SA-227, Reverse Question

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Greetings. With the Speeds in Low, and the Power Levers in reverse the RPM is @80%. With the PL's aft of the flight idle gate the UnderSpeed governor maintains RPM at 71%. I assume there is some mechanical connection with the power levers which cause the RPM's to increase to 80% but I can not find it in any of the books. Thanks in advance!

 

[avbug]

Engine operation in the TPE-331 is maintained by the fuel control unit (FCU). In flight, the FCU's primary component, the fuel metering valve, is set by the pilot, using the power lever. During ground operation, the fuel metering valve is controlled by the underspeed governor (one of two governors which direct the function of the engine).

The TPE-331 uses two guiding control systems throughout it's operating range, depending on it's function at any given time. The minimum speed of the engine is set using an underspeed governor. This is a fuel metering valve which has direct control of the engine in beta and reverse, and protects from RPM decay beyond the minimum setting established by the operator in normal flight. The underspeed governor works much like a propeller governor, using spinning flyweights, a pilot valve under spring tension from the cockpit "condition lever" or "speed lever" (depending on the installation). The operator uses the condition leer to set the minimum engine speed in a similiar way that one sets the propeller governor with a propeller control...except that in the case of the TPE-331, it's the minimum engine fuel flow limit that's being set, as a function of RPM.

On the ground, the underspped governor will cause the TPE-331 to operate at 70% RPM at the LO RPM setting of the condition or speed lever. The HI RPM setting will cause the engine to operate at 97% (approx). During beta operation, which includes any engine operation with the propellers aft of the low pitch stops, the underspeed governor will maintain engine RPM and the pilot directly controls propeller blade angle by moving the power lever into the beta/reverse range. The farther the pilot retards the power lever into the reverse range, the greater the propeller blade angle in the reverse range, and the more fuel required of the FCU via the underspeed governor, to prevent the engine from decaying below it's minimum RPM.

In flight, the pilot controls the FCU fuel metering valve, and the propeller controls the engine speed. In beta and reverse operations, the pilot controls the propeller, and the FCU/fuel metering valve controls the engine. This is the fundamental difference between forward thrust or idle, and beta/reverse in the TPE=331.

Throughout the full range of operation, the various components of the engine "talk" to each other through feedback rods, valves, and preset limits in the engine, fuel controller, and propeller governor, as well as the underspeed and overspeed governors. The exact relationship of these components is variable according to the way in which a particular operator desires the engine to be set up. For example, one operator may establish the rigging such that at idle the engine is operating with substantial drag in flight, whereas another may have the engine operating with residual thrust at idle: both can be within the manufacturer's parameters.

The condition levers serve several functions. The chief function is to set the underspeed governor. These are comparable to setting high idle or low idle on the PT6A. In a two-lever airplane, however, the condition lever also serves the function of a propeller RPM control. It sets the operating RPM (within the 96% to 100% narrow operating range), and also sets the underspeed governor. The overspeed governor is independent, and always stays the same. (actual values vary depending on the unit, and whether it's a bendix or woodward governor or FCU).

I understand your question regards the specific relationship between the FCU and governor in the reverse range, but the function depends on which one is installed on that particular engine. With a woodward governor, for example, RPM increases as the power lever is advanced (or reversed), but with a bendix governor the RPM is maintained constant. The function and relationship depends on which components are installed on your specific airplane, and how they're rigged.

The condition lever thus sets the underspeed governor and the operating RPM just like a propeller lever on a piston airplane or on some PT6A installations. It also serves as the cutoff lever, and emergency feathering lever. To feather the propeller and fuel-chop the engine at the same time, the condition lever is retarded all the way to the rear stops.

It’s important to note that this function is specific to 2-lever installations. Some TPE-331’s use a power lever, propeller lever, and speed lever...just like many PT6A installations use three levers. Some PT6A installations just use two, however...and so also the TPE-331. Regardless of whether two or three levers are used, the engine function is the same; it's simply the cockpit controls in basic layout which vary.

Where three levers are used, a power lever, a propeller lever, and a speed lever, the speed function independently controls the underspeed governor, and the propeller lever controls the prop, except when in reverse...when the pilot turns control over the engine to the underspeed governor, and controls the propeller directly through the power lever (n the beta/reverse range. While this occurs, when the pilot is scheduling propeller blade angle in beta/reverse, the underspeed governor steps up speed to a predetermined setting in order to prevent RPM decay. To a minimal point, RPM is allowed to increase to a maximum value; this helps prevent a compressor stall and/or overtemperature.

[FONT=&quot]Mechanically, the TPE-331 is one of the most complex aircraft engines built. From a pilot perspective it's operation is simple. From a mechanic's point of view, it's not.
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[Checks]

Avbug,

You are by far the smartest person on this message board. Thanks for your answer.

My plane has only the Power Levers and Speed Levers. it is a -12 engine.

In Speeds Hi and the Power Levers aft of the Flight Idle gate the RPM is always 97% as controlled by the USG.

In Speeds Low the RPM is 71% unless demanding reverese via the Power Levers then the RPM will increase up to 80%. I cant figure out the mecanics of why. I will private message you on Monday when I review all this information at work

Thanks Again!

 

[SinglePliot135]

Currently making the transition from running PT6s to the MU2 and this is very helpful info! The first few days of ground school were like a foreign language! I'm gonna print this and give it to the next guy thru, it'll be helpful, thanks!

 

[avbug]

[FONT=&quot]From a pilot perspective, the TPE-331 offers more desirable normal handling characteristics over the PT6A, but worse abnormal characteristics. The drag rise following an engine failure, particularly without NTS, is substantial, and the penalties for mishandling are significantly greater. With NTS as a system component, however, the TPE-331 is easy to manage.[/FONT]

[FONT=&quot]The Garrett is capable of producing significantly more drag with a windmilling engine than a PT6A. If an engine fails or is reduced in power to a significantly low value, a situation could be entered in which the slipstream is driving the propeller. In a geared engine such as the TPE-331 this is particularly undesirable A negative-torque sensing system, or NTS, is installed which can sense such a condition, and prevent it. NTS has the ability as low or zero positive torque is sensed, to increase propeller blade angle using a feather valve and pump. This doesn't feather thepropeller, but prevents the propeller from increasing in speed as it's driven by the slipstream. A windmilling propeller, even at idle, could quickly overspeed, and to prevent this from happening, the NTS system drives the propeller toward a feathered condition, aerodynamically loading the propeller disc, and keeping it's windmilling speed in check.[/FONT]

[FONT=&quot]The NTS system will activate repeatedly as needed, driving the propeller toward the feather angle, and then releasing it, as needed to keep positive torque on the motor. The feeling in flight when the engine is in an NTS condition is a pulsing of the engine or a sense that power is repeatedly being pushed up on that engine, then retarded again, about once or twice a second. The NTS condition can be alleviated by increasing power just enough on the engine to stop the NTS action from occurring...by increasing torque to a positive value.[/FONT]

[FONT=&quot]NTS has a similiar function to Auto Feather on some airplanes; it helps eliminate the debilitating performance loss from a windmilling propeller. It doesn't feather the prop, but it does buy some time to either increase power or secure the engine. (It's important to understand that NTS isn't a feathering system; it's function isn't to drive the prop toward feather, but prevent engine overspeed...a very big distinction between feathering on a PT6A and NTS on a TPE-331). This is one area in which the operation of the TPE-331 differs substantially from free turbine operations such as the PT6A. Whereas an airplane such as the King Air 90 or 200 utilizes the same "identify-verify-feather" procedure that's common to most all light multi engine airplanes, aircraft using the TPE-331 do not. Retarding the power lever to "verify" defeats the NTS system, and can cause a very large increase in drag. This is obviously undesirable when operating at low speeds and high power settings, such as an engine loss during takeoff. Accordingly, the drill is "Identify, Feather." Identification of the failed engine is done by power assymetry: torque, engine gauges, and if appropriate, engine fire warning.
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[FONT=&quot]The start cycle is very nearly automatic, with engine functions occurring on speed sense switches, including starter cutout, fuel introduction, etc. Pilot action is largely limited to cutoff using the condition lever in the event of a hung or hot start. One unique feature of the TPE-331 compared to PT6A is the propeller position during engine start. Whereas the Pratt starts from a feathered position, the garrett is actually started with the propeller locked in the beta, or reverse position. This minimizes the aerodynamic drag on the engine start, allowing the propeller and engine to spin up more quickly, with lower start temperatures. The PT6A uses acceleration bleed valves which open at low power settings to unload the compressor and prevent compressor stalls and hot starts; one may think of the propeller blade angle on the TPE-331 as doing the same thing.
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[FONT=&quot]In the Pratt, once the engine is on-speed, allthat's required to move forward is to push the power up and release the parking brake. With the Garrett, one has to retard the power levers into the reverse range first. This allows the start locks to retract. Then taxiing is a little different from the Pratt; one simply advances the power levers past the idle stop.[/FONT]

[FONT=&quot]During reverse operations, the pilot must also be aware of engine speed and temperature. The engine speed shouldn't be allowed to droop below a specified percent during reverse operations. However, because the pilot has direct control over the propeller and is working against the underspped governor, pulling hard into reverse may cause a drop in RPM. If this occurrs, and airflow through the engine is stagnated, compressor stalls or excessive temperature may occur. This isn't a problem on the PT6A because the pratt has a constant flow of gasses without impediment, regardless of propeller RPM. In the Garrett (Honeywell, etc), however, slowing the engine in reverse by improper use of the power lever and propeller balde angle can cause flame-out, compressor stalls, or heat damage.[/FONT]

 

[Checks]

Avbug,

I sent you a private message.

Thanks!

 

[dougdrvr]

I used to do NTS test flights after an engine change, always at a comfortable altitude and 99% of the time, it operated as advertised. The one time it didn't, I ended up nearly on my back and it made a beleiver out of me that if the system didn't work on a low altitude, low speed failure, you were a dead man!

 

[avbug]

I flew a single engine airplane with -10's and -11's on it. We had it set up such that with the power retarded about an inch before the idle stop, an enormous amount of drag was established. The airplanes were routinely used for low altitude steep descents down mountain faces and inside canyons, and the braking effect of the prop and engine was very useful in maintaining a given airspeed during a long descent.

The airplanes were mostly single seat, and with a plane-side checkout. I was told to take the airplane to 1,000' AGL, and slowly retard the power to idle. As the torque passd a low torque point, the drag rise was so rapid that one had to fairly briskly push forward almost to the vertical to keep from stalling. It was just short of breathtaking how fast it slowed down, throwing one forward in the shoulder straps in the process. If you hit it just right on the descent, you could work it in and out of NTS. The airplane pulsed forward and back, speeding up, slowing down.

On the one occasion I got a lesser braking effect during the descent, I arrived at the bottom of a long run-in descent to find no oil left in the engine and a prop I couldn't control. The NTS pulsing also stopped, due to the lack of oil. At the bottom of the canyon I made a right turn and put it on the hillside.

Garrett (Honeywell) states that the TPE-331 will run for 1/2 hour without oil.

 

[Hugh Johnson]

Avbug, I give up. Did you fly a Bellanca Viking or a Toscano?

 

[avbug]

I've flown a bellanca super viking, but it didn't have a turbine powerplant on it. It had an IO-520, I believe. They make a turbine conversion for the Viking?

Never flown a Toscano.

The airplane previously referenced was a PZL M18T.