RC Jets
Jim Hiller
Balsa sport-jet models are gaining popularity
A TREND that is picking up steam in jet modeling is the balsa sport jets intended for grass-field flying. You don't see them at the big jet meets, or if you do, you walk past and look at the beautiful scale models and high-speed, composite sport jets.
These balsa jets aren't curvaceous, fast, or as sophisticated as the mainstream jet models, but they are showing up everywhere and may soon be typical of jet modeling.
What makes these balsa jets different? They are designed around what it takes to fly at local model fields. Their weight is lighter, landing speeds are lower, and finishes tend to be simple iron-on covering. The top speeds of these models are slower—roughly 90-140 mph. This makes them much less intimidating to fly.
The Bruce Tharpe Reaction 54 is designed to accommodate 12- to 14-pound-thrust turbines. The wingspan is an ample 78 inches.
It is a traditional pod-and-boom design with an underslung turbine located immediately behind the wing under the tailboom—a popular setup. The airplane has a one-piece balsa wing for strength, and its thick airfoil leaves plenty of room for retracts.
This kit model is available directly from Bruce Tharpe Engineering (BTE) at www.btemodels.com. The basic kit is $469.95 plus $15 shipping and handling. Available options through BTE include the retract landing-gear system, tires, wheels, and brake. Bruce Tharpe also offers a single nose-gear brake setup utilizing the Kavan electromagnetic brake.
I have some experience with this model since one of our new jet fliers, Dave Rigotti, contracted me to test-fly his Reaction 54. Dave built a solid, straight model, and it was a pleasure to fly. This is the aircraft he used to obtain his turbine waiver.
Dave powered his Reaction 54 with a JetCat P-60: a nice little engine with good thrust and power response. The aircraft is equipped with retractable landing gear and a nose-gear brake from BTE. This brake setup is okay on grass but is not effective for stopping after landing on pavement. It is satisfactory for taxi operations on pavement.
The empty weight of the Reaction 54 covered in MonoKote is 18 pounds—a substantial mass. This weight is offset by the 10-square-foot wing, making for a fairly light wing loading of less than 32 ounces per square foot.
The model is equipped with 3-inch-diameter main gear tires, so grass-field operations are natural for it. The JetCat P-60 proved adequate to get off the ground in approximately 300 feet at my home field that has thick but well-trimmed grass. The model flies so sweetly it will spoil you.
Is flying these balsa models different from flying our high-speed composite sport jets? Yeah, it is—with toned-down stress levels and speeds. The stall speed is so much lower that takeoff and landing speeds are comparable to those of a 1/4-scale aerobatic model.
At speeds of 80-100 mph, the balsa jets fly much like those 1/4-scale models, but you have to throttle back to stay that slow. At full throttle, these balsa jets generally get into the 120-150 mph range, and with their light wing loadings, they get sprightly on the elevator. It's easy to overcontrol and tear them apart. Fortunately, most of the balsa jets are high-drag models, and with the 12- to 14-pound-thrust turbines, speed control isn't too bad.
What about the new breed of small turbines cranking out 17-18 pounds of thrust? They're the same size and weigh the same, but they pump up the power. These new, powerful, small turbines combined with balsa jets can get you awesome power-to-weight ratios exceeding those of many of our composite jets.
Be careful, though. Put a powerhouse into a balsa jet and enjoy the vertical performance, but don't leave the power on in level flight. The higher speeds will make them difficult to fly smoothly; there's too much lift from those big wings at higher airspeeds. Light wing loadings are great to reduce landing speeds. They tighten the loop diameters and make an airplane fly in a livelier manner.
Don't let the slower speeds of this new class of balsa sport models fool you. Throttle management for speed control is required just as much for them as it is for high-speed composite sport jets—it's just that balsa jets are limited to a slower airspeed.
One characteristic I had to learn with my balsa jet is that although they have aerobatic capabilities that are similar to propeller-driven aerobatic models because of their similar wing loadings, be careful. The turbines push them faster and it's easy to overstress these models at the faster airspeeds.
My major change was snap rolls. The pod-and-boom designs—even with twin tailbooms—are not as strong as a conventional fuselage model, and they can be damaged from the twisting forces of snap rolls.
How do I know this? I failed the tailboom on my balsa sport jet on its 42nd flight when I executed a down-line snap roll at a higher airspeed than I had previously been doing. When I pulled out at the bottom, the tail surfaces, elevator, and rudder immediately started to flutter.
I pulled up, throttled back, and gingerly nursed the aircraft around the pattern and landed. The tailboom was badly damaged. No snap rolls for me on these balsa jets. Spins entered from a stall are all right, but no snap rolls.
Another thing to address when flying these models is grass-field operations. For years we have been flying our jets off of paved runways. Hard-surface runways tend to have limits in length and width, so control has been critical. We've learned to use brakes as an effective method to control ground speed and to stop our models after landing.
Now on grass—what a change! The fields are wide open with much space. Crosswind takeoffs and landings are a breeze. The grass is effective at slowing the model, so we really don't need to use our brakes on landing rollout.
It takes a great deal of power to get a turbine-powered model moving on grass, and then you must immediately throttle back to keep from taxiing too fast. Speed control requires finesse.
Those brakes don't cut it on wet or even dry grass. The problem with our brakes on grass is really our tire selection. We have developed some great tires for paved use. They use hard rubber compounds for wear, and the pavement generally wears off the treads almost immediately. These slicks don't work well in grass.
My first experience on grass was at Top Gun on the polo field's tightly mowed, sparse grass with a sandy base. The brakes—using new tires with treads—worked well on that surface.
Years later, when I took my model out to our local field, it was a different story. Our field has a thick coverage of well-mowed grass, and my model had well-worn tires from lots of pavement flying.
I lined up for takeoff, set the brakes, and throttled up the turbine. The brakes didn't hold; I thought they had lost air pressure and failed. When I landed, I tried the brakes again and no luck; they didn't appear to be working.
When I checked the brakes after the flight, they had plenty of air pressure and worked fine on the bench, so what happened? On the next flight I figured it out. The brakes stopped the tires from rotating, but the airplane just skidded along with little effective braking. My well-worn tires—now slicks—just couldn't brake effectively.
If you fly off of grass, use soft tires with treads. They will provide some braking action. Grass will slow a turbine-powered model just as effectively as it does a propeller-driven model. I still like the brakes for taxiing, particularly when parking back near the pits while on grass. Don't kid yourself, though; brakes are not as effective in grass as they are on pavement.
I hope this gives you some insight about the next direction turbine modeling appears to be heading. The modelers aren't as pretty and sophisticated, but they have already introduced a group of new modelers to turbine flight.
Our group of jet modelers has grown quickly since the introduction of these simple balsa jets. MA
Transcribed from original scans by AI. Minor OCR errors may remain.
