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Author Topic: Boston Bonxie  (Read 1967 times)
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Yak 52
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« on: September 29, 2013, 10:08:05 AM »

I've started a new Bostonian design - the Bonxie, which is a follow up to my Beagle.

I hope to incorporate a few new ideas in this one. The Beagle highlighted the need for stability and ease of trimming when you are flying with high torque at a small site. This model has a much shorter nose and longer tail moment arm. The intention is to build the model well under the 14g weight minimum and use the required ballast as nose weight. This weight target will be critical to the success of the design. I had it in mind to add more streamlining to the nose area but for weight reasons I've kept it simple.

The model also has a full length motor, running all the way back to a squared off fuselage. There will be a small drag penalty but the long motor should give lots of turns and a long cruise. I've increased the tail volume to reduce sensitivity to the CG shifts that may occur with a long motor.

The airfoil is thin (5%) with modest camber. Playing around with XFLR5 has shown the theoretical benefits of a 'proper' airfoil (that is, appropriate to very low Reynolds numbers) on a Bostonian.  To give sufficient stiffness the wing has diagonal rib braces. The airfoil should be capable of higher CLs (like 0.7) so I've bumped up the dihedral and spiral stability to cope.

Wing construction has started. With 1/32 ribs and TE it's more akin to peanut building than Bostonian but hopefully it will be worth it. I've attached a low res scan of the plan, if it's any good I'll finish it properly...


... oh and 'Bonxie' is the Shetland name for a Great Skua - a belligerent bird that ain't afraid of nothin' or no one - with a penchant for divebombing walkers Smiley
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Boston Bonxie
Boston Bonxie
Boston Bonxie
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F F modeller
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« Reply #1 on: September 29, 2013, 02:58:13 PM »

Like it Jon ... and the name  Smiley

I had a model teed up a few years ago that was named with a similar train of thought ..... it never surfaced ... so it didn't earn the name I suppose?!
I'm sure yours will do the business however!

No harm in saying what it was ..... Steve Bage had success with his 'Scorpio' design ..... My design for the Flying Aces that year was going to be the 'Honey Badger' ....an animal that eats scorpions  Wink
I will have to dig the drawing out and finish it sometime .... thanks for the inadvertent reminder!
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Pete Fardell
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« Reply #2 on: September 29, 2013, 03:23:49 PM »

I like it.

Now then....what eats honey badgers?
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F F modeller
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« Reply #3 on: September 29, 2013, 03:32:56 PM »

Thankfully it was a Rapier powered model .... otherwise Bonxies probably would!
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« Reply #4 on: September 29, 2013, 05:09:04 PM »

Yak52,

Interesting that you are planning a full length motor. The more competitive US INDOOR 7-g Bostonians use full length motors and often orient the required box on its side for more lift. Many people dislike this orientation of the box but no one seems to doubt its effectiveness. I have never built a flounder type (or a truly competitive Bostonian).

Steve Gardner wrote up an excellent article on the indoor ones. Building Better Bostonians by Steve Gardner posted in Miscellaneous Articles.
http://www.hippocketaeronautics.com/hpa_plans/categories.php?cat_id=76&page=10

Fred Rash
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Yak 52
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« Reply #5 on: September 30, 2013, 04:38:29 AM »

Cheers guys,

Fred, yes the Steve Gardner article is very good. I'd arrived at some of the same conclusions (wish I'd read it earlier) and I've pinched his high(er) aspect ratio tail idea for this one.

Jon
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bernienichols
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« Reply #6 on: September 30, 2013, 06:14:00 AM »

I like it.

Now then....what eats honey badgers?

a very brave critter indeed! Honey badgers are reputed to be THE fiercest animal (pound for pound) on the face of the earth. [/QI]
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« Reply #7 on: September 30, 2013, 07:55:59 AM »

I like it.

Now then....what eats honey badgers?

The honey monster?  Smiley
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Yak 52
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« Reply #8 on: October 04, 2013, 05:01:46 PM »

I've now completed the wing. The main spar and LE were glued up flat and then when dry the TE was chocked up by 5/32" to set the correct angle where it meets the ribs. The LE was notched and fitted. The diagonals were added and then finally the dihedral was set and the centre section built in. Finally the turbulateor spar and gussets were added.

Weight before shaping and sanding is 2.8g and I'm pleased with the stiffness considering the thin airfoil.
Attached files Thumbnail(s):
Re: Boston Bonxie
Re: Boston Bonxie
Re: Boston Bonxie
Re: Boston Bonxie
Re: Boston Bonxie
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tross
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« Reply #9 on: October 05, 2013, 12:10:22 PM »

I know you and I have discussed how daft I am before..... Cool
But what's that bit of TE action going on at the top of that last pic Jon?
 Grin Cheesy Smiley Cheesy Grin

T

Opps, now I see the plan in the 1st post! Cheesy
Very cool.
« Last Edit: October 05, 2013, 01:30:21 PM by tross » Logged

Instructions: Step One...Assemble the pile of sticks shown in pic "A" to look like the model airplane shown in pic "B"........
Yak 52
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« Reply #10 on: October 05, 2013, 05:38:39 PM »

Hey Tony,

Daft as a brush...

As you can see on the plan it's just for the aileron trim tab. Flying at a small site like Impington means you need a tight circle - too tight for just rudder so the aileron tab gives some 'hold off' bank.


Jon
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« Reply #11 on: October 05, 2013, 06:01:13 PM »

Jon,
The aileron tab is a good inclusion ... I've built in rubber tabs in the past, but always think I'm going to get away without tabbing the wing. Rarely do.
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tross
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« Reply #12 on: October 05, 2013, 07:37:40 PM »

Daft as a brush...

......... and not half as useful! Grin :

Silliness aside, I was glad to see that you are continuing with the development of your Bostonian series.
I will follow with interest. Smiley

Tony

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Instructions: Step One...Assemble the pile of sticks shown in pic "A" to look like the model airplane shown in pic "B"........
F F modeller
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« Reply #13 on: October 06, 2013, 05:29:32 PM »

..... of course I meant rudder tabs! My mind has a mind of it's own.
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« Reply #14 on: October 07, 2013, 03:36:51 PM »

John, I like your new Bostonian. The diagonal ribs look great. I plan to use something similar for my PC-6 which I plan to build. I am curious to see how much torsion stiffness they will add.
Will follow your build closely, cross my fingers that it is an other great flyer.
What colours will you add to the bones?

roman
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Yak 52
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« Reply #15 on: October 07, 2013, 04:11:49 PM »

Thanks Roman, the diagonal ribs do add some stiffness but there is a slight weight penalty. The Beagle wing at this stage weighed 0.5g less. The real benefit is in stiffening thin wings (the Beagle was 7% thick, this is 5%) You could get stiffer and lighter wings with a thicker wing section (10% or more) but that would really increase the drag of the wing and harm duration. For a sport or scale model it may be worth the compromising the aerodynamics.

I have a couple of sheets of new light blue esaki earmarked for this one. Nothing flashy, just the one colour - but at least it's a new colour!
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Yak 52
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« Reply #16 on: October 07, 2013, 06:40:05 PM »

...always think I'm going to get away without tabbing the wing. Rarely do.

Russ, this is the first time I've built a tab in too. I have a feeling the acetate tab on the Beagle was a factor in it's stall behaviour, opening up the turn as it dropped the right wing. I thought I'd try this especially as it's a constant chord wing.
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Yak 52
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« Reply #17 on: October 13, 2013, 10:43:53 AM »

Plodding along.

I now have the tailplane and fuselage sides done. The sides weigh 0.75g each and I've committed to the one motor peg position  Undecided

The windscreen frames are laminated from two strips of 1/32" - they need to hold the curve to give the required side window area. I like to laminate pieces like this overwidth because I find quite a bit is lost in the sanding.

The tissue is a shrinkin'...
Attached files Thumbnail(s):
Re: Boston Bonxie
Re: Boston Bonxie
Re: Boston Bonxie
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« Reply #18 on: October 14, 2013, 02:38:07 AM »

Looks good!
Do you prepare the scrap pieces you press against the lamination with adhesive tape? Otherwise it would stick to the lamination?
Roman
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Yak 52
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« Reply #19 on: October 14, 2013, 07:29:08 AM »

Hi Roman,

I didn't bother this time - they stuck very slightly and left a little wood grain on the lamination but I was happy to scrape this off as I knew the piece would need sanding down anyway. For more delicate/important bits I would tape the scraps (just being lazy.)

Jon
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Yak 52
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« Reply #20 on: October 16, 2013, 01:37:46 PM »

The fuselage is just about done, using my self-building lego-matic jig  Smiley The cut off stern(?) is naturally less stiff than a normal fuselage so it is built from wider wood.

I can never resist a mock up shot. This lot weighs just over 7g.
Attached files Thumbnail(s):
Re: Boston Bonxie
Re: Boston Bonxie
Re: Boston Bonxie
Re: Boston Bonxie
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bernienichols
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« Reply #21 on: October 17, 2013, 05:12:01 AM »

Looking really good Jon, you're way ahead of my effort. Mine won't make Bushfield, but I'm confident hopeful for Impington.

looks like you'll have to ballast it perhaps? either way, I like this one very much. good designing mate.

b
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« Reply #22 on: October 17, 2013, 08:03:33 AM »

Thanks Bernie. I'd really like to get it done to trim at Bushfield but it will depend on work and I'm supposed to be sailing Sat.

Yes hopefully it will need ballast. The whole concept depends on using the ballast as nose weight. I'm aiming for 11-12g finished.


A bit more on the airfoil choice and thinking behind it. It's actually Mark Drela's AG19 glider airfoil with a slight mod to widen the TE to permit 1/32 wood. I spent a long time playing with Nordic tip airfoils, modifying them for thickness and camber to find out what would be best on a Bostonian. I ended up with something that was very similar to AG19 - 5% thick and modest camber of 2%. (Actually the best camber depends on the ability to turbulate the wing) So I used the stock airfoil to see how it would do. Of course the building and sanding limits the accuracy but the ribs are cut with french curves and not sanded for consistency. I've given the wing a turbulator spar at 10% and flattened it off to give an Eggleston vortex nose. Xfoil does like these a lot. My Lizzard foil (as used on my Ferryman wing tip) was slightly better in the analysis but I wanted to try a stock airfoil (makes recommendations easy  Roll Eyes)

I ummed and ahhed over the planform. The Beagle had tapered tips. This was an attempt to reduce induced drag by providing a more elliptical lift distribution. However I did a lot of work on whether this was worth the increased wing loading and I don't believe it is when span and chord are limited.

When you do a detailed break down of the drag components of a normal Bostonian (using something like the Neelmeyer airfoil) you find that the induced drag is about 15% of the total, 20% is parasite from the fat fuselage but a whopping 65% is profile drag from the poor airfoil. This runs counter to the reasoning in larger aerodynamics that best endurance speed occurs where induced drag is larger than profile drag. This is simply because at low Reynolds the airfoils are so horribly draggy that even at the speed for best 'power factor' (endurance) the profile drag is a major part of the whole.

So this made it clear that rather than chasing a few percent of 15% by playing with the planform it would be much better to concentrate on the profile drag by using a skinny airfoil, even if this meant a gram or two diverted to the wing extra for the structure needed. Significant savings in drag should be available. Which means less rubber will fly it, which makes for more turns and better endurance.

I've attached an XFLR5 plot comparing the sink rate of Bostonians with various airfoils. Neelmeyer is a typical 9% flat bottomed airfoil used in small rubber models. Clark Y is included for a laugh Grin. Gottingen 795 is there for Andy Wink. The Beagle is there complete with tapered tips and the BE50 airfoil. The AG19 of the Bonxie does outperform the rest - in theory at least.

Sink rate (Vz) is the performance measure you want to target for rubber power endurance (in spite of it not sinking as a glider would, it's a measure of minimum power required. Less power means more turns for the same weight of rubber.) Sink rate combines the effects of wing loading and power factor. Where span and chord are limited (by the rules) any 'improvement' of the planform means increased wing loading. Under these conditions the constant chord wing with a degree or so of wash out actually performs better than a more elliptical shape with higher wing loading.


The proof of the pudding will be in the flying of course. I have my egg/face scraper at the ready.

I know this seems a lot of trouble to take with a Bostonian but I'm an unashamed nerd  Grin
Attached files Thumbnail(s):
Re: Boston Bonxie
Re: Boston Bonxie
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Yak 52
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« Reply #23 on: October 17, 2013, 12:10:45 PM »

Here's a few construction details.

The 'stern' is a little different to the plan but should be self explanatory.

The UC (22 swg wire) is fitted in a balsa sandwich with thin CA. The gussets are added afterwards.

The trim tab hinges are from thin florists binding wire (similar to sandwich bag tie wire.) The holes are carefully drilled with a tiny drill bit. When you cut the wire it's almost inevitable that it gets bent, but I found you can straighten the pieces out by rolling them on the building mat under a flat surface (yes, you guessed it - a lego brick.) The tabs will be glued in after covering.
Attached files Thumbnail(s):
Re: Boston Bonxie
Re: Boston Bonxie
Re: Boston Bonxie
Re: Boston Bonxie
Re: Boston Bonxie
Re: Boston Bonxie
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« Reply #24 on: October 17, 2013, 03:58:21 PM »

 I am amazed at the perfection of your glue joints. Your construction is truly beautiful! And the wood you have used is so much more regular grained than what I have used. Where do you purchase yours?  And Legos? Wonderful! I am becoming intrigued by Bostonian planes, and will most certainly use Legos for my first build.

And this is to say nothing about your analysis process... so I just spend the last hour trying to decipher the use of Xflr5 via tutorials on YouTube... not productive at all for me. I was going to see if I could use it to analyze my No-Cal models - but that notion will have to wait while I gear up for next week's flying day.

For now, my mind is wrapping around your lowering drag with thinner airfoils... on these lightweight, slow flying models, it sounds like a fine approach. I am anxious for your results. It seems to me that the entry radius is also important, especially not leaving the lower corner of the leading edge sharp - as with most of these lightweight rubber planes - mine included.

Richard Ranney
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