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Mar 25, 2019, 05:44 AM
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Thread OP

Carbon Fiber Boom Repair

I've had an accident on maiden of my new 1M Toro DLG
Looking back I can see this was down to pure inexperience but, lessons have been learnt.
I had it nicely built, controls moving in the correct directions with CG bang on at 57mm although I had only a very rough idea of the required control throws!
So I decided to test throw it from a small hill because my fear was that if it nose dived - I'd have a chance to pull up and save it.
My first couple of gentle tosses showed the controls were too responsive with WAY too much travel. I dialled the Aileron and Elev rates down but that didn't help much.
So - I dialled them down a lot (too much), threw it again and following a gentle stall - it went into a dive and I didn't have enough up elevator to save it - CRACK!!

So - how do I go about fixing this? I've seen a post where a guy slid a slightly smaller diameter of CF tube on the inside of the boom 1st. Then wrapped 1 layer of CF cloth and finishing resin and covered in heat-shrink.
I'm not sure where he got a CF tube that thick but I'm hoping it was only necessary because it was a large 1.5M job.
Can this be mended without any inside tubing? Should I use CF splints down the side? CA - Epoxy - I have a few ideas but not sure about weight/ strength...

Any help/ advice on the repair would be appreciated,

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Mar 25, 2019, 07:57 AM
Registered User
Hello Graig,

look at this video.

Jan-Henning repariert einen Rumpf mit GFK und CFK - Teil 3 - Schienen, Stabilisieren, Geschenkband (15 min 59 sec)

It is in German, but I think you will find/see everything you need.
Mar 25, 2019, 10:49 AM
Registered User
Here is a good video of repair work. I have used this method a couple of times and it is strong and if done using 'judicious' amounts of epoxy and carbon can make a very strong light weight repair.
Good luck.
Mar 25, 2019, 11:26 AM
Transplant Chey WY from Reno
Thermaln2's Avatar
Here's a zip file I previously posted which details how to repair a tailboom damaged as your is. Look at the word doc.

Mar 28, 2019, 09:15 AM
Registered User
Thread OP

Thanks guys

Thanks for the replies guys (Charly, Cary and Chris), I managed to complete the repair using the method that Chris posted in the end. It seemed simple and quick enough however, its not the tidiest repair but, is straight and 'should' stand up to my launches. I think like building - it may take a few attempts to get a nice, tidy, light finish on it.

So - now its back in one piece and the weather is getting better, I'm dying to try it out... I just need some 'safe' initial settings that I can tweak later on. Is anyone able to steer me in the right direction regarding control throws etc. I initially followed the settings from here : but they were way too much and made it fairly uncontrollable.

cheers Craig
Mar 29, 2019, 07:17 AM
Registered User
Are you using any expo on the elevator/ ailerons?

Mar 29, 2019, 07:29 AM
Registered User
Thread OP
Hi Jack, yes I was using some expo (Spektrum) around 20% or so, possibly a bit more on the elevator as I know this needs to be smooth to maintain a good glide. From my knowledge of helicopters and limited plane experience, I'm pretty sure the controls were moving too far so not so much twitchy as overly responsive.
Mar 31, 2019, 02:07 PM
Registered User
+/- 3/8" ailerons.
+/- 1/2" rudder.
Elevator about as much down as you can reasonably get for the push-over at the top of the launch. Up elevator is a parameter you tune.

That will set you up for soaring flight.

Longer answer - read the sticky I put out for answering this sort of question. There is a lot more detail there.

I don't recommend expo on elevator. For much of a successful flight, a sailplane is turning. During the turn, the elevator is pulled up. Now you have a more sensitive up elevator and decreased sensitivity down elevator. That makes flying a smooth level turn more difficult. Why make flying more difficult?

Tune the max up elevator throw as I posted in the sticky. Using more than what that method indicates (loop test) doesn't buy you anything as all you are doing is pulling the wing into the stall region. Not efficient...

Mar 31, 2019, 11:21 PM
Registered User
I should probably modify my above post a bit. For narrower wings you probably only want about +/- 5/16" throws. Drag goes up if the throws are too excessive and the pilot uses full stick range. For a 1M wing, it is possible that even 5/16" might be excessive.

For the rudder, if the root chord is conventional, then the throws I mentioned are fine. If the chord is smaller, then the throws should be reduced proportionally.

The reason elevator throw is a tuned parameter is that the overall design makes a great deal of difference in the answer. The longer the tailboom in proportion to the span, the greater the required deflection for the same radius turn. Unfortunately if the deflection required is too great, the elevator starts behaving as a speed brake! That is not good. There are wing designs which can turn quite tightly (Edge in particular, and generally most of my designs). Putting a long tailboom on such a design sacrifices at least some of the tight turn performance. Some wing designs do not turn tightly or do so poorly. There, a longer tailboom is not quite as much of a sacrifice. Tight turns are most important if you like to practice low level saves, or if your thermals sometimes have rather tight cores. Flying in the core trumps all else!

But in general, the longer the tailboom, the more elevator deflection required for the same turn radius, or loop radius if one is stunting. Full flying horizontal tails do not suffer the speed break phenomenon. But the long tailboom adds extra drag for tight turns, out of proportion to the area increase. I'd rather see shorter tailboom with a bit larger tailfeathers as I think that is a better compromise.

I went a bit OT as I think some designers go overboard for improved straight line pitch control and more tolerance for improper mechanical setup and mediocre CG choices, at the sacrifice of handling in thermal turns. Speaking at least for myself, I'm going to be spending more time doing thermal turns than straight line flight. YMMV!

Apr 02, 2019, 03:57 AM
Registered User
Thread OP
Thanks for those settings Gerald-
I wasn't sure if/ when anyone would chime in so I decided to give it another go last Saturday morning as the weather was perfect. Using some fairly tame settings (similar to those suggested) I gradually tuned them in mainly reducing both aileron movement and up elevator. I also had to drop all but a hint of up elevator in launch mode to stop it looping and got some fairly descent launches in and even managed a 70 second flight!

This is my first full house DLG after a big vick and lightning bug and being composite is clearly a different beast altogether! It just wants to go so fast all the time and makes a cool swooping noise as it goes past. I'd rather it floated though so I guess I should have another play with the camber settings because moving from cruise to thermal didn't make any noticeable difference.
Also, I didn't want to put in any differential settings until I got it flying/ trimmed in and also understood why they are needed. Every time I banked - I had to hold in a load of up elevator to keep the nose up so I will start looking into that.

Anyway - after one of my 'last few throws' - it went into a spin and plummeted to the ground - nose first! Needless to say - the boom has snapped in exactly the same place although not as cleanly as last time. So - back on the bench it goes. Turns out one of the aileron pushrods slipped within it's outer carbon tube and stuck on full!

I wonder how many repairs it can survive because after the last one I had to add an extra 6g to the nose!

Thanks again for the advice Gerald, I'll re-read that sticky now that I've had a go as it should make more sense this time.
Apr 02, 2019, 06:36 AM
Wayne Wimbish
wdwimbish's Avatar
If the new break is in the same location as before you can carefully sand off the old repair before adding the new one. That should help keep the weight increase to a minimum.

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