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Lower Kv motor with bigger prop to maximize flight times

Old 11-08-2014, 05:04 PM
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Eindecker_pilot
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Default Lower Kv motor with bigger prop to maximize flight times

I'm new to giant scale electric power systems and I'm looking for a gut check...

I would like verification that I'm using Motocalc correctly & interpreting its results right...

I've enjoyed using Motocalc to optimize my power system and I'm impressed with all of the different motors & components already loaded into the software, makes it very easy to run various simulations. I ran a bunch of different combinations trying to maximize flight time (that's what I'm defining as better in this case).

My airplane project is a WWI biplane, 1/4 scale, about 19lbs total weight. As such, I'm not after high power or high speed. I want relatively slow scale speed, but long duration.

My baseline power system is a RimFire 1.20 brushless motor, running a single 5000mah 6S lipo and a 15x8 prop. That gave me about 11 minutes of flight time (predicted by Motocalc). I wanted more flight time.

So I've used Motocalc to study a lot of different motor and prop combinations to improve flight time without adding the weight of additional parallel batteries.

I found that I could gain an additional 3 minutes of flight time just by going with a larger motor & prop. For instance, with a RimFire 1.60 and a 19x12 prop, I can get over 14 minutes of flight time, with the exact same 6S 5000mah lipo. (at similar throttle positions and flight performance)

The RimFire 1.60 is a lower kv motor (it's 250 vs. the 450 kv of the RimFire 1.20) and it can take much higher voltages than 6S, but according to Motocalc's "Opinion" analysis, the simulation is predicting that I will still get pretty good in-flight performance running this bigger motor (the RimFire 1.60) on the 6S lipo.

With all that background, here are my questions:

1.) What am I missing by combining a 6S Lipo with the RimFire 1.60 (a motor which is capable of running much higher voltages)? I realize I'm not getting the full power-output of the motor, but that's fine by me in this application. Will there be problems with throttle response?


2.) Is it a common tactic to run lower Kv motors with bigger props to gain flight time & improved efficiency (for the same battery voltage)?


thanks,
Greg
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Old 11-08-2014, 05:15 PM
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JetPlaneFlyer
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Yes, large slow turning props are more efficient. That's why helicopters have huge rotors that spin relatively slowly rather than small high RPM props.

The down side is you lose pitch speed (like driving in a low gear).. so check that your pitch speed is adequate, Motocalc gives you a warning if it's too low.
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Old 11-08-2014, 07:31 PM
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Originally Posted by Eindecker_pilot View Post
I'm new to giant scale electric power systems and I'm looking for a gut check...

I would like verification that I'm using Motocalc correctly & interpreting its results right...

I've enjoyed using Motocalc to optimize my power system and I'm impressed with all of the different motors & components already loaded into the software, makes it very easy to run various simulations. I ran a bunch of different combinations trying to maximize flight time (that's what I'm defining as better in this case).

My airplane project is a WWI biplane, 1/4 scale, about 19lbs total weight. As such, I'm not after high power or high speed. I want relatively slow scale speed, but long duration.

My baseline power system is a RimFire 1.20 brushless motor, running a single 5000mah 6S lipo and a 15x8 prop. That gave me about 11 minutes of flight time (predicted by Motocalc). I wanted more flight time.

So I've used Motocalc to study a lot of different motor and prop combinations to improve flight time without adding the weight of additional parallel batteries.

I found that I could gain an additional 3 minutes of flight time just by going with a larger motor & prop. For instance, with a RimFire 1.60 and a 19x12 prop, I can get over 14 minutes of flight time, with the exact same 6S 5000mah lipo. (at similar throttle positions and flight performance)

The RimFire 1.60 is a lower kv motor (it's 250 vs. the 450 kv of the RimFire 1.20) and it can take much higher voltages than 6S, but according to Motocalc's "Opinion" analysis, the simulation is predicting that I will still get pretty good in-flight performance running this bigger motor (the RimFire 1.60) on the 6S lipo.

With all that background, here are my questions:

1.) What am I missing by combining a 6S Lipo with the RimFire 1.60 (a motor which is capable of running much higher voltages)? I realize I'm not getting the full power-output of the motor, but that's fine by me in this application. Will there be problems with throttle response?


2.) Is it a common tactic to run lower Kv motors with bigger props to gain flight time & improved efficiency (for the same battery voltage)?


thanks,
Greg

Please check out the line of Hacker A60 series motors for your application. These motors come in a whole range of KV specs from 420 KV to 190 KV. They also come in different "Lengths" of motor winding area.
https://www.aero-model.com/8_72/Brus...60-Series.html For the Hacker line of motors, motocalc is pretty close on their estimated Power/Amps/Volts/RPM and so on.

If this is your first go at high powered electric power systems, be prepared for a little shock on the size of those motors. Hanging a 19 or 20 inch diameter prop on a "Tiny" electric motor makes you wonder if it will even work. But, be assured, a Hacker A60-16M motor, running at 3000 watts will get your attention. My two Hacker A60 motors will haul their respective models (weighing in at 13.5 and 18 pounds) straight up, out of sight.

When working with these power levels, absolute reliable control of your motor is mandatory. An unexpected power up by bumping the transmitter throttle with of one of these things is going to cause some damage. They need a positive disconnect from the battery, and along with that, if your transmitter has a throttle kill switch function, be sure to program it in.

I've got two of them, the Hacker A60-5S, and A60-16M. Both are running at 80 Amps, full throttle on a 12S2P A123 pack for the 16M motor, and 10S2P for the 5S motor. They are only slightly warm after a landing. The A60-16M motor turns an APC-E 19X12 Wide blade prop at 6800 RPM on the ground, hitting 7200 RPM in the air, as recorded by my Castle Creations 80 Amp HV ESC's and their data recording feature.

These A60 series motors don't use propeller adapters. Their 8 mm shaft is directly threaded with an 8 mm thread like a glow engine. Spare nuts also available from a well stocked hardware store.

The A60 prices are a bit higher, but these are very well made motors. I've got 8 of them, ranging from a little A30, two A40's, three A50's and the two A60's. You can actually run them at their spec'd power levels without overheating. Should anything happen (Crash?), replacement parts such as shafts are readily available from the Hacker supplier.

Take a look:

Giant Scale Power System: (IMHO, in giant scale models, you absolutely have to have dual receiver power supplies for reliability. Just to many $$$ involved to depend on just one receiver power system.)
Battery Backup System (The Castle Creations 10 amp uBEC handles 98% of the flights, the A123 backup is topped off after every 20 or so flights. Only takes a hundred milliampere hours or so)
http://www.wattflyer.com/forums/showthread.php?t=63794

Giant Scale electric motors vs Gasoline Engines
http://www.wattflyer.com/forums/showthread.php?t=58035

Great Planes Giant Big Stick Electric Conversion
http://www.wattflyer.com/forums/showthread.php?t=65052

Giant Scale Cessna Model (Sold it after some 50 or 60 flights)
http://www.wattflyer.com/forums/showthread.php?t=66414

Redwing MXSR Model
http://www.wattflyer.com/forums/showthread.php?t=72225

A123 cells for receiver power
http://hangtimes.com/a123_batteries_for_giants_faq.html

Last edited by kyleservicetech; 11-08-2014 at 07:56 PM.
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Old 11-08-2014, 08:12 PM
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Results vary with the aircraft... and what you are trying to do.

For a WWI bipe you have a very draggy airframe. Its easy to install a system that seemed fine for something else and get marginal results. High drag slow flying aircraft bennefit more from relatively low pitch, large dia and moderate RPM than some other types would.

Any time pitch speed (RPM X pitch (inches) / 1000 = mph is close estimate) is more than 1.5 X max levell flight speed you have the wrong prop. Closer to 1.2 X is good for most sport-aerobatic types. WWI bipes you'll be lucky to do much better than pitch speed 1.3 X level flight speed. Pylon racers can approach 1.0 X

Just changing the prop (maintaining the same load on the motor) can make a large difference in aircraft performance.
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Old 11-09-2014, 03:07 AM
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kyleservicetech
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Originally Posted by fhhuber View Post
Results vary with the aircraft... and what you are trying to do.

For a WWI bipe you have a very draggy airframe. Its easy to install a system that seemed fine for something else and get marginal results. High drag slow flying aircraft bennefit more from relatively low pitch, large dia and moderate RPM than some other types would.

Any time pitch speed (RPM X pitch (inches) / 1000 = mph is close estimate) is more than 1.5 X max levell flight speed you have the wrong prop. Closer to 1.2 X is good for most sport-aerobatic types. WWI bipes you'll be lucky to do much better than pitch speed 1.3 X level flight speed. Pylon racers can approach 1.0 X

Just changing the prop (maintaining the same load on the motor) can make a large difference in aircraft performance.
Yeah
You'd be after a big diameter, low pitch prop for a slow flying bipe. One thing that will help after you've got the model flying is one of those data recording types of Electronic Speed Controls. Castle Creations has them, among perhaps many other brands of ESC's.

With the data recording, you will be able to take a look at the current and watts pulled by your model at about 3/4 throttle. And see just how much it varies while doing slight climb outs, and slight dives. If there isn't much difference between the two, you might want to go to a lower pitch prop.

Be sure to check your power system with a wattmeter while trying different props. What might seem to be a minor change in prop diameter can result in a very significant change in power output.

If you don't have a wattmeter, what I use is a Sears Craftsman #82369 digital clamp on AC and DC clamp on ammeter. Simple to use, just set the meter to DC Amps, and clamp the jaws around ONE of the battery leads. No adaptor required. This unit has two ranges, 0-40 Amps, and 0-400 Amps. Note that most clamp on ammeters are AC only, not useable for our stuff.

http://www.sears.com/craftsman-digit...p-03482369000P
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Old 11-09-2014, 03:38 PM
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Eindecker_pilot
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Thanks for the input & tips. I am powering the receiver with a pair of parallel NiCd packs, independent from the propulsion system. Each Nicd Pack for the Rx is on it's own switch harness as well. For safety I have installed a physical arming switch for the brushless motor.



Here's some more information from Motocalc on the flight performance with the Rimfire 1.60 on 6S lipo. For this project file, I did modify the airframe coefficients to simulate lots of struts & wires. Read below... What do you think? Seems encouraging...


MotOpinion - DH2 - Rimfire 1.60 - 19x12
500ft above Sea Level, 29.92inHg, 57F

Motor: Great Planes Rimfire 63-62-250 (#4795); 250rpm/V; 1.5A no-load; 0.0294 Ohms.
Battery: Thunder Power TP2500 (G4 ProPower 45C) (45C); 6 series x 2 parallel cells; 2500mAh @ 3.7V; 0.004 Ohms/cell.
Speed Control: Castle Creations Phoenix 80; 0.001 Ohms; High rate.
Drive System: DH2 drive; 19x12 (Pconst=1.31; Tconst=0.95) direct drive.

Airframe: Dh2; 2100sq.in; 308.5oz RTF; 21.2oz/sq.ft; Cd=0.077; Cl=0.41; Clopt=0.57; Clmax=1.1.

Stats: 55 W/lb in; 49 W/lb out; 23mph stall; 32mph opt @ 76% (18:06, 89F); 37mph level @ 86% (14:11, 95F); 682ft/min @ 14.2; -466ft/min @ -9.7.


Power System Notes:
The full-throttle motor current at the best lift-to-drag ratio airspeed (44.9A) falls approximately between the motor's maximum efficiency current (33.2A) and its current at theoretical maximum output (368.4A), thus making effective use of the motor.
The voltage (21.6V) exceeds 12V. Be sure the speed control is rated for at least the number of cells specified above.


Aerodynamic Notes:
The static pitch speed (56mph) is within the range of approximately 2.5 to 3 times the model's stall speed (23mph), which is considered ideal for good performance.
With a wing loading of 21.2oz/sq.ft, a model of this size will have very sedate flying characteristics. It will be suitable for relaxed flying, in calm or very light wind conditions.
The static thrust (192oz) to weight (308.5oz) ratio is 0.62:1, which will result in short take-off runs, and no difficulty taking off from grass surfaces (assuming sufficiently large wheels).
At the best lift-to-drag ratio airspeed, the excess-thrust (81.9oz) to weight (308.5oz) ratio is 0.27:1, which will give strong climbs and rapid acceleration. This model will most likely readily loop from level flight, and have sufficient in-flight thrust for many aerobatic maneuvers.


General Notes:
This analysis is based on calculations that take motor heating effects into account.
These calculations are based on mathematical models that may not account for all limitations of the components used. Always consult the power system component manufacturers to ensure that no limits (current, rpm, etc.) are being exceeded.
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Old 11-09-2014, 09:24 PM
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Larry3215
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I have some doubts about that report. 55 watts in and 49 watts out is close to 90% efficiency which very few outrunners can actually do - especially a RimFire.

Also, 55 watts/pound is going to be flyable but hardly as aerobatic as MotoCalc predicts.

In my experience, that kind of power loading means you are flying very much "on the wing" rather than being able to rely on power to get you through loops etc. The predictions for strong climbs and short take offs on grass seem pretty optimistic.

I have flown models on 1/2 that much power loading, but they were far more aerodynamically "clean" and efficient than your model.

I think 55 watts/pound is definitely flyable - just dont expect it to be a hot flyer and dont expect to be able to power out of trouble.

I personally would want more power - closer to 75 watts/pound peak just for emergencies - but thats me

Good luck!
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Old 11-09-2014, 09:31 PM
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By the way, the nice thing about your choice of motor is it will allow you to prop up to get more power out without having to worry about the motor being able to handle it.

That will of course lower your flight times - IF - you fly at higher power levels. You may still be able to fly around at the same average power level even if you prop for a higher peak power level. So your actual flight times may not change much at all - especially if you go with a larger diameter prop.

If flight times are your main goal, then you will have to put up with more battery and the resulting weight. Increasing efficiency in the power system will only go so far - especially with a draggy airframe like that bipe.
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Old 11-09-2014, 09:57 PM
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Originally Posted by Larry3215 View Post
I have some doubts about that report. 55 watts in and 49 watts out is close to 90% efficiency which very few outrunners can actually do - especially a RimFire.

Also, 55 watts/pound is going to be flyable but hardly as aerobatic as MotoCalc predicts.

In my experience, that kind of power loading means you are flying very much "on the wing" rather than being able to rely on power to get you through loops etc. The predictions for strong climbs and short take offs on grass seem pretty optimistic.

I have flown models on 1/2 that much power loading, but they were far more aerodynamically "clean" and efficient than your model.

I think 55 watts/pound is definitely flyable - just dont expect it to be a hot flyer and dont expect to be able to power out of trouble.

I personally would want more power - closer to 75 watts/pound peak just for emergencies - but thats me

Good luck!
Yeah, agreed:

Only 50 watts per pound of airplane is rather low for any giant scale model. Don't think you'd be very satisfied with its performance. This is one case where motocalc might be a bit off. You definitely do not want a model that gets off the ground, and is barely able to fly.

Better to get close to 100 Watts per pound. After the maiden flights, you can always go to a smaller diameter, lower pitch prop to extend the flying time.

For me, I've been setting up my models with A123 packs, aiming at a safe flying time of 6-7 minutes, while using about 2/3 of the battery pack. Those numbers have worked out well with my models.
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Old 11-10-2014, 12:59 AM
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Thanks for the input folks. I ran a series of simulations, swinging battery from 6S to 8S lipo & the prop sizes in terms of diameter & pitch. The trends are really interesting...

Comparing one of the 6S scenarios to a scenario at 7S, I can increase power loading W/lb from 55 to 71 with little effect on in-flight excess thrust to weight ratio goes from .27:1 to .28:1 (which I assume is the parameter we really care about for reserve "get me out of trouble" power, right?). That turned out to be due to a less efficient prop combination with those two batteries.

I was hanging my hat on the W/lb guidelines (I was originally concerned about the low 50W/lb range I was in), but clearly there's more going on here. Is it "in-flight excess thrust" that we really care about, but maybe W/lb is a much more simple guideline that works 99% of the time?
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Old 11-10-2014, 01:11 AM
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Thanks for the input folks. I ran a series of simulations, swinging battery from 6S to 8S lipo & the prop sizes in terms of diameter & pitch. The trends are really interesting...

Comparing one of the 6S scenarios to a scenario at 7S, I can increase power loading W/lb from 55 to 71 with little effect on in-flight excess thrust to weight ratio goes from .27:1 to .28:1 (which I assume is the parameter we really care about for reserve "get me out of trouble" power, right?). That turned out to be due to a less efficient prop combination with those two batteries.

I was hanging my hat on the W/lb guidelines (I was originally concerned about the low 50W/lb range I was in), but clearly there's more going on here. Is it "in-flight excess thrust" that we really care about, but maybe W/lb is a much more simple guideline that works 99% of the time?
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Old 11-10-2014, 01:36 AM
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Sorry for the double-post, not sure what went wrong there.

Here's another question for folks familiar with Motocalc: What is the significance of the throttle positions that it gives as output from the analysis? Is there a rule of thumb there? I could envision a rule of thumb where I'd want 50% throttle position to be the point at which I have level, hands-off sustained flight.

thanks,
Greg
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Old 11-10-2014, 04:04 AM
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I dont know about the throttle position predictions in MotoCalc.

In my experience, MotoCalc is good as a very very general guide only. It can get you in the ballpark, but thats about it. I would not trust it to the degree you seem to want to trust it

There are a number of reasons why its predictions are not all that accurate. One of the main ones is that motor specs are almost never accurate - especially for the cheaper motors. Then there are variations in props, controllers and especially batteries.

Even when you use the better motors and controllers and batteries - like Hacker, Neu, Castle, ThunderPower, etc, the results are still not all that close to what you will actually see when you put a meter on you personal model. You will be doing very well indeed to get within 10% of the predicted performance. I have rarely been within 20% and I dont use cheep stuff.

So, use the MotoCalc results as a rough guide - BUT - test your actual motor/prop/esc/battery combination with a watt meter to be sure your results are not going to burn something up and that you have enough power to fly.

I personally would nit trust the throttle settings MotorCalc suggests. There are just too many variables that can vary by too much.

Like I said before, I would shoot for a minimum power loading of around 75 watts per pound with 100 being much safer. You can always fly at a lower throttle setting - if the model will stay in the air - to increase run time

To double check those motorCalc predictions for power out, try searching for posts/threads discussing your motor here and on RC Groups. You will find threads/posts where people have used that motor with specific props and battery combinations and there will be reports with actual real world results.
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Old 11-10-2014, 06:55 AM
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Also bear in mind that 50% throttle (as in half power) usually isn't anywhere close to 50% stick travel. I'm not sure why you would particularly want the stick to be in the middle for cruise but to do so would either involve speccing a system that had a large surplus of power (such as around 75+W per lb as suggested above) or using a throttle curve.
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Old 11-10-2014, 07:05 AM
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Originally Posted by JetPlaneFlyer View Post
Also bear in mind that 50% throttle (as in half power) usually isn't anywhere close to 50% stick travel.

Very good point. The horsepower or watts input to a propeller is very definitely NOT linear with its RPM. In fact, the power is related to the 3rd power of the RPM.

So, going from 3500 RPM to 7000 RPM takes 8 times more watts. This shows up in motocalc, when dialing back the throttle to 50% or so.

More information is available through the APC propeller web site. They have a LOT of info on their props. And, from what I've determined, their info is pretty close.

Right now, they're updating their files, so check back with them in a day or three. FYI, their data shows horsepower, thrust and a lot of other stuff on every prop they sell, starting at 1000 RPM and going past 10,000 RPM for each and every prop.
http://www.apcprop.com/v/downloads/P...B/datalist.asp
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Old 11-10-2014, 07:12 AM
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Originally Posted by Larry3215 View Post
I dont know about the throttle position predictions in MotoCalc.

In my experience, MotoCalc is good as a very very general guide only. It can get you in the ballpark, but thats about it. I would not trust it to the degree you seem to want to trust it

There are a number of reasons why its predictions are not all that accurate. One of the main ones is that motor specs are almost never accurate - especially for the cheaper motors. Then there are variations in props, controllers and especially batteries.

Even when you use the better motors and controllers and batteries - like Hacker, Neu, Castle, ThunderPower, etc, the results are still not all that close to what you will actually see when you put a meter on you personal model. You will be doing very well indeed to get within 10% of the predicted performance. I have rarely been within 20% and I dont use cheep stuff.
Yeah, with all the variables out there, especially with the various propellers, getting within plus/minus 10% with motocalc, or any other similar computer program would be pretty good results.

As for me on my A123 battery packs, they put out about 2.8 Volts DC per cell while pulling about 40 Amps on a 2.3 Amp Hour battery pack.

It would really be interesting to get real voltage under load data with the current lot of LiPo batteries, such as those 2300 Mah packs, while also pulling 40 Amps. I believe these LiPos run around 3.5 volts per cell while running at about 20C, but would really like some real data in this subject.

Nice thing about the A123's, they will still put out 2.8 Volts DC per cell at 40 Amps after 400 flights. And, these A123's maintain the same RPM at the beginning and at the end of a flight.
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Old 11-11-2014, 12:44 AM
  #17  
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Thanks for the tips everyone & for sharing your experiences with Motocalc. Good news is that the program has demonstrated that going to a larger motor / lower kv value is going to be more efficient & extend flight times and I'm hearing that part of the result is believeable. I'm also taking the direction to bump up the battery voltage to something in the 7S - 8S range to get closer to the 75W/lb. From there, I'll experiment with the props using my watt meter.

thanks,
Greg
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