electric motor and propeller questions....

[jim gilmore]

How does propeller size affect motor speed ?
I we use a smaller propeller will the motor still run acording to the governor speed but draw more power ,less power or ??
and on the same thought could the motor be run faster to make up the loss in thrust ?

[John Cralley]

How does propeller size affect motor speed ?

Jim the propeller  needs to be matched to the motor. 

If we use a smaller propeller will the motor still run acording to the governor speed but draw more power ,less power or ??

Yes, the RPM is set by the timer and expressed via the ESC in our setups and will remain constant (with in reasonable limits) This is especially true in governed setups. The setup with a smaller propeller will draw fewer amps assuming the propeller is the same pitch.

And on the same thought could the motor be run faster to make up the loss in thrust ?

Yes, But there are lots of potential variables and so no easy answers.

[Bob Hudak]

Jim,
 Step back and take a deep breath because most airplane designs already kitted will fly the same diameter prop on electric power as it will on I.C. power. Pitch is where you will find different opinions. Some like a low pitch high rpm setup saying it gives a braking action in the down hill part of a maneuver. Others like a high pitch similar to the Fox days at a low rpm. I find that a prop with the same diameter will use more battery at low pitch(4.5) than high pitch(6). Try a few props of the same diameter but different pitch and let your findings be the guide.

P.S. don't ask about pusher props or standard props that subject has run it's course in older posts, do a search in this category and you'll find many references.
Bob

[Target]

Can you explain how a high pitch prop uses less battery than a lower pitch prop? I'm a little confused by your statement.
I might not be understanding you, but normally that is the opposite in propeller charts.

I think though that maybe you are saying that turning a higher pitch prop at lower RPM to give the same lap times is more economical on battery than higher RPM on the lower pitch prop for same lap times? Just a guess. Hard to understand otherwise!

Thanks for any reply and happy Easter to all.

Regards,
Chris

[Bob Hudak]

.

I think though that maybe you are saying that turning a higher pitch prop at lower RPM to give the same lap times is more economical on battery than higher RPM on the lower pitch prop for same lap times? Just a guess. Hard to understand otherwise!

You got it. I played with different pitch props and found what you said holds true.
Bob

[Crist Rigotti]

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I think though that maybe you are saying that turning a higher pitch prop at lower RPM to give the same lap times is more economical on battery than higher RPM on the lower pitch prop for same lap times? Just a guess. Hard to understand otherwise!

You got it. I played with different pitch props and found what you said holds true.
Bob

Yeah, I noticed that as RPM goes up the battery drains faster.

[jim gilmore]

Hmm. My question is this....
let's say I have a 7-4 propeller, and it runs at 9600 rpm.
Would it be fair to say that that same propeller could be spun a bit faster at a smaller diameter to give the same battery usage ?
Say cut down to a 6.25-4 and run at 10300 or so for instance ?

[Tim Wescott]

Hmm. My question is this....
let's say I have a 7-4 propeller, and it runs at 9600 rpm.
Would it be fair to say that that same propeller could be spun a bit faster at a smaller diameter to give the same battery usage ?
Say cut down to a 6.25-4 and run at 10300 or so for instance ?

Maybe, maybe not.  Assuming that you've got enough kV and battery voltage to actually spin the thing that fast it may or may not get more efficient -- I suspect that there's some sweet spot that you'd hit, and that going past it would just make things worse.  But I'm not smart enough to tell you what that sweet spot might be, even if you told me what the rest of your setup is.

[Target]

Thanks Bob and Crist for the confirmation of the statement.
That is really something that I hadn't considered much.

Regards,
Chris

[Keith Renecle]

The big thing to keep in mind here, is that your main objective is to fly the plane as well as possible, and then to do this as efficiently as possible, or even as practical. Unless you make your own props, you are stuck with what's available, and fortunately as far as good electric props go, we have a huge range from APC, and guys like Dennis Adamisin that keep APC in touch with making the best possible props for us round & round folks.

For an equivalent airspeed a lower pitch will require more power. Sure with our governor systems we are spoilt so we can play with rpm, but the trade-off is in finding the most efficient system to do this, while keeping in mind our main objective. There are many beliefs about fine pitch giving better braking and other benefits, so just choose whatever fits your belief structure. Keep in mind that the finer the pitch, the higher the rpm, and of course.........the higher the noise as well. Diameter rules as far as thrust goes and once again this has its own side effects, like more gyro effect and lowering the turn rate. The simple way through all of this is to check out what everyone else is using successfully, and you will find that certain APC props have become the standard. For example the 11 x 5.5 for a .40 size, the 12 x 6 thin blade for the .55 to .60 size, or the 13 x 4.5 etc. This forum is an absolute gold mine of info on e-stunt, and I have no reservations in stating that it is by far the best source on the internet!

I've had a ball recently with my .15 size model experiments, and I was convinced that the 8 x 6 E prop would fit the bill perfectly. I was partly right in that it did work very well and the battery consumption was low. I was happy that my calculations worked so well.........that is until I tried the 9 x 6 E prop. It obviously used more power, but I found that my lowering the rpm by around 1000 rpm I got almost the same battery usage and the extra thrust was just sooo much better. It did slow down the turn rate a little, but my Elec-Trick design was a little twitchy, so this helped a lot and now the model flies really well and also at lower lap times which is rather nice at my age! Like many on this forum have stated, e-power is so good because it is relatively easy to measure all of the data.

Keith R

[Howard Rush]

For an equivalent airspeed a lower pitch will require more power... so just choose whatever fits your belief structure.

Not necessarily, according to my belief system, if I interpret the Word of my high priest, H. Glauert, and his followers accurately. 

[Bob Hudak]

I'll bite. What was Herman's theory on airscrew propulsion?

[jim gilmore]

Not truly sure but here is a little bit of interpetition ....
A larger propeller is more efficient at a modest speed than a smaller propeller at a higher speed.
So the smaller propeller needs to spin faster to accomplish the same amount of thrust. Which in turn means it has to spin faster than it's more efficient modest rpm.
Is this what we are saying

[Howard Rush]

I'll bite. What was Herman's theory on airscrew propulsion?

It's a little more complicated than most people dispensing model airplane propeller theory advice think.  For the straight stuff,

Adkins, C.N. and Liebeck, R.H., Design of optimum propellers, AIAA Paper 1983-190,
January 1983, https://www.aiaa.org/IframeTwoColumn.aspx?id=4745

http://www.supercoolprops.com/

http://www.mh-aerotools.de/airfoils/javaprop.htm

[Tim Wescott]

It's a little more complicated than most people dispensing model airplane propeller theory advice think.  For the straight stuff,

Adkins, C.N. and Liebeck, R.H., Design of optimum propellers, AIAA Paper 1983-190,
January 1983, https://www.aiaa.org/IframeTwoColumn.aspx?id=4745

That link just gets a generic search page.  If you search for the title, you find that the paper is behind a paywall.

So, is it a paper on designing propellers optimized for stunt, or for other things?

[Howard Rush]

For anything.  It also includes analysis of existing props.  One can make a spreadsheet from the formulae in the paper and design or analyze props.  A stunt prop probably involves looking at off-design conditions to see how the thrust varies with airspeed and maybe other stuff.  You can use Javaprop to do that too, but I don't think it has as good as explanation of the nitty gritty, which I think is Bob's question.  You can plug numbers into either without understanding the theory and get some insight as to how props work and what changes to them do.

I don't know a cheap way to get the Adkins and Liebeck paper.

[Bob Hudak]

Howard,
 Thanks for the link to Joe Supercool. He has alot of in depth articles on props and other items of interest.
Bob

[Keith Renecle]

Not necessarily, according to my belief system, if I interpret the Word of my high priest, H. Glauert, and his followers accurately. 

O.K......... I can accept that because it can also be like comparing apples to peaches, but from my tests the APC 9 x 4.5 running at 1200 rpm higher than the APC 9 x 6 used more power and flew slower, so I would guess that if I could get to 12 K rpm it would perform well and at the same sort of airspeed, but it would use even more power. This could also be a function of the motor windings, but I would certainly prefer to run at a lower rpm than 12 000 if possible, even if it is just for less noise.

Keith R

[Igor Burger]

It is true that prop with higher pitch has better efficiency, but it must be clear that:

1/ "the pitch" used in that expression is not pitch itself, but P/D ratio. It means the we have to speak about P/D parameter of the prop instead of pitch itself, so better written is "prop with higher P/D has better efficiency" ... and clear that it is only from-to there is certainly some optimum, but it is over our usual props with P/D around 0.5 ... means for example if we compare 10x5 and 10x10, the second will have probably better efficiency. Another thing is that we are speaking about MAXIMAL efficiency at its optimal speed. I can easily find speed for 10x5 prop which will give better efficiency then 10x10.

2/ Fact that 10x10 will need less power than 10X5 does not mean that we have better efficiency, it can easily be that the prop will transfer that less input power also to less output power (efficiency is ratio of those two values). Model flying at the same speed will need the same power does not matter what is pulling its nose, so it is clear that lower input power means better prop efficiency AT THAT SPEED. But it could be that the prop with lower input power has also lower speed stability, means it will slow down up hill, and in that case saved power does not apear because of better efficiency, but because of less output power in time when we need it. And that is the case of props with high pitch, they have worse thrust at low speeds, so if we want the same speed stability (better thrust at lower speed) we need larger diameter for the same flying quality like that smaller prop with lower pitch. I would say that the speed stability is very close to linear to input power (with "normal usefull" props).

The picture shows what is going on, it shows that higher P/D can reach better max efficiency, it also shows that prop can work at speed where efficiency is very bad, it shows also that the difference of max efficiency is not so large as we can measure on battery consumption and it also means that the saved power comes from another source then only prop efficiency.

[Keith Renecle]

Thanks for that explanation Igor. That makes a little more sense to me now. Like I said up front, the main objective is for the model to fly well, and the "normal useful" props that are readily available give us a nice selection to play with. It seems like the range of 4.5" to 6" of pitch will suit most of us.

Keith R

[Target]

What about airframe drag and its relation to prop slippage??
I would not believe that a 10x10 prop could pull the same plane at exactly half the rpm of a 10x5 prop due to airframe drag.
That is a why I was a bit surprised to hear that lower pitch props used more juice. I thought that they would have better efficiency.
I guess in ECL, the planes speed hopefully doesn't change that much, but I would have to believe the drag changes a lot during square cornered maneuvers? After maneuvers I would think a slightly lower pitch would be preferable.

Maybe someone needs a variable pitch setup integrated to the timer/governor?!

Regards,
Chris

[Igor Burger]

What about airframe drag and its relation to prop slippage??
I would not believe that a 10x10 prop could pull the same plane at exactly half the rpm of a 10x5 prop due to airframe drag.

That is what I wrote ... prop is not air screw with fixed pitch, it always works at some AoA (slippage) ... if it works closer to its best gliding AoA, it also works at better efficiency, exactly like wing, you will not give good mileage with gliter if you fly too slow at high AoA and also not if you fly at high speed at too low AoA, you must to fly AT its best glide AoA. Speaking about props, best points are tops of those curves (regarding pitch) on that my graph.

So tou your question - 10x10 prop cannot work at exactly 1/2 rpm of that 10x5, because it will deliver only 1/4 of power to the air frame, and we need the same power, so RPM must be little higher, thus prop AoA will be little higher (due to that higher RPM at the same speed) and prop will deliver the same power to the air frame (which will exactly cover drag loses of that plane at that particular speed which is the same in both cases) ... the only question is, if that new AoA is close to its best gliding AoA or not, that will tell us if motor needs to deliver more or less power TO THE PROP. The point of that higher pitch storry is, that props like 10x10 will reach better numbers then 10x5 in its best regime (see my graph).

[Motorman]

If you optimize the airspeed/rpm and thus AoA for each prop (10x5, 10x10) the prop with the higher pitch will have a disadvantage. The airfoil of a prop produces lift at right angles to the chord so, the higher the pitch the more lift is working against the turning force of the motor. This is also why larger diameter props are more efficient, they have less angle the longer they are so, more of the lift is pulling forward.

The popular brand is designed by computer program to have pitch that perfectly screws through the air at all stations without taking the above into consideration. Not the best design as proven in racing circles but they work ok for less demanding situations.  

However, if the aircrafts airspeed is the same for both props (10x5, 10x10) the lower pitch prop of the same diameter will be turning faster and will have more drag which trumps all. That's why you see more power usage with a high rpm set up.

MM  

[jim gilmore]

some thing i really do not truly understand is this.....
With a 1/2A gas. we normally flew with 6-3. At more rpm on average than the same planes using electric.
Are we saying that the gas engines are that much more inefficient ?
Also were we getting a lot more noise just from the propeller but could not hear it over the engine ?

[Howard Rush]

If you optimize the airspeed/rpm and thus AoA for each prop (10x5, 10x10) the prop with the higher pitch will have a disadvantage. The airfoil of a prop produces lift at right angles to the chord so, the higher the pitch the more lift is working against the turning force of the motor. This is also why larger diameter props are more efficient, they have less angle the longer they are so, more of the lift is pulling forward.

The popular brand is designed by computer program to have pitch that perfectly screws through the air at all stations without taking the above into consideration. Not the best design as proven in racing circles but they work ok for less demanding situations.  

However, if the aircrafts airspeed is the same for both props (10x5, 10x10) the lower pitch prop of the same diameter will be turning faster and will have more drag which trumps all. That's why you see more power usage with a high rpm set up.

Go back and read those references.  It would also help if you understood what we define as lift and drag.  See the first sentence of http://en.wikipedia.org/wiki/Lift_%28force%29 .

[Dennis Toth]

The theoretical prop stuff is very interesting BUT for our purpose we have so many operating regimes that we need to compromise prop efficiency with pulling power we need for a particular aircraft. If you look at the IC pipe set ups they run low pitch and high rpm with the pipe regulating rpm from running away. Many of the modern IC engines will run very solid at the high rpm (11 - 12K) on a normal diameter/low pitch prop. The low pitch at the right rpm will just pull you through the corners and up top while holding back in the wind. With IC if you need more power or longer run time you can adjust the venturi or nitro level and just add a bit more fuel.

For E power we are limited by battery capacity. For a high rpm/low pitch the E power draws more amps. Amps are weight, we have to leave ~25% in the tank (dead weight). When we need a few 100 mah more it more often then not forces us to make a big jump 300 - 500 mah to the next available pack size which can add over 1 to 2 oz. Once our battery technology advances a bit more and if we have better choices on weight/capacity we will see more high rpm / low pitch set ups in E power.

My approach is to use the smallest diameter / lowest pitch prop that will pull the ship consistently in both dead air and higher winds and trim the ship to handle it.

Best,     DennisT

[Curare]

For those who aren't savvy to it, there's a good online motor/prop calculator online here at:

http://www.ecalc.ch/motorcalc.php

It doesn't take long to figure out that a the amps to swing say a 12x5 at 55mph, are a lot more more than 12x6 at 55mph (for example a e flite power 15 on 4s will draw roughly 55amps on a 12x6 and is maxxed out full voltage and 60 amps on a 12x5.

So economically speaking, it'd be great to run a 12x10 at a measly 37 amps, but everyone has said, would it make a flyable stunter?

[Curare]

I just had another thought. (they come whenever they please).

Ol' Joe Supercool's page was mentioned (Stuart is a family friend), and I recall he's run som pitchy props in the past, I think he was trying 11x8.5 props for F2B at one stage. Maybe this is worth revisiting form the electric perspective.

[jim gilmore]

Ok, some random thoughts here...
a 10-6 spun at the same rpm as a 10-4 should have lower amps but higher resistance ?
Or is the resistance of swinging a higher pitch marginal ?

 

[Curare]

OK, lets play with that.

One engine, same voltage input: one on a 10x4 and one on a 10x6.

10x4 - 10927rpm - 42mph - 24.57A - thrust 1285g (45oz)
10x6 - 10565rpm - 60mph - 33.4A - thrust 1802g (63oz)

[Dennis Toth]

Jim,
The 10x6 at the same rpm as the 10x4, the 10x6 will pull more amps. IF we are trying to fly at say 53 mph flying speed (~5.0 sec lap time 60' C to C) then the 10x6 will run at a lower rpm then the 10x4 and the 10x6 will pull less amps.

This is our dilemma in E-stunt because we are limited by the available pack sizes.

Best,       DennisT

[jim gilmore]

Dennis here is where i get a little confused...
Assuming your using a speed controller and timer.
Wont both propellers run at the same speed unless you change a setting
I would assume that the higher pitch propeller would cause the resistance to go up, as opposed to the smaller pitch prop.
Which would result in an increase in amps ?

[Target]

Adding pitch or diameter at the same RPM will cause the amps to rise, in general. You are increasing the load on the motor. If you raise pitch, you should get more flight speed at the same rpm.
If you raise diameter, you should get more thrust at the same RPM. Also, the voltage is very likely to drop a bit when the load is increased.
But all of this assumes a motor with excess power that isn't working too hard, and which you haven't come near to maxing out. When you do, things can change and more heat is produced. Heat is lost power from battery to prop.


But, if you are looking to keep the flight speed the same, adding pitch (not diameter) and reducing rpm will save you some amps, is what these guys are saying Jim. And, when the amps drop, usually you are in a more efficient range in most of the motors out there, and that helps reduce power wasted as heat.
At least, that is the way I understand it.

Hope that helps.

[jim gilmore]

ok,from my time with gas engines though when i would go up in ptch we usually fount the plane flew faster but then lost speed on manuvers is that the same here in electric ??

[Curare]

The air is the same, phyisics haven't changed, so yeah.

Cruising around with a low pitch prop is speed limiting, but also can provide better thrust at low speeds, effectively forcing the plane to stay at a similiar speed, re-accelerating the plane quickly after a sharp change in direction, and braking when it goes too fast (downhill). it can be likened to drivign around in second gear, you won't go fast but you'll be able to climb steep hills and not whizz down the other side.

In the land of electrics (and in glow) this is extremely innefficient, but very effective. to do this though, you need a big battery pack, to keep that motor swinging a prop fast, and going nowhere. You could run a very low amp setup snd hence small batteries, but you'd have less speed manangement, more probably you'd have none.

I guess the question from me is: What's more important, all up weight or speed manangement? ...Moreso if weight is more critical, can we generate the speed management another way, like parasite drag, fat trailing edges, and big pressure cowls?