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Old Aug 25, 2005, 08:14 PM
321
mixed styles flight
central Florida USA
Joined Dec 2002
125 Posts
MC1 microfan rc jet design/build thread

I have a 50mm fan in hand that arrived today so now it's time to build a jet around it. It's going to take awhile, as it's my first design and scratch-build. My goal is to draw plans for the entire airframe and then build it mainly with traditional balsa and plywood.

Starting from the top, the first step was to draw a concept (two views of it are in my gallery). Then some questions about ducting on another thread, and a little shopping and the fan was chosen. The fan has been drawn up on the computer and now the trick is to fit the concept to the fan.

Already a few problems have cropped up. When the plane was resized, the wingspan turned out to be all of 22" with a 107sq. inch wing area. Too small, and not enough wing for the 14 to 16 oz. I'm expecting this bird to be. After all, I want to handlaunch. So, some taper had to come off the wings, which also had to get a little bit larger. They're up to 26" and 146 sq in. now, but only after some serious reworking and repositioning to maintain a "jet" look. No wonder why models are often so ugly - it's easy to make them that way.

The next thing that became apparent, considering FSA rules of thumb, was my intake ducts were much too big! They had to be narrowed down somewhat, causing the fuselage shape to lose some of its angular look. Oh well, it should streamline the fuse better. Building these little ducts could prove difficult and I must keep this in mind when deciding how the plane will be framed and assembled.

Anyway, it's back to working on the plans.
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Old Aug 25, 2005, 09:20 PM
EDF rules... :)
AirX's Avatar
Joined Nov 1999
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Hi 321,

Just signing on to follow your design and build...

Eric B.
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Old Aug 26, 2005, 04:13 AM
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yeah pics..please
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Old Aug 27, 2005, 09:15 AM
321
mixed styles flight
central Florida USA
Joined Dec 2002
125 Posts
Quote:
pics...please


The top drawings are the first draft. An interesting look, but I felt the nose could create turbulence and it was pretty lean in terms of interior space for the radio gear. The intakes were also too large and their angular side profile extended the duct unnecessarily.

The lower drawings further refined the outer shell. What I'm doing is working on one area at a time (basically from front to back) using 3 views to make improvements on the areas of function, ease of construction and appearance. There's already some more changes, especially on the tail, but thankfully the outer shell is close to its final form now.

This morning I've been playing with a 1/16" sheet of balsa and a 1/32" sheet, trying to figure out which will be best to line the duct with. The 1/32 has won my vote. It will be lighter and much easier to build with. The risk is it will be more prone to collapsing under the vacuum. But from what I can tell, with the wood arched cross-grain, it is pretty stiff. Unfortunately this is not very scientific, but sometimes you've got to give things a try. What I could do later on to avoid a terminal design problem is build the duct and test it with the fan at full power before applying the outer skin to the fuselage. That way, it could be strengthened if it had to be without compromising the duct area.
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Old Aug 27, 2005, 09:28 AM
Airx at work.... :)
Joined Nov 2004
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321,

Looks great, been folowing along on the inlet thread as well. Hopefully I can have something to post from my project tomorrow.

Cheers,

Eric B.
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Old Aug 27, 2005, 11:06 AM
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orlando fl
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I like it, esp the new nose, very nice. One thing to keep in mind is the wing incedence, I would advise you to make it easy to alter, not much, but allows a little fine tuning of the flight atitude. It almost looks like your slightly negative, with a sym or semi sym airfoil I would think a little positive would work better. Keep it coming.
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Old Sep 02, 2005, 10:30 PM
321
mixed styles flight
central Florida USA
Joined Dec 2002
125 Posts
I spent a lot of time working out the inlet dimensions. I was shooting for 120% of FSA at the lip, but with a minimum 3mm (.12") radius on the lip as recommended by Wemotech, the throat area had to be squeezed smaller than 90% of FSA - something I felt uncomfortable with. In the end, I decided on a 90% FSA throat area, resulting in a 130.5% of FSA inlet lip area. The way the duct is planned out now, it will be 130.5% and 90% as stated above, diffused to full diameter of the fan and then squeezed to 85% of FSA at the nozzle. It's looking like the nozzle will be removable and so there will be an opportunity to experiment with its diameter later on. An Excel spreadsheet helped make the area calculations.

More and more, the question of where the balance point will be keeps coming up. To come up with an approximation, I decided to tile print the outline of the shell and use it to make a cardboard mockup.



The printout at actual size.



The cardboard mockup. Some pennies were added to the nose to keep it from pitching up. It flies surprisingly well and I'm pleased with both the appearance and size. Note the mockup is mid-wing for ease of construction but the model will be highwing.
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Old Sep 03, 2005, 07:23 AM
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looks good. have you got the layout of the servos, pack and rx worked out?
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Old Sep 03, 2005, 11:46 AM
321
mixed styles flight
central Florida USA
Joined Dec 2002
125 Posts
Quote:
have you got the layout of the servos, pack and rx worked out?
So far, only a general plan. The front fuselage pod is purposely wide enough to allow a lot of flexibility with battery placement fore and aft to be used as ballast. The receiver will be as far forward as the wiring will allow to keep it away from the speed control, which will be at the very rear of the pod and closest to the motor. Which reminds me the motor wire routing can't be overlooked. If need be, there is enough room to stack battery and receiver (3s).

I'm considering different wing airfoils and have it narrowed down to two different ones. On this plane, the choice of airfoil will affect the shape and space available on top of the fuselage, important for servo and linkage placement. Also, I have to decide whether to make the wing removable or not, so the aileron servo is still a big question mark at the moment. It's a critical time in the design.
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Old Sep 15, 2005, 11:59 AM
321
mixed styles flight
central Florida USA
Joined Dec 2002
125 Posts
The airfoil is going to be a symmetrical naca 0010-1.50 or similar (10% airfoil). Hunting it down was fun and educational. Is it the right one? Heck, I don't know but on some calm day in the future we'll find out if it's in the ballpark, right? (!)

There's been no idle time and some of the fuselage formers have been drawn, starting at the nose, and each one was a challenge even though the stringers were being omitted for simplicity. The task at hand was and is to finalize the fuse former shapes and dimensions and to arrange the sheeting, deciding on thickness and any sand-to-shape requirements all the while.

A number of questions had to be answered to proceed with the mid-section formers:

Removable wing? (yes; it doubles as a servo access hatch)
How can it be built? (in five main sections - front pod, right and left mid-fuse/intake ducts, wings and rear fuse/empennage; each buildable on a flat surface)
What will hold it together? (stringers and sheeting with cross-grain orientation over connections)
Fan installation? (underside access hatch)
Canopy? (lexan bubble on balsa sheeted frame)

It all sounds good, but there is growing complexity and the trick will be to keep the weight under control. So, weight will be considered every step of the way.

One thing that remains unanswered is whether to use a single aileron servo or an outboard pair? There's room for either setup. Any thoughts?
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Old Sep 15, 2005, 01:39 PM
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orlando fl
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out board pair, weight is negligable (5gm servos), eliminate a single point of failure, less complex as far as route for torque rods or cables, less intrusion on valuable fuse space (spread the componets around as much as possible, that allows more adjustability of cg thru pack position), prob better control resolution (esp if you used a thin cable single servo setup like I did on the MB v1-3)

I plan to do on the next micro bandit wing with twin nano size servos and solid pushrods.
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Old Sep 18, 2005, 07:39 AM
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any updates?
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Old Sep 18, 2005, 11:56 AM
321
mixed styles flight
central Florida USA
Joined Dec 2002
125 Posts
Dave, outboard servos sound good. They will open the door to some extra flight configurations as well.

The ducting shape had to be determined in order to draw the fuse formers. I used the blend feature of CorelDraw to do this and it was a son of a gun to figure out both the process and how to join the left and right halves (they converge at the rear at a small angle) without messing up the sheetability of the forms (the price of compound curves). The main trick with the software was to manually match up the vector nodes on each of the end forms for linear "interpolation" where it needed to be linear. Aft of the front fuselage pod, linear wasn't possible on one side, so some manual modifications were necessary. Nothing fancy, an elongated final form was used and then the forms aft of the pod were trimmed to match up with the fan.

The software generated profiles, before the aft section was manually trimmed:



I created a large set of 50 profiles in order to have plenty of flexibility when positioning formers. Obviously in the end, all the spares simply get omitted. You CAD guys out there - please don't laugh, this is the software I know at the moment and have at my disposal.

Now the three view drawings include some of the sheeting and most of the formers. The former front view profiles are done up to F5 so far.

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Old Sep 18, 2005, 01:45 PM
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what is the inlet area FSA %? it looks like a gradual increase in area through out the cross sections, as opposed to a constant area ruled duct, (could just be an optical illusion). Are you planning to terminate the split in the duct well ahead of the fan or are you planning to keep the ducts separate right up to fan? How about the spinner? on some models I have built in a tube that matches the od of the spinner, then blended it into the duct work, on others I simply notched the last 1" of the duct splitter (sorry) to clear the spinner.

Looking good.
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Old Sep 18, 2005, 03:17 PM
321
mixed styles flight
central Florida USA
Joined Dec 2002
125 Posts
As drawn above, the inlet is 90% (divide by 2) of FSA. But a little 1/8" radius on the front lip makes it 130% <gulp>. Then the duct gradually grows (diffuses) from 90% of FSA to the full diameter of the fan. I'm a little surprised by the wemo spinner, which is rather flat-nosed. It's going to cause a rather abrupt reduction in area, but I can't see any way around it short of mating it with a tube in front from the front fuse pod - as you mentioned. But for now, this will make the model a little more difficult to build and instead I want to make it as simple as possible to begin with.

The fuse pod ends 1/2" in front of the spinner. Me bad? If that needs to be reworked, I would prefer to take the pod all the way to the fan and make a cutout for the spinner, rather than terminate it further forward and sacrifice space for radio gear or the speed control.
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