface coat to adhere the cloth (you can use a roller), followed by a 2nd epoxy filler coat, after the cloth is anchored by the partially-cured first coat, so that it cannot float and cause added weight.

   Some kinds of epoxy are stiffer than dope. Dope has the advantage of shrinking on its own and tensioning the surface.

      There isn't a whole lot of improvement from just putting things on the outside, particularly if it is put on loose and brushed down. If you vacuum-bag it, you can use much less glue and/or much more fiber for the same weight increase. And you have to use an epoxy that cures hard, which most consumer epoxy does not do. EZ Lam or finishing epoxy, or some variety of industrial unfilled epoxy would be my suggestion.

    Brett
« Last Edit: June 09, 2018, 06:24:32 PM by Brett Buck »

Online Howard Rush

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Re: Stiffening Sheet Flaps
« Reply #11 on: June 09, 2018, 06:15:45 PM »
The JCT is coming to town.
We’re plenty stiff from the flaps on down.
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Offline Serge_Krauss

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Re: Stiffening Sheet Flaps
« Reply #12 on: June 10, 2018, 11:45:57 AM »
I use the West Systems #105. I made the mistake of using "Smooth 'n Easy" with carbon veil to make landing gear legs, and found them to be rubbery, even with many plies. I don't think you'd need multiple layers of biased glass to get a significant stiffening, espectially with hinges alligning the flap. I use this technique on profile fuselages, and they are indeed stiff and twist-resistant. - SK

Offline Motorman

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Re: Stiffening Sheet Flaps
« Reply #13 on: June 13, 2018, 08:52:39 PM »
Use the lightest balsa you can find and insert this between the LE (1/4-3/8) and the TE section. Think of the tube as a torsion tube.   https://alofthobbies.com/carbon-fiber-tubes.html there is a 6mm OD with 5MM ID in that list.
The build will be solid balsa LE tube solid balsa TE. Shape to suit after that.
Don't worry if you have to notch through to hinge flaps to the wing. You can add some tow in the hinge area to stiffen if you are really worried about slicing through the tube. 
This will be as light if not lighter than the plain sheet will never warp and will be consistent.

Cool idea, have you done this? How did you glue the tube in, what glue did you use? Did you use a ball end router to make a cavity or just glue it to flat edges and fill it with something.


I use the West Systems #105.

Do you think #105 is hard enough to make a prop?

Thanks,
Motorman
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Offline Serge_Krauss

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Re: Stiffening Sheet Flaps
« Reply #14 on: June 14, 2018, 12:22:57 AM »
Motorman - I don't really know what props require, but my guess is that it is OK. They make FG boat hulls out of that stuff.

I can't believe I forgot this, since the plane is still unfinished, but I DID use glass and West Systems 105 on flaps! I made my peacemaker's stationary flaps full-span to pass through a trailing edge slot in its fuselage. The glass was not biased, but those flaps are VERY stiff chord-wise. Several had expressed concern about warping Peacemaker flaps with dope. Mine are flat and stiff. They will not be budging much with aero loads. - SK

Online Howard Rush

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Re: Stiffening Sheet Flaps
« Reply #15 on: June 14, 2018, 10:57:34 PM »
Use the lightest balsa you can find and insert this between the LE (1/4-3/8) and the TE section. Think of the tube as a torsion tube.   https://alofthobbies.com/carbon-fiber-tubes.html there is a 6mm OD with 5MM ID in that list.
The build will be solid balsa LE tube solid balsa TE. Shape to suit after that.
Don't worry if you have to notch through to hinge flaps to the wing. You can add some tow in the hinge area to stiffen if you are really worried about slicing through the tube. 
This will be as light if not lighter than the plain sheet will never warp and will be consistent.

This might do the trick.  You want a tube that's stiff torsionally, but wimpy in bending.  Another consideration is that the tube should be as far forward as possible: mass behind the hinge of a control surface is bad. 

If the tube is at the front of the flap, it may have spar slots in it.  Rather than prophylactically adding extra tow, you can do an experiment to see if the slotted tube retains sufficient strength.  The pictures show a torque test rig and the test.  Preston is pouring sand into a bucket to see how much torque the tube will take.  In this case, a 3/8" tube with .014" wall and about a half-inch slot took plenty of torque without reinforcement. 
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Online Bobs your Uncle

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Re: Stiffening Sheet Flaps
« Reply #16 on: June 15, 2018, 08:16:15 AM »

Do you think #105 is hard enough to make a prop?

Thanks,
Motorman

No it is not adequate for props.

Online Ken Culbertson

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Re: Stiffening Sheet Flaps
« Reply #17 on: June 15, 2018, 03:19:51 PM »
This might do the trick.  You want a tube that's stiff torsionally, but wimpy in bending.  Another consideration is that the tube should be as far forward as possible: mass behind the hinge of a control surface is bad. 

If the tube is at the front of the flap, it may have spar slots in it.  Rather than prophylactically adding extra tow, you can do an experiment to see if the slotted tube retains sufficient strength.  The pictures show a torque test rig and the test.  Preston is pouring sand into a bucket to see how much torque the tube will take.  In this case, a 3/8" tube with .014" wall and about a half-inch slot took plenty of torque without reinforcement.

Howard:

Do you have a picture of an unfinished flap done this way?  Your way may be better than mine.  I use target arrow shafts for flap LE on embedded flaps and elevators.  I notch them for Robarts and center the hinge inside the tube.  Lots of work but what is a hobby for if not to create lots of fun work.  They don't twist.  I am wondering if the placement in the flap makes any twisting difference?  I would think that the further back from the LE that the tube is the less protection from twisting it offers.  I really never considered the torsion effect (I may be severely reducing it by using Robarts), since I use the tubes to get a perfectly round LE to fit into the wing.

Ken
« Last Edit: June 18, 2018, 11:54:54 AM by Ken Culbertson »
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Online Lauri Malila

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Re: Stiffening Sheet Flaps
« Reply #18 on: June 15, 2018, 03:49:36 PM »
Torsion tube is fine, but if you want it right it has to be made especially for this purpose. Ready tubes with ideal fibre orientation for our use are not really available.
Maybe an easier option would be the ThinSkin from www.creativecompositetechnologies.com
Also, there are really nice lightweight UD- or woven carbon products easily available. L
« Last Edit: June 16, 2018, 01:11:43 AM by Lauri Malila »

Online Howard Rush

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Re: Stiffening Sheet Flaps
« Reply #19 on: June 16, 2018, 01:34:48 PM »
Here's what I do: https://stunthanger.com/smf/building-techniques/carbon-fiber-rods-on-flaps/msg244782/#msg244782

I am wondering if the placement in the flap makes any twisting difference?  I would think that the further back from the LE that the tube is the less protection from twisting it offers.

It doesn't make any difference on stiffness.  I think Paul Walker made some flaps with the tubes farther aft. 
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Offline Curare

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Re: Stiffening Sheet Flaps
« Reply #20 on: June 18, 2018, 12:00:43 AM »
Just to throw a spanner in the works, would we be better suited making the entirety of the  extreme fibres (i.e. top and bottom surfaces ) of the flap super strong and leaving the core to be, oh I don't know, high density foam?
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Online Brett Buck

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Re: Stiffening Sheet Flaps
« Reply #21 on: June 18, 2018, 10:12:48 AM »
Just to throw a spanner in the works, would we be better suited making the entirety of the  extreme fibres (i.e. top and bottom surfaces ) of the flap super strong and leaving the core to be, oh I don't know, high density foam?

  Yes. The ultimate result of covering the flaps with something rigid would be to create a tube that is shaped like a flap, at which point the filler is there just to hold the shape (or is unnecessary, and should be melted away with solvent).

  There are probably more practical methods for effectively making the flaps "rigid" using conventional construction (like driving it from multiple points, instead of a single bit of steel wire in torsion), but at some point you have to wonder about what you are really achieving compared to some of the more common problems.

     Brett
« Last Edit: July 23, 2018, 09:36:33 AM by Brett Buck »

Online Lauri Malila

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Re: Stiffening Sheet Flaps
« Reply #22 on: June 18, 2018, 10:45:16 AM »
That was exactly my point when I posted the link to ThinSkin page. As core material I'd use lightest Rohacell or honeycomb. Both can be easily shaped with simple tools.
An important thing with such a thin structure  is that the top- and bottom skins must be joined well together in l.e. and t.e. of flap. Especially front edge can be tricky. In our flapper glider we make it by using a separate rohacell spar, covered with diagonal carbon sleeve. It gives plenty of glue area and also distributes nicely the loads from hinge(s) and stoppers.
I think exactly the same structure (Rohacell 30 core and HM 44g flat tow skin) would be quite perfect in stunt flaps too.

Lauri

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Re: Stiffening Sheet Flaps
« Reply #23 on: June 18, 2018, 11:45:39 AM »
If you once have negative mold, it is already easy :- ))
Such flap is like a rock.

Online Ken Culbertson

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Re: Stiffening Sheet Flaps
« Reply #24 on: June 18, 2018, 11:49:51 AM »
  .... at some point you have to wonder about what you are really achieving compared to some of the more common problems.
Brett is on to something - I think we may be trying to solve a problem here that ultimately only results in different trim elsewhere.  IMHO Flaps should not warp but twisting in flight is going to happen to some degree as long as we use flap horns.  If it is not excessive and is equal both between the 2 flaps and more important, upright vs inverted then what is the problem?   In fact, it may actually be more stable having the outer portion of the flap deflect slightly less than the inboard but that is just conjecture.  In many ways this is like making your wheels perfectly round so that the plane rolls without any extra friction when you fly over grass or trying to stop your outboard wing from dropping when you hit the edge of a thermal.  Some things you just learn to live with.

Ken
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Online Lauri Malila

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Re: Stiffening Sheet Flaps
« Reply #25 on: June 18, 2018, 01:04:19 PM »
Brett is on to something - I think we may be trying to solve a problem here that ultimately only results in different trim elsewhere.  IMHO Flaps should not warp but twisting in flight is going to happen to some degree as long as we use flap horns.  If it is not excessive and is equal both between the 2 flaps and more important, upright vs inverted then what is the problem?   In fact, it may actually be more stable having the outer portion of the flap deflect slightly less than the inboard but that is just conjecture.  In many ways this is like making your wheels perfectly round so that the plane rolls without any extra friction when you fly over grass or trying to stop your outboard wing from dropping when you hit the edge of a thermal.  Some things you just learn to live with.

Ken

I kind of disagree with this. Of course, current models have evolved to what they are by using traditional materials and construction techniques, so you may not directly find happiness by making a well-proven design 10x stiffer. I mean flaps and so on. Most propably you will have to re-optimize things like flap movement and size.
But what is clear, is that with stiffer structures, both in bending and in torsion, models perform much better. They are easier and more logical to trim and fly.
My examples are from free flight and the way we fly them is more violent, but exactly same principles apply in F2b.
But perhaps in stunt we will not see such big revolutions like in free flight, such that new technology comes and literally walks over the old. L

Online Ken Culbertson

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Re: Stiffening Sheet Flaps
« Reply #26 on: June 18, 2018, 03:08:15 PM »
I kind of disagree with this. Of course, current models have evolved to what they are by using traditional materials and construction techniques, so you may not directly find happiness by making a well-proven design 10x stiffer. I mean flaps and so on. Most propably you will have to re-optimize things like flap movement and size.
But what is clear, is that with stiffer structures, both in bending and in torsion, models perform much better. They are easier and more logical to trim and fly.
My examples are from free flight and the way we fly them is more violent, but exactly same principles apply in F2b.
But perhaps in stunt we will not see such big revolutions like in free flight, such that new technology comes and literally walks over the old. L

I am not proposing that we do not try and make flaps stiffer.  What I am saying is that conventional methods can make them stiff enough that any incremental stiffness may require radical construction techniques that are simply out of reach for 99% of us.  Perhaps one day some CF fabricating company will start selling flaps in various sizes but until then......  Just out of curiosity, is there a source for light weight honeycomb?   1/4 or even 1/2" honeycomb core covered by a thin hardwood vainer would be both light and stiff.

Ken

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Offline Dave Hull

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Re: Stiffening Sheet Flaps
« Reply #27 on: July 23, 2018, 01:42:44 AM »
Honeycomb material is readily available--in a few sizes--for the hobbiest.

Try Aircraft Spruce and Specialty:

   http://www.aircraftspruce.com/catalog/cmpages/01-01052.php?clickkey=9108

If you want to look at data sheets, and the wider selection that can be purchased if you wanted hundreds of square feet....

   https://www.hexcel.com/Resources/DataSheets/Honeycomb

If I wanted to be totally cool and disrespect my patience, budget, and hobby time, I would try the phenolic or nomex core, probably 1/8" cells to avoid print-thru, in a standard 1/4" thickness for small planes and probably 3/8" for bigger planes. If you can't get material densities below 2 psf, then you are probably wasting your time with honeycomb as compared to structural foams, except for the aforementioned coolness quotient. I would not attempt to taper the core or re-thickness it--period! I do not have a milling machine. You do not need/want the overexpanded core material for flaps. Use the standard stuff. Again, if I was going to mess with this, I would premake and fully cure the skins on a platen by either press method or vacuum bagging, and then as a secondary operation bond the skins, LE/Spar and TE closeout in one operation. You do not need a structural closeout at the TE, all you need is something to protect the edges and avoid any exposed core. This is for durability and to avoid high ugliness coefficients. Because model flaps are so small relative to the available material sizes, it would be easy for the construction to be far heavier than it needs to be. (Black aluminum syndrome, or in our case, black balsa syndrome....)  Careful design might offset some of this. For example on the TE, consider using a 1/16" x 1/4" balsa strip inset to the skins (ie. the core is set back 1/16" as the closeout. You will also need material inserts where the horn is going to go, and so forth.

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Re: Stiffening Sheet Flaps
« Reply #28 on: July 23, 2018, 08:52:46 AM »
Lauri the new technology is here now, and is here to stay. I am already using it in the REvolutions.
One thing was of a concern to what I said earlier to use the 6mmOD 5mmID tube was the cut outs for hinge in the tube. First the cut is on one side of the tube only. Second when you want to assemble the hinges you fill in the tube with Epoxy microballoons mix which creates a hard cork like plug inside and when you inset the hinge inside it also glues the hinge in place.  Because the cut section is filled in with the epoxy stuff it will be stiff.

Since we talk composites here lets discuss the subject.
About the Rohacell and Honeycomb. Rohacell is very expensive but excelent material and hnoeycomb is heavy. An alternative to that is the Home Depot foam. This foam is typically 40PSI strength and works very well for a solid core flap. Flaps can be easily done in vacuum bag without tubes etc. How is it done is similar to this project you can investigate to your hearts desire https://www.rcgroups.com/forums/showthread.php?634176-Tabooish-Build. You make a foam core with hot wire then using the technique described there you get a stiff flap that is super light. Kevlar can be substituted with Fiberglas 1.6oz per square yard https://www.thayercraft.com/1.6-oz-1280.html. You want the cloth balanced (same number of threads in both directions) and you want the cloth in at 45 degrees bias.
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Online Howard Rush

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Re: Stiffening Sheet Flaps
« Reply #29 on: July 23, 2018, 12:16:20 PM »
Kevlar can be substituted with Fiberglas 1.6oz per square yard

No, you have to use Kevlar.  Working with Kevlar builds character.
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Offline Motorman

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Re: Stiffening Sheet Flaps
« Reply #30 on: July 23, 2018, 06:35:47 PM »
TDM, thanks for the link. Just pulled the trigger on 5 yards of 1.6oz 1280 cloth.

Motorman 8)
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Online TDM

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Re: Stiffening Sheet Flaps
« Reply #31 on: July 23, 2018, 08:58:35 PM »
Howard there is a method to my madness. Kevlar for skin is a pain in the ass to cut to trim and finish. Try to sand that and let me know how it went.
In gliders we use it because flaperons love to warp and they need a stiffer material that is stiffer than fiberglass. Actually we face the LE of the flaps with glass then laminate with Kevlar top bottom and then insert it in the bag. Another advantage to using Kevlar is the the hinge material is already there. Lastly there are a bunch of midair collisions and Kevlar makes for easy repairs (hot water inflates the foam back to original shape and a little epoxy re attaches the skin in place).
For our uses because the cross section is symmetric (not much warping to be worried about), and because we generally use plastic hinges (we don't generally use Kevlar hinges) fiberglass will be just fine.
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Online Howard Rush

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Re: Stiffening Sheet Flaps
« Reply #32 on: July 23, 2018, 09:35:50 PM »
Howard there is a method to my madness. Kevlar for skin is a pain in the ass to cut to trim and finish. Try to sand that and let me know how it went.

Not to mention when you’ve laid everything in the mold and pull your hands away with one spider-web-fine strand on your hand, and the whole layup pulls up. Or after a few months when the weave reappears on your nice, smooth surface.

I hear that it sands nicely using liquid nitrogen as a lube. I haven’t tried that myself.

Yep, you gotta use Kevlar. No wimping out.
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Online TDM

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Re: Stiffening Sheet Flaps
« Reply #33 on: July 24, 2018, 06:44:06 AM »
Howard LOL. Well bud the only place I consider it is in the hinges and that where it stops. You forgot to mention the fact that Kevlar doesn't draw the resin in like the other composite materials and you really have to get the resin to penetrate the fibers.
 
https://www.rcgroups.com/forums/showthread.php?1083322-Build-Log-Evolution/page8
https://www.rcgroups.com/forums/showthread.php?1800565-Evolution-3-the-E-Pluribus-Unum-DLG-group-build-log

I think I am hijacking the thread.
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Offline Target

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Re: Stiffening Sheet Flaps
« Reply #34 on: July 24, 2018, 08:29:53 PM »
No, you have to use Kevlar.  Working with Kevlar builds character.
Especially if you try to sand it!
Regards,
Chris
AMA 5956


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