|Continued from page 1
|Rohr 2-175 (71X) Propulsion Unit Design Philosophy
|The following is a direct quotation (in blue italic print) from an internal Rohr Industries,
Inc. memo from Mike Voytish dated May 8, 1974, as was directed to Dr. Jack Edwards
and others. The entire memo is in Richard’s possession, because he was on the
Figures referred to in this memo are attached in another table below these two articles.
This notice is given again after this article.
Both Mike and Richard arrived at Rohr Industries, Inc. from the Garrett Airesearch
Manufacturing Company, located at the west end of Century Boulevard, at the entrance
to the Los Angeles International airport at the time. The reason this is mentioned is
because the propeller blades and hub attachment parallels the design and construction
of Garrett’s Ram Air Turbine (RAT) blades.
These RAT’s are still manufactured by Garrett’s successor, Honeywell International in
their Torrance, California facility (from which Richard retired at the end of July 2007).
The RAT’s are emergency bladed propeller hydraulic or electrical generators that flip out
into the airstream in the event of an aircraft hydraulic or electrical failure. Most of
Boeing's jet liners have these. Many of the US Armed Forces aircraft also have them
Additionally, Mike and Richard paid a visit to Montgomery Field in San Diego, California in
the early part of the 71X propeller development to view what the Cessna Aircraft people
had done. In viewing the aircraft that were parked along the line, it was evident that
Cessna had a hydraulic encapsulated piston forward of the propeller that moved back
and forth to change the pitch, hence, the 71X did also.
“The 71X propulsion unit can be best described as a ducted propeller, multi-bladed (5),
quiet (low tip speed – 700 ft/sec), variable pitch (forward 40 deg. and reverse 20 deg.)
mechanical pitch input with hydraulic boost, with no stored energy in the propeller and
pitch change assembly.
The propeller has been around a long time, predating aero-planes (flying, that is).
To get a better range of performance out of the propeller, a variable pitch, two position
was developed and used successfully for wide speed range engines.
With larger horsepower engines, higher altitudes, and increasing air speeds, it became
essential that a variable pitch prop be used to get the propeller efficiency over a wide
range of conditions. Several types were made and used successfully over the years.
With the advent of the turbo prop installations with hindsight, it is safe to say that the
existing governor schemes at the time were modified to be more compatible with turbine
engines. The basic logic was to use as much proven technology based on existing
designs and tooling. Therefore, it is of no great amazement to see the complexity of the
averaged turbo prop governor and control schemes.
As you know, we were requested to review existing control schemes and to try to
implement an all mechanical input into the propeller pitch change mechanism that would
be compatible with a single shaft turbine engine. After a thorough review, it appeared
obvious that the mechanical input control system could be used if supplemented with a
power boost. In the event of loss of power boost loss, it was still possible to change
pitch with mechanical force. Most current variable pitch propellers use some form of
stored energy, either springs and/or centrifugal twisting moment force, to drive the blade
pitch change in one direction and a controlled power source through a governor control
scheme to direct the pitch change mechanism in the opposite direction. With this
approach, failure of the governor or hydraulic pressure would activate the propeller pitch
change mechanism to drive the propeller into one direction by the restoring force.
Since these blade pitch changes could be uncontrolled, they could cause serious
problems on propulsion and safety of flight. Therefore, a series of stops were required to
prevent the blade from going into undesirable pitch positions. In essence, these are the
crutches for the basic design philosophy. With the advent of the single shaft turbine, the
prop blade position for zero pitch also became critical for starting. Therefore, more
complexity and another detent or stop was required.
Since no mechanical input was available on blade pitch position, a beta angle indicator
had to be implemented to give blade position to the governor in the beta feedback
Being exposed to some of the above problems and being addressed to a new design to
be compatible with a single shaft turbine engine, some of the basic design philosophy
became self evident.
1. No stored energy in the pitch change mechanism with a deliberate input required
for a blade pitch change.
2. Infinitely variable pitch desired response to a deliberate mechanical input.
3. Power boost responsive in the same direction as the command mechanical input.
4. Loss of power boost would not cause a change in blade pitch setting.
5. With loss of power boost, the blade pitch change could be accomplished
mechanically with sufficient input force.
6. Capability of setting blade pitch position mechanically without power assist when
the engine is not operating.
7. When operating, the blade pitch not to change from an external disturbance.
The basic design requirements were to provide a propeller that would be compatible with
1. Essentially a 160 HP single shaft turbine.
2. Capable of operation over an altitude range of sea level to 10,000 ft.
3. Capable of operating over an airspeed of zero to 153 knots.
4. Maximum diameter of 40 inches to work in a duct mounted integral with the airplane.
Determine and specify best duct configuration.
5. Unit to be a pusher prop with variable pitch from full forward pitch to full reverse.
6. Propeller RPM – 4000
7. Propeller spinner diameter not to exceed 15 inches at prop blade centerline.
8. Develop propulsion efficiencies for the following design points:
Takeoff - 65 knots at sl 60% plus
Climb - 90 knots at 5000 feet 70% plus
Cruise - 152 knots at 10,000 feet 80% plus
9. Try to maintain a tip speed of 700 ft/second to obtain a quiet propeller.
10. Design capable of being certified by required demonstration testing to FAA.
11. Use engine lube oil for boost, 60 psi and 1 gallon per minute.
The propeller configuration for the 71X propulsion system can best be defined by the
sketch and data of Figure 1, attached. This sketch defines the duct, the stators, the
propeller, and the blades.
The method of balancing out the blade and aerodynamic twisting moments and hence no
stored energy in the governor is shown in Figure 2.
The primary position of the blade change mechanism is shown schematically in Figure 3.
The hydraulic valving schematic which illustrates the power boost function, is
schematically shown in Figure 4.
The major elements of propeller and pitch change mechanism are shown in
The propeller interface with respect to the 71X airplane is shown on Drawing
189-4025, sheets 1 and 2.
Since the propeller was specifically designed to be compatible with the 71X single shaft
turbine engine and aerodynamic configuration of the airplane, a better understanding of
the 71X propulsion breakdown is required and can best be explained by the 5 sheets of
Figure 6. This identifies the assemblies and subassemblies of the 71X propulsor as they
are assigned to the airplane, the propeller, and/or the 71X gear box.
The predicted propeller efficiency is shown on Figure 7 with no stator vanes. This
efficiency must be corrected by the efficiency ratio due to stator vanes, Figure 8.
Some useful formulas and examples for use with the propeller for determining shaft
horsepower and advance ratio to use with the predicted efficiency curves are shown in
Figure 9 and Figure 10. If horsepower is known, then Cp can be calculated. With the Cp
and Jo known, then one can enter the predicted efficiency charts and obtain predicted
blade angle and efficiency.
A predicted actuator force-rate curve is included as Figure 11 for the 189-4001 propeller.
It is felt that the current propeller 189-4001, when fully developed, will be a satisfactory
compromise for use with a single shaft turbine engine for use on the 71X airplane at
essentially 160 shaft horsepower design points. It will also be capable of satisfying the
start, idle, taxi, and reverse thrust requirements. The general overall propulsion design
will be considerably quieter than a conventional propeller design.
A. B. Billet
R. W. Fraser
(end of quotation)
It is very sad that this propulsion system never reached the test flight 71X aircraft.
The 5 - bladed variable pitch propeller was tested in the spin chamber shown on page 1
and on the static test aircraft, but because the single shaft turbine engine resulted in
lower power than what was required, the development of a recuperator to capture more
horsepower was developed to late in the schedule, and also would have given the engine
too much added weight.
The variable pitch 5 bladed propeller, sometimes referred to as a fan, ala the “Fan-Jet”
terminology, worked very well, but was never used beyond static testing.
The key players on this propulsion variable pitch propeller, stator vanes and duct design
were Mike Voytish, Paul Ritenour, and consultant Henry V. Borst (Hank) of Henry V.Borst
The Figures mentioned in the above article are shown below in a separate Table
following the next "a Slight Extension of Life" article.
Single Shaft Turbojet Engine, a Slight Extension of Life
|Not long after it was evident that the newly designed engine would not make the required
power or weight requirements for the 71X aircraft, the very resourceful Rohr engineering
team thought they could venture to use the engine in different ways. This would at least
recover some of the development costs associated with the engineering and design of
this single shaft turbojet engine.
What was ventured beyond the 71X program involved the introduction of a small
compact static emergency electrical power generation system for small and large
businesses when city electrical supplies failed. Installations on roof tops of buildings was
a much sought after commodity at that time. Several units were manufactured and sold,
one in remembrance to a local telephone company.
One of the problems with the turbojet engine in Richard’s memory was that one of the
main bearings leaked so bad that the turbine engine was labeled as using more oil than
jet fuel. This sleeve (Babbitt style) bearing was introduced into the engine design early
on by VP Chuck Hill (C.C. Hill), who brought the design from what Richard remembers
was from the Williams Research Corporation (now Williams International), not General
Electric (GE) as is mentioned in Bill Chana’s “Over the Wing” publication.
The time period is so far back that it doesn’t really make any difference now.
It is just as well that the turbojet engine did not make it into the final 71X program as the
aircraft would have never flown.
Rohr’s CEO Bert Raynes had very good insight. He didn’t think much of the single shaft
turbine engine and ordered Walt Mooney to purchase two high rpm Lycoming engines
that were experimental at the time. The 71X flew with one of the Lycoming piston engines
and a 4- bladed wood propeller. Don Westergren retrieved the wood propeller, and the
nose wheel (as previously mentioned) before the 71X aircraft and data were destroyed
and taken to the Otay dump.
Since the Lycoming engine replaced the Rohr designed single shaft turbine in the test
flight aircraft, there was an obvious gap in length from where the interface of the turbine
engine would have been and where it now was with the Lycoming piston engine. So, an
extended shaft was designed and fabricated. That initial result gave such an unbalance
resulting in tremendous vibrations that the extended shaft had to again be redesigned.
As described in Bill Chana’s book, it was Mott Taylor’s design suggestions (he was
consulted, probably because of several extended pusher engine shaft aircraft designs
that he had done) that actually worked. Thus, the 71X successfully took to the air.
Ah, so much energy and devotion from all of our co-workers on these projects, and alas,
gone because of politics.
Fond memories of hard work and shared friendships will never die.