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Author Topic: Boston Beagle  (Read 2957 times)
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Yak 52
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« on: November 28, 2012, 08:16:15 AM »

I'm back on a new Bostonian design - the Boston Beagle, so called because it has a long but square nose  Smiley

I seem to be mildly obsessed with Bostonians, they are nice and easy to draw but the rules provide an interesting design challenge. This is an attempt at a winning design, incorporating some new ideas.

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. The last Impington was won by a model that did 70 odd seconds, this is really impressive in such a small hall as it's so easy to drift off and hit things. Scigs' Cub is really stable and flies beautifully. It has a long tail moment arm but a short nose which means the hook to peg length suffers.

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 Impington used a 30" motor of 1/8 rubber. My 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 HtP 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 experiment to see if a proper airfoil offers any noticeable advantage at this size. The planform has tapered tips.


Jon
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Boston Beagle
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Boston Beagle
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« Last Edit: November 28, 2012, 08:57:02 AM by Yak52 » Logged
Sky9pilot
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« Reply #1 on: November 28, 2012, 01:07:33 PM »

NICE Grin Cool

Tom
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« Reply #2 on: November 29, 2012, 09:34:44 AM »

Well Jon, it looks good on paper  Grin . Seriously, your design is well thought out and should be a champ.

I'm not familiar with UK Indoor Bostonian rules and wondering how much consideration the judges give to "charisma" points, i.e. wheel pants, exhausts, likeness to a real A/C and all that.

When I saw your fuselage side view my first impression was a Wittmann Tailwind with a somewhat longer nose, seems that you could go with a homebuilt aircraft theme if you chose to.

This will be fun to watch as it progresses  Smiley .


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« Reply #3 on: November 29, 2012, 10:43:48 AM »

Thanks Tom, Jim.

I'm not familiar with UK Indoor Bostonian rules and wondering how much consideration the judges give to "charisma" points, i.e. wheel pants, exhausts, likeness to a real A/C and all that.

There's no static judging for charisma, it's the normal rules but to the 14g minimum. This is intentionally a boring design  Smiley Just focusing on the flying  Cool

Done a couple of fuselage sides. When compared to the Cub, the tail moment isn't too bad and the nose is quite a bit longer.


Jon
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« Reply #4 on: December 04, 2012, 07:24:21 AM »

I've been playing with my lego again  Smiley

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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« Reply #5 on: December 04, 2012, 02:50:44 PM »

Getting there... Weight is about 5.5g at this stage.

Sorry about the rubbish pics. Better to follow in day light.
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« Reply #6 on: December 05, 2012, 08:21:05 AM »

A few more pics...
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« Reply #7 on: December 05, 2012, 09:43:51 AM »

Yak52,

The bones are really neat! Can you please share how do you get such smooth joints? Not even a little glue mark! In my builds, the glue marks are very apparent!

Thanks.

Regards
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« Reply #8 on: December 05, 2012, 02:32:13 PM »

Yak52,

The bones are really neat! Can you please share how do you get such smooth joints? Not even a little glue mark! In my builds, the glue marks are very apparent!

Thanks. Regards

Shadow stole my thunder.  Great wood working.  I'm envious of your skill!

Tom
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« Reply #9 on: December 05, 2012, 05:18:28 PM »

Thanks Shadow and Tom.

I'm not doing anything out of the ordinary, but I do pay attention to the dry fit of the joints.

Here's a few points, most of which I've picked up from other people:

I use a scalpel to cut sticks. I do change blades quite regularly to keep them sharp. I cut the stick over a second copy of the plan (not the one on the board, a second print out) this gives you an accurate angle. I always cut oversize and trim several times a hair's breadth at a time. I find it easier to get the end square when removing a shaving rather than a single full depth cut so I usually trim both ends of the stick get them square (in both planes.)

I dry fit most of the fuselage side (pinned to the building board) before gluing. Using bits of scrap wood protects the sticks from pin marks. The glue I use is Speedbond PVA (white glue) Glue control is important. I use a very fine paint brush to apply the glue. I thin the glue as necessary for the job with water, squeezing a small blob onto a plastic jar lid and mixing in a drop or so of water. For every joint I wet both sides with a weak solution (to ensure the contact and overcome the surface tension of thicker glue) and then apply a slightly thicker glue for the actual joint. A dry paint brush removes/dilutes any excess. It's important to wash the brush every time to keep it in good condition.

I also consider which side is to be on the outside of the model - often sheet wood is thinner than the strips I use so I pad it out with a piece of card. Finally, I sand the sides gently with fine grade wet n dry on a wide flat block.

The most important thing is snug but not over tight joints. When I muck it up I sometimes glue a shaving of balsa in to fill a gap, let it dry and then remove the excess with a sharp blade. This is most useful when stringer notches are moved a bit. You can see where the wing ribs have a gap in the first pics. This was filled in this way.

Started covering tonight - hoping to fly this one Friday night  Shocked Here is the wing, showing the wing tips. A second 1/16th rib is added as a cap to give a 1/8" thick wing tip.

The weight of the bones, UC and wheels was exactly 6g after being prepared with sanding sealer. It could be lighter but I've used quite hard spars to compensate for the skinny wing.
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Re: Boston Beagle
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« Reply #10 on: December 05, 2012, 05:53:41 PM »

Nice work Jon .... mildly obsessed with Bostonians myself.
I hope to be back on form by the next Impington ie. actually bring some models with me! .... look forward to some tough competition .... in an Impington fun kind of way that is.
I have not passed the 70 second mark myself at Impington ... though 3x69 seconds at one Impington is close I suppose. I have only ever used an undercambered wing on one of my designs though ... I will be very interested in the performance of your model in this respect. Where will you be flying on Friday? (guessing at Oundle .... have flown there a few times but it is a bit small for getting a new model sorted).
It is still a 'quest' of mine to see how well a 14g Bostonian can go at Impington ... I've done 75 seconds at Digby but that is a lot larger ... I did not retrim from 'Impington' trim though.
Not bothered if it's not me that achieves this 'peak' (lies!) ... I just enjoy watching them.


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« Reply #11 on: December 05, 2012, 06:24:31 PM »

Bring it on!  Grin But seriously I look forward to flying with you Russ. Oundle on friday if I make it and get the model finished, it is a bit small but it's the only indoor date we have in Dec since Whittlesey was cancelled next week.

Re: the 70 second mark, I haven't actually bothered timing the Cub yet. Was getting something like 40 seconds but at Whittlesey there are some downdrafts from the ventilation and it kept drifting into the wall after a few circuits. Excuses, excuses!

What are you building/designing then?
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« Reply #12 on: December 05, 2012, 07:10:50 PM »

Jon,
I think your Cub and Bede both have great potential.

I've got loads of things floating around in my head still ... it's part of my problem I think.
I still want to 'revisit' my 'Kipper' lifting body design ... there are a couple of things that John Barker suggested that I have still to try out.
That was a very weird performer ... but the way it flew did suggest that the body was doing something ... just not in the right way sometimes!
It would do a peculiar short takeoff and climb at quite an angle but with the body level ... there was a conflict of 'pitching moments' between the body and the wing that made trimming a bit on the edge.
It was here that John had some great advice.
(I won't mess up your thread by posting a pic here)
Another build of Mike Stuart's 'SortaSenator' would be nice too ... after my first got trodden on!
Also, another build of my NE14T (that would be the 4th!) and perhaps even a lighter OO-LA-lA ... though I do keep getting drawn on to new ideas.
My 'Chesty Jeep' gave me the same issues as your Bede ... it harmed itself on some gym equipment though ... but I would have liked to have cracked that too!
.... and these are just the Bostonians that I have made!

One thing I would mention with your Beagle ... I have seen a few designs with a swept back rear of the fin. I have always thought of this as losing some of the length allowance ... though I have seen designs like this fly very well, so it is probably of limited significance. Except perhaps for that 'ultimate' performance I romance about? Roll Eyes
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« Reply #13 on: December 05, 2012, 10:48:07 PM »

I'm not doing anything out of the ordinary, but I do pay attention to the dry fit of the joints.

Here's a few points, most of which I've picked up from other people:

I use a scalpel to cut sticks. I do change blades quite regularly to keep them sharp. I cut the stick over a second copy of the plan (not the one on the board, a second print out) this gives you an accurate angle. I always cut oversize and trim several times a hair's breadth at a time. I find it easier to get the end square when removing a shaving rather than a single full depth cut so I usually trim both ends of the stick get them square (in both planes.)

I dry fit most of the fuselage side (pinned to the building board) before gluing. Using bits of scrap wood protects the sticks from pin marks. The glue I use is Speedbond PVA (white glue) Glue control is important. I use a very fine paint brush to apply the glue. I thin the glue as necessary for the job with water, squeezing a small blob onto a plastic jar lid and mixing in a drop or so of water. For every joint I wet both sides with a weak solution (to ensure the contact and overcome the surface tension of thicker glue) and then apply a slightly thicker glue for the actual joint. A dry paint brush removes/dilutes any excess. It's important to wash the brush every time to keep it in good condition.

I also consider which side is to be on the outside of the model - often sheet wood is thinner than the strips I use so I pad it out with a piece of card. Finally, I sand the sides gently with fine grade wet n dry on a wide flat block.

The most important thing is snug but not over tight joints. When I muck it up I sometimes glue a shaving of balsa in to fill a gap, let it dry and then remove the excess with a sharp blade. This is most useful when stringer notches are moved a bit. You can see where the wing ribs have a gap in the first pics. This was filled in this way.



Thanks for the explanation above...I'll try to see if I can achieve the same on my next build.

Regards
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« Reply #14 on: December 06, 2012, 03:06:24 AM »

I agree with Shadow Jon - very neat and tidy work - typical of your builds. Your techniques are worth bottling Grin I like your lego ideas and the second plan approach.

The design info was interesting as well. Htp? - rubber motor length perhaps?

Incidentally I have never regarded canards or lifting tail/tandem wings as effecient as conventional layouts in terms of drag etc. I feel that the less tail you have to do the work required - the better. Thats another topic though.

I also like the spar arrangement of 2 on the top surface only. I have just recently built, with the kids I teach, Bill Warner Sky Bunnies with a similar arrangement (although he has them a little further forward) and on shrinking it seems to induce a little washout. The turbulator efffect and lack of sagging on the top surface must help as well. Probably all old hat to you FF fellows!

BE50 - thats an old F1C foil I think - it looks nice.

It should definitely be a nice flying model.
John
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« Reply #15 on: December 06, 2012, 05:11:03 AM »

It was here that John had some great advice.
(I won't mess up your thread by posting a pic here)

Please do Russ, I've never seen it (just the plan). I'd like to hear John's ideas too.


... though I do keep getting drawn on to new ideas.

Tell me about it. I decided I wouldn't touch another model until this was finished and I've actually managed to bring it through from the initial idea to nearly done in a couple of weeks. That's VERY unusual for me  Embarrassed


One thing I would mention with your Beagle ... I have seen a few designs with a swept back rear of the fin. I have always thought of this as losing some of the length allowance ... though I have seen designs like this fly very well, so it is probably of limited significance. Except perhaps for that 'ultimate' performance I romance about? Roll Eyes

Russ the trailing edge of the fin isn't swept, but I'm glad I managed to trick you  Grin


The design info was interesting as well. Htp? - rubber motor length perhaps?

Hi John, thanks for the kind comments. Yes, HtP is the 'prop Hook to motor peg length'.


Incidentally I have never regarded canards or lifting tail/tandem wings as effecient as conventional layouts in terms of drag etc. I feel that the less tail you have to do the work required - the better. Thats another topic though.

I agree about efficiency but I'm not sure that minimum induced drag is the be all and end all in Bostonian. I haven't explored the idea fully but I think that the wing loading it permits might be worth the reduced efficiency? (Kind of like the power factor?) There's no shortage of power as the rubber weight is unlimited. The rules limit wing area but a larg(ish) tail is permitted. This is how the old time rubber models developed such large lifting tails after all. I'd be interested to hear what you think though.


I also like the spar arrangement of 2 on the top surface only. I have just recently built, with the kids I teach, Bill Warner Sky Bunnies with a similar arrangement (although he has them a little further forward) and on shrinking it seems to induce a little washout. The turbulator effect and lack of sagging on the top surface must help as well. Probably all old hat to you FF fellows!

Yes the spar is there for turbulator reasons only. There's no D-box obviously so it's not going to be the best representation of any airfoil  Roll Eyes
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« Reply #16 on: December 06, 2012, 11:55:50 AM »

Very nice build, Jon. Beagle's bones are beautiful  Grin .

I'm curious about your PVA method. Modellers that use it for general construction report that the joints are more resilient and "shock-absorbing" in a crash than other glues, have you found that to be true?

Other than sandability, the only downside I've heard about PVA is that dihedral joints have been known to weaken over time to allow the wings to sag enough to make the model unflyable. I've believed that was more likely due to poor joinery, improper glue application, or lack of gussets at dihedral joints. Looks like you've eliminated all of these in the Beagle build, but still wondering what your thoughts are on this issue.

Last, what approximate glue-to-water ratio do you aim for in the two different mixtures that you use?

TIA  Smiley ,


Jim (6aw6)
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« Reply #17 on: December 06, 2012, 12:47:46 PM »

Quote
Russ the trailing edge of the fin isn't swept, but I'm glad I managed to trick you 


How can this be? .... is it some trick of light or other deception?  Wink
I demand a profile shot!  Grin
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« Reply #18 on: December 06, 2012, 01:06:51 PM »

It's on the plan Russ  Grin Post #1
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« Reply #19 on: December 06, 2012, 04:52:57 PM »

I'm curious about your PVA method. Modellers that use it for general construction report that the joints are more resilient and "shock-absorbing" in a crash than other glues, have you found that to be true?

Other than sandability, the only downside I've heard about PVA is that dihedral joints have been known to weaken over time to allow the wings to sag enough to make the model unflyable. I've believed that was more likely due to poor joinery, improper glue application, or lack of gussets at dihedral joints. Looks like you've eliminated all of these in the Beagle build, but still wondering what your thoughts are on this issue.

Last, what approximate glue-to-water ratio do you aim for in the two different mixtures that you use?

Jim, thanks. I would agree that PVA is less brittle than CA for instance. I use different glues for different jobs but I steer clear of CA most of the time especially where things might need re-gluing. PVA is less of a 'contact' adhesive but where there is a good mechanical joint it works well. Particularly when you have a jig or can clamp joints. I've never had anything sag although none of my models are that old.

I don't mix by ratio so it's hard to say. I just thin for the consistency I like. I would guess that the weak solution for wetting out the joint would be two thirds water and the thicker stuff in the joint itself would be one third water. But it's not precise. With some lighter wood it wicks in a lot of thinned glue, so a thicker mix is better and actually lighter.


Jon
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« Reply #20 on: December 07, 2012, 05:57:50 AM »

I offer you: The Stealth Beagle  Smiley Covered in special radar absorbent esaki.

Here's a profile shot of the fin for Russ too. The base rib on the fin has warped 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 have 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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Re: Boston Beagle
Re: Boston Beagle
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Re: Boston Beagle
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« Reply #21 on: December 09, 2012, 01:27:22 AM »

[I agree about efficiency but I'm not sure that minimum induced drag is the be all and end all in Bostonian. I haven't explored the idea fully but I think that the wing loading it permits might be worth the reduced efficiency? (Kind of like the power factor?) There's no shortage of power as the rubber weight is unlimited. The rules limit wing area but a larg(ish) tail is permitted. This is how the old time rubber models developed such large lifting tails after all. I'd be interested to hear what you think though.]

My main concern with Canards and for that matter tandem wings is that the rear surface is always effectively the tail running at a lower angle of attck than the wing. The consequence of this is that the larger surface is not abnle to work as hard as the smaller surface nad could spend most of its working life in a high drag area depending on the section. Another way of saying this is that the rear surface should have far less camber than the front.

Tandem wings would not appear to be much better – longitudinal dihedral still requiring less angle of attack than the front surface.

Thus for any given required amount of lift overall – I feel it is always best to minimise the lift developed by the tail.

Old timers with their fairly high cambered wings(6% or so) would have required aft CG’s to establish trim at low angles of attack during their fast climbs and low longitudinal dihedral to minimise looping. This would then have led to large tails on the relatively short fuselages to obtain sufficient stability. At lower speeds during the glide – the tails would then have been lifting. That’s how I feel the tails sizes ended up as 30-40% or more of the wing.

John
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« Reply #22 on: December 09, 2012, 07:13:10 AM »

John, thanks for clarifying this for me. I was aware of the stability reasons for large tails but I thought that the rules limiting wing area were also a factor in their development. I went back to the text books and you're right, I had overlooked the fact that a lifting tail would be operating at a low lift coefficient for the required balance.

I plugged in some figures to Hepcat's NP and Trim Calculator using the Beagle wing and a tandem tail at the largest size permitted (24in^2). With a 10% static margin and the wing at CL0.8 the tail was at CL0.23. Working backwards, this represents a lift contribution of 2.4g.
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« Reply #23 on: December 09, 2012, 01:04:09 PM »

Quote
Here's a profile shot of the fin for Russ too

Still not convinced!  Roll Eyes .... but it is more vertical than expected (yup ... I'm a sore loser  Wink)

I have usually used cambered tails .... not gone too far into the theory of it though, I just 'feel' they are of advantage. Not had monster trimming problems with them either.


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« Reply #24 on: December 09, 2012, 06:44:14 PM »

John, thanks for clarifying this for me. I was aware of the stability reasons for large tails but I thought that the rules limiting wing area were also a factor in their development. I went back to the text books and you're right, I had overlooked the fact that a lifting tail would be operating at a low lift coefficient for the required balance.

I plugged in some figures to Hepcat's NP and Trim Calculator using the Beagle wing and a tandem tail at the largest size permitted (24in^2). With a 10% static margin and the wing at CL0.8 the tail was at CL0.23. Working backwards, this represents a lift contribution of 2.4g.

Yes its a lot of area being dragged around that isn't actually doing much in the way of lift.- contributing drag.
In any case i prefer the look of your conventional Beagle which I hope  will tame your  model bashing Ippington site Grin

John
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