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Turbine Awareness Document
By
The Jet Pilots Organization

 

The advent of the model aircraft turbine engine has brought with it new safety concerns. Models powered by these engines travel at higher speeds, carry a large quantity of highly flammable fuel, and provide a high temperature ignition source. These three elements combined can result in a large, and possible uncontrollable fire in the advent of a crash. It can not be stressed enough that if trouble is encountered in flight shutting down the engine can within seconds remove the ignition source and prevent a fire from starting. All of the above means that safety must be foremost on the pilots and helpers minds when operating these models.

The Turbine Engine

A turbine engine draws air in through the compressor; all current model turbine engines use a centrifugal compressor. The compressor, as its name implies, compresses the air and sends it to the diffuser. In the diffuser the airflow is expanded and the pressure increases even more. Next the air moves into the combustor. In the combustor part of the air is mixed with fuel and ignited. At this point the temperature is high enough that it would damage the turbine wheel, thus the remaining air is mixed in to lower the temperature of the high temperature gas. This relatively high temperature gas then flows through the turbine nozzle, also called turbine guide vanes, which directs the flow to the turbine wheel. The flow of the gas through the turbine forces it to spin, which in turn spins the compressor. The gas then exits the engine though the nozzle.

Throttle lag

As stated, not all of the air that goes into the engine is used for combustion; some of it must be used for cooling. When a turbine engine is commanded to accelerate more fuel is added, however, the airflow is a function of engine RPM, so if a lot more fuel is added all at once, more, or all, of the cooling air will be used for combustion, which in turn will greatly increase the temperature of the gas going to the turbine nozzle and turbine wheel. So, when the engine accelerates the fuel flow can only get a little ahead of the airflow so that proper cooling flow can be maintained. When a turbine engine decelerates the fuel flow needs to be reduced at a rate that prevents the mixture in the combustor from going so lean that the engine flames out. The Electronic Control Unit (ECU) used with the engine controls the fuel flow so that the engine is not damaged from to much fuel on acceleration, or allowed to flame out on deceleration, however, the pilot must be aware at all times of how throttle lag will effect operations. Takeoff and landing are the areas that are most effected by throttle lag. If a take off must be aborted the delay in the power coming back down must be taken into account, thus full power should not be applied until a good takeoff roll is established. On landing if a go around is required it can take several seconds for the engine to go from low power to a point that will allow the model to climb. Thus if the decision to abort is made to late the model could impact the ground while the engine is still spooling up. If a model is equipped with speed brakes it can be advantages to use them on final so that more power can be carried, and then if more speed is needed the speed brake can be retracted, which has the effect of adding a step increase in thrust, and, if needed, the engine will take less time to achieve full power. The opposite problem on landing is if to much power is carried and the throttle is reduced to late the time that it takes for the engine to spool down could carry the model past the end of the runway.

Residual thrust

A turbine engine requires a certain amount of air going through it just to run. This results in a fair amount of thrust with the engine at idle, for most model turbine engines this is about one pound. Most models that roll reasonably well on the ground will not sit still with this much power, plus when landing, the model must generate more drag, either by devices on the model or by pilot technique, to slow down and land in a controlled manner. Once on the ground the inability of most turbine engine powered models to stop by them self make the use of wheel brakes necessary. When taxiing, residual thrust combined with throttle lag requires more care so as to not allow the model to take off at high speeds across the taxi area. In most cases a model will start to roll with the engine at idle, if not the throttle should only be increase a few clicks and normally returned to idle within a few seconds.

 

Turbines, ducted fans and propellers

All current forms of air breathing propulsion work by accelerating a quantity of air to a higher speed in order to generate thrust. In general the amount of thrust developed can be considered to be the product of the quantity of air accelerated and the velocity to which it is accelerated. A propeller accelerates a large quantity of air to a low speed, around 100 MPH, a ducted fan accelerates a smaller quantity of air to a higher velocity, around 200 MPH, while a turbine engine accelerates a small quantity of air to very high velocities, over 400 MPH. In general the thrust developed is proportional to the difference between the flight speed and the velocity the air is accelerated to by the engine. If a propeller powered model generates 10 pounds of thrust on the ground it would only produce around 5 pounds at 50 MPH, a ducted fan model generating 10 pounds of thrust on the ground would hit the half thrust point at around 100 MPH, while a turbine engine powered model would not hit this point until over 200 MPH. Because the turbine engine losses its thrust at a much lower rate it is necessary to fly at lower throttle settings so as to not over speed the model. It should be noted that full-scale aircraft manufactures have teams of engineers making sure that the airframes can handle the high speeds that a turbine engine make possible. The model industry simply cannot afford this level of effort, thus it is the responsibility of the pilot to operate the model at a safe speed.

Ground Safety

For propane fueled engines the fueling process must take place away from crowds and spectators and preferably down wind. Since propane vapor must be vented during the fuel process care must be taking to assure that no ignition source are close by. When starting all turbine engines the model should be turned into the wind. This will help in carrying combustion products out the tail pipe. Pointing the model into the wind will also at times make the engine easier to start since the wind will not be trying to blow the fuel out the front of the engine. Care must be taken when starting an engine to make sure that the tail pipe is not pointed at any flammable material, or people, and that loose objects will not be blown around which create possible hazards. Spectators must be kept at least 25 feet away during startup and operation. When the engine is shut down the model should be turned into the wind so that the high temperature air in the engine is blown out the tail pipe rather than through the less temperature capable inlet.

Radio setup

The addition of the ECU and more metal parts in the model can create problems for the radio receiver. Care must be taken when routing the antenna so as not to place it close to components, such as the ECU, fuel pump, and connecting wires, which might generate radio noise, while also making sure that the antenna is not shielded by large metal objects, such as the engine and tail pipe. Receivers and servos should be flight tested in non-turbine models before installation in a turbine model. While in this less demanding environment the ground range check distance should be recorded, and once the radio equipment is installed in the turbine powered model the ground range check distance should be compared to the earlier value. If a notable reduction in range occurs the installation should be reworked so as to get back to the earlier range. Once the ground range has been verified it should be rechecked with the engine running. At this point a small loss in range is acceptable (around 10%), but if the range is reduced very much the installation should be reworked again.

Failsafe

The pilot should carefully read the manual for the radio being used, and be sure to understand how the failsafe function operates. There are many opinions as to what is the best setting for control surfaces when setting up failsafe. The pilot should use his/her best judgment when setting this up. Some pilots choose to have failsafe extend the landing gear to give an indication that something is going wrong. The throttle, however, should never be set to hold, it should at least be set to go to idle. Setting the engine to shut down on failsafe is possible, however, it is not unusual for a properly functioning radio to lose a few frames over the period of a flight. If the pilot were contemplating setting failsafe up in this manner he/she should check with the ECU manufacturer to see if a single frame loss would cause a shutdown if failsafe was set to shutdown. In all cases should the model start experiencing radio trouble; the pilot or a helper should shut down the engine. If the problem is ECU noise, shutting down the engine will normally clear up the trouble, and allow the model to be landed dead stick safely.

Emergency procedures

Once again, in the advent of an emergency shutting the engine down should be the first thing the pilot does. In the air if there is chance the emergency might cause a crash shutting down the engine can prevent a disaster by allowing the engine to cool down. On the ground shutting the engine down at the first sign of trouble can save an expensive model, and prevent injuries to the pilot and helpers. All persons assisting in the operation of the model should know how to shut down the engine. In general there are several ways to shut turbine model engines. These range from commanding a shutdown from the transmitter, shutting down from a start box, or just turning off a fuel valve in the model. The pilot should be sure that all helpers are aware of all of the methods to shut down the engine.

Fire extinguishers

It is mandatory that a fire extinguisher be present when operating a model turbine engine. Should an engine or tailpipe fire occur during engine start either a CO2 or Halon extinguisher should be used. The extinguisher should be aimed at the inlet of the model, never at the tailpipe. Firing an extinguisher up the tailpipe will push the flame forward, towards the fuel lines and fuel tanks. By firing the extinguisher down the inlet the engine will be shut down and the flame will be blown out the tailpipe. CO2 and Halon can be used to extinguish small grass fires, however they provide no protection against the grass relighting. Water is one of the best extinguishers for grass fires, and water-fighting equipment should be on hand if there is a risk of grass fire. In the advent of a grass fire the local fire department should be contacted, it is better to have them come out to find the fire has been dealt with than to have a fire get out of hand.

Spotters

A spotter is an assistant for the pilot, when starting the engine they man the fire extinguisher, keep and eye out for fire at start up, watch for people entering the area, and in an emergency help shutdown the engine. In the air they communicate with other spotters to help provide safe separation and to make sure that the pilots intentions are understood by others, and generally keep the pilot informed of the area conditions. The jet community has used spotters long before turbine engines; they are a key element in flying any high-speed model.

AMA regulations

All turbine engine pilots must read and understand the AMA regulations for model aircraft turbine engines. These regulations place specific limits on model sizes, engine sizes, speed limits, operating limits and more. All AMA members operating turbine engines must abide by these regulations.

Conclusion

Turbine engine powered models present a unique safety concern. All turbine engine pilots must exercise vigilance in operating these models. Always seek the help of an experienced turbine pilot before your first turbine flight. In case of an in-flight emergency shut the engine down.

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