Did John Stringfellow ever build an aeroplane, powered by a steam engine, that really flew?
I was not able to discover articles of 1848, or even up to 5 years after, that give details about the 1848 flight of Stringfellow's model. However, I found 3 first hand articles, published on July 3, 24 and Oct. 9, 1868, which say that, at an exhibition which took place starting with June 25, 1868, at the Crystal Palace in London, John Stringfellow exhibited two engines. For the larger one, that generated, when started, a little more than 1 HP, he took a prize of 100 pounds. The smaller motor was installed on a working plane which apparently flew but the description is vague being unclear whether the author talks about an event that happened in 1868 or he refers to previous flights, like the one of 1848. Anyway, the conclusion was that despite taking the prize (for the larger standalone engine) it was not clear whether that motor would serve for any practical purposes in aeronautics.
From the numerical values furnished by the commission that awarded the prize, it results that Stringfellow's motor needed:
$150 ft * 3 in^2 = 88.5 liters$ of steam at $80 lbf/in^2 = 5.4 atm$ per minute to generate 1.09 HP continuous power.
Nothing is said about the interval of time during which the boiler was able to maintain this considerable flow of steam. Another remark would be that the steam was expelled from the cylinder at 5.4 atm which means the engine had a very poor efficiency.
&quot;The Aeronautical Society of Great Britain&quot;, The Mechanics' Magazine, July 3, 1868, col. 2-3, p. 3.

THE AERONAUTICAL SOCIETY OF GREAT BRITAIN. ...
We have said that there are a few exhibits which are a step in advance
of the rest at the Crystal Palace. Amongst the rest may be noticed a
working model of an aerial steam carriage, by Mr. J. Stringfellow, the
whole, including engine, boiler, water and fuel, weighing about 12
lb.; cylinder, 1 3-16ths inch diameter; 2-inch stroke; works two
propellers, 21 in. diameter, about 600 revolutions per minute; gets up
steam of 100 lb. pressure in five minutes. ...

&quot;The Aeronautical Society Late Exhibition&quot;, English Mechanic and Mirror of Science, July 24, 1868, col. 3, p. 378.

THE AERONAUTICAL SOCIETY'S LATE EXHIBITION.
THOSE who predicted that the exhibition of the Aeronautical Society at
the Crystal Palace would prove a failure may, in part be said to have
been true prophets; but the exhibition was not all a failure — far
from it: enough, we think, was given in evidence to cause at least one
section of the members to be of good heart. True, the expected
demonstration of the possibility of flying was not made, and the
rewards offered for flying have not been gained. The flying man put in
no appearance with bis wings, and the only model of this class which
actually worked was the ingenious steam-propelled machine of Mr.
Stringfellow, which, running along a a wire, has enabled our facetious
contemporary, &quot;Punch,&quot; to change the inventor's name to Wirefellow,
and which elicited the remark from sundry spectators of a certain
class that &quot;he was wiring into it.&quot;

&quot;The Aeronautical Exhibition&quot;, The Mechanics Magazine, October 9, 1868, pp. 282-284.

THE AERONAUTICAL EXHIBITION.
IT will be fresh in the memory of our readers that the Aeronautical
Society of Great Britain held their first exhibition at the Crystal
Palace on the 25th of June last and ten following days. ...
The only engines, therefore, which competed for the prize were three,
— one by M. Camille Vert and two by Mr. J. Stringfellow. M. Vert's
engine was a small model of a steam engine, weighing 2 lb., for
propelling a hydrogen gas balloon. Mr. Stringfellow's engine No. 1 was
a light engine and boiler for aerial purposes; it was of 1-horse
power, and weighed 13 lb. No. 2 was a similar engine on a smaller
scale. M. Vert's engine was found to be too small to enable any
accurate test to be applied for ascertaining its power. The jurors
state that it appeared to keep up steam well in the repeated
demonstrations in propelling a model balloon in various directions
within the Crystal Palace building.
Mr. Stringfellow's engine No. 1, from its size and power, may be
considered something more than a mere model. The cylinder was 2 in. in
diameter, stroke 3 in., and it worked with a boiler pressure of 100
lb. per square inch, the engine making 300 revolutions per minute. The
time of getting up steam was noted; in three minutes after lighting
the fire, the pressure was 30 lb., in five minutes 50 lb., and in
seven minutes there was the full working pressure of 100 lb. When
started, the engine had a fair amount of duty to perform in driving
two four-bladed screw-propellers, 3 ft. in diameter, at 300
revolutions per minute.
In the jurors' report, the data for estimating the power are taken as
follows: — Area of piston, 3 in., pressure in cylinder 80 lb. per
square inch, length of stroke 3 in., velocity of piston 150 ft. per
minute, 3 x 80 x 150 = 36,000 foot pounds; this makes rather more than
1-horse power (which is reckoned as 33,000 foot pounds). The weight of
the engine and boiler was only 13 lb., and is probably the lightest
steam engine that has ever been constructed. The engine, boiler, car,
and propellers together, were afterwards weighed, but without water
and fuel, and were found to be 16 lb. The council of the Aeronautical
Society agreed that this engine, as a complete working machine, met
the condition of the Society's award for &quot;the lightest engine in
proportion to its power from whatever source the power may be
derived.&quot; The prize of £100 was accordingly allotted to Mr.
Stringfellow. ...
The exhibition, however, proved that the advocates for aerial
locomotion by mechanical means have greatly increased in number, and
their designs for accomplishing this end were as varied as they were
ingenious. But still we have no practical result by which we can
measure the progress of aeronautics in this direction; we cannot say
we have even attained the first step in actual practice. The nearest
approach that was made to this long-desired condition of things was
the demonstration that by means of vertical screws a weight of 100 lb.
might be supported by a constant force of about 90,000 foot-pounds or
three-horse power. Then we have Mr. Stringfellow's engine of one-horse
power and weighing only 13 lb., which, taken in conjunction with the
vertical screws, would appear to indicate the direction in which
mechanical flight may be realized; and this is as far as we have got —
just on the borders of possibility. But we must bear in mind the
enormous expenditure of power the system of vertical screws requires,
and the very flimsy character of an engine and boiler sufficiently
powerful to work them, and sufficiently light to allow of their
working result, if, and when attained, being made available for
transit.
It practically comes to this, that Mr. Stringfellow carried off the
£100 prize for the lightest engine in proportion to its power, but of
what real service that engine will prove to the science of aeronautics
remains still to be seen.