RADIO CONTROL JETS
Jim Hiller, 6090 Downs Rd., Champion OH 44481
We are deep into our building season here in the Midwest. It's cold, windy, and snow-covered outside, but the shop is warm and projects are nearing the painting stage.
Crow Aviation is now offering Rhom Air retracts on its sport jets. I’ve just received a set for my Razor. These Rhom Air retracts appear to be a solid gear. All surfaces are machined, with rugged aluminum and steel retract links, and bushed pivot pin holes.
The design includes positive up and down locks — a must for today’s high-performance models. The air cylinders are sized to pull up our heavier gear struts, brake systems, and heavy tires that many of us are now using.
The nose gear retract unit is particularly impressive. It is a fully machined piece with extremely tight clearances — more like a jewel than a rugged unit to be abused by hard landings. Likewise, the main gears are machined to tight tolerances, virtually eliminating side play in the up or down position. They should provide long life and durability from those hard landings we never admit to.
Rhom’s heavy duty struts were included with the Rhom Air package, and they too are of high quality. The loose fits of the old, lighter struts are gone. This set is definitely built to tight tolerances, with almost no side play in the knuckles. Tire alignment is tight, and should result in improved ground handling.
Last week I was talking to Bob Wilcox and Tad Krzanowski of Golden West Models, and they updated me on their next turbine engine project: the JetCat P-120. Final testing of the production units will have occurred by the time you read this, followed by submission to the AMA for approval by spring.
Like the Hammer, this turbine will have onboard starting systems. The P-120 is expected to be certified in the 26-pound thrust range, but at an increase in weight of only two ounces from the Hammer.
Jet modeling and the Internet just seem to go together, with most of the manufacturers having some real quality Web sites. Check them out if you have the chance. One good place to start is at the Jet Pilots Organization Web site, with links to many of the manufacturers' sites. JPO’s Web site is www.jetpilots.org.
Turbine installation types
The installation of turbines in model airplanes is new to most modelers, and appears daunting. There really are only three basic types of installations. All work well, with advantages in performance as the complexity increases.
1. External mount
- The simplest installation, as on the Kangaroo and Hot Shot.
- The turbine engine itself is mounted external to the aircraft, with only the fuel system and ECU mounted inside the aircraft.
- No heat shields are normally required, and no engine cowlings are used.
- You can watch the condition of the turbine during startup, observing any problems that may occur (such as a hot start, leaking fuel or oil line connections).
- The glow plug or spark plug is easily accessible for maintenance.
A hot start occurs when unburnt fuel or oil is expelled out the engine during the starting process, and then burns behind the engine, usually in the tailpipe.
2. Internally mounted turbine without full ducting
- The turbine is mounted inside the fuselage, without a full inlet ducting system leading to the turbine (similar to Byron ducted-fan setups).
- Air is pulled into the compressor from inside the fuselage, after entering through simple inlets.
- The hot exhaust from the turbine is routed to the back of the model through a tailpipe.
- A hatch is normally provided to fully expose the turbine and support systems with easy access for maintenance and starting.
- This makes a great installation, provided one does not mind the resulting drag limiting a model’s cruise speed to about 150 mph — many consider this natural speed limit an advantage.
3. Fully ducted turbine installation
- The highest level of complexity, offering the most aerodynamic, clean installation.
- The air comes in through the inlets, is ducted to and around the turbine, and the hot turbine exhaust is carried away by the tailpipe.
- Inspection of the turbine during starting and running operation is not practical, so the installation must be done right: no fluid leaks, and use caution to avoid hot starts.
- Inspection will require removal of not just the external hatch but also the duct hatch to get to the turbine engine itself.
- Some manufacturers use clear duct hatches over the turbine to aid inspection, but actual access requires removal of the duct hatch.
Tailpipes and cooling
A tailpipe is required in both internally mounted turbine installations. This routes the hot exhaust from the turbine to the back of the airplane. You may notice how hot the jet exhaust is — but not hot enough for the turbine blades to glow cherry red. So why doesn't the tailpipe also glow cherry red?
All tailpipes, including full-scale aircraft, introduce cool bypass air along with the hot jet exhaust into the tailpipe. This cool bypass air:
- Serves to cool the jet exhaust.
- Increases the mass of air traveling out the tailpipe, resulting in little or no loss of thrust.
This is why tailpipes do not attach directly to the turbine engine; instead, the turbine exhaust is directed toward the center of the tailpipe opening. Cool air is pulled around the engine and into the tailpipe by the low-pressure area set up by the high velocity of the jet exhaust — similar to the old physics demonstration of sucking liquid up a straw by blowing over the top of the straw. This bypass air is the secret to a cool tailpipe.
The tailpipe can be tuned to improve performance and temperature by adjusting:
- The shape of the curve leading to the tailpipe.
- The diameter of the tailpipe entrance.
- The fore-and-aft position of the tailpipe.
Early tailpipes for the eight- to 11-pound-thrust turbines were made from .005 stainless steel and performed well. As turbines developed into the 15- to 17-pound-thrust range, problems with tailpipes arose, particularly when a hot start was encountered. Hot starts cause excessive heat in the tailpipe, with very low tip-to-tip velocities.
The solution was to increase tailpipe thickness. Most of today's tailpipes are made from .007 to .010 stainless steel, safely handling most hot starts, but any tailpipe should be immediately inspected for distortion following a hot start.
All that heat from the jet exhaust will still heat up the tailpipe tube, thereby heating the inside of the fuselage. Cool air must be kept moving through the model as much as possible, even while the model is sitting on the ground. This is done with a simple trick at the aft end of the tailpipe:
- Leave a gap between the round opening of the fuselage and the tailpipe to pull air out the back of the fuselage, using the low-pressure area set up by the high velocity of the jet exhaust.
- The tailpipe should stop 3/4 to 1 inch short of the rear of the fuselage; the fast-moving jet blast pulls internal air from inside the fuselage.
- Tuning the air pulled through the fuselage can be controlled by the diameter of the fuselage opening and the fore-and-aft position of the tailpipe.
Proper movement of air is critical to controlling model temperatures. Spend some time with this area to realize cooler fuselage temperatures.
Heat protection and tailpipe design notes
The Xterminator I flew for the last few years did not have full ducting, but instead used a simple internally mounted turbine with a 20-inch-long single-wall tailpipe. A layer of aluminum foil was contact-cemented to the inside of the fuselage to reflect the heat from the tailpipe, and was quite effective. The fiberglass around the tailpipe got warm, but presented no problems.
The reflective aluminum foil was the key to success; an incident during the last year I flew this model proved how important it was: a piece of aluminum foil came loose, so I just removed it and went ahead and flew another flight. During turbine runup on the ground, the paint blistered from the heat.
A better solution to this is Heat Shield, Bob Viollet Model's water-based ceramic barrier coating. Simply paint it on the inside of the fuselage. This is a great solution to what is otherwise a nuisance.
My new model, the Razor, will have a double-wall tailpipe. This came about from conversations with Bob Wilcox of Golden West Models. He has been using double-wall pipes in his latest models, and is a strong supporter of their use.
My Razor kit was originally set up for the RAM 750. I am using a GWM Hammer, which has a slightly different setup on the jet exhaust end, so I need to make some minor changes anyway. The outer layer is an .048-inch aluminum wrap about 1/8 inch larger in diameter, providing an insulating layer of moving air between the hot tailpipe and the interior of the fuselage. The inside of the outer aluminum layer is shiny, to reflect the heat.
This type of tailpipe construction has been catching on lately, though it is not felt necessary by many modelers and manufacturers, as the outer tube does add considerable weight to the tailpipe. The nice part is that setting the gap at the aft of the tailpipe to the outer tube for proper airflow is controlled during the construction of the tailpipe. The tailpipe assembly must still be set in from the aft of the fuselage 1/8 inch to provide some pull of air from the fuselage for additional cooling. I'll let you know how it works.
That's it for this month; see you at the Jet Meet. MA
Transcribed from original scans by AI. Minor OCR errors may remain.
