Re: Double springs on a VP hub?

From: <RLBailey_at_care4free.net>
Date: Mon, 14 Aug 2006 12:49:23 +0100

The model spends most of its air time at minimum pitch, so it's logical to design the prop for this region ie cruise and let down.

A soft spring will delay the pitch change for a given preload torque so that the model could descend bfore the torque drops to cruise.

I use a flexible top stop (ie the second spring) as opposed to a solid top stop which allows a small amount of extra pitch and hence allows the use of more torque at launch.

One doesn't want to reduce the pitch at high torque; the climb rate goes up!

Bob

  ----- Original Message -----
  From: Tapio Linkosalo
  To: Indoor_Construction_at_yahoogroups.com
  Sent: Monday, August 14, 2006 8:48 AM
  Subject: [Indoor_Construction] Double springs on a VP hub?



  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-



   

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Received on Mon Aug 14 2006 - 16:00:06 CEST