Electrically Powered Ultralight Aircraft

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