Carbon Resin Bell Bypasses... Good or Bad?
 

My 5th jet turbine which I recently completed is a CARF Eurosport with the CARF vectored tumble tube. I first saw this jet on a few Youtube videos awhile back and I thought it cool enough to get one.

It showed up in an extremely large box along with its wings and Electron retracts. Fit and finish looked nice. The part of the jet that concerned me was the carbon resin wound bypass bell and the fact that there is no removable top hatch over the turbine.

The CARF instructions specifically said to place the carbon bell within the included thrust pipe and place the turbine so that the back edge of the turbine exhaust was between 20 and 25mm distant. This placement seemed a bit close, but given that the maker knew better than myself I placed the back edge at 22mm.

If everything works correctly the bell bypass sucks in plenty of air which keeps the bell cool. The problem is what happens when things don't go perfectly? Keep in mind that the exhaust gas of the turbine is around 700 C and the flash point of epoxy type resins is 170 C. The aluminum used in aluminum bells melt at around 660 C. That ~500 C difference between resin and aluminum has go to be a great safety factor. Also keep in mind that solid rocket fuel such as used on the Space Shuttle and SLS system use a type of rubbery organic polymer similar to the resin used in these carbon wound bells.

After completing the CARF Eurofighter build I placed the jet on my driveway and proceeded to fire up the turbine and check out the robustness of the vectoring. The first run was routine and I used the time to wind up the turbine and check the centering of the vectors. To my surprise the deviation from centered cold to centered hot was around 30 degrees in both yaw and pitch! I centered the vectors hot with my radio sub trims. After cooling the vectors returned to 30 degrees off center, Same amount of deviation just the opposite direction. I then asked myself how much will the thrust tube temperature/expansion vary during flight and how easy will it be to keep the plane trimmed during flight?

So far, so good other than the questionable vectoring deviations from cold to hot. I allowed the turbine to cool and had my wife record the second run where I planned to do a short taxi test to check the steering. The start was relatively normal at the very beginning but I quickly noticed some yellow puffs of the exhaust which intensified in the coming seconds. After it was obvious that things were not going to get better I shut the turbine down and saw black smoke. I grabbed the extinguisher and put out the fire.

Here's a link to the Youtube video of the second run:


While it wasn't immediately obvious to me at the time of the run I apparently had a "hot" start. After inspection I found that the carbon resin had burned through and the yellow puffs I was seeing were actually a result of the resin burning off. The only thing holding the carbon resin bell to the stainless was about 1 or possibly 2 three mm bolts since the majority of the carbon bell had burned through. My guess is that the resin caught fire and the exhaust blow by from the damaged bell scorched the inside of the fuse. I'm certain that had I had an aluminum bell this problem would not have happened.

I have since bench tested the turbine and it runs fine. I cleaned the inside of the fuse and have lined the entire rear section with aluminum HVAC tape. I have also ordered a new non-vectored straight pipe replacement with an aluminum bell.

Conclusion: This will be my first and last carbon fiber resin bell. This type of carbon resin bell can work perfectly fine, UNLESS something unplanned happens, and as we all know the unexpected can, has and will happen. My understanding is that CARF has redesigned their tumble tube but I wonder why they ever chose this design given the large 30 degree angular deviation from cold to hot.


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Why not just replace the bell with a proper metal one?

That starter sounds like it's stuffed. Years of flights on the carbon has been fine for me.

I have carbon fiber bells on a CARF SU27. The insides are painted with BVM heat shield and have been working fine.

Damage can certainly be done to a Carbon Cone, or reversed lip…..but the same can be said for the aluminum cone….they are very soft metal, and some have been almost over stretch. I have build with both, the motor was the problem not the bell….that’s also the reason I run every motor I install new or used on the bench before they are run in the model…I have had a new engine that refused to go into cooling that would have been really bad in the model. Since I’m religious about running them all on a few starts….

The turbine ran fine during the first run. The turbine runs perfectly on the bench post burn. Amazing that no one seems to think that a 30 degree hot/cold deviation in the thrust vectors is a problem. Consider that exhaust temps vary over 200 C and ask yourself how you're going to keep the TV trim centered during a flight? Crappy engineer who didn't know that metals expand when heated and CARF still continues to sell this trash "tumble tube".JUNK...

Now, I'm not going to try and say its this or that, but you, yourself, questioned the distance from the bell to the turbine.

You are the builder, they, the manufacturer. If you questioned the distance did you a. Call and verify, or b, adjust to what you thought it should be? According to your post, you did not do the latter.
The measurement could have been a simple typo, not caught, before printing.
Have you contacted CARF, at all, about the incident.
To my knowledge, they do support their products, but, some common sense also has to be used.

Its no different if you mount an engine for a prop driven plane, if the spinner is too close to the cowl and it rubs, you going to leave it?
They cannot account for, in their build manual, every single power plant that could possibly be put in a plane. I know of, at least 6 different companies that make rc turbines off the top of my head, at varous diameter, thrust, and weight. To do so would require months of very expensive testing of each and every possibility.
You followed the manual, I don't fault you for that, but when that red flag went up, further research to be sure needed to be done. The lack of it almost cost you thousands of dollars. Right now, at this moment, a phone call to CARF with stills from your video, as well as pictures of the turbine spacing you had would go a long way to getting parts replaced by CARF AND making sure that the manual is updated so it doesn't happen to others.

I'm not going to brow beat it, but CARF is only, partly, to blame.

Our hobby is costly, sometimes dangerous, any red flag, such as your "that can't be right" moment should stop progress until further information from the manufacturer clarifies it and either resolves the doubt, or fixes the problem.

I have, yet, to build a turbine. Its on the to do list, but I have had a few of those moments on the current project and, every time, the tools get set down and phone calls and emails go out until I am sure of the right procedure, measurements, amp loads, etc. It moves no further if there is any doubt.

We are as responsible for building a good plane thats as safe as we can make it, yes, manufacturers SHOULD shoulder some of that responsibity, but it is, ultimately our ball, not theirs. Had they shipped you a completed aircraft, everything installed so all you had to do was fire up the reciever and bind, then its all on them.

This, is not that. They design a plane, pick an engine and design the included systems based on engine, servos, linkages, weight and balance.

One thing I have noticed in 20 years, is that most aircraft manuals are based on one, maybe two, engine choices out of many.
Might be worth the time to drop an email and ask which they used prior to clicking buy. The engine they used in prototype may have required that spacing. Yours requires more..that simple.

Its my understanding, that each turbine manufacturer has a recommended spacing for each of their engines. IMO, unknowledgeable as it is, that would be the measurement to use, no matter the bell material. The guys that designed and built ypur turbine know exactly what spacing would be needed based on exhaust temp at WOT to keep from melting the bell and charbroiling the aircraft. It wouldn't have mattered what CARF had in the manual, at that point and we wouldn't be reading about a fire, but a maiden. The reason for the spacing, in part, is to let the exhaust gasses cool a bit before hitting the tube and bell. I imagine diesel runs cooler than jet a, kerosene somewhere in between.
I'll be honest with you, other than finding rough linkage lengths, throws, and cg...other than a "how did they do it" because I'm not coming up with it is about all the creedence I give to an ARF instruction manual. Everything is done per the component manufacturer recommendations for install. Everything.
exceptions to that, anything thats hinged.

There were multiple ways to resolve your concern. 1. Most difficult: trim the tube down and space the bell back as much as you could, fashion new mounts.
2. Slide turbine forward and redrill the rails.
3. Combination of both.

These things are up to you, as the builder, not CARF

My .02

Never understood why people always have to make excuses for the inexcusable.

CARF hasn't updated their website in years. This was my first carbon fiber bell and I had concerns from the start of the build. First the bell doesn't fit and you literally have to grind off 1/2 of the thickness just to get it to fit inside the stainless. Either that or cut up the stainless into multiple tabs and slip it over. Both are bad given that the folks in Thailand that made the bell and plane should be able to make it so that it fits. But when CARF doesn't give a rats pa toot about quality it gets shipped regardless.

I did check and the turbine manufacturer says 22mm is the minimum distance between nozzle edge and stainless.

Absolutely inexcusable that CARF sells a TV setup that defies simple physics. Stainless expands, a cold carbon rod doesn't expand. Pretty simple concept.

My first and last CARF model.

Last I checked, they had both a sales and a support email listed

My turbine manufacturer say 25-30mm so that might be your problem…..there’s literally 100’s of these out there….

And going minimum distance isn't neccesarily always a good idea. Probably can get closer with aluminum vs carbon fiber. I'd have put it at close to max distance, myself. ..I tend to be conservative onnthose types of things until I see how it acts later. Just seems to me that if you have any object that purposely generates flame and heat that more is better at first...

Can always move it closer later, hard to move it closer if the plane went up in smoke..

The OP wants to blame CARF because he thinks a designer is going to design something that works for every single application. Its like saying a Chevrolet car should be able to use a ford engine direct bolt in without requiring modification. It don't work that way.

I'm betting, with a bit of exploration, we'd probably find a set of specs for that bell that states max temp, and I'm betting at 22 mm his turbine exceeds that threshold by a ton. The exhaist gas temp starts to really shed temp when it exits the turbine, but the closer to that nozzle you are, the hotter it is. During his bench run, might have been a goid idea to check it to figure out how to mount to keep the bell from melting.

Boils down to wrong application and lack of knowledge maybe? Or just in a rush to fly...

wondering out loud if you had more space b/w turbine and pipe, how much would that affect the tail temp being able to draw more cool air in the efflux.

This may be just a drunken rambling, but could you buy some gold heat shield, stick it inside the bell and end up with a nice, lightweight insulated product?

He posted this rant in the S.A.D. group on facebook as well.

His starter stuck engaged on the second start up. If you listen close to the vid, you can hear it.

The stuck starter was the beginning cause. He swore, up and down, there, the turbine ran fine on the bench after this happened (doubtful)

He also had the turbine at 22 mm from the bell, recommended from turbine manufacturer, thats the minimum distance on an ALUMINUM bell. No safety margin. At that distance its recommended that a water cooling sustem be used.

He got called out so hard by guys who flew turbines when you needed a propane tank to start them, the admins deleted the thread. CARF, upon seeing how everything was installed, put blame where it really was and told him they would gladly sell him parts to fix it, but no warranty. He was also overheating the tube, causing the thrust variation cool to warm. He's basically trying to badmouth CARF until they relent and replace his turbine and fix his plane for free. CARF won't because not manufacturer defect, but builder/owner error.

He's actually lucky the plane didn't turn into a fireball and take half his house with it.

Mr."I've owned five turbines" was in a rush to fly his new jet and didn't do his due diligence.

Wow! Speak of alternate reality trolling.. When you don't like what you see simply just start making stuff up. I'm expecting a proper straight pipe complete with aluminum bell to be delivered in a few days. I don't want anything from CARF. This is my first and last CARF.

Here's the distance from the less than stellar CARF PDF which spec's the turbine distance between 20-25mm and also tells you to start grinding the carbon bell to get it to fit the pipe. Simply piss poor quality control. I also find it really strange that CARF shows a picture of an aluminum bell in the set of instructions that talks about grinding and spacing their carbon resin bell. Is this just another sign of the outright lack of CARF quality control or just laziness on the part of CARF?

The turbine works fine post fire, just like I said before. If it didn't it would be on the road back to the maker. Keep up the trolling since you sound like the idiot CARF rep from Massachusetts who told me I destroyed the resin bell on my first run by idling it too long. :D Typical type of "don't blame us excuses".

And here's the email reply from Andreas Gietz in German where he acknowledges the problem with the TV change of centering with temp changes. He claims to have changed the TV design in the email but shows the same crap TV pipe on his website that is now collecting dust in my garage. ..


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wir haben bislang die Kohlefaser-Einlauftrichter ungelogen fuer mehr als 10,000 Modelle verwendet und nie Probleme damit gehabt. Die Temperatur in dem Bereich ist absolut kalt. Nur nach dem Abschalten des Triebwerks kommt dort etwas Stauwaerme auf. Diese bewegt sich aber im Bereich von 50 Grad oder so. Gar kein Problem fuer das Material.

Wenn dieser Trichter gebrannt hat, dann gibt es ein grosses Problem an Ihrer Turbine, die sicher einen ordentlichen Heissstart hingelegt hat. Hier sollten Sie beim Turbinenhersteller Rat suchen, wie das in Zukunft vermieden werden kann. Es ist keine Seltenheit, dass ein Heissstart passiert, ist aber immer die Folge einer Fehlfunktion des Triebwerks oder dessen Elektronik oder peripheren Geraete (shutoff valves etc)

Die Laengenausdehnung des Schubrohrs ist in der Tat ein Problem, daher verwenden wir dieses Schubrohr seit mindestens 3 Jahren nicht mehr. Wir haben nun einen getrennten Schubvektor, der an den hinteren Spant montiert wird und keine direkte feste Verbindung zum Schubrohr mehr hat.

Ich hoffe, Sie bekommen Ihre Probleme in den Griff. Wenn sie einen neuen Kohletrichter brauchen, dann koennen Sie diesen ueber unseren Shop bestellen, die Lieferung sollte kurzfristig moeglich sein.

Letztlich bitte ich Sie, umgehend das Video vom Netz zu nehmen, sonst sehen wir uns gezwungen, unseren Support einzustellen. Ihre Schlussfolgerungen sind inadequat und laden zu weiterer Spekulation der vielen Spezialisten in unserem Hobby ein, die letztendlich grossen Schaden anrichten koennen.

Wir sind in der Lage, Ihr Problem zu analysieren und zu loesen, insbesondere auch ohne die Oeffentlichkeit auf diese Weise einzuschalten.

Viele Gruesse
Andreas Gietz

You posted this rant in a total if ten forums, got ripped in every single one if them for a. Setting it up wrong, and b. Blaming CARF for your stuck starter motor which started it all, dude.

You simply failed to follow the recommendations by CARF and your turbine manufacturer, set up your turbine at the minimum distance from the bell according to turbine spec, no margin for safety for things like a stuck starter motor, and overheated the tube and bell in the process.
This was all fully explained to you by people on FB with the same aircraft, same tube and bell as what you melted, yet, you still try to bash CARF.

Maybe you would be better off sticking to edf conversions...I have never flown a turbine, or owned one, but even I, at first glance, knew the turbine install combined with that bell was not done right ...at all.

You can be mad at CARF all you want, but the person at fault is staring you in the mirror every morning.

Here is whats worse. You stated in your opening post that the instructions with the bell were for a distance to turbine outlet of 20 to 25 mm. You stated you thought that was too close, but instead of setting it at the recommended max, you set it at 22 mm,

Umm..its a matter if liosening the bands and slipping the turbine 3 mm..you didn't even have to drill holes..but instead of setting it at 25 mm and adjudting from there to closer, if needed, you set it to the minimum safe distance accirding to your turbine manual. Minimum safe would be the hottest part of the exhaust plume where an aluminum bell could survive. In your case, 25 mm would have probably been marginal at best. You failed to research enough to figure out how hot that bell could get and not melt. Take a match..light it, and hold it under your hand, theres a point where it goes from feeling warm to burning your skin. Now apply that analogy to your turbine and your melted bell. Its not rocket science. That turbine is a flame thrower and you put the bell directly in the flame by setting it at 22 mm spacing..it may have even survived if your starter motor hadn't hung.
How many people have told you this now?

Guys, what happens to a turbine if the starter motor fails to disengage and overheats...let it go long enough and chunks of starter motor start hitting compressor blades, but it still causes the turbine to run hotter than it should from that point on.
The only thing that saved your turbine from spitting blades was the bell melting from the overheating and you shutting the turbine down. Now, stop blaming CARF because things went sudeways.
1. Turbine too close to bell
2. Inadequate cooling
3. Starter motor malfunction.
Result, bell melted, almost lost your jet, your yard, but avoided burning half your house down. Congratulations.
The thrust line variation, they explained that to you too..your turbine being too close was overheating the thrust nozzle causing it to expand more than it nornally would. 3 guys in the fb thread told you that too that have that pipe IN THE SAME AIRCRAFT. And told you that was the first sign your turbine was too close.

I'm done. Go fix your jet and happy flying. Mind the spacing to the bell and check out your starter motor or we'll see a redux to this thread, soon.

Man, you're just too funny.... You just stated: "I have never flown a turbine, or owned one, but even I, at first glance, knew the turbine install combined with that bell was not done right ...at all."

For a guy who has never owned or flown a turbine you sure do seem to think you know everything about turbines and their install. You know the science of turbines literally down to the mm! :)

Why take my post so personally? Take a chill pill and relax. You just got to love the internet and the "experts" who populate it.

Over and out. The pictures and my original post speak for themselves.

They speak to the fact you know as much about turbines as you infer I do.

BTW, I showed your little video to someone who does commission RC turbine builds. His opinion was the same as mine. Your mistake, your cost.

I view this as no different than setting the conyrol surface throws, you work up to perfect so you don't end up slaming them past the limits and breaking things. In the case of a turbine, or anything else, big margin of error and dial it in. You failed to heed it. Pure and simple.

Night.