Scratch-One Technical Update

January 2010

Background:

My Scratch-One design appeared as two parts of my “From the Ground Up” series (actually parts 9a and 9b) in 2004. The index for the entire series can be found at this AMA Web site: www.modelaircraft.org/mag/FTGU/titlespageftgu.htm.

I hadn’t given much thought about a Scratch-One update, but the plane always flew so well, it could certainly benefit from all the new technology. I checked back to my original articles. The plane was originally powered by a Speed-400 brushed motor running direct drive with a Gunther 5 x 4 prop on 8 X1100 mAh NiMH cells.

That battery pack was heavy, as you stated, nearly 6 ounces. The original plane weight with that battery and motor was close to 17 ounces. I actually had much more power than really necessary. Something like 10 amps at 8 cell (9.6 volts). That probably worked out to about 90 watts. Way too much!

In all honestly, I feel today you could easily get away with about 50 watts input power which would yield something like 50 watts/pound power loading. You would want to figure on a brushless outrunner motor, brushless ESC, and a two-cell 1500 mAh Li-Poly battery pack.

Although there are many sources for supplies, I personally like dealing with BP Hobbies in New Jersey. The person you might want to talk to at BP is Jim Minadeo. Their company’s Web site is: www.bphobbies.com. I was able to select three relatively inexpensive components for my needs:

(1) Motor: Brushless outrunner catalog No. BP A2208-14 at only $15.95. Plan to use an APC Slo-Fly prop, either an 8 x 4 or a 9 x 5.

(2) ESC: A brushless catalog No. BP 10A MKII at $16.95

(3) For the battery, a Poly Quest XP two-cell 1500 Li-poly pack ($24.90) that weighs 3.1 ounces or half of what the original eight-cell NiMH pack weighed. Since the motors will be a wash (in weight), you could end up with a Scratch-One at 3 ounces lighter or 14-ounce total.

You could only obtain roughly a six-minute motor run with the NiMH pack, but with the new Li-Poly pack, I calculate close to a 13-minute motor run time at full throttle. Throttling back you might expect 20 minutes or more.

Although the Scratch-One has a long nose, the lighter battery may make the model slightly tail heavy, so make sure you get that battery as far forward as possible, even if you Velcro it to the top of the fuselage just behind the motor.

This new power/battery arrangement should have a motor current now of roughly 7 amps and on two cells (7.4 volts) that’s very close to the desired 50 watts input power.

Before closing there are two other technology improvements that you should consider at this same time. In the last four years our hobby has gone almost 100% toward spread spectrum RC system technology that operates on a band in the 2.4 GHz region.

All the popular radio system manufacturers have gone this route. The primary advantage is that each and every system sold (regardless of the manufacturer) employs a digital code.

Since one modeler can not interfere with another modeler, frequency control at the local flying field is no longer a concern. You can even have several flying fields in adjacent sites and not suffer any interference.

These new radios can be purchased as complete new system or in many cases you can buy just a module for your existing transmitter along with a new receiver. The same servos work with these new systems. I personally recommend the Horizon Spektrum brand system and in particular the company’s DSX-6I and DX-7 transmitters. Horizon also has receivers that range from only 2 grams on up.

This is a typical Horizon Web site reference: www.horizonhobby.com/Products/Default.aspx?ProdId=SPM2710

A second item you must consider is a safe and easy-to-use battery charger. Toward that end the FMA Direct Multi4 for less than $100 can safely charge nearly any battery chemistry presently on our hobby market. For details, visit www.fmadirect.com/Detail.htm?item=2306&section=45.

I hope this will help!

Bob Aberle
AMA 215
Technical Editor
Model Aviation

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