RE: O.S 55AX vs. 61FX?  
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RE: O.S 55AX vs. 61FX? - 5/27/2008 8:28:45 AM   
DarZeelon



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From: Rosh-Ha'Ayin, ISRAEL
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quote:

ORIGINAL: XJet


quote:

ORIGINAL: DarZeelon
But I have seen an electric powered (KD A50-14L) Katana V2 do very nice 3D, using a 16x12 prop.


The difference with electric is that the torque curve is *much* different to an IC engine.

An electric motor develops its maximum torque when stalled (ie: 0 RPMs). An IC engine usually develops its maximum torque at substantial RPMs.

So electric motors respond to hi loads much better than IC engines do and a hi-pitch (for a given diameter) generally means a higher load.

One of the factors for a 3D prop on an IC engine is that it should represent a light enough load that the engine can spool up to an RPM where it makes good power with the minimum delay. Too coarse a pitch means that it may have trouble getting up onto the peak of the torque curve so flatter props are often used to help. The other factor is that most 3D planes aren't designed for hi-speeds so a flat-pitch prop will avoid overspeeding the airframe and the resulting risk of damage that would arise.

But, given the RPMs that a modern 2-stroke can pull, a fine pitch doesn't necessarily mean *slow*. Check out this YouTube video for proof of that.



Bruce,


I know... I know... that clip which has the 12x4 prop spinning and showing quite a brisk pace, on a high-drag trainer type...

As to the effect of high pitch vs. large diameter...
As far as load on the engine, diameter has roughly 4 times the effect of pitch...

If you take the load of a given prop and double its diameter, the load will grow as much as if you multiply its pitch by 8...


But... diameter has another very profound effect. Inertia.

The inertia of a larger diameter prop is much greater than that of a smaller diameter prop.
Pitch has no effect at all on prop inertia.

So, although 'spool up' is a term only usable for turbine engines, prop pitch is an immaterial criterion for RPM change...

Load is, but it is more effective as higher RPM sets in...
I.e. even with an internal combustion engine, acceleration from 2,000 to 4,000 RPM is almost unaffected by prop load (pitch and diameter), but is very heavily affected by prop inertia (diameter only).
Acceleration from 11,000 to 13,000 RPM, however, is quite heavily affected by prop load (pitch and diameter), because aerodynamic forces are a function of air-speed (RPM) squared; and it is also affected by prop inertia (diameter only).


Do you really think that trainer of yours would fly as briskly, if you put a 15x2 prop on its nose?! I don't think so.

Also, I think very few 3D 'artists' use a flat prop on their planes, in order to protect their planes from the harmful effects of over-speeding... They would exercise judicious throttle control for that...



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(in reply to XJet)
       Post #: 26

RE: O.S 55AX vs. 61FX? - 5/27/2008 10:01:30 AM   
XJet


 

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quote:

ORIGINAL: DarZeelon
I know... I know... that clip which has the 12x4 prop spinning and showing quite a brisk pace, on a high-drag trainer type...

Which makes the speed even more impressive - no? It's easy to get a low-drag airframe up to speed but a dragy one requires more work.


quote:

But... diameter has another very profound effect. Inertia.

All else being equal, yes. But if you compare (say) an APC 11.5x4W with an APC 12x4, the 12x4 has much less inertia. The blades of the 12x4 are very thin at the tip and the overall mass of the 11.5x4W is far greater (including at the tips). Which is to say that here are several factors that determine the inertia.

quote:

The inertia of a larger diameter prop is much greater than that of a smaller diameter prop.
Pitch has no effect at all on prop inertia.
Except that a hi-pitch prop often has a thicker blade because the flex loading on those blades is higher due to their greater angle of attack and the higher loading that this places on them.

quote:

So, although 'spool up' is a term only usable for turbine engines, prop pitch is an immaterial criterion for RPM change...
Ah the English language is a wonderful thing it is constantly changing. In 3D circles, spool-up is comonly used to refer to the rate at which an engine increases its revs in relation to opening the throttle.

quote:

Load is, but it is more effective as higher RPM sets in...
I.e. even with an internal combustion engine, acceleration from 2,000 to 4,000 RPM is almost unaffected by prop load (pitch and diameter), but is very heavily affected by prop inertia (diameter only).

True, but that's not really the part of the power-curve that's important to 3D fliers.

quote:

Acceleration from 11,000 to 13,000 RPM, however, is quite heavily affected by prop load (pitch and diameter), because aerodynamic forces are a function of air-speed (RPM) squared; and it is also affected by prop inertia (diameter only).

That also is true but you'll find that a hi-pitched prop is likely to have a greater amount of induced drag for a given amount of thrust when compared to a larger diameter fine-pitched one producing the same thrust. The l/d plot of most airfoils shows that the induced drag increases faster than the lift over much of the operating curve. This is why gliders often have the best glide ratio when flown at a speed greater than the one which produces the lowest rate of sink.

So, a coarse-pitched prop is likely to be less efficient in a 3D application because for a given amount of thrust, it's creating more drag. Thus, when you throttle up to pull out of the hover, the engine has more load (in the form of that induced drag) to overcome therefore "spools up" less quickly.

Sound good? ;-)


quote:

Do you really think that trainer of yours would fly as briskly, if you put a 15x2 prop on its nose?! I don't think so.
. No, and I did say there was a point where increasing the diameter and decreasing the pitch actually produced a worse result.

quote:

Also, I think very few 3D 'artists' use a flat prop on their planes, in order to protect their planes from the harmful effects of over-speeding... They would exercise judicious throttle control for that...

Ah, I fly a lot of 3D and a lower-pitched prop is actually quite useful (in conjunction with prudent throttle management) in maintaining a safe airspeed. A big low pitched prop makes a damned fine airbrake on the downlines too.


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       Post #: 27

RE: O.S 55AX vs. 61FX? - 5/27/2008 10:42:13 AM   
suzonka


 

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From: calverton Long Island, NY, USA
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quote:

ORIGINAL: DarZeelon


quote:

ORIGINAL: XJet


quote:

ORIGINAL: DarZeelon
But I have seen an electric powered (KD A50-14L) Katana V2 do very nice 3D, using a 16x12 prop.


The difference with electric is that the torque curve is *much* different to an IC engine.

An electric motor develops its maximum torque when stalled (ie: 0 RPMs). An IC engine usually develops its maximum torque at substantial RPMs.

So electric motors respond to hi loads much better than IC engines do and a hi-pitch (for a given diameter) generally means a higher load.

One of the factors for a 3D prop on an IC engine is that it should represent a light enough load that the engine can spool up to an RPM where it makes good power with the minimum delay. Too coarse a pitch means that it may have trouble getting up onto the peak of the torque curve so flatter props are often used to help. The other factor is that most 3D planes aren't designed for hi-speeds so a flat-pitch prop will avoid overspeeding the airframe and the resulting risk of damage that would arise.

But, given the RPMs that a modern 2-stroke can pull, a fine pitch doesn't necessarily mean *slow*. Check out this YouTube video for proof of that.



Bruce,


I know... I know... that clip which has the 12x4 prop spinning and showing quite a brisk pace, on a high-drag trainer type...

As to the effect of high pitch vs. large diameter...
As far as load on the engine, diameter has roughly 4 times the effect of pitch...

If you take the load of a given prop and double its diameter, the load will grow as much as if you multiply its pitch by 8...


But... diameter has another very profound effect. Inertia.

The inertia of a larger diameter prop is much greater than that of a smaller diameter prop.
Pitch has no effect at all on prop inertia.

So, although 'spool up' is a term only usable for turbine engines, prop pitch is an immaterial criterion for RPM change...

Load is, but it is more effective as higher RPM sets in...
I.e. even with an internal combustion engine, acceleration from 2,000 to 4,000 RPM is almost unaffected by prop load (pitch and diameter), but is very heavily affected by prop inertia (diameter only).
Acceleration from 11,000 to 13,000 RPM, however, is quite heavily affected by prop load (pitch and diameter), because aerodynamic forces are a function of air-speed (RPM) squared; and it is also affected by prop inertia (diameter only).


Do you really think that trainer of yours would fly as briskly, if you put a 15x2 prop on its nose?! I don't think so.

Also, I think very few 3D 'artists' use a flat prop on their planes, in order to protect their planes from the harmful effects of over-speeding... They would exercise judicious throttle control for that...



@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
The video is quite impressive ,however what size engine is that????

(in reply to DarZeelon)
       Post #: 28

RE: O.S 55AX vs. 61FX? - 5/27/2008 6:03:11 PM   
blw



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Joined: 3/15/2004
From: Auburn, AL, USA
Status: online
quote:

ORIGINAL: XJet


quote:

So, although 'spool up' is a term only usable for turbine engines, prop pitch is an immaterial criterion for RPM change...
Ah the English language is a wonderful thing it is constantly changing. In 3D circles, spool-up is comonly used to refer to the rate at which an engine increases its revs in relation to opening the throttle.


Ah, I fly a lot of 3D and a lower-pitched prop is actually quite useful (in conjunction with prudent throttle management) in maintaining a safe airspeed. A big low pitched prop makes a damned fine airbrake on the downlines too.



If you have been around turbines and hear recip guys say that, it sounds wrong. Spooling up refers to the compressor section delay, and usually is more pronounced than with any prop effect. One of the differences is that you can hear the turbine slow down momentarily when you increase throttle demands drastically.

A lot of pattern guys are going to 3 bladed props now for the same braking action on down lines.

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       Post #: 29

RE: O.S 55AX vs. 61FX? - 5/27/2008 8:24:54 PM   
asmund


 

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Joined: 11/20/2005
From: Floroe, NORWAY
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quote:

ORIGINAL: XJet
I would have expected more than that.
Even my oldest TT46Pro will turn a 12x4 at nearly 14K RPMs on 5% nitro fuel.



Maybe the 61 FX can swing the APC 12-4 a little more than 13500 rpm, I didn`t try that prop on my 61 FX. I do know however that the 61 FX is one of the weakest 61`s out there and mine could only swing 12.25-3.75 at 13100 rpm. All my other 49-52 engines usually swing the 12-4 at 4-500 rpm faster than the 12.25-3.75, so I "guestimated" that the this also goes for the 61 FX which I no longer own. It might make it to 13700 or something but I seriously doubt that it can break 14K on that prop.

My tuned 52 has broken the 15K barrier that prop and is 200 grams lighter than the 61 FX, and that is exactly why I got rid of my 61 FX.

It will fly fine on a 12-4 and it will fly fine on a 13-4 too, try both and stick to the one YOU like the most

(in reply to XJet)
       Post #: 30

RE: O.S 55AX vs. 61FX? - 5/27/2008 9:43:02 PM   
XJet


 

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Joined: 3/31/2003
From: Tokoroa, NEW ZEALAND
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quote:

ORIGINAL: suzonka
The video is quite impressive ,however what size engine is that????

It's a Thunder Tiger 46 Pro with a Tower/GMS47 muffler and fuel blended with just 12% of Coopers Plus-C oil and 5% nitro.


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       Post #: 31

RE: O.S 55AX vs. 61FX? - 5/27/2008 10:34:28 PM   
suzonka


 

Posts: 643
Joined: 11/22/2004
From: calverton Long Island, NY, USA
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quote:

ORIGINAL: XJet

quote:

ORIGINAL: suzonka
The video is quite impressive ,however what size engine is that????

It's a Thunder Tiger 46 Pro with a Tower/GMS47 muffler and fuel blended with just 12% of Coopers Plus-C oil and 5% nitro.


THANKS THAT REALLY CRANKS WITH THE 12X4 PROP ,HE COULDNT SLOW IT DOWN

(in reply to XJet)
       Post #: 32

RE: O.S 55AX vs. 61FX? - 5/27/2008 10:36:33 PM   
suzonka


 

Posts: 643
Joined: 11/22/2004
From: calverton Long Island, NY, USA
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quote:

ORIGINAL: asmund


quote:

ORIGINAL: XJet
I would have expected more than that.
Even my oldest TT46Pro will turn a 12x4 at nearly 14K RPMs on 5% nitro fuel.



Maybe the 61 FX can swing the APC 12-4 a little more than 13500 rpm, I didn`t try that prop on my 61 FX. I do know however that the 61 FX is one of the weakest 61`s out there and mine could only swing 12.25-3.75 at 13100 rpm. All my other 49-52 engines usually swing the 12-4 at 4-500 rpm faster than the 12.25-3.75, so I "guestimated" that the this also goes for the 61 FX which I no longer own. It might make it to 13700 or something but I seriously doubt that it can break 14K on that prop.

My tuned 52 has broken the 15K barrier that prop and is 200 grams lighter than the 61 FX, and that is exactly why I got rid of my 61 FX.

It will fly fine on a 12-4 and it will fly fine on a 13-4 too, try both and stick to the one YOU like the most



tHANKS AGAIN, WELL i DONT HAVE TO WORRY ABOUT OVER REVING THE 61 os ACCORDING TO YOUR FINDINGS ,THATS GOOD NEWS

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< Message edited by suzonka -- 5/27/2008 10:37:39 PM >

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       Post #: 33

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