Prop pitch speeds
08-29-2007, 07:11 PM
#27
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RE: Prop pitch speeds
ORIGINAL: dick Hanson
Ok -but efficiency and max power for max speed are not the same things
More power input is more power output till you blow the thing up
Been there--You keep loading the prop till something gives .
Electrics are THE way to go for max speed (this will kill some old diehard IC fans) because there are NEW battery chemistries which produce more power at elevated temps and the output per pound is simply mind boggling .
Different subject of course - BUT these cells allow us model flyers to prop for max speed and stil have a prop which will get us into the air and up to speed - I use A123 cells now
On racing watercraft - -one of the problems (of many) was setting up the prop position to get the boat moving then -when up to speed -somehow take advantage of the fact that the boat then has less drag and IF the prop could somehow become more efficient - -the rig would go faster . The really quick stuff use really high pitch props which literally come up out of the water at speed and the props are running along in the surface
On really fast planes the engine/motor prop are never all working at peak till power is at max and prop is at max speed efficiency- tricky-
If you don't blow one up - you don't know if you are trying hard enough.
ORIGINAL: adam_one
So, an increasing of Input Power doesn't necessarily mean increasing Prop's Output Power.
Also, every electric motor type has an ideal voltage, current and RPM at which the motor's max efficiency is obtained.
That gives the max flight duration but not the max Power Output.
So, an increasing of Input Power doesn't necessarily mean increasing Prop's Output Power.
Also, every electric motor type has an ideal voltage, current and RPM at which the motor's max efficiency is obtained.
That gives the max flight duration but not the max Power Output.
More power input is more power output till you blow the thing up
Been there--You keep loading the prop till something gives .
Electrics are THE way to go for max speed (this will kill some old diehard IC fans) because there are NEW battery chemistries which produce more power at elevated temps and the output per pound is simply mind boggling .
Different subject of course - BUT these cells allow us model flyers to prop for max speed and stil have a prop which will get us into the air and up to speed - I use A123 cells now
On racing watercraft - -one of the problems (of many) was setting up the prop position to get the boat moving then -when up to speed -somehow take advantage of the fact that the boat then has less drag and IF the prop could somehow become more efficient - -the rig would go faster . The really quick stuff use really high pitch props which literally come up out of the water at speed and the props are running along in the surface
On really fast planes the engine/motor prop are never all working at peak till power is at max and prop is at max speed efficiency- tricky-
If you don't blow one up - you don't know if you are trying hard enough.
I agree with you that using the new A123 cells it may be possible to build a real quick airplane using an electric motor.
I have one of Mike Connor new deltas that I think given proper prop and motor setup may be able to go well beyond the 200 mph range.
My question is with a clean airframe such as Mikes "Screamin Demon" what pitch and size would you start with using a 1.5 to 2KW power system (2to 2.35 hp power)?
With the airframe and power system under 3 lbs, target weight is 2lbs 80z. but let just say 3 for a ball park figure.
I am thinking I need to look at a 14 to 16 pitch prop am I close?
08-29-2007, 09:37 PM
#28
RE: Prop pitch speeds
wow 2 KW- well try the 14/16 pitch - one thing for sure the batts will likely come thru OK at least for a run
Here is how I look at it --
Figure the time needed for the speed trial run- and then shut down and measure energy spent on the run and monitor heat
Your speed controller better be ready for big loads
If the cells were not badly depleted add more prop load -try more dia also
The name of the game is to extract all the power you can short of blowing up th motor andcontroller
The cells will handle way over 100 amps for the speed part of the run .
If you have good data on stuff running the best biggest NiMh packs - start with the props they use for that plane
I can run 600 watts on my puny 32 EFlite motor on 5 cells -and 700 watts for a short run - -in the 3.5 lb models aerobatic types tho
use as many volts as weight will permit - that cuts back on conductor heating for same amps input
Here is how I look at it --
Figure the time needed for the speed trial run- and then shut down and measure energy spent on the run and monitor heat
Your speed controller better be ready for big loads
If the cells were not badly depleted add more prop load -try more dia also
The name of the game is to extract all the power you can short of blowing up th motor andcontroller
The cells will handle way over 100 amps for the speed part of the run .
If you have good data on stuff running the best biggest NiMh packs - start with the props they use for that plane
I can run 600 watts on my puny 32 EFlite motor on 5 cells -and 700 watts for a short run - -in the 3.5 lb models aerobatic types tho
use as many volts as weight will permit - that cuts back on conductor heating for same amps input
08-29-2007, 11:58 PM
#29
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RE: Prop pitch speeds
Dick,
I plan on using the LI-ion a123 cells for the power they can dump lots of amp and survive. As far as an ESC the Phoenix HV-45 will handle 45 amps at 50 volts so that would give me the power handling. Thus far the one thing I can't find is a motor that can take that type of power that is light enough....
I plan on using the LI-ion a123 cells for the power they can dump lots of amp and survive. As far as an ESC the Phoenix HV-45 will handle 45 amps at 50 volts so that would give me the power handling. Thus far the one thing I can't find is a motor that can take that type of power that is light enough....
08-30-2007, 01:50 AM
#30
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RE: Prop pitch speeds
ORIGINAL: dick Hanson
Ok -but efficiency and max power for max speed are not the same things
More power input is more power output till you blow the thing up
Been there--You keep loading the prop till something gives .
....
ORIGINAL: adam_one
So, an increasing of Input Power doesn't necessarily mean increasing Prop's Output Power.
Also, every electric motor type has an ideal voltage, current and RPM at which the motor's max efficiency is obtained.
That gives the max flight duration but not the max Power Output.
So, an increasing of Input Power doesn't necessarily mean increasing Prop's Output Power.
Also, every electric motor type has an ideal voltage, current and RPM at which the motor's max efficiency is obtained.
That gives the max flight duration but not the max Power Output.
More power input is more power output till you blow the thing up
Been there--You keep loading the prop till something gives .
....
Let’s assume that you’ve got an AXI 2212/26 Gold Line powered by Kokam 2100 20/40C 3s1p
In attempt to get the highest power input you use a relatively big prop, let’s say:
APC E 14x10
The Power consumption (Input Power) will be about 300W giving about 110W Power Output.
Static efficiency 37%
Now you choose a smaller prop, let’s say:
APC E 12x6
The Power consumption (Input Power) is now about 210W giving about 120W Power Output.
Static efficiency 57%
Note that I’ve chosen loads above the recommended in order to show that just increasing the power input in order to obtain increased power output may be misleading….
08-30-2007, 06:57 AM
#31
RE: Prop pitch speeds
well - if you lock the motor from turning you also get more draw -
I see your point -but in racing - you still must find a way to use more power to go faster .
more power in is still more power out -
it may go out in heat as well as spinning the prop but it is up to the operator to sort it out.
I see your point -but in racing - you still must find a way to use more power to go faster .
more power in is still more power out -
it may go out in heat as well as spinning the prop but it is up to the operator to sort it out.
09-02-2007, 12:55 AM
#32
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RE: Prop pitch speeds
This is a very interesting discussion and I would like to add my two cents.
Propellers provide thrust while absorbing power from the engine or electric motor. For an internal combustion engine there is an engine RPM that corresponds with a peak horsepower. Let's make the following assumptions that the engine produces 1.0 hp at 12,000 RPM. So what is the "best propeller to fly at 60 MPH? We can mathematically model the characteristics of propellers and make the following calculation.
For a 10" prop, it turns out the "best" design is a 8.2" pitch that produces a thrust of 4.72 pounds at an efficiency of 75.6%. The shape is such that the chord at the 70% of the diameter is 3/4" at a Rn of 150,000 and a tip Mach number of 0.47. Okay, maybe you are not satisfied with a 75% efficiency so you jump the diameter up to 14" …. What is the "best" prop? Using the same parameters, but with a 4" larger diameter now produces 5.06 pounds of thrust @ 81.0% efficiency with a 7.8" pitch. However the blade chord shrinks to 0.32" (a toothpick) and the tip Mach jumps to 0.65 and the Rn drops to 87,000. Remember, both of these two propellers absorb 1.0 horsepower at 12,000 RPM at 60 MPH. The first case, the 10x8, is a reasonable size and shape propeller. The second case, the 14x7.8 toothpick, is not a practical design.
The other factor is the airplane, specifically the airplane drag. If at 60 MPH the drag is about 4 3/4 pounds then prop #1 is a good match. If however the drag is more that 4 3/4 then the plane will not reach 60 MPH and a different propeller design is required.
In summery, the propeller needs to match the HP, RPM and drag of the airplane. As I said at the start …. an interesting topic.
Propellers provide thrust while absorbing power from the engine or electric motor. For an internal combustion engine there is an engine RPM that corresponds with a peak horsepower. Let's make the following assumptions that the engine produces 1.0 hp at 12,000 RPM. So what is the "best propeller to fly at 60 MPH? We can mathematically model the characteristics of propellers and make the following calculation.
For a 10" prop, it turns out the "best" design is a 8.2" pitch that produces a thrust of 4.72 pounds at an efficiency of 75.6%. The shape is such that the chord at the 70% of the diameter is 3/4" at a Rn of 150,000 and a tip Mach number of 0.47. Okay, maybe you are not satisfied with a 75% efficiency so you jump the diameter up to 14" …. What is the "best" prop? Using the same parameters, but with a 4" larger diameter now produces 5.06 pounds of thrust @ 81.0% efficiency with a 7.8" pitch. However the blade chord shrinks to 0.32" (a toothpick) and the tip Mach jumps to 0.65 and the Rn drops to 87,000. Remember, both of these two propellers absorb 1.0 horsepower at 12,000 RPM at 60 MPH. The first case, the 10x8, is a reasonable size and shape propeller. The second case, the 14x7.8 toothpick, is not a practical design.
The other factor is the airplane, specifically the airplane drag. If at 60 MPH the drag is about 4 3/4 pounds then prop #1 is a good match. If however the drag is more that 4 3/4 then the plane will not reach 60 MPH and a different propeller design is required.
In summery, the propeller needs to match the HP, RPM and drag of the airplane. As I said at the start …. an interesting topic.
