Propeller STC’s
DeltaBurn is currently in the process of establishing the first STC for the turbo-propeller use of the technology. The aircraft for this is a Part 23 Merlin, fitted with Garrett TPE331’s and turning Hartzell props. The testing to date has baselined the performance of the aircraft/power-plant/propeller, and then worked on assessing suitable geometry for the modification to the propeller.
This process has included developing our own proprietary thrust measurement system, to permit measurement of the thrust of the propeller in a static case, but on the engine, and on the aircraft. This system is suitable for measuring the thrust of all aircraft propulsion systems with scaling of the design.
Ground testing has completed for now, and flight testing has commenced.
The TPE single shaft turbo-prop is the least effective type for our application, but it is analogous of the Allison T56A/D502 type engine, and the application to these engines is the most limiting case for our technology. We reduce the torque required to get the thrust that is needed, and for the TPE and T56A type engines, as they run at a single RPM at all times, as far as the core & compressor are concerned, then the lower fuel flow and temperature from demanding lower torque while significant, is not as great as when the core can operate at a lower Ng, which would further lower the fuel and temperature of the engine.
However, we see high order changes in repeat tests on the static testing, and in flight, for engine out climb capability, there is a world of difference between the standard propeller, and the modified case. How much you may ask, for a test case, we have a sink rate of 700 FPM for the standard case, and a 500 FPM climb rate for the modified case. Now, 1200 FPM may not sound like much, but that happens to be exactly the same change that occurs between the standard case max climb capability at the test altitude, and the engine out case for the same weight, altitude and temperature. We think that is an interesting change in outcome.
The STC will affect the torque setting for the takeoff case, it is our intention to not increase the load on the thrust bearings at any time, we match the thrust, but that occurs at a far lower torque than normal, reducing the core torque output from the turbine, the input load into the reduction gear box, RGB, and reducing the torque load out of the gear box to the propeller hub. The video of the blade angle and bending is shown below:
Standard Propeller case…
Modified Propeller case….
Compare these videos for a given torque…
look at the blade angle, and look at the blade bending change. Then look at the difference in thrust for the same torque as measured!.
This isn’t magic, but it is fine art.
But, this is a propeller, what about the turbo-fan?
The turbo-fan is essentially a refined fixed pitch, low aspect propeller blade. Not only does the fixed beta angle result in a rapid drop off of thrust from the bypass system as speed increases (…TAS… , not IAS) but it also suffers from sonic shocks as well as mass flow rate choking. Propellers don’t get choked, their wake expands and contracts naturally, but the turbo-fan, cannot do that, it gets to a flow choking condition.
We happen to circumvent mass flow rate choking by design, which we think is a pretty neat trick.