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Skin repair of molded models (60" glass and rohacell racer)

Konrad

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Truth be told rattle can would be fine. Like I said I wasn't to concerned about hanger rash while developing this repair. But I think getting set up for the Klass Kote system would be an investment in the future.
 

Konrad

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I was able to get the bottom inlay patch installed. And much to my surprise I was able to get a lot of glass fibers across the seam from the inside. I used 2 pieces of 2 oz cloth that were heavily soaked in epoxy. I put these patches against the inside of the bottom wing skin allowing half the patch to sag out the opening. I then placed the inlay part down on top of these sagging pieces of glass cloth after adding a thick bead of epoxy and micro balloons to the leading edge. This results in there being only epoxy on the inside glass skin and on the leading edge. I then used foam safe CA to tack glue the sides of the patch where needed to keep the patch aligned with the outside surface. I then placed the wing so that gravity would help hold the glass patches down against the inside wing skin. It was hoped that the heavy soaked glass would drape across the joint. Once this epoxy set I filled and glued any areas around the perimeter of the patch with a very dry mixture of Epoxy and micro balloons.

I've been using this real dry (10:1 by volume) micro balloon mixture as a filler for its ease of sanding.

While color sanding I see that I have a bit of a low spot in the top wing (compression damage towards the root). I see this by the wet candling of the surface. This area is about .25mm out of profile. So I'll have to mixup some more very dry epoxy micro balloon mixture. After this is block sanded I think I'll be ready for the top glass (outer skins).

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Wayne

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Got to see this repair yesterday, and I have no concerns, it looks and feels great. Like everything Konrad does, it is well overdone. :)
 

Konrad

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Since I'm hacking away on this wing I decided to change the flap actuation from the original bottom drive to top drive. I did this in an attempt to lower drag. This decision was based a lot on the write up by Martin Hepperle.
Top drives keep a lot of the linkage inside the wing
https://www.mh-aerotools.de/airfoils/index.htm
See Aerodynamics and then Drag of Aileron & Flap Linkages

I'd like to ask which way do you set up your linkage? I know that the DS guys set up there flap linkages to be "locked" while in profile to minimize the load on the servo gear train at high speeds.
I've got my flap linkage set up so that the linkage is locked with the flaps fully down. This was done to try to save the flap servo gear during landings. The concern I have is that any gear slop I have is now showing up with the flap in profile. This hasn't shown up as a problem as the top speed at the bottom of the split "S" start is probably less than 100 mph.

I know the ultimate solution is to use servos with tighter gear trains, such as the MKS or KST.
https://alofthobbies.com/mks-hv6110-mini-servo.html
https://alofthobbies.com/kst-x08h-servo-2-8kg-38-oz-in-09-sec-8-grams-wide-voltage.html

But for the price of one of the above mentioned servos I can fit the wing with 4 of these economy KS Servos.
https://alofthobbies.com/jx-pdi-0902mg-servo-19kg-264-oz-in-10sec-94-grams.html

I've been using the HV version (PDI-0903MG) and these have been working well in this ship for about 2 hours of high speed flight.

At one time I was involved in trying to keep youths involved in this hobby. This often meant helping them with repairs and basic construction methods. I remember one insightful youth say to me; "It's not that you don't make mistakes, It is that you know how to fix your mistakes". That realization that we make mistakes during the repair process went a long way towards that youth becoming a great modeler.

I've seen far too many models abandoned because there were mistakes made during the repair attempt.

As a form of encouragement I'll detail the two mistakes I've made during this repair. The first I've already covered in detail and that was the failure to realize that stiff structures (carbon spars) fail at three points as a result of impact damage. This cost me about twice as much carbon as would otherwise have been needed (read added weight to my repair).

The second mistake was the sloppy use of CA glue, and the subsequent soaking with acetone!
During the initial piecing together of the wing with CA glue, I folded over the aileron to use it as a splint. This was great as it defined a straight trailing edge upon which I used as a datum to piece together the wing. The mistake I made was not placing a release layer between the aileron and the top wing. Yep you guessed it, I had glued the aileron to the top of the wing! Now as the structural foam was imperious to CA glue I thought, wrongly, it was safe to use acetone. In a panic I soaked the glued area between the aileron and the top of the wing, giving it time to soak into the joint to loosen the CA. Much to my horror I noticed the wing tip distorting as the structural foam was dissolving. I shook off the acetone and with my fingers and pry bars lifted the aileron off the top of the wing. You can see the extent of the damage as the block sanding clearly shows the low spots. I got lucky in that after the acetone dried the structural foam remained stiff. To repair this damage I will have to use a bit of glazing putty and a lot of block sanding.

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Konrad

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Well, I declare her airworthy! I had to make my own blister servo covers from 4 layers of .75 oz cloth. These should help drop the drag from the flap linkage a lot, compared to the original configuration. They are about twice as large as need be. But as I was against Sunday's time constraint for the MoM race, I thought better a bit too large than too small.

The repair is looking real good in that it looks to have added about 35 grams to the total weight of the wing. This is real good when you realize that half of this weight is counter balance lead added to the opposite wing.

I'm a bit shocked in that the radio programing looks the same even with the reversal of the flap drive. In OpenTX all I've done is reversed the flap servos rotation. I'll go through the calibration routine again as Mike Shellim recommends with his templet.
http://rc-soar.com/opentx/setups/f3f/index.htm

I hope the winds come up for a test flight at Mussel Rock but it is looking rather calm right now.

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jvaliensi

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Klass Kote has a quick cure hardener for their primer. I've had good success with it. I cannot remember exactly how long before it was cured, but maybe an hour or so.

Klass Kote - Primer Catalyst White Fast #420 - Side-B Catalyst
 

Konrad

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I've now got about an hour of test flying on the repaired wing. The wing performs just as well after the repair as she did before meeting the pylon pole. High speed flight with 300 grams of ballast results in no trim changes from the same flight envelope but without ballast. Low speed stalls, clean and in crow, are stable and predictable. Just like they were before the wreck. The wing has handled hard landing and the full ballasted split "S" diving start. So I think I found all the breaks in the carbon spar.

With the help of Mike Shellim's templet I only had to make trim changes of maybe three clicks from what I had in the radio before the crash. I think most of this as a result of changing to top drive flaps, not the repair.

Now at the airframe level I've noticed a few things. First I think the plane actually is flying better after the crash! Not because of the repair but because I'm getting the aileron differential and snap flap expo dialed in much better.
Some might say it is also psychological effect from the first dent.

And second, I think I noticed a bit of yaw trim change as the speed changed. This might be because the Starboard wing has a surface finish from block sanding with 180grit sandpaper and the Port wing still has the nice gloss factory finish. I can't really say as the air was rather rough today at the slot. I also noticed that she now has a stronger whistle at high speeds. This surprised me, I can only think it is coming from the holes for the top drive of the flaps. I think I'll need to add some more fairings to the wing.

Talking with the guys, I think there is a misconception of how or what a hollow core wing is and how it works. The issue might very well be with my understanding.
My understanding is that the molded hollow core wing is much stronger than the classic stress skinned foam core wing. This is because the wing skin is made up of two stressed skins working as load bearing members with a foam shear web between them. The performance of the wing (weight savings) isn't from the hollow spaces. But rather this structure (two skins verses one) allows for a lot less glass (carbon) and epoxy to be used in the skins for the same amount of strength.

All the best,
Konrad

P.S.
AUW without ballast is 910 grams

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