Double springs on a VP hub?

From: Tapio Linkosalo <tapio.linkosalo_at_helsinki.fi>
Date: Mon, 14 Aug 2006 10:48:59 +0300 (EEST)

Last week spent a couple of days paddling on the lake Saimaa, and had good
time to think and ponder model-related things. I started to wonder why
some VP-hubs that I have seen show double springs to resist the pitch
change. I also made up a hypothesis, which is as follows:

An ideal spring should have a linear change of twist as the function of
the torque, thus if the spring coefficient was right, it would result in a
constant power output independent of the prop turns. However, the prop
efficiency is not constant, but for a prop that is designed in the
mid-range of the pitch, the efficiency will decrease at higher pitch, and
then even though the input power to the prop was constant, the output was
reduced at higher pitch. Thus a model with "proper" spring coefficient,
and which was trimmed for level flight at mid-torque, would actually
_sink_ at higher torque. ( I suppose we have seen this, as some people
report models sinking at the start of the flight when trimmed at low
sites?). One solution would be to reduce the high pitch setting, but the
setting is probably highly sensitive, and to avoid climbing to the ceiling
then one would also need to back-off some. So to make the high-pitch
setting more flexible, one adds another spring that kicks in at high
torque, and reduces the amount of pitch at high torque. This way one can
maintain positive climb even at the high TQ, and still be able to wind the
motor to the max, without a need for backoff?



-Tapio-
Received on Mon Aug 14 2006 - 00:50:31 CEST