Just curious, why do we use carbon fiber pipes instead of spun aluminum pipes? Are they lighter, no aluminum pipes available?

Thanks,
MM

My understanding is weight

Quote from: Motorman on March 27, 2026, 08:34:38 AMJust curious, why do we use carbon fiber pipes instead of spun aluminum pipes? Are they lighter, no aluminum pipes available?

   Much lighter.

    Brett

   What I always wanted to see, was what the carbon pipes looked like on the inside. Especially as the designs progressed and changed. Now that things have evolved more and more electric is being flown, maybe there are some older ones that are not usable any more, and could be cut open so the guts could be seen? The later pipes that had a blunt end and a angled stinger for the exhaust kind of reminded me of the old Hartmann Blimp pipes that we ran on racing karts. They were pretty quiet but really helped with the power delivery. Tuning the Blimp was a matter of different length flew tubing sections between the header and the pipe for the different tracks we ran, so some similarities there.
  Type at you later,
  Dan McEntee

Were the aluminum pipes the real thick ones from Macc? I'm looking at the thin spun F2A pipes and hard to believe CF would be much lighter.

Quote from: Dan McEntee on April 09, 2026, 08:14:44 AMWhat I always wanted to see, was what the carbon pipes looked like on the inside. Especially as the designs progressed and changed. Now that things have evolved more and more electric is being flown, maybe there are some older ones that are not usable any more, and could be cut open so the guts could be seen?

    I have done it, the texture/surface finish looks like carbon weave/epoxy pushed up against a rigid mandrel until it set. It has some baffles (usually 3) with holes in them glued to the inside. Eather pipes are exactly what you think from looking at the outside, too, and there is an aluminum insert with a nest of baffles spaced on standoffs poking up about 1.5-2" in the back. Whether it was intended that way or not, the multiple baffles create a series of closely-space reflections that have the same effect as the tapered tail cone on, say, a Randy/Derek pipe.

    The only other variant is the stinger. Some of them look like they do on the outside, and are just a tube flush to the interior to let the gas out. Some of them extend up into the pipe by some amount (like an inch or more) to try to pick up the exhaust but avoid the combined effects of the direct pressure wave and the reflected wave, which *sometimes* makes it quieter is you can put it at the right depth so have them at a local node where it cancels the incoming and outgoing waves.

    The smaller you make the stinger (for a given pipe diameter) the stronger the reflection and the stronger the tuning effect, but with reduced exhaust flow. There is some optimum combination of stronger return waves VS exhaust flow. Increasing the pressure by restricting the flow also raises the density and the temperature of the exhaust in the pipe and raises the acoustic velocity, which alters the resonant frequency. For example, if you take an optimal system, and then just restrict the stinger, it is also now out of tune (effectively too short) and will have more boost/brake in the maneuvers. You have to pull it out to get the same engine response.

     There is nothing very mysterious about how are are made or how they work, it's just an adjustable echo chamber. What you want the engine to do (and thus how you want to adjust it) and how to tell from flying it what to adjust to get the best score is another far more complicated matter - because most people don't know how the engine response affects what they feel at the handle or how they react to it. The overwhelming tendency is to run *far too much* boost and brake because it makes the engine feel powerful at the handle and you know it is doing something, OR, to get misled by whether it is breaking from 4-2 and back or not.  You can have WAY too much boost/brake while the engine note hardly changes - the classic 40VF systems are usually described as "constant speed" because they stay in a two-stroke the entire flight - but can go from almost no response to  excessive with 1/2" of pipe length adjustment - and from the outside listening to it, you can barely tell it is doing anything. 

   The other common issue is the same as any new "feature" - once you get the "new" thing, it becomes the focus of your activities, and then everything that happens is somehow related to pipe length. So people spend hours pushing it in and out trying to fix every percieved engine run problem they have. Once you have it right and don't change anything like above (stinger length/diameter, running RPM) the pipe length is the very last place you need to look for adjustments, the optimum doesn't just change for no reason. Until I started messing around with the stinger again recently, I ran the same pipe length for my Jett 61 system since 2003 and while I did experiment with it, the preferred length for my pipe and prop setup hasn't changed once in well over 20 years.

     Brett

Sure you could do an aluminium pipe with more or less same weight, but sheet metal/spinning work requires quite heavy tooling, and brazing/welding so thin walls is not an easy task. Carbon composite is much easier for small series/cottage industry. You can wrap the carbon around a banana if you want, like Windy put it.
Another thing is that Aluminium has quite a big thermal expansion coefficient, when carbon has it near zero. That has an effect on acoustic length/volume. That is one reason why they are moving/have moved from aluminium to galvanically made nickel pipes, or bar stock-machined steel pipes in F2A. Also the very good thermal conductivity of aluminium may well be counter productive in stunt.
It's a little sad that IC is slowly dying away because I feel that pipe development is not quite ready yet. It is absolutely the best way to go, but now when it works well I'd like to see some improvement in the acoustics, too. L

Quote from: Lauri Malila on April 10, 2026, 01:11:32 PMI'd like to see some improvement in the acoustics, too. L

   Of course "improvement" is in the eye of the beholder. I don't think the sound hurts *at all* at the Nationals, it may actually help. FAI, you would get killed, but that is a small issue for a few people.

    The system and pipe I have been running for many years is on the loud end of the tuned pipe scale, but still under the FAI limit. David (and others) says it sounds "angry" and the static thrust, is, reportedly "like a tractor" - even by people used to launching PA75s and electrics. The only one that is in the same ballpark is the Frank Williams "no baffle" pipe that Doug Moon was running, it is substantially harsher than mine although probably not any louder in absolute terms.  The 40VF is way quieter but gets *really tiresome* listening to 20 flights in a row (which often happened in the mid-90's).

   I have experimented in the past with an aftermuffler and have recently been looking for one of the old molded silicone rubber FAI combat mufflers to use as an aftermuffler, just to take the edge off. Past experiments worked.  But, as you say, that is like studying advancements in buggy whip technology.
 
    Brett