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Old Nov 02, 2006, 07:08 PM
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AndrésMtnez's Avatar
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RPM: lower´s better for traction but, were´s the limit?

Hi all


A friend of mine is going to use a setup for a glider (well, I´m not sure if I should name a Pike Brio like a glider ) were the prop is going to spin around 4000-4500rpm, and that makes me doubt about the minimum advisable rpm...


About the max, I know f5d setups spins around 30k, even more, so I supose that´s marking the max limit but, where is the opposite? 2k? 3k?



Thanks
Andres
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Old Nov 02, 2006, 08:10 PM
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You can figure out the pitch speed. You need to fly fast enough so it will stay airborne. This one is for a boat, but put in the pitch in inches give it a 1 to 1 ratio and a slip of .01 and you have pitch speed.

http://www.rbbi.com/folders/prop/propcalc.htm

For example: 4000 rpm with a 6" pitch yields 22.5 mph pitch speed. Sail planes don't need to fly very fast to stay airborne I don't think.

Len
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Old Nov 03, 2006, 03:13 AM
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EDF will do 30k-50k..

I have found that the sensible upper limit on diameter is at aboute 1/3rd the model wingspan..much more than that and the torque makes it a bit ugly to fly.

Drag on the model blades however will get lower the slower it rotates..so bigger is always better from that pint of view..the old conrest rubber models used to have huge very coarse blade props doing a few hundred RPM only.
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Old Nov 03, 2006, 03:35 AM
hul
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Quote:
Originally Posted by vintage1
the old contest rubber models used to have huge very coarse blade props doing a few hundred RPM only.
that might be because the rubber motor only has a limited number of revolutions.
The limiting factor is possibly Reynolds number of the blade.

It's a good question.

Hans
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Old Nov 03, 2006, 04:34 AM
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But the rubber powered contest world became very sophisticated, technically, knows all about gearing (after all they use plenty of geartrains to get the motors wound up!). So they could easily choose to use their tremendous torque to turn a small prop fast. Except for scale models, they go for very large slow-revving folding props - it's a matter of choice, not necessity.
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Old Nov 03, 2006, 04:54 AM
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..and they were the quietest power models ever.

I remember at the 1968 or 9 Nationals watching rubber contset models..just a woof-woof-woof as the single bade propallor took the model up in a 60 degree climb.. almost NO prop noise at all. Has to be efficient.

I would guess tip velocities no more than 30-40mph. On a ten inch prop around 110 RPM if I have it right.

I seem to remember that 2-300 turns and a motor run of about 30 seconds?..thats 400-600RPm..Hmm. Anyway well below 1000 RPM.

Just foir fun I took one of my stock planes that fles pretty well on a fgeared 600 at about 5500 ROM prop and set it up for a 20x30 prop..o 13:1 gearing..50% more rate of climb on this 60" spanner..slightly better on a 35:1 geared 30x60 prop doing 705 RPM

Beyond that it really got a bit silly.
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Old Nov 03, 2006, 06:45 AM
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Germany, BY, Gräfelfing
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Hey look for optimisation of an humanpower propeller for an aircraft like this (it´s in german,maybe there is some similar in englisch by Prof.P.Macready , http://www.mh-aerotools.de/company/paper_2/ulprop.htm .
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Old Nov 03, 2006, 08:07 AM
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My old duration aircraft ran a 20x20 prop at 2000RPM WOT but the 20" pitch was the key. At 2000RPM this works out to 60km/hr.

In the case of the pike at 4-4500 RPM to get a reasonable speed and climb aim for 80km/hr which translates back to 12 or 13" of pitch and as much diameter as your motor/battery will handle. If you have less pitch you will get less speed.

Tim
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Old Nov 03, 2006, 09:13 AM
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He´s going to use a 18,5x12 prop, so I´m sure he won´t have any problem, but I had always look for 6k-9k rpm setups (except when I´m only looking for speed) and this made me think about it



Then, I supose there´s no limit more than the pitch speed theese setups could achieve or the physical restrictions for the BIG prop you´re going to need.




Many thanks all
Andres
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Old Nov 03, 2006, 10:01 AM
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In sacle land, to get the right prop diamteres we seem to be nedeing to go down to 3500 or less RPM, especially on WWI types..and go for square or even over square props...judiging by what data I can glean, that's what the full size used as well, at similar RPM.
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Old Nov 03, 2006, 10:35 AM
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The scale work we;ve been doing shows the difference between namby-pamby limp-wristed modern direct drive light aeroplane engines and proper manly geared aero engines. A scale model of a direct-drive Lycoming or Continental powered light aeroplane works fine using a scale prop and "conventional" geared 400 / 600 setups or softish outrunners used DD. On a typical Lycoming with a fixed pitch prop, on a 180 horse Cherokee for example, you will get about 2100 rpm static, about 2500 rpm at full throttle low level in the climb, and will cruise at anything between 2300 and 2500 rpm with the throttle well back. Max power is theoretically at 2700 rpm, and redline is 3000. At higher levels the revs rise significantly for a given power setting.

In contrast my Yak, with its geared radial, used to rumble along at low cruise at 1800 rpm on the engine, only 1200 on the prop. High cruise was 2100rpm, 1400 rpm on the prop.
The Merlin on fixed pitch Watts is the real shocker for low prop revs, though. Even at climb speed it can only hit 2200 engine rpm at low level, so I don't think it can be much over 1800 rpm static and redline / peak power is 3000 rpm, which it just can't reach in level flight near the ground - the pitch is super-steep to let you apply full throttle without over-revving at the intended combat altitude of 20,000 feet plus. But the engine is geared 2.13:1, so we're probably looking at less than 1000rpm on the prop at take-off. Which at 22 feet pitch means the pitch speed is still 240 or 250 mph. Eek and double eek.

We don't fly our models at 20,000 feet so we can do the same as Supermarine would have done - and actually did do - for a Spitfire that was only intended to fly at low level - we can reduce the pitch/diameter ratio from 2:1 to 1.5:1 The first Spitfire prototype did exactly that for its first few flights.

However, even the best case low wing loading scenarios we can work out for practical outdoor RC models mean we are stalling at perhaps twice geometric scale speed, and if we have enough power we will have a higher than geometric scale top speed too. Plus, we are undoubtedly losing overall efficiency from low Reynolds number, so it seems reasonable that we are going to end up with a scale-ish prop revving about two or two and a half times as fast as the original, and the maths all reconciles.

Anyone really interested in gearing for serious torque applications should have a look at this detailed engineering analysis of the Merlin reduction gearing - here
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Old Nov 03, 2006, 12:52 PM
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Quote:
Originally Posted by Work in Progress

Anyone really interested in gearing for serious torque applications should have a look at this detailed engineering analysis of the Merlin reduction gearing - here
Actually hindsight is a wonderful thing. I have met with and talked to engineers who designed both Formula one engines and gearboxes. The basic principle is 'keep lightening it till it breaks, and then add back the last bit you machined off'

Analysis of the result shows that this sort of stepwise iterative design evolution often leads to the optimal solutions of many engineering problems.

If the overall graph of performance is more or less continuous, with no local minima., it doe in time lead to the exact minimal solution.

It's like water finding its way to the bottom of a valley. You don't know where you will end up, but as long as there is a slope you follow it down.....local 'puddles'. are catered for by occasionally making large jumps out of 'low places' - if you end up lower than where you were, you carry on down.

I forget the name, of this algorithm..but its ben used in many design processes with computer simulation.

Prior to computers, you just did a lot of prototyping..
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Old Nov 04, 2006, 05:47 AM
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Quote:
Originally Posted by Work in Progress
The scale work we;ve been doing shows the difference between namby-pamby limp-wristed modern direct drive light aeroplane engines and proper manly geared aero engines. A scale model of a direct-drive Lycoming or Continental powered light aeroplane works fine using a scale prop and "conventional" geared 400 / 600 setups or softish outrunners used DD. On a typical Lycoming with a fixed pitch prop, on a 180 horse Cherokee for example, you will get about 2100 rpm static, about 2500 rpm at full throttle low level in the climb, and will cruise at anything between 2300 and 2500 rpm with the throttle well back. Max power is theoretically at 2700 rpm, and redline is 3000. At higher levels the revs rise significantly for a given power setting.

In contrast my Yak, with its geared radial, used to rumble along at low cruise at 1800 rpm on the engine, only 1200 on the prop. High cruise was 2100rpm, 1400 rpm on the prop.
The Merlin on fixed pitch Watts is the real shocker for low prop revs, though. Even at climb speed it can only hit 2200 engine rpm at low level, so I don't think it can be much over 1800 rpm static and redline / peak power is 3000 rpm, which it just can't reach in level flight near the ground - the pitch is super-steep to let you apply full throttle without over-revving at the intended combat altitude of 20,000 feet plus. But the engine is geared 2.13:1, so we're probably looking at less than 1000rpm on the prop at take-off. Which at 22 feet pitch means the pitch speed is still 240 or 250 mph. Eek and double eek.
In the world of engines, we need a PSRU (prop speed reduction unit - aka "gearbox") to spin a prop at the most efficient speed. This speed is around 1/2 of the speed where the engine makes its best horsepower and torque - which is why you need a gearbox. I personally don't like the added weight and complexity of gearboxes, although they do allow you to fine tune things.

You can get the same sort of effect as a gearbox by altering the wiring on an outrunner electric motor. More winds = more torque and less rpms. There are some charts and calcs that will get you into the ballpark, but the fine tuning is up to you. Or you can just put the question up in the power systems forum and ask other folks to chime in

I'm not going to get into the gearbox vs DD argument - I've stated my preference. Both have their advantages and disadvantages - the choice is yours to make.

Brad
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Old Nov 04, 2006, 06:06 AM
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Quote:
Originally Posted by arx_n_sparx
In the world of engines, we need a PSRU (prop speed reduction unit - aka "gearbox") to spin a prop at the most efficient speed. This speed is around 1/2 of the speed where the engine makes its best horsepower and torque - which is why you need a gearbox. I personally don't like the added weight and complexity of gearboxes, although they do allow you to fine tune things.

You can get the same sort of effect as a gearbox by altering the wiring on an outrunner electric motor. More winds = more torque and less rpms. There are some charts and calcs that will get you into the ballpark, but the fine tuning is up to you. Or you can just put the question up in the power systems forum and ask other folks to chime in

I'm not going to get into the gearbox vs DD argument - I've stated my preference. Both have their advantages and disadvantages - the choice is yours to make.

Brad
Sadly I have to disagreee in quantity, though not quality, with what you say.

I would say that teh best regime for props on most models is 600-6000 RPM. Outrunners are not really efficient below 5000, and most inrunners develop best power in the 15k-50k region.

That means very few outrunners ungeared are optimal, whereas gear ratios should be more like 10:1 on average.

W.I.P. and I will let you know what happens with our exploration of low RPM steep pitched props...one thing I have established is that outrunners are very inefficient at low RPM - in terms of power to weight anyway. You can get the electrical efficiency, but not at good power levels.

Only very fast models need more than 10K RPM at the prop.
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Old Nov 04, 2006, 08:10 AM
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Quote:
Originally Posted by vintage1
Sadly I have to disagreee in quantity, though not quality, with what you say...

I would say that teh best regime for props on most models is 600-6000 RPM. Outrunners are not really efficient below 5000...

That means very few outrunners ungeared are optimal, whereas gear ratios should be more like 10:1 on average.
Regrettably, I have to disagreee in quantity, though not quality, with what you say as well.

Outrunners are very efficient below 6000 rpm, but their power-to-weight ratio is not very good below 6000 rpm.

I'd say that the best regime for props on most models is 6000-12,000 RPM... Good pitch speed, high efficiency and excellent rpm match w/o gearbox to outrunners. Inrunners are also geared to this rpm range, since very high gear ratios need two reduction stages and loose efficiency!
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