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National Advisory Committee for Aeronautics, Technical Notes - The Dynamimeter Hub

Following the earlier reports entitled “Preliminary results of Free
Flight Tests with Dynamometer HuhK and “Propeller Thrust and Measuring
Hub," the present report will give a. description of the measuring hub ac—
tually used.

The instrument takes the place of the propeller huh on the engine
shaft and carries the propeller in the usual way. There are two princi-
pal moving parts of the measuring hub: one is connected with the engine
shaft, the other with the propeller. Between these two parts are insert-
ed dynamometer devices. -

For the forces that are to be measured, the dynamometer devices used
are the most convenient, because:

There is no lag; when the direction of the load. changes there
are no mutual displacements between the measuring and. force-transmitting
parts.

These properties are of the greatest importance; because of the highly
periodical forces of the motor, which, on account of absence of a fly-
wheel, are transmitted to the hub; and it is those properties that make
the measurements possible.

Description of Device.

The propeller dynammeter device' of Professor Bendemann is represented
in Fig. 1. It consists of a closed ended cylinder, with a tight but very
light piston p to which forces can be transmitted by aid of a piston rod
9. On the cylinder 3. pivoted lever d is fixed and guided at the two ends,
which are connected with the regulating pistons g and :1.

When the rod is loaded, the pistons first go down, the regulating
piston g admits at g the liquid under pressure which flows in till the
piston g is stopped. In this manner the flow is first throttled down by
the regulating piston g and finally stopped when the piston has reached
its original position. The pressure under the piston is then (neglecting
piston friction) proportional to the loading of the piston and can be
measured by a manometer connected. The same action takes place
when some liquid is lost by the piston's not being completely tight.