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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, greutert_ed@bah.com, 206-794-7526

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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

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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.

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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.

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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.

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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

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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

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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,

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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

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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.

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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.

REFERENCES

1. Aerospace America (July/August 2013), UAV Round Up 2013, retrieved 10 March 2014,

from: http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=14&cad=rja&uact

=8&ved=0CDQFjADOAo&url=http%3A%2F%2Fwww.aerospaceamerica.org%2FDocuments%

2FAerospaceAmerica-PDFs-2013%2FJuly-August-2013%2FUAVRoundup2013t-AA-Jul-

Aug2013.pdf&ei=SNskU_GRNsrkoASloIDoAw&usg=AFQjCNH5xhQI7XsrhDmmpbQJlRceRh

W4gQ&bvm=bv.62922401,d.cGU

2. RTCA, Operational Services and Environmental Definitions (OSED) for Unmanned

Aircraft Systems (UAS), Section 6.5.4, Table 5, RTCA Inc., Washington, D.C.,

June 2010.

3. Forbes, Drone Sales Soar Past $16 Million on eBay, retrieved 23 March 2015 from:

http://www.forbes.com/sites/frankbi/2015/01/28/drone-sales-soar-past-16-million-

on-ebay/

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4. Gundlach, Jay, Designing Unmanned Aircraft Systems, Section 2.1 – 2.10, American

Institute of Aeronautics and Astronautics, Education Series, Reston, VA, 2012.

5. FAA. Unmanned Aircraft Propulsion Technology Survey, Washington D.C., Federal

Aviation Administration, DOT/FAA/AR-09/11.

6. Rittman, Max., The Rotary Combustion Engine, video (2008), retrieved 13 March 2014,

from:

http://www.youtube.com/watch?v=6BCgl2uumlI

7. Gas2.org, The Best Rotary Engine Vehicles You Can Buy, retrieved 23 March 2015 from:

http://gas2.org/2013/08/12/the-best-rotary-engine-vehicles-you-can-buy/

8. Cubewano Inc., Cubewano Heavy Fuel Engines, retrieved 23 March 2015 from:

http://cubewano.com/products/engines/

9. Rotron Power Ltd., Rotron Heavy Fuel Engines, retrieved 23 March 2015 from:

http://www.rotronuav.com/engines/rt-300hfe

10. Smithsonian National Air and Space Museum. Wright Aeronautical (Wankel) RC2-60

Rotary Engine, retrieved 23 March 2015 from:

http://airandspace.si.edu/collections/artifact.cfm?id=A19870228000

11. Wikipedia.org, Cessna 177 Cardinal, retrieved 23 March 2015 from:

http://en.wikipedia.org/wiki/Cessna_177_Cardinal

12. Wikipedia.org, Cypher-UAV, retrieved 15 March 2014, from

http://en.wikipedia.org/wiki/File:Cypher-UAV.JPG

13. Rotron, Rotron Rotary UAV Engines, retrieved 23 March 2015 from

http://www.rotronuav.com/engines/rt-300

http://www.rotronuav.com/engines/rt-600

14. HPWizard, Estimating Engine Power, retrieved 15 March 2014, from:

http://hpwizard.com/engine-horsepower-calculator.html

15. Kweon, Chol-Bum M., A Review of Heavy-Fueled Rotary Engine Combustion

Technologies, Army Research Laboratory (ARL), ARL-TR-5546, Aberdeen

Proving Ground, MD, May 2011.

16. Adapted from Greencarcongress.com, Mazda to Display Hydrogen Rotary Engine-

Hybrid Concept at Tokyo Show, retrieved 23 March 2015 from:

http://www.greencarcongress.com/2005/10/mazda_to_displa.html

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17. Phillip R. Meng, William F. Hady, and Richard F. Barrows, An Overview of the NASA

Rotary Engine Research Program, National Aeronautics and Space

Administration (NASA), NASA Technical Memorandum 83699, Lewis Research

Center, Cleveland, OH, 1984.