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Old Jul 15, 2012, 10:29 PM
Thermal, where art thou?
BavarianCharles's Avatar
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Originally Posted by G_T View Post
...
If a custom cut for the opening is needed, then I'd need to know that. The fuselages come with the required 6-32 screws of appropriate lengths.

All this is just FYI. Gerald
Hi Gerald,

are you selling fuselages to the general public, or are these part of a kit?

Cheers, Charles
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Old Jul 16, 2012, 12:59 AM
G_T
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Hi Charles,

General public. These fuselages are a perfect match for an Edge-2P or Edge-4P (I still like flying them, though it takes a while to learn how - they grow on you). Or a JJEdge wing that needs a new fuselage. Other wings will require a bit of splooge.

I have a few reasons for making these. (1) Get something durable out there for builders to use (2) Get good with the process of making these (3) See if I want to make fuselages for a kit of my own design (4) Get a little cash return for my effort.

I've now gotten the quality to be minimally good enough IMHO (better than average in the industry) though they are not perfect and probably never will be.

So far, even though the price isn't cheap, it isn't worth my time to do the work. Too much effort goes into each fuselage. So I'm working to streamline the process.

Were I starting again, there are some things I know to do which would save time. Use a semi-permanent mold release. It is faster to apply than PVA, doesn't show as much in the part, and dries much faster. So that would save a bit of time. CNC mold with hatch recess machined in, or a separate mold for a nosecone. That would save a bit of time as well. Different fabric selection - that will be happening when I run out of fabric relatively soon and end up placing an order.

Unfortunately one of the things which would help is a fatter fuselage. I'm not willing to go that direction. I don't want the extra drag and I don't need to do it for stiffness. Really I don't even need to do it for weight... Mine are getting lighter as I go along, but they are still super tough compared to most. That toughness is really what is costing the small weight increase compared to some other fuselages out there. I could drop a gram in a heartbeat, out of the tailboom (best place to drop weight!), but then the tailboom would be as fragile as most others out there. It would still be stiff enough. It would even be easier/faster to make, probably saving me 10 minutes per fuselage.

Actually, all of you paying attention to this thread can let me know if you would like the benefit of a little lighter tailboom at the cost of a tailboom which is not shatterproof. Personally I doubt it is worth the tradeoff for the majority of pilots. I'm not even sure I'd make the change. But I am curious what you think.

I could certainly reduce fabric up front, but then it would be as weak as most other fuselages out there.

Reducing fabric layers would be the biggest thing that would reduce my production time. Fewer pieces to cut; fewer pieces to apply. There have been fuselages out there that have gone that direction. If they are light they are fragile. Right now I have 6 pieces of tow and 26 pieces of fabric - soon to be 27. Plus some stuff that goes into the nose to make it tough, and two batches of specialized splooge. The prep time for one of these is a killer. Every Kevlar piece has to be cut with scissors (7) ... At least the glass and carbon pieces can be cut with a rolling disc cutter that looks like a high-tech pizza slicer.

The mold generally gets waxed every other fuselage and that is another thing which takes time.

How long I continue making fuselages available will depend on how fast I can make the process.

Gerald
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Old Jul 17, 2012, 01:35 AM
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Gerald,

While your fuse isn't as skinny as some, I'm one that would vote for a "fatter pod". I have no way of calculating the tradeoffs, but the difference in frontal area that a 1/4" wider pod would make has to almost be lost in the "noise" level of the total drag of the airframe.

Installation and repair of equipment would be so much easier.

Just my opinion.

Gary
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Old Jul 17, 2012, 06:57 AM
Thermal, where art thou?
BavarianCharles's Avatar
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Quote:
Originally Posted by G_T View Post
... These fuselages are a perfect match for an Edge-2P or Edge-4P (I still like flying them, though it takes a while to learn how - they grow on you) ...
Gerald
I'm assuming it will accomodate a Zone V2 with a root chord as published. BTW, which planes are you flying - Zone V2 with Edge tails?

- Charles
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Old Jul 17, 2012, 07:12 AM
Father of Fr3aK, DLG Pilot
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USA, OH, Worthington
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Originally Posted by GaryO View Post
Gerald,

While your fuse isn't as skinny as some, I'm one that would vote for a "fatter pod". I have no way of calculating the tradeoffs, but the difference in frontal area that a 1/4" wider pod would make has to almost be lost in the "noise" level of the total drag of the airframe.

Installation and repair of equipment would be so much easier.

Just my opinion.

Gary
I'm testing exactly that right now Gary. I just finished a "V3" Fr3aK fuse that is big enough for four hyperions in the pod, plus a receiver, bigger battery, telemetry module, and altimeter. It looks huge but when compared to my old fuse (which is moderately thin) there's not a big difference. We'll see very soon how it flies.

BTW Gerald, good work on the fuse. These are the hardest part of a molded DLG to make, with (at least for me) the highest scrap rate. Tails are a pain to do "right" but it's quite easy to make flyable tails. Not so much the case for fuselages.
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Old Jul 17, 2012, 09:51 AM
G_T
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People would have thought at one time that the drag of control surface linkages would also be lost in the total noise... The lower drag the plane has from other sources, most notably at high speeds, the greater any single drag source stands out. Drag gives you sink rate and costs L/D.

The flow past the widest point of the nose of the fuselages is going to be turbulent. On a poorly designed fuselage, or one that is rough, it will likely be turbulent even in front of that. That turbulent wake has drag, and it has width. It hurts the performance of the wing and tails downstream as it guarantees they cannot have any region of laminar flow. Think of it as the equivalent of having very dirty surfaces. On the Supergee-II the wing was located above the turbulent wake from the fuselage. There are costs associated with such an approach, but it has that benefit.

Even just from a surface area perspective, one is adding a fair bit when one goes fatter.

During a turn, and particularly during a launch, the fuselage does not present a perfectly point forward perspective to the airflow. On launch, there is the fishtail. During a turn, the relative airflow is curved. Bigger does not have to be draggier for arbitrary shapes but smaller of the same shape will be less draggy.

Gerald
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Old Jul 17, 2012, 10:09 AM
Just fly it!
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Originally Posted by G_T View Post
The flow past the widest point of the nose of the fuselages is going to be turbulent.
Why do you think that? Is the flow past the widest part of an airfoil turbulent?
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Old Jul 17, 2012, 10:23 AM
G_T
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I think that from some wind tunnel data I saw perhaps a decade or two ago. It is in some ways a harder problem than the wing, as the fuselage loses size in two dimensions rather than one as is the case for a wing. We also have constraints on nose radius which are not aerodynamically optimal. Had I the tools that I thought would do the job, I'd be designing the fuselage much the same way I design wings. Perhaps someday...

If one just sketches up an airfoil, then likely its flow isn't going to be laminar much past the thickest part either, not towards the root of the wing.

Gerald
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Old Jul 17, 2012, 10:27 AM
G_T
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BTW, the bolts I send out with these fuselages are steel. As such, they are not particularly light. One of the easiest weight savings one could do is replace them with some aluminum or titanium bolts. One may have to use a spot of anti-seize compound for titanium. The aluminum ones might be ok without this, if they are anodized and the anodizing hasn't worn off. I don't recommend nylon as even in larger sizes it would allow the wing to squiggle around on the throw, robbing power.

Gerald
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Old Jul 17, 2012, 10:44 AM
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I have access to Star CCM. Do you know how well it handles low Re flow analysis? I'm not familiar with it, but as a student I can access it for free. When I have some time I could learn it and run some analyses on fuselage shapes.

Brandon
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Old Jul 17, 2012, 11:15 AM
G_T
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From what I can tell, it looks suitable for at least a first order analysis. It may not get the transition quite right, but it does model transitions and it apparently has multiple options for the turbulence model it uses. So it should be better than just using a panel method with inviscid assumptions which are pretty worthless for our uses.

Gerald
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Old Jul 17, 2012, 12:31 PM
Aurora Builder
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Gerald,

I too have access to some basic CFD programs. The primary software program I have access to is CMARC (http://www.aerologic.com/cmarc.html), which is better than the freebies, but getting the model loaded into the software is a pain and a half as all the points must be defined in text files (no CAD import).

The steig fuselage was designed in CFD and tested in the wind tunnel, at least according to the designers. Not sure how many iterations they went through.

-Sam
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Old Jul 17, 2012, 12:42 PM
G_T
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CFD if it doesn't handle the viscous effects correctly won't be worth much for us. From what I've seen, the results tend to be way off.

I actually have access to a small demonstrator windtunnel but it isn't big enough to be all that good for the job and the turbulence level will be a bit on the high side. I could get a fuselage in, though I'd have to cut off much of the tailboom. But the tunnel is small enough that a wing section would alter the flow too much. If it weren't for these limitations I'd have used it a year ago.

Hmmm, perhaps I'll do something with it when I get around to finalizing my next plane design (don't hold your breath, it may not be soon).

Gerald
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Old Jul 17, 2012, 02:36 PM
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Gerald, when it comes to simulating low Reynolds number flows I would rather trust the results of a wind tunnel over CFD. I am not up to speed with the latest research in this area, but from what I remember from 2/3 years back was that a LOT of effort was being put into this area. Most of the interest came from the need to study low Re flows found in micro air vehicles. Researchers were using fairly complex CFD models, but even these had limitations when it came to accurately modelling general flows at low Reynolds numbers.


Most decent commercial CFD packages (read Fluent/CFX/Star-CCM+/Star-CD) have some ability to model laminar to turbulent transitions. WIthout getting into too much technical detail, they typically use either a Spalart-Almaras or a variant of the k-omega model to account for the effect of turbulence. The problem with both these models is that while they can predict laminar-turbulent transitions they cannot model relaminarization adequately (if at all).

While you may get reasonable accuracy at low AOA, you will not be so lucky at higher AOA as the turbulence model cannot resolve the flow details adequately. Even at low AOA you will have to be careful in setting up the CFD model. This raises another question: how do you know that you have an adequate CFD model in the first place? Normally this is done by comparing the simulated results to reliable experimental data obtained from wind-tunnel measurements. The bottom line is that you have to tread VERY carefully if you want to get meaningful results.

CFD is one of the better GIGO (Garbage In Garbage Out) machines out there. I also find it amazing how people will readily believe the colorful plots this method produces without questioning the results. I am not saying that you cannot get some use out of CFD for our problems, just that you have to do so knowing that the models used are currently far from adequate (at least the ones we have access to).

Anyway, just my 2 cents. What was this thread about again?

Regards,
Imraan (who likes calling himself the PPE - Pretty Picture Engineer
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Old Jul 17, 2012, 05:06 PM
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Spoken like my FEA teacher in college...

James
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