An Aerobatic Design for Indoor Flying

Introduction:

One of the few kits that I have ever built is the Tiny from Todd's Models. I am mainly an ARF flyer but in my search for indoor aerobatic models, the results were slim. The Tiny came up as a choice for a proven outdoor performer in light winds. By building a kit, I knew that I could make it light and incorporate my own design changes.

 After building my kit, I discovered an almost identical ARF version of the Tiny called the Triangle RC Street flyer. This is manufactured by TelTech in the Czech Republic and distributed in the USA by Hobby Lobby Int.

 I wanted to use my spare Astro Flight "Firefly" motor to power the plane and I felt that if I kept the design very light that it should work just fine. The new metal gearbox on the 4:1 Firefly has doubled the power output of this tiny coreless motor. By choosing to use  only 2 servos, one for both ailerons and one for elevator control, a light Firefly coreless motor, and 2 of the new 3.6 gram, 140mAh LiPoly cells, my Tiny was RTF (Ready-To-Fly) at only 3.6 oz!

Specifications:

Framing the Wing:

The instructions that came with the Tiny contained details for multiple variations to be built. The design itself allowed plenty of freedom for the builder to do their own customizing. In my design efforts to keep the plane light, I choose to use only a single servo for the ailerons and another for the elevator.

The wing is built up from CNC cut parts and checked for alignment

Building the Fuselage:

The carbon rod fuselage was very light yet strong. Initially, I mounted my Astro Flight  Firefly motor using a rubber band, but later, after I discovered that I had a problem balancing the plane, I replaced the rubber band with some black trim tape and a little epoxy. This gave my model a better look and the motor a better mount. To combat my heavy tail balance problem, I extended the motor forward by inserting a second piece of carbon fiber tubing inside the main tube. This balanced the plane by extending the weight of the motor and 2-cell Lithium pack.

The carbon fuselage runs through the center of the wing

Using Lithium Polymer Cells:

I recently received a pair of the new Lithium Polymer (LiPoly) cells from Bob Selman Designs. One of these cells is almost equivalent to 3 of the 50mAh NiCd cells but only weighs about the same as a single cell. Put 2 of these LiPoly cells together in series and it can suddenly power my Firefly motor with only a 0.3oz increase in weight. Very light!

(Photo compliments of Graham Stabler)
The new Lithium Polymer cell has the voltage of 3 NiCd cells but only 1/3 the weight!

When you compare this LiPoly pack to a 6-cell 50mAh NiCd pack that weighs 0.8oz, you can see why this new technology is exciting the world of micro-flight. My Tiny Ready-To-Fly at only 3.6oz including the LiPoly pack I was happy with this good start for my little indoor aerobatic flyer project.

My 2-cell 140mAh "LiPoly" pack weighs 0.3oz

For more information on the LiPoly cell and some other international charger vendors, please visit the Web site of  Graham Stabler. LiPoly cells can also be purchased from Graham Stabler, Dynamics Unlimited, and Dynamic Web Enterprises.

Potenski Lithium Ion Charger for 1 or 2 cells

To charge my 2-cell LiPoly pack, I used the Potenski Lithium Charger sold at WildRC.com. This is Dan Kreigh's newly named company at the well known www.flyifo.com site. Same great service and management!

The Potenski 2-cell charger is also available at Hobby Lobby Int. (POT010)

Only two servos are used; one for the elevator and one for both ailerons

Ready-To-Fly:

My finished Tiny from Todd's Models weighed only 3.6oz Ready-To-Fly (RTF)! The plane itself weighs 3.3oz and my new 2-cell 140mAh Lithium Polymer (LiPoly) pack a mere 0.3oz.

A key component to my very light Tiny is the 4:1 Astro Flight Firefly motor. It draws about 700-800mA with a Gunther (5x4) "spoon" prop. It made for a perfect match with the new 140mAh LiPoly cells since they can deliver up to 1000mA.

I recently (after the maiden voyage) replaced the Gunther prop with a GWS 6x5 prop and a 2mm adapter from BMC Model Products. I have had trouble with a good fit and balance using the Gunther prop on the Firefly due to the small 2mm shaft. The Gunther prop works better on the Speed 400's 2.3mm shaft, which is also much longer. BMC Model Products also makes a 2mm adapter that perfectly fits the Gunther prop to the Firefly without using the plastic spinner. Hobby Lobby also sells a Graupner prop shaft adapter for 2mm motor shafts (GR605320).

The Firefly-powered Tiny uses a GWS 6x3 prop

My new GWS 6x5 prop has perfect balance and feels like good power when throttled up. I measured the current draw at 850mA on a DC power supply with full power on the GWS 6x5 prop. This also includes the R4-P receiver and AF ESC since the measurement was done at the battery connection. The current rose to about 950mA when I moved both servos together at full power. This looks like a good design for both the Firefly and the LiPoly cells.

Flight Testing:

Another plane that I often fly indoors is my 5.8oz Ultra-Lite Stick (ULS). The GWS S1 powered ULS has more power than my AF Firefly-powered Tiny but my goal here is to create an indoor aerobatic flyer than can be flown very slow yet still perform some simple aerobatic rolls and loops. The ULS has rudder/elevator controls.

The 3.8oz Tiny sits next to my 5.8oz Ultra-Lite Stik and 5.9oz Tilde wing

The back parking lot where I work is about twice the size of a full court basketball gym. I can fly my ULS within the paved area very comfortably and within 1/2 the lot with concentration. I had flown this model indoors last winter very successfully. My Tiny can comfortably fly in 1/4 of the lot! It can fly very slowly due to the light wing loading. It is a similar size to my ULS but has slightly more wing area, more lift, and weighs 2 ounces less.

Note that my ULS is now powered with the 2-cell 950mAh LiPoly pack from Dan Kreigh's WildRC site. I also use it to power my GWS direct drive Tilde wing from Birdworks. These new Lithium Polymer packs will fly the models for 30 minutes or more!

I flew my Tiny in the back lot at work on a calm evening and my 1st and 2nd test flights of the Tiny were very successful. It loops and it rolls! Even at the slower speeds, I could roll the ship without losing altitude. The Tiny flew for about 10 minutes. The duration surprised me!

I could loop in a 3' diameter and easily circle in a 30' diameter. By using the dual rate settings, I could perform some rapid rolls and loops without losing much height. For normal flight, I would switch back to the reduced rates for more relaxed control.

I measured the LiPoly pack when I got home and it was still at 7.5v. Nowhere near the 3v cutoff mark (7.5/2cells=3.75v). This was the same experience that I saw with the Ultra-Lite Stik the day before using my 950mAh LiPoly pack. It stopped flying the plane well at around 7.5v. This data makes the LiPoly cells look very promising.

Verification with the Astro Flight Micro-Wattmeter:

I had ordered a new Astro Flight Micro-Wattmeter, and when it arrived, I re-measured the current draw using several props and the 2-cell 140mAh LiPoly pack. I also measured the current using a 6-cell, 50mAh NiCd pack.

           PROP                 PACK           CURRENT

I decided to use the GWS 6x5 prop. If it proved to be to brittle on those unexpected nose-ins, I would switch to the more durable Gunther 5x4 "spoon" prop.

Conclusion:

I felt that I could actually fly my Tiny in a half-court size gym using the Astro Flight Firefly coreless motor. I was very happy with the duration and it ability to slow fly in a very small area. The light ship was not very powerful but had enough to perform simple aerobatics for indoor flying.

At quick check of the voltage on the 2-cell LiPoly pack after a 10 minute flight showed that it had a great safety margin from the cell bottom threshold of 3.0v. Using the 2-cell, 140mAh LiPoly pack would not be a problem for either current delivery at 1amp or for a minimum discharge limit of 3v.

I had a great deal of fun with this project! My rare opportunity to build a kit from balsa parts and then cover it felt very rewarding. Using a new combination of power components proved to work very well on the model and transform it into the indoor aerobatic flyer that I had been searching for.

Article References:

Todd’s Models at: www.toddsmodels.com/index.html

Astro Flight Inc. at: www.astroflight.com/

Hobby Lobby International at: www.hobby-lobby.com/

Gordon Stabler at: www.indoor.flyer.co.uk/

Dynamic Web Enterprises at: www.smallrc.com/

Bob Selman Designs at: http://users.joplin.com/~bselman/

WildRC.com at: www.flyifo.com/

Birdworks at: www.harborside.com/~birdworks/bwtilde.html