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BOB v4 Build for FF East 2019

#1
It's been a while. I'll admit, I haven't flown fixed wing since last Flite Fest and certainly haven't built anything. But here we go, now that school is over BOB v4 is in progress. Let's jump right in.

First things first, here are my thoughts on last year's build:

Pros:

Biggest I'd gone yet, with decent construction quality. I was specifically happy with the fuselage and tail, but the wing was okay.

It was a tank in the air, almost impossible to take down. See photo: ( That's 3 planes stuck, a nutball that just bounced through a motor, and contact imminent)

bob rekt.jpg

Landing gear allowed takeoffs. This was a big one, hand launches in the previous year had just gotten too sketchy.

Cons:

Super lacking in power, keeping it in the air was a constant struggle.

Insufficient control surface size made it challenging to fly and necessitated planning of flight paths, made recovery and wind gusts hard to manage.

SUPER heavy tail, made the center of gravity a huge struggle and I was flying with POUNDS of dead weight through the entire event.

The landing gear was sketchy and broke frequently. Did its job, barely.

MY SOLUTIONS:

So, to address the cons (and because I never bring BOB home) I'm doing a full redesign.

First, the biggest thing I want to have this year is sufficient power. To this end, I'm getting rid of the Emax 3520 925kv motors that served me faithfully for 2 years in favor of Propdrive 5060 380kv motors (double the size baby) on 6S. This should get me the thrust I need to keep this thing in the air, and keep things exciting.

Unfortunately, this means I need to buy new batteries. I'd been running two 4S 6600mah packs for two years, I'm going to be swapping to four 3S 5000mah packs in parallel and series for a net 6S 10000mah pack, with a dedicated 6S 5000mah on each side. ESCs can stay the same though, which spares my wallet slightly.

I'm starting the build earlier this year, which has given me more time to plan. Larger control surfaces are certainly in the works, and proper landing gear will be a priority.

Expect more updates soon, for now, I'll leave you with some rough sketches I did while on a plane a few days after last year's event.

IMG_20180720_063335545_HDR.jpg
Rough Stats:
Wingspan: 150 inches
Chord 26-30 inches
Length: 96 inches
Width: 18 inches
IMG_20180720_063325651_HDR.jpg
This tail is still hopelessly overbuilt, I'll fix it in Fusion 360 later.
IMG_20180720_070007888_HDR.jpg
Don't like this spar design, but I'll expand on my plans in follow up posts.
 

Fluburtur

Cardboard Boy
#3
I think you could make the control surface larger yet, also you probably dont need dihedral, that joint will be a weak point but do as you want on that.
For easier balance you can make the fuselage longer and put the wings further back.

also BOB vs STAN, everyone wants to see that
 

Tench745

Well-known member
#4
I think you could make the control surface larger yet, also you probably dont need dihedral, that joint will be a weak point but do as you want on that.
For easier balance you can make the fuselage longer and put the wings further back.

also BOB vs STAN, everyone wants to see that
Certainly not that much dihedral. A degree or two would be plenty.

Also, not having to carry around all that dead weight for balance would help solve your under-powered situation too.
 
#5
Certainly not that much dihedral. A degree or two would be plenty.

Also, not having to carry around all that dead weight for balance would help solve your under-powered situation too.
One thing that I think you two might be forgetting is that I don't use ailerons on BOB. It's pure rudder, elevator, and differential thrust.

Since it's high wing I agree I probably don't need much, but it's never been an issue before. That picture is definitely exaggerated, as I said the sketches are from over a year ago and my design plan has changed since then.
 

Tench745

Well-known member
#6
One thing that I think you two might be forgetting is that I don't use ailerons on BOB. It's pure rudder, elevator, and differential thrust.

Since it's high wing I agree I probably don't need much, but it's never been an issue before. That picture is definitely exaggerated, as I said the sketches are from over a year ago and my design plan has changed since then.
Ah, I had not realized BOB was rudder only.

I would consider adding ailerons for increased maneuverability and leaving out the rudder since you have differential thrust for yaw control. These are my personal preferences, so please feel free to ignore them.
 
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willsonman

Builder Extraordinare
Mentor
#7
Cons:

Super lacking in power, keeping it in the air was a constant struggle.

MY SOLUTIONS:

First, the biggest thing I want to have this year is sufficient power. To this end, I'm getting rid of the Emax 3520 925kv motors that served me faithfully for 2 years in favor of Propdrive 5060 380kv motors (double the size baby) on 6S. This should get me the thrust I need to keep this thing in the air, and keep things exciting.
Ok, so I agree with this, given that this is about the size motor I fly on almost all my airplanes now. I'm super stoked to maybe put out a fire from BOB!!

But seriously, There may be something available on-site this year to address this in the 420kV variety. I have word from the man himself that there is a goal to have it there for these kinds of builds.
 
#8
Ok, so I agree with this, given that this is about the size motor I fly on almost all my airplanes now. I'm super stoked to maybe put out a fire from BOB!!

But seriously, There may be something available on-site this year to address this in the 420kV variety. I have word from the man himself that there is a goal to have it there for these kinds of builds.
While I'd love to avoid the expense of the motors, one thing that sets BOB apart from a lot of the big builds that we see every year is that I'm flying, from day one.

It's awesome to know that some variety of spares might be available for the inevitable, but I don't want to leave any of the build (or hopefully a few test flights) all the way until I'm at the event.

Awesome to know there might be more support for big builds from event organizers. I'd also love your input on the CAD I post soon. (ish)

Cheers
 

foamtest

Toothpick glider kid
#9
I cant wait to see Bob in action again! It also might be worth putting som ailerons on him just to make the wing easier to build. And since you'll have the power to pull off some more interesting manuvers, and have more control over Bob in general, I think it should be something to think about.
 

kdobson83

Well-known member
#10
I agree with stated above. Flatten out that wing, and add Ailerons. Don't know if differential thrust will be adequate enough for yaw and such a big/heavy bird but adding ailerons will give you SO much more control. Unless your goal is to have a large/slow easy target for people to destroy their planes on. Lol
 
#11
Sounds like the general consensus is to add ailerons. (Not surprising)

Right now I'm inclined to stick with the formula that has worked in the past and stick with the goofy style and dihedral. I'll reveal what I go with when I post the initial CAD.
 

b-29er

Active member
#14
some thoughts i had on BOB after seeing (and audibly hearing) the flight last year:
-Take a look at how full-size aircraft do their controls. They usually weight out control surfaces so the hinge is just behind the center of mass. In other words, creating a scoop or section that sticks forward and counterweighting it may not be a bad idea to prevent flutter.
-having a CF tube or something of the line on the LE to offset head-on attacks to the wing (the highest energy attacks possible) may not be a bad idea. You clearly have this well implemented on the horizontal stab/vert stab
-Home depot rulers make good wing spars. The grains are in a uniform direction, unlike plywood, and material quality tends to be very good. If you are around a Menards, their rulers are thicker, stronger, and don't have the indentations for the inch marks, making them even stronger
-If you have the $ for a 4-engine setup, i'd more suggest a fore and aft engine on a nacelle that extends in front of and behind the wing. That way the loss of an engine leads to less of a potential torque issue.
-You can also go bigger on your engines. If you want to run a 6s setup, i'm currently going to be running some 5055 600kv motors on my b-36 build that should put off around 5-6kg of thrust each. (link, its hobbyking). In a counter-rotating setup, this should null torque and give you the kind of power for some sick vertical or very efficient cruising, since a set of higher pitch props will put off a good cruise power while keeping a good pitch speed.
-BEFORE YOU SET UP WING PLACEMENT: Cad this all out. If you are using Autocad LT, you can use Hatch to figure out the 2d area of various components you are using. Combine this with density analysis of materials (put your material on a scale, and figure out how much a square inch weighs, then multiply by the square inches of the material in your component) and you can get a rough analysis of how much your tail weighs. do a weight and balance on your aircraft without the wing, and use that to determine where to put your wing.
-If you're putting hinges somewhere, put them on wood, even if its just balsa, and screw the hinges in. This will help you distribute shocks from impact better.
-Place reinforcing elements to connect the rudder and elevator. like fishing line or even a strut. That way, if something does hit the elevator, the force will also be pressed into the rudder and pull on the other side of the elevator, and energy might be discipated enough to save your control surfaces
-mini rudders on the tip of your elevators. Give yourself enough authority so that if someone rips off your main rudder, you have enough stability to stay in the fight
-you may also want to attach all your control surfaces to a single piece of wood or structure to prevent discipating energy into foam.
-Split your elevator! Using 2 elevator servos means redundancy if one fails or gets ripped off.

Also on your dihedral, something to consider: Judge your piloting skills. Its a lot stronger with 2d materials like foamboard and wood sheet to reinforce a 2d structure and keep your wing joint from becoming a weak spot if someone strikes from below/above. If you think your skills are good, i'd build out as much dihedral as you can. If you want a plane that flies itself, use that high dihedral.
 
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#15
some thoughts i had on BOB after seeing (and audibly hearing) the flight last year:
-Take a look at how full-size aircraft do their controls. They usually weight out control surfaces so the hinge is just behind the center of mass. In other words, creating a scoop or section that sticks forward and counterweighting it may not be a bad idea to prevent flutter.
-having a CF tube or something of the line on the LE to offset head-on attacks to the wing (the highest energy attacks possible) may not be a bad idea. You clearly have this well implemented on the horizontal stab/vert stab
-Home depot rulers make good wing spars. The grains are in a uniform direction, unlike plywood, and material quality tends to be very good. If you are around a Menards, their rulers are thicker, stronger, and don't have the indentations for the inch marks, making them even stronger
-If you have the $ for a 4-engine setup, i'd more suggest a fore and aft engine on a nacelle that extends in front of and behind the wing. That way the loss of an engine leads to less of a potential torque issue.
-You can also go bigger on your engines. If you want to run a 6s setup, i'm currently going to be running some 5055 600kv motors on my b-36 build that should put off around 5-6kg of thrust each. (link, its hobbyking). In a counter-rotating setup, this should null torque and give you the kind of power for some sick vertical or very efficient cruising, since a set of higher pitch props will put off a good cruise power while keeping a good pitch speed.
-BEFORE YOU SET UP WING PLACEMENT: Cad this all out. If you are using Autocad LT, you can use Hatch to figure out the 2d area of various components you are using. Combine this with density analysis of materials (put your material on a scale, and figure out how much a square inch weighs, then multiply by the square inches of the material in your component) and you can get a rough analysis of how much your tail weighs. do a weight and balance on your aircraft without the wing, and use that to determine where to put your wing.
-If you're putting hinges somewhere, put them on wood, even if its just balsa, and screw the hinges in. This will help you distribute shocks from impact better.
-Place reinforcing elements to connect the rudder and elevator. like fishing line or even a strut. That way, if something does hit the elevator, the force will also be pressed into the rudder and pull on the other side of the elevator, and energy might be discipated enough to save your control surfaces
-mini rudders on the tip of your elevators. Give yourself enough authority so that if someone rips off your main rudder, you have enough stability to stay in the fight
-you may also want to attach all your control surfaces to a single piece of wood or structure to prevent discipating energy into foam.

Also on your dihedral, something to consider: Judge your piloting skills. Its a lot stronger with 2d materials like foamboard and wood sheet to reinforce a 2d structure and keep your wing joint from becoming a weak spot if someone strikes from below/above. If you think your skills are good, i'd build out as much dihedral as you can. If you want a plane that flies itself, use that high dihedral.
Definitely some good ideas in here. Some of the input feels a little insulting, (this ain't my first rodeo) but it was all definitely written with the right intent. Great reminders as I work on the CAD.

I had four (maybe five) good flights last year with the plane, but it was absolutely a rush build. I went for something ambitious and had about a week to do it, which certainly wasn't enough time.

To address the motor concerns, if you read the post above you'll see that I'm planning to run a much larger 6S setup this year. In previous years, my wing was also a folded construction, with a central spar and a simple folded outer covering. This year I'm planning to have proper ribs, which will make adding leading edge reinforcement significantly more elegant.

At the end of the day as I design this thing I try to remind myself that it's supposed to be a silly airplane, and that it's going to crash (multiple times) so hopefully y'all can keep that in mind before you judge some of my design choices. All that being said, here's a full parts list:

BOB v4 PARTS LIST:

Motors:
NTM Propdrive 5060 380kv: https://hobbyking.com/en_us/propdrive-v2-5060-380kv-brushless-outrunner-motor.html
Going with these because I'm familiar with the NTM motors, generally have liked them, and trust the build quality. Solid price, should get me at least 5kg of thrust, and I can get away with keeping my current ESCs.

ESCs:
Emax BLHeli Series 80a ESC: https://www.readymaderc.com/products/details/emax-blheli-series-80a-esc
I've used these since FFE 2017 and they've served me well. Large, generally well built, should be exactly what I need, and I halready have them. XT90s soldered on from last year.

LiPos:
A mix of RMRC 3S 5100mah 35C and Turnigy 3S 5000mah 25C
I already have three of the RMRC batteries, so it would be a shame to put them to waste. Buying the Turnigy batteries to supplement my supply (Running 4 batteries). I'm slightly concerned about the mismatch but running them in series it shouldn't be an issue as long as I keep the flight times in check. Combining batteries for two 6S 5000mah packs, one to run each side.

Receiver:
FrSky D8rii: https://hobbyking.com/en_us/frsky-d8r-ii-plus-2-4ghz-8ch-receiver-with-telemetery.html
Already have it, a receiver I trust completely, and with a maximum input voltage of 10V I can eliminate the need for a voltage regulator.

Servos:
Trackstar TS-500HD: https://hobbyking.com/en_us/trackst...al-gear-racing-servo-27-3kg-0-22sec-188g.html
Going with some big servos this year instead of a dual servo setup, I think these should do the trick. Can run straight 2S power, at a reasonable price with more torque than any irresponsible pilot could ever need.

Props:
I'd love some input here. I've never used anything this big. Open to suggestions.

Wheels:
Hobbyking Scale: https://hobbyking.com/en_us/scale-alloy-hub-rubber-wheel-3-75inch.html
These looked cool, not much more to say.

Other than that, there will be some long wire runs for motor wires and servo wires, but it shouldn't be too crazy. I have some other fun parts that I'm going to be keeping close to my chest for now, but it should be a fun build process.

Please shoot any ideas my way, all the input will be invaluable as I try to put on a better show than in previous years.
 

b-29er

Active member
#16
Definitely some good ideas in here. Some of the input feels a little insulting, (this ain't my first rodeo) but it was all definitely written with the right intent. Great reminders as I work on the CAD.

I had four (maybe five) good flights last year with the plane, but it was absolutely a rush build. I went for something ambitious and had about a week to do it, which certainly wasn't enough time.

To address the motor concerns, if you read the post above you'll see that I'm planning to run a much larger 6S setup this year. In previous years, my wing was also a folded construction, with a central spar and a simple folded outer covering. This year I'm planning to have proper ribs, which will make adding leading edge reinforcement significantly more elegant.

At the end of the day as I design this thing I try to remind myself that it's supposed to be a silly airplane, and that it's going to crash (multiple times) so hopefully y'all can keep that in mind before you judge some of my design choices. All that being said, here's a full parts list:

BOB v4 PARTS LIST:

Motors:
NTM Propdrive 5060 380kv: https://hobbyking.com/en_us/propdrive-v2-5060-380kv-brushless-outrunner-motor.html
Going with these because I'm familiar with the NTM motors, generally have liked them, and trust the build quality. Solid price, should get me at least 5kg of thrust, and I can get away with keeping my current ESCs.

ESCs:
Emax BLHeli Series 80a ESC: https://www.readymaderc.com/products/details/emax-blheli-series-80a-esc
I've used these since FFE 2017 and they've served me well. Large, generally well built, should be exactly what I need, and I halready have them. XT90s soldered on from last year.

LiPos:
A mix of RMRC 3S 5100mah 35C and Turnigy 3S 5000mah 25C
I already have three of the RMRC batteries, so it would be a shame to put them to waste. Buying the Turnigy batteries to supplement my supply (Running 4 batteries). I'm slightly concerned about the mismatch but running them in series it shouldn't be an issue as long as I keep the flight times in check. Combining batteries for two 6S 5000mah packs, one to run each side.

Receiver:
FrSky D8rii: https://hobbyking.com/en_us/frsky-d8r-ii-plus-2-4ghz-8ch-receiver-with-telemetery.html
Already have it, a receiver I trust completely, and with a maximum input voltage of 10V I can eliminate the need for a voltage regulator.

Servos:
Trackstar TS-500HD: https://hobbyking.com/en_us/trackst...al-gear-racing-servo-27-3kg-0-22sec-188g.html
Going with some big servos this year instead of a dual servo setup, I think these should do the trick. Can run straight 2S power, at a reasonable price with more torque than any irresponsible pilot could ever need.

Props:
I'd love some input here. I've never used anything this big. Open to suggestions.

Wheels:
Hobbyking Scale: https://hobbyking.com/en_us/scale-alloy-hub-rubber-wheel-3-75inch.html
These looked cool, not much more to say.

Other than that, there will be some long wire runs for motor wires and servo wires, but it shouldn't be too crazy. I have some other fun parts that I'm going to be keeping close to my chest for now, but it should be a fun build process.

Please shoot any ideas my way, all the input will be invaluable as I try to put on a better show than in previous years.
My apologies, wasn't trying to insinuate anything. Judging by the HK reviews, looks like folks are using 18 and 19" props. I'd keep pitch speed low, unless you're looking for speed.
 

rockyboy

Skill Collector
Mentor
#19
You might be able to take some weight out of the tail by switching your wood structural support pieces to fiberglass arrow shafts instead. You can get a dozen shipped from China for about $18 on eBay - or a dozen carbon fiber arrow shafts for about $28
 

Fidget

Active member
#20
@rockyboy, how would you hold the arrow shafts together in that case? Wood you can glue or screw or plate together, but I can't picture how @remzak would directly replace his wood design with cf or fiberglass. (Asking because I feel like I'm missing something, not to imply it's not a good idea.)