Hello,

Quite simply:

If you have a 12x8 prop running on 4s and you want to lower the max speed, then you have two ostensibly similar options:

12x6 on 4s

OR:

12x8 on 3s

Which would be more energy efficient from the battery at the same speed?

Additional info:

At 12x8 on 4s, the ESC is near its limit, the battery is at 80% of its C rate, and the motor is ~at its practical current limit. I have more accurate numbers if necessary but I want to keep this discussion general if possible so it can be more useful to different people and situations in future. If more info is needed even to give a decent generalised response, please let me know.

For completeness of discussion it would be good to discuss the parameters of either system at full throttle steady state, partial throttle steady state, and during acceleration/low speed where pitch becomes interesting. It might also be interesting to compare to 12x8 on 4s at 75% throttle.

My own understanding:

A few months ago I would have said 12x6 on 4s, but now it's somewhat up in the air due to what I learned in a previous thread regarding pitch and prop efficiency. I don't feel my understanding is strong enough ATM, so I will wait till I can think about it more and maybe read some responses before I contribute too much personally .

Efficiency is a tricky word in my opinion.

Is it motor 'efficiency', i.e. power in to power out ?, (may have to be measured on a dynamometer).

Or is it 'efficiency' of the flight, i.e. the maximum flight duration.?.

What if the motor was quite efficient, but the model wasn't ?, or the model was quite efficient at flying, but the motor was the wrong one for the required flight ?.

As I said, to me efficiency is a tricky word when related to model aircraft.

There is no comparison. Going from a 12x8 to a 12x6 on 4s will decrease your pitch speed to 75% with a modest drop in power consumption. There will also be some decrease in propeller efficiency. But any change in the airplane's performance, which will be slight but noticeable, can be attributed more to the decreased power consumption and pitch speed.

However, going from 4s to 3s with the same propeller will cause a drastic decrease in power consumption and airplane performance. The motor's RPM, and thus the propeller's RPM, is proportional to the input voltage. A propeller's load, and thus the motor's power consumption, is proportional to the change in RPM cubed. Therefore, going from 4s to 3s will decrease your RPM to 75% of the 4s RPM. This means your power will be reduced to approximately 42% of the 4s level.

You will definitely notice a difference in airplane performance.

Larry

Originally Posted by eflightray

Efficiency is a tricky word in my opinion.

Is it motor 'efficiency', i.e. power in to power out ?, (may have to be measured on a dynamometer).

Or is it 'efficiency' of the flight, i.e. the maximum flight duration.?.

What if the motor was quite efficient, but the model wasn't ?, or the model was quite efficient at flying, but the motor was the wrong one for the required flight ?. I gave those rough %s of loading in the op to deal with these questions, I hoped they would be sufficient?

As I said, to me efficiency is a tricky word when related to model aircraft.

It's efficiency of the whole system, battery, wires, ESC, motor, prop - which should roughly equate to efficiency of the flight I assume?

Originally Posted by Lnagel

There is no comparison. Going from a 12x8 to a 12x6 on 4s will decrease your pitch speed to 75% with a modest drop in power consumption. There will also be some decrease in propeller efficiency. But any change in the airplane's performance, which will be slight but noticeable, can be attributed more to the decreased power consumption and pitch speed.

However, going from 4s to 3s with the same propeller will cause a drastic decrease in power consumption and airplane performance. The motor's RPM, and thus the propeller's RPM, is proportional to the input voltage. A propeller's load, and thus the motor's power consumption, is proportional to the change in RPM cubed. Therefore, going from 4s to 3s will decrease your RPM to 75% of the 4s RPM. This means your power will be reduced to approximately 42% of the 4s level.

You will definitely notice a difference in airplane performance.

Larry

Well, the added power of the 12x6 will mostly be relevant during acceleration, and it should slightly reduce the difference in speed between the thrust speed and the plane speed.

This is interesting and worth exploring (but I think you have covered it in almost as much detail as you can with so little information already).

But what about in a situation of same speed, or both at full throttle? If both were at full throttle the 12x6 I believe would go slightly, slightly faster in steady state. It would also get to that speed faster. But what about at the same speed? Which would be drawing more current to go at X km/h in steady state? I think that is an interesting question.

What about both at 50% throttle? Would that be appreciably different to the relationship when both are at 100%? Etc.

Originally Posted by Nereth

It's efficiency of the whole system, battery, wires, ESC, motor, prop - which should roughly equate to efficiency of the flight I assume?

You forgot the airframe.

Without knowing which motor, props and airframe are being considered, we cannot say for sure which combination will be best. However I have found that in general, a lower pitch prop at higher voltage produces more thrust and acceleration for takeoff and aerobatics, but a higher pitched prop at lower voltage cruises more efficiently.

Well, the added power of the 12x6 will mostly be relevant during acceleration, and it should slightly reduce the difference in speed between the thrust speed and the plane speed.

What is 'thrust speed'?

The 12x6 on 4S will produce more thrust at low speed, but the 12x8 on 3S will produce more thrust at high speed. If the airframe is draggy then the 12x6 will win, but if it is slick then the 12x8 will win.

If both were at full throttle the 12x6 I believe would go slightly, slightly faster in steady state. It would also get to that speed faster.

Not necessarily. The 12x8 has slightly higher pitch speed and its in-flight thrust drops away slower as airspeed increases, so it should go a bit faster (assuming the airframe permits it).

But what about at the same speed? Which would be drawing more current to go at X km/h in steady state? I think that is an interesting question.

Again, this depends on the airframe. If the model is heavy with a small wing area then it will need to fly fast, and the 12x8 will be more efficient (the 12x6 will be working closer to pitch speed and therefore losing more power to blade drag), but if it is light with a large wing area then it can fly slower so the 12x6 may work better.

What about both at 50% throttle? Would that be appreciably different to the relationship when both are at 100%? Etc.

At 50% throttle the pitch speed is halved. Most props achieve best propulsive efficiency at about 70% of pitch speed. If you cut the throttle back to 50% then you should be cruising at 35% of full throttle (static) pitch speed, but you also want to be at the bottom of the model's 'drag bucket', ie. not too fast or too slow (induced drag rises dramatically as you approach stall speed).

At 50% throttle the theoretical pitch speed of both props is similar (the 12x8 having a slight edge), but the 12x6 is spinning about 30% faster so it will suffer more blade drag and waste more power as it approaches its (lower) pitch speed. If the model has a high stall speed then the 12x6 may not even be able to keep it in the air!

Originally Posted by Bruce Abbott

You forgot the airframe.

Without knowing which motor, props and airframe are being considered, we cannot say for sure which combination will be best. However I have found that in general, a lower pitch prop at higher voltage produces more thrust and acceleration for takeoff and aerobatics, but a higher pitched prop at lower voltage cruises more efficiently.

It's difficult to adequately describe the airframe unfortunately, all I can really say is the plane can easily take off on both props at less than full throttle, and can double if not triple stall speed at full throttle.

Originally Posted by Bruce Abbott

What is 'thrust speed'?

The 12x6 on 4S will produce more thrust at low speed, but the 12x8 on 3S will produce more thrust at high speed. If the airframe is draggy then the 12x6 will win, but if it is slick then the 12x8 will win.

I am probably wrong, but I thought thrust speed was the term for the speed at which air is ejected from the spinning prop, i.e. it's the pitch speed plus some more to account for the prop blades aerofoil. Perhaps it is called the zero thrust speed, at which the prop can no longer make thrust?

The idea with these props and batteries is they both theoretically have the same pitch speed/thrust speed. Why would the 12x8 get it slightly faster in a slick airframe? Does it actually produce slightly more 'thrust speed' than the 12x6 due to aerofoil considerations?

Originally Posted by Bruce Abbott

Not necessarily. The 12x8 has slightly higher pitch speed and its in-flight thrust drops away slower as airspeed increases, so it should go a bit faster (assuming the airframe permits it).

see above question, "Why would the 12x8 get it slightly faster in a slick airframe? Does it actually produce slightly more 'thrust speed' than the 12x6 due to aerofoil considerations?"

Originally Posted by Bruce Abbott

Again, this depends on the airframe. If the model is heavy with a small wing area then it will need to fly fast, and the 12x8 will be more efficient (the 12x6 will be working closer to pitch speed and therefore losing more power to blade drag), but if it is light with a large wing area then it can fly slower so the 12x6 may work better.

I think this comes back once again to the afforementioned question?

Originally Posted by Bruce Abbott

At 50% throttle the pitch speed is halved. Most props achieve best propulsive efficiency at about 70% of pitch speed. If you cut the throttle back to 50% then you should be cruising at 35% of full throttle (static) pitch speed, but you also want to be at the bottom of the model's 'drag bucket', ie. not too fast or too slow (induced drag rises dramatically as you approach stall speed).

It was my understanding that props are most efficient at precisely their pitch speed, which is something like... 80% in turn of their thrust speed (a term I have no choice but to keep using until someone lets me know what the correct one is :P )

Originally Posted by Bruce Abbott

At 50% throttle the theoretical pitch speed of both props is similar (the 12x8 having a slight edge), but the 12x6 is spinning about 30% faster so it will suffer more blade drag and waste more power as it approaches its (lower) pitch speed. If the model has a high stall speed then the 12x6 may not even be able to keep it in the air!

Both props should be able to pull the plane at ~3x stall speed at full belt so that isn't an issue =D.

The note about the 12x6 having more blade drag is interesting, that's probably quite a significant effect, isn't it?

Considering the things you cannot change, the plane and motor, (or do not want to change, other than prop and battery), the only way to really determine the best 'efficiency', is to fly the model with different props and different Lipo packs, (number of cells).
Then determine from the flight results, duration, speed, rate of climb, etc., which was the best combination to meet your requirements.

I don't think there is a way of coming up with an definitive answer here, other than try and see. The plane itself is a large part of the overall 'efficiency' of the system, choosing the right one to fit the flight parameters is only the start. Well actually defining the flight parameters is the start, then choosing all the components to meet those parameter.

Even then, flight testing will always be the final proof of meeting or failing them, and of determining if any changes are required. Starting with what may be equipment that may not be suitable will always end in compromise.

Originally Posted by eflightray

Considering the things you cannot change, the plane and motor, (or do not want to change, other than prop and battery), the only way to really determine the best 'efficiency', is to fly the model with different props and different Lipo packs, (number of cells).
Then determine from the flight results, duration, speed, rate of climb, etc., which was the best combination to meet your requirements.

I don't think there is a way of coming up with an definitive answer here, other than try and see. The plane itself is a large part of the overall 'efficiency' of the system, choosing the right one to fit the flight parameters is only the start. Well actually defining the flight parameters is the start, then choosing all the components to meet those parameter.

Even then, flight testing will always be the final proof of meeting or failing them, and of determining if any changes are required. Starting with what may be equipment that may not be suitable will always end in compromise.

I've learned in my industry that theory can serve a lot better than practical tests in situations where it is difficult to control variables etc in the practical tests. I would imagine this applies even more truly to this hobby since it is very difficult to accurately measure performance of a plane in flight and account for variables.

Even though my plane has an OSD and a bunch of measuring equipment on board that gives me a decent advantage for example, my GPS based ground speed measurement is neither accurate enough, nor can it account for wind, for me to get the same speed twice in a row with two different setups (full throttle will not lead to the same speed with both props almost certainly). There are other limitations as well. I have an accurate altimeter for flying level but cannot account for thermals helping my plane out. I won't bother continuing to list issues

Meanwhile, Bruce Abbot for example has started to touch on some interesting stuff, I think for example that he was saying that aerofoil based lift from the props gives the 12x8 prop a significantly higher (zero?) thrust speed which is a very important issue. Going into more detail here could be fruitful. Profile drag from the prop at higher speed impacting efficiency and so on are also both interesting points to bring up.

I really don't think it is worth throwing away theory in this case just because we cannot guarantee it's correctness in one particular case. If you cannot find the truth in a specific case due to lack of information, it is still worth it to cover what you can so that when more information can be found, or reasonable assumptions can be made, the theory can become useful.

I think I annoy a few people on these forums with these threads they don't see the point in, but I have actually learned a lot from every thread I have made so far, and the last paragraph is just a strong opinion I hold that I would like to share so that people can understand where I am coming from in future

My best wishes for happy and successively flying achievement in the coming new year.

I hope the plane does meet all your requirements, on paper, and in the air. My apologies if you found my comments unhelpful.

Originally Posted by eflightray

My best wishes for happy and successively flying achievement in the coming new year.

I hope the plane does meet all your requirements, on paper, and in the air. My apologies if you found my comments unhelpful.

Please don't take my post personally, I was just justifying my somewhat unorthodox approach that people have questioned before, I understand where you are coming from, and I really don't want to discourage you from posting in my thread at all, I'd be very happy to see you keep posting whether or not we agree, since opposing opinions are often the most valuable

The planes requirements will likely be met with either option, I'm just trying to figure out which one will most likely meet it better, since I will only be purchasing either a lower cell count battery or a lower pitch prop for now, rather than purchasing both to be able to test.

Originally Posted by Bruce Abbott

Most props achieve best propulsive efficiency at about 70% of pitch speed.

This is not correct.

Props are designed to have the best lift-drag ratios at Geometric Pitch Speed (RPM X Pitch).

Props might develop their best efficiency at 70% of VIRTUAL PITCH SPEED or Zero Thrust Speed which is 20 to 40% higher than Geometric Pitch Speed depending on the P/D ratio.

Originally Posted by Nereth

Which would be more energy efficient from the battery at the same speed?.

My interpretation of this question was that the aircraft in both cases was flying at the same speed.

Doesn't this imply that the power dissipation in both cases is the same and if the motor were being operated in the flat portion of its efficiency curve that the power requirement would be approximately the same?

Originally Posted by Martyn McKinney

This is not correct.

Props are designed to have the best lift-drag ratios at Geometric Pitch Speed (RPM X Pitch).

Props might develop their best efficiency at 70% of VIRTUAL PITCH SPEED or Zero Thrust Speed which is 20 to 40% higher than Geometric Pitch Speed depending on the P/D ratio.My interpretation of this question was that the aircraft in both cases was flying at the same speed.

Doesn't this imply that the power dissipation in both cases is the same and if the motor were being operated in the flat portion of its efficiency curve that the power requirement would be approximately the same?

But wouldn't the props and the rest of the system have differences in efficiency?

Also I feel like one or the other prop (not sure which) would get the plane going slightly faster at a given throttle setting due to being able to overcome the planes drag with slightly less difference between plane speed and zero thrust speed, or having a higher zero thrust speed. Mind you, 'same speed' is probably a more important comparison than "same throttle" for partial throttle cruise situations (which is a lot of what a lot of planes do).

Originally Posted by Nereth

But wouldn't the props and the rest of the system have differences in efficiency?).

The 12X6 prop would be a few percent less efficient than the 12X8 prop.

Prop efficiency increases with increasing P/D ratio.

Originally Posted by Nereth

The planes requirements will likely be met with either option, I'm just trying to figure out which one will most likely meet it better, since I will only be purchasing either a lower cell count battery or a lower pitch prop for now, rather than purchasing both to be able to test.

Apparently you missed the whole point of my post. Using the same battery and changing the pitch from 8" to 6" you will see a slight change in performance. Maybe better, maybe worse depending upon your desires and the airframe. Staying with the 8" pitch and dropping a cell in the battery, you will see a dramatic change in performance, most likely for the worst. You will have dramatically less speed, acceleration, and vertical.

Your choice.

Larry

Originally Posted by Martyn McKinney

The 12X6 prop would be a few percent less efficient than the 12X8 prop.

Prop efficiency increases with increasing P/D ratio.

Yep that's one thing I expected to hear.

Bruce seemed to be indicating that the 12x8 would have a higher zero thrust speed, does that sound right to you? They should both have more or less the same geometric pitch speed, I'm interested to hear more about differences in zero thrust speed if you're aware of them.