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Old Nov 29, 2012, 04:00 PM
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Boston Beagle - Design and Build

I've started a new Bostonian design - the Boston Beagle, so called because it has a long but square nose

This is an attempt at a duration comp design, incorporating some new ideas. It's all about the flying performance so it's pretty boring and characterless! There are a few important aims and striking the balance between these factors is what I hope to achieve here:

1) A stable design. I've seen the importance of this with the BD5. A long motor is only helpful if you can trim the thing.

2) A long motor. This is a balancing act between trying to get the CG as near the 7" mark for a full length motor but without compromising the length of the tail moment arm for dynamic stability. I studied various ideas including a full size lifting tail and canard configurations to try and get the CG at the halfway mark. It is possible with a canard but only by reducing the size of the foreplanes to the point where I lost confidence in the trimming again. A full 24in2 lifting tail again means that the tail moment is reduced, because the CG is back at 75-80% the tma suffers and you have to move the wing forward to preserve the dynamic stability. A lifting tail might be good in that it reduces wing loading but it offers no real HtP advantage.

I think the best way to get a long motor and a decent tma is to build underweight and use nose weight in a conventional design.

3) A roomy fuselage. The winner at the last Impington comp used a 30" motor of 1/8 rubber. My Boston Cub ended up with similar 21" of 1/8, so a decent wide fuselage is preferable to let it all flop around. Bunching is baaad.

So I've ended up with a conventional lay out with a longish nose and the tail moment arm is 2.5 times the mean chord. The tail volume is 0.7, big enough but not 'lifting'. The hook to peg length comes out at 10" if balanced on the CG, but if I can afford 1g of noseweight I should be able to use the second peg position giving 11.5". The windscreen and side windows are the minimum permissible in the rules.

The airfoil is BE50, the first time I've built an 'undercambered' wing. It's a bit of an experiment to see if a 'proper' airfoil offers any noticeable advantage at this size. It will be open stick and tissue (no D-box!) but a turbulator spar should help. The planform has tapered tips for better lift distribution, less drag and better strength to weight.


Jon
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Old Nov 29, 2012, 04:05 PM
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The wing is almost done. The ribs were cut from a sheet with a hard streak in it to maximize strength in the thin bit towards the TE. The grain was also oriented to have the same effect.

I did compromise slightly on the weight but it has come out surprisingly stiff in a shearing direction. Pretty bendy in torsion but the tissue should sort that.

The fuselage is coming along. Compared with Scig's Cub the nose is quite long but the tail moment arm is not too bad either.
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Old Nov 30, 2012, 06:07 AM
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Very nice plan drawings Yak! The structure looks excellent also

A thing I never could master without any proper instructions - How do you plot the tapered airfoil sections?

Regards,
Dragan
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Old Dec 01, 2012, 05:34 AM
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Thanks Dragan.

I've just started using this online plotter, it's not a totally comprehensive list of airfoils but it has some good ones and seems to print out quite accurately on my printer. I printed on to tracing paper and then used french curves to transfer (and cut) the rib shapes. Not the most accurate method ever but good enough for something like this.
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Old Dec 01, 2012, 09:57 AM
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Yak

Great looking design. Seems to be built for outdoor flying. How much did the airframe weigh?

I'm flying Bostonians indoors with the Bong Eagles in Wisconsin, and managed a couple flights near a minute in their 40 ft ceiling Category II site. I modified the plans for a Beancraft Bonanza to give it a straight tail. The airframe weighed 5 grams without covering and prop. Finished it weighs 12 grams without rubber. I got it climbing from the floor with an 11 inch loop of 1/8 inch rubber and a 2L bottle prop.

Usually fly Embryo style airplanes outdoors for the small cabin type events. I find 4 strands of 3/32 inch rubber drives the 6 inch prop fast enough for card table R.O.G. The fuselages are narrow. I can't seem to manage the long motors you're talking about.

Enjoy flying your Bostonian.

Joe
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Old Dec 01, 2012, 10:42 AM
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Nice Beancraft

This one is still in build but it will be for indoor. Over here there is a regular competition at Impington and a new one at the BMFA Indoor Scale Nats in Nottingham in April. They are built to the 14g minimum so I'm aiming for 12 or 13g plus a bit of nose weight.

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Originally Posted by DuPageJoe View Post
... managed a couple flights near a minute in their 40 ft ceiling Category II site.
Impington was won at over 70 seconds I think. Impressive in a small hall. I'm not sure of the dimensions at Impington but here is a video of my Cub - (not anywhere near competitive yet ) which gives you an idea of the size. Stretch winding is the key but it does land with quite a few turns still on (If it doesn't hit the wall! )
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Old Dec 01, 2012, 12:02 PM
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Yak
Thanks for the info. Looks like Impington is roughly 30 feet high. Your Cub was flying well untill the wall flew into it. The 14 gram minimum makes a decently robust plane, probably closer to the original intent of the event.

Over here, most winning indoor Bostonians are closer to the 9 gram AMA minimum. The winner at the Racine meet was Anton Telford, with an average of 88 seconds for the 2 best flights. A photo of his plane was posted on the Eagles' web site. It seems smaller than mine, with a smaller tail. Otherwise, Tony's workmanship and experience counts a lot. Nobody was flying the flat style Bostonians that day. I've seen a 2 minute flight at the Racine site.

http://kenanne.50megs.com/BongEagles...s/Slide027.htm

Keep 'Em Flying

Joe
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Old Dec 04, 2012, 06:31 AM
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I've been playing with my lego again

The fuselage is just about done. As of now it weighs exactly 2 grams. The wing is a scratch under 2g but needs a couple more gussets and some sanding.

The undercarriage will be of the balsa sandwich type. The first piece of the sandwich added is the wider part as shown in the pic. I'm pleased with the Warren construction, the nose has come out remarkably stiff already and I'm becoming a fan of the technique.


Jon
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Old Dec 04, 2012, 01:46 PM
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Getting there...

Sorry about the rubbish light in the pics.

Weight is about 5.5g at this stage. It balances at 33% of the chord right now so with covering and the prop it won't need too much nose weight.
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Old Dec 04, 2012, 10:09 PM
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Framework looks great, and weight pretty good too. Happy covering!
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Old Dec 05, 2012, 04:51 AM
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Thanks Joe, I could have built it lighter but the airfoil is skinny (7% thick) so it needs beefy spars. I'd rather have the weight in there than a blob of ballast to bring it up to 14g. Although now I've said that I'll probably miss the target anyway
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Old Dec 05, 2012, 07:17 AM
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A few more pics...
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Old Dec 07, 2012, 05:04 PM
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I offer you: The Stealth Beagle Covered in special radar absorbent esaki.

The base rib on the fin has warped a bit so I need to sort that and tweak the plan accordingly. I haven't pre-shrunk the tissue as I wanted it taught but maybe I should've done on the empennage. I'm pleasantly surprised at the weight, 8.5 grams at the moment. Banana oil, prop assembly, windscreen and motor peg will add a bit though.

The undercambering went quite well. It hasn't pulled off the rib bottoms anyway. I anchored the tissue right over the centre section ribs which made applying the tissue on the centre much easier. When doing the overlaps on ribs I always end up loosening one side while applying pressure to the other but this has solved that problem.
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Old Dec 18, 2012, 06:42 PM
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Here's a couple of final pics (tissue a bit saggy in the damp). AUW before rubber or ballast is 11.35g. It's slightly nose heavy so I could have got away with a longer motor or a longer tail moment for stability. Plenty of ballast to play with so I've gone with a Peck Prop for starters and will add tail weight. Glides over the bed were surprisingly low on drag - hit the wall rather than the bed
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Old Dec 19, 2012, 05:29 PM
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Your Bostonian is looking good. What sort of motor are you planning? I'm assuming it's a 6 inch prop, so maybe a long loop of 3/32 would work.

I've posted a thread on the Comet Phantom Fury I've built with a similar airfoil. Ribs are shown below. It needs a bit rearward CG, but some of that might be from the tail volume, which I need to calculate. I added a top spar but don't have your front turbulator. It's much bigger than my Bostonian, and i've gotten a good climb but I need to improve the glide.

http://www.rcgroups.com/forums/showp...21&postcount=1
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