Most radio control R/Gs are designed to have some pilot control on boost. It's the most efficient design, since RC affords you the option of control and you don't have the added mass and/or drag penalty of a trim-change device (slide-wing, slide-pod, etc.) I opted for a trim-change device for several reasons. First, I'm rusty as a pilot and didn't want to have to control the boost of a fast-moving model like a B R/G. Also, if things work out, my teammate (Chuck Weiss) might be flying a similar model at NARAM. He has never flown RC before, so a simple model with the RC benefits of easier return and thermal-seeking without the skill requirement of control on boost was desired. Second, the RC gear I selected to use (Parkzone DSM2 brick) is a rather weak unit, intended for low-speed aircraft, and there is some doubt in RC R/G pilot circles whether it would have sufficient strength to fully control the flying surfaces at boost speeds. Finally, I wanted something that could be "grafted onto" a freeflight-capable model, one that could be reverted to freeflight with minimal alteration if desired. I choose a slide-pod design as it seemed it would have the relatively low drag and mass penalties and its implementation would not interfere with the RC implementation.
There have been several previous adaptions of the Parkzone gear to boost/ and rocket/gliders. Several people used it at NARAM-51 for the B R/G event. Keith Vinyard won C Div. with a ~70-80 sq. in., 53 gram model. Scott Branche of Meatball Rocketry flew a 48 sq. in., 37 gram (empty mass) model to second place in Team Div. Bob Ferrante of the Pod Bay Doors Team used a foam flop-wing to place 4th in Team Div. The Flying I-Beam Kids placed 6th in Team Div. with a ~60 sq. in. model. Most of these R/Gs have relied on pilot control on boost. On the B/G side, Chris Flanigan has flown a 75 sq. in., 43 gram (glide mass) model. Alyssa Stenberg has also set records with a more-or-less stock Ember with micro or mini motors strapped on to the glider.
As mentioned, I selected the Parkzone DSM2 brick for the project. It is a small circuit board (approx. 1 inch square, 1/4" thick) that contains a receiver, electronic speed control, and two linear servos. (This is similar to the Spektrum 6400 unit.) This unit comes in at 3.7 grams. The Parkzone battery is a single-cell 70mAh lithium polymer battery that weighs 2.7 grams. As suggested by Chris Flanigan, I opted for a BNF Ember2 ($90) to get a pre-built micro indoor/park-flyer aircraft for the same price as the airborne gear, battery, and battery charger purchased separately.
The BNF (bind'n'fly) model requires the user to provide his own DSM2 transmitter. I choose this option to use a full-range Spektrum DSM2 transmitter. (My understanding of the range of this receiver is that it has a fairly long range. The primary limiting factors to its range [and other Spektrum DSM Rx] is: 1) having a full-power, full range transmitter [versus the limited unit included with the RTF version of the Ember2] and 2) limiting the amount of conducting material [metal, carbon] in the aircraft that would attenuate the radio signal.) I selected a Spektrum Dx5e transmitter ($60). It provides the full range desired and basic functionality at a minimal cost. (Those interested in serious pursuit of RC would do well to consider a Dx6i or higher Tx, which include more computer-programmable functions.)
Other materials used are those common to model rocket gliders.
The glider is based on a trihedral wing of 60 sq. in. (19" span) made from 5.7 lb/ft^3 density 3/16" balsa. The airfoil is a typical model rocket-style flat-bottom airfoil. (Dimensions can be found in the accompanying neutral point spreadsheet created by Blake Goddard.) The stab is 10 sq. in. made from 8.5 lb/ft^3 density 3/32" balsa. The rudder is 2.5 sq. in. made from 7 lb/ft^3 density 3/32" balsa. Hinging is a simple mylar-like tape on one side, with the surface beveled to pivot in the opposite direction. Control horns are 1/32" plywood. Finishing of the flying surfaces was several coats of Design Master ColorTools orange and flat black lacquer (similar to spray dope.)
The fuselage is 1/8" x 3/8" x 19.38" spruce, tapered to 1/4" from the wing to the tail. The 18mm pod rides on a 1/2" high 1/8" balsa pylon with a basswood/spruce box around the fuselage. The pod has about 4.75" of travel from boost to glide, accomplished via a burn string and rubber band. The RX/servo brick is mounted on the underside of the wing, in a cut-out in the fuselage (but still open to the breeze -- crude but effective.) The battery is taped to the side of the fuselage, under the wing and ahead of the Rx. Pushrods are 0.025" music wire, riding inside Sullivan 507 Gold'n'Rod pushrod sleeves. Launch guidance is a 3/16" launch lug mounted on the pod. In glide configuration (with a used motor casing), the model weighs 54 grams.
The models maiden (and only, to date) flights were made at the ASTRE Final Contact Open Meet, September 11, 2010. It was a beautiful (that is to say, atypical) day at our field, with sunny skies, the temperature in the 60s and low 70s, winds at 5mph or less, and frequent, gentle thermal activity. On the first flight I attempted to apply down-elevator to minimize any tail camber, but this proved the wisdom of a hands-off boost (it was safe, but a bit erratic.) Further flights were down with the full elevator down trim allowed by the transmitter, then neutralized for glide. The models were launched almost vertically, with a slight pitch down angle. Two A8-3 flights were made. There was a slight roll that I tried to dial out with trim, but the pitch trajectory was good. Glide control and performance was good (about what you'd expect from a slightly heavier and draggier R/G. No times were recorded for the test flights. One more test flight was done with a B4-2. With the faster boost, the roll was more pronounced, but the boost was still vertical in pitch mode. One interesting thing was that the fast roll degenerated into a 3D tumble near apogee. I believe it was the rearward movement of the CG when the pod slid back that changed the ballistic roll into a full-fledged kerfuffle (technical term. :) The model recovered quickly and glided well. Next, two contest flights with B4-2 motors were made. The first flight was nominal and was made with significant lift in the area. I did a number of figure 8's over the range and scored a time of 2:04 (setting a new U.S. record.) The second flight did not have the benefit of the lift and all I could manage was a 42 second flight.
This implementation of a slide-pod R/G with the Parkzone gear was successful. I need to fly it some more to fully dial out the boost roll. Once becoming more familiar with the glider, I would like to try to work more on piloting skills in the glide phase. I would also like to test the model with a B4-4 to see if that delays the onset of the 3D tumble near apogee.
The success of this model and the lack of any apparent boost flutter or control problems make me wonder if it is feasible to use the same gear for a C R/G model (this year's NARAM event.) I am considering a different model design that might be both less draggy on boost and better accomplish the trim change, while still allowing a hands-off boost.
|Overall view of the slide-pod B RC R/G with transmitter and LiPo charger|
|Overhead view of the RC R/G with the pod in boost (forward) position.|
Under-wing view from right wingtip. Shows the the RC brick mounted
in a fuselage cut-out (with a piece of spruce added to reinforce the
fuselage) and the LiPo battery taped to the side of the fuselage.
Under-wing view from left wingtip, again showing the the
RC gear installation.
Closeup of the RC gear and the pushrod attachments.
The Rx antenna is the small wire under the scotch tape at upper left.
There was also a companion free-flight B R/G model built at the same time.
The FF model (left) is similar to the RC model (right) with the following differences:
The business end of the two models, showing the RC tail surfaces
(left) and pop-up stab DT (right.)