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Old Aug 28, 2012, 09:21 AM
Balsa&Tissue
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United States, OR, Beaverton
Joined Jan 2011
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Build Log
Bird of Time Kit Build

Update: I am converting this to a build log. I have decides to wait on a powers system decision until the build is nearly complete. Also decided to be totally obsessed with taking weight out of the plane. I started looking at some of the balsa and the sides of the fuse for example, are really heavy and have big grain swirls and knots. I am going to replace them with better balsa.

O am also going to cut ellipse shaped reliefs in the trailing edge parts and some of the ribs. I am going to look hard at all the pieces for places that weight can come out without reducing strength.


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One of my first RC planes was a Bird of Time back in about 1980!. I want to build one with a brushless motor and folding prop but all my experience with conversions and power systems has been in scale aircraft. I searched for a more recent thread but didn't find a lot of information. I am not looking to build a super performance plane because I can't afford one anyway (man, the latest sailplane technology is expensive). I just want someting I can cruise around with and hunt for thermals.

Here is a nice image of one:


Wingspan: 118 in (3000 mm)
Wing Area: 1070 inē (69 dmē)
Length: 49 in (1245 mm)
Wing Loading: 5.5 oz/ftē (17 g/dmē)
Weight: 41 oz (1160 g)


With a 2200 - 2600 3s battery and a 300-400 watt motor I am estimating 12 additional ounces. I am just guessing at the 300-400 watt system with a 10-11 inch prop but my real question is: Am I in the ballpark for a well balanced power system for this plane?

I plan to take some measures to lighten the plane but I am of course unsure how much I can remove before I get into it a ways. Also, the BoT has a short nose and I hear it ends up needing a lot of nose weight to balance anyway so if I keep the battery as far forward as possible it will help. I am also considering moving the nose forward some amount to help. However this assumes that I could get by with a lighter power system. If a 300-400 watt system is really needed then the nose probably won't need to move as the plane will be fairly easy to balance.
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Old Aug 28, 2012, 03:28 PM
FPV Desert Beta Test Center
Mesa, Arizona
Joined Nov 2006
2,303 Posts
Mine is quite differant but it may give you some point of reference.
I fly the ARF version (fiberglass fuse) FPV which makes for more weight than you would have to deal with. Mine balances perfectly with the following equipment.

EFlite 25BL 550W motor w/folding prop.
3S 5000 battery installed right behind the firewall.
Small elevator servo mounted in the tail.

The factory nose weight was removed and the fuse cut back to where a 40mm spinner would fit.
While it will not climb vertically the rate is more than I need.
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Old Aug 29, 2012, 09:16 AM
Balsa&Tissue
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United States, OR, Beaverton
Joined Jan 2011
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Quote:
Originally Posted by Gary Evans View Post
Mine is quite differant but it may give you some point of reference.
I fly the ARF version (fiberglass fuse) FPV which makes for more weight than you would have to deal with. Mine balances perfectly with the following equipment.

EFlite 25BL 550W motor w/folding prop.
3S 5000 battery installed right behind the firewall.
Small elevator servo mounted in the tail.

The factory nose weight was removed and the fuse cut back to where a 40mm spinner would fit.
While it will not climb vertically the rate is more than I need.
Thanks, I am thinking a .10 equivalent motor may be a pretty good starting point for me. I have some 3800 3s batteries that would probably be enough power for that setup as well. I need to get into the build a ways and start to see what my overall weight is going to be for the plane.

Dave
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Old Aug 29, 2012, 09:23 AM
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Hi, Dave.

I recently saw a finberglass fuse for a BOT for about $35.00. Assuming that it would accept your wings and tail it might be worth looking around for a deal. At that price it wouldn't cost much, if any, more than materials and you could end up with both an electric and a pure sailplane. Good luck either way.

Pete
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Old Aug 29, 2012, 10:40 AM
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United States, NJ
Joined Aug 2011
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I think it would be worth you tinkering in eCalc, a very handy tool for assessing battery/ESC/Motor/Prop combos.

http://www.ecalc.ch/motorcalc_e.htm

Very useful for working out a suitable prop after you pick your motor, ESc & battery capacity/ C rating from the drop down menus.

I put in the EMax BL2815 & it seems to be ok on an 11 x 6 Aeronaut CamCarbon folder. If I try an 11 x 8 then there are warnings about prop stall & motor temp.

11 x 6:
Mechanical Power Out: 222 watts, 53 oz model weight, 3S 20/30C Lipo, max current 26.54 amps.

If you want about 100 watts per pound on the 53 oz model then you are looking about 330 watts output, so 222 watts will fly it but it will not climb as rapidly as you might be wanting.

If you haven't used eCalc, Google it & you should find the free program & some 'How To Use It' material.

Also, check Ray Hayes Skybench Aerotech web site as there may be examples of electrified BOT setups there. he sells a BOT type sailplane that has been converted to E {Big Birdy]

http://www.skybench.com/index.html?h....com/home.html

http://www.skybench.com/index.html?h....com/home.html

Googling "Electric BOT motors ' & variants of that phraseology should turn up some power-train info.

Takes a bit of digging, but its there.


G2
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Old Aug 29, 2012, 01:30 PM
Balsa&Tissue
payne9999's Avatar
United States, OR, Beaverton
Joined Jan 2011
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Quote:
Originally Posted by Gemini2 View Post
I think it would be worth you tinkering in eCalc, a very handy tool for assessing battery/ESC/Motor/Prop combos.

http://www.ecalc.ch/motorcalc_e.htm

Very useful for working out a suitable prop after you pick your motor, ESc & battery capacity/ C rating from the drop down menus.

I put in the EMax BL2815 & it seems to be ok on an 11 x 6 Aeronaut CamCarbon folder. If I try an 11 x 8 then there are warnings about prop stall & motor temp.

11 x 6:
Mechanical Power Out: 222 watts, 53 oz model weight, 3S 20/30C Lipo, max current 26.54 amps.

If you want about 100 watts per pound on the 53 oz model then you are looking about 330 watts output, so 222 watts will fly it but it will not climb as rapidly as you might be wanting.

If you haven't used eCalc, Google it & you should find the free program & some 'How To Use It' material.

Also, check Ray Hayes Skybench Aerotech web site as there may be examples of electrified BOT setups there. he sells a BOT type sailplane that has been converted to E {Big Birdy]

http://www.skybench.com/index.html?h....com/home.html

http://www.skybench.com/index.html?h....com/home.html

Googling "Electric BOT motors ' & variants of that phraseology should turn up some power-train info.

Takes a bit of digging, but its there.


G2
I am going with a all wood build from the kit so with some changes it will hopefully be light but at least lighter than the fiberglass version. My hope is this will keep the power requirements reasonable.

I used some motorcalc type program for a while but it was'nt so intuitive and was only free for a short evaluation period.

Thanks,

Dave
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Old Aug 29, 2012, 01:48 PM
Balsa&Tissue
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Looks like a .15 size motor and an 11X6 folder would be about right, comes out overal to 80% efficiency:
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Old Aug 29, 2012, 02:17 PM
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San Gabriel Valley, CA
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Quote:
Originally Posted by peterlngh View Post
Hi, Dave.

I recently saw a finberglass fuse for a BOT for about $35.00. Assuming that it would accept your wings and tail it might be worth looking around for a deal. At that price it wouldn't cost much, if any, more than materials and you could end up with both an electric and a pure sailplane. Good luck either way.

Pete
+1

I would not sacrifice a 41oz BOT, it is a breed in extinction. The Dynaflight glass fuselage and bigger motor is what you want for an e conversion. My e-P(ig)OT weights 70oz.
Tai
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Old Aug 29, 2012, 02:17 PM
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United States, NJ
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Quote:
Originally Posted by payne9999 View Post
Looks like a .15 size motor and an 11X6 folder would be about right, comes out overal to 80% efficiency:
Figures look pretty good other than bottom left 'Entire Drive Weight: 20.6oz, Which means for a 50 oz glider you need to build it to 30oz - very tight I think.

I would suggest the battery pack size is the problem, suggest you drop that to 3S Lipo, 2100mah ,25/35C.

& Top left 'elevation', I used 100' ilo 1641'

Prop at 11 x 7 & you will get 84.9% motor efficiency & pitch speed 56mph. They are key indicators. You want to try & get motor efficiency over 80% & pitch speed above 50mph, I read somewhere.

The 2100mah Lipo's won;t give the motor run duration of the 3700's, but you shouldn't need it if you consider the BOT a sailplane used for thermalling & the power system a means of launching to thermal catching height. Motor burst then off, ilo running it constantly.

The 2100mah battery gives you a power train weight of 15.5oz, which is about 25% less than the 3700's & you can build the BOT to 35oz ilo 30oz, which is probably not achievable.

Would put the eCalc table in , but don't know how to do that

As an addendum I would suggest you check the motor diameter specs of anything you are considering to make sure it will fit in the front of the BOT.

G2
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Old Aug 29, 2012, 03:22 PM
Balsa&Tissue
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Quote:
Originally Posted by tai626 View Post
+1

I would not sacrifice a 41oz BOT, it is a breed in extinction. The Dynaflight glass fuselage and bigger motor is what you want for an e conversion. My e-P(ig)OT weights 70oz.
Tai
I don't quite understand what you mean about extinction and the use of a glass fuse. For me, I made patterns of all the parts and if I build another one it will also be from scratch.

Now that I own the plans and have made my own back up copies (I never use the originals cause they get trashed during the build) I don't foresee ever being without a BoT cause I can build whatever I want. In fact I want to build a single dihedral version with ailerons eventually.

Anyway, what I love about this glider is it is a classic and can still be built to perform well. The one I had 30 plus years ago was a blast. That wqas when it was "Mark's Models".

Dave
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Old Aug 29, 2012, 04:49 PM
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I had assumed that you were going to whack the nose off of an extant and irreplaceable fuse. Scratch built from plans you should be able to build one that would be much better for your purposes than a glass fuse. I still have, I think, a fairly intact set of plans for a Mark's Models Windfree and I have toyed with the idea of making an ep version. With modern materials and electronics I would think one could make one that's not much, if any, heavier than the original and even add ailerons and some carbon. The wings of my original would flap a bit at high speeds. I would also probably go with a modern airfoil. The original was good but I think it could be better. It's the looks, much like the BOT, that make me pine for the old days.
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Old Aug 29, 2012, 10:55 PM
Balsa&Tissue
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United States, OR, Beaverton
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Quote:
Originally Posted by peterlngh View Post
I had assumed that you were going to whack the nose off of an extant and irreplaceable fuse. Scratch built from plans you should be able to build one that would be much better for your purposes than a glass fuse. I still have, I think, a fairly intact set of plans for a Mark's Models Windfree and I have toyed with the idea of making an ep version. With modern materials and electronics I would think one could make one that's not much, if any, heavier than the original and even add ailerons and some carbon. The wings of my original would flap a bit at high speeds. I would also probably go with a modern airfoil. The original was good but I think it could be better. It's the looks, much like the BOT, that make me pine for the old days.
I almost forgot about the "Windfree". I built one of those when I was 17.

Dave
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Old Aug 29, 2012, 11:56 PM
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My hat is off for scratch builder. I can only dream about a 41oz. BOT. My electrify Pig of Time is an ARF and started heavy ~60+oz. With motor/esc/batt, 70+oz. I will fly it in a Cross country so I need weight anyway to fly it fast.
Good luck with you built!
Cheers, Tai
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Old Aug 30, 2012, 08:54 AM
Balsa&Tissue
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Ok, starting off with the wings. I started laying out all the parts and to begin with I took the trailing edge pieces and added elliptical holes. It doesn't really affect the strength but takes out a little weight. Some of the balsa parts are quite heavy. I will replace the worst pieces.

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Old Sep 03, 2012, 06:43 AM
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I flew a kit built BOT in the late ninety's with an Astro FAI brushed motor and nicads. UGH, it was a heavy motor system but it soared well enough to place at a LMR event. Don't take any structure out. This airframe is quite different from the ARF BOT out there today.

You should put spoilers in the wing, layout the battery, rx, servos etc as even todays components need room. The ballast compartment size can be changed and the bulkhead under the LE opened up to allow a LIPO to pass thru. Your biggest challenge will be the nose because it depends on the motor type. Since I exclusively use geared inrunners I would go with a NEU 1105 w/4.4GB. If you want to reduce cost maybe there is an MVVS outrunner with a fully enclosed can and narrow profile may work. They are heavier but cheaper and run well. Do not lengthen the nose. The motor and battery will allow balancing the tail. This is a gas bag of an airframe so stop worrying about loading.

I managed to put the rx and servos in the rear most compartment under the wing.

I have a new-in-box original BOT as well as a copy of the original mag plans.

Ray
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