Pieliker96
Elite member
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
-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:
-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).
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
-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:
-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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