Unloaded rpm of glow engine?
12-09-2010, 11:56 AM
#26
RE: Unloaded rpm of glow engine?
http://www.youtube.com/watch?v=RvM2lwWN7sc
Listen and learn. Props are typically 16-18" square. I'm not sure what power would have been during this time, but probably on the order of 2+kW.
Listen and learn. Props are typically 16-18" square. I'm not sure what power would have been during this time, but probably on the order of 2+kW.
12-09-2010, 02:02 PM
#27
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RE: Unloaded rpm of glow engine?
I know props can be stalled on the ground. look at the Square props like 12x12 12x14 16x16 and so on. they are stalled while the plane is setting on the ground. in high HP engines we can spin them at a higher rpm on the ground. when the plane takes off and gets on step the props "unstall" and then load the engine down more.............
ever listen to a anyturbo prop plane with the quite carbon blades. listen when the shut down the engine and the blades feather. you can hear them stall.
the electric guys use data recording speed controls go look at some of thier date and see some props unload a lot and some load up just after take off then unload again after air speed rises.
i flew my small revolver one time that has a OS 120 AX in it with a 11x14 APC prop. high rpm on the ground around 13,000 and in the air didnt seem to change at all in rpm. in a very high straight down dive it would increase rpm some as it reached 160+ but in level flight of 135 it was putting a big load on the engine.
ever listen to a anyturbo prop plane with the quite carbon blades. listen when the shut down the engine and the blades feather. you can hear them stall.
the electric guys use data recording speed controls go look at some of thier date and see some props unload a lot and some load up just after take off then unload again after air speed rises.
i flew my small revolver one time that has a OS 120 AX in it with a 11x14 APC prop. high rpm on the ground around 13,000 and in the air didnt seem to change at all in rpm. in a very high straight down dive it would increase rpm some as it reached 160+ but in level flight of 135 it was putting a big load on the engine.
12-09-2010, 02:10 PM
#28
RE: Unloaded rpm of glow engine?
Lnewqban,
Thanks for the illustration for the 15x8 and 14x10 prop flight speed estimate and for the other member's well researched and thought of posts. My goal is to determine why a particular prop is gaining more rpm in the air (15x8) versus other props with similar load (14x10) while having almost the same rpm on the ground (i.e 15x8 and 14x10), thus having a higher airspeed in the air. Using some speed calculator, an apc 15x8 turning 8700 rpm on the ground will have an estimated speed of around 66mph, but when my plane's speed was measured by a radar gun... it was around 82mph on a level flight. So it only means that the rpm actually increased significally during flight. I guess all I want to know is how to determine which prop would be increasing rpm the most in the air so we could all apply this data to our prop selection.
thanks,
ton2di
Thanks for the illustration for the 15x8 and 14x10 prop flight speed estimate and for the other member's well researched and thought of posts. My goal is to determine why a particular prop is gaining more rpm in the air (15x8) versus other props with similar load (14x10) while having almost the same rpm on the ground (i.e 15x8 and 14x10), thus having a higher airspeed in the air. Using some speed calculator, an apc 15x8 turning 8700 rpm on the ground will have an estimated speed of around 66mph, but when my plane's speed was measured by a radar gun... it was around 82mph on a level flight. So it only means that the rpm actually increased significally during flight. I guess all I want to know is how to determine which prop would be increasing rpm the most in the air so we could all apply this data to our prop selection.
thanks,
ton2di
12-09-2010, 06:11 PM
#29
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RE: Unloaded rpm of glow engine?
Your 15-8 prop may have wound up a bit in the air, sure. Your results do not necessarily mean the rpm increased to 82/68 times 8700rpm though. It is hard to apply a hard and fast rule like you suggest. Too many variables.
- how is the pitch measured? True aerodynamic pitch, i.e. the angle between the prop blade airfoil zero lift angle of attack and the plane of rotation? Lower surface datum line (like pitch gauges measure)? There can be a few degrees difference there.
- is the pitch number accurate (in whatever system of measure)? Or is it a round number?
- what is your ground rpm and in flight rpm in relation to the engine's power curve?
- does the engine have a resonant exhaust system? For the most extreme example of this watch a YouTube video of an F2A speed model.
- how well is the prop matched to the airframe and engine combination?
What I suspect is happening is that your 15x8 is well matched to the drag of the airframe and available power - whereas the 14x10 is overpitched and is wasting power due to excessive blade AoA in the speed range where the airframe drag is reaching the limit for the power available. This may be a classic case where the next "faster" prop in terms of equal static load factor is not faster in the air, but slower. In which case stick with the 15-8. The fact that the aircraft only flies 82mph or so tends to bear this out.
The ground rpm is about the same because although the 15-8 has less pitch, it has more blade area. At zero forward speed they use the same horsepower to spin 8700 rpm. But to achieve higher forward speed with the 14-10 you need less drag. Then.. the 15-8 may top out due to pitch whereas the 14-10 can continue to accelerate the aircraft to a higher forward speed until drag absorbs all the available power times efficiency (power converted to thrust).
Think of a list of prop choices, somewhat arbitrary here but let's assume they have the same static (zero forward airspeed) load on the airframe - 12-12, 13-11, 14-10, 15-8, 16-7. Which one will fly your aircraft the fastest? Good question.. not necessarily the 12-12. The prop that will fly the fastest on your aircraft and engine combination, is the one that converts the power to thrust most efficiently as you converge on maximum forward speed as dictated by drag and available power.
Ever notice that you may know what you mean to say, but it's real hard to put it into words..? [sm=spinnyeyes.gif]
- how is the pitch measured? True aerodynamic pitch, i.e. the angle between the prop blade airfoil zero lift angle of attack and the plane of rotation? Lower surface datum line (like pitch gauges measure)? There can be a few degrees difference there.
- is the pitch number accurate (in whatever system of measure)? Or is it a round number?
- what is your ground rpm and in flight rpm in relation to the engine's power curve?
- does the engine have a resonant exhaust system? For the most extreme example of this watch a YouTube video of an F2A speed model.
- how well is the prop matched to the airframe and engine combination?
What I suspect is happening is that your 15x8 is well matched to the drag of the airframe and available power - whereas the 14x10 is overpitched and is wasting power due to excessive blade AoA in the speed range where the airframe drag is reaching the limit for the power available. This may be a classic case where the next "faster" prop in terms of equal static load factor is not faster in the air, but slower. In which case stick with the 15-8. The fact that the aircraft only flies 82mph or so tends to bear this out.
The ground rpm is about the same because although the 15-8 has less pitch, it has more blade area. At zero forward speed they use the same horsepower to spin 8700 rpm. But to achieve higher forward speed with the 14-10 you need less drag. Then.. the 15-8 may top out due to pitch whereas the 14-10 can continue to accelerate the aircraft to a higher forward speed until drag absorbs all the available power times efficiency (power converted to thrust).
Think of a list of prop choices, somewhat arbitrary here but let's assume they have the same static (zero forward airspeed) load on the airframe - 12-12, 13-11, 14-10, 15-8, 16-7. Which one will fly your aircraft the fastest? Good question.. not necessarily the 12-12. The prop that will fly the fastest on your aircraft and engine combination, is the one that converts the power to thrust most efficiently as you converge on maximum forward speed as dictated by drag and available power.
Ever notice that you may know what you mean to say, but it's real hard to put it into words..? [sm=spinnyeyes.gif]
12-09-2010, 06:58 PM
#30
RE: Unloaded rpm of glow engine?
ORIGINAL: Sport_Pilot
This is overly simplistic. The first figure shows the AOA as the same as the pitch angle when static. This is incorrect as the prop is pulling air in so that there is an incoming air velocity. The second figure shows that the incoming velocity as the same as the planes speed. That would also be wrong as the prop is pulling air in and the incoming air will exceeed the planes foward speed.
This is overly simplistic. The first figure shows the AOA as the same as the pitch angle when static. This is incorrect as the prop is pulling air in so that there is an incoming air velocity. The second figure shows that the incoming velocity as the same as the planes speed. That would also be wrong as the prop is pulling air in and the incoming air will exceeed the planes foward speed.
I believe that you refer to the upwash in front of the LE of the blade, which increases the effective AOA.
I just tried to explain graphically the reason for which the AOA of the blade changes greatly with the increasing forward speed of the plane.
For the case of the 14 x 10 prop, that change of the AOA can be as big as 18 degrees.
I believe that any prop blade will be working in stalled condition beyond around 10 degrees.
Note that the schematics were made to scale, regarding angle of the blade and proportion between the velocities.
The incoming velocity that you mention could be very small compared to the other represented velocities, and I don't know how to estimate it.
Did you check the schematic #3?
According to your description, the AOA would become negative for full forward speed.
However, practical results indicate the opposite, since values above the theorical rpm x pitch have been measured.
12-10-2010, 12:08 AM
#31
Senior Member
RE: Unloaded rpm of glow engine?
Lnewqban,
Just look at it this way; a prop with a 2/1 diameter-to-pitch ratio, would have its static AOA at the tips of the blades, at ~9.2°.
For a full-size plane's prop blades, operating at higher Reynolds numbers, the AOA of stalling is ~12°, so the effective, high-speed outer part of the blades can produce thrust very efficiently...
But for very small chord prop blades, which also have a rather sharp leading-edge, stall occurs at around 9°...
For a 12x6 prop, this would mean the entire blade is theoretically stalled, because the AOA at the tip is the lowest and even it is stalled!
About how the vortex formed by the prop decreases the prop blades' AOA; you pretty much explained it to yourself, in post #23.
Just look at it this way; a prop with a 2/1 diameter-to-pitch ratio, would have its static AOA at the tips of the blades, at ~9.2°.
For a full-size plane's prop blades, operating at higher Reynolds numbers, the AOA of stalling is ~12°, so the effective, high-speed outer part of the blades can produce thrust very efficiently...
But for very small chord prop blades, which also have a rather sharp leading-edge, stall occurs at around 9°...
For a 12x6 prop, this would mean the entire blade is theoretically stalled, because the AOA at the tip is the lowest and even it is stalled!
About how the vortex formed by the prop decreases the prop blades' AOA; you pretty much explained it to yourself, in post #23.
12-10-2010, 03:23 AM
#32
RE: Unloaded rpm of glow engine?
Thank you very much, Dar.
I was reading about these props angles last night and I learned that APC propellers have around 4 degrees difference between the zero lift line and the flat back of the blade (to which the nominal pitch stamped on the prop is measured).
Some other propellers may have up to 6 degrees of difference.
On top of that, because all these airfoils are non-symmetrical, the zero lift angle is higher than the geometric pitch.
Hence, the AOA that we calculate based on the nominal pitch is always smaller that the angle that the blade "feels".
Then, a much "unloaded" prop (right engine-prop-rpm-frame combination) can move an airplane forward faster that nominal pitch x rpm's.
As pointed above, the stream of air, pressure differentials and by-passes that flow thru the prop all change for different forward velocities and rpm's, which may lead to the blades "feeling" very different angles.
(Edited to correct statements in blue)
I was reading about these props angles last night and I learned that APC propellers have around 4 degrees difference between the zero lift line and the flat back of the blade (to which the nominal pitch stamped on the prop is measured).
Some other propellers may have up to 6 degrees of difference.
On top of that, because all these airfoils are non-symmetrical, the zero lift angle is higher than the geometric pitch.
Hence, the AOA that we calculate based on the nominal pitch is always smaller that the angle that the blade "feels".
Then, a much "unloaded" prop (right engine-prop-rpm-frame combination) can move an airplane forward faster that nominal pitch x rpm's.
As pointed above, the stream of air, pressure differentials and by-passes that flow thru the prop all change for different forward velocities and rpm's, which may lead to the blades "feeling" very different angles.
(Edited to correct statements in blue)
12-10-2010, 06:25 AM
#33
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RE: Unloaded rpm of glow engine?
ORIGINAL: Lnewqban
I was reading about these props angles last night and I learned that APC propellers have around 4 degrees difference between the chord line (to which the actual AOA is measured) and the flat back of the blade (to which the nominal pitch stamped on the prop is measured).
On top of that, because all these airfoils are non-symmetrical, the zero lift angle is even higher.
Hence, the AOA that we calculate based on the nominal pitch is always smaller that the angle that the blade ''feels''.
I was reading about these props angles last night and I learned that APC propellers have around 4 degrees difference between the chord line (to which the actual AOA is measured) and the flat back of the blade (to which the nominal pitch stamped on the prop is measured).
On top of that, because all these airfoils are non-symmetrical, the zero lift angle is even higher.
Hence, the AOA that we calculate based on the nominal pitch is always smaller that the angle that the blade ''feels''.
12-10-2010, 06:30 AM
#34
RE: Unloaded rpm of glow engine?
I thought both pitch and AOA were referenced from the apex of the leading edge and trailing edge. I also thought this was what APC was using and claiming that they are using the correct reference and others are using something else. Though I may have my manufacture mixed up.
12-10-2010, 07:44 AM
#35
RE: Unloaded rpm of glow engine?
ORIGINAL: Lnewqban
I believe that you refer to the upwash in front of the LE of the blade, which increases the effective AOA.
I believe that you refer to the upwash in front of the LE of the blade, which increases the effective AOA.
The downwash in your diagram is a small factor. You have a downwash angle and then the 'mean deflection of airflow'. This is half of the downwash angle. This pushes the lift vector aft because of induced drag.
A fixed pitch form will have a variety of speeds along the span. That gives you 'effective pitch'. You also have 'geometrical pitch' and 'experimental pitch', or 'zero thrust pitch'. Geometrical pitch is a measurement at a certain point that gives you have far it will advance in one revolution. Geometric pitch is always greater than effective pitch.
12-10-2010, 06:18 PM
#36
RE: Unloaded rpm of glow engine?
Thank you guys; I stand corrected regarding APC props.
I have corrected the wrong statements of post #32.
I read that in the late Andy Lennon's book "Basics of RC model aircraft design" (page 89).
The schematic of post # 32 applies to many props but to the APC.
Now I have the book in front of me.
The book shows four propeller airfoil sections.
For the APC prop, the nominal (red) and geometric (blue) pitches are parallel and the zero lift angle is -4 degrees.
For the other props the nominal (red) and geometric (blue) pitches have angles of 0.75 and 2.75 degrees and the zero lift angle is -6 degrees.
The book explains that:
-Nominal pitch is measured across the flat back surface, usually at 75% of the diameter; being that the value stamped on the prop.
-Geometric pitch is measured across the chord line (LE-TE).
-True pitch is the actual forward advance per revolution.
-Geometric pitch – True pitch = Actual AOA (also known as propeller slip).
Regarding the upwash and downwash I am not sure.
I just believe it should happen in similar way to a regular wing or helicopter blades.
I understand that there is an accelerating air flow before and after the disk, and that there is low pressure over the curved surface of the blade and high pressure over the flat surface.
I have corrected the wrong statements of post #32.
I read that in the late Andy Lennon's book "Basics of RC model aircraft design" (page 89).
The schematic of post # 32 applies to many props but to the APC.
Now I have the book in front of me.
The book shows four propeller airfoil sections.
For the APC prop, the nominal (red) and geometric (blue) pitches are parallel and the zero lift angle is -4 degrees.
For the other props the nominal (red) and geometric (blue) pitches have angles of 0.75 and 2.75 degrees and the zero lift angle is -6 degrees.
The book explains that:
-Nominal pitch is measured across the flat back surface, usually at 75% of the diameter; being that the value stamped on the prop.
-Geometric pitch is measured across the chord line (LE-TE).
-True pitch is the actual forward advance per revolution.
-Geometric pitch – True pitch = Actual AOA (also known as propeller slip).
Regarding the upwash and downwash I am not sure.
I just believe it should happen in similar way to a regular wing or helicopter blades.
I understand that there is an accelerating air flow before and after the disk, and that there is low pressure over the curved surface of the blade and high pressure over the flat surface.
