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MPC3: Motorized prop-cart, version 3
A top speed of 1.5 m/s makes this 0.42 kg, 4.8W propeller-driven remote control cart my fastest vehicular MOC ever. Quite an improvement, considering that the original MPC was easily my slowest MOC of all time.
About this creation
Please feel free to skip the verbiage and look over the photos

See also:
MPC2, the previous version, and MPC1, the original prototype

On this pageWarning! 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.




Introduction
The motorized prop-cart (MPC) presented on this page (hereafter, MPC3) represents the 3rd and final version of the prototype (hereafter, MPC1) posted in March, 2014.

A top speed of ~1.5 m/s makes MPC3 ~25% faster than MPC2 (3rd video below) and at least 50% faster than Trackrat, the fastest of the rest of my motorized LEGOŽ vehicles.

∨ MPC3: Full-power acceleration to top speed in a more or less straight line.



∨ MPC3: Maneuvering via limited but useful reverse thrust (~20% of forward thrust).



MPC2: Previous previous MCP with original 2-blade prop turning laps at full power.



Both previous MPCs used #5 or #6 Technic long smooth fairings as prop blades. The photo below shows MPC2's best prop (yellow) mounted coaxially over MPC3's prop for comparison. When I replaced the 2-blade prop seen on MPC2 in the video above with the larger yellow 3-blade prop seen below, MPC2 gained ~20% in straight-line top speed.



Without glue at every join susceptible to failure in tension due to centrifugal force, the fairing-based props would routinely disintegrate above a few hundred RPM -- especially the larger, lighter versions developed for MPC2.

NB: Eye protection is an absolute must when testing props like MPC2's prior to gluing.

MPC3's new prop, on the other hand, easily survives speeds in excess of 2,000 RPM without glue. Each of its wind turbine blades -- hereafter, WTBs (89509, from the LEGOŽ Education Renewable Energy Add-on Set, 9688) -- offers 6 strong centrifugal force-resistant attachment points. When used as blades, the fairings provide no such attachments.





The heavier hub needed to take advantage of the WTB attachment points led to a 10% increase in overall prop mass (hub included) WRT MPC2's best prop (0.023 vs. 0.021 kg, respectively).

Much more important than total mass from a performance perspective, however, is the radial distribution of mass about the prop shaft as measured by a prop's rotational inertia (aka moment of inertia). With more of its total mass concentrated near the hub, MPC3's prop ends up having a lower rotational inertia and hence spins up faster.




Design goals

Early on in my work with propeller-driven carts, it became clear that an MPC with any performance to speak of would need the following attributes:
  • High delivered power: A high-power propulsion motor and efficient drivetrain carefully matched via gearing to the prop and the MPC's total resistance to forward motion
  • Efficient prop: A safe and efficient prop capable of producing adequate thrust from the shaft power delivered to it
  • Minimal mechanical losses: Low bearing and rolling friction -- especially within the drivetrain and at the heavily loaded rear axles
MPC3 represents a revolutionary step forward on the propeller front, a major step forward WRT installed power and drivetrain efficiency, and evolutionary progress everywhere else. Because the need to minimize weight is less critical now, MPC3 gained a few cosmetic frills here and there.



What was retained
Though revised substantially in detail in most cases, the following basic features were carried over from MPC1 and MPC2:
  • Propulsor: Single pusher prop
  • Chassis and tower: Still based on 5x11 and 5x7 Technic frames
  • Steering linkage: Sariel's simple design retained
  • Motor location: Steering motor low and far forward; propulsion motor high and far aft
  • IR receiver: V2 at front of nacelle
  • Battery: 7.4V Li polymer rechargeable battery below nacelle
The last 2 items are unchanged.



What changed from MPC1
The main changes WRT MPC1 are listed below. Some were already in place in less refined form in MPC2.
  • Prop: Replaced previous fairing-based props with a much more efficient, high-thrust 3-blade propeller based on wind turbine blades (WTBs, 89509) from the LEGOŽ Education Renewable Energy Add-on Set (9688)
  • Propulsion motor: Replaced the L motor used through MPC2 with the more powerful high-speed 9V RC Race Buggy (RCRB) motor (5292) and revised motor mounts and prop shaft supports accordingly
  • Drivetrain: Optimized a new 1-stage transmission with 1:67 overdrive gearing
  • Wheels: Greatly reduced rolling friction and sensitivity to small surface irregularities by replacing MPC1's wedge belt pulley wheels with much larger hard plastic Mindstorms/motorcycle wheels
  • Rear axles: Greatly reduced rear axle bearing friction and bending by splitting rear axles and beefing up rear axle support
  • Steering performance: Vastly improved responsiveness and return-to-center accuracy by replacing the previous (M) steering motor with a PF servo and eliminating the steering reduction gear entirely
  • Steering mechanism: Replaced knob wheel-based final steering drive with rack and pinion (more efficient, less play) and reduced overall steering linkage size and mass
  • Tower: Stiffened structure to reduce power losses due to vibration and torque reaction -- in part by using the propulsion motor casing to better advantage as a structural member
  • Frills: Added tower fairings and fenders just for looks
  • Color scheme: Replaced yellow accents with white ones
Together, the non-cosmetic changes brought about a ten-fold increase in top speed relative to MPC1 and greatly improved steering. Although most of the credit for the speed gain goes to the new prop and propulsion motor, the wheel and rear axle changes contributed substantially.




MPC3 overview

The next 6 photos show MPC3 wearing the best MPC prop ever -- a 3-blade unit built up from the wind turbine blades (WTBs, 89509) from the LEGOŽ Education Renewable Energy Add-on Set (9688).













Top view for scale: The light-colored floor tiles were 305 mm (12 in) squares before corner cuts.




New prop and drivetrain

MPC3 is mostly about the new prop and motor it introduced to the MPC series. (The new prop also makes a pretty darn good rotor for a working horizontal-axis wind turbine.)





Two long-awaited events allowed MPC3 development to proceed: (i) Acquisition of a set of 6 wind turbine blades (WTBs, 89509), and (ii) receipt of an RC Race Buggy (RCRB) motor (5292) on loan from Shawn Kelly.

Theoretical considerations having to do with blade length and shape on the one hand and the mechanical power vs. torque trade-off involved on the other cast some doubt as to whether the new blades and motor would actually improve on MPC2 performance, either alone or in combination. However, they were certainly worth a try.

The MPC3 propulsion test matrix involved 2 motor choices (L vs. RCRB) and 3 props: (i) MPC2's best 3-blade prop (the best-performing fairing-based prop I've ever produced), (ii) the largest-diameter WTB-based prop I could muster (the one now used by MPC3), and (iii) a slightly lighter WTB-based prop with a diameter 2 LU shorter.

The need to find the optimal gearing for each prop-motor combination added to the tedium of the testing, but a clear winner emerged: MPC3's current prop on an RCRB motor geared up 1:1.67 produced ~4.0 Newtons of thrust -- twice that of MPC2's best prop on an L motor geared up 1:9. The worst combination: The RCRB motor and MPC2's fairing-based prop.





The RCRB motor's victory here was by no means a foregone conclusion. Its most endearing quality by far is its unrivaled peak mechanical power output: ~4.8W at 7.4V and ~586 RPM (50% no-load speed on the lower-speed, higher-torque power take-off used by MPC3). The L motor peaks at less than half that output at 7.4V and ~160 RPM.

On the downside, the RCRB motor (i) produces ~20% less stalled torque, (ii) weighs 31% more, (iii) takes up much more room, and (iv) offers fewer and much less convenient mounting options than an L motor. Shawn and I had already tried RCRB motors in several motorized boats, but similar boats driven by L or XL motors always left them far behind.

Nevertheless, the RCRB motor still had potential with props working in air rather than water. Being ~1,000 times less dense and less viscous than water, air demands a lot less torque from a propulsion motor. Hence, an MPC might allow a power-rich, torque-poor motor like the RCRB to shine. That turned out to be the case with MPC3 -- but only because I happened to have a well-matched prop for it.



MPC2 used an L motor geared up 1:9 for propulsion.




New wheels and rear axles

Together, MPC3's beefed-up rear axle supports, separate rear wheel axles, and large-diameter, small-contact patch hard plastic wheels netted a huge performance gain over MPC1 despite the added mass involved. These key improvements were already largely in place when I shot the MPC2 video but were further refined in MPC3.







In the process of photographing an early MPC2 (above), I discovered 2 major design flaws bearing much of the responsibility for its lackluster response to a new prop that should have made it a lot faster: (i) Lousy rear axle support and (ii) a single rear axle. I should have known better but was too focused on prop design at the time.



The basic chassis and tower design used in all MPCs puts most of the weight on the rear axles and wheels. Cantilevering MPC2's rear wheels 3 LU out from 1 LU-deep supports side allowed the rear wheels to bow upward and the single 12L axle connecting them to bow downward between the supports. It takes a good bit of power to maintain a constant flexure in a spinning axle -- power that could have gone into forward motion instead.

Putting both of MPC2's rear wheels on the same axle was an equally bad idea. Its hard plastic wheels had all the traction needed to translate rear wheel skidding in turns due to the single rear axle into a significant power sink. And worse still, any surface irregularity hindering one rear wheel affected both.

Much fiddling with wheels revealed the following: (i) Hard plastic wheels with very small contact patches greatly reduced rolling friction. (ii) Large-diameter wheels vastly improved the ability to get past small surface irregularities.




New steering system

As with previous MPCs, the steering mechanism was patterned after a simple but effective design found in Sariel's Technic bible (see references).

However, MPC3 is the first to use a PF servo motor for steering, and what a difference it's made WRT steering accuracy and response! Better yet, of the many return-to-center steering systems tried, this is only one with truly accurate return-to-center behavior -- a must in a vehicle this fast.





Steering accuracy still requires a PF speed control like the one below (mods shown posted here). Small steering inputs in the ±2/7 range generally work best.



Previous MPCs used M motors with heavy reduction gear trains for steering. The photo below shows the 3-stage, 27:1 steering gear train used on MPC2.






Specifications



Dimensions and speed
Overall, including prop:290 x 276 x 276 (LxWxH)
Overall, excluding prop:258 x 134 x 164 (LxWxH)
Mass:0.420 kg (0.90 lb)
Top speed:~1.5 m/sec




Design features
Version:3
Construction:Studless except for battery mounts
Motors:2 -- 1 RC Race Buggy (high-speed take-off) for prop ; 1 PF servo for steering
IR receiver version:V2
IR receiver connections:2 -- 1 for each motor
Electrical power:7.4V PF Li polymer rechargable battery
Modified LEGO® parts:None
Non-LEGO® parts:None
Thanks:Shawn Kelly for loaning me his RC Race Buggy motor
Credits:Sariel for the steering mechanism; otherwise an entirely original MOC




Propeller
(Speeds and flows taken at full power, 7.4V input)
Design:3 wind turbine blades
Mass:0.023 kg
Diameter:276 mm
Chord:40 mm at 70% radius
Angle:25° at 70% radius
Pitch (nominal):[] mm at 70% radius
Air inflow speed:~1.5 m/sec
Rotational frequency:827 RPM
Angular frequency:86.6 s-1
Tip speed:12.1 m/s
Tip-speed ratio:8.1
Advance ratio:0.12
Air outflow speed:[] m/s peak at 0.30 m
Gearing:1:1.67 overdrive
Motor shaft speed, actual:496 RPM
Motor shaft speed, ideal:586 RPM
Static thrust:4.0 N
Reynolds number at tip:~2.8 x 103 (based on chord)





Selected references
  • Kmiec, Pawel "Sariel", 2013, The Unofficial LEGO® Technic Builder's Guide No Starch Press. Sariel's one of the most talented and prolific technical LEGO® builders around, period. I can't recommend this book highly enough. IMO, there should be a well-worn copy in every technical LEGO® junkie's workshop, regardless of experience and skill level. You can also learn a great deal from Sariel's well-illustrated web site and countless YouTube videos.

  • LEGO® 9V Technic Motors compared characteristics by Philippe "Philo" Hurbain, the undisputed guru of all things electrical in the LEGO® realm -- especially the motors.

  • WikpediaYou're likely to find a good explanation of anything discussed here on Wikipedia. You can't believe everything you read there, but it's a superb resource for LEGO® engineers. In my experience, the science, engineering, and math articles are all very good to excellent. I have yet to find any real misinformation under those headings, and you're unlikely to find a topic that isn't covered.

  • 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.

  • NASA's FoilSim III Student Version 1.5a interactive airfoil simulator. Among the available airfoil profiles is a curved plate simulation that sheds some light on the aerodynamic behavior of the Technic #5 and #6 long smooth fairings when used as prop blades.





Comments

 I made it 
  December 22, 2016
Quoting The Actionfigure That´s pretty cool!
Thanks!
 I like it 
  December 21, 2016
That´s pretty cool!
 I made it 
  April 24, 2015
Quoting jds 7777 One other thing, the large diameter wheels mean the axles connected to them actually spin at a lower RPM than they would w/ the smaller wheels. This means reduced friction in the drivetrain.
Excellent point! Hadn't thought of that, but clearly a trick to keep in mind for heavily loaded axles.
  April 24, 2015
One other thing, the large diameter wheels mean the axles connected to them actually spin at a lower RPM than they would w/ the smaller wheels. This means reduced friction in the drivetrain.
 I like it 
  January 21, 2015
Awesome job! The most detailed report on a Lego creation I have ever seen! Keep up the good work!
 I made it 
  December 31, 2014
Quoting Dr. Monster Amazing MOC and write up. Is it just me or does this model remind anyone of the Beverly Hillbillies jalopy?
Unfortunately, I can totally see that!
 I like it 
  December 31, 2014
Amazing MOC and write up. Is it just me or does this model remind anyone of the Beverly Hillbillies jalopy?
 I made it 
  December 30, 2014
Quoting Gabor Pauler It is almost like a scientific paper on how intolerably low is PF sets power/weight ratio...
Couldn't agree more about the low PF motor and battery power/weight ratios we're stuck with -- largely, I think, for safety reasons. (Mindstorms ratios are even lower.) The safety-motivated current limiters built into all LEGO electricals also cut deeply into performance. (Given your interest in helicopters, you must find all of this downright exasperating.) On the bright side, this MOC shows that testing-guided optimization of the moving components driven by the motors can pay off big-time. As you know, careful matching of motors to their ultimate loads -- in this case, via prop design, gearing, and fastidious power loss reduction -- is a tedious iterative process, but an absolutely essential one in any MOC intended to push LEGO batteries and motors to their limits. Propeller-driven vehicles (in my case, boats and prop-carts) push the batteries and motors especially hard.
 I made it 
  December 30, 2014
Quoting Yann (XY EZ) Really great and well documented moc. Well done & keep building!
Yann, thanks for the encouragement. Most visitors are probably put off by all the documentation, but I keep adding it in the hope that it might help folks interested in trying their hand at similar MOCs to come up with even better designs without repeating my mistakes.
 I like it 
  December 30, 2014
It is almost like a scientific paper on how intolerably low is PF sets power/weight ratio...
 I like it 
  December 28, 2014
Really great and well documented moc. Well done & keep building!
 I made it 
  December 28, 2014
Quoting matt rowntRee Impressive speed there, doesn't look like it could keep up with the cornering abilities of a cat though (we all know that's really why we get RC cars. XD ) Love how you reduced a bunch of friction and drag with those hubs, and those blades have an amazing surface area to create some outrageous thrust. Awesome!
Thanks, Matt. Yeah, no future in chasing cats with this thing -- partly for lack of grip, and partly because the steering's still too twitchy, especially at speed. I =thought= I was reducing rolling friction with the skinny little wedge pulley wheels on the original, but apparently not. As hoped, the big white Mindstorms wheels sans tires greatly improved the original's miserable performance WRT small surface irregularities like grout lines. What I didn't see coming was the huge reduction in rolling friction.
 I like it 
  December 28, 2014
Impressive speed there, doesn't look like it could keep up with the cornering abilities of a cat though (we all know that's really why we get RC cars. XD ) Love how you reduced a bunch of friction and drag with those hubs, and those blades have an amazing surface area to create some outrageous thrust. Awesome!
 
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LEGO models my own creation MOCpages toys shop MPC3: Motorized prop-cart, version 3Technic


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