# Methods of Powering Stock Props- Application and Efficiency

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Before Reading I'd recommend that you'd watch Bradley Whistance's video on his stock prop speed test. The method he outlines is not relevant to the actual math work, and simply affects the values I will be plugging in.
I will only discuss the resulting data in this thread to keep things simple, if you have specific questions about the math, let me know.

With that out of the way, let me preface this discussion. The dominant method of powering stock props is by using RTGs, which makes sense. They continuously generate power, allowing stock props to run forever, this is great for Duna, Eve, and Laythe exploration. However, I commonly see this used on props built for fighter craft, transport aircraft, and others. While there are some cases where endless flight is desirable on Kerbin, such applications certainly do not qualify. Most people don't fly a single stock prop fighter for hours on end (without crashing). This leads to the question, are RTGs the most efficient way of powering these props? Short answer, no, but I'll explain in more detail. I am taking efficiency to mean weight in this conversation, although other uses of the term, such as part count, may also be referenced.

First we must establish all of the possible methods of powering a stock prop: RTGs (duh), batteries, and fuel cells. RTGs we already discussed. Batteries would simply run the engine using their stored electric charge, and the flight would end once they run out. Fuel cells would burn LFO (Liquid Fuel + Oxidizer), continuously generating the engines power needs whilst draining LFO from on board tanks.

Of course, I wouldn't even consider the fuel cells to be an option unless I had a good reason. Doing some simple math, we can find that all batteries hold 20,000 units of electric charge per tonne. How much electric charge, then, is LFO equivalent to per tonne? It depends on which of the fuel cells you are using, but for the small fuel cells this comes out to 79,934 units of electric charge per tonne of LFO, and for the large fuel cells it is 81000 units per tonne.

Looking at the raw data, fuel cells are the obvious winner. However, the additional weight associated with fuel cells throws a wrench into the works, and in either case this tells us nothing about how RTGs stack up. So I will analyze these three choices in an applied setting, where I will test their mettle in a hypothetical prop that utilizes 10 of the 1.25 meter reaction wheels with 2 dumpling fuel tanks used as bearings. I did these calculations presuming that this engine would also utilize the trick described and demonstrated by Bradley Whistance's video. Using this method the reaction wheels consume roughly 2.73 times their normal power (according to my own testing), I'm using ball park numbers so any small discrepancy is irrelevant.

RTGS: The 10 reaction wheels consume 13.65 units of electric charge per second, as such the engine would normally require 19 RTGs to run continuously, which themselves would weigh 1.52 tonnes.

BATTERIES: To run this engine for 1 hour you would need 49,140 units of electric charge. This would require 2.457 tonnes of batteries, so those certainly aren't the best solution, although using the largest battery bank available, this would only require 13 batteries. Since the weight of the batteries required scales directly with time of flight, they are likely the most efficient in very short flights.

FUEL CELLS: The smaller fuel cells generate 79,934 units of electric charge per tonne of LFO, meaning only .615 tonnes of LFO is required to run the engine for 1 hour. This amount of LFO is almost perfectly held by the 2 dumplings in the bearing, plus two oscar tanks which altogether hold .62 tonnes of LFO. In total the tanks would weigh .698 tonnes (including dead weight), and this engine would require 10 of the small fuel cells to run continuously. This adds another .5 tonnes to the total weight, bringing it up to 1.198 tonnes. As well as 14 parts, but really that's 12 since the dumplings have to be there regardless.

In every possible way, the fuel cells are more efficient, while the use of dumpling bearings may seem to bias the fuel cells, in reality this does not affect the weight, and only affects part count. Using a different bearing type would only add 1 additional part, making it on par with batteries and superior to RTGs. None the less, it is far superior to both in terms of weight. This leads me to conclude that Fuel cells are the superior method for powering stockprops intended for short to medium flight times.

It should also be added that it is of course possible to mix these methods together. In this one instance, it is actually beneficial. While 10 fuel cells are required to meet the continuous power generation needs, this is only by a small amount, the raw value is 9.1 fuel cells. This adds the equivalent of 540 additional electric charge generation required over the hour of flight. Since fuel cells come with 50 units of electric charge storage each, this means that 9 of them would have 450 units total, leaving us 90 units short. These 90 units can be accounted for with a single of the smallest battery pack (which has 100 units). Weighing in at a mere .005 tonnes, this change leaves us with the original part count of 12 and a reduced mass of 1.153 tonnes.

So yes, it is possible that adding batteries will lead to a net increase in efficiency, however, the instance described above is the only type of scenario where this is actually the case. So limited is this possibility that even if the real value turned out to be just 9.3 instead of 9.1, then using batteries would result in a net increase in weight. Again, these results may be different for engines of differing sizes. however I can only see fuel cells being knocked from their throne with very different total flight times. For instance, short flight times where batteries' ability to instantly deliver power gives them a major advantage, and long flight times where RTGs' endless power generation eventually overcomes their initial mass costs.

This may be obvious already, but just to clarify, the fact that the math above uses reaction wheels affected by Bradley Whistance's technique IN NO WAY affects the actual math or conclusions. The math above is roughly equivalent to if you had simply used 27 reaction wheels functioning under normal conditions.

The objective of this thread was to demonstrate that fuel cell powered props are a lighter and more part efficient approach to powering certain stockprops. While it took a decent amount of complex work to arrive at this conclusion, in actual application fuel cell powered props are no more difficult to build or use then RTG props.  There's also the small fact that a fuel cell powered prop will not only start out lighter than a RTG powered prop, but it will also get even lighter as LFO is drained throughout the flight, further increasing performance.

I appreciate any questions or feedback you may have. If I made any mistakes, please let me know, and I would love to hear your thoughts on the topic.

@klond I think this may interest you.

Edited by Kronus_Aerospace
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Well, the problem with that is  that LFO stock props need to be refuelled.... which can defeat the point in a lot of cases. Using batteries, you can land, reconnect the rotor, and recharge it while timewarping.

Spoiler

https://i.imgur.com/gnqA2BE.png

https://i.imgur.com/UisLa8I.png

Above I used RTGs, just 'cause. All I wanted was a biome hopper that I could use to go wherever without worrying about fuel.

The "infinite range" aspect of stock props is very appealing, and solar is not attractive because of the difficulty using it with rotors, so that leaves RTG. Any LFO solution is going to lose if you require a long enough range.

So what you said " While there are some cases where endless flight is desirable on Kerbin, such applications certainly do not qualify. Most people don't fly a single stock prop fighter for hours on end (without crashing).

...

i﻿ntended for short to medium flight times. "

Also... aren't many of these stock props not quite "stock", isn't there a change in one of the config files to allow higher RPMs. IIRC, regardless of torque, reaction wheels won't spin a part past a certain number of radians/sec, and many "stock" props use stock parts but the game isn't stock because they change the max rad/sec limit.

I'm pretty sure that is happening in that video.

I think I'll stick to RTGs or batteries and solar, or stock jet engines. Basically anwhere that stock jets don't work, I want unlimited range.

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@KerikBalm that is why I prefaced the discussion by saying that this would be used for planes where unlimited range is unnecessary. Like fighter craft, where increased performance due to lower weight and part count is preferable. I can guarantee you that most people fly a stock prop fighter for half an hour at most.

Edited by Kronus_Aerospace
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5 hours ago, KerikBalm said:

Also... aren't many of these stock props not quite "stock", isn't there a change in one of the config files to allow higher RPMs. IIRC, regardless of torque, reaction wheels won't spin a part past a certain number of radians/sec, and many "stock" props use stock parts but the game isn't stock because they change the max rad/sec limit.

I'm pretty sure that is happening in that video.

No, the prop is not spinning faster than the max allowed rotation speed. It is simply able to achieve a higher thrust to weight since Bradley Whistance is making use of an exploit that allows reaction wheels to increase their power output by 71 percent, in exchange for increasing power consumption by 173 percent. The low weight, high thrust, and aerodynamic efficiency of the engine is what allows it to reach those high speeds. Try the exploit yourself in a stock game, it works.

Also, even though the max rotation speed hurts stock props high speed performance, the rotational speed isn't actually what matters, what matters is the actual velocity of the prop blade, so the low rotation speed cap is compensated for by making the blade diameter very large, hence why his blades are so far out from the engine itself, this also further improves TWR.

Edited by Kronus_Aerospace
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17 hours ago, Kronus_Aerospace said:

I think this may interest you.

Thank you for pinging me.  I am very interested.

I watched Bradley's vid you linked.  What a good find!  Not sure if exploit is the right word, but this is abuse that I can get behind, especially since more torque uses way more resources so it doesn't feel as cheaty as say leg-powered stuff.

I'm not a math guy, but I did some testing for kicks.

One RTG

With one RTG and 1-axis I can run 2 reactions wheels at 100%, and a 3rd at about 87ish.

Spoiler

With one RTG and 2-axis (a 45 degree tilt on the core, re-centered for balance) I can run 1 reaction wheel at 100% and another at 44%

Spoiler

With one RTG and the core set for 3-axis like in Bradley's video I can only do one wheel @ 96%

Spoiler

I don't know if these numbers lineup with the math.  How do the numbers come out theoretically for 2-axis?

I didn't measure power, but you can see the red arrows getting smaller as theorised and power requirements growing as I have to drop wheels and lower their power.  Each axis added allowed me to drop a .05t wheel, but I'm not sure the loss of power was worth the weight savings.  I guess it would depend on design requirements.  I have one tiny plane that may benefit from this discovery.

Anyways, fantastic idea Mr. Bradley and nice write-up Kronus.

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1 hour ago, klond said:

I didn't measure power, but you can see the red arrows getting smaller as theorised and power requirements growing as I have to drop wheels and lower their power.  Each axis added allowed me to drop a .05t wheel, but I'm not sure the loss of power was worth the weight savings.  I guess it would depend on design requirements.  I have one tiny plane that may benefit from this discovery.

Bradley's technique was not really central to the idea of this thread, but I will say that it is better with larger wheels. Since larger wheels are much more power efficient, the increased power generation requirements will be easily overshadowed by the increased thrust. It is likely not worth it for the smaller more inefficient wheels.

That area may be helped by using LFO and fuel cells as I described, as they offer a better weight efficiency. While I'm too tired to even attempt the math now, I believe it is possible to produce a higher TWR .625 meter prop using Bradley's method if combined with fuel cell power, although part count would likely be higher as well.

Regardless of the props size, or whether or not Bradley's trick is used, using fuel cells instead of RTGs will result in a higher TWR.

Edited by Kronus_Aerospace
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On 3/7/2019 at 8:19 PM, Kronus_Aerospace said:

Bradley's technique was not really central to the idea of this thread

You're right, sorry I got a bit off topic.  Too excited about the multi-axis thing.  If your goal is 'most power for least amount of parts/weight' then multi-axis is a good compliment to your fuel cell argument.

I was thinking about the math for 2-axis again.  A line in one vector and a perpendicular line in another vector would make a right triangle.  If both lines were 100% power and A sqr + B sqr = C sqr I would think then a probe core at a 45degree angle for 2-axis should use 200% electricity and give 141% power (so 41% more).

I wouldn't mind doing multi axis myself, and for videos, but putting a craft on kerbalx then trying to tell peeps how to spin it up properly, I'm not sure it would go super well.  Better to keep it simple then.

Sorry if I'm still off-topic, but this was presented early in the first post and I'm still stuck on it.  Hard to disagree with your fuel cell arguement; you provide strong evidence, especially if you're already using fuel tanks as bearings.

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@klond Your math makes sense, looking at the numbers you reached it seems far more practical to go for a 2 axis design, as 3 axis gives diminishing returns in terms of torque per power consumption, +41% to +30%. For the sake of part count I would personally go with a 2 axis prop if at all.

While it may be a bit off topic, this thread is meant to discuss the more complex and in-depth side of stock props, so I don't really mind all to much.

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Very interesting thread! I've always used fuel cells because they lower part count and it feels less cheaty than magic infinite electricity RTGs. I wasn't aware it was the most efficient option though.

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1 hour ago, Ol’ Musky Boi said:

Very interesting thread! I've always used fuel cells because they lower part count and it feels less cheaty than magic infinite electricity RTGs. I wasn't aware it was the most efficient option though.

Thanks!

@klond Keeping with the off topic discussion, I decided to do the math on whether or not 2 axis rotation is more weight efficient if one is using 1.25 meter reaction wheels and fuel cell power. I won't bother describing the math since I imagine that you are perfectly capable of recreating it yourself (and I'm too tired right now to bother). Through testing I found that 2 axis rotation consumes 180% of the reaction wheels normal power consumption, instead of the 200% theoretically proposed. Presuming your 141% torque value, and an ideal scenario, 2 axis rotation produces about 20% more torque per tonne (that per tonne value is the weight of the entire prop assembly). For this math i kept the power generation constant, so I compared a 10 wheel 2 axis prop, to a 18 wheel single axis prop. The results would likely be different if one were to keep # of wheels constant instead of power consumption.

It's not all that significant, unless you're striving for that little bit of extra performance, 2 axis rotation with 1.25 meter props is not all that fantastic. This value would be higher with 2.5 meter props as they are around 39% more power efficient (the reverse is true of .625 meter props).

Considering the previously mentioned diminishing returns of the 3 axis rotation I'd speculate that it would have maybe 30-33% more torque per tonne.

Edited by Kronus_Aerospace
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Upon further in-game testing I have found that Large Fuel cells are especially useful, as they produce 12 times the power for less than 5 times the weight compared to fuel cells. They also operate at a higher efficiency.

Edited by Kronus_Aerospace
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• 1 month later...

What of solar power? Perhaps it's not as reliable but it's very much doable during the day, and can often be incorporated into an inverted bearing design, effectively cutting part count of the power unit. In small aircraft with weak engines possibly even cutting it to zero.

And if it's not an inverted bearing it also allows turboshaft backup power to push on the solar panels from inside.

On 3/7/2019 at 4:48 AM, Kronus_Aerospace said:

@KerikBalm that is why I prefaced the discussion by saying that this would be used for planes where unlimited range is unnecessary. Like fighter craft, where increased performance due to lower weight and part count is preferable. I can guarantee you that most people fly a stock prop fighter for half an hour at most.

I've actually flown them for over an hour before breaking the engine by exceeding the altitude cieling. It was a super low power biplane so not a lot of risk of random explosions.

Edited by Pds314
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What I like about fuel cells is that aircraft typically are combustion engines, not electrically driven.

So a aircraft without combustion that has no solar panels and/or thermo electric generation makes little of an airplane unless the aircraft is obviously covered in solar panels. But that's impossible since solar panels have to be attached to the bearing. Or just place extra solar panels on the wings for cosmetics and make it look as if it's so.

In any case, if part of the bearing isn't shielded (makes for a not so fast propeller plane) you can dock a claw to it to refill the dumpling tanks. I've seen people put solar panels in radial around the bearing casing, can you attach a claw to a solar panel

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@Pds314 I have tested solar panels before for that usage, for fighter craft they are rather un-ideal as the TWR and power requirements of stock prop fighter craft mean a huge amount of solar panels would be needed, panels generate very little power for their size and on a prop would operate at very low efficiency as only half of them would be facing the sun at any one time and even then only some of them would be facing the sun directly. Mounted on the bearing shaft for the engine the panels would have only 31% the area that they would mounted flat, this is the best you can get by the way there is no magical way of mounting them to exceed that number. However, a plane will not always be flying perfectly horizontal, and over the course of a 1 hour flight the sun will not always be directly overhead, so taking those factors into account I will be instead using an efficiency of 25%.

Using the smallest panels, which would be the only ones that you could easily incorporate onto the prop itself which they would need to be attached to, you'd get .0875 units of electric charge per second per panel, which works out to 8.75 units of electric charge per tonne of panels. The hypothetical prop engine outlined in this topic had power requirements of 13.65 units per second, meaning that 1.56 tonnes of panels would be required, which is 156 individual panels. This would put it on par with RTGs in terms of weight, but far behind them all in terms of part count. It is still much heavier than fuel cells.

Being even more generous let's use the XL panels, even though they are so large as to be rather difficult to incorporate into most fighter craft. Using the 25% efficiency number, they generate .7 units of electric charge per second per panel, which is 17.5 units per tonne. Putting them to use in our hypothetical engine .8 tonnes of panels would be required, coming out to 20 panels total. This puts them ahead of fuel cells in terms of weight but still behind in part count.

However, if we are giving the solar panels the luxury of being able to use their larger, more efficient variants, for the sake of a fair comparison we must do the same to fuel cells. The larger fuel cell arrays are around the same size as a single XL panel, however while a single XL panel generates .7 units of electric charge per second in this application, a single fuel cell array generates a whopping 18 charge per second. This comes out to a colossal 75 units of charge per second per tonne of fuel cells. A single large fuel cell would easily meet the requirements of out test engine, this would make the total weight come down to .86 tonnes to power the engine for 1 hour. It not only matches the panels' weight, but could power the engine using just 5 parts, versus the panel's 20.

Keep in mind this is the most ideal scenario, the panels power generation would vary wildly as the craft maneuvered, so realistically the engine would need batteries and extra panels for auxiliary power in such a scenario, the efficiency of the panels would also only decrease if flown earlier or later in the day, and would drop to zero effectively at dawn or twilight as the sun would be shining parallel to the axis of rotation in which the panels are mounted.

Panels may have some advantages for smaller engines, however since I've spent enough time on this already I won't bother doing the math for that and will simply concede the point. However the strict limitations they pack compared to all of the other power generation methods discussed, as well as being more difficult to incorporate make them simply obsolete. Of course looking at the numbers I found, even in poorer conditions for the panels they would still work, I have in the past made fully functional aircraft using them. Doesn't change the fact that they aren't the best performance wise, but outside of applications where that is important, they are perfectly applicable. Though part count wise there is no reason to use them over RTGs.

Edited by Kronus_Aerospace
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19 hours ago, Kronus_Aerospace said:

@Pds314 I have tested solar panels before for that usage, for fighter craft they are rather un-ideal as the TWR and power requirements of stock prop fighter craft mean a huge amount of solar panels would be needed, panels generate very little power for their size and on a prop would operate at very low efficiency as only half of them would be facing the sun at any one time and even then only some of them would be facing the sun directly. Mounted on the bearing shaft for the engine the panels would have only 31% the area that they would mounted flat, this is the best you can get by the way there is no magical way of mounting them to exceed that number. However, a plane will not always be flying perfectly horizontal, and over the course of a 1 hour flight the sun will not always be directly overhead, so taking those factors into account I will be instead using an efficiency of 25%.

Using the smallest panels, which would be the only ones that you could easily incorporate onto the prop itself which they would need to be attached to, you'd get .0875 units of electric charge per second per panel, which works out to 8.75 units of electric charge per tonne of panels. The hypothetical prop engine outlined in this topic had power requirements of 13.65 units per second, meaning that 1.56 tonnes of panels would be required, which is 156 individual panels. This would put it on par with RTGs in terms of weight, but far behind them all in terms of part count. It is still much heavier than fuel cells.

Being even more generous let's use the XL panels, even though they are so large as to be rather difficult to incorporate into most fighter craft. Using the 25% efficiency number, they generate .7 units of electric charge per second per panel, which is 17.5 units per tonne. Putting them to use in our hypothetical engine .8 tonnes of panels would be required, coming out to 20 panels total. This puts them ahead of fuel cells in terms of weight but still behind in part count.

However, if we are giving the solar panels the luxury of being able to use their larger, more efficient variants, for the sake of a fair comparison we must do the same to fuel cells. The larger fuel cell arrays are around the same size as a single XL panel, however while a single XL panel generates .7 units of electric charge per second in this application, a single fuel cell array generates a whopping 18 charge per second. This comes out to a colossal 75 units of charge per second per tonne of fuel cells. A single large fuel cell would easily meet the requirements of out test engine, this would make the total weight come down to .86 tonnes to power the engine for 1 hour. It not only matches the panels' weight, but could power the engine using just 5 parts, versus the panel's 20.

Keep in mind this is the most ideal scenario, the panels power generation would vary wildly as the craft maneuvered, so realistically the engine would need batteries and extra panels for auxiliary power in such a scenario, the efficiency of the panels would also only decrease if flown earlier or later in the day, and would drop to zero effectively at dawn or twilight as the sun would be shining parallel to the axis of rotation in which the panels are mounted.

Panels may have some advantages for smaller engines, however since I've spent enough time on this already I won't bother doing the math for that and will simply concede the point. However the strict limitations they pack compared to all of the other power generation methods discussed, as well as being more difficult to incorporate make them simply obsolete. Of course looking at the numbers I found, even in poorer conditions for the panels they would still work, I have in the past made fully functional aircraft using them. Doesn't change the fact that they aren't the best performance wise, but outside of applications where that is important, they are perfectly applicable. Though part count wise there is no reason to use them over RTGs.

Well yeah, the large fuel cell array will always beat everything basically.

I agree that the main use case for solar panels would be weak engines.

Edited by Pds314
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Have you attempted powering them via jet engines?

Edited by Avera9eJoe
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1 hour ago, Avera9eJoe said:

Have you attempted powering them via jet engines?

I've built some of the most powerful turboprops out there. But that was not the topic of this discussion. That's a whole nother can of worms.

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3 hours ago, Kronus_Aerospace said:

I've built some of the most powerful turboprops out there. But that was not the topic of this discussion. That's a whole nother can of worms.

xD It is indeed. I saw your title and thought it worth referencing- I did a lot of tests to find the optimal efficiency on blade angle vs number of blades vs number of jets and that video was part of the results. 237-ish tons lifted vertically by 10 basic jets

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11 hours ago, Avera9eJoe said:

xD It is indeed. I saw your title and thought it worth referencing- I did a lot of tests to find the optimal efficiency on blade angle vs number of blades vs number of jets and that video was part of the results. 237-ish tons lifted vertically by 10 basic jets

That is pretty impressive! It goes to show the incredible potential of turboprops in this game. Their efficiency and power is really unrivaled when done right.

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4 hours ago, Kronus_Aerospace said:

That is pretty impressive! It goes to show the incredible potential of turboprops in this game. Their efficiency and power is really unrivaled when done right.

Yeah! The problem I ran into was trying to rotate the whole mechanism- I wanted to make a super-sized osprey but wasn't able to get the engines to rotate and maintain speed and stability.

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On 4/30/2019 at 10:49 PM, Avera9eJoe said:

Have you attempted powering them via jet engines?

My PFP is an old turboprop plane I made. One of the first large turboshafts I'm aware of which didn't rely on wheels in the bearing design yet could still reach the rpm speed limit.

Edited by Pds314
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8 minutes ago, Pds314 said:

My PFP is an old turboprop plane I made. One of the first large turboshafts I'm aware of which didn't rely on wheels in the bearing design yet could still reach the rpm speed limit.

You reached the rpm speed limit without using landing gear in the bearing?!

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13 hours ago, Avera9eJoe said:

You reached the rpm speed limit without using landing gear in the bearing?!

Yup. I tried to design non-expanding props so I could do 10000 degrees per second because without blades my designs could spin arbitrarily fast but just ended up proving the ~50 rads/s limit is in fact not easily beaten by an actual prop. You can dump infinite amounts of power into the RPM-limited prop and it just cycles through violent modes of oscillation and expansion.

Edited by Pds314
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On 5/2/2019 at 12:33 AM, Pds314 said:

Yup. I tried to design non-expanding props so I could do 10000 degrees per second because without blades my designs could spin arbitrarily fast but just ended up proving the ~50 rads/s limit is in fact not easily beaten by an actual prop. You can dump infinite amounts of power into the RPM-limited prop and it just cycles through violent modes of oscillation and expansion.

My rotor hit the limit too, we actually did a whole bunch of data collection on it as well: https://docs.google.com/spreadsheets/d/143IrolQa6QJanAz8UFcFsZQv5D_7QLEX_68QLswrL0E/edit?usp=drivesdk

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