RC Jets
Jim Hiller, 6090 Downs Rd., Champion OH 44481
Wren MW54 turbine — review and AMA process
There is good news for the modeler who is interested in gaining a stronger knowledge of the makeup and operations of a turbine jet engine. AMA is continuing to review the kit turbines for future approval, and this process is beginning with a serious analysis of a Wren Turbines Ltd. MW54 turbo-jet turbine.
We have a Wren MW54 kit on loan for review, building, and flight testing, compliments of JD Enterprises. Experience gained from this will hopefully lead to a set of recommendations for the AMA Safety Committee to consider.
So far the Safety Committee, while being careful, has been very good to the turbine community by providing us with the opportunity to fly under AMA guidelines.
Joe Amato will be heading up this effort; I wish him the best in his volunteer service. I have this Wren MW54 turbine kit in my possession for review before Joe builds the engine.
The kit contains all the parts necessary to assemble the turbine engine itself; it does not contain auxiliary items, such as a fuel pump, electronic control unit, or any ground-support equipment.
A modeler who elects to build the MW54 needs the knowledge to find suitable components to turn this basic turbine engine into a running turbine. I am sure additional help in these areas will be available from JD Enterprises in their marketing of the turbine.
Rotating assembly
- The rotating assembly comes in an unassembled condition.
- The compressor wheel is a Garrett unit—a proven part, manufactured under quality control.
- The turbine wheel itself is made expressly for this application to Wren Turbines Ltd. specifications.
The manufacturing processes used, the quality-control measures, and even the inspection methods of the turbine wheel are critical to the safety of all of us on the flightline when a turbine is running. Wren has taken this responsibility seriously and can provide a solid history on the turbine-wheel manufacture.
The turbine builder will need to properly balance the rotating assembly, and the instructions provide a makeshift method to balance the assembly, but Wren does say that the ideal way to balance the rotating assembly is with an electronic dynamic balancer.
I located a suitable balancer at a local race-engine machine shop, so I guess finding someone to balance your turbine's rotating assembly is not an impossible task.
Combustion chamber and fuel piping
- The combustion chamber parts on the MW54 are stainless-steel fabrications, leaving the builder with little more to do than assemble the components. All the forming and spot-welding is completed.
- The fuel-pipe assembly is especially difficult, and Wren completes it for you. These are complex parts that require a lot of work to make; it sure is nice that they come prefabricated and ready to use.
There are some drilling and tapping operations to be performed on the diffuser, bearing tunnel, and similar parts. None of these operations should require other tools than those a typical modeler would have in the shop. This turbine kit was designed for a modeler to build in a home workshop, with machining limited to drilling and tapping-type operations.
The buildup and operation of this turbine will be an interesting endeavor. I wish Joe Amato the best on the project.
Tools and tips for composites
I found a new tool bit last year for my Dremel® tool that is ideal for cutting fiberglass and all the other composites we deal with. Try a RotoZip® 5/8-inch-diameter carbide bit (RT2125) for wall tile and cement-board use.
This tool is rated for 25,000 rpm or higher and holds up well to trimming not only fiberglass but carbon fiber. Don't skimp on tools; a carbide tool bit will hold up well and is one of the few items that will not dull immediately when cutting carbon-fiber laminates.
RotoZip® tools are available from most hardware stores.
Hopper Tanks
What are they and why do we use them on ducted-fan models?
That is a typical question new ducted-fan modelers ask, and it leads to the question, do we need them in turbine models?
We use hopper tanks to solve fuel-flow problems caused by air bubbles in the fuel line, due to our strange-shaped fuel tanks.
The typical ducted-fan fuel-tank system consists of a pair of 5/8-inch-wide saddle tanks curving around either side of the tailpipe duct. These tall, narrow tanks allow us to cram fuel into the tight space left in the fuselage after the fan, engine, inlets, and tailpipe use up all the space.
This system—two saddle tanks plumbed with a "T" connector, then to the engine—is a typical ducted-fan setup. It works well if care is used to keep the fuel lines from both tanks equal, then fuel burn will be equal from both tanks.
Our problem comes when we do aerobatic flying—particularly when the fuel tanks are half empty of fuel or less. The clunk does not keep up with the fuel in these tall, narrow saddle tanks as we roll from right-side up to inverted and back again.
The engine burps as the resulting air bubble makes it to the engine. This is more than an annoying situation; it can result in a dead-stick landing. I know; I've been there.
How a hopper tank solves the problem
The solution to our problem was simple: a small hopper tank, one to two ounces in size. The two saddle tanks are plumbed to the hopper tank, then a single fuel line leads from the hopper tank to the engine.
The function of the hopper tank is to separate the air bubble, allowing it to float in the hopper tank. You can tell how bad your fuel feed is from the saddle tanks by how much air is in the hopper tank at the end of a flight.
The normal fuel-feed pickup from the hopper tank to the engine is often a brass tube extending roughly halfway back into the hopper tank. A clunk is not used on this line, to avoid the problem of the clunk not moving freely in such a small tank.
This basic system has worked well for us, though some of us still use a clunk in the hopper tank. I prefer to use a clunk in mine. Why?
It's simple: the engine will suddenly go rich when you quit drawing from the saddle tanks and get down to only the hopper tank. I use two-ounce hopper tanks, so this gives me just enough fuel to circle in for a quick landing. The engine may flood out at idle, but I have some warning in case I fail to observe the discipline of using a timer.
This is only effective if you are flying alone. It's not good at jet meets; you will never hear your engine go rich.
Be careful if you decide to use a clunk in a hopper tank; it must travel freely to avoid the same air-bubble problems that saddle tanks experience. A two-ounce tank is much easier to set up with a clunk because of its larger size.
Should you use a hopper tank on a turbine-powered model?
That's a good question and one worthy of discussion.
Most of my jet models used two tanks feeding into one main tank prior to the turbine fuel pump. Each setup has its unique problems with air bubbles, and a header tank was one possible solution to the problem.
My first jet was an X-Terminator with two 24-ounce tanks feeding a 10-ounce flat tank mounted under the inlets. I could suck a large enough air bubble from the header tank after a few minutes of flying to shut down the turbine just from doing a slow roll. My main tank was a poor shape for the clunk to travel in; hence many air bubbles. The solution was a four-ounce hopper tank after that 10-ounce flat oval tank. There were no more air bubbles to the turbine, but why could I fill that hopper tank with air?
Last year I started flying a Crow Aviation Razor with a fuel system consisting of two saddle tanks surrounding the inlets, feeding a 40-ounce rectangular main tank. This main tank had a lowered floor in the aft end, to assure completely draining the fuel from the tank.
I had a terrible beginning with this tank. I used a piece of stiff Tygon® tubing on the clunk in the main tank, and the clunk would not stay in the fuel as I rolled upside down when I was down to half tank or less in the main tank. A hopper tank could have helped, but my real problem was the clunk in the main tank.
Crow Aviation, Inc. has quality flexible tubing and oversize clunks developed to solve this problem. I installed them, and my main tank has not had any more problems with air bubbles since. There is no hopper tank in this model, and I have had no reason since to even consider installing one.
Look at your particular fuel system—primarily your last fuel tank prior to the fuel pump—then make your decision. Most manufacturers will give you recommendations regarding the best setup. Listen to them; they have the experience with their products. If in doubt, install a hopper tank; it does help.
New hopper-type product
Bob Violett Models has a new type of hopper tank: the Ultimate Air Trap. SWB Turbines developed this tank, and it is said to be as effective as a hopper tank. It is basically a four-ounce tank with a special pickup arrangement; the pickup is a shaped bag. The tank is said to be effective even with almost three-quarters full of air.
Be careful if you try this system; read the instructions and follow them. New systems require new methods to make them effective.
It's time to go flying, so let's get to it. See you at Superman. MA
Transcribed from original scans by AI. Minor OCR errors may remain.
