Propeller STC Development

Propeller STC Development

Whats up?

What are we testing?

At present, we are in the process of the turbo-prop engine on our demonstrator aircraft. We have developed new techniques to measure exact thrust that the propeller can achieve for static testing, on the engine, on the plane, on the ramp.

Take a minute to look at what we are achieving. First up, this is a standard propeller run…

Standard Propeller…

And the same propeller, modified…

And that is the largest change in propeller efficiency since … ever.

With a simple modification that reduces the torque required to achieve the standard thrust, we are working towards the certified modification of existing propellers. This is aerodynamically similar to what we are doing on the turbofan, without the complications of mass flow rate choking, which we also solved for the turbofan.

Our original research began with propellers, (some time after the bikers from Dayton put on welding goggles and turned their cap around backwards, starting a fad that continues to this day). And… many years later, we are back again, but this time, concentrating on certification rather than experimentation.

Original experiments were undertaken on piston powered propeller aircraft, first starting with a Piper Aztec, which served well for research on the propeller, wings, flaps and various other areas of interest. The propeller was remarkable in its effectiveness, but for sheer fascination, modifying the flap, flap cove and outer wing was quite breathtaking. The only twin aircraft that I have flown that came close to matching the Aztec so modified was a Robertson Stol Cessna 337G which could land and stop before the numbers, and take off in the same distance. The Aztec stall speed was below the air speed indicator minimum reading when modified, and for a flap that had a 50-degree deflection, it was quite something to see fully attached flow over the full flap, approaching what took the place of the stall.

The Aztec propeller was suitable to experiment with, and it exhibited the effects that we are now certifying for the aircraft.

In our testing on propellers, apart from increasing thrust at all occasions, the notable change was a lowering of vibration. Alway. This characteristic occurs even when we have modified only one of two blades, and it holds true for turbofans as well. ¹

The plane

Today, we are working on a timeless aircraft as the first of series for turbo-propeller STC’s, Ed Swearingen’s Merlin III aircraft. The Merlin evokes strong views from all those that cross paths with them.

• There is no doubt when a Merlin may be running an engine on a ramp, within the country that you may be in…;
• It can fly long and far enough that both coffee and bathroom facilities are a planning consideration;
• The long body versions earned the nickname to the maker as the San Antonio sewer pipe company;
• Heavier versions climb capability reminiscent of an Airbus… the A340-300, an effective instrument to determine the curvature of the earth;
• The extended wing versions had ailerons that politely suggest any roll.

For the purposes of a propeller modification, the plane is perfect. The Garrett’s turn various versions of propellers, either Hartzells, McCauley’s or Dowty’s, and they can turn in either direction, depending on what siren was fitted to the aircraft. The blades are, blades. Perfect.

Merlin’s and Metro’s have nose wheel steering that will be familiar to Lear pilots, with some quirks. Early testing suggested we might need wider runways to be working with symmetry testing of the power plants, and so, we invented a means to measure static thrust that didn’t result in farmers complaints. A thrust plate was developed that the aircraft can be parked on, and the thrust is used to ensure that the plate will not slide. This system will work for all size aircraft, in fact we are setting one up now to run a 4-engine jet with full power on 2 engines at a time; symmetrically…

Our technology alters the aerodynamics of the propeller, and we get a “slight” change in the amount of thrust that the propeller provides for the amount of power that is needed to spin the prop. Simple stuff. That describes my activity since 1988, working out how to do the simple. So, now we have said what we are trying to do, how does it look so far?

We love squiggly lines, and color, so the above set of test results for the modified prop compared with the standard prop are colorful, and… clear as mud. From the same data set, we can rearrange the data, and we get… this next chart.

Now, this chart isn’t as cool, its lost the wiggly bits, but it shows two things of interest.

1. When torque increases, thrust increases, but it starts to plateau.
2. The modified propeller produces “a bit more” thrust for any level of torque that is put out by the power plant.
3. [just went past 2 things, but…] have a look at what torque is required to get equal thrust from the engine… the standard engine puts out 1,907 pounds of thrust for 1,908 foot-pounds (pounds-foot) of torque. We happen to know the engine is producing this torque at 99.8% RPM for the standard case, and the modified case is running 99.2% RPM, and 1,198 foot-pounds of “talk” or torque. Same engine, same prop, a bit of aero magic.

Hmmm.

We have enough data here to run some numbers and work out what we have actually done.

	STANDARD	MODIFIED	change (abs)	%change
NP,%	99.8%	99.2%	-0.6%	0.6%
RPM	1996	1984	-12	0.6%
Thrust, lbf	1907	1907	0	0.0%
Torque, ft-lbs	1908	1198	-810	-42.45%
FF, pph
ITT, Co
HP	725	453	-272	-37.51%

“A bit more”

Through the above test, we seem to have quantified what “a bit more” is, for the static condition. Static is a unique case, quite close to a “divide by 0” condition, but, we are certainly measuring the thrust of the propeller system, and we seem to get a “a bit” of a change. Is this change consistent with the aerodynamic change that we are doing? Actually, it is, very close to the analysis of what the change in C_L, & C_D is for the prop mod. The difference of C_L/C_D is on the money for what we see in the change of torque required. The static thrust chart shows clearly that we are getting greater thrust for the torque applied, this is repeatable, and is seen on every propeller we have ever tested, although there was this one case…. long boring story².

In flight, we are still clearing the test limitations, and we will shortly be able to undertake evaluation with both propellers modified, which will simplify life a lot, turbo-props need a fair bit of process to remove variables, more than turbofans do. We have seen that the candidate modification we are evaluating at present does work nicely, but one test point was quite telling, and is one we want to revisit in various manners to get a better data set for the cruise performance case. What complicates this is that thrust from propellers alters with true airspeed, TAS. Go fast enough, and your propeller will not give any thrust at all, yet will take more power to spin around. Jet engines are different, and efficiency drops off slightly as TAS increases, but then increases markedly above Mach 1. Turbofan engines are fancy propellers, with a bit of jet thrown in, but by the time the aircraft is cruising at FL350 and Mach 0.8 or so, the fan blade is not so happy. Jet engines, thrust specific fuel consumption, TSFC, is about the same at the end of the runway as in the cruise. The “efficient” turbofan engine, the TSFC at brakes release is as good as it gets, and at FL350, Mach 0.8 or so, somewhere shy of double the fuel burn is required for each pound of thrust that was achieved at static. That this is the case is clearly in manufacturers own information, (Eloude Roux has a good book out on the subject of engine datasets).

So, the interesting little test point so far was checking that the aircraft in test is safe to shut down an engine for the testing. To do that, we bring the modified engine back to zero torque, approximating a feathered propeller, and set a power on the standard engine and check the rate of climb we can get. This isn’t pushing the engine to a limit, it is using a repeatable power setting below max continuous thrust, MCT. (its torque, but… ). For the conditions, the book says the plane would descend at 180 FPM, going downwards… OK, and we get a 700FPM steady descent, which is OK, we are using a set torque for care. So returning the mod engine to normal power, and reducing the standard prop to zero torque (thrust), we are looking at 500FPM, climb, a change of 1200 FPM. That is an interesting number, as the flight manual also has data on expected rate of climb for all engines operating, and for these conditions, the all engines case should achieve 1000 FPM (we are hot, heavy and quite high). The math is not too involved to work out the difference in thrust that is occurring in this case, and we have to do a couple more tests to verify that this geometry is our go-to-guy for the STCs, but it is not a bad outcome. It was repeatable, and not far removed from expectations, but it certainly was quite something to see.

We record our instruments for these types of tests, as we do on the static tests on the ground, where we are recording the power instruments, thrust and we are imaging the propeller itself, to be able to measure the blade angle, and the deflection of the blade out of plane under live loads.

Single Shaft Turbo-propeller engines

There are 2 flavors of turbo-propeller engines, ones that have a single shaft, where every rotating component is turned at the same time, but at varying gearing ratios, and the rest which use a free power turbine that is turned by the “gas generator” which produces the power output, that is then picked up by the power turbine which is directly coupled with or without a gearbox, to the propeller itself. Examples of single shaft engines are the Garrett TPE331, Allison T56A or D501 and the free power turbines are the rest, such as the PWC PT6A, PW120 series, and the EA2000’s etc. From an STC standpoint, the most difficult propeller modification to fit on any aircraft is to the single shaft engine, and specifically, the TPE331. The single shaft engines run the power turbine and the compressor at the same speed in operation at all times, lowering the torque output does lower fuel flow and temperature, but, the core is still spinning at the same speed, absorbing power just to be. The PWC PT6A etc, where there is a lower torque required, the core can operate at lower RPM, and that energy expended in the compressor is lower. This results in the best fuel efficiency gain we will see being for PT6A/PW120/engines etc.

The logic for the STC is, such as it is, the TPE331 is the most demanding case to certify, any changes to engine starting would show up readily on TPE331 engines, and apart from that, Textron KingAirs are rather expensive to play with out of the box. It also happens that we have a fair amount of experience on Lockheed Orions, so the vagaries of the single shaft are reasonably well known, if not lost in the sands of time.