Searching for the best match.........

Friday, February 25, 2011

Introducing the Z83D Scissor Prop

Using the principles of elliptical wing design to protect lift from tip losses and vortex aero to energize flow over a sub-wing, I have invented a new prop optimized for low speed 3D maneuvers and hovering called the Z83D Scissor Propeller.  This prop is an asymmetrical 4-blade, where the leading blades are of both smaller diameter and shallower pitch.  The purpose of the two smaller, leading blades is to create some additional thrust/lift, but in a way so their vortex can be used to energize the airflow over the top of the two, larger, primary blades.   Now there is lift from all four blades, but a useless vortex is only left behind by two.

Here are some initial, empirical test results:

 11x4.7 SF
11x5.5 APC
11x5.5 APC + 8x3.8 SF

The first installed static test shows the Z83D Scissor Prop gives a 23% Thrust/Amp boost over a similar size slow fly propeller, and a 5% advantage over the same APC propeller.  I expect greater gains when used on aircraft with a less draggy engine cowling.

The vortex aero theory behind the Z83DSP is the same used by an Eagle during low speed, high thrust maneuvering:

The Eagle splits his relatively low aspect ratio wing into a procession of high aspect ratio winglets, where each leading edge winglet's vortex rolls over the trailing winglet to energize top flow. There are two complex effects at play at once:

  1. The elliptical plan form of the Eagle's main wing and winglets reduce lift to zero at each fractal tip, maximally separating the bulk of lift from the finite wing tip, and thus, associated vortex losses.  This is the same principal used in any elliptical wing aircraft design, perhaps the most famous of which is the Supermarine Spitfire.
  2. The procession of Eagle winglets, to include several mid-span wrist winglets, use their sub-tip vortices to energize airflow over the trailing winglet.  This probably does not reduce the intensity of the overall wing tip vortex, it simply puts some portion of it to good use.  This is the same principle copied in any vortex aero aircraft wing design, perhaps the most famous of which is the greatest fighter in aviation history, the F-16.

The Z83D Scissor Prop's smaller sub-wing vortex is rolled over the top of, and thus recycled into more lift for, the trailing primary prop blades.  There is probably a more optimal alignment, diameter and pitch ratio than my prototype scissor.

Sunday, February 20, 2011

Twin Engine Edge 540

Motor 1: Super Tigre .10
Motor 2: Super Tigre .10
Twin Weight: 4.3 oz
Prop: 11 x 6
Max RPM: 10,300
Amps: 58 (29/motor)
Watts: 620 (310/motor)

Tuesday, February 15, 2011

Super Tigre .20 & Super Tigre .30

This multi-engine set up runs as smooth and strong as I'd hoped.

ST .30

The trick to getting the multi-engine setup to run is to use a dedicated ESC per motor.  Interestingly, there isn't much additional cost to outfitting the airplane this way.  In fact, summing smaller ESCs can be less expensive, and you get multiple power sources to the receiver and servos.

With low-friction brushless systems, there is surprisingly little overhead when running only one motor in the group.  I think the jury is still out on the best battery configuration.  It works fine with multiple batteries or a single battery.

What apparently doesn't work great is to attach a battery on only one ESC, and leave the other(s) unpowered.  When one ESC is unpowered (= nothing attached to the battery connector) it's idle motor acts as a generator being spun by the other motors and it sends power back to the ESC from the motor side.  The unpowered ESC gets quite hot when other motors are running.  If a battery is attached, even if the ESC is not being controlled by the receiver and thus the motor is idle, the voltage from the attached battery apparently doesn't allow current to flow back into the ESC from the motor/generator, and it does not heat up.

ST .20

Three ST motors with fit nicely on a 100mm shaft.  The shaft diameter must be exactly 4mm.  Building the motor is pretty easy with a good strong vice: 
  1. Place a spark plug socket (from a ratchet set) firmly in a vice, vertically oriented.  The top opening should be about an inch above the vice jaws.  The top should be the end with the smaller hole.
  2. Pry off the tiny black c-clamp from the rear of the ST .10 shaft.  The second or third (or fourth) motor bell to be mounted on the longer shaft can perform the function once served by the C clamp: to keep the propeller from pulling the bell forward and out of the motor.
  3. Grab the front of the shaft and pull the shaft/bell assembly forward and out of the coil/mount section.  The strong magnets will try to hold it in place.
  4. Place the shaft/bell assembly on top of the spark plug socket, with the shaft protruding down the middle of the spark plug socket.  The spark plug socket should form complete circular support underneath the bell's cooling holes, around the shaft.
  5. With the motor bell centered on top of the spark plug socket (it will be hard to center due to the magnets wanting to pull it to one side), and while holding it firmly in place, lightly tap on the top of the shaft with a hammer, so it slides down and out through the bell.  You will need a punch or a nail to tap out the final portion of the shaft through the bell.  The shaft will fall out.
  6. With the bell still on the spark plug socket, tap the new, longer shaft down into the front of the bell as far as necessary.  If the bell is a bit loose on the shaft, tap the shaft all the way through to start over.  Use a medium pliers and (lightly) squeeze the rear circular opening of the bell's shaft collar (on the magnet side).  It is made of pliable metal, so be careful not to squeeze too hard and deform it too much.   Using any Dremel-type roughing tool to lightly roughen the shaft only where it will meet the bell, will help as well.  Now tap the longer shaft into place through the bell.
  7. Slide the motor coil/mount assembly back onto the new motor shaft.  Make sure the bell spins without slightly wobbling, or gently align it with your hands by bending the steel slightly with your hands until it spins perfectly true.   One motor is done.
  8. Repeat as many times as necessary.
  9. If you do a reverse motor orientation (like the middle motor of my .30 photo), be sure to reverse the red and black wires for that motor when it hooks into its ESC.
  10. Test for proper spin direction, one motor at a time.

    Monday, February 14, 2011

    OrangeRx - Spektrum DSM2 Compatible Receiver

    Update - 11 Aug 2011:  My OrangeRx has been performing well in conjunction with my Dx6i.  Seems the problem was with sub-standard Spektrum transmitters.

    Update: The model crashes I've experienced using this Rx were later proven to be due to a defective DX8 radio. There seem to be other issues, but I am uncertain if they are due to Horizon Hobby's predictably substandard quality or the OrangeRx. I am continuing testing now and will post an update if I can clear this Rx of all wrong doing. One thing is certain: the Spektrum DX8 is a gravely dangerous product--don't let your kids anywhere near one. Original post follows:

    Lost the link twice on first two flights, crashing the model both times.  Avoid.
    On a side note, I think it is great that a company is making Spektrum/Horizon Hobby pay for their policy of not allowing price competition among resellers. Horizon's prices are simply ridiculous for the cheap China-made products they peddle at premium prices.  But they do have a near monopoly on a lot of desirable stuff, so for now they get to sell $5 worth of electronics for $50+.

    Update: February 14, 2011:
    I bought 3 OrangeRx receivers. My first experience is in the previous blog entry . Since there was less at stake, I decided to try a second one in my Techone Mini Christen Eagle, just in case the first sample was defective. I works fine and continues to work fine. It is a micro that stays within 200 feet or so.

    So I put the 3rd sample in my Art-Tech Yak 54, pretty inexpensive, slow plane, so I thought it would be a good 3rd test. That was a mistake, the rx continually wants to reset and recenter the trim or the servos in all channels. This caused the model to fly well out of trim - trimmed back up - way out - trimmed back up - way out - and so on.

    Perhaps the most interesting thing happened on the ground, afterward. A construction worker from a nearby site came over to chat while I was flying the Yak. After I landed, we talked about getting into rc. So I switched my DX8 over to my Champ to show him a great starter plane (I forgot the Yak still had a live battery). To my surprise, as the Champ bound to the DX8, I heard the Yak servos recenter too, then the Yak beeped in a double-bind along with the Champ. Needless to say, I quickly yanked the Yak's battery.

    My recommendation is to stay very far away from this clone rx, especially if someone else is flying one.

    Sunday, February 6, 2011

    E-flight UMX Extra 300 BNF

    The E-flight UMX Extra 300 is a very lightweight half foam/half mylar offering in the indoor 3D arena (or to a much lesser extent and no wind whatsoever, outdoor). 

    To make the super light and super flimsy construction technique viable, thin carbon fiber rods reinforce the underside of the wing in truss form, with a single rod protruding forward from the CG to the tip of the nose.  Despite the most vague resemblance to an aircraft that actually has an interior, the "Extra 300" branding looks nice with the white star field on red mylar fused to one side of the foam stencil frame.  The other side of the wing and fuse are left uncovered in bare white foam.

    Overall, the construction method simply doesn't work.  The plane is fairly light, but incredibly weak in certain areas which become apparent very quickly.  The wings are so pliable, that the slightest miss, generating just a few ounces of force, perhaps on a landing or getting sucked toward a wall by Bernoulli, will twist them to the point of tearing the foam and the mylar.  Within an hour of hovering practice, I had ripped both wings at the point where the leading edge meets the fuse, and one wing at the tip.  One wheel pant simulator had torn.  A few pieces of glossy Scotch tape a tiny dab of foam safe CA made those problems go away.  Why couldn't E-flight do that?  Or better yet, sell airframe designs that aren't known junk.

    The specs for the 300 are as follows:

    Wingspan: 16.8 in (427mm)
    Overall Length: 19.4 in (494mm)
    Wing Area: 74 sq in (4.8 sq dm)
    Flying Weight: 1.2 oz (32.5g)
    Motor Size: 8.5mm Coreless Brushed
    Trim Scheme Colors: Red, Grey, Clear, Black
    Prop Size: 130mm x 70mm
    Recommended Battery: 3.7V 1-Cell 150mAh LiPo
    Approx. Flying Duration: 4-8 minutes
    Scale: Ultra Micro
    Minimum Age Recommendation: 14 years
    Experience Level: Advanced
    Recommended Environment: Indoor/Outdoor
    Assembly Time: Less than 1 Hour
    Is Assembly Required: No

    No radio is included.

    It's all a bit strange.  Compare to the Hobbyzone Champ, which is an extremely strong, molded foam truly scale build:  The Champ has a 22.5" span (1/3rd longer), more wing area, and the same flying weight as the UMX 300 when flying times are equalized.  The Champ has a higher top speed but doesn't have quite the same T:W, but it could with the same gear reduction and larger prop.  The Champ sells for half and includes a 2.4 GHz radio.  Something is terribly wrong with the design philosophy unpinning this airplane.

    E-flight needs to reevaluate their ridiculous pricing, there is $10 of airframe and maybe $10 of electronics here, retail price is $210, street price is $160, 9.5 to 1 retail markup.  A 200% markup would bring E-flight inline with industry, and suggest a fair street price of about $40.  I assess this plane as a monumental rip off.  

    Strength Comparison:

    HobbyZone Champ
    Wingspan: 22.5 in.
    Wing Area: 71 sq in.
    Weight 1.3 oz
    Flying Time: 12-15 minutes

    E-Flight UMX Extra300
    Wingspan: 16.8 in.
    Wing Area: 70 sq in.
    Weight 1.2 oz
    Flying Time: 4-6 minutes

    Coke Bottles
    Span: 16.5"
    Contents: 16.9 Oz of black chemical joy

    The Champ wing barely flexes under it's on weight.

    Extra 300 wants to fold under it's own weight.  This is after Scotch taping all edges. 
    The mice add up to 7.5 oz.  I successfully added a hard drive for a total of 17.8 oz.
    Forget 7.5 oz, a AA battery is enough to fold the Extra 300. 

    E-flight needs to go back to the drawing board to figure out why larger airframes weigh the same, are 20x stronger, and sell for 1/3rd the price.

    The Extra flies well, and has no problem hovering for the first half of the battery life.  E-flight claims "t is slow and light enough to fly extreme 3D maneuvers in spaces no larger than a dining or conference room."  That is not true.  You can hover the plane in a dining room, hanging on the prop, but it is too fast to fly on the wing inside a large house.  That leaves indoor community center or outside in dead calm winds as the primary workable venue for this plane.  

    In fairness, E-flight says the plane is only for experts.  So in non-fairness, that's probably to keep them from paying money out to the intermediates and advanced novices who can effortlessly handle this extraordinarily easy-flying airplane, but tear it apart after a few delicate  mishaps.

    While I have been hovering around the house with the 300's rather loud, aggressive buzz, the winds have not cooperated to complete the flying portion of this review.  This only emphasizes my point that this in not a satisfactory "dining room" flyer.

    (...time passes...)

    So at this point, I know readers are thinking, well, this plane is going to get an F for sure.  Not so fast.  The plane performs beautifully outdoors, and would truly excel in a gym.  Speed is well controlled with the large, gear-reduced prop, lines are straight, hovering is controlled and powerful, and turn radius is, well, non-existent by any definition that requires the center post of the circle to be outside the dimensions of the aircraft.   Flat turns progress indefinitely, given a little more radius slack than is initially possible.  Throws are exceptional and inputs are mostly linear to the stops, not causing a lot of unexpected, off-axis side-effects.  Full back stick can snap roll into a tapered-wing stall, but stall speed is fairly difficult to find and under-fly.  Multiple aileron rolls stay axial and uncoupled. Knife edge loops happen without much drama, indicating an excellent thrust to weight ratio.   Parachute landings are easy.  The plane is one spectacular 3D flier. 

    Simply put, the UMX is the best micro 3D flier I've flown outside of a simulator, and it hangs, literally (and for real, which is worth a lot), with any bizarre contraption I've flown in a sim.  Stunning fun in a cul-de-sac or small field.  The lack of Mylar on one side is no doubt cheap, but also seems to serve as a decent speed brake to keep speeds relatively slow even with full power applied.  High alpha flight was easy to stop cold about 45 degrees nose high at 60% throttle, into tonight's light breeze (~4 knots).  

    This plane should be relatively easy the novice-enthusiast to handle, as long as throws are reduced by a third or more with 50%+ expo applied.  If you understand what that means, new aerobatic fliers shouldn't hesitate to try the UMX Extra 300 over grass.   The light mass of the airplane makes any old attitude grass landing very forgiving, even with the planes embarrassingly low build quality.  But tape around the wing edges, and reinforce the leading edge where it meets the fuselage, first.

    Not surprisingly, the plane quickly developed a bit of a rough spot in the reduction gear.  This is fairly common with cheap brushed motors and gears, even within the cheap Horizon micro servos, which are hanging on so far.

    This plane is a tough grade. The build quality absolutely sux, making the price point just plain stupid. As I've shown above, other planes with more wing area and the same flying weight, test up to 20x stronger. Horizon is to be admonished for their new trend toward dirt poor product quality, ionospheric pricing, corner-cutting and resulting terrible value. At the same time, it is reasonably difficult to crash the 300 in a large gym-type of venue or outside based on a magical turn radius and low stall speed, and hitting grass with the light mass should prove forgiving indeed. So with a solid F- for build quality, D for price, and A+ for flight characteristics, I average that to a B+ (recommended) since airplanes are meant to be flown.

    Choosing the Right Motor - Reality vs. Sales Pitch

    There are a lot of people mundanely peddling motors on the common RC message boards. If you were in the business, wouldn't you?  The sales people are easy to identify, they make 1,000s upon 1,000s of posts, often 10s of thousands on many different boards to subtly, or more often brashly push their products to mostly unsuspecting victims.  The favorite method of collusion between board and salesman is the simple rigged review, usually fluffed with all kinds of silly to ridiculous claims that make no aerodynamic sense whatsoever.  Their tactics wouldn't be as bad if they didn't so flagrantly trash their competition, often taking aim at products which are demonstrably better than their merchandise.

    So how do you distinguish a sales pitch from reality?  It's easier than you think. If it is permitted to appear on one of the larger forums, it is a sales pitch.  Just because a product is being pushed without revealing associations to paid sponsors doesn't make it bad, though it does make one wonder why they feel the need to misbehave. 

    With that in mind, is there anything one can do to determine which motor might be the best choice for a particular airplane?  Yes!  Understand the common misconceptions, usually spread by the salesman/reviewers themselves who lack any technical background to make sound judgements, and it is easy to separate the wheat from the chaff.

    Let's start by debunking the sales pitch of choice, the ground tested "data base" of good and bad motors, complete with charts and graphs of statistical noise.  Some people have seemly dedicated their lives to creating these long lists of meaningless gobbledy gook.  Let's be clear, you cannot determine anything useful from a ground test of any motor in isolation if that motor is intended to fly.  Looking at data bases of ground test data is a total waste of time, even if the creators' intent wasn't to trash superior competition.  Why?

    Where to start?  How about if we start by assuming ground test data bases are not complete garbage, and see how far it gets us?  Let's be intelligent, and start by defining some notional requirements.

    Let's say we want to power a model with the goal of (A) flying very casually, and (B) achieving the longest possible flight times.  We need (C) a power reserve for takeoff and landing with a go around option in a field boxed by trees.  The battery compartment is (D) fairly small, so we have a very limited selection of suitable batteries.  The ESC is up to us, we'd like to (E) minimize cost.

    So let's go to a motor data base and absorb all the wonderful misinformation.  Here's one that takes the time to describe how they rate engines (big mistake), let's examine their own example:

    The above graph (minus annotations) was produced with the following link here

    1) Vertical labels indicate the current the noted prop would draw at the given voltage. Note the diamond is drawn on the blue efficiency curve, but they could have been correctly placed on any of the curves since the diamond indicates the current draw that prop will draw on the noted motor at the noted voltage. In this case, putting a Graupner Miniprop 4.3x2.0 motor on a HiMaxx HA2025-4200 motor and powering it from a 10V supply will result in the motor consuming just over 18A, the prop producing 962g of static thrust at a pitch speed of 69 MPH. Prop data used in the graph is measured data imported from the excellent work done in Drive Calculator.

    If the prop label is in black, then the prop RPM is estimated safe. If the prop label is gold/yellow, then the prop might be operating at an unsafe RPM and you should consult the manufacturer's specifications on that prop. If if the prop label is red, then the prop RPM is exceeding the manufacturer's RPM recommendation.

    2) Reading motor efficiency is done by noting intersection of current and the blue efficiency curve. With the 4.3x2 prop, efficiency is about 83%. Note that ESCs and batteries and the the prop are not factored in here. This is simply the Pout/Pin of the motor, which is a measure of how good the motor is at converting electrical energy into mechanical energy.

    3) Noting where the green line intersects the operating current will yield the power input to the motor. With the 4.3x2 prop, it's about 185W of point going into the motor.

    4) Noting where the gold line intersects the operating current will yield the RPM for the prop. In the case of the 4.3x2 prop, it'll consume just over 18A at 10V on this motor, and turn about 36K RPM at that voltage.

    5) Thermal power is a measure of how much power is converted into heat. The closer you operate to peak efficiency of a motor, the harder you'll be able to push the motor because less of the power is being converted into heat. Currently, the motor weight is used to determine how much heat the motor can dissipate and for how long it can dissipate it. Ideally we should be characterizing various motor cases to determine Theta for case-to-ambient. T = Ta+ Pd * Thetaca. For now, the closer you operate to the red region, the hotter the motor will get. Inside the red region may damage the motor. I really depends on an enormous number of variables.

    6) These labels note the parameters used to generate the graph.

    7) A text summary of the red region of the graph.

    9) The title idicates the manufacturer, model of motor, and the voltage.
    So let's start with assertion # 1 that this motor will draw just over 18A and generate 962g of static thrust.  True?

    Of course not.

    The motor will pull far less than 18A once airborne, because any fixed pitch prop is nowhere close to its design AoA at zero airspeed.  Every prop is optimized for a certain combination of airspeed and RPM (and all are different).  The combination of airspeed (the forward vector) and RPM (the horizontal vector) determines a resultant relative wind vector.  When the resultant wind vector is compared to the prop airfoil's chord line, a certain Angle of Attack (AoA) is achieved.   When the AoA is correct, the prop is operating as designed.

    So when the airspeed is zero, no useful data can be gleaned or compared, especially if tests are conducted in a trade space that includes various props.  Every prop will operate at a dramatically different delta from it's design lift and drag, depending on it's pitch, airfoil, chord and twist, and there is no constant relationship inherent to any "data" we might measure.  In general, the steeper the prop's pitch, the farther out of whack the test will be.  High pitch airfoils, or parts of them, will simply be stalled during the test, while lower pitch props will be closer to but never achieve their design AoA, so they'll artificially perform completely differently on a test stand, usually generating more thrust in a relative sense but with no measurable relationship to reality.

    Even though all the "information" extracted is wrong in the first place, the longer the "test" is allowed to run, the more the air column will start to move, like a hovering helicopter creates re-circulatory downwash.  If the test is conducted inside a house or room, the size and shape of the room will change results your results, because an RC motor typically creates airflow between a few mph and 100 mph or more, requiring a massive venue to avoid changing the garbage data output.  To quantify this one of so many compounding errors, I did a test in my 4 car garage (3 spots are in tandem).  Placing the test airplane in the middle of the long lane colored the known-wrong thrust measurements by almost 10% (roughly 3 oz delta over 40 oz thrust) simply by opening and closing the garage door.

    To conceptualize this concept, imagine a motor boat anchored in the middle of a still pond. Run the throttle to full. The force on the anchor chain is substantial, but the propeller is mired in turbulent water, the engine is artificially taxed, and the prop blades are largely stalled even through the drag from the boat is zero. If I anchor the same boat in the middle of a flowing river, where the water current is perfectly matched to the design speed of the boat's power system, then you instantly see the absurdity of static "thrust" testing. If the motor was working as designed, then there would be no measurable thrust (i.e. the anchor chain would go slack).

    So much for making any progress on requirements A, B and E.

    So stop right  there, ground test data bases are positively useless. But let's keep going just to humor the novices that create these impressive monuments to aeronautical engineering ignorance.

    Thrust is 962 grams, right?   Of course not.  The net force pushing an airplane forward is "thrust." The motor only accounts for part of that equation.  Whenever you blow air on something, it feels a force.  That force is called drag.  Whenever you run up a motor, especially in a conventional tractor config, there is a lot of airframe drag.  Since the propeller has to pull the plane (too bad it cannot proceed along by itself) the plane is usually attached to it.  When the prop pulls one way on the shaft, the airplane pulls back the other way.  The net result, not the prop's contribution taken in isolation, is "thrust."   What this data base is telling us is UNinstalled thrust, which has no relationship whatsoever to installed Thrust, or Thrust Available, T(A).

    If you mount the motor in the middle of a flat board, the motor will produce no thrust.  If you mount the motor using a tiny suspended nacelle that has no aircraft form behind it, you might come closer to the uninstalled number, in exchange for possible  handling issues and increased structural requirements.  A conventional nose-mount might lose 30% to 80% of uninstalled thrust to installation error, depending on the aircraft form and the relative scale, twist, and airframe match of the selected prop.

    Propellers must be well matched to the aircraft, first.
    This one is designed to produce maximum thrust outside of the
    cowling boundary, while producing just enough inside air flow for 
    cooling during ground ops with the cowl flaps open.  The 3-blade 
    design transmits a lot of engine power to the air  while reducing
    the necessary diameter of the prop to avoid prop strikes with
    the ground.  3-blade (and 4, 5, 6...) props are easier to spin, per
    blade, during static runs because they create more turbulent
    wake that reduces drag (and produce less lift), but are overall
    more difficult to turn once forward speed allows 
    each blade to bite clean air. 

    So stop right  there, again.  All ground test data bases are positively useless.  But let's keep going anyway to see what other misconceptions are pervasive when trying to push ill-suited merchanside. 

    The next discussion is about how "efficiency" is measured, because it is presumably good.  Then they go on to embarrass themselves by using motor weight as the driver for heat reduction.  In other words, the heavier the motor, the better it is for aviation.  Smart.  Not.

    But even if this wasn't, obviously, backwards, the very assumption that efficiency is good is preposterous.  Good for what?  Well, good for being efficient, I guess.  But who says we desire efficiency?  Did they use telepathy to divine that everyone wants to drive a Prius?  Some people actually want high performance and are willing to compromise efficiency.  Why, some people even enjoy driving '60's muscles cars around!  Shame on you!!  Don't you know the car is not efficient?!

    But let's humor their absurd assumption, because I want to show that their methods are backwards even if their assumptions were sound, as laymen musings often are (just look at the Prius driver trying to save trees by minimizing their output of life-giving CO2).   What makes an electric motor efficient?  They've tried to estimate it with their egregious assumption that motor weight is good for efficiency, noting that a motor's ability to "dissipate heat and how long it can dissipate it" is somehow a good thing.  So... if I take a motor that converts 100% of it's fuel into it to heat, and incorporate a 90 lb heat sink, then I've achieved efficiency Nirvana, right?  Efficient motors don't dissipate heat well, they create a minimal amount of heat in the first place.

    So let's ignore that backwards logic, too.  Well, we already disposed of the notion that everyone wants an efficient plane, that's bunk, but what if you actually do want an efficient plane - kind of like our requirement A, which I graciously aligned with their false assumptions and methods for a reason. 

    There is another level where they've boarded the opposite direction train. How do electric motors generate less heat per unit power?  Generally speaking, by minimizing resistance to lots of current.  For any given voltage, that means heavy gauge, highly conductive coils (not heavy cases or heatsinks as they assert).  Ah ha, so we doooo want some heavy parts inside the motor, so maybe generating less heat during a ground test could be an ok metric on some level, right?  Wrong.  What we want, even for our carefully chosen, efficiency-oriented requirement A, is the smallest motor that remains reasonably efficient spinning the prop at it's design AoA (which is, remember, a combination of forward airspeed and RPM), where that airspeed also happens to match the most efficient airspeed for the airframe.  The motor should not incorporate coils so heavy as to be too efficient at full power.

    The "efficient airspeed" of the airframe depends solely on our requirements.  In requirement B we specified our goal is long flight duration, so we want to optimize motor efficiency at the airplane's L/D)max airspeed, not at full speed.  L/D)max is the airspeed that minimizes the sum of Induced Drag (caused by lift creation) and Parasite Drag (caused by aircraft form, skin friction, interference, and with fast airplanes, wave drag).

    Does the motor "data" base know any of this?  No.

    Turbojets are very efficient.  Turbofans are very efficient.  Turboprops are very efficient.  See a pattern?  All motors are very efficient within certain performance parameters.  All motors are very inefficient within certain performance parameters.  Efficiency is a meaningless word without aircraft design parameters.  If one is dumb enough to attempt to optimize motor efficiency in a vacuum, there is about a 100% chance they will drive the overall solution toward woeful inefficiency.

    Even if we ignore the fact that it is a clueless recommendation, and blindly choose the heaviest motor assuming that heavy casing or a heavy shaft is somehow related to efficiency in addition to the purity and conductivity of the design the motive parts, we'd always choose a motor that is massively over-designed (read: too big, too heavy and too power hungry) for a given aircraft.  The assumption that max motor efficiency (both incorrectly defined and incorrectly measured) is somehow good, drives us to install anvils that aren't working nearly hard enough to be light, and thus efficient, as part of the overall flying solution.
    Smaller/lighter motors = higher efficiency in the overall flying solution.
    The motor on the left weighs 100% more and draws 13% more power,
    while producing 11% more installed thrust.  As the shadows indicate,
    the silver front and back portions of the left motor's case are empty.
    Worse, much worse, is the aerodynamic design reality that for every 1 unneeded oz, you must add about 6 ozs of unneeded support.  More weight means you need bigger wings, that means more lift, more lift means more drag, more drag mandates more thrust, more thrust needs more fuel and beefier support structure and power control systems, more wing, motor, and fuel and support systems means even more weight, and that additional weight means more wing, motor and fuel and support systems, and so on, and so on, and so on....  The effect of adding that extra oz finally ends as these infinite integral summations mathematically settle, and that happens around 6 to 1.

    Point being, for efficiency's sake, we don't want a heavy motor that feels particularly relaxed most of the time, or we are hauling more dead weight all of the time.  We want a light engine that must work very hard, hard enough to pull its own weight.  We also don't want a motor that overheats and dies on takeoff.  But truth be told, and for true efficiency sake, under-design, not over-design, is generally the mundane path that leads to high efficiency, complete flying solutions.  In other words, the most efficient solutions are at the opposite end of the spectrum from what most toy motor testers are erroneously trying to optimize, in the name of efficiency.

    We've already "stopped right here" enough, so I won't repeat it anymore.

    What about the whole idea of characterizing motor performance without understanding the scale and type of aircraft.  Duh.  If one motor/prop produces 50 oz of static thrust and another produces 40 oz, is that good?  Maybe.  Maybe not.  We have no idea without tons more information.  Even if we assume a static test has some unknown but useful relationship to actual flight, if some motor/prop combo pumps out 50 oz and weighs 10 oz more than the 40 oz solution and also requires 5 oz more battery, then it probably sucks by comparison--especially after the 6:1 rule mandates increasing the size of the airframe to achieve proper handling.  You can't put little balsa wood fins on a 1,600 hp Rolls Royce Merlin and expect it to fly right.  Not caring a whit about the scale and weight of the power system as a whole (= weight of the motor, prop, battery, Rx, ESC, wiring, and all airframe structure required to do more than glide without it) is actually quite bizarre. 

    And what about that wide scale radial cowl, perhaps designed to minimize drag using cooling airflow over conventional cylinders?  Or what about the airplane's particular design and form? Gee, does the airplane, and the purpose of its full scale parents matter at all when selecting an RC motor?  I wonder...

    I once read  a toy tester who goes by the name of "Dr" Kiwi (tens of thousands of mostly erroneous posts on several internet chat boards = salesman) suggest that the best diameter prop for a certain person's 480 motor was 9", since it was "the best performing prop" on his homemade static thrust tester.   I saved the person from having to repair a crashed airplane by pointing out that a 9" prop would hardly clear the large scale radial cowl of his plane.

    The airplane actually matters. 

    But let's not even pick apart the idea that you can choose a motor and prop without knowing the desired purpose, type, or scale of aircraft, and finish up with this:  Matching a prop to the design goals and desired purpose of the airplane is a black art that almost always produces some unexpected results.  Anyone, anyone, who tests an isolated aircraft motor or prop on the ground, without knowing the exact airframe and performance objectives, and the weight of the power system as a complete flying solution, is beyond clueless.

    So how do you pick the right motor for your application?  Unfortunately, its a complex decision and the process itself changes from aircraft application to application.  The answer starts with defining your own unique requirements for the overall solution.  It ends with ignoring anyone who thinks they've solved all or part of your problem in advance--that is a sure path to picking the pig of the litter.

    Lastly, I suppose I might go easier on ignorant toy testers if they weren't mostly slimy salesmen.  Slimy is to be expected in sales, but I really hate when I know a gazillion times more than a salesperson who's only job is to learn about what they are selling.
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