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Scratch Built me-262

Old 07-04-2006, 06:57 AM
  #1  
CraigO
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Default Scratch Built me-262

I am struggling for inspiration fellas. give me a hand
i am about 1/2 way through a 1350mm span scratch built 262.

I am looking at a weight of about 2.5kg with 2 wemo mini fans and the het 2w 20 motors and 2 nice new 3700MAh 4s packs for grunt. i thought you cant build a 262 this size and not have retracts so off i went and built a nose retract to suit and i am using springair 603's for the mains.
all nice and dandy but i started to look at the wing loading and the wing cube loading and it was pretty high.

The first foam core wing i built was a disaster so i built a second. that is getting heavy, the wing alone without aileron servos weighs 300Gms. so 350 with servos and it hasnt been glassed yet.
It also didnt have flaps and looking at the wing loading i thought i would build a 3rd wing this time with flaps. To be scale the flaps need to be fowler type so they move back and down, increases the wing area, lowers the wing loading and stall speed for landing and take off and increases the lift.
I am about 1/2 way throught the first panel of the built up wing and i am running out of inspiration. I have put a fair bit of effort into the cockpit to make it look the part and have pulled the canopy i need and at the moment it is all seeming like a big hill to climb.
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Old 07-04-2006, 04:50 PM
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Bill G
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Originally Posted by CraigO View Post
I am struggling for inspiration fellas. give me a hand
i am about 1/2 way through a 1350mm span scratch built 262.

I am looking at a weight of about 2.5kg with 2 wemo mini fans and the het 2w 20 motors and 2 nice new 3700MAh 4s packs for grunt. i thought you cant build a 262 this size and not have retracts so off i went and built a nose retract to suit and i am using springair 603's for the mains.
all nice and dandy but i started to look at the wing loading and the wing cube loading and it was pretty high.

The first foam core wing i built was a disaster so i built a second. that is getting heavy, the wing alone without aileron servos weighs 300Gms. so 350 with servos and it hasnt been glassed yet.

It also didnt have flaps and looking at the wing loading i thought i would build a 3rd wing this time with flaps. To be scale the flaps need to be fowler type so they move back and down, increases the wing area, lowers the wing loading and stall speed for landing and take off and increases the lift.
I am about 1/2 way throught the first panel of the built up wing and i am running out of inspiration. I have put a fair bit of effort into the cockpit to make it look the part and have pulled the canopy i need and at the moment it is all seeming like a big hill to climb.
Pics, man!?

Scratch built 4 of these so far. Some fly, some don't, some I know better (beautiful little 27 incher), and 1 I gave away to a guy who claimed to get it in the air.

I would go as light as you can, and make it a grass lander. I know this sounds odd, but these twins get heavy faster than an Oprah cycle. The retracts add weight real fast. Another thought would be a nose retract only. You may actually need the nose weight. The mains don't look that bad when down, its the nose that looks bad down, in flight. Never seen a half-retract plane, but it would make some sense with this plane.

I just built a new foam core wing for my large sheeted fuse foam 262. The first one got heavy fast. 20+oz with motored EDFs and ESCs on the wing too, is still just too heavy. I used CF rods asd spars for reinforcement, and didn't sheet this new one. I actually covered it with Coverite, basically weightless. Paints fine, but does chip a bit. I've found a very thin coat of semi-flat clear makes any finish tougher, and adds little weight. Point is you just can't get this twin light ENOUGH. I'd forget the glassing and sheet with the lightest 1/32 balsa you can find, and adding something like the Coverite if you want a sheeted/glassed effect. Same thing goes for the flaps, they're nice, but weigh.
Consider an off-scale wing. I know it deters looks, but the high taper ratio wing and the nacelles reduce wing area, fully effective wing area, and stability. Concentrating on good looking nacelles makes a better looking 262 than a perfect scale wing. At least this is what my latest wing rebuild tells me. I've extended wing surfaces, which works fine, as long as they closely follow the airfoil. Now a slightly different airfoil, but just needs to be a correct airfoil.

I've built rolled sheet nacelles, light cardboard core nacelles, but I'll just buy pre-made foam nacelles from now on. Nacelles are a weight killer, and just can't be light enough. My analogy is that if it wasn't a 262, you could cut a tiny bit off the nose, mount a small outrunner, and be down the road 10oz lighter, less the twin EDFs and nacelles, with a much smaller battery to boot. I grafted together 2 sets of GWS A10 nacelles for this rebuild, and made the farings with foam, shaping near perfectly to minimize filler. Normally I would have sculpted a bunch of light balsa for this, but again, still not light enough.

Canopies weigh. Those Sig canopies (and others) are nice, but weigh about 1/2oz, on a plane this size. I may strip mine and use a GWS A10 canopy. The thinner plastic weighs only several grams, looks close to correct, and will look fine when detailed. Its not that much savings, but elimination of DEAD weight.

This plane is a plane where specs matter, and it taught me to build to spec. More on this after I get my coffee.

The EDF40 version is unflown, due to the small 12mm brusless motors in a 16oz 27"span plane. May fly, but I'll probably build a paper light wing for it, for flight. Where the wight comes from: Figure in 2 motored EDFs with nacelles and 2 sets of 3wire motor wires, a stronger batt, and an additional ESC, and you have an easy 6oz. Using the prop plane analogy, it could be an 11oz geared LPS brushless plane, that should fly well. Makes you think where the weight comes from.

Building to spec: Needs to be a goal with this plane. Hopefully we don't get any of those "why so heavy" idiots like Bare at rcg, after all the weight explanations. The first 262 I built was a specification disaster, because it had none. My second 40" was better, since it had specs, but didn't meet them. Rebuilding a lighter, larger off-scale wing, and now there finally barely in the ballpark.
AUW: 34oz
Wing Load: 20oz/sq-ft
Watts total: 240 after level off
Watts per pound: 113

Still not a high-po plane, but should fly. More goals were hand launch and using a 3s-2100 Pro-Lite, instead of buying a larger lipo. Same goes for the Himax 2015-4100 motors. I could pirate another plane for my Hacker (Eflite) 4900 motors, but then they would need a larger batt to see the benefits, again entering the viscous cycle. Still may happen.

Wow, long post, but good thread dude.

The one analogy I keep in mind is the design of the (around 1990) Mazda RX7. An engineer (not Mazda) told me the prototype was 10% too heavy. Instead of trying to go after heavy/highly weight inefficient components, they required redesign of a large number of components, even small ones, to shave off 10% or more weight. Same mentality needed for building. Its finally starting to sink in for me, which is why I touched base on the canopy.

Enough for now. Bill
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Old 07-04-2006, 11:06 PM
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Ed Waldrep
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5.5 lbs and 53" span doesn't seem unreasonable. You can fly at higher wingloadings with a plane this size vs something smaller, especially if you have enough smooth runway to get to takeoff speed.
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Old 07-05-2006, 04:10 AM
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CraigO
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pics are coming i thought i had some with me but didnt. there are a couple of pics in my gallery but none that show the whole model as it stands at the moment.

at this stage 53 inches has about 315 sq inches of wing area which is a horrible high 30 wing loading. wing cube loading is an acceptable 28 but still its high. i have a mate how has built a 72 inch 262 with midi fans and that weighs in at almost 6kg. his wing loading is higher than mine but the wing cube loading is about the same.

You are right Bill a tight spec is needed and that is what i am working towards. The nose gear is heavy, weighing about 50gms, under 2 oz with the wheel. I am going to lighten it some more but one of the heaviest parts is actually the piano wire leg, the cylinder and the mechanism is only 29gms.
at this stage based on ground tests of the motors in the nacelles i am looking at about 220W/lb all up if i hit the 5.5 lb. the test also seemed to indicate that it was producing about 1 kg of thrust per motor. so if that is maintained i should get a TW ratio 2:2.5 or 0.8 which is pretty hopeful and more than my friends bigger model.
i did have a pilot figure that i cast but it weighs an extra ounce so he is out. If i was to loose the retracts that would save about 6 oz. and bring me back into more acceptable weight levels.
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Old 07-05-2006, 08:20 AM
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I'm not sure how the cubic wing loading figures in, but it is probably one of the most grey areas. We try to keep models at 1.5lb per sq-ft or less. Full size planes go way into at least the 20s or more, in fighter jets. Where this curve in relation to span starts kicking in, is what I'd like to know more about.

The 40" plane in my previous pic has a hand carved balsa pilot. He's light, looks ok, but doesn't have to be perfect, as the idea was to have a "fully" scratch built plane, including the pilot. It adds character.

I don't know how high 30oz/sq-ft is, for your 53" span with decent area. You do seem to have a powerful setup. I'm sure quite a bit takes place between 40" and 53" span. I reworked mine with in off-scale wing and shaved a few ounces, to get an even 20.0oz/sq-ft load. I've found on the 40" span, they don't do well above 20, begin to need considerably more power, and don't hand launch well . Looking at your 5.94" average chord, your wing is probably pretty scale. They look great that way, but the cheating can buy you a lot more area. Boosted to say 8", it wouldn't look that scale, but would give you almost 3sq-ft of area, similar to warbirds that size with much lower taper ration wings. Would solve all issues overnight, but admittedly not look as good. This plane has some things that work against us. Twin nacelles (EDFs, motors, wiring ESCs) add massive weight. Increasing span doesn't really help much, as the weight of the plane jumps up along with the span, since the high taper ration wing doesn't provide that much more area, with span. That's where the cheating comes in and helps out.

The nose retract weight doesn't seem that bad. I have a mech rotating Robart nose retract, waiting to go in something. I wanted to put it in my 262, but just too much weight. Where I always seem to get burned, is in all the related weight to building features. I'll weight the part, but don't consider the added weight of things like mounts, reinforcements to the structure (such as reinforcing a wing for EDF nacelles), related hardware, etc. Its all that stuff that just seems to come out of the woodwork. On some builds, I'm still really at a loss, as too where some of the weight came from.

I used to try to find someone who has had success with similar specs, for encouragement. Problem is, you always can seem to find great success stories, even with high weights and wing loadings, that almost seem unbelievable . I do less and less of that referencing now.
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Old 07-05-2006, 02:19 PM
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CraigO
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use a thick walled ali tube and use a lighter spring wire on the nose gear that will reduce the weight by about 1/2 an ounce.

here are a couple of pics i took with wing 2, the foam cored one without flaps on. it is basically complete it just needs finishing. then i start another wing. doh!


and

i am getting a second wind so to speak. thanks bill i will keep on posting pics as i make progress. I will finish the new built up wing with the flaps. I cant get out to do much flying at the mo cos my wife is expecting and has been really sick, so i stick around the house for her.
cheers all
Craig
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Old 07-07-2006, 05:34 AM
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Bill G
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Looks good. Looks like a lot of work too. I did a fully sheeted built-up scratch build before. I didn't get the nose quite right on mine, and ended up giving it away, to motivate me to build another lighter one. Still remember the work, though.
Those gear wires are heavy, and the 1/2 oz counts. If I build another 262, I'll even be trying to take it easy on the glue. Gotta shave weight anywhere you can.
I see the nose is not on yet. I've been building them using a rubber spinner cone for the nose. When I glue the circular ply mounting plate to the sheeting, I put a piece of foam rubber behind it and compress it, as I CA glue and activate it in place. Its on a sheeted foam core build, but something similar could be done with a built up. The idea is that the compressed foam under pressure will provide some "resistance" to the nose crushing in, if hit hard on the nose. It proved itself well on one of my sheeted foam builds. I simply stretch the iron-on over the rubber spinner, to eliminate the seam. The GWS spinners work well, since they can be mounted on a push-on screw. Just more thoughts.
I guess the "saying around the house" thing has given you a lot of build time. This is definitely one plane, that for whatever reason, you always want to complete. I'm always motivated to build 262s. Its one of those addictions where each time you think "I'll take all the experience from the last one to make a better, lighter one".
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Old 07-07-2006, 07:42 AM
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CraigO
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love the look of the models in your pics. i seem to have trouble downloading the movies that people have posted showing their models. i dont know why, but then again i cant work out how to post a thumbnail here either.

I was looking back through my collection of models and i have actually built 2 other 262's both used direct drive speed 400's the first was from a plan by mark rittinger and the second was a bit more scale from my own plans. they were both good. the Rittinger model was more stable but not as fast.

i am going to pull an ABS plastic nose for the current 262. i built a number of parts for my Mosquito like that and they worked really well. in fact the spinner plug for the mosquito is almost exactly the right size but it is a little stumpy. all i did for that was mount a piece of soft pine in my drill press and sand it back to the shape of a cardboard profile template.
i think i will complete both wings which also gives me a chance to build another set of nacelles. i am not 100% happy with the first ones but i learnt heaps making them and will improve the design then too. they are really light already but i can see ways of making them both stronger and lighter. i set them up so that the whole front is removable giving better access to the fan. here is what i mean.

i am hoping that i can get the flap mechanism completed in the first wing over the weekend.
cheers

Craig
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Old 07-07-2006, 11:21 PM
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I made removable access nacelle covers for 2 of my builds. The recent rework now has them in the front of the nacelle, so it can pull right out. Much easier to build that way, and needs forward cg weight anyway. To retain the fans, I simply use a small set screw, that threads into a small hole in the fan housing, behind the blades.

The only reason I now see to mount them rearward a bit, is so the nacelles can be tapered a bit toward the front, to look more scale, without having to enlarge the entire nacelle diameter even more, to get the same effect. I never split them as you did, just a straight seam. Looks like you've do a lot of work, in making those nacelles.
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Old 06-16-2007, 05:29 AM
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Even with the GWS262 out there now, the good ol' scratchbuilt 262 is still one of my favorites, although I rarely take it out. Great flyer, but always worried about crash and repair, which is the EDF curse.

Had a lousy solder connection come undone on a motor connector. Caused the motor to overload the ESC, which in turn shut down BEC power. JU87 Stuka dive.

Well the plane may as well benefit from the repair process. Put in real turrets this time, and an airbrush, not "spray bomb brush" finish.

...and yes, I've now learned to be smart enough to test fly them After the repairs., Before repainting.

Edit: From the view count, I see there's more interest in my Pomeranian Foxy than in the plane......
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Old 06-16-2007, 05:53 AM
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I had a major think about what was light & what was not about 11 years ago when I built a B-36. The inspiration was a six engined EDF Antonov from europe. This was basically a model made of a carbon fibre tube fuselage frame and a light weight BUILT UP wing.

Thin foam shells gave it shape. It worked for me, just a basic flying stick puffed out with hollow foam to make it look like a plane. Being electric I test flew it before fitting retracts & the shapely bits. If the concept didn't work the outlay in time would have been much less. First thing to go was the scale section foam wing, being 110 inch span & almost 3 inches thick at the root the weight was considerable. - John.
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Old 06-16-2007, 08:23 AM
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Originally Posted by Boomerang View Post
I had a major think about what was light & what was not about 11 years ago when I built a B-36. The inspiration was a six engined EDF Antonov from europe. This was basically a model made of a carbon fibre tube fuselage frame and a light weight BUILT UP wing.

Thin foam shells gave it shape. It worked for me, just a basic flying stick puffed out with hollow foam to make it look like a plane. Being electric I test flew it before fitting retracts & the shapely bits. If the concept didn't work the outlay in time would have been much less. First thing to go was the scale section foam wing, being 110 inch span & almost 3 inches thick at the root the weight was considerable. - John.
You know with some modification, you could make that into an EDF B52.

My EDF scratchbuilds are all of foam construction, mostly sheeted with CF rods/spars and iron-on covering. Its proven quite crashworthy.

Bill
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