Stubby SR-71 for 40mm EDFs — Build Guide

At a glance

• Difficulty: Beginner to build, intermediate to fly
• Wingspan: 755 mm (29.7")
• Length: 762 mm (30.0")
• Flying weight: 800–900 g (1.8–2.0 lb)

Overview

Thank you for purchasing a model by Lofted Aero! 3D printed aircraft are an exciting new segment of the hobby, and we've got no shortage of ideas for new designs. Your support helps us make those reality.

The Stubby SR-71 is a playful, cartoonish take on the legendary Blackbird – the same unmistakable silhouette squashed into a "fattie" that's a ton of fun to fly. Twin 40mm EDFs tucked into the nacelles give it character and performance, while the compact airframe keeps things light and easy to handle.

Skill meter

Build: With a simple design and low parts count, this build is just about as easy as it gets! It is well suited to be an R/C pilot's first transition into 3D printed models.

Flight: This model has good handling characteristics and a stable delta wing planform. But it's still a fast, nimble jet that is best flown by pilots of intermediate skill or above.

Specifications

Spec	Value
Wingspan	755mm (29.7")
Length	762mm (30.0")
Wing Area	24.7dm2 (2.66ft2)
Print Weight	~383g
Flying Weight	800-900g
Wing Loading	32-36g/dm2 (11-12oz/ft2)
Airfoil	PW-75 modified

Recommended equipment

The following hardware & electronics are required to complete this model. In addition, you'll need some CA glue and activator, your R/C transmitter and receiver, and a LiPo battery charger.

Power system & avionics

Item	Details
EDF	2× XFly 40mm EDF for 4S
ESCs	2× 20A ESC
Battery	4S 1500 – 2200mAh LiPo
Servos	9g Metal Gear (only 2x required) or FMS 9g Metal Gear with 300mm Wire
Servo Extensions	200mm/8"

Filament

Good filament matters more here than on most prints. The materials below have been tested with our filament and process settings and produce good results. Alternate options are listed as well, though weight and durability may vary.

This model is often finished in black, which gets hot in the sun – so it leads with the higher-temperature materials rather than the usual foaming LW-PLA.

Use	Filament	Amount
LW-ASAHeat and UV resistant. Needs a heated enclosure.	ColorFabb LW-ASA or Bambu ASA Aero	~340 g
PETGHigher heat tolerance.	Bambu PETG Basic or Creality PETG	~43 g
Alternate material options
LW-PLA replaces LW-ASALighter, and no enclosure needed. Prints on the 0.28mm LW-PLA profiles — see Profiles used by this model.	3DLabPrint PolyLight or ColorFabb LW-PLA or Bambu PLA Aero	—
PLA replaces PETGStiffer, but less heat tolerant. Same process profile — load a PLA filament preset.	3DLabPrint PolyAir or eSun PLA+ or Bambu PLA Tough+	—

Hardware & structure

Item	Details
Wing Spar Tube	Carbon Fiber 6mm x 4mm x 500mm Tube (only 1x required)
Pushrods	1.2mm Pushrods with Linkage Stoppers (only 2x required)
Fan and Hatch Mounting Screws	#2 x 3/8" Tapping Screws (or equivalent, only 8x required)
Hatch Latch	Pen spring (available here)

Printing thin-wall models

Desktop 3D printers are perfectly capable of producing great-flying R/C models durable enough to withstand hangar rash and general use. However, aircraft designs contain a unique mix of thin surfaces and intricate solid supports that require some practice to print perfectly. Keep the following tips in mind when printing your model.

Slicing the project files

The model download includes .3MF project files for both Bambu Studio and OrcaSlicer – modern, free, and open source slicers with wide support for popular printers and the ability to utilize multiple plates and varying settings within a single project. These project files include print settings tailored to the model as well as filament settings with adjusted temperature and retractions. The provided settings have been tested to suit a wide range of printers – in most cases, no changes should be necessary aside from choosing your printer.

Open the folder for the slicer you're running. The two sets are equivalent and the workflow is the same in both, but each is written in its own slicer's settings dialect, so they aren't interchangeable.

The process and filament presets these projects use are our own. You don't need them to print – the project files already carry the settings – but importing them once from the Print Settings page keeps them in your slicer for future projects, and that page explains what each one is for.

Generic example screenshots

These happen to show the LW-PLA profiles being picked. The profiles this model uses are listed just below.

Profiles used by this model

Project file	Process profile	Filament profile
LW Parts	Lofted Aero 0.20mm LW-ASABambu Studio OrcaSlicer	Lofted Aero Tuned ColorFabb LW-ASABambu Studio OrcaSlicer
Non-LW Parts	Your slicer's own 0.20mm Standard	Your slicer's own Generic PETG

LW-ASA needs a heated enclosure. Hardware parts like the battery tray, mounts, skids and latch ride your slicer's own stock profile, which is already matched to your printer.

Printing the airframe in LW-PLA instead? Substitute Lofted Aero 0.28mm LW-PLA for the LW-ASA process profile, with the Generic Foaming LW-PLA filament profile from the Print Settings page – no enclosure needed. The non-LW parts print the same way on a PLA filament preset. Everything else in the table is unchanged.

Modifiers and per-object settings

Both slicers allow per-object modifications to slicing settings, and this method is used heavily in the provided .3MF files. Where a part needs extra perimeters, different infill, or a relocated seam, that's applied as a per-object modifier on top of the profile rather than as a separate profile. When making settings changes, be mindful of these per-object settings as well as any other modifiers applied to each part. Switch the Process panel from Global to Objects to see every part in the project, the settings overridden on it, and any modifier volumes attached to it.

(example screenshot shown – may not represent this model)

Mirrored parts

You'll notice that the included STL files often contain parts for only one side of symmetrical components like wings, tail surfaces, and landing gear. Don't worry – you can simply mirror these parts in your slicer to produce the other side. When using the included .3MF project files, this mirroring has already been done.

Other printing options

If you'd rather configure and slice the STL files yourself with the software and method of your choice, refer to the print table below for the profile and modifiers applied to each part.

A few things to keep in mind:

• Retraction should be just enough to prevent stringing between features
• Extra length on restart should be just enough to prevent sparse extrusion at layer start
• Extrusion ratio should be adjusted until print weight equals the suggested part weight

Print table

Reference print profiles, per-part modifiers, and weights for each part.

Part	Weight	Print Profile	Modifiers
LW-ASA Parts
Fuselage 1	19g	Lofted Aero 0.20mm LW-ASA0.20mm Height 0.42mm Extrusion Width 1 Perimeter 3 Top / 2 Bottom Layers 2D Lattice @ 4% Back Seam Classic Wall Generator 255°C Nozzle / 90°C Bed	Cubic Infill
Fuselage 2	25g
Fuselage 3L	48g
Fuselage 3R	48g
Fuselage 4L	44g
Fuselage 4R	44g
Fuselage 5	11g
Hatch 1	11g		Cubic Infill Cube Modifier with Localized 10% Fill
Hatch 2	10g		Cubic Infill
Hatch 3	2g
Fan Hatch L1 & R1	3g each		Cubic Infill Cube Modifier with Localized 25% Fill
Fan Hatch L2 & R2	2g each
Wing L & R (x2)	18g each		—
Elevon L1 & R1	2.5g each
Elevon L2 & R2	6.6g each
Vertical Tail L	6.3g
Vertical Tail R	6.3g
Vertical Tail Tabs L & R	0.4g each		Cubic Infill
Total LW-ASA Parts	340g
PETG Parts
Fan Mount (x4)	0.3g each	Your slicer's own 0.20mm Standard0.20mm Height 0.42mm Extrusion Width 2 Perimeters 5 Top / 3 Bottom Layers 15% Infill (Grid in Bambu Studio, Cross Hatch in OrcaSlicer) Aligned Seam Classic Wall Generator 255°C Nozzle / 70°C Bed	100% Fill
Fan Hatch Mount (x4)	0.35g each
Hinge Point (x4)	0.75g each		—
Nozzle L & R (x2)	9g each		1 Perimeter 0% Fill Back Seam No Brim
Nose Skid	2.4g		—
Ventral Skid (x2)	2g each
Hatch Latch	4g		100% Fill
Battery Tray	9g		—
Total PETG Parts	43g

Joining parts

Unless otherwise specified, medium CA and activator are recommended for joining printed parts. In addition, many joints use ~18mm lengths of 1.75mm PLA filament for alignment. Preparing a handful of these ahead of time can help speed up the build.

Fuselage assembly

1. Use a hobby knife to remove the support structure from the hatch cutout area of fuselage section 4L.

   

2. Using filament guide pins and medium CA, join fuselage sections 3L and 4L.

   

   

3. Repeat with the opposite side of the fuselage.

   

4. Join the left and right fuselage 3/4 sub-assemblies together, also using CA.

   

5. Add fuselage section 2 to the assembly with guide pins and CA.

   

6. Similarly, attach fuselage section 1.

   

7. Glue the nose skid into its slot on the underside of fuselage sections 1 and 2.

   

8. Attach fuselage section 5 at the rear of the assembly.

   

9. Finally, attach the nozzles to complete the fuselage assembly.

   

   

Wing and control surface assembly

1. Slide the 6mm x 4mm x 500mm carbon tube spar into its channel in the fuselage assembly. Ensure that it protrudes evenly from both sides. Glue is optional but not necessary.

   

2. Glue the left and right wing sections to the fuselage assembly.

   

3. Join the two sections of each elevon.

   

4. Glue two elevon hinge points into each wing.

   

5. Hinge the elevons to the wing using a length of filament. Trim the end and secure with a drop of CA.

   

   

Power system installation

Check fan direction

Test each fan to ensure it rotates in the correct direction before installing. Install fans only after confirming safe and correct R/C setup.

1. Glue the fan mounts and hatch mounts into their slots in the nacelles.

   

2. Mount each EDF unit to the tabs, securing it with self-tapping screws.

   

3. Join the two halves of each fan hatch. Now is also a good time to glue the ventral skids in place.

   

   

Servo installation

1. Prepare and center the two elevon servos – this model uses 9g metal-gear servos, one for each elevon.

   

2. Install an elevon servo in its pocket and connect its pushrod to the elevon using your preferred method.

   

3. After securing the wiring for both the servo and fan, screw the fan hatch in place with self-tapping screws.

   

Avionics and final assembly

1. Install a pen spring and hatch latch between hatch sections 2 and 3. Be sure not to accidentally glue the moving parts.

   

   

2. Glue on hatch section 1.

   

3. Install the battery tray with straps or a fastening method of your choice.

   

4. Install the ESCs and receiver, routing and securing the wiring as needed.

   

5. Glue the small protruding vertical tail tabs to the underside of each vertical tail.

   

6. Glue a vertical tail onto each nacelle.

   

Pre-flight setup

The recommended CG is marked on the underside of the wings. Ensure that the model balances at this location with the battery, electronics, and all hatches installed.

The elevons require upward reflex. The neutral elevon position for the recommended CG is such that the upper elevon surface is parallel to this groove near the nozzle.

Adjust control deflections using the suggested throws below. Dual rates are optional depending on your preferences.

Suggested control throws

Flight Control	Travel	Exponential
Aileron	15 mm up / 15 mm down	30-50%
Elevator	20 mm up / 20 mm down	20-30%

Congratulations - you're ready to fly!

Contact us

Have a question, issue, or just a cool idea for the next aircraft we should model? Drop us an email at: info@loftedaero.com