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Technical/Specs | Hydromatic Propeller

The Hamilton Standard Hydromatic Propeller

by John D. Cugini, Aviation Mechanics Journal, November 1988, (Used here by written permission from Hamilton Sundstrand)

Twenty-one June, 1944. Over 1,300 bomb-laden B-17 Flying Fortresses wing their way in close protective formation to this day's designated target, deep within the heartland of the Third Reich: Berlin. Every pilot, gunner, navigator, and bombardier share one common, ominous thought: it will be defended furiously.
Approaching the beleaguered city, the able AAF captain, looking for all the world like a young Jimmy Stewart, tightly grips the controls as a veritable storm of flak bursts engulf his heavy, four-engine bomber. Suddenly-BAAM! A shell explodes just off the port wing, sending shrapnel cutting through the number one engine.
Oil flies, spattering the windscreen, as dark, threatening smoke trails the wounded radial. Reflexively scanning the gauges, our pilot notes with distress the oil pressure dropping rapidly. Number one is gone. He steals a quick glance at his copilot, whose dire expression reveals thoughts completely in unison with his own.
With one engine out we lose vital airspeed; power-up the other three and we might be able to keep within the safety of the formation. If we drop behind, the 109s and 190s will pounce on us like wolves on a wounded lamb.
Only one magical event stands between us and a safe ride back to the English countryside - number one must feather. The last thing we need now is that big, three-bladed monster windmilling in the breeze, sapping our life-saving airspeed.
Hanging onto the buffeting controls, the captain shouts above the noise of the war, "Cut and feather number one." With an intensity reflecting the gravity of the situation, the feathering push-button is jammed into the panel, energizing the system and sending an emergency supply of oil to the propeller hub.
In excited anticipation, both flyers watch the triad of mighty, lumbering blades slowly knife into the slipstream in perfect, wondrous harmony. Fully feathered and offering no wind resistance, the propeller slows to a silent, gratifying halt. With sighs of relief, glances are exchanged along a common thread - we just might make it after all.

Hamilton Standard 500,000 props, eBay.jpg (43349 bytes)Repeated countless times during the course of the war, Hamilton Standard's unique, but simple, Hydromatic design can undoubtedly be credited with saving hundreds of aircrews, aircraft and engines - a grand achievement.

But the Hydromatic wasn't created to save man and machine, that was just an added benefit, albeit a very noble one. According to Hamilton Standard, it was originally designed for airplanes with engines from approximately 450 to 4,500 hp.

Its revolutionary constant-speed feature helped such famous aircraft as the Boeing B-29 bomber and the Grumman Hellcat span the vast distances of the Pacific Ocean during the Second World War, lending U.S. aircraft a decided edge over opposing Japanese forces.

In fact, the Hydromatic, in both its three- and four-bladed versions, powered more military aircraft than any other comparable propeller. Just what made this wonder-prop so darn good? Simple, basic, common sense engineering.

Hydromatic propellers are composed of four major assemblies: the hub, blades, dome and engine shaft extension (non-feathering single-acting, or non-reversing double-acting installation) or distributor valve (feathering single-acting installations) or stop lever assembly (reversing installations).

Hydromatic Schematic - (Copyright Hamilton Sundstrand, used with written permission)

The terms "single-acting" and "double-acting" refer to the propeller's operational characteristics. In the single-acting system, governor oil is metered to only one side of the piston, and engine oil delivered to the other side of the piston.

In the double-acting system, governor oil is metered to both sides of the propeller piston. The back and forth motion of the piston changes the pitch of the blades.

The hub and blade assemblies consist of three major parts, which are the spider, the barrel and the blades. The spider may be considered as the foundation for the entire propeller. Its central bore is splined to fit the engine shaft, and it is through these splines that the engine's torque is transmitted to the propeller.

It is equipped at either end with a precision ground cone seat and at its outer end provision is made for the propeller retaining nut and front cone by means of which the spider is attached rigidly to the engine shaft. Integral spider arms with two bearings on each arm support the blades, taking the greater part of the thrust and torque loads from the blades.

The barrel is supported on the spider by means of phenolic blocks located between the spider arms. Shoulders, machined in the barrel, take the centrifugal loads from the blades, which are transmitted to the barrel through heavy-duty roller bearings.

Chevron-type oil seals are used between the blades and the barrel and between the spider and the barrel. The barrel thus forms an oil tight casing, which houses the entire hub mechanism and provides support for the attachment of the dome unit.

The dome assembly contains the pitch changing mechanism by means of which oil forces on a double-acting piston are translated into blade twisting movements. It consists of four major parts, namely two cylindrical co-axial cams forged from high-strength alloy steel, a double-walled piston and a dome cylinder, which also serves as a housing for the entire unit. The piston and cylinder are machined from aluminum alloy forgings.

When the dome unit is installed in the hub assembly, the outer or stationary cam becomes rigidly fixed in the barrel and provides support for the remaining parts of the dome unit.

The inner or rotating cam, with which the main drive gear is integral, is supported within the stationary cam by means of roller bearings, which take the gear reactions and piston oil forces. The piston motion is transmitted to the rotating cam by means of four sets of cam rollers carried on shafts, supported by the inner and outer walls of the piston.

This construction provides a very rugged, simple, pressure lubricated mechanism that can be quickly assembled and disassembled and is easily attached to the propeller by a single retaining nut.

The distributor valve assembly basically controls the pitch changing motion of the blades. During constant speed operation of the propeller, it provides a passage through which engine oil, boosted in pressure and metered by the governor, is led to or from the inboard side of the piston and a passage through which oil, under engine pressure, is conducted to or from the outboard side of the cylinder.

During feathering, the same two passages provide means for delivering high oil pressure (from the auxiliary pressure system) to the inboard side of the piston, and a means of leading oil from the outboard end of the cylinder to the engine lubricating system.

The pressure differential that exists across the piston moves it toward the outboard end of the cylinder and feathers the blades. Thus, during constant speed operation or feathering, there is no movement of the distributor valve, and the assembly merely provides passages through which oil may flow to and from the cylinder.

In unfeathering, the function of the distributor valve is to reverse the above-mentioned passages. The high pressure oil from the auxiliary system is then led to the outboard side of the piston, and the inboard side is connected to the engine lubricating system, thus reversing the pressure differential and moving the piston toward the inboard end of the cylinder in order to unfeather the blades.

The hydromatic propeller blades themselves are manufactured from high-strength aluminum alloy forgings and are of semi-hollow construction. This design allows the use of thin, solid tips of high aerodynamic efficiency and a hollow, upset shank for attaching the blade to the hub.

The upset shank incorporates as integral aluminum-bronze bushing, which supports the blade on the spider arm, and transmits the blade thrust and torque loads to the spider. The roller bearing assembly, which transmits the centrifugal load to the barrel, consists of two steel races that are not removable from the blade and a split type bearing retainer, which incorporates roller bearings of special design to carry large forces with a minimum of friction.

The all-important piston is an aluminum forging, machined to very close tolerances and independently balanced. It is the medium by which oil pressure forces actuate the cams to turn the propeller blades to their various pitch settings.

Auxiliary Pressure

During constant speed operation, the control forces required to actuate the pitch-changing mechanism are derived entirely from dynamic twisting moments of the blades and from governor and engine oil pressures. No part is played by the auxiliary pressure system.

Feathering and unfeathering, on the other hand, require a source of energy of the engine, since the forces used during constant speed operation are not available when the propeller has been feathered and the engine stopped. For this independent source of energy, an auxiliary pump is employed to provide oil under higher pressures than those required for constant speed operation.

Application of this high-pressure oil to the propeller for feathering automatically disconnects the governor from the system. Discontinuance of the high-pressure oil, during unfeathering, automatically returns the propeller to the control of the governor.

There are several types of auxiliary pressure systems that may be used to operate the feathering feature:

  1. The individual electrical motor-driven pump system in which a motor pump unit, installed in each engine nacelle, is used to supply oil from the engine tank to the propeller.
  2. The central pump system in which a single pump unit is used to supply low viscosity oil, through a selector valve, to any of the propellers.
  3. The airplane's hydraulic system where advantage is taken of the high pressure oil system already available in many aircraft; the low viscosity oil being delivered through a selector valve, or individual valves, to the propellers.

There is an interesting footnote concerning the Hydromatic's feathering system used in the Boeing B-17G bomber of the second world war. It seems no matter how sophisticated the feathering system, a prop just won't feather without the oil to do it. If an engine-propeller system was shot out and its oil supply lost, lack of propeller control and deadly windmilling resulted.

The remedy? The oil supply tanks were equipped with simple standpipes to hold a reserve of oil, enough to feather the dead engine's propeller. The Hamilton Standard Hydromatic propeller has, over the past 50 years, seen service on aircraft of legendary proportions. In truth, it has helped make them so.

The list is impressive. The P-51 Mustang, P-47 Thunderbolt, Grumman Hellcat, Vought F4U Corsair, B-17, B-29, Consolidated B-24, the British Mosquito, and the DC-3/C-47 Gooney Bird, and the list continues on.

Well documented is the Hydromatic's reliability and ability to withstand copious amounts of abuse. There are numerous accounts of combat aircraft flying so close to the ocean's surface that their propeller blades struck and churned through the water only to be slightly bent and subsequently returned to service.

There is also an almost unbelievable story reported in the July 20, 1944 edition of the Hamilton Standard Blade newspaper concerning a 29-year old P-51 pilot who, upon strafing a Nazi airport, flew so low as to plow through about 30 yards of enemy airdrome.

Major George L. Merritt Jr. reported, "With a terrific noise and vibration that like to jar my teeth out, I nursed my aircraft home 600 miles to my Eighth AAF Fighter Station in England".

Hamilton Standard even received special commendation from the British Air Commission stating that the Air Ministry was highly pleased with the exceptional performance of the Hydromatic propellers used on their heavy four-engine Lancaster bombers utilized in the bombing campaign over Nazi Germany.

Over 50 years after the Hydromatic's incorporation into the aviation industry, we tend to take for granted its exceptional abilities. We have benefited from its good performance, maintainability, reliability and cost-effectiveness for so long that it seems rather run-of-the-mill.

To really appreciate that gem of a propeller, we must realize what was around before its inception. Only then can we absorb the fact that the Hamilton Standard Hydromatic was light years ahead of its time.

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Last modified: September 18, 2012

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