Author Topic: Power (Read 920 times)

Larry Wong · « **on:** October 05, 2009, 03:36:47 PM »

Just think, the IC guys have gone to large engine's and small planes and control power with a pipe. What would Electric guys do to control power? like a Plattenberg in a Vector?

Dennis Adamisin · « **Reply #1 on:** October 05, 2009, 05:20:13 PM »

Larry I think this is going to be a pretty HOT thread!

I think in a nutshell the motor almost doesn't matter. It matters if it is too small and overheats or dies trying to to its job but otherwise....

Put it this way, if you have a 5 oz class motor set-up for 9000 RPM on a given prop, then the motor will do its best to deliver. If it is not overloaded it should happily deliver 9000 RPM until....? Put on a larger or smaller prop the motor will still run 9000 RPM. Take the prop OFF completely and---still 9000 RPM.

IF you enlarge the motor to say a 7 oz class motor AND enlarge the prop then reset the rpm for the desired speed then the nature of the thrust has changed (larger disc, lower force enacted on the air = more efficient thrust). I think there is a diminsihing return on prop size too - reduced corner, inability to penetate in the wind, overstress bearings, etc. I think this is where the guys putting 60's in Vectors get surprised.

Bigger motors will run easier (cooler) to do the work required, but even that reaches diminishing returns. However, I am starting to get more conservative on motor selection - erring to larger rather than lighter motors. However, I think the motor is of secondary importance in an ECL machine.

I think it is more important to buy motor FEATURES; large shaft, good bearings, good prop hub, lower current draw - along with things like correct kv for the intended application - THOSE things matter.

All of the thrust and braking is in the prop - and the battery supplies the power. The motor is the converter...

Alan Hahn · « **Reply #2 on:** October 06, 2009, 07:57:26 AM »

Larger isn't necessarily better when it comes to motors

. There is a point that a lightly loaded large motor is less efficient than a smaller motor.

We are already carrying props as big (diameter) on Noblers as people are using on the 60's. I could go bigger, but I have prop clearance problems as is. (as an aside, our props are so light, that I doubt they are effecting cornering very much)

Bigger is better just seems to be a "Tim the Toolman" mindset of the American male.

And I'll bet you in a year or two, the pendulum will have swung the other way. Just follow the 4 stoke history over the last 10 years.

Larry Wong · « **Reply #3 on:** October 06, 2009, 07:52:55 PM »

This gives me an idea I think if the RPM matches the Prop Pitch for lap time , then why not try 3bld props for lap time -RPM. this way we can run smaller props (3bld )

But now we need is 3bld pusher props!

Alan Hahn · « **Reply #4 on:** October 06, 2009, 08:20:19 PM »

Quote from: Larry Wong on October 06, 2009, 07:52:55 PM

This gives me an idea I think if the RPM matches the Prop Pitch for lap time , then why not try 3bld props for lap time -RPM. this way we can run smaller props (3bld )

But now we need is 3bld pusher props!

I thought that was a good idea too, but a comment by Dennis---or at least one of the Adamisins! in the 13x4.5 pusher prop thread--said that APC had considered it but rejected it due to a loss in efficiency over a 2 blade prop.

According to the developer of Prop Calc, you lose efficiency

......
The well-known effect of (slightly) lower efficiency of props with higher number of blades (with same diameter and same disk loading) results simply from the fact that more blades produce more drag, whereas the lift is (almost) the same; there is too much blade area. This could be avoided by using blades with less blade chord, but then Reynolds Number or structural problems may arise.

This is approximately the same as with a normal wing, when fast flying is the objective and wing area is too large. The direct counterpart of wing loading of a normal wing to a propeller blade is “blade loading” (do not mistake “blade loading” for “disk loading”). If this happens (wing loading too low), the lift/drag-ratio (L/D) is not as good as it could be.

So, if maximum efficiency is the main objective, always the minimum number of blades should be used which can do the job. On the other hand, lower blade loading (achieved by higher blade number or larger blade chord) results in a less critical “wide-band” prop. Maximum efficiency will be a few percent lower, but the maximum will be “wider”, what makes matching of motor/prop/model easier and provides a greater range of flying speeds with good prop efficiency. I think such experiments are well worth the effort; in PropCalc you can use the “blade number” and/or the “chord factor” option for this.

I hope I didn't take his quote out of context. I know that the propeller calculations I made
( see http://stunthanger.com/smf/index.php?topic=7574.msg64055#msg64055 )
using his program (PropCalc), that a 3 blade version needed 17% more input power to make the same thrust as a 2 blade prop at ~54mph (nominal flight speed). Of course the 3 blade prop also made more thrust when the airspeed droped and more brake when it increased which might make the tradeoff of efficiency worth while.

Of course if they don't exist what can you do!

By the way, a similar tradeoff exists (AFAIK) from using low pitch props at high rpm. Everything is a compromise.

Larry Wong · « **Reply #5 on:** October 06, 2009, 08:33:58 PM »

Wonder how Bob Hunt got his plane to work with AXI and a Bolly 3bld to work!

Alan Hahn · « **Reply #6 on:** October 06, 2009, 08:56:03 PM »

If you can haul a big battery around, you don't care if you waste a few watts.

Like I said, everything is a compromise. Sometimes it matters, sometimes not.

By the way, I'm talking about props that cost less than $10. You can always buy carbon fiber (2 or 3 blade) which have many of the characteristics of our electric props (namely thin) but then your prop is almost as costly as my airframe!

We all do what we like!

That's why it is a hobby and not a job.

Larry Wong · « **Reply #7 on:** October 06, 2009, 09:04:52 PM »

Well I guess I'll have to try some of my Bill Lee 12x4 3bl and see what happens and I have some other 3 bld for my P/a 40 and P/a 60 props carbon type.

Dean Pappas · « **Reply #8 on:** October 06, 2009, 09:52:24 PM »

Hello Larry,
The answer to how Bob got the Genesis to work with the 3-blader is that it ended up working better with the 12-6 APC-E than it ever did with the 3-blader on the AXI.

3 blades are not only less efficient, but the reason for running the 3-blader in the first place is that the torque curve of the wet setup required the extra blade in order to absorb the engine's power without noisy tip speeds. RPM * Diameter < 130000 ! Additionally, a larger diameter prop at higher wet RPMs tended to kill the turn, where the APC 12-6E at 9000 RPM does not.

Dean Pappas

Dennis Adamisin · « **Reply #9 on:** October 06, 2009, 11:37:50 PM »

When we were discussing the recent 13x4.5 pusher venture with APC we also inquired about 3 blades. They politely said "HELL NO" because of the loss of effiency that comes whn the 3rd blade is compeiting for air space with the other two.

However the ultimate effeicincy is the EFFECTIVENESS of the prop in helping the airplane do its tricks. Larry, if you give those 3 bladers props a try I hope you can report back the good or the bad...

jim gilmore · « **Reply #10 on:** October 07, 2009, 12:12:16 AM »

Hmmmm there is something here that's been glossed over. A motor will run 9000 rpm with or with out the prop because of how the motor is driven. That being the escape. But doing so changes the actual load and in turn changes the amount of amperage and actual amphrs drawn from the battery.
Though with out a prop the motorwouldhave the least amout of load but no actual amount of cooling air drawn over it.

Is this correct?
As for multiple blades being less effecient that is true but does lower effiency actually mean that it will not work ?
After all multiple blades have been used on real aircraft. Possible the amount of power that has to be added to use a 3 blade prop over a 2 blade prop is a function like horsepower and higher speeds as being an expontial of added power to increase speed?

Alan Hahn · « **Reply #11 on:** October 07, 2009, 07:31:26 AM »

Jim,

Just to be clear, I would love to have a 3 bladed 11-5.5 APC TE prop. I have even gone so far as to figure out how to kludge one of my own (haven't actually tried to do it though).

That all being said, it would be an experiment. The gain---I think---is that I would get a better climb in the vertical and better braking in the dive than with the 2 blade 11-5.5, and most of all, I would get more clearance and stop knocking the nose off my planes!

. Ignoring the last advantage, I am guessing I am getting at least the equivalent climb and braking by using my 12-6 TE prop.

The loss is maybe my current batteries wouldn't be big enough to make the full pattern with the 3 blader . However I am an experimentalist by heart and would like to see.

I am sure they would work, and maybe darn well. But you would have to carry a slightly larger battery if you are already using the minimum necessary with a 2 blade prop.

That's all.

Dennis Adamisin · « **Reply #12 on:** October 07, 2009, 08:23:07 AM »

Quote from: jim gilmore on October 07, 2009, 12:12:16 AM

Hmmmm there is something here that's been glossed over. A motor will run 9000 rpm with or with out the prop because of how the motor is driven. That being the escape. But doing so changes the actual load and in turn changes the amount of amperage and actual amphrs drawn from the battery.
Though with out a prop the motorwouldhave the least amout of load but no actual amount of cooling air drawn over it.

Is this correct?
As for multiple blades being less effecient that is true but does lower effiency actually mean that it will not work ?
After all multiple blades have been used on real aircraft. Possible the amount of power that has to be added to use a 3 blade prop over a 2 blade prop is a function like horsepower and higher speeds as being an expontial of added power to increase speed?

Jim G:
No load = (nearly) no heat. So cooling demand will be pretty low too. Some outrunners have directed openings, some don't; however one end of the motor is churning up air prety good not matter what, and the outrunner shell is turning at speed, so the air inside the motor is not staying static. It will dissapate the no-load heat pretty well.

Of course it we are worried about it there is ALWAYs heli motors whcih oftern have fans on them!

One of my favorite prop comparisons is this: the Supermarine S6-B had a (roughly) 2300 HP engine driving a large 2 bladed FIXED PITCH prop. The late series Supermarine Seafires had roughly 2300 HP pushing 2x 3-bladed contraprops. Efficiency be damned I KNOW which one I would rather fly!

Igor Burger · « **Reply #13 on:** October 07, 2009, 08:34:25 AM »

Quote from: Dennis Adamisin on October 07, 2009, 08:23:07 AM

No load = (nearly) no heat.

:-) ... unloaded motor has stilll approximately 50% loses (iron loses) ... but yes, 50% is less that 100%

Quote from: Dennis Adamisin on October 07, 2009, 08:23:07 AM

one end of the motor is churning up air prety good not matter what, and the outrunner shell is turning at speed, so the air inside the motor is not staying static.

rotor, also without any explicit blades etc is still relatively good fan, it is easy to check if you put it to plastic bag and if you seal it around the stator (rotor inside the bag) - it will fill the bag visibly :-) (stator must have holes for air)

Author Topic: Power (Read 920 times)

Larry Wong

Power

Dennis Adamisin

Re: Power

Alan Hahn

Re: Power

Larry Wong

Re: Power

Alan Hahn

Re: Power

Larry Wong

Re: Power

Alan Hahn

Re: Power

Larry Wong

Re: Power

Dean Pappas

Re: Power

Dennis Adamisin

Re: Power

jim gilmore

Re: Power

Alan Hahn

Re: Power

Dennis Adamisin

Re: Power

Igor Burger

Re: Power