Re: Re: Double springs on a VP hub?
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_earthlink.net> wrote:
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> 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:
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> 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.
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> 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-
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Received on Mon Aug 14 2006 - 16:18:21 CEST
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