mini Mach (Sanda Models)

[Konrad]

One of the benefits (or curses) of having Aloft Hobbies as my local hobby shop is that I get to go through customer returns. I went in to Aloft Hobbies looking for a 60” racer other than a Sunbird and walked out with a customer returned Sanda Mini Mach. I’m still in need of a good value 60” racer for “Man on Man” racing.

Mini Mach Sport Glider Carbon Version

A nice and stiff carbon composite glider from Sanda Models.

alofthobbies.com

This customer return was for a damaged wing. The damage is so difficult to find that the customer didn’t find it until well into the assembly process. This wing is showing delamination for approximately 1/3 the span on the starboard wing.

By the way this is why it is so nice to deal with Wayne at Aloft Hobbies. He is also a real modeler and fully understands how damage ( shipping or manufacturing) might not be found until well into the assembly process. (Just try to explain to some other importer that damage was present before you started assembly. All you are like to get from them is motion sickness from all the run around)

I don’t think this delamination is a manufacturing issue as the epoxy bead leading edge is broken into three pieces. I looked for signs of an impact point and couldn’t find one. I suspect that the wing was subjected to a shock load directly on the leading edge And it is this shock that broke the LE and delaminated the top wing skin. I don’t think this delamination has anything to do with the manufacturing of the wing (dry layup).

To repair this damage I’ll need to inject a very low viscosity epoxy under the delamination. I will have to rely on capillary action to ensure that the epoxy fills all the voids under the skin. Time will tell if I got a killer deal on this model or if I payed top dollar for scrap carbon and foam!

All the best,
Konrad

 

[Konrad]

Ok, I hit my first real snag with this model. The customer had installed servo mounting trays. This was fine other than he did not add any carbon to the underside of the top skin. But the real issues is that these trays are not for any standard servo mounting spacing that I’m aware of. These look to be Hitec proprietary spacing. I think the planned servo was the D145SW.

D145SW, 32-Bit, Wide Voltage, Steel Servo | HITEC RCD USA

With 32-bit high resolution, ultra-response technology and wide voltage operation, the slim wing D145SW...

hitecrcd.com

I’d like to ask is there a known way to remove servo trays that have been CA’ed in without damaging the foam core of the wing skins? As I think the answer is no I would like to know if there are any 10mm thick wing servo other than Hitec that can be made to work with the Hitec D145SW servo mount?

I don’t think the D145SW is a great value at $60 for a plastic cased servo. Then there is the added $70 expense for the programer. Having to buy 4 D145SW will eat into the deal I got for a damaged aircraft and I’d still be left with Hitec servos. (Hitec isn’t a bad brand it’s just that it would be another brand of servo I’d have to maintain).

All the best,
Konrad

 

[Wayne]

In a word - DREMEL

 

[Konrad]

Yep, I'm thinking for $240 I can do a lot of repair to the inner skin!

I'm thinking of putting a wheel coller as a depth stop on the small straight burr.

 

[Wayne]

Take your time, it is not too hard.

 

[Konrad]

Patients is a virtue I don't have!

 

[Glenn Beer]

Sad to say that was my Minimach.
Part of me wishes, I’d kept it, except I just couldn’t come up with a repair method I would trust, or that wouldn’t butcher it up. I think thin CA would have worked but might have made a true mess of the finish.

FWIW the intended servo was Hitec’s HS-7115TH. It’s an 8mm thk servo. And yes four of them cost almost as much as the plane.

On removing the existing mounts. I think I would use a dremal with a these 60 grit quick release sanding disks.
They will make quick work of the mount and when using a light touch, remove fine amounts.

snip.... I think the planned servo was the D145SW.

D145SW, 32-Bit, Wide Voltage, Steel Servo | HITEC RCD USA

With 32-bit high resolution, ultra-response technology and wide voltage operation, the slim wing D145SW...

hitecrcd.com

I’d like to ask is there a known way to remove servo trays that have been CA’ed in without damaging the foam core of the wing skins? As I think the answer is no I would like to know if there are any 10mm thick wing servo other than Hitec that can be made to work with the Hitec D145SW servo mount?

All the best,
Konrad

 

[Konrad]

Thanks for the heads up on the EZ lock discs!

The repair of the LE was easy. All I did was open up the delamination with tooth picks. Mixed up some strong slow setting epoxy.
Now comes the tricky part. I heat a small amount until is has a viscosity lower than water. I then brushed it into the delaminated area. Letting it run all over the place. Remove the tooth picks. Open and close the delamination a few times. Wipe off the epoxy that is on the outside on the skins with denatured alcohol. Tape the seam shut. Wait for the epoxy to set but not fully cured. (I think this is often called tack free) un-tape the area again clean off any epoxy that might have oozed out with a rag soaked in denatured alcohol. When the alcohol has evaporated re-tape the wing and set aside. 24 hour latter I have a bond that is stronger than the original as I don't have any fillers in the epoxy.

The trick is that set epoxy can be cleaned up before it fully cures.

BTW; CA is a very weak glue only about 2K PSI at best.

 

[Glenn Beer]

Those disks are far easier to control than any burr. You can go from wholesale reduction to fine removal just by regulating the pressure applied.

 

[Konrad]

LOL.
Isn't that how a burr works?

But I know what you mean about controllability.

 

[Konrad]

The use of the Dremel EZ Lock 60 grit sanding disks worked great! It only cost me one sanding disk. They tend to load up with the melted plastic from the high RPM of the rotary tool. I think this had a nice benefit as the heat failed the CA glue. I could hook a fingernail under the servo tray and pop the trays off after I sanded them down to about 2mm from the skin.

I did have one flap servo tray not pop off. I noticed that before an attempt at removal of the tray that there was ever so slight amount of distortion in the top skin. After sanding off this tray, the process took a little of the inner top skin of glass. I think this process has resulted in slightly more distortion in the top skin than at the beginning of the removal attempt. I hope that after adding the carbon reinforcements to the top inner skins there might be more stability in the area that this distortion might be reduced. This is an esthetic issue only as the distortion is way back on the airfoil well aft of the start of the separation bubble (in the turbulent flow).

I think that there was some porosity in the area of this flap servo mount. This may have allowed the CA to get under the glass and bind (attack?) the foam skins, causing the slight distortion to begin with.

The other three servo pockets cleaned up real easily with the 240 grit disks after removal of the trays and are ready for the carbon and epoxy wing skin stiffeners.

All the best,
Konrad

 

[Motowncali]

Do you have any Monoject syringes? They work great for injecting epoxy into a leading edge.

 

[Konrad]

Yes, but with the elevated heat I suspect they would melt or distort. I had great luck saturating the debonded zone with heated epoxy (low viscosity) and flexing the area open and shut.

 

[Konrad]

Earlier I had voiced some concern about the value of this customer return. Well I’m happy to say I got a great deal!

I was able to repair the delamination. This wasn’t in doubt, but I had concerns about removal of the customer installed servo trays. There was nothing really wrong with the customer installation of the trays. It was just that I didn’t have the servos that fit these proprietary trays. It wasn’t until after removal of the trays and the installation of carbon backing to the insides of the upper wing skin that I considered the wing repaired and ready for the installation of servos of my choosing.

I’m happy to report that the distortion of the top wing skin around the servo mount is just outside that of optical distortion. I could not get this distortion to show in any of my photos. Yes, I can feel it but it really is very slight. Should I damage the model and need to break out the black paint I’ll block sand the distortion out. But until then, few will notice the distortion.

Now I’d like all OEMs of moldies to add this carbon stiffener in the servo pocket. It doesn’t need to be the latest spread tow, just something to stiffen and add support for the mounting of the servos.

All the best,
Konrad