Two Speed Propeller Reduction Gears

[Piper106]

One of the 'next great ideas' in the 1940s was going to be two speed propeller reduction gears.

The 24 cylinder Lycoming H-2470-5 and -7 engines. and the Lycoming 36 cylinder R-7775-3 engine were among the engines scheduled to have this feature.

To the best of my knowledge no two speed reduction gear was ever reliable and no engine with this feature was ever mass produced.

Does anyone have any data / links to technical papers on what the intent was for this arrangement??

I have a photocopied page from one of Wilkerson's Aircraft Enines of the World that shows a 1500 HP low cruise on the R-7775 engine at 1300 (!!) crankshaft rpm in 'high gear' of the two speed propeller gearbox. In this case I assume the higher gear (less reduction between crank and prop rpm) allowed the crankshaft to turn very slowly at cruise while still turning the propeller fast enough to convert the HP to thrust efficiently.

But was this the case with all the engines that were to use a two speed reduction gear??

Piper106

 

[basil]

Try Google Patents; there are several proposals of two speed reduction gears (for example from Rudolph (Rudy) Daub of Curtiss Wright, and others). Focus the serach by restricting the filing date on f. e. 1940 - 1946. By the way, there are also some interesting proposals for radials with "true motion connecting rods" (eleminating the master rod and slave rod construction in radials).

 

[Piper106]

Thanks very much basil.

Reading the patents I see my speculations in my first post is very wrong.

From the patents, the intent with the two speed reduction gearing was to turn the propeller faster (more rpm) at take-off and climb i.e. low air speed and low allitude. This would allow a flatter blade pitch and better conversion of horsepower into thrust. Once air speed had built up, the gear ratio would be changed to turn the propeller more slowly (less rpm). This minimizes the velocity at the tip of the blades (i.e. trying to keep the combination of forward speed plus the spiral path of the blade tip below sonic velocity) again to best convert horsepower into thrust.

As indicated in my first post, despite the advantages, this was never prefected for military size engines, and to my knowledge no production engine during the WWII era was ever equiped with a two speed reduction gear.

Piper106

 

[nuuumannn]

Hi Piper, From an engineering standpoint, that doesn't sound all that good for the engine/gear box. Surely a constant speed governor that changes the blade angle to give a greater amount of thrust for a given rpm (say, 1,000) is more efficient than a device that increases and decreases the prop rpm through the rgb automatically. You'd need a pretty clever rgb to do that. Part of the reasoning for a C/S governor is that it places less wear on engine components, bearings etc. This is what most WW2 and modern aircraft were/are fitted with.

 

[basil]

Piper106, that was exactly the aim of the concepts.

Nuuumannn, same thoughts as I have - I never really understood the proposed advantages of a two speed drive. If you have a constant speed unit with a fully variable blade change mechanism, you should be able to achieve all that without a two speed drive. Nevertheless some of the patent filers were very experienced aero engine engineers.

 

[Shortround6]

Just guessing but perhaps the Propellers didn't have the range of adjustment that was wanted? As in trying to adjust from transmitting well over 2000hp at low speed and low altitude to cruising at something under 1000hp at a speed 3 times higher in air 1/2-1/3 as dense? The Prop blades are going to be effective over part of their pitch range even if they can turn 90 degrees (or more for props that can reverse). Too flat gives nothing 5^ vs )6 for example and how many degrees before full feather?

Prop blade design was also advancing at the same time the gear boxes were being worked on. Perhaps the newer blade designs reduced/eliminated the need for the gear boxes.

 

[nuuumannn]

Perhaps the newer blade designs reduced/eliminated the need for the gear boxes.

Possibly. Early Ham Std hydromatic fully feathering props like the 23E50 on the DC-3 had quite a big range, something like 80 deg. These were licence built by de Havilland. Ham Std had been doing C/S Hydromatic props since the early 1930s.

I'm just guessing but perhaps the two speed rgb was designed to take into account two position only props, to increase their range of effectiveness. Novel idea, but sounds like designing in needless complexity.

 

[wuzak]

Coarse pitch can't be all that efficient at slow speeds?

I would imagine that the 2 speed boxes were to allow the same thrust using smaller blade angles and higher rpm.


The 2 speed boxes were conceived for some later engines too. The Lycoming H-2270 and R-7755 have both been mentioned, the Wright R-2160 Tornado was also to have a 2 speed reduction box on one of its variations. These engines would never have been envisioned with simple 2 position props.

 

[Piper106]

It wasn't the range of adjustment on a modern constant speed propeller that was the problem, the prop could be set to an angle that would absorb the engine power at a speed, it was the efficiency that was the problem.

If you think of a propeller blade as a piece of wing section (which it is), what you are trying to do is maximize lift (thrust) compared to the amount of drag (horsepower to turn the propeller). I assume that beyond an 'optimum' angle of attack increasing the propeller pitch gives less and less thrust increase for each extra degree of pitch while the horsepower drawn from the engine keeps going up.

My take away was that at low air speed during the take-off roll and initial climb, a propeller turning slow enough to avoid Mach problems at high allitude and high speed turned too little of the engine power into thrust because the blade pitch (angle of attack) had to be set beyond some aerodynamic 'optimum'. A propeller turning faster could have used a flatter pitch and turned more of the horsepower into thrust, improving take-off and climb performance.

Agreed, a two speed propeller reduction gear was a complicated piece of equipment. In this case too complicated, a 'bridge too far' for the mechanical engineers and materials of the time.

Had the mechanical engineers been able to do it, my guess is that it likely would have become widespread on heavy bombers (think B-29 and B-36) and long range maritime patrol aircraft (Shackleton and P2V).

Piper106

 

[basil]

I still don´t get it. For take off and climbing you can choose a flatter pitch with higher rpm without a two speed gear box. The flatter pitch leads to a lower power absorption (and higher efficiency for this speed) and will let the prop turn faster with the same engine power. For high speed flights when tip speed (Mach 1) becomes critical you will choose a steeper pitch which will absorb more engine power at reduced rpms. I don´t see any necessity for a two speed gearbox. Nevertheless there must be some advantage because some highly respected engineers evaluated this idea even after WW 2.

 

[Shortround6]

Too flat a pitch won't do much good and there is a point at which too great a pitch is is moving the blades sideways through the air paddle wheel fashion and not doing a lot for forward thrust. You also have to remember that the air density at about 25,000ft is 1/2 the air density at sea level and you need a lot more blade area and disc area at 25,000ft to transmit the same amount of power. The two speed drive was viewed as one more 'tool' in the search for higher efficiency which might not mean speed but better range. could the two speed drive offer enough better range to compensate for it's weight and cost. Say each gear box weighed an extra hundred pounds. Could a four engine plane fly further using the gear boxes than it could with an extra 400lbs of fuel on board?

Of course an advance in propeller blade design (material, shape, airfoil, etc) might make the propeller more efficient over a wider range of conditions making the two speed box an unnecessary complication.

 

[davidjgall]

Let's look at it from the standpoint of design. In particular, let's start with the cruise-flight design condition.

At high altitudes the air is thin (lower density than at sea level) and cold (lower temperature than at sea level). The first causes a loss of engine power, the second causes a reduction in the speed of sound (approx. -14% from sea level to airliner flight levels). The first can be overcome by supercharging (especially with two-speed superchargers) to "maintain" sea-level power at altitude (give-or-take). This power works against the aerodynamic drag of the airplane which is a function of dynamic pressure, that is, at higher altitude in thinner air the airplane can "go faster" (CAS/EAS) on the same power. In other words, with "given" power, TAS increases with altitude (approx. +2% per thousand feet at lower altitudes, decreasing with altitude).

The TAS increase conspires with the decreasing speed of sound at cold temperatures to manifest as a mach number increase with altitude which is disproportionate to the TAS increase alone. That is, mach number goes up faster than TAS. Jet pilots know this by (in part) the transition from IAS airspeed limitations to mach number limitations at high altitudes.

Propellers are limited to tip speeds that are less than some high subsonic mach number (e.g. 0.92M). The increased TAS at altitude increases the helical velocity of the propeller tip, but reducing the RPM of the propeller can counter that trend. Thus, low propeller RPM is important to high altitude, high speed flight. Additionally, a propeller designed for this high altitude cruise condition will have a slightly smaller diameter than a sea-level prop, again in an affort to limit the tip mach number.

So, we've "designed" a propeller optimized for high altitude, high speed flight.

However, a propeller designed for such conditions is not optimal for takeoff and climb conditions. A compromise (since variable diameter is not an option) is to allow that small-diameter high-altitude prop to spin up to near its limiting tip mach number RPM during low altitude and/or low speed operations. Since air is warmer at lower altitudes, the speed of sound is faster. Since takeoff and climb are usually conducted at speeds slower than cruise speed, the helical tip speed can't approach the limiting mach number of the "little" prop unless the RPM is allowed to be significantly higher than the high altitude cruise RPM. This may require that the engine be operated above its design RPM limits, thus a modification to the engine's (existing) reduction gear is required that will permit higher prop RPM without overspeeding the engine. Presuming that the engine we've been talking about all along already had a one-speed gearbox (optimized for the high-altitude design condition), we've just specified a two-speed gearbox.

Said differently and cast in terms of starting with existing low- to mid-altitude technology, the "normal" engine already having a gear reduction and turbo- and/or super-charging (big WWII-era 'V' or Radial engine) needs to have a slightly lower first gear ratio (less reduction since the prop will be smaller than "standard") and a second gear added that has a higher gear ratio (more redution) for high-altitude high-speed cruise operation.

Notice that at no point in the foregoing discussion was the propeller pitch mentioned, nor was any mention given to how much power the propeller absorbed. For the purposes of the foregoing it is assumed that the engine makes the same horsepower at takeoff, climb, and cruise. The constant-speed (actually, constant RPM) action of the governer+prop pitch change will act to absorb watever power that is. In fact, the takeoff power is usually more than the climb power which, in turn, is usually more than the cruise power. However, the range of power absorption capability of a propeller that is well matched to the engine and mission will encompass those power differences and we need not dwell on the issue.

Finally, we have to acknowledge that the free-turbine turboprop effectively eliminates the need for a two-speed gearbox since the power turbine can be designed to accomodate a range of operating RPMs that encompass the range of anticipated propeller RPMs between sea level and mid- to high- altitudes using a single-speed gearbox (see DH8D/Q400, C130J, TU-95, A400M) and the prop governor selects the prop RPM without concern for the internal engine RPM. Further, the advent of the jet engine killed the need for further development of two-speed gearing altogether for about 50 years, and the development of the turboprop in the interim sealed its fate. However, contrary to Piper106's speculation that it was "a bridge too far," I think it was close to operational. It wasn't needed for maritime patrol aircraft (low altitude) and it was supplanted by jets in the altitude stratum for which it was needed (B-36, B-50).

Disclaimer: Everything above could be wrong.

P.S. Why do the P-51s in the WW2Aircraft.net banner art have their flaps down?

 

[nuuumannn]

Why do the P-51s in the WW2Aircraft.net banner art have their flaps down?

Because, David, to quote a cartoon character, they were drawn that way!

Finally, we have to acknowledge that the free-turbine turboprop effectively eliminates the need for a two-speed gearbox since the power turbine can be designed to accomodate a range of operating RPMs that encompass the range of anticipated propeller RPMs between sea level and mid- to high- altitudes using a single-speed gearbox (see DH8D/Q400, C130J, TU-95, A400M) and the prop governor selects the prop RPM without concern for the internal engine RPM.

Couldn't agree more; I do line maintenance on turboprops for a living and their workings is evidence that the two speed idea was perhaps not the most efficient way of dealing with the issues.

 

[basil]

Davidjgall, your explanation really makes sense; thank you. Btw, in contrast to todayś most commercial turboprops the engine of the Tu-95 did not have a power turbine (free turbine) for the propeller drive but the gearbox was directly connected to the compressor shaft. Another example is the Garrett T76 (TPE331).