Weight Reduction by Static Discharge: An Update on the View-A-Charge
|Last month's column included some remarkable photos taken through the View-A-Charge electrical charge filter, which at the time was still considered 'classified' by the government. However, in honor of the 100th anniversary of the discovery of the electron (Sir Joseph Thompson of England is credited with having discovered this rather important particle on 30 April 1897), the View-A-Charge has been de-classified, so I can now show you a photo of it in use. This photograph shows my lab assistant, Samuel, demonstrating the use of this remarkable instrument. The markings on the instrument, S.A.M., are the initials of Society Atomique D'Masochist, the French organization that actually owns the patent for its design.|
The View-A-Charge camera adapter, which looks like an elliptical tube with parallel (rather than helical) threading, is still classified, so you will have to use you imagination to visualize the View-A-Charge attached to a camera.
I did received a few questions about the April column. Greg Arserio reported that the zero-weight-zero-resistance superconducting wire (Radio Shack part #555-1212) is no longer available. I realized that they may have had problems with the highly radioactive nature of the material, but all I can suggest is that you complain to your local Radio Shack about their tendency to discontinue parts that are hard to find elsewhere.
Pete O'Shea expressed surprise over what electrons actually look like. What can I say? That's what they look like! And Bob Al7ev (AOL users always seem to have weird names!) suggested using a Discwasher static discharge gun originally designed to de-static LP records (remember them?) to discharge the static accumulation on a plane. After a few emails back and forth, we concluded that the best way to use this would be to mount the gun on a 'Static Discharge Plane', set up with a servo on the trigger. The Static Discharge Plane would fly in front of other planes occasionally, and set off the discharge gun. The Static Discharge Plane pilot would, of course, charge for this discharge service, so that they would be able to discharge their hobby expenses on their tax return. What a way to make a living!
Beginner's Corner: Breaking in a 'Can' Motor
If you have been around electric flight for very long, you have probably heard other flyers mention 'breaking in' a 'can' motor. You may have even done it yourself. Some people may make specific claims about what break-in will do for a motor in terms of increased RPM; I will leave the specific claims to others. I think that breaking in a motor is a good idea, and so does Kirk Massey at New Creations R/C, and so do a lot of other people.
This article applies ONLY to brushed motors; brushless motors do not require any sort of break-in. First all, a few definitions are in order:
|What is a 'can' motor? A 'Can' motor is one that typically has the motor housing body and front plate pressed from a single piece of metal, similar to a drink can. The back plate may be either metal or plastic, and is typically held in place by metal tabs and/or crimping. The Graupner Speed xxx series are all 'can' motors, as are the Robbe 'Power' series, virtually all car or 'buggy' motors, and many, many others. Astro Flight, Mega, Plettenberg, and Graupner Ultra motors are NOT 'can' motors. A general rule of thumb is that if you paid less than US$50 for a motor, it is a 'can' motor.|
What is meant by 'break in'? To understand what 'break in' does, you must first understand a little about the insides of a motor. This drawing shows an exaggerate end view of the commutator and brushes inside the motor both before and after break-in. You may be able to see essentially the same thing on your motor by looking into the cooling opening while holding the motor in front of a light.
The brushes in the drawing on the right have been worn down to the point where their shape closely matches the shape of the commutator, thus improving the electrical contact, and reducing resistance. That is all there is to it! 'Break-in' is the colloquial or common term for this process - but perhaps 'wear-in' would be a better description, because what we are really doing is accelerating the wear of the brushes, so they will be more closely shaped to match the commutator. One could accomplish approximately the same thing by running the motor for a long time, but that would also wear out the bearings, which is NOT a good idea.
Power during break in: One of things that varies between descriptions of how to break in a motor is how to power the motor during break in. Some people suggest running down a pair of carbon-zinc D cells, and others suggest using a NiCad pack with one less cell than will be used during flight. I don't like throwing away batteries, and I see no point in keeping odd size NiCad packs, so I use a DC power supply. The next photo shows the complete equipment setup that I use for breaking in motors. The power supply is a Lambda 5VDC 6A linear power supply, with an adjustable output voltage. The fan sitting on top of the supply is powered from 120VAC, and is just a precaution to keep the supply cool and maximize the available current. You will have to look at an electronic surplus or salvage dealer for a power supply of this type - you do not want to know what they cost new. This type of equipment is relatively ageless - mine is at least 20 years old - and you are most likely to find this sort of supply at a company that breaks up old electronic equipment for scrap. You will have to add your own wiring for both the AC line and the DC output to the motor. My supply has terminals that fit crimp-on spade connectors. If you do not know how to attach an AC line cord, this will become a major project, so find someone who can help you. Make sure that your connections are SOLID - this is NOT the place for clip leads or electrical tape.
|If you look very closely at the photo, you will notice that the DC output from the power supply ends in a nice, neat Astro Flight Zero-Loss connector - which is connected to the motor by a pair of clip leads. This is not a display of laziness, but rather is a carefully considered circuit component. Really. When a permanent magnet DC motor is attached to a power source, the initial current can be quite high - 20 Amps for a Speed 400, 70 (!) Amps for a Speed 600, even more for larger motors. This very high initial current will taper off once the motor starts rotating, but it is present long enough to cause a high quality power supply to see it as an overload, and go into 'crowbar' shutdown. The clip leads add just enough circuit resistance to reduce the startup current to below the level that will cause a shutdown.|
The other thing you may notice is the large bottle of distilled water and the plastic cup. The distilled water is used as a break-in lubricant! If you have heard about breaking in a motor in water, and wondered if this idea was for real - rest assured, it is for real. Fill the cup with water, connect the motor to the power supply, and put the rotating motor in the cup. Yes, it feels a little strange - but it does work.
The water serves several purposes - it helps to keep the motor cool, carries away material that is ground off the brushes, and it acts as a cutting lubricant. As the brushes are ground to shape, the water will turn (yeech!) gray from the brush dust. I use distilled water because, well, that is what I was told to do - and it makes sense, because tap water may contain an assortment of minerals that will react with the motor and leave it in worse shape than when you started.
I run the motor for 3 minutes in each direction (reverse first) and then run it out of the water for a minute or so to get rid of any water that is left in it. The total time is mostly a matter of how gray the water becomes - it does not need to turn black! Then I put a drop or two of 3-In-1 machine oil on the bearings, and turn the motor a bit to help it soak into the bearings. Let it sit for a while, and then wipe away any excess oil. That's all there is to it!
Changing an Electric Sailplane from Direct Drive to Geared Drive
After more than 200 flights with a Speed 400 direct drive system, I finally changed my NSP Neon to a Graupner Speed Gear 400 gearbox. The original drive system was:
According to Electricalc, the characteristics of the direct drive system should be 11.5A (89W input, 51 W to prop), 12900 RPM, 43 MPH pitch speed, 600 ft/minute climb at 17 degrees, 23 MPH.
The new geared system was:
If you look closely at the photos, you will notice that the location of the receiver and battery have been swapped to maintain balance. Note also that the 11-8 is actually closer to a 10.75-8 because of the smaller hub.
I wanted some idea of how this setup would perform, so I used several methods for calculating the performance of the system:
Pete's spreadsheet requires the motor parameters, while Bongartz's spreadsheet calculates them from other user data. According to the electric motor data library from Kress Jets, the parameters for the Speed 400 6V are:
According to Electricalc, the characteristics of the 4:1 geared 11-8 system should be 7.9A (64W input, 41W to prop), 4000 RPM, 31 MPH pitch speed, 645 ft/minute climb at 20 degrees, 21 MPH.
I was looking for a faster, more stable climb - what might be described as climbing with 'more authority'. What I got in actual flight testing was a similar climb - but at about 1/3 less power! It would appear at first that we are getting something for nothing, because (according to Electricalc) we get about the same climb rate with less input power. I don't particularly believe the rate-of-climb figures, but they are probably good for comparison purposes. Comparing the 4:1 11-8 to the direct drive 6-3.5, the geared prop is running at a lower current, so the motor is more efficient (lower I2R losses). The prop is probably more efficient at the slower speed, too, but this is probably not accounted for in the calculation. Part of the reason for improved prop efficiency is simply operating at a slower RPM, but part of it is operating at a slower pitch speed, the speed at which the rotating prop is trying to fly the plane. If a big, draggy plane is flown with a a motor/prop combination that results in a pitch speed far higher than the plane can possibly fly at, much of the energy into the prop is wasted.
A very crude calculation of stall speed gives an approximate value of 18.5 MPH - or maybe less, depending on whose crude calculation - so for a slow moving plane like the Neon, the 31 MPH pitch speed sounds a lot better than the 43 MPH for the direct drive.
I made a few important observations during the first few flights with the geared system:
I then wanted to see what would happen if I installed a larger prop on the gearbox to bring the current up to the allowable limit of about 11 Amps.
The modified system was:
The only problem with this setup is that the spinner is about 40mm diameter, and the nose of the airplane is only 30mm diameter. These blades are so heavy compared to the 11-8 that I was not willing to put them on the tiny Robbe hub, and risk having it self-destruct in flight. The oversize spinner looks funny, but flies ok.
A close-up of the spinner, showing how it doesn't quite fit the fuselage.
According to Electricalc, the characteristics should be 10.0A (80W input, 47W to prop), 3500 RPM, 33 MPH pitch speed, 800 ft/minute climb at 24 degrees, 22 MPH.
This is a bit less power than the direct drive setup, but Electricalc claims a 1/3 faster climb. In actual flying tests, this combination really did feel very different from the direct drive setup. It was, in fact, the best climb I have ever seen on a Speed 400 sailplane. A few flight observations:
The geared 12-10 is definitely a nice combination - interestingly, the Bongartz spreadsheet indicates that it should produce about the same static thrust as the 11-8 - with significantly more input power. Pete O'Shea's spreadsheet indicates about 20% more thrust than the 11-8 prop, but the numbers are completely different (and probably more accurate) There is still room for experimentation here - I am limited to 7 cells at present because I have a number of 7 cell packs, and the Neon has no additional room for the battery.
The three props used in the tests
If you look around at your flying field, you will probably find that most of the other pilots hold on to their transmitters with both hands while they fly. A few contrarians may have neck straps, but in the US, that is about all you are likely to see. The current crop of Japanese-designed transmitters are clearly meant to be held in the hand, and have the curves to prove it.
In Europe, the story is somewhat different. In the UK, the JR, Futaba, and Sanwa agents sell essentially the same radios that are sold in the US. The German companies sell transmitters with integral tray attachments, which allow the transmitter to hang from your neck as a stable platform by simply adding a strap and a pair of metal brackets. Graupner (JR) and Robbe (Futaba) both sell this type of tray/transmitter, although they do not sell anything comparable in the US. Multiplex currently sells a tray/transmitter in Europe, but I am not sure of what their plans are for the US market.
Despite the lack of readily available tray/transmitters in the US, it is still possible to adapt a US-market transmitter to a tray. I have been using a Simprop 'Assistent' tray since the mid '70s. It fit my old US made Royal transmitter perfectly, and fits the Futaba 4NBF when blocks of soft foam are placed on either side of it. I only recently purchased trays that are better suited to the Japanese designed transmitters.
Why bother with a tray? Accuracy of control, convenience, and all-around ease of use are a good starting point. A tray provides a place to put your stopwatch - or your beer. A tray also provides you with the opportunity to stretch your arms while circling in a thermal without putting down the transmitter. The transmitter is held in a stable position in front of you, and you can use ALL of both hands for flying. Launching a sailplane on a high-start is particularly easy, because you can launch with one hand, and FLY with the other. The same is true of launching an electric - at least one that does not require a running start. I never adjust trims with my right hand - I fly with my right hand, and all trim adjustments are done with the left - so I never have to take my hand off the right stick. I really find I am much more comfortable flying with my transmitter in a tray, because it leaves both hands free for flying.
I am certainly not the only person to think a tray is a good idea. If you look at magazine articles about international events, you will notice that many of the top helicopter and pattern pilots use transmitter trays.
To the best of my knowledge, none of the radio manufacturers make trays for their transmitters. I have a JR XF622 and Futaba 4NBF, and don't particularly like the design of either of them. Placing the transmitter in a tray is a big improvement.
|My JR XF622 is usually used in a Falcon Expert tray from JV Trading in Norway. See the JV Trading (Norway) Falcon Trays web page for information on a whole line of models. JV does not currently have a US distributor, but you can order directly from Norway by paying with a credit card, and the prices are quoted in US$. The Falcon Expert and Plain trays will fit any of the Japanese brand transmitters sold in the US and UK. Additional models are available to fit the tray style transmitters sold in the German market. These trays are not cheap, but they are very nicely made, and I certainly think they are worthwhile. A transmitter can just sit in the tray, or you can add velcro to the tray and the back of your transmitter. The only problem is that many of current transmitters have no flat spots appropriate for velcro - the XF622 is a good example of this.|
I used the Falcon Expert tray with my Radio Glove through the winter. The tray and glove seemed to go together quite well, and it actually felt strange to fly without the glove when the weather finally got nice enough.
I have also used the 'Petal Saber 6 Transmitter Tray', available from Hobby Hangar. This is a rather minimalist tray, but it is also a lot less expensive. The only attachment to the transmitter is a clamp on the transmitter handle, which is really not adequate. There just is no good way to pick up the tray/transmitter assembly. If I use this tray again, I will add some velcro to attach the transmitter more securely. The photo below shows the optional palm rests, which are attached to the tray bracket with large nuts. A similar tray is available from Performance Pad, although I have not actually seen one.
In most of the above photos, you may be able to see the stick extensions that I use on both of my transmitters. These extensions come from Guidari R/C International in Canada. They are adjustable, and versions are available for almost all transmitters.
|You can, of course, make your own tray. The model magazines have published plans from time to time. The Simprop Assistent may be a good model for making one yourself. The original straps on mine broke a long time ago, and I replaced them with a set of leather straps. I made a wooden tray similar to the Assistent from 1/4" plywood and masonite, but have never used it.|
JV Trading, O.Solbakken 43, 7550 Hommelvik, NORWAY Check web page for prices and shipping.
Radio Glove, 1077 So. 930 W, Payson Utah 84651 USA 801-465-4228 US$34.95 plus US$5 shipping. R. Brent Billings
Guidari R/C International 170 University Ave W, Suite 12-103, Waterloo, Ontario N2L 3E9 CANADA US$17.95 plus US$5 shipping
Petal Saber 6 Transmitter Tray, available from Hobby Hangar, 1862 Petersburg Road, Hebron KY 41048 USA 800-611-3860 US $18.95 plus shipping.
The Harry Higley book "Entering Electrics" includes a description of how to break in motors, although it does not include the water immersion method. There is also a description of how to use an old Astro Flight charger as a power source for break in.
"Entering Electrics" contains a lot of useful material, and its only real shortcoming is that very few of the many planes described in the book are still available in the US. Nonetheless, the descriptions of the planes does provide some idea of the variety of planes that have been available. Discussion in the book is limited to motors no larger than an Astro Flight 05. There is also a lot of discussion of construction techniques.
This book is available from a variety of sources, including Tower Hobbies in the US and Traplet Publishers in the UK.
If you would like to recommend something, please email me at skranish(at)ezonemag.com
I have mentioned this several times before: Wise Owl Worldwide Publications, 4314 West 238th Street, Torrance CA 90505-4509 wiseowl(at)sprintmail.com is the US subscription agent for Aviation Modeler International. This magazine just gets better with each issue (so does the E-Zone!). The May 97 issue includes 2 completely different electric free flights, an article on electric control line combat (the plans are in the April 97 issue!) and an article and plans for an indoor electric R/C plane made largely from cut foam. This is a LOT of electric material for what is supposed to be a general interest magazine!
The nice people at Wise Owl respond promptly to email. Please tell them I sent you.
If you are not in the US, the address is Model Activity Press, 5 George Street, Albans AL3 4ER UK, Tel 01727 840010
The May 97 RC Model World (from Traplet Publications at Traplet Publications On-Line) includes a nice article on electric round-the-pole (RTP) control line, with the promise of more to come in future issues.
The folks at Traplet do respond to email, but much more slowly. Please tell them I sent you.
If you would like to recommend a 'Source of the Month', please feel free to contact me at skranish(at)ezonemag.com
This document is copyrighted (c) 1997 by Steven Kranish, and may not be copied or used in other forms of publication (electronic or paper) without written permission from the author. I will probably grant permission, but I would like to know about it, so go ahead and ask.
If you have any questions, please feel free to contact me at skranish(at)ezonemag.com
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