Re: Double springs on a VP hub?

From: ewmonda <ewmonda_at_hotmail.com>
Date: Tue, 15 Aug 2006 18:11:55 -0000

I might be getting things mixed up here, but I thought I had read
earlier on this list that the most efficient pitch was something
around a 2:1 P/D ratio. From my very limited experience, it seems
that a much lower P/D ratio is needed on the low pitch stop in order
to use all turns on the descent for a full motor in Cat I. Is the
2:1 ratio the most efficient only for a fixed pitch prop? What kind
of P/D ratios are people using for the low stop?

Eric Monda

--- In Indoor_Construction_at_yahoogroups.com, "Fred Tellier" <fred-
tellier_at_...> wrote:
>
> I have to agree with Bill, the gradual slow climb is what one
wants in low sites. This is accomplished with the excessive high
pitch that will actually cause the plane to descend a little at
first and wallow around at very low altitudes for the first few
minutes of the flight. I find that the extremely low RPM causes the
turns to be used slower so a little more spring tension usually is
needed to get the plane to start to climb. I use the same prop hubs
and springs for all ceiling heights but usually need to turn the
spring screw in some for low ceilings. The prop usually is in low
pitch at the top altitude of the flight just before the model starts
to descend. If this happens right you should be able to use up most
of the turns. If you get it right on a F1D the plane should take at
least 10 to 12 minutes to get to the top and stay there at least
another 5 or 6 before starting the slow descent. I only have 18' at
my cat1 site and can get 24 minute plus flights fairly easily if the
air is stable. The problem with cat1 F1D flights is turbulence in
the first 5 minutes as the plane is struggling to fly with such a
high pitch. Another problem is getting enough movement in the hub to
allow such a high pitch
>
> Fred Tellier
> ----- Original Message -----
> From: Bill Gowen
> To: Indoor_Construction_at_yahoogroups.com
> Sent: Monday, August 14, 2006 3:31 PM
> Subject: Re: Re: [Indoor_Construction] Double springs on a VP
hub?
>
>
> Nope. I don't see any advantage in causing a quicker change over
for low ceiling. I would think a lighter spring would be the way to
go for low ceiling to delay the changeover as much as possible
(without landing). I also think that a single climb would conserve
more turns than going up and coming down and going up again but this
is just a gut feeling not based on ever having much success with
either of these plans.
>
> I wouldn't use a second spring for anything.
>
> ----- Original Message -----
> From: Nick Ray
> To: Indoor_Construction_at_yahoogroups.com
> Sent: Monday, August 14, 2006 2:23 PM
> Subject: Re: Re: [Indoor_Construction] Double springs on a VP
hub?
>
> Maybe I'm reading comp challenged, but are you saying to
configure it
> such that he second spring is only active during the first part
of the
> flight? Thereby, causing the the amount of time that the model is
> descending in the initial stage of the flight to decrease and
then for
> the spring to some how deactivate? If that is right, then
wouldn't
> that be more efficient than using a heaver spring the whole way
> through because the change in pitch would be more gradual.
> Also what are the thoughts of gradual pitch change verses sudden
pitch change?
> Nick Ray
> On 8/14/06, Bill Gowen <b.gowen_at_...> wrote:
> >
> >
> >
> >
> >
> >
> > 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]
> >
> >
> >
> >
>
> [Non-text portions of this message have been removed]
>
>
>
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Received on Tue Aug 15 2006 - 11:13:28 CEST