Author: Red Scholefield
Edition: Model Aviation - 2012/07
Page Numbers: 89,90,91
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THE BATTERY CLINIC - 2012/07

There is a wealth of knowledge on the Internet

Many have chosen Eneloop cells for transmitter packs, primarily because of their improved charge retention compared to standard NiMH cells. I decided it was time to find out how much charge would be left after a month on a transmitter pack that had been used for more than two years.

First, I charged the pack and then discharged it. I recharged it and let it sit for 30 days, then discharged it again.

From the included graph you can see it lost roughly 1% of charge per day. If you don't charge your pack for a month, you will still have approximately five hours of flight time left based on the 250 mA load the transmitter puts on the battery. This is a significant improvement compared to the 700 mAh packs we have been using.

Free RCadvisor calculator

Carlos Reyes has launched a new version of his website, RCadvisor.com, with two major changes. His model airplane calculator is now free, and he is posting approximately one high-quality article per day. With more than 10,000 registered calculator users already, he expects the calculator to be more popular than ever. Feedback on the articles has been positive, and he hopes you like the changes.

If you have not visited his website, you are missing out on some excellent modeling material. Here is a portion of his article series, Quick and Easy Model Airplane Sizing, which can be found on his website:

How big of an electric motor do you need to power a 6-foot (1.8 m) Piper Cub model? Finding the answer is easy!

Six-Foot Cub

Let's say we decide to design and build a Piper Cub because the world cannot have enough Cubs. We want to make the wingspan exactly 6 feet (1.8 m) because that is the size of our building table. How big of an electric motor do we need to power it? If that sounds hard, don't worry — let's take it one simple step at a time until we get the answer.

Full-Size Cub

Look up the specifications for the Piper Cub. For the sake of argument, let's say that it weighs 1,000 pounds (454 kg). The wingspan is 35.25 feet (10.75 m) and the wing area is 178.5 square feet (16.6 m²).

The formula for the aspect ratio of a wing is (wingspan²) / (wing area). The units are not important as long as they are consistent.

Plug in the numbers from the specifications and you should come up with the value 7. I know that it's really a little bit less than that. Let's just round it off. For the sake of this discussion, the aspect ratio of a Piper Cub is 7.

Given the wingspan and aspect ratio, let's compute the wing area of a Piper Cub as a check on the numbers. The wing area is (wingspan²) / (aspect ratio). Plugging in the numbers, you should get a value close to 178 square feet (16.5 m²). Again, this refers to the full-size Piper Cub.

Similarly, we can plug in the numbers we have to compute the wing area of our 6-foot Cub. I get 5.1 square feet (0.47 m²).

Airplane Type Constant

There's a very simple formula that relates the weight, wingspan, and wing area of an airplane. The value that you get is an indicator of how that airplane will handle in the air. The formula is:

(weight) / (wingspan × wing area)

The units are not important as long as they are consistent. In this example, we will use ounces as the unit of weight and feet (or square feet) as the units of length and area.

Let us plug in the numbers for a full-size Piper Cub to find its airplane type constant. I get a value of about 2.5. So if we come up with a 6-foot model of the Piper Cub that has an airplane type constant of 2.5, it will behave in the air similarly to the full-size Cub.

Note that I said similarly, not exactly. The point of this article is to quickly come up with some numbers that can be used as the starting point of a detailed airplane design.

The airplane type constant formula may look very simple, but it is extremely handy. In fact, this is the formula from my first book that I use the most.

To read more, visit the RCadvisor.com website.

Ed King

Ed has provided the specifics on his beautiful Constellation, which was pictured in this column in the March 2012 issue. The model was built from Don Smith plans, with many construction changes. The full-scale Connie he modeled is located at a museum in Kansas City, Missouri.

  • Wingspan: 11 ft 8 in
  • Fuselage length: 10 ft 6 in
  • Weight: 55 lb
  • Finish: Fiberglass/epoxy, automotive acrylic enamel, automotive clear urethane
  • Radio system: JR 12X
  • Total servos: 20
  • Battery: Dual 6-volt airborne battery capacity, 5,400 mAh total
  • Motors: AXI 4130-20
  • ESCs: JETI 77 Opto
  • Total motor power: 6,000 W (1,500 W × 4; 30 V × 50 A per motor)
  • Total motor battery (LiPo) capacity: 22,000 mAh
  • Total battery packs: 10
  • Includes: Fowler flaps, operating beacons, navigation lights, landing lights, fuselage lights for checking de-icing boots, retractable landing gear, doors opening and closing
  • Construction time: 2½ years

RC Hall of Fame

The Radio Control Hall of Fame website is worth visiting if you are interested in RC modeling history. This site covers nearly all manufacturers of RC equipment. It is a small organization attempting to do a large job. Anyone who can do it better is welcome to do so, or to join in the cause.

Audible Alarm Module

Gregg Stockdale sent this email:

"Red, I've stopped using my timer on my transmitter in exchange for using a 2-in-1 LiPo Battery LED Low Voltage Meter Tester 1S–8S with Buzzer Alarm.

"This little beauty connects to the balancing plug on your battery pack. It measures the charge in each cell, and when any one cell hits the low (you can set the limit) voltage, it buzzes quite loudly. It's easy to know when you should think about landing rather than just running out of juice.

"I set mine for 3.5 volts. That way, when the buzzer goes off, I have plenty of time left to reduce throttle and wait my turn at landing (we have a very busy field and there are almost always five planes up at the same time). This also solves the issue of consumption not matching the timer. If you fly hard, you'll run out of juice prior to the timer going off. If you are not pushing it, you'll land with too much juice left unused. Also, if you have a cell that's not up to par, you'll be warned about that as well. Good stuff to know!

"I've had the device embedded deep inside a foam plane and I'm still able to hear the buzz. At around $10 (eBay), it's a lot cheaper than a telemetry system and very light. I can't keep mine in stock because I keep giving them to new fliers ... it's a real 'plane-saver.'"

Don't Forget the Females

I messed up. I thought it would be cool to get my great-grandson an AMA Youth Membership. Then it hit me: I forgot someone.

I exited the doghouse by adding my great-granddaughter, Jenna. If you are taking advantage of the free AMA Youth membership program, don't forget the females.

That's all for this month. I enjoy your communications. No email connection? Then drop me a note at:

The Battery Clinic 12219 NW 9th Ln. Newberry, FL 32669

SOURCES

Transcribed from original scans by AI. Minor OCR errors may remain.