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Lunch with Keith - January 2003

This month Keith talks about mimicking a realistic engine/prop sound, timing an AstroFlight 25G, the development of battery technology, and more.

Splash

Mimicking A Realistic Engine/Prop Sound

Keith,
I am relatively new to the RC world. I came across this Q&A of yours that discussed plane/prop noise and the desire to mimic a realistic engine/prop sound. Is there anything on the market related to having recorded sounds of your particular plane being broadcasted from the model? I am thinking of mainly the electric motor world, as the gas motors would be too loud and drown out anything else. It has surely been thought of by someone, and it may be a weight issue that stopped it from being developed. Any related info on this? What are your thoughts?
Thanks!
Travis Watson

Hi Travis,
The idea of making our models sound "real" has intrigued me for a long time. I have designed and built several sound synthesizers with varying results. I have also used a small microphone mounted just behind the prop to amplify the prop noise, as the majority of sound we heard from a man-carrying plane in flight is prop noise. All these methods show real promise, but the BIG engineering challenge is to build a lightweight, efficient loudspeaker. Commercial units are far too heavy and very bulky. I have an interesting design in the works, but not had time to flight test it. To get the sound to carry requires a large diameter unit driven with about 50 to 100 watts. Needless to say, we're taking about big, high cell count planes. Once I get a design to work, it is possible that the idea can be scaled down to smaller size planes. So many projects, so little time...
Keith


The Estrellita

I have a vice, the AstroFlight 010! I love this motor. Since I saw the Estrellita in your hands, I have wanted to build one and put one of my AstroFlight 010 motors in it. Is there a plan to this plane to buy/lend/lease?
I live in Sweden, so a plan in electronic format would be appreciated.
Best regards,
Patrik Delin

Hi Patrik,
The Estrellita was designed and built to make use of a 25-year-old fiberglass fuselage for a Shark Formula One. The 1/2A racing class was very popular in the mid-70s in Detroit, so my friend John Fotiu got the idea for a TeeDee.020 class, and designed the Shark. He was a one-man company that produced very high performance designs for small engines, all based on fiberglass fuselages and sheeted foam wings. I flew his designs in 1/2A pylon, competed in standard pattern with a 1/2A "Fun-X", and over the years have converted most of the designs for electric use.
The good news is that John has gotten interested in E-flight, and has revived the company so that your can now get kits for these wonderful designs. His website is www.jmglascraft.com, please note the ONE "s".
Keith


Timing An AstroFlight 25G

Hi Keith,
I recently purchased an AstroFlight 25G, used but in very good condition. However, upon power-up I realized the motor was timed for direct drive. After retiming (this is an older 25, as it has only one set of holes in the rear endbell), I found I could not get much power. Sixteen cells and an 11x7 prop produced little more then a 550 can. I soon found out that a bad cell was the culprit. However, after trying another set of batteries, I still had to "over-prop" the system. I had to use a 14x6 to get anywhere near the rated power. I was perplexed. After calling Kirk Massey and consulting with him, he suggested that I advance the timing in reverse to get more power. I already had it set as far as it would go for that endbell. After a few test runs, the power has now increased to the point where I now use a12x8 prop and 18 cells. Using my Wattmeter, the input power now measures 590+ watts@ 30 amps. The last time I tested the system, it measured 635 watts input. Was this apparent increase in power due to the brushes reseating to reverse timing? I have become a little confused as to the cell count and prop size, as I have been reading anywhere from 14 to 16 cells and an 11X7 prop. Apparently, this combination is working okay, so unless I hear differently, I believe I will use it. Any comments?
Thanks Keith,
John Zook

Hi John,
We expected much less from our motors back then, mostly because of the NiCads we had to use. Nominal currents were around 20 amps, and only madmen went above 30. Sub-C cell capacity was only 1200 mA-hr, and cell efficiency fell off quickly after 20 amps. At 30 amps, you might only get .9 volt/cell and less than one amp-hour. These early AstroFlight motors only had a very mild timing range, enough to cover the typical expected use. There is nothing wrong with these motors. They are still quite efficient, and they will perform well. If you need additional timing range, you can disassemble the motor, and carefully drill and tap an extra set of holes in the back housing, I did this on some of my motors before it became a standard feature.
The brushes do need some run-in when you change direction, so I'm not surprised that your performance went up after a while. This motor could also have a milder winding than the present day 25, so it would take more cells to achieve expectations.
Keith


De Havilland Comet

I have seen photos of your De Havilland Comet and wondered if the narrow wingtips made it very prone to tip stalling. I was thinking of scratch building a Speed 400 size, but I am leery of the narrow tips. Would a symmetrical airfoil that stalls straight ahead help the situation?
Thanks,
Joel Scholz

Hi Joel,
The tip-stalling characteristics of the Comet are legendary. Everyone loves to see it in flight, and remarks on its grace, beauty, and smoothness, mistaking it as an indication that it is easy to fly. Nothing could be further from the truth, as it is by far the most demanding and least forgiving airplane that I fly. At any point in the flight, whether straight-and-level, gentle turns, or careful aerobatics, just a touch more elevator than is needed makes a tip drop away. The actual Comet had the same manners. When I was researching the Comet before building mine, I found a reprint of the test pilot's report. There were all the usual test results, but in the "comments" section at the bottom was written in all capital letters, "DON'T BUILD ANOTHER ONE!" However, three Comets were built for the MacRobertson race from England to Australia, and before they were allowed to leave the DeHavilland factory, all six pilots were required to do five solo take-offs and landings. These pilots were the elite top-gun level pilots of the day, and all except one screwed up at least one circuit, and several actually caused enough damage that the landing gear needed repair before proceeding. The only pilot that did all tests successfully was Amy (Johnson) Mollison, piloting "Black Magic". Like all racing planes, the Comet was not a bad design, just one highly optimized for its goal. It was designed as a long distance racer of unusually high efficiency. Two mild engines coupled with sailplane-like aerodynamics gave it a very long distance between fuelings, which was DeHavilland's best guess on the way to win the race.
Getting back to the model, it did put -4 degrees of wing twist in the tips starting at the inboard end of the aileron. Long before I built the big Comet, I first made a small one (about 1978) that was powered with two AstroFlight 02 ferrites, virtually identical to 4.8-volt Speed 400s. It flew okay, but was a little marginal on four cells, much better on five cells, but the motors didn't last long. I still use it as a slope soarer. It has a 50" wingspan, about 300 sq.in., and reasonably scale, but I did increase the tip chord about 30%. They are still very tiny even at that! On highly tapered wings, I find it better to keep the same airfoil all the way out, and then use wing twist to minimize tip stall, rather than trying transitioning airfoils. By the way, both the big and little Comet use my favorite airfoil, the Clark Y.
Keith


The Development Of Battery Technology

Dear Dr. Shaw,
Do you care to make any predictions about the development of battery technology over the next few years? What changes can we expect in energy density? What type of cell chemistry will dominate? Is it reasonable to expect useable fuel cells anytime soon?
Sincerely,
Gib Lichstein
Boulder CO

Hello Gib,
There is a lot of research activity in power cell development, driven by environmental concerns, performance, capacity, and cost. While nickel-cadmium batteries (NiCads) have been the staple for the past 30 years of electric flight, there is growing "green" concern about their safe disposal due to the cadmium component, which is considered a toxic substance. For that matter, so is nickel, but much less so. As long as NiCads are correctly disposed, they are safe. A lot of technology has gone into the safety of high-performance NiCads used in e-flight, so that even severe abuse does not release any dangerous materials.
The next cell chemistry to reach e-flight used nickel metal hydride electrochemistry (NiMH). We are on the fourth generation of NiMH, and most of the early failures/limitations have been conquered. These cells still contain nickel, but the cadmium electrode material is gone. The good news is that as long as you have a quality charger, NiMH batteries behave much like NiCads. For sport flying, they offer better capacity per weight than NiCads, as long as you accept slightly lower performance. I seem to be in the minority, as every NiMH I've tested falls short of NiCad performance, while others report equal performance. I see a fairly dramatic voltage depression under load, so that the WATTS to the motor is reduced, requiring heavier propping to regain peak performance, which due to the higher current, eats into the supposed capacity advantage. The other shortcoming seems to be lower cycle life than NiCads. I have 10 year old NiCad packs with 400+ cycles that still achieve 90% of their original performance, while based on tests of a bunch of borrowed NIMH packs, most are dropping to less than 80% of their original performance in about 100 cycles. There are always new NIMH cells coming out, so I can't state conclusively that this problem will remain to be a concern.
Lithium batteries have been around for a long time, even in e-flight use. Before rechargeable cells became available, a few very rich souls used primary (one-time) lithium cells. I remember an early KRC meet where the first two places in All-Up-Last-Down were won by a father and son who taped three "D" lithium primary cells to the bottom of some really ugly, "old-timerish" planes. Yes, they won the event and received nice $2 plaques that cost them about $140 for the two three-cell packs! This is not very practical or economical. Early rechargeable cells used the lithium-ion or lithium-metal electrochemistry. One of my concerns about lithium technology is that several of these cells used heavy-metal poisons, like thionyl chloride, in their formulation. I honestly don't know which brands use what chemistry, as the companies are being very closed about this info. The biggest drawback to lithium-based cells has been very high internal resistance, which severely restricts the amount of current that can be drawn from them with any efficiency. This is mostly due to the positive lithium-alloy electrode, which in the trade is disparaging referred to as "a good insulator". New alloys are being tested that should dramatically reduce the resistance, while holding the cost within reason.
Some new lithium cells have come onto the market recently, called Kokams, distributed by FMA (see their website www.fmadirect.com for extensive details and performance curves, look under SUPPORT for application note AN000001). These cells have some intriguing improvements over other lithium-based cells that I am familiar with currently. Using the correct charger, they can be charged at a 1C rate, and still show decent discharge performance up to about 3C, or even 5C for very short bursts. While playing series-parallel games could get the performance up to feeding an AstroFlight 90, it would be outrageously expensive. Rather, these cells seem to be just the ticket for the few amps up to 10-amp sport regime employing sub-280 to 400 sized ferrites and small brushless motors. I intend to buy some of these cells to play with this winter, so I should have some real-world experience to report on later.
Years ago in my stint at NASA, I was involved in some of the early fuel-cell research. At its simplest, a fuel cell catalyzes hydrogen gas mixed with atmosphic oxygen to produce electron flow, while expelling only water as a by-product. The real dream is to couple this with a converter/filter that will take hydrocarbons like methanol, gasoline, or peanut butter and create the hydrogen gas for the fuel cell. There is still a long way to go before use in e-flight as the most recent "high-performance" fuel cell I've seen looked like a good-sized paperback, needed pure hydrogen gas to operate, and produced a whopping 5 watts of power! Even farther into the future is a biological converter being studied in England that uses E. Coli bacteria to produce hydrogen from sugar cubes. In the future, we will have yet another excuse for not being able to go flying. Sorry Fred, I forgot to feed my fuel cells last night...
Keith


AstroFlight 010 Powered Goon

Keith,
I've followed your scale models over the years, and you always come up with subjects that are interesting. I have an AstroFlight 010 Brushless 801V motor I've been flying in a sport 4-channel model that I designed. I'd like to build a small Thompson Trophy Racer to put it in and one of my favorites is the Chester Goon. I seem to recall a picture in one of the model journals of you flying a small Goon powered by an AstroFlight brushless 010. Do you have any plans available for that plane, and what are its specs? I also saw a picture of a much larger Goon that you built that had retracts. I would really like to get my hands on plans for that one! Do you have any plans available for it? The only source of info I currently have on the Goon is from Charles Mendenhall's book. There is a small three view, but there are no details on airfoils, etc. Thanks for any help you can provide!
Al Clark

Hi Al,
Thanks for the compliments on my Goon. I think it is the neatest looking monoplane ever designed, shear beauty in flight. Now I have some good news and bad news.
The little AstroFlight 010 powered Goon was designed by my good friend Martin Irvine. He lives in Kingston, Ontario and writes for Electric Flight International, now called Quiet and Electric Flight. I test flew it for Martin, and it was a delightful flyer. You can get the plan from Traplet Publishing in England at www2.traplet.co.uk, plan number MW2845. Be sure to install the recommended rudder servo, as these long fuselage, short span racers really need rudder input at times.
Unfortunately, I do not have plans available for any of my planes. There is never enough time to clean up my sketches to publishing level, nor time to write a construction article. Besides, the retractable gear and door system required many handmade parts not easily constructed in most home workshops. Also, the mid-wing design with an unwieldy 82" long fuselage and a 36 cell 1.5 kilowatt power system makes it impractical for any but the most diehard 1930s racer nuts.
However, all is not lost, as there is an excellent documentation drawing available that shows every rib and fuselage station, along with a wealth of info on the design. You can get this drawing in various scales (along with similar high quality drawings of the GeeBee R-1, R-2, Z, Hall Bulldog, Schoenfeldt Firecracker, and many other 1930s designs) from Vern Clements 308 Palo Alto Dr. Caldwell, Idaho 83605, by email at GeeBeeRacers(at)aol.com, or by phone at 208-459-7608. Tell him Keith sent you. My father was a draftsman, and I worked as a draftsman for a while, and based on that, I will tell you that these are probably the finest examples of the drafting art, long before it was usurped by the sterile, comical CAD stuff I see everywhere today.
I used his drawings as the basis for my 31% scale Goon. I wish I would have known about his Gee Bee R-1 drawings in 1984 when I drew mine own to build my R-1. It would have saved me endless hours plotting bulkheads.
Keith


Thin Versus Thick Airfoils?

Hi,
Why do many electric plane designers favor a thin airfoil? Some of the older wet designs used very thick airfoils and seem to fly well. I have a Joe Bridi model that I converted to e-power and I like how it flies. When I change airfoil thickness on the flight simulator, it doesn't offer any advantages that I can see. I'm asking about sport planes, not pylon racers or really hot gliders.
Thanks,
Rob Zinck

Hi Rob,
In the early days of "full house" pattern using 3-axis control, circa mid-to-late 50s, airfoils were actually quite thin, typically 12-14%. This was due to the very high radio weight (around 2# or more) and meager power, a scenario much like early electrics. This was done in an attempt to fly on efficiency and use momentum to help carry the plane through the next maneuver. All that changed with the advent of Ed Kazmerski's Taurus in 1962. It had a 20% semi-symmetrical, a "big" engine in the way of a K&B .45, and a very long tail moment to help dampen out the twitch of the reed radio system of the day. The philosophy was that this high-drag, high-power combination produced a more constant speed airplane that was easier to control through aerobatics. Given sufficient power, it's not a bad design philosophy. Virtually all pattern designs in the 60s used this formula, including Joe Bridi's Kaos and Phil Kraft's Kwik-Fly. It wasn't until knife-edge maneuvers became prominent that higher airspeed and thinner airfoils returned with designs like Jim Martin's Banshee and Ralph Brooke's Crusader in the early 1970s.
Enough history, let's get back to you question. Simply, the thinner airfoils produce the necessary lift at far less drag. I remember reading an NACA journal article that tested a host of airfoils for fighter use. Their conclusion was that the crossover is at about 12%. While thinner sections could have lower drag coefficient, their max lift coefficient were lower or stall characteristics were worse. Thicker airfoils had much higher drag coefficient, with the same or only slightly higher max lift coefficient, and many times a softer stall. Keep in mind, this work was done in the 1930s, so fancy airfoils hadn't been created yet. Most of our models fly at Reynolds numbers that are appropriate to their testing parameters. The grim realities of wing spar design strength usually required a thicker root airfoil, but the tip sections were all about 12%.
We now have so much power available with electrics that outrageously inefficient aerodynamic layouts can be flown with abandon. The bottom line is that if you are enjoying the way your airplane flies, it's a good airplane. If it leaves something to be desired, it's time to consider better design elements.
Keith

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