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Author Topic: Bostonian Bede BD5  (Read 9739 times)
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Yak 52
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« on: February 03, 2012, 09:10:32 AM »

Hi all.

This is my Character Bostonian design, based on the Bede BD5 pusher jet. I've finished drawing the plan and started cutting wood so I thought a build thread might be realistic. Once I've built it (from photocopies) the plan will be finalized then scanned to pdf and anotated.

The BD5 is a cool little plane! This version is stretched and squashed and simplified to fit the Bostonian rules. As a model it has a few interesting features, a short tail means trimming might be tricky but also makes room for a 13" long motor... we'll see I guess.

At some point I'll be looking for advice on making a pusher prop...


Cheers!
Jon
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Bostonian Bede BD5
Bostonian Bede BD5
Bostonian Bede BD5
Bostonian Bede BD5
Bostonian Bede BD5
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tross
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« Reply #1 on: February 04, 2012, 09:04:32 AM »

Nice Jon,
Interesting choice.
I remember driving with my dad to Oskosh back in the early 70s, we did that a couple of times.
He was disapointed that the thing didn't quite make it.
He was building one of his experimental planes and was talking most of the way up about how he was going to get one of those next.
He didn't. He bought a 1948 Piper Vagabond instead!!
I was wondering about the thrust line. I'm thinking you have that covered and I was just curious.
Won't the plane pitch up as the power is reduced? Or are you thinking outside the box.
You know how I love that.

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"........
Yak 52
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« Reply #2 on: February 04, 2012, 09:45:01 AM »

I'm planning to suck it and see! I'm hoping that with a generous amount of stability (forward CG) the tendency to power stall will be countering the high thrustline/downthrust effect. I've just set the thrustline to the datum and I'll add shims as neccesary.

So rather than pitch up as power is reduced, I'm giving it something to pitch down against power on... if you get me?

I'm far from "having it covered" though so I'd welcome thoughts, advice, counselling  Grin

Tobgun did a scale version which trimmed out nicely in the end so I'm optimistic  Smiley
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« Reply #3 on: February 04, 2012, 09:51:44 AM »

Quote
So rather than pitch up as power is reduced, I'm giving it something to pitch down against power on... if you get me?

I get you. You're working the other end.
Balanced with power down, giving something to push against at power up. So it doesn't pitch down at thrust.
It'll be way cool. Can't wait.

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"........
Hepcat
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« Reply #4 on: February 04, 2012, 04:02:58 PM »

Jon
Very cleverly done to get the character so well into a miniature slabsider.  Like you I would worry about the moment arm - would anyone notice if you moved the wing forward a smidgen, or even a tad? 

The drawing looks good.  Is that with your new CAD software?  And what about the pictorial view?  I notice that you treat your Warren girder joints differently on the tailplane to those on the fuselage and the fin.  Am I missing some fine nuance of design here or did you just feel like being different!

John
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John Barker UK - Will be missed by all that knew him.
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« Reply #5 on: February 04, 2012, 06:52:41 PM »

Thank you very much John  Smiley

I seriously considered moving the wing forward but my fear of phugoids was overcome by my fear of nose weight!  Undecided I could just see all the weight piling on at the back with the prop assembly etc and decided to put as much of the motor forward of the CG as I dared. I probably overdid it on the forward structure for the same reason. The tail volume is still reasonable (about 0.8 as I recall) and I'm hoping to get some extra stability from the pusher prop. I hope the risk pays off, if I can get it to fly then the 13" motor space should come into it's own.

The plan is drawn by hand (drawing board, french curves etc), I've put my CAD on hold for the moment. I'm getting somewhere with it but it takes me forever to do the simplist things! I'm actually quicker on the drawing board despite numerous drafts. When I've finalized it I'll scan it and annotate it on the computer. The pictorial view is just an early stage isometric projection with a few sketchy bits thrown in. I do that now and then to help me visualize it.

The warren girder joints weren't particularly nuanced  Grin I did give some thought to where the stresses might come from but mostly I was thinking of what was simplest to build, as in "which bit will be pinned down first and which bit will I be cutting." I'm sure there is a 'proper' way though? The warren trusses just seemed to fall into place with the fuselage, in terms of what fixed points I had. The fin came next and with the sweep it had to be 'warrened' ( Huh) then the tailplane really had to follow suit. I thought it made it look a bit more modern and funky although I'm sure it's overbuilt...

I plan to use the angle of the nosegear to perfect the right attitude for ROG.


Thanks for the interest... I'll be hoping to pick your brains on the prop soon  Cool

Jon
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« Reply #6 on: February 06, 2012, 08:16:10 AM »

Construction has begun...

The sides came together ok, the bent longerons were soaked to get them to hold the bend better. I've added a couple of gussets around the bit where the fuselage 'splits', it seemed a bit vulnerable when the lower bit is bent. And of course I just moved the stresses elsewhere  Undecided


Jon
Attached files Thumbnail(s):
Re: Bostonian Bede BD5
Re: Bostonian Bede BD5
Re: Bostonian Bede BD5
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« Reply #7 on: February 06, 2012, 02:49:41 PM »

Superb! Yet another for the ever growing "to do" list... Sigh.
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« Reply #8 on: February 06, 2012, 03:17:34 PM »

Jon
Very cleverly done to get the character so well into a miniature slabsider.  Like you I would worry about the moment arm - would anyone notice if you moved the wing forward a smidgen, or even a tad? 
I can never quite remember; exactly how many smidgens are there to a tad?
Pete
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« Reply #9 on: February 06, 2012, 05:33:43 PM »

Pete
I am not surprised you can't remember, it's one of those unusual ones three and a half Smidgens to the Tad.  Then of course there are the subdivisions of the Smidgen the Gnats Dickie and the Hole Down its Middle but these are usually taken as variable and appropriate to the particular job.
John
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« Reply #10 on: February 06, 2012, 05:52:37 PM »

Thanks Dan  Smiley

John, thanks for that - I always have trouble converting from 'bodgic' to metric.
I'd heard of the gnat's crotchet as a measurement of time but a gnats dickie was new to me  Grin


Jon
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« Reply #11 on: February 06, 2012, 06:18:37 PM »

That looks like an interesting challenge Jon.


I seriously considered moving the wing forward but my fear of phugoids was overcome by my fear of nose weight!  Undecided I could just see all the weight piling on at the back with the prop assembly etc and decided to put as much of the motor forward of the CG as I dared. I probably overdid it on the forward structure for the same reason. The tail volume is still reasonable (about 0.8 as I recall) and I'm hoping to get some extra stability from the pusher prop. I hope the risk pays off, if I can get it to fly then the 13" motor space should come into it's own.
Jon

It may help when visualising pushers to treat the nose length as what you would have expected to see if it was the tail boom length, and then the tail length on the pusher will then to a degree resemble the nose length. It's only meant to be a quick cross check and obviously tail volumes and static margins still need to be sufficient.
It can be surprising how long the nose may need to be and this approach helps to put your mind at rest.
Those definitions stir some memories John – although I had never heard of the sub divisions of a smidgeon. I don’t think I worked to that level of accuracy. Grin
How did the system allow for angles - flickers of the gnat’s wing or similar?  Grin
John

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Yak 52
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« Reply #12 on: February 07, 2012, 06:41:49 AM »

It may help when visualising pushers to treat the nose length as what you would have expected to see if it was the tail boom length, and then the tail length on the pusher will then to a degree resemble the nose length. It's only meant to be a quick cross check and obviously tail volumes and static margins still need to be sufficient.
It can be surprising how long the nose may need to be and this approach helps to put your mind at rest.

Hi John, not quite with you there, are you talking about the yaw damping effect of the forward fuselage? Or are we talking weight distribution of the motor/prop set up?

Plugging away with the build. Tail surfaces are done, weight of the tailplane was 0.6g and the fin 0.4g. The tailplane will have the TE spar slot cut out when I've covered it. It then slots into the fuselage and is pushed back into position. Then the slot is filled again with a small piece of sheet and patch covered.

The fuselage is jigged up on it's back (the only flat bit). Looks like a beetle...

I'm finding all the structures so far have come out nice and stiff so really light balsa could be used.


Jon
Attached files Thumbnail(s):
Re: Bostonian Bede BD5
Re: Bostonian Bede BD5
Re: Bostonian Bede BD5
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« Reply #13 on: February 07, 2012, 07:12:39 AM »

That's a very technical looking jig, Jon. Did it take a lot of building and wherever did you source the parts for it? Grin
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« Reply #14 on: February 07, 2012, 07:20:28 AM »

It was custom built Pete...  Grin

Investment was made over a long period. Finding storage space for jig materials has been problematic. The management felt we should declutter but the shopfloor union made a pressing case.
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« Reply #15 on: February 07, 2012, 06:22:40 PM »

It may help when visualising pushers to treat the nose length as what you would have expected to see if it was the tail boom length, and then the tail length on the pusher will then to a degree resemble the nose length. It's only meant to be a quick cross check and obviously tail volumes and static margins still need to be sufficient.
It can be surprising how long the nose may need to be and this approach helps to put your mind at rest.

Hi John, not quite with you there, are you talking about the yaw damping effect of the forward fuselage? Or are we talking weight distribution of the motor/prop set up?

It’s more of a rough check on the overall weight distribution Jon. When you consider how a conventional layout with its tail moment will in most cases come close to balancing as required - it helps to use something close to this length in a pusher.
Thus I found your layout to be close to what I would have expected in terms of wing position and nose length.

As your BD5 has a flat sided fuselage this nose length may reduce your yaw stability and require a larger fin although the prop is helping here as you mentioned.

I still envious of all those nice tight joints and your that high budget building jig. Grin

John
« Last Edit: February 07, 2012, 06:34:36 PM by Ratz » Logged
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« Reply #16 on: February 08, 2012, 05:18:10 AM »

Thanks John, yeah it was always going to be a compromise between nose length for weight distribution and the tail moment arm for stability. The one positive about the long nose is that it should make for more spiral stability providing I get the fin area right. I was planning for 1" to 1.25" dihedral under the wing tip. (That's a wadge or a wadge and a tad btw) In fact I have to have a certain minimum dihedral because the flat wingspan is 16 1/8" so that the projected span will be 16".

The fuselage is coming along. It just needs a nose block and some of the structure on the rear end to complete. I soaked the lower rear fuselage and rolled it under a pen to get it to take the tighter curve. It was a bit tricky and I should have used lighter/softer balsa in that area. The fuse weighs 2.75g at this point.


Thanks for the interest,

Jon
Attached files Thumbnail(s):
Re: Bostonian Bede BD5
Re: Bostonian Bede BD5
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« Reply #17 on: February 08, 2012, 05:36:22 PM »

You've solved the mystery, Jon Grin Angles are measured in wadges!
How is the torsional stiffness of the fuselage just in front of the prop Jon? It is reduced cross section and the prop shaft tunnel doesn't seem to have spacers on the bottom?
Fast work.
John
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« Reply #18 on: February 08, 2012, 06:17:13 PM »

How is the torsional stiffness of the fuselage just in front of the prop Jon? It is reduced cross section and the prop shaft tunnel doesn't seem to have spacers on the bottom?

It will have! It will fair into the lower fuselage with something approaching a wing fillet, which includes a former and partial keel yet to be fitted. The stiffness overall is pretty good - I could have gone a lot lighter on the wood  Undecided


Jon
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« Reply #19 on: February 08, 2012, 08:04:43 PM »

Yak52

Love the BD5, been following this over at SFA...nice progress.  Look forward to seeing this one in the air...great work.

Tom
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« Reply #20 on: February 09, 2012, 06:38:38 PM »

Thanks Tom, you settling in ok?


I thought the fin mounting might be worth a bit more detail. It's actually just slotted into two pieces of 1/16th strip but I use the 'jig' to set it up correctly.

First off I cut the two strips to the right length as measured against the gap between the relevant cross pieces on the actual fuselage. Having done this they are pinned to the plan in the correct position laterally, using another strip as a spacer.

Then the fuselage goes back in the jig, aligned over the plan again. Having tested the fit and got the position right it's lifted off, glued up and then re jigged.

This puts the slot in the centre and hopefully aligns it correctly. I might add a strip under the forward end to protect the tissue, if I can afford the weight...
Attached files Thumbnail(s):
Re: Bostonian Bede BD5
Re: Bostonian Bede BD5
Re: Bostonian Bede BD5
Re: Bostonian Bede BD5
Re: Bostonian Bede BD5
Re: Bostonian Bede BD5
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« Reply #21 on: February 10, 2012, 09:01:22 AM »

The partial keel is attached to the rear former and the lower fuselage.
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Re: Bostonian Bede BD5
Re: Bostonian Bede BD5
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« Reply #22 on: February 11, 2012, 05:59:59 AM »

The final bit for the rear fuselage is the small former. I'm hoping I can tissue cover the curved concave bit without wrinkles, but I may have to use a bit of stiff paper like you would for a wing fillet.

The result is that the motor part of the rear fuselage is reasonably stiff in torsion despite the small cross section. The partial keel piece was cut from quarter grain balsa which seems to help a bit too.

With the little scallop the motor has a clear run to the peg at the front. There's not loads of room inside at the back but hopefully enough.
Attached files Thumbnail(s):
Re: Bostonian Bede BD5
Re: Bostonian Bede BD5
Re: Bostonian Bede BD5
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« Reply #23 on: February 11, 2012, 07:57:23 AM »

Looking good Jon.
By all means, I would give it a go.
But if the compound curves start to give you fits, you might try a straight contour at the last former installed and
a small amount of stiff paper at the transition.

Tony
Attached files Thumbnail(s):
Re: Bostonian Bede BD5
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« Reply #24 on: February 13, 2012, 06:24:18 AM »

If I've got it right there should be no compound curve there, just a 'cone'. It's just that with plain tissue it may wrinkle a bit. I was thinking on similar lines but going the whole hog and making the keel bit a straight line from the lower fuselage to the rear former. It would be a lot simpler (more Bostonian?) but this way is working so far....
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Re: Bostonian Bede BD5
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