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On the Development of a Suite of Rotary Engines to Power UAVs
Ed Greutert, P.E.*
An estimated 100,000 small and mid-sized unmanned aerial vehicles (UAV) are expected
to be built and incorporated into the National Airspace System (NAS) in the next decade
and beyond. This paper proposes a propulsion system concept utilizing the Wankel rotary
engine to power some of these aircraft. Due to the rotary engines high power to weight
ratio, its smooth operation, simplicity in design and operation, versatility in applications
for various aircraft and missions, and other desirable attributes, the rotary engine is an ideal
candidate for many UAV propulsion systems. These features, along with unprecedented
scalability and modularity, could provide designers and builders with a unique opportunity
to power a wide variety of UAVs using a small inventory of parts that potentially could
significantly reduce UAV engine development costs and provide the industry with a wide
variety of inexpensive and suitable propulsion systems to power these UAVs. One of the
biggest obstacles preventing widespread adoption of the rotary engine for UAVs is
considered to be problems associated with the rotor apex, corner, and side seals. A potential
funding model is discussed to focus research on solving the outstanding problems
associated with the seals.
INTRODUCTION
Worldwide demand for unmanned aerial vehicles (UAV) is on the rise. It has been estimated that
governmental and commercial entities within 57 countries own and operate UAVs1. The U.S. Military
currently owns close to 15,000 UAVs and the military continues to increase the number of UAVs in
their portfolio2. RTCA estimates the number of military, civil, and commercial UAVs in the U.S.
national airspace system (NAS) to be as high as 50,000 by 20402. However, in accounting for small
UAVs, such as small multirotors and fixed wing aircraft, this number may have already been exceeded.5
Although it is difficult to predict how many UAVs will be in use by military, civil government, and
commercial entities around the world by 2040, it is easy to speculate that the number will exceed
100,000 in the U.S. alone.
Micro, small and mid-sized UAVs are expected to make up the majority of UAVs built and operated
in the coming years, with majority of the fleet of military, civil agency, and commercial aircraft being
micro UAVs, small UAVs (sUAV) and mid-size UAVs. There is currently no universally accepted
system for categorizing UAVs based on size. Micro UAVs are generally defined as having a maximum
dimension of 2.5 feet or less and weighing less than 0.5 pound. sUAVs are generally considered those
that have a gross weight of between 1 and 55 pounds, while mid-sized UAVs generally range from 56
pounds to 1,000 pounds. Large UAVs are generally considered to be those that exceed 1,000 pounds4.
However, there is a lot of discretion used within industry regarding the classification. Currently, the
majority of the heavier sUAVs, mid-sized, and large UAVs are powered by an internal combustion (IC)
engines. Generally micro UAVs and smaller sUAVs utilize an electric engine or propulsion system
other than an IC engine.
With an expected growth of UAVs over the coming years, it’s important at this time to reconsider
the appropriate propulsion system for those UAVs. Although they may be powered by fuel cells or
other developing technologies at some point, an obvious choice for a significant number of UAVs for
the foreseeable future will continue to be the IC engine. Currently a wide variety of IC engines are
used for UAV propulsion including 2- and 4-stroke piston engines, rotary engines, turbine jet and
turbine shaft engines that run on gasoline, heavy fuels, and other hydrocarbons.
____________________________
*Ed Greutert, P.E., Booz Allen Hamilton, 720 Olive Way, Suite 1250, Seattle, WA 98101, [email protected], 206-794-7526
2
It would be most efficient if a standardized engine suite could be designed, and accordingly scaled,
to meet the demand and wide variety of propulsion needs of these UAVs. This paper proposes a process
to develop an IC engine design based on the Wankel rotary engine. It proposes a potential funding
mechanism to address historical challenges associated with the rotary engine, specifically; the apex,
corner, and side seals.
THE WANKEL ROTARY ENGINE
There are a wide variety of IC engines. Currently the most common IC engine for UAVs is the
piston engine which relies on reciprocating pistons to convert heat into mechanical energy. In contrast,
the Wankel rotary (rotary) engine relies on rotational mechanical energy. There are no valves, lifters,
cams, or other moving parts typical of a piston engine.
This paper is not intended to address in detail how a rotary engine operates. However, because a
basic knowledge of the rotary engine is necessary to understand both the benefits of using the rotary
engine for UAVs as well as current challenges with the engine, this section provides a brief overview
of rotary engine mechanics.
The rotor relies on a triangular shaped rotor rotating within an epitrochoid (somewhat oval shaped)
housing (stator) and an eccentric shaft to convert heat into mechanical energy. The primary rotary
engine parts are presented in Figures 1 and 2 below.5
Figure 1. Wankel Rotor and Stator Figure 2. Wankel Rotary Engine Eccentric Shaft
The shaft runs through the center of the rotor and the stator is enclosed by two end plates on either
side of the rotor. The rotor rotates about its center of gravity while at the same time rotates around the
eccentric shafts centerline. The eccentric shaft rotates three times for each 360 degree rotation of the
rotor and the eccentric shaft is driven by a gear mounted on the shaft. It may not appear obvious in the
photograph, but the rotor (equilateral triangle) and epitrochoid shaped stator geometry are such that the
rotor will rotate about the eccentric shaft while maintaining continuous contact with the stator at each
of the three apexes on the rotor. As the rotor rotates, the three chambers within the stator are
compressed and expanded. Fuel and air are injected into the chamber as it expands, the air/fuel mixture
is ignited with a spark plug once it is compressed, the power stroke is the resulting expansion, followed
by a compression cycle which exhausts the combustion gases.
The rotary engine has the unique capability of executing the full 4-stroke cycle with one rotation
of the rotor and because of the 3:1 ratio of the shaft rotation to the rotor, there is one power stroke for
3
each revolution of the eccentric shaft. The video below by Rittman6 graphically demonstrates how the
rotary engine operates and highlights its simplicity and modularity. This video can be viewed at:
http://www.youtube.com/watch?v=6BCgl2uumlI
Figure 3 compares the functions of the 4-stroke piston engine to the rotary engine. Although this
illustration is provided as background of how the rotary engine operates, it also provides context
regarding later discussion of simplicity and modularity of the rotary engine.
.
Figure 3 - Comparison of the 4-Stroke Piston Engine to the Rotary Engine7
As discussed later in the paper, it is the simplicity and modularity of the rotary engine that provides
an opportunity for the application of the engine compared to the piston engine.
To date, the rotary engine has been used for a variety of applications including automobiles,
agricultural equipment, snow mobiles, racing engines, and UAVs, to name a few.
Current rotary engine gasoline configurations in these various applications generally use regular
gasoline. However, there are multiple heavy fuel engines (HFE) that have been developed based on
the rotary engine design8, 9 and there is significant interest in the aviation community to further develop
the technology. However, these efforts related to HFE design will not be discussed in any detail in this
paper for reasons to be presented later.
SUITABILITY OF ROTARY ENGINES FOR UAV APPLICATIONS
Rotary engines offer many advantages for UAV applications compared to reciprocating piston
engines. Advantages include higher power density, smooth operation, low vibration, simple design,
compact size, light weight, fewer moving parts. Although the rotary engine is a 4-stroke IC engine, it
has a thrust to weight ratio that far exceeds most 4-stroke and 2-stroke piston engines. In addition,
rotary engines operate best at higher RPMs and when operated continuously, which is consistent with
many UAV applications.
4
These qualities make the rotary engine more suitable for turning the propeller on fixed wing aircraft
and turning a rotor blade on vertical take-off and landing (VTOL) aircraft. Examples include:
Fixed Wing
Probably the first aircraft to be powered by a rotary engine was a Cessna Cardinal (Figure 4 and 5)
followed by a Lockheed QStar (Wright 2014).
Figure 4 - Wright Aeronautical (Wankel) RC2-60 Rotary Engine10 and Figure 5 - a Cessna
Cardinal11
The rotary engine is also currently used on several other UAVs including:
Shadow 200
Harpy
Hermes
Searcher
VTOL Application
Although only two were ever built, the Sikorsky Cypher II was a VTOL aircraft that successfully
flew 550 test flights before the concept was scrapped (Cypher 2014). It was powered by a 53 hp rotary
engine.
Figure 6 - Sikorsky Cypher II12
The RC2-60 engine was later used to power a Hughes TH-55 helicopter.
5
MODULARITY AND SCALABILITY OF THE ROTARY ENGINE
Another feature of the rotary engine is that it is comparatively easy to stack additional identical
rotor modules to produce 2, 3, or 4 rotor systems compared to a piston engine. This provides a way to
essentially double, triple, or quadruple the horsepower for a given rotor size. Although this will require
a different eccentric shaft for each additional rotor, the rotor and stator would be essentially identical
and would be stacked to increase the displacement and power of the engine. All other parts remain
largely the same. Automobile maker, Mazda, was quite successful in manufacturing such an engine
based on this concept, most recently with the three rotor RX-8 series of automobile engines. Rotron
has also developed a UAV rotary engine that uses this arrangement. It consists of a single rotor 31 hp
engine, or a dual rotor 56 hp engine that uses the same rotor and stator as the single rotor engine. Figure
8 presents the Rotron 1-rotor and 2-rotor solution respectively.
Figure 8 – Rotron One and Two Rotor Engine13
This provides a mechanism to potentially build a suite of high power engines with high power to
weight ratios that cover a wide range of horsepower requirements using a relatively small number of
parts.
The number of moving parts in the rotary engine can be estimated by Equation (1):
Number of moving parts in a rotary engine = Nr = 1 + r (1)
Where r is equal to the number of rotors in the engine. So, a single rotor engine will have two
moving parts and a 4 rotor engine will have five moving parts. A reciprocating piston engine has far
more moving parts and the actual number varies widely between manufacturers and engine types. For
a 4-stroke two piston engine, the engine will likely have on the order 15 to 25 moving parts and a 4
piston engine may have 40 or more moving parts. A 2-stroke piston engine fares much better in this
regard. In its simplest form, a 2-stroke single piston engine has roughly 4 moving parts (piston head,
connecting arm, valve, and crankshaft) with 3 additional moving parts for each additional piston. For
the purpose of this analogy we will compare the rotary engine to the 2-stroke reciprocating piston
engine. We can conservatively estimate the minimum number of moving parts in a 2-stroke piston
engine using Equation (2):
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Number of moving parts in a piston engine = Np = 1 + 3p (2)
Where p is the number of pistons in the engine. So, a single piston 2-stroke engine would have 4
moving parts and a 4 piston 2-stroke engine would have 13 moving parts. What this calculation does
not show is that the 2-stroke piston engine also requires a new engine block and a new crankshaft for
each added piston. These are significant costs items that do not burden the rotary engine and are in
addition to the increased moving parts count for the 2-stroke piston engine. Table 1 summarizes the
moving parts count for a 1, 2, and 3 rotor and a 2-stroke piston engine.
Table 1 - Moving Parts for a 1, 2, and 3 Rotor and Piston Engines
Number of Rotors/Pistons Number of Moving Parts
(Rotary)
Number of Moving Parts (2-
Stroke Piston)
1 2 4
2 3 7
3 4 10
Comments
Each additional rotor requires a
new and unique eccentric shaft
Each additional piston requires
a new and unique engine block
and unique crankshaft
Table 1 illustrates that by utilizing the rotary engine, it is possible to build three separate engines
with single, double, and triple horsepower ratings out of a single sized rotor and stator and 3 different
eccentric shafts with a total of 4 unique moving parts (one rotor and 3 eccentric shafts).
If we take the concept one step further and envision 4 different size rotors, each capable of being
utilized to build a 1, 2, or 3 rotor engine, it would be possible to design a suite of 12 engines with a total
of 16 unique moving parts. The suite of engines could conceivably be designed to cover a very wide
range of power requirements and be suitable for a wide range of aircraft of different sizes and weights.
Table 2 presents a summary of such an engine design concept. It includes examples of horsepower
ratings and displacements based on 1, 2, and 3 rotor engine designs for 4 sizes of rotors. No attempt
was made to optimize the rotor sizes and power ratings for real world applications. The purpose of the
table is to illustrate the modularity and simplicity of the design concept in addition to the range of
horsepower that could be developed based on a relatively small number of unique parts. Examples of
existing UAVs with similar engine power ratings have been included to help provide the reader with
an idea of the range in size and type of existing aircraft that the engine suite might be suitable for. In
addition, it also provides examples of existing rotary engine manufactures that currently provide
comparably powered rotary engines in the respective power range.
7
Table 2 – Rotary Engine Design Concept for Engine Suite
Rotor
Displacement
Per Revolution
(cc)
Number of
Rotors
Calculated hp
@6,700 RPM14
UAV Utilizing
Similar Engine
Power
Examples of UAV
Rotary Engine
Manufacturers in
the Same Power
Class
Engine Size
1
33.33 cc
Rotor/Stator
33.33 1 3.67 ScanEagle Barnard
Microsystems
66.67 2 7.35 Cubewano
100.00 3 11.02 Cubewano
Engine Size
2
83.5 cc
Rotor/Stator
83.5 1 9.2 Cubewano
167 2 18.4 Cubewano
250.5 3 27.6 RQ-2A Pioneer Rotron
Engine Size
3
210 cc
Rotor/Stator
210 1 23.1 Cubewano
420 2 46.2 RQ-7B Shadow*
Elbit Hermes 450**
IAI Harpy***
Rotron
630 3 69.3 General Atomics
GNAT
RQ-5A Hunter
Rotron
Engine Size
4
525 cc
Rotor/Stator
525 1 57.9 Rotron
1050 2 115.7 Predator MQ-1B
EADS Harfang
1575 3 173.6
*Currently uses 38 hp rotary engine
**Currently uses 52 hp rotary engine
***Currently uses 38 hp rotary engine
Table 2 was not developed to suggest that those aircraft listed that do not currently utilize rotary
engines would be better served with a rotary engine, it is simply to give the reader an idea of the range
of aircraft sizes and weights that could be conceivably powered by the a rotary engine suite.
Note the intentional overlap in the calculated horsepower between the rotor displacement sizes.
This was done in order to provide the flexibility and advantages that more rotors provide. As the rotor
count increases, 2 and 3 rotor engines offer smoother operation, higher torque, and can achieve more
power at lower RPMs, which may be advantageous for certain applications. The overlap can easily be
8
adjusted to provide more options within the highest density range of engine utilization based on a
thorough market study of rotary engine applications for UAVs.
DISADVANTAGES OF ROTARY ENGINES
The main disadvantage of the rotary engine compared with the piston engine is inefficiency. The
inefficiency is due in large part to leaky apex, corner, and side seals on the rotors which cause a
reduction in the compression ratio and incomplete combustion. This contributes to increased
maintenance and increased pollutants related issues compared with the piston engine.
In addition, the seal problem has been a challenge for manufacturers to overcome as rotary engines
have been modified to utilize heavy fuels. Heavy fuel engines (HFE) utilize diesel, jet fuel, and other
longer chain hydrocarbon fuels and are of keen interest, especially to DOD. Although rotary HFE
engines show great promise, they are not likely to gain wide acceptance until the sealing and other
problems are resolved. Regular fuel and HFE rotary engines also require additional engineering
improvements related to combustion chamber optimization, fuel injection, spark or glow plug
improvements, exhaust port improvements, issues related to heat loss, main bearing wear, fuel
economy, time between overhauls (TBO) and use of super and turbo chargers15. Many of these issues
are impacted in some way by the apex, corner, and side seal problem. That problem needs to be solved
for gasoline engines first and foremost. Figure 9 presents a diagram that includes the location of the
apex, corner, and side seals for a typical rotor.
Figure 9 – Apex, Corner, and Side Seal Locations16
HOMING IN ON THE APEX, CORNER, AND SIDE SEAL PROBLEM
At first glance, the seal problem is straight forward. However, many companies have worked on
the problem over several decades with mixed success, although incremental progress has been made.
Mazda manufactured the rotary engine for use in certain vehicles from 1967 to 2012 and probably had
the most success solving the seal problems on automobile sized engines. Although Mazda’s problems
9
with the seals plagued the rotary engine in the early days of production, it was largely solved for their
automobile production engine by the time Mazda ceased manufacturing it in 2012.
UAV engines are, in general, smaller than the automobile engine. Unfortunately, this exacerbates
the seal problem compared to automobile sized engines. As the rotary engine gets smaller, the seal
problem and associated issues generally become more significant. This results in lower efficiency and
decreased time between overhauls, among other issues.
Government agencies, such as NASA17, have funded extensive studies on the rotary engine and
looked at areas for improvement. They have also zeroed in on the seal problem.
While there have been significant advances made regarding the seals, the problem is far from
solved, especially for the smaller engines. There are many rotary engine manufactures domestically
and world-wide and there remains a keen interest within the aviation community regarding the rotary
engine for UAV applications. In general, the aviation community considers the seal problem one that
needs to be resolved before the rotary engine can fulfill its potential in the propulsion system
community.
CURRENT MODEL FOR SOLVING THE SEAL PROBLEM
There are many rotary engine manufacturers that continue to work on the seal problem
independently. Naturally, they use their own rotor/stator/shaft combinations to work on issues related
to the seals along with other areas of the engine where they feel there is room for improvement.
Currently each vendor is left to its own devices to develop a seal for their rotary engine. In general,
this model to conduct research has served industry well, it promotes innovation within industry, rewards
those that are successful, and leads to rapid technological advancement. Either a vendor places a bet
by investing in a new technology solution that they believe will allow them to recoup their costs and
make a profit after they are successful, or alternatively, an eager customer may decide to fund the
technology development themselves because no vendor would take the financial risk of developing the
solution on their own.
POTENTIAL FUNDING MODEL TO SOLVE THE SEAL PROBLEM
The seal problem associated with the rotary engine has several unique aspects that might support
an alternative strategy to solving the problem.
Persistence. The seal problem is a decades old problem. Although progress has been
made, the problem is far from solved. It is exacerbated in smaller engines whose size
would serve the UAV industry well. Traditional approaches to solving the problem have
had limited success.
Isolated challenge. The apex, corner, and side seal problem is straight forward to
understand and is an isolated problem. If one solves the seal problem, one solves several
key shortcomings associated with the engine and provides a path forward for optimizing
other aspects of the engine that are hindered because of the seal problem.
Reward. If the seal problem is solved, not only could it make engine concepts proposed
in this paper a reality for UAV applications, it would also be beneficial to other industries
and vehicles that could benefit from utilizing the rotary engine.
Due to the limited success of previous research efforts to solve the seal problem, and the potential
benefits to manufacturers and industry if the problem is solved, perhaps there is an opportunity for an
alternative approach to solving the seal problem. Ideally, a strategy that isolates the seal issue and
focuses the attention of various stakeholders (industry, government, and researchers) on the seal issue,
10
and not on the development and advancement of other aspects of the engine might be in order.
Essentially, this approach would hold other technical aspects of the engine constant for participating
researchers and allow the results of seal research efforts to be more directly comparable to seal research
efforts than they otherwise might be.
Such an effort might consist of the following elements:
1. Identify an entity to sponsor the seal research development effort. Ideally this would be
government entity such as NASA, DOD, DOE, or other federal agency, but could also be
an industry consortium or group.
2. The sponsoring agency would convene a forum of rotary engine experts and consolidate
results from a literature search on the topic. The forum would propose a base model
production rotary engine that was sized to be representative of UAV aircraft rotary engines.
This would be an off the shelf engine selected for size, cost, operating and performance
history, and suitability to have seal testing performed.
3. Solicit proposals from industry, government agencies, and the research community to
prepare cost and technical proposals for the development and testing of new seal designs
that would be tested in the base rotary engine. Proposals would be limited to seal research
efforts to be conducted on the base engine with any other parts substitutions discouraged.
Alterations to the rotor and seals would be encouraged. Demonstrators would be required
to share their results with the public. Results would compare the modified performance
results to the base engine performance and operating history.
4. Multiple contracts would potentially be awarded. The most successful resulting seal
designs would be peer reviewed and additional optional experiments and testing could be
conducted and results shared.
5. Any technology developed and experimental results from the program would be available
to government and industry to use as they saw fit to benefit their own engine performance.
Ideally by focusing the engine community specifically on the seal problem this strategy
would promote the break though technology that the rotary engine and industry need to
realize the full potential of the rotary engine concept.
Each of these steps is discussed in more details below.
Identify an Entity to Sponsor the Development Effort
From a government agency perspective, NASA and DOD may stand to gain the most from the
advancement of the seal technology. Both have a history of interest in the rotary engine.15, 17
Commercial industry also stands to benefit from the technology development. However, success of the
rotary engine will at least to some extent, be at the expense of the reduced number of piston engines
that the rotary engines would be replacing. These are competing issues that may produce a conflicting
dynamic in the commercial space. In addition, commercial industry may not provide the objectivity in
decision making that a government sponsor might. Because of the potential application for the rotary
engine on land, sea, and maritime vehicles, DOD would stand to gain significantly from a rotary engine
technology breakthrough. Even if DOD did not sponsor the research, they would almost certainly have
representation on the forum or governing body of the effort. The same can be said for NASA.
Convene a Forum of Rotary Engine Experts and Consolidate Results from a Literature Search
on the Topic
The sponsoring entity would identify the leading rotary engine experts from government, research,
and industry and form a panel. In addition, a literature search would be conducted to identify the
relevant research on rotary engines, specifically on the topic of seals. This would include a market
11
assessment of the state of the seal technology. There are many specialty applications for the motors
and seals. Currently rotary engines are being made for a wide array of specialty applications including:
Automobiles
Aircraft
Maritime
Other/recreational vehicles
The market assessment would cover each of these areas.
Subsequently, the forum would identify a base motor system that would serve as the base model
for the research program. The purpose of proposing a base motor system would be to discourage
research and development on other parts of the engine not related to this effort, and focus the research
or the apex, corner, and side seals. This would provide a uniform platform against which all research
efforts could be evaluated. Performance, seal wear, and maintenance information would be compiled
for the base motor system under a variety of operating conditions and provided to interested parties.
Ideally the base motor system would have specifications similar to what would be expected for UAV
applications and have an established operating history and supporting data.
Solicit Proposals from Industry, Government Agencies, and the Research Community
A request for proposals (RFP) would be released that describes the purpose of the research program
and solicits an offer from any interested party. There are many ways to structure the contract, but one
method would be to use a time and materials contract with optional tasks that could be executed after
completing certain milestones. The proposal evaluation team would evaluate the proposals based on a
number of criteria such as technical merit, likelihood of success, technical contribution towards solving
the seal problem, cost reasonableness, etc. This would allow the team to further focus the research
effort through selective technical evaluation into areas deemed most important. At key phases in the
contract cycle the sponsoring agency would evaluate progress to date and award optional tasks to
continue the work, or end the effort by closing out the individual contract. For example, Phase 1 of all
contracts might consist of the development of a detailed test protocol. Phase 2 might be conducting the
research and providing a report, and Phase 3 might include the preparation of an additional proposal to
conduct research in an area of interest as a result of the original experiment.
At the conclusion of the project, all research would be made available to the public and could be
used by anyone with an interest for any purpose.
Seal Design Improvements Would Be Peer Reviewed
A summary of the results of the research effort would be peer reviewed by the sponsoring agency
forum and additional areas of potential beneficial research would be identified. The sponsor would
have the option of conducting a further round of focused research using the existing model, applying
some other model to promote the research effort, or do nothing and let market conditions dictate future
research, areas of interest, and level of effort in much the same way it is done now. Regardless, the
sponsor would generate a final report summarizing the research effort and results and advancements
that had been made as a result of the research program.
Technology Developed as Part of the Program Would be Available to the Public
Making the results of the research available to the publics would allow industry to benefit from not
only the original literary search and summary of issues related to the seals, but also the results of the
new research that had been conducted. The vendor community would then be able to take the new
technology back and apply it towards their own engine designs with the intent that it would help all
manufactures make a quantum leap in solving, or at least improving, rotary engine seal performance.
12
In the long run, this not only helps industry, but also helps customers improve the effectiveness of their
missions. This would help promote the full engineering development of the rotary engine technology
that is to some extent stalled by the persistent and stubborn problem associated with the seals.
CONCLUSION
The Wankel rotary engine exhibits several attributes that make it a desirable option for UAVs and
several other vehicle propulsion applications. These attributes include a very high thrust to weight ratio
compared to other IC engines, compact size, light weight, comparatively small parts count (especially
compared to piston engines), and low vibration, which are all sought after features for aircraft
propulsion units and other engine applications.
Another attractive feature is the simplicity and modularity of the engine design which provides the
unique opportunity to build a suite of COTS rotary engines that could be used to provide propulsion
systems to a wide variety of UAVs, other aircraft, maritime systems, and other vehicles. This engine
suite could be built from a relatively small pool of unique parts compared to piston engines, and at least
in theory, for a lower cost.
The primary disadvantage the rotary engines that prevents it from realizing its useful potential is
the stubborn apex, corner, and side seal problem. If this problem can be solved it would provide the
opportunity to further advance the technology so that the rotary engine can be developed to its full
potential which would benefit manufactures and customers. Due to the persistence of the seal problem,
the unique opportunity to build and exploit a suite of rotary engines, and the potential reward associated
with solving the seal problem, an alternative to isolating the seal problem and utilizing a creative
contracting and research mechanism to focus and fund the development of the seal problem is in order.
Allowing market forces alone to solve the problem has achieved only limited success and a new
approach could potentially be more cost effective and efficient. Without a change in approach, the
rotary engine concept may continue to languish and never achieve its full potential.
ACKNOWLEDGEMENTS
The author would like to that Jeff Radcliffe at Northwest UAV for guidance and technical input
during the development of this paper.
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