Electrically Powered Ultralight Aircraft

On Tue, 7 Aug 2007 11:48:50 -0500, "Neil Gould"
wrote in
:

Recently, Larry Dighera posted:

[snip]
I see what you mean. Unfortunately, the highest power requirements of
aircraft engines are during the takeoff and climb phases of flight.
Power requirements are even greater when the ambient temperature rises
resulting in less air density or a higher density altitude. That is
when the most power is required for takeoff, but that would be a
situation where the Stirling engine would have its minimum power
production.

If an engine's minimum power production is greater than the power required
for takeoff, would it matter?

Probably not, but it would mean you'd have significantly more power
available at altitude if the Sterling engine were sized to provide
takeoff power at high density altitudes.

What I was getting at was the author of the articles emphasis on
overcoming the reduced power output of IC engines at lower atmospheric
pressure overlooks its possibly anemic performance (due to minimal air
movement through the heat exchanger and higher ambient temperatures on
the ground) when it is needed most, at takeoff. I find it revealing
that the author failed to mention that point, and it reduces my
confidence in the assertions he made in that article.

It would seem that if this could be achieved, the operating conditions
of the Stirling engine would be mostly understressed.

I am unable to infer your meaning by that statement. Do you mean
under emphasized or less mechanical stress on the engine, or what?


I would also like to see a comparison of the efficiencies of IC and EC
engines and their relative weight and size per horsepower compared.

Unlike electrical motors, that must be constructed with heavy iron, IC
and EC engines can be constructed of lighter materials like aluminum,
but electrical motors are usually 80% to 95% efficient. With the
Stirling aircraft engine there is a requirement for what I would
imagine would be a large heat sink or heat exchanger located in the
slip stream. The weight of this heat exchanger and its drag penalty
must also be considered.

Why couldn't the heat exchanger be an integral part of the airframe? Wings
come to mind... ;-)

I'm thinking there would be necessity for some means of conducting the
heat from the engine to a remote heat exchanger, and the resulting
complexity and weight increase would negatively impact the potential
advantages of a Stirling aviation engine. In any event, in addition
to the Stirling engine and its fuel, a heat exchanger of some type
needs to factored into the weight, cost, performance, and efficiency
equations.


There might be one advantage to using Sterling external combustion
engines for aviation: the use of atomic energy as a fuel source if the
weight of the lead shielding were not too great. Imagine an aircraft
that effectively never runs out of fuel! There'd be no more fuel
exhaustion mishaps.

One downside would be the hazardous materials that could be dispersed in a
crash.

There are a lot of down sides to atomic power, but NASA uses it to
power Stirling engines in space.


Here's some information about what NASA successfully has accomplished
with nuclear power:


http://www.grc.nasa.gov/WWW/tmsb/index.html
The Thermo-Mechanical Systems Branch (5490) is responsible for
planning, conducting and directing research and technology
development to advance the state-of-the-art in a variety of
thermal systems for space, aerospace, as well as non-aerospace
applications. The systems of interest include thermal energy
conversion for power systems and solar thermal propulsion systems.
The effort involves working at the component level to develop the
technology, the subsystem level to verify the performance of the
technology, and the system level to ensure that the appropriate
system level impact is achieved with the integrated technology.
System analysis is used to identify high-impact technology areas,
define the critical aspects of the technology that need to be
developed, and characterize the system level impact of the
technology. Specific technology areas of interest include:


Dynamic Power Systems: Brayton, Rankine and Stirling Convertors,
Solar Receivers and Thermal Energy Storage
Primary Solar Concentrators: Thin film, SRP and Rigid
Secondary Solar Concentrators: Refractive and Reflective
Thermal Management: Radiators, Electronics Packaging, and Heat
Pipe Technology


http://www.grc.nasa.gov/WWW/tmsb/stirling.html
Animation of a 55 We Stirling TDC
(click on image to view)


http://www.grc.nasa.gov/WWW/tmsb/sti...adisotope.html
AVAILABLE TODAY FOR TOMORROW'S NEEDS
NASA Glenn Research Center and the Department of Energy (DOE) are
developing a Stirling convertor for an advanced radioisotope power
system to provide spacecraft on-board electric power for NASA deep
space missions. Stirling is being evaluated as an alternative to
replace Radioisotope Thermoelectric Generators (RTGs) with a
high-efficiency power source. The efficiency of the Stirling
system, in excess of 20%, will reduce the necessary isotope
inventory by a factor of at least 3 compared to RTGs. Stirling is
the most developed convertor option of the advanced power concepts
under consideration [1,2].


http://www.grc.nasa.gov/WWW/tmsb/sti...ng_bckgrd.html
However, about this time NASA became interested in development of
free-piston Stirling engines for space power applications. These
engines use helium as the working fluid, drive linear alternators
to produce electricity and are hermetically sealed. These 12.5 kWe
per cylinder engines were intended for use with a nuclear reactor
power system; the Space Demonstrator Engine (or SPDE) was the
earliest 12.5 kWe per cylinder engine that was designed, built and
tested by MTI. A later engine of this size, the Component Test
Power Convertor (or CTPC), used a "Starfish" heat-pipe heater
head, instead of the pumped-loop used by the SPDE. Recently, in
the 1992-93 time period, this work was terminated due to the
termination of the related SP-100 nuclear power system work and
NASA's new emphasis on "better, faster, cheaper" systems and
missions.


http://www.spacedaily.com/news/outerplanets-00a2.html
Europa Orbiter was replanned to use a new "Sterling" nuclear
generator design which would use less plutonium



http://www.cndyorks.gn.apc.org/yspac...heed_offer.htm
Boeing, Lockheed Offer NASA Two Choices for Nuclear Power

Recently, Larry Dighera posted:

On Tue, 7 Aug 2007 11:48:50 -0500, "Neil Gould"
wrote:

Recently, Larry Dighera posted:

[snip]
I see what you mean. Unfortunately, the highest power requirements
of aircraft engines are during the takeoff and climb phases of
flight. Power requirements are even greater when the ambient
temperature rises resulting in less air density or a higher density
altitude. That is when the most power is required for takeoff, but
that would be a situation where the Stirling engine would have its
minimum power production.

If an engine's minimum power production is greater than the power
required for takeoff, would it matter?

Probably not, but it would mean you'd have significantly more power
available at altitude if the Sterling engine were sized to provide
takeoff power at high density altitudes.

Exactly, but I don't see that as a negative... ;-)

What I was getting at was the author of the articles emphasis on
overcoming the reduced power output of IC engines at lower atmospheric
pressure overlooks its possibly anemic performance (due to minimal air
movement through the heat exchanger and higher ambient temperatures on
the ground) when it is needed most, at takeoff. I find it revealing
that the author failed to mention that point, and it reduces my
confidence in the assertions he made in that article.

I understand your perspective, which is what prompted my reply. If there
is sufficient power to take off, then the issue should be moot, unless I'm
overlooking something. It should be reasonable to presume that any
practical aircraft engine would have sufficient power to take off, right?
;-)

It would seem that if this could be achieved, the operating
conditions of the Stirling engine would be mostly understressed.

I am unable to infer your meaning by that statement. Do you mean
under emphasized or less mechanical stress on the engine, or what?

Less mechanical stress due to operating well below maximum power settings
under normal cruise. That should provide plenty of reserve power at
altitude and increase the fuel efficiency as well.

I would also like to see a comparison of the efficiencies of IC and
EC engines and their relative weight and size per horsepower
compared.

Unlike electrical motors, that must be constructed with heavy iron,
IC and EC engines can be constructed of lighter materials like
aluminum, but electrical motors are usually 80% to 95% efficient.
With the Stirling aircraft engine there is a requirement for what I
would imagine would be a large heat sink or heat exchanger located
in the slip stream. The weight of this heat exchanger and its drag
penalty must also be considered.

Why couldn't the heat exchanger be an integral part of the airframe?
Wings come to mind... ;-)

I'm thinking there would be necessity for some means of conducting the
heat from the engine to a remote heat exchanger, and the resulting
complexity and weight increase would negatively impact the potential
advantages of a Stirling aviation engine. In any event, in addition
to the Stirling engine and its fuel, a heat exchanger of some type
needs to factored into the weight, cost, performance, and efficiency
equations.

Of course, but I don't see a lot of reason why that couldn't be
incorporated into the overall design. My point is that heat exchangers
need not be heavy, and could probably double as structural and/or
aerodynamic components, further reducing (and possibly enhancing) their
impact.

There might be one advantage to using Sterling external combustion
engines for aviation: the use of atomic energy as a fuel source if
the weight of the lead shielding were not too great. Imagine an
aircraft that effectively never runs out of fuel! There'd be no
more fuel exhaustion mishaps.

One downside would be the hazardous materials that could be
dispersed in a crash.

There are a lot of down sides to atomic power, but NASA uses it to
power Stirling engines in space.

Understandable, but their expectation is that catastrophic destruction
would disperse the nuclear material harmlessly. That can't be presumed for
light aircraft.


Neil

On Tue, 07 Aug 2007 17:51:27 GMT, "Neil Gould"
wrote in
:

Recently, Larry Dighera posted:
[...]
I'm thinking there would be necessity for some means of conducting the
heat from the engine to a remote heat exchanger, and the resulting
complexity and weight increase would negatively impact the potential
advantages of a Stirling aviation engine. In any event, in addition
to the Stirling engine and its fuel, a heat exchanger of some type
needs to factored into the weight, cost, performance, and efficiency
equations.

Of course, but I don't see a lot of reason why that couldn't be
incorporated into the overall design. My point is that heat exchangers
need not be heavy, and could probably double as structural and/or
aerodynamic components, further reducing (and possibly enhancing) their
impact.

How would you get the heat from the Stirling engine to the heat sink?
If you use liquid coolant, it would be heavy and prone to leaks.

There might be one advantage to using Sterling external combustion
engines for aviation: the use of atomic energy as a fuel source if
the weight of the lead shielding were not too great. Imagine an
aircraft that effectively never runs out of fuel! There'd be no
more fuel exhaustion mishaps.

One downside would be the hazardous materials that could be
dispersed in a crash.

There are a lot of down sides to atomic power, but NASA uses it to
power Stirling engines in space.

Understandable, but their expectation is that catastrophic destruction
would disperse the nuclear material harmlessly. That can't be presumed for
light aircraft.

If the rocket detonated in the atmosphere, it might not be so
harmless. I would guess the reactor is jacketed with sufficient
strength to preclude its destruction. Presumably, that could be done
for a Stirling aircraft engine also.

Recently, Larry Dighera posted:

On Tue, 07 Aug 2007 17:51:27 GMT, "Neil Gould"
wrote in
:

Recently, Larry Dighera posted:
[...]
I'm thinking there would be necessity for some means of conducting
the heat from the engine to a remote heat exchanger, and the
resulting complexity and weight increase would negatively impact
the potential advantages of a Stirling aviation engine. In any
event, in addition to the Stirling engine and its fuel, a heat
exchanger of some type needs to factored into the weight, cost,
performance, and efficiency equations.

Of course, but I don't see a lot of reason why that couldn't be
incorporated into the overall design. My point is that heat
exchangers need not be heavy, and could probably double as
structural and/or aerodynamic components, further reducing (and
possibly enhancing) their impact.

How would you get the heat from the Stirling engine to the heat sink?
If you use liquid coolant, it would be heavy and prone to leaks.

I'm not a Stirling engine designer, so I can't answer that factually. I
have been reading up on it a bit since the article was referenced in this
thread, but I haven't seen such things as the required rate of dissipation
for the engine to work efficiently. If the heat sink needs to be large and
close to the engine, perhaps a design where the engine is mounted on or
even incorporated into the wing is a way to go.

There might be one advantage to using Sterling external combustion
engines for aviation: the use of atomic energy as a fuel source if
the weight of the lead shielding were not too great. Imagine an
aircraft that effectively never runs out of fuel! There'd be no
more fuel exhaustion mishaps.

One downside would be the hazardous materials that could be
dispersed in a crash.

There are a lot of down sides to atomic power, but NASA uses it to
power Stirling engines in space.

Understandable, but their expectation is that catastrophic
destruction would disperse the nuclear material harmlessly. That
can't be presumed for light aircraft.

If the rocket detonated in the atmosphere, it might not be so
harmless.

I don't see why it would be nearly as bad as a "dirty bomb" would be, as
the material would be dispersed over a pretty large area.

I would guess the reactor is jacketed with sufficient
strength to preclude its destruction.

My guess is that NASA et al are just hoping for good fortune. Having a
reactor land from orbit intact in the middle of a city wouldn't be all
that desirable. ;-) So, my bet is on there being no good plan for
dealing with such a catastrophe *other* than wide dispersal of the nuclear
material or the luck of landing in the ocean. Not that *that* outcome is
desirable either...

Neil