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Author Topic: VP Hubs how do they work?  (Read 782 times)
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Jorgefly
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« on: April 02, 2020, 06:17:39 PM »

Hello from Spain, during the corona problem I’m flying in my 9 foot ceiling garage with my f1M-l 3gr with a VP hub. So I’m trying to understand how VP works, how trim it and how much affect the other two settings when you trim only one.
I think the biggest problem is that the VP spring works linear and the rubber band has his own torque curve.
Nonetheless I could fly for 10 minutes with 2 whole height climbs hitting and 8 minutes no hitting.
Could you help me?
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VP Hubs how do they work?
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Tapio Linkosalo
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« Reply #1 on: April 03, 2020, 12:55:25 AM »

I think the biggest problem is that the VP spring works linear and the rubber band has his own torque curve.

Actually, the VP works linear as the function of the torque, not the angle, so it should match the rubber torque ok.

Having said that, the type of the spring has big impact. If you have a so-called "soft" spring, with multiple turns, this has a little change in the force as a function of angle. With such spring the VP has kind of on-off action. The prop holds on maximum pitch until rubber torque drops down to a threshold torque, then the pitch drops down to minimum. And typically a second climb starts. Your prop sounds like it has a soft spring.

On the other hand, you may make a "hard" spring. This has only few turns (of thicker wire), and has big change of force per angle change. With such spring he model starts again on high pitch, but as the rubber torque drops, the prop pitch drops a little. However, as only a small angular change reduces the spring torque a lot, the prop does not go to low pitch right away, but only reduces the pitch a little. Until rubber torque further reduces and the pitch drops a little more. This way the prop pitch reduces steadily along a large range of motor torque, and the prop stays "on spring" for a longer time. This way you may be able to trim the model to fly level or climb ever so slightly for a long time, but probably need to try many different springs to find the one that fits your model and rubber, as much of the characteristics depend on the geometry of the spring.

     
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Jorgefly
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« Reply #2 on: April 03, 2020, 08:59:19 AM »

Hi Tapio, first of all, thanks for your quick reply.
I am very surprised that there is not much information on how VP Hubs work. However there are hundreds of different models.
I can't quite understand why a hard spring is more progressive and a soft spring is like on / off, it not seems quite logical for me.
On the other hand, I think the logical thing would be that the spring was not linear, i think progressive is better, I know progressive springs in the suspension of racing motorcycles.
I think you could use a piece of rubber instead of a spring to work with the same torque curve.
What you think?
I would also like to talk about the two types of flights with VP: the one climbing or the two type.
Thinking about energies, the most logical thing would be a single climbing but it seems that in low ceilings it is better 2 climbs ...

http://www.ukrflight.narod.ru/pages/vp_502.htm

i think the perfect VP should be able to be adjusted to be able to fly as long as possible at 2 meters for example with no climbing or diving, that is to say that it adapts the pitch according to the torque goes down and maintains the necessary revolutions to fly as slow as possible.
Finally I have seen in the forum a VP model with two different springs, what do you think of that design, in principle the best thing I have seen
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Jorgefly
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« Reply #3 on: April 03, 2020, 09:03:01 AM »

http://www.ukrflight.narod.ru/pages/vp_502.htm
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frash
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« Reply #4 on: April 03, 2020, 12:00:58 PM »

I have no experience with VP hubs and am quite unlikely to ever try one.

If the spring were wound on a cone rather than on a cylinder (or wire), would it have a longer range of movement or action?

Fred Rash
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Olbill
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« Reply #5 on: April 03, 2020, 01:26:06 PM »

I've always used a single climb for my F1M. In a low ceiling situation my motor will be wound to over 1 in-oz of torque. The prop blades will be roughly parallel to the motor stick at the root of the blades. The prop generates just enough thrust to keep the model flying at the beginning of the flight and the pitch change is very gradual. I try to have the prop at low pitch when the model is as high as it can go, the object being to hold on to that altitude as long as possible.

Here is a photo made several years ago just before launching in a Cat 1 site. You can see the blade angle at launch.
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Jorgefly
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« Reply #6 on: April 03, 2020, 02:25:09 PM »

I've always used a single climb for my F1M. In a low ceiling situation my motor will be wound to over 1 in-oz of torque. The prop blades will be roughly parallel to the motor stick at the root of the blades. The prop generates just enough thrust to keep the model flying at the beginning of the flight and the pitch change is very gradual. I try to have the prop at low pitch when the model is as high as it can go, the object being to hold on to that altitude as long as possible.

Here is a photo made several years ago just before launching in a Cat 1 site. You can see the blade angle at launch.
Your plane looks really nice, my blades are close to be parallel. Now I’m flying on two climbs mode reaching 10 mins with this basic no covered prop plane in 9foot ceiling, maybe I will try one climb mode.
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Jorgefly
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« Reply #7 on: April 03, 2020, 02:30:06 PM »

I have no experience with VP hubs and am quite unlikely to ever try one.

If the spring were wound on a cone rather than on a cylinder (or wire), would it have a longer range of movement or action?

Fred Rash


I think so, a progressive spring will work better than two different rate springs but don’t know the fisics of excentrical springs
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Tapio Linkosalo
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« Reply #8 on: April 04, 2020, 12:10:56 AM »

I can't quite understand why a hard spring is more progressive and a soft spring is like on / off, it not seems quite logical for me.

It is all about the spring, more spesifically the change of force (torque) as a function of angular change (in engineering terms delta torque divided by delta angle). If you take a spring in rest, it shows no torque, but the more you rotate it, the stronger torque it gives. With soft spring, this change per angle is small, and with hard spring it is large. Now consider that the change of pitch in the VP takes place when the spring angle changes. With hard spring within the range of pitch the spring sees a large change in torque, and with a soft spring the change is small.

I think you could use a piece of rubber instead of a spring to work with the same torque curve.
What you think?

I tried that with my very first VP. It works, but as the rubber develops fatique, the settings change. SO for a consistent VP, the steel spring is better.

Finally I have seen in the forum a VP model with two different springs, what do you think of that design, in principle the best thing I have seen

The ukranians have used the double-spring design. The idea is that even though you could match the (linear) spring to the rubber perfectly, the efficiency of the prop goes down at high pitch. Therefore in low ceiling where the initial pitch is really high, the VP may reduce the output power too much. You may actually need to launch really high, and the model will loose altitude until motor torque reduces, thus prop pitch reduces, and at lower motor torque the model starts to climb(!). The ukrainian double spring design solves this by adding some extra spring torque to the top end of VP anfle change.
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Jorgefly
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« Reply #9 on: April 04, 2020, 03:31:55 PM »

Hello Tapio,

I have already understood, I have had to make a couple of drawings to remember my old studies in physics hehehe.
It seems that the Ukrainian VP is one step ahead in low ceiling?, is it used in competitions?
Could you explain the differences between the two types of flight, one climb or two?. Thinking about energy, the logical thing is a single climb and stay flying as high as possible. I don´t understand why two climbs flight is used if it costs more energy to do two climbs than to do only one and stay at altitude.
In high ceilings is VP used? why?
Two more question, how could I measure when (torque and turns) my VP start the transition between low and high pitch and when is resting in low pitch?
How affect when you trim for example the hight pitch to the other two settings I mean:
When you change HP setting  how the preload and LP should be trimmed?
when you change Preload setting how the Hp and Lp should be trimmed?

thanks a lot
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Olbill
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« Reply #10 on: April 04, 2020, 08:49:09 PM »

You probably can't see the details in these photos but I made this apparatus to calibrate one of my early VP hubs. Basically you turn the dial on the torque meter and read the angular change of the VP for a corresponding torque. Then if you know the torque curve of the rubber motor you can determine the VP angle at any given point on the torque curve.

I don't know the advantages of a 2 climb VP. Possibly the prop is more efficient if it is not completely stalled like it is in a one climb setup in low ceilings.
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Tapio Linkosalo
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« Reply #11 on: April 05, 2020, 05:23:02 AM »

Could you explain the differences between the two types of flight, one climb or two?. Thinking about energy, the logical thing is a single climb and stay flying as high as possible. I don´t understand why two climbs flight is used if it costs more energy to do two climbs than to do only one and stay at altitude.
In high ceilings is VP used? why?

I do not know if there is definite answer whether two climbs is better or worse than one. As Bill noted, the baseline feature is prop efficiency - you want to spend as much time as possible on the pitch where the prop is most efficient. Now, what that pitch setting is, is another question. A two climb trim is easier to set up, as you can see more clearly when the pitch change takes place. But the problem is that it is also sensitive to the setting. Change you early and you end up scrubbing the ceiling. Change too late and your second climb is not good enough. Whatever you trim setup is, you want to fly next to the ceiling the very moment that the motor torque is not sufficient to maintain level flight... All throught the use of VP you all the time trade energy between altitude and motor, and the final question is how to use that energy on most efficient way.

In highest ceilings you do not use VP. Like when flying F1D in the Romanian saltmine. But if you can hit the ceiling using a fixed pitch prop, then using VP is more efficient.


Two more question, how could I measure when (torque and turns) my VP start the transition between low and high pitch and when is resting in low pitch?
How affect when you trim for example the hight pitch to the other two settings I mean:
When you change HP setting  how the preload and LP should be trimmed?
when you change Preload setting how the Hp and Lp should be trimmed?

In F1D you find out the working of the VP by reading the prop RPM with a stroke watch. On high pitch the RPM drops, then starts to increase as pitch is reduced and reaches another maximum when the prop reaches minimum pitch setting. In F1M that is more difficult as the rpm is so big that it is not easy to count (I have considered taking video clips of the flights and counting RPM from those at lower replay speed).

About the settings - I suppose you could write an entire paper on how to set up your VP, so there is no short answer to that...
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Jorgefly
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« Reply #12 on: April 05, 2020, 07:00:35 AM »

Quote
In F1D you find out the working of the VP by reading the prop RPM with a stroke watch. On high pitch the RPM drops, then starts to increase as pitch is reduced and reaches another maximum when the prop reaches minimum pitch setting. In F1M that is more difficult as the rpm is so big that it is not easy to count (I have considered taking video clips of the flights and counting RPM from

Do you think it could be a good test to put the propeller in a jig with the rear end of the loop in the torque meter and take a video? I know is not the same that real flight conditions because the real propeller incidence is less in flight that static on a jig... then I could check the changes between rpm and torque.

I think is very difficult to make a video in my f1M-l during flight and check the rpms but maybe I will try because I have so much time
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Tapio Linkosalo
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« Reply #13 on: April 05, 2020, 11:49:25 PM »

Static testing differs way too much from flight conditions, so the times for pitch change would be quite different. You might get a better evaluation of the changes using a whirling arm testing (where the prop "flies" around in a circular path), but even that would take considerable effort and the results might not match reality.
 
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Skymon
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« Reply #14 on: April 09, 2020, 05:21:14 AM »

If you have a smartphone then download any BPM app.
That allows you to watch the plane and tap each revolution of the prop.
I use one to count RPM and log when it changes.
You can get quite sophisticated graphs if you take a reading regularly.


If you statically test then you can easily get a comparative set of data.
The same as rubber testing.
It won't reflect exactly what is happening on your airplane, but it will show you the changes over the torque curve.
That info might be useful or it might not.

You are correct though - the path of the rubber torque reduction is a curve, whilst the path of spring torsion is linear.

If you set the VP to change on the knee of the rubber curve it will operate like a switch to move from the rapidly dropping pat of the rubber curve to the flatter cruise.
If you back off the rubber after winding down the sharp drop to the cruise section then maybe you can match the hub spring to the rubber gradient..

All very interesting stuff Smiley
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Jorgefly
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« Reply #15 on: April 09, 2020, 02:26:03 PM »

If you have a smartphone then download any BPM app.
That allows you to watch the plane and tap each revolution of the prop.
I use one to count RPM and log when it changes.
You can get quite sophisticated graphs if you take a reading regularly.


If you statically test then you can easily get a comparative set of data.
The same as rubber testing.
It won't reflect exactly what is happening on your airplane, but it will show you the changes over the torque curve.
That info might be useful or it might not.

You are correct though - the path of the rubber torque reduction is a curve, whilst the path of spring torsion is linear.

If you set the VP to change on the knee of the rubber curve it will operate like a switch to move from the rapidly dropping pat of the rubber curve to the flatter cruise.
If you back off the rubber after winding down the sharp drop to the cruise section then maybe you can match the hub spring to the rubber gradient..

All very interesting stuff Smiley


I think the most important information when you buy or build a VP from a plan is the range that the VP works on which is the spring range (torque/linear or angular displacement.
I mean: the torque range that the spring supports from maximum high pitch to minimum low pitch. Thus each user could adjust the VP to the torque curve of his rubber.

Attached mi VP study
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