TPE331 initial testing

We are working on a direct measurement of static thrust for an installed propeller, which is underway at this time. Measuring installed thrust is not a simple matter when looking at accurate figures, but, it is possible

Our prior method on the jet engines has been based on determining asymmetry that results from a change in thrust. That is possible for the propeller case as well, but is less accurate in principle, due to the difference that always exists when all propellers are turning in the same direction, due to what amounts to P-factor. 

With jet engines, there are very little effects from variations other than thrust from the engines, once tyre pressure, wind, runway camber etc are taken into account. 

While waiting for the static thrust measurement system to be built, we looked at the engine performance in general, and there are a number of matters that are of interest. 

The Garrett TPE331 has been chosen as it is an analogue of the venerable T56A-14/15 that are used on the C-130 Hercules before the J model, on the P3 Orion, and on a number of other civil and defence aircraft. This is a direct drive, single shaft layout which has upsides, and downsides.... From our perspective it is a good system to work on, as everything else is easier. 

Running the standard system to the target torque setting for the conditions which is found in performance charts as a lbs-ft value that the performance of the aircraft is determined on, we can observe that the torque target is attainable, with margin on both engines, one of which is fresh from overhaul, and the other is mid life, with a lower ITT margin available at the target. 

While remaining within the torque limit for the engine, we are able to exceed the target for the performance by a reasonable margin, this takes up some of the ITT temperature margin we had at the target torque value.
At the higher torque setting (2100 vs 1839) the best engine is starting to get to the limits of tyre friction to stop movement with the wheels locked.
Modified, we expect to have higher thrust at all cases, and even at low speed ground idle, which is normally zero thrust, on releasing brakes the aircraft moves forward noticeably. In taxi, the aircraft continues to gather speed with the power levers at ground idle, and with the speed levers at low RPM,  with a slight tendency to steer away from the modified propeller. This is easily managed, but it is identifiable as a change in thrust, and occurs relative to any modified propeller.
With the stronger engines prop altered, on powering up with the brakes locked, things become more interesting, as we cannot hold the aircraft on the brakes at a very obviously lower torque than was able to be held when unmodified. This change in torque that can be held is "interesting", it is larger than we were wishing to see, and is in the range of CFD potential outcomes, but is towards the high side.

The high order change is a minor issue, as we can calculate the residual forces that will occur on this propeller  when feathered, and that is problematic with the TPE engine. Unlike the T56A, the TPE does not have a propeller brake that stops reverse rotation when feathered... and a quick consideration on engine design will indicate that reverse rotation is not desirable, not by a long shot! The starter generator carbon brushes are angled in their contact to the rotor, and so reverse rotation risks increased wear of the brushes, or taking material off the carbon contact. Not expensive, but irritating. Looking at the oil system however, that is not so benign, as the scavenge pumps are twice the capacity of the pressure pump, and so an effective hydraulic lock is in effect, if you are fortunate, as hydraulic locks have a way of resolving themselves by blowing seals... and that is a notable nuisance. The engine manuals provide cautions against reverse rotation, this is why.

Having a propeller brake makes life easier, and we can achieve a greater effectiveness without having the concern of reverse rotation when feathered. 

Why would the propeller rotate in the opposite direction you may ask.... As the modification alters the coefficient of lift for a given angle of attack, the angle that the propeller normally achieves against the feather stops is giving a total force due to the flow from the aircraft forward motion, that is more or less zero as a residual torque. The mod increases the lift for any angle of attack, so the blade is effectively over pitched aerodynamically at the normal stop angles, and will then result in a residual torque that differs from standard case. 

The good news is, the mod is able to be tuned to a given speed, so we do want maximum effectiveness when the propeller is operating, at which point the Mach number is quite high. When feathered, the forces on the propeller are due to the TAS of the aircraft. 

The TPE and T56A engines conceptually share a negative torque sensing (NTS) system, and that makes life easier for our testing. 

Normally, you are obliged to feather a propeller when an engine stops giving power out to the prop, the NTS system acts to limit the drag associated with the air loads on the propeller trying to drive the single shaft/direct drive turbo-prop engine.   

The engine drag is reduced by the NTS to around 20 horse power equivalent which coarsens the propeller blade angle to balance the torque. It is ingenious and has been around for 70+ years, and works well. The propeller while NTS-ing  is rotating at a modest RPM, normally around 25-30% of normal RPM, and is actually properly set up to be restarted, if desired if ignition and fuel are supplied. 

Fully feathering the propeller is however desirable and normally mandated,  so we have some balancing work to do on making the modification match the feather stop. 

When all else fails, we can change the setting on the propeller to alter the stop angle, it will be around 10 degrees lower than standard, but, heck, that means opening up the spinner and the dome on the propeller, and it then makes it mandatory to have the mod in place, the modified feather angle will cause a standard blade to continue rotating at about 10% RPM, which is time limited.