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The Other XB-70 Build

#1
As you may have seen, Mid7Night has recently started an impressive XB-70 build here.
Coincidentally, I have been planning an extremely similar build to his for the past month or so.

My goal is to build a 1/20 scale (9.45 foot) XB-70 with 4 EDFs and retracts with a total AUW under 4.5kg (10 lbs) out of mostly DTFB.

Electronics__________

-4x 70mm EDFs of some sort. They need to run on 4s, and would preferably be counter-rotating. My "smaller" scale test model only turns to the left well because of prop torque. I've got my eyes on these freewing units with two of these counter-rotating blade sets. They should be decent quality and a pretty low cost. They produce a total of 6kg of thrust (probably closer to 4kg with ducting and real-world performance) at full tilt.

-4x BLHeli 80A ESCs. I've had good experiences in the past and they're nice and cheap.

-1x Castle Creations 10A BEC. It's a nice, low cost BEC from Castle, who makes really good R/C car stuff. I'll need the extra amperage because the BLHeli UBECs are known to be weak and I'll be running retracts :cool:

-5x Emax ES3154 servos. one for each elevon, one for each rudder, one for the nose wheel. These are the servos that Flite Test used in the new piggyback glider mechanism. They're durable, torquey, and should have more than enough power to move the surfaces and nose gear. plus, they're metal-geared.

-Some sort of retract set. I'm a kid in school, so Robarts are out of the question. I looked online and found these beauties, but they're too short to be scale (~100+mm mains). I might cobble some stuff together from hobbyking and make my own bogies, or buy these and put longer struts on them.

-4x Gens Ace 2s 5000mah LiPos. I already have 2 of these from my R/C Truck (Traxxas Summit). I could run just two in series, but at a discharge rate of 50c, my EDF units would be pulling extremely close to the 250A limit at full throttle. As ducting increases current draw, I'll be safe and double my (already short) flight times by adding another two. This will give me a 4s 10000mah pack which can put out 500 amps. I'll be able to add two more fans in the future if i want to.

-Thick wires. There's going to be a long wire run from the batteries to the ESCs, and I'll be pulling just shy of 250 amps through them. I'll also put some capacitors right in front of the ESCs to stablilize the voltage.

Airframe__________

-The majority of the plane will be made out of DTFB, some (complex curvy bits) with the paper removed. Wood will only be used in high-stress or mission-critical areas (landing gear mounts, servo holders, wing hardpoints).

-The general style will be much different from the traditional FT style: I'm going for scale looks, not ease of assembly. If you're going to make a bird this big, you might as well make it look pretty. I'm going to use the techniques from the fuselage of my smaller model on a larger scale. Here's the extent of the plans so far:

Img.jpg

-It'll break into four sections for transit: the front fuselage, rear fuselage/engine section, and the two wings.

Miscellaneous Stuff__________

-I haven't flown an EDF before. Some of you are probably freaking out and convinced that I will crash. The thing is, I've identified the three things that generally separate the flying characteristics of an EDF from a prop plane:
1. No propwash over the control surfaces
2. High wing loading
3. Slow throttle response (spool up/down) times
and made sure that they will be at least partly mitigated in the design. I've already flown my pusher-prop XB-70, and know how it feels to have no propwash on the surfaces. This model actually has really low wing loading, which should be not that much higher than my test model, which is excellent at high-alpha and low speeds (see the flare at the bottom of the hill). The high thrust-to-weight should allow for quicker accelerations than a lower one. If all else fails, I can just buy two of the EDFs and make an EDF trainer model while I wait to get the money for the other two.

-My radio is more than a bit limited. I have the base-model Graupner Tx/Rx combo (MZ-10). It's very reliable, but I only have one switch which will be used for the gear. So no flaps, parachutes, drooping nose, or folding tips until I get a better radio. Gear doors will still be a thing though, you can pull some magic with levers and springs. Also no expo. Dual rates are 70% or 100%, on a switch. I'm not planning to go fast with this model (hence 4s instead of 6s) and have flown with no expo all my life, so I think I'll be fine. I can always kick it down to 70% for a high-speed pass.

-It'll be at Flite Fest East 2018, assuming I don't crash it or anything ;)

That's all, I guess. I'll try and finish the plans tomorrow (not tonight, I have to study for the PSAT).
 
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#2
Well well well! (that's three holes in the ground, as my grampa would say...) :p

I love that this idea was sparked in someone else's head at the same time. Good luck on your plans and your build, gotta watch the competition close around here!! ;)
 
#4
A word on retracts

The main reason for having 4 EDFs instead of 6 in my design is to allow for a faired-in section on each side of the fuselage where the main gear will sit. This keeps the retracts from interfering with the intake air, which reduces performance.

Here's the finalized parts list for my retract set:

Mains. Yes, they're $8 a piece. But I've heard that they're actually pretty decent over on RCgroups, and I'll be keeping weight at least 0.5kg under their max capacity. If they break, then what the heck. It's another $8 to replace.
Main Oleos. These are some beefy oleos with a decent pin size (5mm). I may have to modify the retracts to get the gear to swing back, not in, although this should be as trivial as just grinding another flat in the shaft.
Nose. Again, this is cheap, but the reviews are good and it can carry the weight.
Nose Oleo. Not much to say about this, other than that it's 10mm longer than the mains: I want some angle of incidence on level ground for easier take-off.
Tires. These are some nice cheap wheels that are slightly larger than scale (I want to be able to take off from multiple surfaces, not just the flattest of runways).

I'll be making my own bogies from some 4mm wire I have in my garage, either wrapped in copper and soldered or just spot-welded. The total retract cost is just $60, compared to $100 more some better-looking but less scale ones.

I'm debating putting a "servo saver" on the steering servo to increase the lifespan in the event of a hard landing.

Time to go to class. Plans are still on the way, life has been interrupting them for a while now.
 
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JimCR120

Site Moderator
#5
Ooh, very interesting! Pieliker where will you be flying this once built? Any plans of bringing it to one of the Flite Fests? Regardless of where, video posts will be necessary. Looking forward to it.
 

JimCR120

Site Moderator
#7
Nice. I'm lived in Lake County all my life until my Navy days, now in Maine and looking to relocate to NC. I'll look forward to seeing you and your Valkrie at FFE next year.
 
#8
Got the first build log done. It's not the best quality (especially the audio) because of how I recorded it. Future build logs will be more refined and show progress on the build, while this one was mainly me talking about the plans.

 
#9
Here's the progress so far this weekend:

I got the front fuse built and about halfway skinned. I also built the Vstabs- they're two sheets of foam thick and have a hinge on both sides.

This was my first time covering with foam. I removed one side of the paper to make it easier to bend, and to save weight.I'm happy with the results overall, although there are some nicks and small creases in the surface. The fuselage is very light, due to the paper being removed from the formers. The two prongs that will be used to connect the front fuse to the main body are definitely strong enough to hold the front fuselage, as I can swing it around by them.

The distance between the vertical "spars" is 2", which is just enough for me to fit 2 of the batteries I'm planning on using. I'm still not sure where I will have to put them though, so I'm building the entire plane and then cutting the battery hatches.

2017-10-22 13.05.49.jpg
 
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#10
Got the nose done, although I don't like the shape. It seems a little bit too thick and doesn't seem to follow the curves of the real XB-70 (when we see it on the ground, at least). This is because my CAD model that I based the plans off of is of the plane configured for supersonic flight (windscreen elevated). I'll fix this when I can, but I won't be able to go home for another two weeks now. During that time, I'll also put some fixed landing gear on my test model and practice landings.

Here's the second build log. It's closer to what I want them to be, although I feel this one was too fast-paced.

 
#11
Made some more progress this weekend, although not enough to warrant a proper build log. I built the canopy / windscreen and the left wing, as well as re-forming the nose. The wing consists of some ribs on a spar wrapped in foam. I also laid out the existing pieces to get an idea of how massive this thing will be - human for scale.

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I've also been experimenting with making my own bogies for the main gear - I was originally going to use some thick steel wire, but I have found a better solution. 4mm diameter brass rods solder very well, and are respectably stiff. Joints can be reinforced by wrapping them with small-gauge wire, which also helps to hold them in place prior to soldering.

We're progressing slowly but surely...
 
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#12
I ended up spending most of my time over break working on retracts for my mini FT-22, and didn't get around to the XB-70 until today.

I got the other wing done and the center fuse/nacelle started. I don't think a foam spar is going to cut it, so I'll probably end up adding one or two wooden dowels, probably around 1" in diameter.

The plane itself takes up around half of my usable room space when fully assembled, but stows to a much smaller footprint when disassembled. Hopefully I won't have to rent a trailer in order to take it to Flite Fest.

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The next time I'll have to work on this beast is during winter break, about a month away.
 
#14
F106: Heck yeah it will! ;) This bird is going to have a huge presence in the air, even larger than my test model.

I've got the ducting all figured out. The intakes are 105% FSA (I've read that cheap Changesun-style fans tend to be hungry for air). The side fairings are for the main gear, and the center splitter is for the nose gear and possibly some batteries, depending on where the CG ends up. This weekend, I'll finish up the top mid fuse and maybe get started on the nacelle.

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FoamyDM

Building Fool-Flying Noob
#15
Looking great! Based on that size how are you planning to strengthen this behemoth. It seems the flight forces may be too much for simple DTFB.
 
#16
There are a couple of areas that concern me with regards to structural rigidity:

Landing gear mounts
_____
These will be connected to the main wing spar and front reinforcement structure with double-layered foam and plywood reinforcement, in a box that rests on the bottom of the mid-wing.. These need to be very structurally sound, especially the nose gear, as it has to carry the weight of the long nose and (potentially) four batteries.

Wing attachment_____
The main spar is currently a 4"x1" foam box spar which connects to the wings. This will have to be reinforced with wooden spars(s) (thinking around yardstick dimensions) in order to properly bear the load of a 5kg plane. The wing has two locating strips, at the leading and trailing edge, that keep the wing from twisting around the spar and keep the leading and trailing edges in line.

Neck attachment area_____
This region of the plane is going to have to be very structurally sound, as it supports the nose gear and nose itself. This was a concern on the real B-70, which is why the nose gear uses the same tires as the mains: it had to support a lot of weight. I may incorporate ply into this area, or just use double-layered foam. The formers for the fuselage and ribs for the wing will form an overlapping lattice, which should be miles stronger than the wings' sheeted rib structure. The formers will hold the receptacles for the two foam prongs of the front fuselage, which may have to be reinforced depending of battery placement. The nose gear mount will push up against the lattice from the bottom and be reinforced longitudinally by attachment to the inner lower nacelle.

It's sort of difficult to translate this idea into words, so here's a crude drawing/labelling of the load-bearing structure in MS paint:

Structure 1.jpg

Structure 2.jpg
 
#18
I finished the load-bearing spars, center wing ribs, and fuse formers in the wee hours of the morning. Tomorrow (well, technically today), I'll work on building the lattice structure in the wings, sheeting both the fuselage and the wing, and determining if the front fuselage is secure enough with 2 foam spars and a friction fit.

Side note, I installed new lighting in my room.

Spars are 3/8" poplar.

Time for some sleep.

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2017-12-10 00.23.55.jpg
 
#19
Here it is so far. I still need to finish sheeting the fuse and center wing, but I've run out of time.

I finally assembled this thing fully for the first time, and I'm quite impressed with the size.

Total AUW without electronics is 1.4kg so far. Electronics weigh in at ~2.5 kg, so I still have a kilo or so to work with (10 sheets of foam). This should be enough to finish up the existing parts and build the underbelly nacelle, although it'll be close.

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Build Log #3 should be up by friday.
 

AkimboGlueGuns

Biplane Guy
Mentor
#20
That sucker is really moving along. I love the application of traditional former/skin construction on the swan neck. It really brings out the XB-70ness in the design.