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Author Topic: All Foam Bostonian  (Read 2534 times)
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korale
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« on: November 28, 2014, 06:02:25 AM »

I've been playing with some foam sheets that I got my hands on and decided to try building a bostonian out of foam,

weight at the moment is 9.8 g, final weight with prop should be at least 11 g which is about 2g more than the observer Bostonian.

not too happy about the tail might try one with a little less span. 

Korale
Attached files Thumbnail(s):
All Foam Bostonian
All Foam Bostonian
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Maxout
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« Reply #1 on: November 28, 2014, 09:08:41 AM »

Looks really nice. Do keep us posted on how it flies.
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Crabby
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« Reply #2 on: November 28, 2014, 02:21:42 PM »

pretty model korale...can I ask what kind of paint you used here? there seems to be a little drama...I am worried that the paint attacked the foam a little too aggressively and bled under the mask....
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TimWescott
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« Reply #3 on: November 28, 2014, 04:45:25 PM »

You can decorate Depron with Sharpie markers.  They attack the foam a little bit, but not enough to cause problems.  It's not good enough for scale unless you're really, really good, but it's nice to put a bit of color on a model.
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TimWescott
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« Reply #4 on: November 28, 2014, 04:45:55 PM »

Koral:

What thickness foam?  1mm?  2mm?
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korale
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« Reply #5 on: November 28, 2014, 05:38:42 PM »

I used a permanent marker to decorate, no drama it's foam safe,  but I used one of the cheaper brands and it did bleed under the mask. I have used a different brand (on another model)  and that has not bled under the masks.

The foam is 1/16 foam from 'woodland scenics' http://woodlandscenics.woodlandscenics.com/show/Item/C1174/page/1 (link is not the place I bought it) .

Its not as stiff as Depron and can't be heat formed in the oven (like you can with Depron). However it was a lot cheaper than Depron. I did use some 2mm Depron for the internal bulkheads.

Korale 
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danmellor
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« Reply #6 on: November 28, 2014, 05:44:41 PM »

Interesting build! I've built a few models using 2mm wallpaper lining foam and it's good to see what can be done.

Cheers,

Dan.
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korale
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« Reply #7 on: November 29, 2014, 07:09:07 AM »

It was the first time I built in foam, I found that the fuselage was really floppy until all four sides had been glued in and dried. once the 'box' was firm the fuselage was stiff and strong.

The problem was gluing up a straight and true fuselage when the sides were floppy. After two failed attempts, I tried taping a strip of balsa to the outside of each fuselage side, that provided the needed stiffness until the glue had dried. 

This was an attempt on my part to see what the advantages and disadvantages of Foam construction are for Rubber powered models. I'm still interested in seeing how well the fuselage will withstand buckling with a fully wound motor inside.



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C/L Gee Bee
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« Reply #8 on: December 01, 2014, 09:13:25 PM »

Yes, the tension from the wound motor will want to 'stress the risers'...but what an interesting project!

I've been 'observing' the Observer here for a couple of days...Your foam 'Coupe' is furthering my desire to get cutting!

Alluring tip on backing the sides with balsa sheet. Love it when you guys share hard-found secrets!
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Russ Lister
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« Reply #9 on: December 02, 2014, 06:32:32 PM »

Looks good Korale  Smiley

I've built a couple of all foam Bostonians ... didn't get any real advantage or disadvantage over built up other than being able to take the knocks at Impington a bit more (mine were Depron).

One thing though is the loud embarrassing rattling when you have a motor break!

 
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korale
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« Reply #10 on: December 03, 2014, 06:39:02 PM »

Russ,

  I agree with you there is no great advantage in the final product between a foam and balsa Bostonian, however it seems to me I built the foam bostonian quicker than I finished the built up balsa one, mostly because there was a very small parts count and no covering phase.

  Hopefully it will be more 'ding resistant' than the balsa one as well.

  The proof of the model is in the flying. If all goes well I will get a chance to give it a go this Sunday.

Korale
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DavidJP
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« Reply #11 on: December 04, 2014, 06:23:00 AM »

I have not built a foam Bostonian but built a mini Magna and a Molly Hawk, and flown both at Impington - the latter a bit big!

I notice that you have used a rib to create an section and wondered if this showed any advantage as I have not simply sanding a section as bet one can.  A couple of other chaps I fly with have not bothered to create any noticeable section either and their models (own designs duration) perform!

I also feel inclined to pretension my motors -to keep the motor tidy, again can I ask if this is useful?

Thanks.  Sorry for the diversion.
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korale
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« Reply #12 on: December 04, 2014, 07:05:01 AM »

The curve on the wing is more for structural reasons than aerodynamic.

From my extremely limited understanding of low Reynolds number aerodynamics, there is not much difference between a curved and a flat surface at these scales. However the foam that I was using was very floppy and the slight curve on the wing stiffens up the wing.

As for pretensioning motors, I personally have never got it to work satisfactorily, I will continue trying. 

Korale.
 
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TimWescott
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« Reply #13 on: December 04, 2014, 03:09:11 PM »

I agree with you there is no great advantage in the final product between a foam and balsa Bostonian, however it seems to me I built the foam bostonian quicker than I finished the built up balsa one, mostly because there was a very small parts count and no covering phase.

I know modelers that won't touch balsa construction with a stick, but who happily build with foam.  FWIW.
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korale
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« Reply #14 on: December 05, 2014, 12:06:10 AM »

I'm the other way,

   I'm very happy to work with Balsa, but this is the first time I've built anything in foam.

Korale
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TimWescott
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« Reply #15 on: December 05, 2014, 12:48:25 AM »

I build with balsa almost exclusively.  Ironically, my one published design is a flat foamy (a Laird Solution).
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DavidJP
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« Reply #16 on: December 05, 2014, 04:52:20 PM »

I think it depends upon the type of model you want to produce but each of the foam models I have built the main reason was speed.  The Molly H awk for example as no more than 3 days which for me is supersonic!  And of course they bounce very well!
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lincoln
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« Reply #17 on: December 05, 2014, 11:25:57 PM »

The curve on the wing is more for structural reasons than aerodynamic.

From my extremely limited understanding of low Reynolds number aerodynamics, there is not much difference between a curved and a flat surface at these scales. However the foam that I was using was very floppy and the slight curve on the wing stiffens up the wing.

As for pretensioning motors, I personally have never got it to work satisfactorily, I will continue trying. 

Korale.
 
Camber actually makes a big difference. Most really light indoor planes use some camber. The lower the ceiling, the more camber. Of course, even a limited penny plane has a Reynolds number much lower than these. I just used Profili/Xfoil to make polars* for two 2 percent thick plates, one flat and one 4 percent camber.  Above a Cl** of about 0.3, the cambered plate is better, and it quickly gets MUCH better. It also goes to much higher maximum lift. Of course, in order to get just these results in real live, you'd need to use exacly the same airfoil, which wouldn't be easy. However, lots of stick and tissue models have too much camber or too much thickness in their wings. Xfoil says that the Clark Y, RAF32, and NACA 6409 are  really terrible at the same conditions as the other two, worse than the flat plate.  If you go up to a Reynolds number of 45,000 (think 8 oz. Wakefield), the thicker airfoils don't look nearly as bad. With turbulation, they're in the same ballpark as the cambered plates, and for high lift, the 6409 looks superior. A lot probably depends on the details, though, and the possible variations are endless. I wonder how an airfoil with slotted flaps would do compared with these?

Lincoln
now returning to nominally useful activities, or at least that's the plan

* type 2, i.e. as the lift coefficient goes down, the speed and Re go up, just as they would in a aircraft maintaining steady flight.
**lift coefficient, i.e. higher Cl, all else equal, is flying more slowly
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korale
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« Reply #18 on: December 07, 2014, 01:13:44 AM »

well, it flies. its a little underpowered at the moment I think it needs slightly thicker rubber, I made a video, will upload soon.

Korale
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korale
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« Reply #19 on: December 07, 2014, 05:15:25 AM »

Here's the Video clip https://www.youtube.com/watch?v=EcE-zNG1UkY

It refuses to climb with the present setup , I have another prop that I want to try and will increase the thickness of the rubber next time.

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Ross J
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« Reply #20 on: January 02, 2015, 06:07:01 PM »

I like the model! It is underpowered as the video shows, but it looks like a motor with more cross section will solve the problem.

I built an all foam Bostonian many years ago, using a hot wire on an adjustable fixture to cut the foam to specific thicknesses. I used the wire to cut an airfoil as well so no ribs were necessary to achieve a curve and yet it was stiff enough without a spar. Weight was around 11 or 12 grams as I remember.

Camber in the wing is very important for flight and the advantage of the foam over balsa is a very thin under cambered section, perfect for very low Reynolds numbers but also very light. In some ways a foam wing on a stick and tissue fuselage might be ideal.

Keep up the great work!
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danmellor
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« Reply #21 on: January 03, 2015, 01:21:37 PM »

I find the 2mm wall lining foam to be much lighter than Depron. The trick is to iron it fairly firmly with a modeller's film iron. The temp. takes a bit of experiment; too cool and there's no effect-too hot and there's a molten mess! Get it right and the foam stiffens appreciably. I built a CO2 powered HM14 that was a great flier using this technique.

Cheers,

Dan.
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korale
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« Reply #22 on: January 31, 2015, 10:00:56 PM »

Got a chance to do some flying today. I replaced the previous rubber with a new loop and it looked better in the air, took out some downthrust and then it climbed.

I started adding winds and it started flying over and between the various odds and ends hanging off the roof. At that point I shortened the rubber by 6 inches.  It flies quite well now, ROG is still sometimes an issue.

This is one of the better flights of the day.  http://youtu.be/IIqFHLXTNrU

Its really noisy, I think I need a new thrust button, and the whole model vibrates because of the rubber thrashing around, should braid the motor a little more. Other than that I'm very satisfied with this 'foam bostonian' experiment.
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