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Tank Tests of a Full Size Dynamically Similar Model of the Army OA-9 Amphibian with Motor Driven Propellers - NACA Model 117

By John B. Parkinson and Roland 1. Olson
' sUMMAaI

The influence of -running propell-ers on t-he hydrody-
namic characteristics of a model of a seaplane were in-
vestigated in the NADA tank to evaluate 'the importance of
power in tests of dynamically similar models. Various in-
crements of power, including that sufficient for self— '
propulsion, were applied: and a gear allowing fore—and-
aft freedom of the model with respect to the towing car-
riage when self—propelled was provided.

It was found that. as in wind-tunnel work, the pow-
ered propellers have a large effect on the aerodynamic
characteristics of the model and consequently on the hy-
drodynamic stability. which depends to a certain extent
on those characteristics.. Furthermore, the interference-
of the propellers and the slipstream with the wave sys-
tem around the hull at taxying speeds is the moat sig-
nificant factor in the problems of spray control and lim-
itation in load imposed by the spray. Hence the use of
powered models is desirable in tank tests of new designs
for a more precise prediction of stability and spray
while taking off and landing.

In general. the magnitude of the effects of a given
increment of power in such tests decreases as the power
is increased. The use of powers and revolution speeds
that are less than the scale values would be preferable
to neglecting entirely the effects of .the running propel-
lers. Fore-and~aft freedom of the modhl has a negligible
effect on the trims at which porpoising begins but changes
the character of the motionlsomewhat.

INTRODUCTION

The influence of running propellers on the aerody—
namic characteristics of highly powered and heavily
loaded airplanes has become of fundamental importance in
design. The general effects of the. slipstream are to in-
crease lift, to increase the effec'tiveness of the con—
trols, and to decrease stability. The phenomena involved
are of a complex nature, which precludes at the present
time either an exact theoretical treatment or empirical
research extensive enough to' caver all cases. Consequent—
ly, powered models are being widely used in wind—tunnel
tests of new designs for a more precise determination of
stability, control, and flying qualities (reference 1).

In the case of the seaplane during take—offs and
landings, the effects of the powered propellers should be
basically the same except as modified by the proximity of
the surface of the water. These effects are therefore
factors in the determination of hydrodynamic characteris-
tics. such as hydrodynamic stability and resistance, which
are functions of the aerodynamic forces and moments par—
ticularly in investigations of the porpoising characteris-
tics of multiengine long—~range flying boats' for which the
percentage of wing area affected by the slipstream is very
large, .

.Of equal importance with the aerodynamic effects bf
the slipstream is the.profound influence of the rotating
propellers on the spray characteristics, which in contem—
porary seaplanes constitute a limitation on maximum take-
off load. The objectionable spray is greatest at slow _
speeds and. full power when it is picked up by the_propel—'
ler tips and the slipstream and blown back over the en—"'
gines, wing, and tail. _The influence of the propellers
is therefore a factor in the determination of limitations
in load imposed by spray and in studies of methods of con;
trolling the spray. .

The foregoing_considerations point to the desirabil—
ity of the use of powered models in tank tests of models
of seaplanes as well as in the wind~tunnel tests since
the effects of the propellers on the aerodynamic charac-
teristics or on the spray cannot be adequately taken into
account by other means. In addition, the use of power-
driven propellers permits tests in which the model is
self-propelled instead of pulled by the towing carriage
so that its behavior as a free body can be investigated.
Furthermore, the increase in lift and in elevator effec-
tiveness with power enables dynamic maneuvers, such as
take-offs and laidings, to be reproduced at water speeds
and trims corresponding more closely with_full—sizs values.
=-

"' The present investigation was made in the NLGd‘tank
to"determineuthe-magndtude of thereffects.9f powered,pro— _
pellers on the hydrodynamic stability and the spray char—
acteristics of a dynamic model. For this purpose, the

1'6 fu-ll—siae model of the Army Od—Q amphibian was fitted
with model -airplane propellers driven by direct-current

'motors that had sufficient' power for self—propulsion and

low enough weight to 'retein dynamic similarity with the

=full-size craft. The provision of scale powei and pram

peller speed, as in the more precise wind—tunnel tests.
wa's not considered essential for the investigation- and.
would have involved additional delay" and cost. -

The means for investigating the effect of the longi—
tudinal restraint imposed by the usual towing procedure
were provided by a modification of the gear that permit—
ted fore—and—aft movement of the model with respect to
the towing carriage. For convenience, the usual restraint
in roll and yaw was retained in the gear. -