Fuselage self-propulsion by static-pressure thrust - CAFE Foundation

I. Introduction
This paper attempts to focus the attention of
aeronautical engineers on the great potential of
aerodynamic static-pressure thrust AKA form thrust
AKA negative pressure drag AKA negative form drag
for fuselage self-propulsion. Over thirty years
have elapsed since the initial aerodynamic research
phase in the 1950's; younger engineers today seem to
be totally unaware of this early work after HW 11.
It is still relevant to quote J. B. Edwards
1
(1961) : "Our aeroplanes still follow the old
concept of the powered glider, although thcy have
become more refined as time has passed. It is truc,
of course, that the acceptance of this concept
results in a great simplification of the aircraft
design procedure. One set of problems and
considerations is divorced from the other and all is
well, provided that consistent definitions of drag
and thrust are accepted by the people working on
these two separated sets of problems. The airirame
group measures its achievement by the lift/drag
ratio .... The engine group measures its cruising
achievement in terms of specific fuel
consumption ..." It would appear downright
iconoclastic to even suggest that power can be
usefully and efficiently applied to the airframe
itself to increase lift and to eliminate drag, even
to the point of creating "pressure thrust"; this is
why some authors have employed the curious semantic
appellative of "negative drag" for pressure thrust.
Pressure thrust has been documented experimentally
in several known cases. Perhaps the best-known case
is that of the shrouded Dropeller: .I. E. Fanucci, et
a1 (1974)' carried out an excellent experimental
investigation at West Virginia University. Figure 1
presents the test set-up and Figure 2 presents the
measured thrust vs. power, with the three categories
of propeller thrust, shroud thrust and total thrust.
At max. power it can be seen that the shroud
pressure thrust is 60% of the total, while the
propeller reaction thrust is 40%. Another well-
known case is that of the airfoil with jet-flap.
I. M. Davidson (1961)3 discusses the jet-flap
airfoil and states: "Naturally the jet reaction J
itself has a thrust component of only J cos $ but
the difference J(1-cos #) is recovered in the guise
of an aerodynamic pressure thrust distributed over
the surface of the airfoil'.
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Fig. 1 ~
Wind-Tunnel
Layout of Experimental
Shrouded Propeller (From Ref. 2)
c-----I
0.25 0.5 0.75 1.0
POWER HP
Fig. 2 - Experimental Shrouded Propeller
Performance: Thrust YS Power (From Ref. 21
N A. Dimmock (1957) 4 presents experimental
wind-tunnel results of a 12.5% thick airfoil with
jet-flap at 0 = 58" from the chord line, as
presented in Figure 3. The measured total thrust
was 76.5% of the jet thrust, while the axial
component of the jet thrust was only 52.5%. Thus 31%
of the total measured thrust was pressure thrust and
69% was reaction thrust.
B. S . Stratford (1956)5 was very aware of the
potential for reducing or reversing the pressure
drag of an airfoil with jet flap. The following is
quoted from his Ref. 5 (pp. 181 and 182): "The form
drag (which is that part of the two-dimensional drag
transmitted by the normal pressure forces and not by
skin friction) may be associated with the boundary
layer displacing the main flow from the true profile
of the aerofoil and thereby preventing the full
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