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General Turbine Theory
Aircraft

There is plenty of good information available on the Internet and in books that describe how turbine engines work, but there is always a lack of information that applies to our model engines and installations. This article was published several years ago by a Phil Cole who is flies both airplanes and turbine helicopters who has a keen insight on the theory as it applies to our models.

        

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Model Jets, Gas Turbine Theory

Written by Phil Cole. First published in the Bayside R/C Club newletter, the Flightline.
November 2003.

You've seen jets flying at model airshows, fun flies, or just out at a flying field one day. This article attempts to explain a bit about what's going on in a turbine powered jet model aeroplane.

How Much?

This is the first question everyone asks. New engines vary in price from around $2200 to $5000. There are kits which you assemble at home starting at $1800.

Jet model kits start at a few hundred for a simple sport model to $4000 and up for the top-of-the-line scale models including retracts and fuel tanks. These are kits, so many hours of building and finishing are required. If you not inclined to build, there are ARFs available from $400 to $4000. The price of a model + engine + on-board electronics (bought new and then assembled) ranges from $2800 to $15000.

Now for the interesting stuff...

How Do They Work?

Model jet engines work pretty much the same way as full scale turbine engines:

  1. Air is drawn into the front of engine by the compressor.
  2. The compressor increases the pressure of the air.
  3. The compressed air is heated. The most convenient way to heat the air is to burn liquid fuel.
  4. The hot, compressed air now has a lot more energy than it took to compress it.
  5. Some of this extra energy in this hot air is extracted by the turbine to drive the compressor.
  6. The remaining energy in the hot air is used to squirt it out the back of the engine through some sort of nozzle, creating forward thrust.

There are a lot details associated with each of the processes listed above that affect the operation of the engine, or even whether it will run.

Figure 1.

  1. Air intake.
  2. Compressor blades.
  3. Diffuser blades.
  4. Combustion chamber.
  5. Nozzle guide vanes.
  6. Turbine blades.
  7. Tail cone.
  8. Main shaft.

Compressor and Diffuser

The centrifugal compressor used in model turbines works by flinging air outward, increasing its velocity. To increase the pressure, the air must be slowed down. This slowing down is the job of the diffuser. The diffuser consists of a number of narrow passages that gradually open out. If they open out too fast, the air will become turbulent, wasting the energy put in by the compressor. Increasing the size of the diffuser will result in an increase in engine diameter, making it harder to fit into a model.

The amount that the air is compressed is known as the pressure ratio. Pressure ratio is the pressure at the output of the diffuser divided by atmospheric pressure. In model engines it will have a value of around 2 to 3. This means the pressure inside the engine housing will be between 30 p.s.i. and 45 p.s.i. These pressure ratios are all that is practical with a single stage centrifugal compressor. Full scale engines have pressure ratios up to 30's (450 p.s.i.). However, these engines use large, multi-stage compressors which are not feasible for model use, if only because of the cost. Most model engines use compressor wheels from automotive turbochargers, which are precision cast in large quantities. Without them, model turbines at realistic prices might not be possible.

Combustion Chamber

The job of the combustion chamber is to control the flame so that it remains in the front part of the chamber, known as the primary zone. The secondary zone is where more air is mixed in, cooling the exhaust before it exits the chamber. The chamber must be large enough for the fuel to burn completely in the primary zone - another compromise, as a longer chamber makes the main shaft longer, reducing the RPM at which destructive vibrations can occur. The smaller the engine, the less room there is for the combustion chamber. The design of a combustion chamber is generally a trial and error process - contruct one, put it in the engine and see how it works. The process is repeated until good results are obtained..

Combustion chambers are constructed from bent and folded sheet metal. The material is a high temperature steel alloy such as stainless steel or Inconel.

Nozzle Guide Vanes and Turbine

The nozzle guide vanes (NGV) impart a rotating motion to the air as it is expelled from the combustion chamber. This rotating motion is absorbed by the turbine blades, thus rotating the turbine, main shaft, and compressor. The angle of the NGV and the angle and size of the turbine blades are matched to absorb just the right amount of energy to power the compressor. If too much energy is absorbed the engine's performance is reduced. If too little energy is absorbed the engine won't run, or it has to be run with an excessive exhaust temperature. The turbine must also be efficient, otherwise the exhaust temps will be high. The main factor in turbine efficiency is the gap between the ends of the blades and the inside of the turbine housing. In models, this is generally around 0.4 mm, depending on the diameter of the turbine wheel. Larger gaps reduce efficiency, and smaller gaps can result in the blades scraping on the inside of the housing as they expand with temperature. The blades heat and cool much faster than the housing - as temperatures increase during acceleration the turbine blade clearance will decrease until the housing temperature catches up with the turbine blade temperature. The smaller the engine, the more critical the clearance is, as a greater portion of the exhaust can 'escape' around the ends of the blades.

The NGV and turbine are subject to the highest temperatures in the engine. Generally they are cast and then machined from Inconel, a high temperature steel alloy. Inconel is expensive, the casting process moderately difficult, and Inconel is not a pleasant material to machine.

Tail Cone

The job of the tail cone is to speed up the exhaust air by gradually reducing its area. The faster the exhaust air exits the engine, the higher the thrust, within limits. If the nozzle squeezes down too much, airflow is reduced, leading to higher temperatures.

If Jets are so Expensive and Difficult, Why Use Them?

Propellers and ducted fans can be used to power jet models, and they cost a lot less than turbines. You can't really see the prop on a prop jet once it's in the air, and ducted fans solve the problem of the visible propeller (and the engine plus exhaust system hanging out the side) completely. However, jets are meant to be fast. They are relatively small and have high wing loading, and the more true-to-scale they are the higher the loading. Hence piston engine powered jets tend to have high-revving two stroke engines. The engines typically require tuned exhausts to make enough power. In particular, ducted fan engines are little different from all-out racing engines. In flight mixture-controls are common. An all-out lean setting is required for take off, and then the engine is richened to a setting that allows it to survive through the flight without overheating or burning the glow plug.

The reason the engines have such a hard time in jet models is the wide speed range. Full scale piston powered aircraft generally don't have a wide speed range. The ratio of flying speed to landing speed is generally not more than 2.5:1. Jets extend this to 4:1 and more. Why is this a problem? The answer lies in the thrust vs.air speed characteristic of the respective propulsion systems, as shown in the diagram below.

It can be seen that the turbine engine thrust is relatively constant over a wide range of airspeed. The turbine powered plane has more thrust available at higher speed than any other power plant. The reason lies in efflux velocity. The efflux velocity is the speed of air exiting the propulsion system. If there no aerodynamic drag, an aeroplane's maximum speed would be equal to the efflux velocity.

The propeller efflux (propwash) is relatively low (between 100 and 200 m.p.h. for this class of model). The DF efflux is somewhat higher, due the high pitch of the fan blades. There is a penalty in lower thrust at low speeds, compared to a propeller, and in reduced efficiency. A model turbine jet will have an efflux velocity in the region of 600 m.p.h.

So, you can see that if you want your plane to go fast, jet turbines are the best engines to use. There are some disadvantages, or course. The cost is one that everyone knows about. Some of the others are: high fuel consumption, increased complication, and a higher fire risk.

Fuel Consumption

Turbine powered model jets typically carry between half and one gallon of fuel on board. The fuel weighs between three and seven pounds. This will be over 10% of the model's weight, so the tanks need to be near the CG if the model is to be flyable through the range full to empty.

Even with this amount of fuel, flight times are short: between six and ten minutes is the usual range. This is usually enough, as flying a jet is quite intense. The reason for the high fuel consumption is that small turbine engines are not very efficient.

The fuel used these days is kerosene, or Jet-A. Jet-A is basically kerosene with better quality control. The characteristics of kerosene that make it desirable for use as jet fuel are its high energy content per lb. (similar to gasoline, twice that of methanol), high density (smaller fuel tanks), its high boiling point (making its flammability low), and its wide boiling point range (making it easy to vaporize over a wide range of temperatures, leading to stable combustion).

Complication

The main complication come from the fact that the fuel supply must be regulated. Piston engines with carbs are able to draw fuel as required. The carb simply supplies the amount of fuel that the engine wants at its current RPM and throttle setting. To much or too little fuel will prevent a piston engine from producing optimum power.

Turbines are different. They can always burn as much fuel as you can feed them. The temperature rises, the turbine drives the compressor harder, compressing more air, allowing the engine to burn more fuel. Very soon the turbine overheats, the blades scrape on the housing and then break off, ejecting red-hot chunks of metal out the back of the engine. So, all model turbine engines have electronic control units (ECUs) that monitor RPM and exhaust temperature (EGT). The ECU reads the throttle signal from the receiver and translates it into a power setting. The ECU then controls the fuel supply to maintain the power setting. If RPM or EGT exceeds allowable limits then the fuel supply is cut back. If the condition persists, then the fuel supply is cut off.

The ECU can either measure RPM or case pressure to determine the engine's current power level. ECUs are also used to control the start procedure. Some ECUs can start the engines completely automatically by controlling the starter motor and starting gas, others just monitor the operation, and feed in liquid fuel when required.

Starting

No doubt you've all seen or heard about the procedures required for starting turbines required compressed air from scuba tanks and propane tanks. Later model turbines have full auto start capability, but the procedure is still the same as far as the actual engine is concerned. The propane is required to preheat the engine before it can run on liquid fuel. Liquid fuel must be vaporized before reaching the combustion chamber. This is done using heat from the combustion chamber, so propane is used to heat the combustion chamber to get the process going.

The actual start process is:

  1. Spin the engine up to 5000 RPM or so with the starter or compressed air and then let it spin down.
  2. Feed in the propane while the engine is spooling down and ignite it. Typically a glow plug is used for ignition, though a gas lighter up the tail cone will work (and maybe result in some singed fingers).
  3. Spin the engine again, feeding in more propane. Wait until the EGT begins to rise.
  4. Start pumping a small amount of liquid fuel. The engine should accelerate and the EGT should rise smoothly. Too much fuel will result in large flames out the exhaust which can set fire to anything in their way. Too little fuel and the engine won't accelerate. Once the engine is accelerating nicely, the propane can be shut off.
  5. Wait until the engine reaches self-sustaining RPM. This is the RPM at which it can run by itself without assistance from the starter. Generally this will be around 30,000 RPM for a model engine. Disengage the starter (either electric or air).
  6. Wait until the engine has accelerated to idle speed and stabilized.

Flying

Flying model jets is different from propeller powered aircraft. The obvious difference is that they are generally much faster, so you need to be on top of the situation all the time. The higher speed means turns are much wider, so a lot of room is required.

Due to the thrust characteristic (see above), full throttle is not used for level flight. This would result in extremely high speeds, possibly beyond the capability of the airframe. Beyond the capability of the airframe can mean ripping the wings off due to high G-loads, or the elevator is unable to pull out of a dive or split-S.

The throttle response of turbines is quite slow due to the time it takes for the rotating parts to respond. The time from idle to full power can be between three and six seconds, depending on the engine. Reducing power has a similar response time. The pilot must anticipate the need for power changes ahead of time, e.g. powering down before the top of a loop, and powering up ahead of time before entering a steep climb.

Landings can be difficult because of the weight of the models (high landing speeds, and long run outs), the delayed throttle response (the decision to go around needs to be made before you stall and crash) and high fuel consumption (limited number of go arounds before dead stick). Flaps and wheel brakes help landing (and are necessary in some models), but they are extra workload for the pilot. In the event of the engine stopping, jets make very poor gliders.

AMA Waiver

Due to the difficulty of flying jets, the AMA has decided that it won't provide insurance for just anyone who wants to fly jets. You have to have a 'Turbine Waiver' to fly jets with AMA coverage. To get the waiver there is written test covering the points I've made here (and some that I didn't due to space and time), and pass a practical test. The practical test involves demonstrating your ability to fly a high performance model to the satisfaction of two CDs. One of the CDs must be a 'Turbine CD', i.e. a CD that is experienced in turbine operations. In addition, you have to sign a declaration that you have flown 50 flights with a high performance model, capable of 150 m.p.h. For more details you can go to the AMA web site www.modelaircraft.org. Go to the 'Membership Services' menu and select AMA Documents. Document #513 contains the basic rules, and refers to other documents for various details.

More Information

If you want to learn more, there are a number of books on model jet turbines. They are available from Traplet Publications, www.traplet.com, 1 800 695 0208. 'Model Jet Engines, Second Edition' by Thomas Kamps explains the functioning of turbines at level to suit the interest modeller. There are some formulas if you really want to get into the details, but the descriptive material is enough to get a good understanding of what's going on inside the engines.