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Flying rotor launcher, reversible
This very simple reversible wind-up launcher sends various right- and left-handed ultralight flying rotors (flyers) to heights of up to ~15 m! The updated short-tailed flyers still spin on their tails like tops after landing.
About this creation
Please feel free to look over the images and skip the verbiage.

Several months ago, my ongoing fascination with high-speed rotary motion in the LEGOŽ realm turned to flying LEGOŽ rotors -- "flyers" for short.1 This page presents the 2nd in a series of flyer-related MOCs.



This very simple reversible wind-up launcher for ultralight flyers started out as a top spinner. All I had to do was add a suitable reversible pull-out winder.



The idea of using a LEGOŽ pull-back motor (PBM) to launch LEGOŽ flyers came from the only other instance I've run across: The elegant and much more elaborate LEGO Technic Heli Launcher (2015) by favorite builder Desert752_Kirill.

The video below shows my original PBM launcher lofting its heaviest flyer. This one works in a similar way, but with some important differences.



The 6x5x3 PBM in the launcher featured here is once again TLG's latest and most powerful (12787c01), but this particular unit seems to have a slightly stronger spring than the one in my original PBM launcher. Hence, most of my flyers see a 5-10% gain in maximum altitude with this launcher for that reason alone.

PBMs of this type have more than enough torque and speed to launch ultralight flyers weighing under 10 g. Heavier flyers require ripcord-powered launchers2 (stay tuned).

On this page:


Operation

Warning! Always wear eye protection when working or playing with high-speed LEGOŽ rotating machinery and keep valuables and bystanders (including pets) a safe distance away -- especially when testing new designs. Really.



Step 1: Determine the handedness of the flyer to be launched. (For example, place the flyer tail-up on a table with one blade at 12 o'clock. If the right edge of that blade is closer to the table, the flyer is right-handed.)

∨ Step 2: Grasp the launcher handle with the PBM away from you. The PBM's orange axle hole should be on your right for a right-handed flyer (e.g., the red one below) and on your left for a left-handed flyer (e.g., the yellow one below). If not, just flip the launcher over.





Step 3: Insert the flyer's tail into the PBM's orange axle hole.

∨ Step 4: Retract the winder's safety pin (without bevel gear bearing) into its handle and insert its winding shaft (with bevel gear bearing) into the lower end of the PBM.



∧ Step 5: Push or pull the winder's winding shaft in or out as needed to seat the flyer's tail at its optimal depth within the PBM.

Step 6: Wind up the PBM until it resists strongly. Then push the safety pin into the nearest open hole in the launcher's yellow frame to lock the winder in place. (Counting your turns will help you avoid over-winding.)

Step 7: To launch the flyer, firm up your grip on the launcher handle (to help you keep your aim) and yank the entire winder out of the launcher as sharply as possible.

Step 8: Keep ultralight flyers in sight from launch to landing as best you can, as they're very easy to lose.

As explained here, leaving no portion of the winder behind insures that the PBM has nothing to spin up but the flyer itself.




Overview

The reversible launcher featured here couldn't be simpler.



Here, the launcher is seen here with right-handed Flyer4 mounted.










New and improved flyers

The right-handed ultralight flyers presented with my original launcher all work just fine in this new one. This section presents (i) some new left- and right-handed long-tailed flyers and (ii) refined versions of some short-tailed originals. The flyer IDs below pick up where the originals left off.

Before proceeding, however, some flyer anatomy is in order. Every flyer has (i) a thrust-generating "rotor" consisting of a LEGOŽ propeller with 2 or more blades, and (ii) a cross-axle "shaft" connecting the rotor to the launcher during spin-up.



Flyers with rotors that can't be keyed directly to their shafts (e.g., Flyer4 above) also need (iii) a "hub" connecting rotor and shaft with as little slip as possible. The portions of the hub and shaft below the rotor make up the flyer's "tail".

The updated short-tailed flyers still spin on their tails like tops after landing, but the new long-tailed flyers topple on impact.



Yes, I'm still obsessed with spinning tops.

∨ Flyer2I: This impure update of originally pure Flyer2 replaces the original shaft (a stopped 3L axle) with a 5.5L stopped axle cut down to 3L.



The result (left and center below) is a 1L tail that won't retract into the rotor on impact. The bush hub, 13L 4-blade rotor, and total mass (7.2 g) are unchanged.



This update was motivated solely by the original's tendency to end up with litte or no tail after landing. This detracted from its otherwise excellent performance in top mode.

If you prefer to keep your flyers pure, swapping out Flyer2's original shaft for a black 3L axle (far right in last photo) will reduce this tendency slightly.

∨ Flyer3S: This updated impure 2.5 g flyer (center) replaces the 3L axle serving as shaft in the original Flyer3I (far left) with a 2L notched axle.



The result is a 2L tail and a weight loss of 0.2 g (~7%) good for a small (5-10%) gain in maximum altitude relative to the original. Maximum altitude remains in the 4-5 m range. The taped bush pin hub and 9L 3-blade rotor are unchanged.

The shorter tail also enhances performance as a top after landing by lowering the center of mass.

∨ Flyer3T: This impure 2.3 g update (far right below) is just Flyer3S (center) with most of the excess hub trimmed from its nose.



The resulting weight loss -- 0.4 g or 15% relative to Flyer3I -- adds nearly 1 m to Flyer3I's maximum altitude!

Lopping off the nose of Flyer3S further enhances performance as a top after landing by lowering the center of mass even more.

∨ Flyer6S and Flyer6A: A maximum altitude of ~15 m makes the new and very impure long-tailed flyer at lower left (Flyer6A) my highest PBM-powered ultralight flyer of all time! Flyer6S is at upper right.



The yellow rotors here are modified left-handed 5.5L 2-blade LEGOŽ "twisted" propellers3; the shafts, 8L stopped axles. The hubs consist of half-bushes and (gasp) superglue.

Flyer6S (below right) retains the prop's original slab-like blade profile -- hence the "S" suffix for slab. It weighs 2.4 g and climbs ~10 m with the PBM launcher featured here -- i.e., ~4 m or 67% higher than previous champ Flyer4.



Flyer6A (above left) differs from Flyer6S only in having rotor blades (gasp) hand-sanded into proper airfoils. It weighs a mere 2.2 g and reaches a maximum altitude of ~15 m with this launcher. The gain of ~5 m over Flyer6S stems almost entirely from the increased thrust brought by improved blade aerodynamics.

In both cases, the 7L tails are just long enough to eliminate wobble in flight. Both would also work well with a left-handed version of my original PBM launcher.

Making a torque-friendly flyer out of a 5.5L 2-blade prop isn't easy. Mounting the prop on a taped bush pin doesn't work, as the mounting hole inside the unmodified hub is slightly larger than a standard Technic pin hole.

To make Flyer6S, I first removed the cap from the prop's original hub so that an axle could pass through. Then I applied superglue to the stopped end of the 8L shaft and slid on the prop and the half-bush, making sure (i) that the prop was tightly sandwiched between the axle stop and bush, and (ii) that glue was forced between all mating surfaces.



To make Flyer6A, I first hand-sanded the prop's original blade profile into a typical airfoil profile using a series of ever-finer emery boards. Then I proceeded as with Flyer6S.

NB: Shaping LEGOŽ prop blades into airfoils is rather tedious. Flyer6A is one flyer you don't want to lose in a tree, as I've done more than once.

∨ Flyer7: This new 2.4 g long-tailed purist flyer (upper left below) has a 9L 2-blade LEGOŽ propeller for a rotor and an 8L stopped axle for a shaft.



With the launcher featured here, Flyer7 goes 20-30% higher than the recreation of the flyer used in Desert752_Kirill's LEGO Technic Heli Launcher at bottom right. Most of that difference is due to Flyer7's much lower mass (2.4 vs 3.2 g).

The rotor's central axle hole obviates the need for a hub. As with Flyer6S and Flyer6A, the long 7L tail is necessary to stabilize the 2-blade rotor, but Flyer7 still wobbles slightly in flight. The axle stop on the shaft keeps the rotor from leaving the tail behind.

Flyer7 (far left below) has a maximum altitude of 5-6 m with the launcher featured here. By comparison, Flyer6S (center) nearly doubles that height, and Flyer6A (far right) nearly triples it. The much more efficient 5.5L 2-blade prop used by the latter 2 flyers deserves most of the credit here.



All of the long-tailed flyers above land tail-first but topple immediately. Their centers of mass are far too high, and their axial radii of gyration are too small to allow them to spin like tops after landing.




Reversible launcher

The real beauty of this launcher, as opposed to the original, is its ability to launch both right- and left-handed flyers without modification.





The red flyer above is right-handed; the yellow one, left-handed. Note the orientation of the PBM in each case.

To switch between right- and left-handed flyers, one simply flips the launcher over and winds in the opposite direction.

As with my original PBM launcher, jerking the entire winder out of the launcher initiates flyer spin-up. The launcher's long handle keeps fingers clear of the rotor and helps steady the launcher during abrupt winder detachments.



As before, the job of holding the flyer's tail during spin-up falls to the PBM itself, here seen here with Flyer1 seated.

The PBM's loosely fitting 24 mm-deep power take-off (PTO, orange axle hole at far right) transmits torque to the flyer's tail while keeping it upright.






Pull-out winder

Flyers with their rotors solidly keyed to their tails like Flyer1 and the original Flyer2 below can be used to wind the PBM directly.



However, winding with a flyer takes more effort than with a dedicated winder, and a way to keep the PBM from unwinding prematurely is still needed.

With this launcher, the anti-unwinding tool below works with flyers with tails longer than 4L (e.g., Flyer7), but I find it rather awkward.



Moreover, winding the launcher with an impure flyer like Flyer3S, Flyer3T, or either Flyer6 might shorten the life of the tape or glue used to keep its rotor from slipping on its shaft or hub.

The simple pull-out winder below addresses all of these issues. The same winder works for both right-handed (upper photo below) and left-handed flyers (next photo).





As explained here, altitude and range are maximized by leaving nothing attached to the lower end of the PTO when the winder comes out.



The winder was designed with 5 goals in mind:
  • Reversibility without modification
  • Adjustability of tail seating depth over the entire useful range
  • Ease of winding
  • Secure protection against premature unwinding and accidental discharge
  • Ease of detachment to minimize the impact of the launching process on the user's aim
  • Sufficient internal strength to withstand the torque of the fully wound PBM.
The winder at lower left has 2 dark tan 5L stopped axles. The "winding shaft" with the single-bevel gear bearing (to reduce wiggle during winding) goes into the PTO. Fully extending the winder shaft, as seen here, leaves a 4 mm-deep "seat" for the flyer's tail at the other end of the PTO.



The other winder axle, the "winder safety pin", slides into one of the open holes in the yellow frame nearby to lock the winder in place once winding is complete.

The next photo shows the safety pin retracted in preparation for winding.





Flyer tails definitely encounter friction against the motor's PTO as they slide out during take-off. How much friction is a complicated function of flyer mass and "tail seating depth" (the length of tail inside the PTO at the start of spin-up) that I have yet to sort out.4

Tail seating depth is adjusted by changing the length of winder axle inside the lower end of the PTO. As discussed here, maximum altitudes turn out to be quite sensitive to seating depth in all my flyers.




Specifications
Flyer spec symbols and units: R ≡ radius (mm); L ≡ axial length (mm); Z ≡ blade count (--); M ≡ mass (g); A ≡ maximum altitude (m).

Launcher overall dimensions:128x48x60 mm (LxWxH) including winder
Launcher mass:0.080 kg including winder
Winder type:Pull-out
Flyer2I:R = 52, L = 24, Z = 4, M = 7.2, A ≈ 1
Flyer3S:R = 36, L = 31, Z = 3, M = 2.5, A ≈ 5
Flyer3T:R = 36, L = 24, Z = 3, M = 2.3, A ≈ 6
Flyer6S:R = 22, L = 64, Z = 2, M = 2.4, A ≈ 10
Flyer6AR = 22, L = 64, Z = 2, M = 2.2, A ≈ 15
Flyer7R = 36, L = 64, Z = 2, M = 2.4, A ≈ 5
Non-LEGOŽ parts:Superglue or tape to prevent slippage between the rotors and tails of Flyer3S, Flyer3T, and Flyer6
Modified LEGOŽ parts:Shortened 5.5L stopped axle in Flyer2I; 5.5L 2-blade prop in Flyer6
Credits:Original MOC





Footnotes

1 Motorized LEGOŽ devices will remain too heavy to fly under their own power for the foreseeable future -- at least with LEGOŽ motors and props. (See, for example, Sariel's "Why LEGO can't fly" video.)

However, lightweight LEGOŽ flyers of various kinds can be made to fly with all-LEGOŽ external launchers powered by ripcords and wind-up and electric motors.

2 My more powerful ripcord-based launchers and flyers will be posted in the coming weeks. In the meantime, here are some excellent examples by 2 favorite builders.3 The blades of this so-called "twisted" propeller really aren't twisted at all. Rather, their uniform blade angle is just greater than those found on most LEGOŽ props, including all 9L props.

4 I sense that tail color also plays a role in tail-PTO friction -- hence, the preponderance of dark tan among the 3L stopped axles used in most my ultralight flyers.




Selected references

Bouadbdallah, S., 2007, Design and control of quadrotors with application to autonomous flying, published doctoral thesis.

Bouadbdallah, S. and Siegwart, R., 2007, Full control of a quadrotor, IEEE/RSJ International Conference on Intelligent Robots and Systems, p.153-158.

Powers, C., Mellinger, D., Kushleyev, A., et al., 2013, Influence of aerodynamics and proximity effects in quadrotor flight, Experimental Robotics, STAR88, p.289-302.

Wald, Q.R., 2006, The aerodynamics of propellers, Progress in Aerospace Sciences 42:85-128 (an excellent treatment if you have access and don't mind the heavy math)




Comments

 I made it 
  October 19, 2018
Quoting Bacon Tomato This looks like fun!
Thanks! It gets lots of oohs and ahh at LEGO shows.
  October 19, 2018
This looks like fun!
 I made it 
  August 24, 2015
Quoting VAkkron ™ These detailed experiments give me great appreciation for the ability of Lego. Thank you for writing them; they are interesting to read for as much as I can understand. I'm looking forward to coming back to all of your analyses after studying physics in more depth. This particular design, and the design of Desert752_Kirill seem like they could have several practical uses that I want to experiment with. Hats off to you for sharing your studies with us! Also, I might have missed it in the writeup, but is the motor manually reversible, or do you invert the assembly to achieve the opposite direction?
Thanks for those very kind words, VAkkron. I'm particularly delighted that you'll be doing some experiments of your own, as I share all this stuff mainly in the hope that others will pick up where I've left off and report their own findings. The reversibility of the launcher resides in the ability to use the entire assembly with either side of the motor up. The motor itself winds up in only one direction relative to its own casing, so it has to be flipped over to reverse its action. Put another way, this launcher has the same symmetry as the motor, whereas the previous launcher has less.
 I like it 
  August 23, 2015
These detailed experiments give me great appreciation for the ability of Lego. Thank you for writing them; they are interesting to read for as much as I can understand. I'm looking forward to coming back to all of your analyses after studying physics in more depth. This particular design, and the design of Desert752_Kirill seem like they could have several practical uses that I want to experiment with. Hats off to you for sharing your studies with us! Also, I might have missed it in the writeup, but is the motor manually reversible, or do you invert the assembly to achieve the opposite direction?
Jeremy McCreary
Matt Bace
  August 19, 2015
Another great LEGO engineering tour de force. You realize, of course, that any great engineering work like this is bound to be weaponized at some point. It won't be long before kids are firing flyers at each other in nursery school. :-)
 I made it 
  August 19, 2015
Quoting Matt Bace Another great LEGO engineering tour de force. You realize, of course, that any great engineering work like this is bound to be weaponized at some point. It won't be long before kids are firing flyers at each other in nursery school. :-)
Thanks, Matt. I believe in making angular momentum, not war, but I'm afraid you're right. It's the technological imperative writ in ABS rather than plutonium or tritium or anthrax spores: What can be done, will be done. Unfortunately, this information has already fallen into the wrong hands -- my own. I understand that carrot cake has recently been weaponized, and that Congress is about to pass strict cream cheese icing control laws. There goes half my diet.
 
By Jeremy McCreary
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LEGO models my own creation MOCpages toys shop Flying rotor launcher, reversibleTechnic


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