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May 15, 2006, 12:26 PM
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MikeAnderson's Avatar
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F5B Winning Strategies - Climb Rate or Pitch Speed?


A recent discussion on "what prop should I use in plane x" got me thinking about what combination of thrust and speed wins contests.

We want planes with the highest rate of climb (RofC) with the highest pitch speeds possible so we consume the fewest seconds getting to altitude and enter the course at the highest possible speed.

But pitch speed and RofC are usually mutually exclusive - RofC costs Pspeed & Pspeed costs RofC. (For a given motor/pack/ratio combo, changing only props)

So I pose this to the experienced competitors: If you have to make a choice between RofC and pitch speed, which would you choose?
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May 15, 2006, 12:32 PM
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Andy W's Avatar
Quote:
Originally Posted by MikeAnderson
But pitch speed and RofC are usually mutually exclusive - RofC costs Pspeed & Pspeed costs RofC. (For a given motor/pack/ratio combo, changing only props)
For a given motor/pack yes, but the idea is if thrust>>weight, then pitch speed is DIRECTLY related to rate of climb..
..a
May 15, 2006, 12:58 PM
I do not know if any one else noticed but, most of the top F5B pilots use home made hubs and props. One thing in common is the pivot pin on the hub is at a fairly steep angle, this would allow for a change in the pitch with load/rpm change.
May 15, 2006, 02:04 PM
I plan to take over world
sayno2glo's Avatar
Quote:
Originally Posted by easy fly
I do not know if any one else noticed but, most of the top F5B pilots use home made hubs and props. One thing in common is the pivot pin on the hub is at a fairly steep angle, this would allow for a change in the pitch with load/rpm change.
Or just more streamlined design- less drag.
May 15, 2006, 02:27 PM
Registered User
Quote:
Originally Posted by MikeAnderson
A recent discussion on "what prop should I use in plane x" got me thinking about what combination of thrust and speed wins contests.

We want planes with the highest rate of climb (RofC) with the highest pitch speeds possible so we consume the fewest seconds getting to altitude and enter the course at the highest possible speed.

But pitch speed and RofC are usually mutually exclusive - RofC costs Pspeed & Pspeed costs RofC. (For a given motor/pack/ratio combo, changing only props)

So I pose this to the experienced competitors: If you have to make a choice between RofC and pitch speed, which would you choose?
This is the million dollar question, and really requires self-test based on the particular factors of your plane. You may have all the pitch speed but not enough bite to carry up a heavier airplane. Alternatively, you may not have enough pitch speed if the weight is lower and the prop gets on step far before entering the course. Lately my strategy has been to hear the motor wind up just before power off (while doing a 3-3.4 sec climb for 4 legs). I like to hear a zip up in RPMs just after you level out after the climb.
Last edited by tdearman; May 15, 2006 at 02:32 PM.
May 15, 2006, 02:27 PM
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Kyri's Avatar
Quote:
Originally Posted by easy fly
I do not know if any one else noticed but, most of the top F5B pilots use home made hubs and props. One thing in common is the pivot pin on the hub is at a fairly steep angle, this would allow for a change in the pitch with load/rpm change.

The offset and twisted parts do not allow pitch changes, because with any rpm the prop opens out. It does however maximise the cleanliness of the front section when the prop is folded, therefore reducing drag.

With the higher pitches that we are experiencing nowadays, the increased twist (eg 19deg as opposed to 5 or 7) just allows the props to fold flatter.
May 15, 2006, 03:04 PM
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Kyri's Avatar
Quote:
Originally Posted by MikeAnderson
So I pose this to the experienced competitors: If you have to make a choice between RofC and pitch speed, which would you choose?

As the diameters of props being used has got bigger, this has helped a lot with static thrust. However to maintian the pitch speed the prop pitch has had to increase. The ratio of pitch to dia used to be 1:1, not usually more, however with the 18 and 19 inch props having more pitch than dia seems OK.

The other aspect is thin vs wide vs whatever (many blade designs). The issue is not simple and in most cases, judging a prop by trying it on the course can be the only way to see whats best. Even then, there are more varibles that can affect the performance more than the prop (battery temp, condition, pilot....)

Generally, it is a case of propping to get the best out of your cells, if the cells are not up to it then underpropping can actually get better use of the available power (the battery voltage wont drop as much with the smaller prop). However this is not recommended as a winning strategy

Regards

Kyri
May 15, 2006, 04:48 PM
Quote:
Originally Posted by Kyri
The offset and twisted parts do not allow pitch changes, because with any rpm the prop opens out. It does however maximise the cleanliness of the front section when the prop is folded, therefore reducing drag.

With the higher pitches that we are experiencing nowadays, the increased twist (eg 19deg as opposed to 5 or 7) just allows the props to fold flatter.

From the photos I saw the pin angel on the home brew hubs was around 25 to 30 degrees. If the blades had no mass the aero pull would fold the blades to the front, the cone angel is determined by the balance of the centrifugal force and the aero pull forward, if you change the longitudinal centre of mass ( in front or behind the pin) the cone angel will change, if aero load changes the cone angel will change, if the centrifugal force (RPM) changes the cone angel will change, the pitch changes more with a higher pin angel. If the production off the shelf stuff is good enough why are the top guys building their own hubs and props?
May 15, 2006, 11:45 PM
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bhchan's Avatar
Quote:
Originally Posted by easy fly
If the production off the shelf stuff is good enough why are the top guys building their own hubs and props?
It is the " I got something that you don't have" thing in the competition. Or "My work better than yours"....

And A lot of the pilot wants to have "their own" in stead of everyday stuff.

Brian, an EAJ
May 16, 2006, 01:20 AM
f5b-uk
Mike Seale's Avatar
Quote:
Originally Posted by easy fly
the cone angel is determined by the balance of the centrifugal force and the aero pull forward
Have you done any calculations of centrifugal force vs. thrust. I think you'll find that centrifugal force is so much greater and the blades are always creating the same shape / pitch.
Quote:
Originally Posted by easy fly
If the production off the shelf stuff is good enough why are the top guys building their own hubs and props?
A lot of the production stuff is copied from the bespoke units made by the top pilots/teams. Modellers without the time or skill will have to buy the commercial units and hope they find a good match for their model. The top guys will always be one step ahead because they will continue to make their own props and spinners to match the motor/battery perfectly.

Mike - glad to know there's a cone angel looking after me
May 16, 2006, 02:43 PM
How about one blade with less pitch than the other, best of both worlds?
May 16, 2006, 02:53 PM
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Kyri's Avatar
That would make an interesting sound, one blade would have to be slightly longer to make sure the blades are balanced (statically, don't know what would happen when you spin it up

What about this?
May 16, 2006, 03:08 PM
f5b-uk
Mike Seale's Avatar
Quote:
Originally Posted by easy fly
How about one blade with less pitch than the other, best of both worlds?
I think Dave Dixon did this by accident once (16x16 and 16x17). Didn't notice any problems until he came to swap the blades to 16x16 and realised he was already half way there!!
May 16, 2006, 03:14 PM
Quote:
Originally Posted by Kyri
That would make an interesting sound, one blade would have to be slightly longer to make sure the blades are balanced (statically, don't know what would happen when you spin it up

What about this?

Achieving static balance is a non issue, just look at single bladed props, the cother issue is the outer half of the prop does 90% of the work and look at were the largest surface area is.
May 16, 2006, 03:37 PM
Registered User
Quote:
Originally Posted by easy fly
Achieving static balance is a non issue, just look at single bladed props, the cother issue is the outer half of the prop does 90% of the work and look at were the largest surface area is.

I am not sure about the 90% thing. Some props like the Italian F5B props can lose 1.5 inches off the tips without any significant difference in current draw or performance.
Kyri has some pics of my version of the Italian letter opener prop.

Dave


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