Re: Double springs on a VP hub?
Here's a contrarian view (possibly but not necessarily correct), that will be called Bill's Theory of VP's. (all you guys who beat me all the time can go ahead and laugh at this point).:
There's a point in any flight where the energy left in the motor is unable to maintain level flight no matter what the pitch of the prop is. At that point in the flight the prop pitch should be at it's most efficient value. If the prop is running at it's most efficient pitch then having a higher or lower pitch will not make your flight any longer and in fact should make it shorter. So, IMHO, the prop should be at it's lowest pitch when the model is still at the highest possible point in the flying site, AND that the low pitch setting should be carefully adjusted for the best possible cruise.
Just for purposes of illustration let's say that this point is at an altitude of 100 feet and that you have 1000 turns left at this point in your flight. Let's also say that you started with 2500 turns. What's left for the VP to accomplish is to feed those initial 1500 turns to the model in a way that puts the model at 100 feet altitude at the same point in time that it reaches 1000 turns remaining. Under normal (fixed pitch) circumstances these 1500 initial turns are going to be much more than is needed to reach the target altitude. The job of the VP is to force the prop to run at an inefficient (higher) pitch in order to reduce the model's climb so that all of the initial 1500 turns can be safely used. You also want the prop to run as slowly as possible during this phase of the flight so it is desirable to have the highest possible pitch at all points of the climb phase - keeping in mind that you have to get to your 100 foot target altitude.
It follows that there are 2 distinct parts of a flight using VP. The descending portion is controlled by the quality of the model, rubber, prop, flyer skill, weather conditions, thermals and whatever else you can think of that makes some indoor flights better than others. The ascending portion is controlled by how well the VP does it's job of power modulation.
If the above statements are factual, then I'm led to the following conclusion:
One of the most critical attributes of a VP hub is that the spring is strong enough to give absolute low pitch at absolute maximum altitude.
The downsides of having a spring strong enough to do this are:
1. a strong spring is heavy and
2. a strong spring is going to want to change pitch earlier than a weaker spring. If the change is too early then RPM's go up and time is lost.
I don't really see a way that a second spring can be helpful. It seems that a second spring would make problem 2 worse instead of better and would also make the adjustment of the hub even more complicated than it already is. What you really need is a way to delay the change from high pitch as long as possible and then have a quick change to low pitch.
Oh yeah... that's already been done - the folding VD props of Richmond and Doig!
----- Original Message -----
From: Tapio Linkosalo
To: Indoor_Construction_at_yahoogroups.com
Sent: Monday, August 14, 2006 3:48 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-
[Non-text portions of this message have been removed]
Received on Mon Aug 14 2006 - 08:22:18 CEST
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