Power Pod Revisited
Back in the May 1999 issue of Forum 400, I brought up the idea of a self-contained Power Pod for electrifying Hand Launch Gliders (HLG). The backbone of this idea was to utilize a high-Kv motor using just 2 or 3 cells for power, which kept the weight addition to an absolute minimum but afforded a surprisingly high level of output. Additionally, being "self-contained", it would allow someone to electrify their favorite HLG without any significant modification. Or with a little ingenuity, perhaps NO modification whatsoever. Either way, the unit could simply be rubber-banded or taped on, then easily removed to revert back to glider-only mode.
Ive flown hand-launch and love it, but after a while my shoulder gets tired and it stops being fun. Once I got into the electric side of airplanes and gliders, the concept of having electric assist meant utilizing existing HLGs and turning them into what people are now calling Park Flyers and such. And the calm summer evenings outside became more enjoyable because the pain in my shoulder went away!
Most of the HLGs on the market today barely have room for the radio gear, much less tagging along an extra 7-cell battery pack for conversion to electric flight. The alternative offered by the power pod approach is that you still rely on the 4-cell receiver pack in the glider to maintain control of servos and such, and just add two cells to the power pod with a manual on-off switch. As mentioned above, running off of just two cells kept the converted gliders light, but my concept suffered one drawback . No ability to control the motor once you let it go! And thats where I left it, until now.
To update my original concept of a "switch and go" pod, the task now would be to find a way to control the motor from the ground. If youre using a 7-cell battery for motor AND radio control then this is a no-brainer; its already built into the speed controller. But if you stay with a 4-cell battery running the glider receiver, and add a 2-cell battery for the motor, how would you find a controller that could handle as low as 2 cells? Well, once again "Micro-Man" comes to the Rescue! (a.k.a. Pat delCastillo of Castle Creations). Already known for bringing the 20-25 amp speed controllers to a new definition of "smallness" with the Sprite series, he went a jump further and brought the 5-10 amp controllers to microscopic levels with the Pixie series at Toledo 99. This year at Toledo 2000 I finally picked up a Pixie 7 from Pat, and spoke with him about modifying it for what I was hoping to do for controlling a two cell motor.
Now I hope I dont confuse the issue any further (or confuse my self in the process ) but understand there are three different modes of operation for the Pixie. Two are in the instructions, the third one is "unofficial" and is what Im talking about doing.
- a Pixie 7 can operate in BEC mode on 3-6 cells (up to 8 cells if you limit to two servos) and provide power to the receiver and motor with a low-voltage cutout to prevent the batteries from being discharged too far.
- a Pixie 7 can operate with BEC disabled (which means you need a separate receiver pack) and will handle up to 18 cells this way , and again with a low-voltage cutoff.
In these two modes of operation, the low-voltage cutout happens at about 2.5 volts. This is okay for a 3-cell motor battery pack which is at our nominal 2.7 to 4.2 volts, but not for the 2-cell one Im after which would be 1.8 to 2.8 volts.
- a "modified" Pixie 7 controller will still need a separate receiver pack, but will control the motor clear down to essentially zero volts whether youre using one, two, three or more cells. In this mode, using the 4-cell receiver pack already existing in the glider to power the Pixies "brains", you could configure whatever battery condition you wanted to make, clear down to 1-cell and clear up to an 18-cell motor pack.
This is what I needed! So, armed with Pats recommendation for "surgery" to the Pixie 7, I warmed up the soldering iron and went to work. Ill make this VERY CLEAR at the beginning that any modifications to a controller will void its warranty. Pat delCastillo strongly cautions against this, as he doesnt want controllers coming back on warranty repair bearing the scars of an attempted re-wire!
*** These modifications will void your warranty ***
As you can tell from the pictures, its going to be nearly impossible to show the modifications made, so Ill have to resort to WordArt or something. But its a real simple operation, if you know how to solder. First, referring to Figure 1, arrange the Pixie so it looks like that shown in the picture. I didnt draw all the detail, but pay attention to the chip size and its "legs" and to the orientation of the mounting pads for the Rx BEC connector wire. Just to make sure youre starting on the right side of the controller!
With this in mind, refer to the "leg" of the main chip that the big black arrow points to. Youll need to heat this leg up and gently pry it off of the printed circuit board once the solder melts. This particular chip is the BEC regulator, and youre removing the power input leg which also feeds the microprocessor. Use a fine point, 20-30 watt soldering iron for this operation. You only need to raise the leg off the pad by about 1/16", just enough to make sure it never springs back into place against the pad. Secondly, unsolder the red lead of the BEC output from the far right circuit pad and solder it to the chip leg you just raised off the board. This allows the 4.8 volt receiver battery to feed voltage right to the microprocessor. Otherwise, with only 1 or 2 cells feeding the BEC regulator and microprocessor, youd never have enough voltage to keep the microprocessor alive. Thats it for this side, turn it over now.
For the second half of the modification, refer to Figure 2. Again note the chip type and leg configuration and the orientation of the mounting pads for the BEC output to make sure youre doing it right. You need to jumper a wire from the ground of the BEC output to the ground of the Drive FET output. Pay special attention to the fact that the lower 3 legs of the Drive FET are joined together, but NOT the 4th one. This ties the ground of the receiver battery in with the ground of the motor battery, so you dont have a "floating ground".
Thats it. Your modifications are finished. Wrapping the controller with electrical tape or heat-shrink tubing will help protect your components and prevent short-outs.
*** These modifications will void your warranty ***
From here just wire up the battery and motor connections as laid out in the instructions for the Pixie. Of course, once you mount the power pod to the glider you snake the throttle control wire down to the receiver and push it into the throttle channel. Then it works like any other controller. As far as making the final connections, ensure that your receiver is turned on and functioning before you plug in the 2-cell motor battery.
The nice thing about the Pixie is its high amperage rating. Since youre only dealing with 2 cells, its nice having the ability to draw 7 amps to make up for the lack of voltage in the wattage equation. This will give you about 20 watts of power, and will only add a few ounces of weight. If you substitute a smaller motor and smaller batteries, you can even power the indoor types of planes, or hand-launch gliders in the 4-8 oz range. If you have a heavy hand-launch, you can go up to 3 or possibly 4 cells on the Graupner 300 power pod arrangement.
Whether you use this for a Speed 280 or 300 motor, a slot-car motor, or even one or two of the Wattage B-2 micro-motors this arrangement will work nicely. Its capable of 7 amps continuous, so that outdoes the capability of most every motor I know of in this size category and smaller.
Your design of the "pod" itself should take the glider design into account. Whats the best way to mount? How much prop clearance do I need? Forward facing or rearward facing? Since you need to maintain CG location, you can accomplish this by moving the entire pod fore or aft. Or if the base must remain in place due to the mounting method, you can sweep the pod standoff fore or aft as well.
The pod pictured above weighs 2.75 oz and uses two 270 mAh NiMH batteries for power. Your choice of motor will depend on how much power is required to keep the glider aloft, and the choice of battery capacity will depend on a balance between the duration required versus the weight gain.
What follows below is a repeat of part of the May 1999 column in which this was covered, and some of the rationale behind the motor and cell count choice:
The idea came to me for a simple lightweight power pod while looking at some speed 300 motors Ive had laying around. They have a very high Kv (rpm per volt) rating, and are not good candidates for direct drive applications. If you intend to run 6-7 cells on it and had any hopes of keeping the amperage within motor-smoking limits, youd have a hard time finding a small enough prop. Conversely though, this high Kv value works in your favor when considering the usage of 2 or 3 cells. My idea for a "stand-alone" power pod would work well if it was light, was independent of the Rx battery, and was installable or removable without affecting the glider it was on. Given the number of people that own hand-launch-gliders, and couple that with the growth in "Park Flyers" lately, I figure many people would be interested in such a device! I threw an aluminum bracket, a speed 300 motor, prop, and two 300 mAh cells on my scale and it teetered between 1.75 and 2.0 ounces. Now, on with the experiment I installed a 5-2 prop on the speed 300 and hooked up two fully charged 300 mAh batteries to it. The thrust generated felt surprisingly strong for only 2 cells, but the numbers would tell for sure. Unfortunately, my Whattmeter doesnt function at that low a voltage, so I had to use the old fashioned method. Best I could measure, I was pulling 4 amps at 2.4 volts for a grand total of about 10 watts and turned the 5-2 at 8700 rpm. Theoretically, this would give me a pitch speed of about 17 mph. The pod weighed 2 ounces, which brought the flying weight of the glider up to 7 ounces, so at 10 watts this gave me about 23 watts per pound. Certainly looked like it would fly! Since my hand-launch has a ¼-20 nylon hold-down bolt directly at the CG, it worked out well to use this bolt to mount the power pod. The CG of the glider was unaffected by the mounting of the power pod, as it bolted right at the CG. I figured the high thrust line would need a little up-elevator to compensate, but with a 5" prop thats only a shade over 2.5" above the wing I didnt need to give it more than a few clicks of trim. The 2 cell 300 mAh battery was Velcroed to the bracket and the connection to the motor was made by a simple in-line connector. I went out to the field, plugged the battery in, tossed the glider and proceeded to have a very enjoyable 4-5 minutes of very relaxed, true "park flyer" type flying! The 2-cell system seemed to be about right, as the power was enough to climb slowly, but the top speed was a very comfortable "lazy" speed. I figured the pitch speed calculation was a little high, but still rather close. I called the trial a success! I also tried it with 3 cells (300 mAh) and power and speed rose accordingly, but duration dropped as one would expect. I went back to the 2 cell pack for the remainder of the test flights. It worked well, and I felt the best feature of this power pod was the ability to add or remove it without affecting the glider in any way. (End of repeat of May 1999 Column)
The modifications to the Pixie 7 were easy to make, and perhaps the power pod will fill a niche that someone out there is looking for. No, its not for everyone, but it does offer a unique solution to someone who doesnt want to cut into their glider just to make it electric! And now the capability is there to run a "controlled" setup for a 2-cell pod.
Last month I only had ONE of the Vario-Props available from Christian Ramoser, so I was delightfully surprised to have another one arrive. Now, armed with a PAIR of these beauties I mounted them on my Twinstar. Yea, I know, scale props on a very non-scale looking foamie
I ensured both props were set to their 6x2 pitch setting and checked amperage. On an 8 cell pack, the two motors drew about 11.5 amps each for a total of 23-24 amps. The Twinstar flew just fine, although at the 2" pitch setting the air speed was lacking somewhat. Flight time was longer, but not appreciably.
Setting the Varios at the 3" setting, the amperage climbed to 14 amps each and the flight speed picked up nicely. It flew as nicely or slightly better at this setting than with using the Gunther props.
Setting them at the 4" pitch, the static amperage was now a shade over 17 amps each for a total of nearly 35 amps. A little much, but I decided to try it at reduced throttle settings. Half-throttle and the Twinstar was off in a hurry! It was more than enough to fly it, and when I throttled it up full throttle for a short burst the performance was awesome! I flew most of the flight at ¼ and ½ throttle and flew for nearly 10 minutes. I could back off to a 7 cell pack at the 4" pitch setting and be okay with the amp draw, but I prefer to use the 8 cell pack and set the VarioProps back to 3" pitch settings for the long term. I like this setup, and the sound is great. Matter of fact, there isnt much sound at all as the props are extremely quiet. These props are nicely engineered, and would be a fantastic addition to your favorite scale plane.
And dont forget he has other blades and hubs other than just for 400 motors.
In case you missed last months column, Christian Ramoser can be reached through the mail at:
Ramoser Technik and Design
Fellow electric flyer Brian Steele has been keeping me updated on the progress of one of his latest projects, and I will say there is more behind the scenes in terms of the work and analysis hes done than what shows here. Brian provided an incredibly in-depth analysis of motor, prop, and battery utilization complete with screen-shots from Motocalc of various graphs, charts, predictions, etc. What he provided could well be a single column submission all by itself! Hes finding the 1100AUs a very practical battery to be using in 400 applications.
But the pictures below show the true mark of his artistic ability. Now maybe a "Positron-ish" airplane is going to appeal to me more than it would to anyone else but someone who raises the looks of a flat-plate delta to this level needs to be recognized!
His letter is attached here:
From: Brian Steele
Subject: Delta Experiments
Greetings from Canada:
I thought Id send you a final update of my Delta experiments of which there have been many. Keith Shaw and I converse on about a monthly basis where he attempts to keep me on the straight and narrow path.
I mentioned that I was pleased at how the Positron performed, but was looking for improved flight duration. I believe that after many tests, I have achieved this objective. Attached are several JPG files of my most successful attempt to this point.
The Positron was certainly the geneses of my Delta. The span was increased to 22" and the length to 17" which nets an area of 233 sq. in. I used 1/8" x 1" leading edge to resist warping. I chose pusher propulsion and shoulder wing configuration to achieve a generic military delta appearance. The motor tube is a simple tube of 17" typing paper rolled around the motor and bonded with thick Cyano. This cheap, light but sufficiently strong tube is supported at four points (wing, fin and support from bottom fuse). The nose was band sawed and sanded blue foam which was finished with lite filler, water urethane, and water based spray paint. This finished nose added 5 grams of weight. The motor is a 6v speed 400, timed 5mm advance and brushes seated properly. The best prop to date is a Master Airscrew 5.5 4. The cells are (8)Sanyo 1100 AAUs. A pack of these cells is about one ounce more than 600 AE cells. The ready to fly weight (covered with standard Monokote) is 17 0z.
The first flight was made in 25+kph winds. The launch resulted in a very rapid 70 degree climb to about 200 feet until the correct trim was achieved. The power was kept between 80 and 100 percent for most of the flight, which consisted of several rolls, a couple of loops, inverted flight and many high speed low passes. The flight was terminated before the pack BECd and lasted 7 minutes and 45 seconds. Pack run down indicated that we could have remained aloft for 30 more seconds. Subsequent flights have produced consistent 7 to 9 min sorties. Even the 9 minute flights are at sufficient throttle to achieve scale " jet-like" displays, with the exception of power hungry vertical maneuvers. The several glow pilots in attendance all came over and were very excited about the performance, I hoped they didnt notice my knees still knocking.
The initial static current is the 12.5 amps that Motocalc predicts. This is of course unkind to the speed 400 and the cells. The strategy is to achieve a solid launch and 15 second full power steep climb out with an almost vertical roll, then throttle back to about 80 percent before things start to melt down. This power setting provides 50+ mph performance. As the voltage drops and thus the power begins to fade (about 5 min) the throttle is gradually advanced to maintain high performance. These cells have a capacity of 66 amp minutes, therefore an 8 minute flight results in an average current draw of 8.25 amps. The 1100AU cells maintain voltage much better than the 600 AE cells. I charge them at 2C with no ill effects. The Delta can be landed with zero power at a reasonable airspeed with plenty of control to round out and protect the foam nose. So there you have it, I am now working on a "sorta" Avro Arrow using some very light but robust 3/16" D carbon arrow shafts (for spars) that I picked up at Toledo. Keep up the good work
Last month I mentioned picking up some spare Wattage B-2 motors and props for an experiment. I had also picked up a Pixie 7 and a Hitec Feather radio system at the Toledo R/C Exposition this April. Now, never in a million years did I figure myself getting into "micro" systems, but its a natural progression as the 280-480 sized equipment gets smaller and smaller. I already had some MPI MX-32 servos, and picking up the Feather system still fell into the "400 category". Actually, given that the Pixie 7 handles 7 amps continuous and 10 amps for short bursts, you could run a 400 with it so long as you watched your current levels. So, all of this equipment still fits my 400 sized focus.
So, with an idea in mind and an evening with a hot-wire bow, heres what I came up with for a vehicle to carry all this micro stuff:
I already had the white foam wing cores, and the fuselage is just 1" thick white foam profile cut on my jig saw. A few strategically placed cutouts for equipment, and equipment installation was a breeze. Light weight pushrods (1/8" sq. balsa stick) actuated thin-foam tail surfaces. The B-2 motors were sticky-taped to the underside of the wing. A Pixie 7 was also taped into place, and a 6-cell 170 mAh NiMH battery pack was stuck in the hole at front. Two holes were made in the fuselage for batteries as I wasnt sure of the final balance point.
Final specs are:
- Wing span 32 in.
- Wing area 208 sq.in
- Fuselage length 25"
- Flying weight 4 oz.
And it flies well. It wont handle much wind outside, but its spent most of its airborne life in my back yard rather than a gymnasium. Actually, the only indoor flying its seen so far is the cafeteria where we hold our monthly R/C Club meetings. You could have considered it indoor pylon racing, although the term racing is used loosely here. Top speed isnt much, which made it a lot easier to negotiate the "pylons" in the cafeteria, which were actually hanging planters. Its a fun little plane, and at that weight it rarely suffers damage even in a collision with something.
Its amazing what electronics are coming out these days in support of electric flight in general, but especially so for the real lightweights. And the advantage of it for me is it still fits my mainstay usage in the 280-480 realm for equipment.
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|Article Forum 400 - February 2000||Pat Mattes||Electric Plane Talk||0||Feb 07, 2000 01:00 AM|
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|Article Forum 400 - May 1999||Pat Mattes||Electric Plane Talk||0||May 01, 1999 01:00 AM|