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Groundhog Gravy

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Question: What is the currently preferred airfoil for Fleas?
The HM-293, like most recent Fleas, uses a NACA 23112. If you look at the moment coefficient, it doesn't change much except at extremes of AOA. It's what I'm using on mine. Of course, if you do the moving rear wing, that's probably less important, as you'll have plenty of pitch authority — maybe more than you want! Is that still your plan?
 

Protech Racing

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Any of the reflexed foils will work. The Fraser 15 , the 23112 are often used . I chose the 43015 due to it's thicker section near the TE and my foam ribs. I thought that I could use the extra 1in of foam for a little stronger rib.
 

rtfm

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Hi guys,
Thank you for taking the time to answer my question. I'll take a look at both the 23112 and the 43015.

Cheers,
Duncan
 

rtfm

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<snip> Of course, if you do the moving rear wing, that's probably less important, as you'll have plenty of pitch authority — maybe more than you want! Is that still your plan?
The current plan (given my decision to configure the FleaBike as a bog-standard Flea) is to make the rear wing incidence ground-adjustable so that the pilot can experiment, and then choose for himself what incidence he prefers.
 

rtfm

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Hi guys,
After reading Orion's little paper about airfoils, I decided to go with the NACA 747A315 (as I mentioned elsewhere). It is a very different looking airfoil, with max thickness way back at 40% C but it has almost ZERO Cm over almost the entire flight, only increasing slightly above 8 deg AoA.
Now, question time:
My 20mm Dow Blue foam ribs are spaced 250mm apart and my huge D-tube (right back to the spar at 40% C) is a wrap-around sheet of 1.5mm Gaboon ply. First question: Is there a need for some sort of leading edge reinforcement? This is the screenshot from DevWing (a brilliant piece of software BTW)
1616710592505.png
As you can see, it insists on some sort of LE strip. It gives me five Type options and three Placement options. I need some advice here. My gut feel is to go with the Round option?

Second question:
Placement: Is there an optimal placement for the 747? I don't want to bugger up the LE by shoving a circular bit in the wrong place. And I don't know what diameter to use either.

Thanks in advance,
Duncan
 

Tiger Tim

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See if there’s any significant difference in output for all the leading edge options but I would think that ‘stock’ is probably the closest to the sheeted leading edges we’re used to seeing on full size aircraft. All of the options given look like popular ways of making a leading edge in models and I suspect that your software assumes some amount of covering sag aft of the LE and between ribs. Leading edge stock (it comes pre-shaped for models) maintains the airfoil profile the farthest aft of all the options.
 

BBerson

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Strip is simple if the plywood is wetted and molded and wrapped all the way around. The strip is just to hold the ribs in place.
 

rotax618

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I would plan on using intermediate ‘D’ ribs, they wont weigh much in foam but will support the ply, which from my experience is prone to buckle as it expands due to moisture and spar bending loads.
 

rtfm

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Hey Rotax - that's a good suggestion. Still use the LE strip though?

A good local builder mate (well known on this forum) warned me off my terribly cool wing I'd designed, and strongly suggested I keep it simple and go with a Hershey bar planform. So I compromised, and produced a slightly tapered wing.
1616732040287.png
Those ribs look awfully "busy" though...

Is this what you mean by intermediate D ribs?

Here's the whole wing...
1616732634427.png

Duncan
PS 5m^2, 13.5kg all up.
 
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Victor Bravo

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Duncan, look into using a carbon fiber tube as your leading edge, underneath the plywood.

You can steam and form and mold the plywood around this carbon tube ahead of time, to match the tube exactly. You will not only get a perfect glue joint, but when the time comes to put on the plywood permanently... it fits perfectly with no effort, no Herculean tasks of stretching and clamping, no desperate battle against time while the glue sets up, etc. etc.

The carbon tube will make the leading edge much more dent resistant and damage tolerant compared to Spruce bars, dowels, strips, etc.

Lastly, you will be guaranteed an even leading edge radius that is uniform. The thin plywood can crack easily if it is not supported, every time you bump into the wing or roll a toolbox next to the aircraft is a potential for a crack in unsupported plywood.

The front of the ribs is easy enough to make with a semi-circular "cup" shape, which will give you a slightly larger/stronger glue bond line. The cupped ribs will also guarantee your leading edge is straight, and that there is no accidental twist built into the front of the wing.
 

rotax618

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Good luck getting the Gabon ply to bend around the tight radius, I tried everything when I built my fles and ended up using a shaped timber moulding and putting it on as top and bottom sheets, my outer wing panels were elliptical, but I was unable to get the ply to bend around the straight centre section.32002287-2797-4CD8-98B4-3CD6A2BC31A4.jpeg
 

rtfm

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Hi Rotax618,
Interestingly, I left all my scrap plywood outside the shed and they got rained on recently. REALLY rained on. When it came time to chop the pieces into manageable chunks so I could put them in the dumpster, I tried to break the ply by simply bending it. I was amazed that even 4mm ply bent almost 180 degrees before it broke. So I'm hopeful that with some encouragement (i.e. hot water, cloths, an iron etc) I'll be able to bend my 1.5mm Gaboon over my formers. Wish me luck.

Duncan
 

rtfm

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A question for all you Flea-bitten,
Does anyone know what the hell is going on inside the red circle of this pic (taken from the HM360 manual)?
1616799979931.png

It seems that the control rod is attached to a double-sided lever, with two pull-pull cables, which run through an eye, and then pull on a double lever (hidden) somehow attached to the wing control rod. But the details are not apparent. Anyone actually built a HM360 with this control arrangement? Or perhaps someone here with more engineering visualization skills can help me out.

The "normal way to control the wing is to connect the control rod directly to the wing, like this:
1616798956088.png
...but being a sit-on fuselage, my legs which are on the outside, get in the way, so I can't use this method. I could, of course, extend the control torque tube six inches or so and slip my legs between the control rod and the fuse. But it's a bit constricting...

1616799789861.png

Thanks,
Duncan
 

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rtfm

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Perhaps something like this? Streamlined rods placed snugly against the fuselage so as not to interfere with the legs too much, and from the second lever, the control rod can angle outwards where it doesn't interfere with either my legs or my vision? I could angle the control horns in such a way that the movement of the initial rod is almost in a straight line.

Just trying to keep things simple...
1616801177333.png
 

rtfm

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Duncan, look into using a carbon fiber tube as your leading edge, underneath the plywood.
Hi Victor Bravo. Great idea, but not cheap. 2m lengths of 20mm CF tubes go for about $140.

I understand that the guys use aluminium tube for this? Cheaper, but no real bond with the ply. The alternative is to use 30mm pine dowling - cheap, good bond with the ply, but not guaranteed to be straight, and heavier. There are no simple answers when designing a plane...

Duncan
 

Tiger Tim

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A question for all you Flea-bitten,
Does anyone know what the hell is going on inside the red circle of this pic (taken from the HM360 manual)?
Here’s the HM380 at the Lane Motor Museum without the fairings shown on the plans. I assume the point was either streamlining or to use up a spool of wire Mignet had kicking around because yikes that seems needlessly complicated.
 

Malcolm C

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I made a shaped bending form for my Fleas leading edge, soaked the 1/16 th birch ply in cold water for a couple of hours and poured boiling water on the extreme radius once it was nearly there. Had one crack at the end but was able to trim that bit off. Glad to lend the form to anyone who needs it, I am in Torrance and it's much too heavy and bulky to ship.
 

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